US3553547A - System for aligning and synchronously driving units of a press without mechanically interlinking them - Google Patents

Abstract

A DRIVE SYSTEM FOR OPERATING THE LOADER, UNLOADER, AND CONVEYOR OF A POWER PRESS IN SYNCHRONISM WITH ITS SLIDE. THE ROTOR OF A POLYPHASE INDUCTION MOTOR IS COUPLED THROUGH A GEAR TRAIN TO THE PRES SLIDE. THE ROTORS OF SECOND AND THIRD POLYPHASE INDUCTION MOTORS ARE SIMILARLY COUPLED TO THE UNLOADER AND CONVEYOR OF THE PRESS. BY CONNECTING THE STATOR WINDINGS OF ALL OF THE MOTORS TO A COMMON POLYPHASE POWER SOURCE AND BY INTERCONNECTING THEIR ROTOR WINDINGS WHEN THE ROTORS ARE IN A DESIRED POSITION RELATIVE TO ONE ANOTHER, THE UNLOADER AND THE CONVEYOR ARE MADE TO MOVE IN SYNCHRONISM WITH THE PRESS SLIDE BY THEIR RESPECTIVE INDUCTION MOTORS. ALSO DISCLOSED ARE INTERLOCKS FOR EACH OF THE ELECTRICALLY DRIVEN MOTORS TO INSURE THAT THEIR ROTOR WINDINGS ARE CONNECTED TO THAT OF THE MECHANICALLY DRIVEN MOTOR ONLY WHEN THEIR ROTORS ARE IN THE SAME ANGULAR POSITION, A SECOND SERIES OF INTERLOCKS FOR DISCONNECTING THE ELECTRICALLY DRIVEN MOTORS FROM THE POLYPHASE POWER SOURCE WHEN THEIR ROTORS FALL OUT OF STEP WITH THAT OF THE MECHANCALLY DRIVEN MOTOR, AND INDICATING EQUIPMENT SHOWING THE ANGULAR POSITIONS OF ALL OF THE ROTORS AT ALL TIMES.

Description

j2m- 5, 1971 F. E. HEIBERGER 3,553,547 Y `SYSTEM FOR LIGNING AND SYNCHRONOUSLY DRIVING UNITS OF A PRESS WITHOUT MECHANICALLY INTERLINKING THEM Filed Aug. 7, 1967 8 Sheets-Sheet 1 Jan. 5, w71

F. E. HEIBERGER SYSTEM VOR ALIGNLNG AND SYNCHRONOUSLY DRIVING UNITS OI" A PRESS Wl'I'HOUT MECHANICALLY- TNTERLINKING THEM iff- 12 Jan. 5, 1971 A PRESS FiledAug. '7, 1967 F. SYSTEM FOR ALIGNING A E. HEIBERGER ND SYNCHRONOUSLY DRIVING UNITS OF WITHOUT MECHANICALLY INTERLINKING THEM 8 Sheets-Sheet 5 Jan. s, um

SYSTEM FOR ALIGNING'AN Filed Aug. 7,

F E. HEIBERGER D SYNCHRONOUSLY DRIVING UNITS OF A PRESS Wl THOUT MECHAN ICALLY INTERLTNKING THEM 8 Sheets-Sheet 4 `Eau. 5, 197i F. E. HEIBERGER 3553,54?

' SYSTEM FOR ALIGNING AND SYNCHRONOUSLY DRIVING' UNITS OF' A PRESS WITHOUT MECHANICALLY INTERLINKING THEM ,WM/f i Aff/mwa@ F. E. HEIBERGER Jan. V5, E97@ SYSTEM FOR ALTGNING AND sYNcHRoNoUsLY DRIVING UNITS 0F A PRESS WITHOUT MECHANICALLY TNTERLINKING THEM 1967 Filed Aug. '7,

"8 Sheets-Sheet 6 Jn 5, 1971 F. E. HEIBERGER 3,553,547

`. SYSTEM FOR ALIGNING AND SYNCHRONOUSLY DRIVING UNITS OF A PRESS WITHOUT MECHANICALLY INTERLNKNG THEM FiledAug. 7, 1967 8 SheetS-Sheet 7 NX WX www Jan 5, 1971 F. E. HEIBERGER 3,553,541?

' n SYSTEM FOR ALIGNING AND SYNCHRONOUSLY DRIVING UNITS OF A PRESS WITHOUT MECHANICALLY INTERLINKING THEM Filed Aug.- 7, 1967 8 Sheets-Sheet United States Patent O U.S. Cl. 318-85 4 Claims ABSTRACT F THE DISCLOSURE A drive system for operating the loader, unloader, and conveyor of a powerrpress in synchronism with its slide. The rotor of a polyphase induction motor is coupled through a gear train to the press slide. The rotors of second and third polyphase induction motors are similarly coupled to the unloader and conveyor of the press. By connecting thestator windings of all of the motors to a common polyphase power source and by interconnecting their rotor windings when the rotors are in a desired position relative to one another, the unloader and the conveyor are made to move in synchronism with the press slide by their respective induction motors. Also disclosed are interlocks for each of the electrically driven motors to insure that their rotor windings are connected to that of the mechanically driven motor only when their rotors are in the same angular position, a second series of interlocks for 9 A. INTRODUCTION The present invention relates to power presses and more particularly to a control system for synchronizing the operation of several transfer mechanisms with the slide of the press.

It is a general object of the invention to synchronize several units of a press without mechanically interlinking them.

A more specific object of the invention is to provide a rugged, relatively simple press synchronizing system wherein units of the press are driven by electric motors having interconnected windings energized by current generated directly and without the need for amplification in response to a lead or lag between their position and the position of a unit with which they are to run in synchronism. It is a related object of the invention to maintain the operating speed of the synchronizing motors high relative to the speed of the units which they drive so as to insure eicient operation, yet low enough to prevent the motors from approaching synchronous speed and breaking synchronism with the rest of the system.

A further object of the invention is to provide in a system of the above type means for controlling the interconnection of the windings of its electric motors so as to assure that they will lock .in phase with one another.

A further related object of the invention is to provide means in a system of the above type whereby any motor in the system which falls out of step beyond a preset limit is automatically disabled so as to prevent any damage which such an out of step motor might do to the unit which it drives.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

ICE

B. LIST oF FIGURES FIG. 1 is a general block diagram showing the principal units of a press and the arrangement of electric motors 4whereby they are driven in synchronism;

FIG. 2 is a schematic diagram showing a preferred way of connecting the windings of a pair of motors to a source of power and to one another for operation in the system shown in FIG. l;

FIG. 3 is a more detailed -block diagram additionally showing control circuitry for use with the system generally shown in FIG. 1;

FIGS. `4a4e comprise a single schematic diagram vertically divided into ve portions and showing in detail the synchronizing system and its associated controls represented in block form in FIG. 3;

FIG. 5 is a table indicating the contacts of one of the selector switches of the system of FIG. 4 which are closed at various positions of the switch;

FIG. 6 is a table similar to that of FIG. 5 indicating the contacts of a second selector switch of the system of FIG. 4 which are closed at the different settings of the switch; and

FIG. 7 illustrates the manner in which various controls and displays of the system of FIG. 4 may be arranged on an instrument panel for ease of operation.

While the invention has been described in connection with a preferred embodiment, it will be understood that I do not intend to limit the invention to the embodiment shown but intend to cover the various alternative and equivalent constructions included within the spirit and scope of the appended claims.

C. GENERAL DESCRIPTION OF A PRESS WHICH MAY BE SYNCHRONIZED BY A CONTROL SYS- TEM INCORPORATING FEATURES OF THE IN- VENTION The principal elements of an automatic power press 10 are shown in FIG. 1. They include a base 11 and a vertically reciprocable, cyclically driven slide 13. Upper and lower dies (not shown) are mounted on the slide and on the base for cooperative engagement during downward strokes of the slide. Workpieces are moved into the working area between the dies by a loader 15 and are removed therefrom after the slide has completed its down stroke by an unloader 17. The workpiece is then placed by the unloader 17 upon, and carried away by, a conveyor 19. The press 10 may be an intermediate one in a line of several presses, in which event the workpiece is fed by the conveyor 19 to the loader of the next press in the line. For a more detailed showing and description of a press of the above type, reference should be made to U.S. Pats. 3,199,- 4431and 3,199,439 issued to James C. Danly and assigned to the assignee of the present application. Suice it to say that the loader 15, the unloader 17, and the conveyor 19 must operate in synchronism with the press slide 13 so that each of these transfer mechanisms performs its function in cycles which are precisely timed in unvarying relationship with the cyclic reciprocal motion of the press slide 13. The press includes appropriate mechanisms (not shown), but well known in the art) to convert rotary motion into cyclically reciprocating motion of the slide 13. For purpose of this description, it may be assumed simply that rotary input power is applied to the press by a drive 21 through an input drive shaft 14, which shall be referred to herein as the input drive shaft of the press slide 13.

In accordance with the invention, the press slide drive 21 also drives a polyphase electric motor 23 having a set of windings electrically connected to corresponding Windings of a similar series of polyphase motors 25, 27, and 29 so that their rotors will turn in synchronism with the rotor of the rst motor 23. As will become clear from the explanation which is to follow, the rst polyphase motor 23 acts as a transmitter of electric power received by the other motors 25 through 29 and, to reect this relationship, the terms power synchro transmitter and power synchro receiver are used in the drawings and in the specication. Through suitable mechanical drive trains, the rotors of the respective synchro receivers 25, 27, and 29 are connected to the drive shafts 16, 18, and 20 of the loader 15, the unloader 17, and the conveyor 19 respectively, driving them in synchronism with the rotor of the power synchro transmitter 23 and therefore with the press slide 13.

D. THEORY OF OPERATION OF A PAIR OF POLY- PHASE INDUCTION MOTORS CONNECTED TO OPERATE IN SYNCHRONISM The manner in which a pair of polyphase electric motors connected according to the present invention provides synchronous drive at one or more follower drive shafts is best understood with reference to FIG. 2 which for purposes of illustration shows the motors as being powered by a three phase A.C. supply. A type of motor which is best suited for operation as a power synchro in accordance with the invention is the polyphase wound rotor induction motor of which two are shown interconnected in FIG. 2. The motor 23, operating as the power synchro transmitter of FIG. l, has a set of three phase stator windings 31a, b, c connected to a source of three phase power through lines 33a, b, c, respectively. A rotor, mounted on a shaft 35, carries a set of three phase rotor windings 37a, b, c connected to one another directly at one of their ends and connected at their other ends to a set of three slip rings 39a, b, c. The purpose of the slip rings in the conventional wound rotor induction motor is to permit the rotor windings to be connected to a bank of speed control rheostats by which the torque characteristics of the motor may be changed. As will be seen from the following description, the presence of slip rings in the wound rotor induction motor makes it particularly suitable for use in carrying out the present invention.

Turning now to the polyphase motor suitable for use as a synchro receiver, this motor may be identical to the motor 23 having the same combination of stator windings, rotor windings, and slip rings, which are therefore identied in FIG. 2 by the same reference numerals, primed, as those used to denote the corresponding elements of the motor 23. In accordance with the invention, one set of corresponding windings of the two motors are connected to a common source of polyphase power and the other set of corresponding windings of the motors are connected to one another. In a particular installation incorporating the invention, and illustrated in FIG. 2, the stator windings of the motors were connected to the power source and the rotor windings were connected to each other. In this embodiment of the invention, the stator windings of the motor 27 are connected to the same three phase lines 33a, b, c to which the corresponding windings of the motor 23 are connected, and the rotor windings of the motors 23 and 27 are interconnected, this being readily accomplished through their respective slip rings 39a, b, c and 39a', b', c.

In keeping with the invention, the rotor windings of the motor 2'3, acting as a power synchro transmitter, and of the motor 27, acting as a power synchro receiver, are interconnected at the instant when the rotor windings are in approximately the same position relative to their respective stator windings. :Assuming that such an interconnection is made when the rotor windings are in precisely the same relative position, the flux eld A created by current flowing through the stator windings of motor 23, and the second rotating flux field @B created by similar current owing through the stator windings of motor 27, induce in the respective rotor windings of the motors 23 and 27 voltages which are exactly in phase with one another. Consequently, under this set of conditions, no current flows in either of the rotor windings, and no torque is exerted on either one of the rotors. If the rotor of the motor 2'3 is now turned, its stator windings 31a, b, c, will assume a position relative to its rotor windings 37a, b, c, which is different from the relative position of the stator windings 31a', b', c of the motor 27 relative to their associated rotor windings 37a', b, c so that, depending upon the direction in which the rotor of motor 23 is turned, the voltages induced in its rotor windings will lead or lag those induced in the rotor windings of motor 27. As a result, current will flow between the rotor windings of the motors 23 and 27 and the direction of the current will be such as to cause a torque to be exerted on the rotor of the motor 27 in a sense which will tend to minimize the current, i.e., in a sense which will minimize the positional dilference between the rotors of the motors 23 and 27. Thus it is seen that as the rotor of the motor 23 is turned, a current is transmitted from its rotor windings and received by the rotor windings of the motor 27 so as to cause the rotor of the motor 27 to follow the rotor of the motor 23.

To prevent excessive current flow through the stators of the motors 23 and 27 their rotors should be in approximately synchronous position before they are interconnected. To this end they are connected through a set of relay contacts MR41 whose control forms an important feature of the present invention and which will be eX- plained in detail with reference to FIG. 4. Further limit on the amount of current permitted to flow through the rotors is set by a set of resistors 41a, b, c connected between their respective rotor windings 31a-37a', 37 [1-37 b', and 37e-37C. Alternatively, the rotor windings themselves may be made to have suicient resistance to limit current ilow therein.

E. GENERAL DESCRIPTION OF A SYSTEM INCOR- PORATING FEATURES OF THE INVENTION Turning next to FIG. 3, additional features of a system incorporating the invention will be described with particular attention to various circuits for controlling the interconnection of a power synchro transmitter and a power synchro receiver. The means 21 for driving the press slide 13 are shown in somewhat more detail as including a press drive 43 whose output shaft 44 is connected to the press slide drive shaft 14 through a clutch 45 and a speed reducing drive train 47, which may be a set of gears. In a typical press, the press drive 4.3 includes a synchronous motor operating at 1800 r.p.m. geared down by 6:1 gear ratio to 300 r.p.rn., the rate at which the output shaft 46 of the clutch 45 is rotated when the clutch is engaged. The speed reduction ratio of the drive train 47 is 15:1 so that the drive shaft leading to the press slide is rotated at 20 r.p.m. and the press slide is cycled at a rate of 20 strokes per minute. These figures are a matter of design choice, but will be used in describing the system embodying the invention for sake of clarity in explaining its operation.

According to a feature of the invention, the power synchro transmitter 23 has its rotor mechanically coupled to the press slide 13 through a speed changing drive train so that the rotor will revolve at a significantly higher speed than the drive shaft of the press slide 13, thereby allowing the synchro transmitter 23 and its associated receivers to operate at higher etliciency. Yet the speed at which the motors are driven is kept safely below their synchronous s peed, so as to prevent them from operating as synchronous motors independently of one another. While a separate drive train might be provided for coupling the synchro transmitter 2'3 to the slide 13, in the preferred embodiment the rotor shaft 24 of the synchro transmitter 23 is connected to the output shaft 46 of the clutch 45 so that the speed ratio between the cycling frequency of he press slide 13 and the frequency of rotation of the synchro transmitter rotor is determined by the drive train 47 and is 15:1, making the rotor speed 300 r.p.m., which is safely below the synchronous speed of the motors. Through sets of connecting lines, schematically represented by the lines 49 and 50, the rotor windings of the power synchro transmitter 23 are selectively connected to the corresponding windings of the unloader power synchro receiver 27 and the conveyor power synchro receiver 29 respectively. Because the circuitry relating to the loader synchro receiver 2'5 may be identical to those associated with the unloader and conveyor synchro receivers 27 and 29, the loader power synchro receiver and its associated control circuitry are not shown in FIG. 3 and will not be separately described.

In normal operation, in a manner to be described in detail hereinafter but explained generally with reference to FIG. 2, the rotor windings of the unloader power synchro receiver 27 and of the conveyor power synchro receiver 29 will be connected to the corresponding windings of the power synchro transmitter 23 at appropriate instants and will ybe locked in synchronism therewith, whereupon they, too will rotate in step with the synchro transmitter rotor at 300 r.p.m. or at whatever other speed the synchro transmitter rotor is turned. Through individual sets of speed reducing drive trains 51 and 53, preferably but not necessarily gears, the outputs of the power synchro receivers 27 and 29 and in particular speeds at which their respective rotor shafts 28 and 30 rotate, are reduced by the same factor as the speed reduction ratio which is maintained between the power synchro transmitter rotor shaft 24 and the drive shaft 14 of the press slide 13 so that the outputs of the drive trains 51 and 53 will turn at the same speed as, and in synchronism with, the drive shaft 14 of the press slide. The outputs of the gear trains 51 and 53 are mechanically coupled to the drive shafts 18 and 20 of the unloader and the conveyor 17 and 19 respectively, thus driving them in step with the press slide 13.

In accordance with an important feature of the invention, means are provided for preventing a connection from being made between a power synchro receiver and the power synchro transmitter 23 so long as the position of the power synchroreceiver rotorwinding relative to its stator winding differs by more than a predetermined amount from the position of the rotor winding of the power synchro transmitter 23 relative to its stator winding so as to insure that when the rotor windings of the synchro transmitter 23 and one of the synchro receivers 27 or 29 are interconnected, the resulting current will not be so large as to cause the power synchro receiver rotor to over shoot and run up to near the synchronous speed of the receiver motor. Toward this end, and with particular reference first to the conveyor power synchro receiver 29, means are provided for producing a signal whose magnitude is indicative of the difference in the positions of the rotor windings of the power synchro transmitter 23 and the power synchro receiver 29 relative to their respective stator windings. The means for performing this function includes a monitoring synchro transmitter 55 having a rotor mechanically coupled to the press slide drive shaft 14 and a conveyor monitoring synchro receiver 57 having a rotor mechanically coupled to the conveyor drive shaft 20. The synchros 55 and 57 may be of the well-known type illustrated and described in the above-referenced Pat. No. 3,199,439. As there illustrated, each synchro has a set of three star-connected stator windings and at least one rotor winding. In practice, the stator `windings of the synchro transmitter 55 and the synchro receiver 57 are individually interconnected through three wires, and the rotor winding of the transmitting synchro 5S is energized by a source of alternating current. When so connected, the rotor winding of the synchro receiver 57 produces a voltage across its terminals which by its sense and magnitude is indicative of the sense and magnitude of the difference between the positions of the synchro rotor shafts, and therefore Ibetween the positions of the press slide 13 and the conveyor 19 relative to their respective prescribed positions.

Means are also provided for producing an additional signal whose magnitude is indicative of the diiference in the positions of the rotor windings of the power synchro transmitter 23 and the unloader power synchro receiver 27. These means are shown as a second set of monitoring Synchro devices 59 and 61 which may be constructed and connected in the system identically to the synchro devices 55 and 57 but with the rotor of the unloader monitoring synchro receiver 61 being mechanically coupled to the drive shaft 18 of the unloader 17.

In the block diagram of FIG. 3, only two of the transfer mechanisms, the unloader and the conveyor 17 and 1'9, are illustrated, and signals representing their position relative to the slide appear on output lines 63 and 65 of the conveyor and unloader monitoring synchro receivers 57 and 61 respectively. However, it will be understood that additional transfer mechanisms would be accommodated lby additional position monitors.

In carrying out the invention, there is additionally provided a rst plurality of switching means, and means for opening each of them in response to the signal produced by a respective one of the positions monitors 55, 57 and 59, 61 reaching a first predetermined value, with each of the switching means being individually connected between the rotor windings of the power synchro transmitter 23 and the rotor windings of a respective one of the power synchro receivers 27 and 29. In the preferred embodiment, the plurality of switching means include sets of tie-in interlocks 67 and 69, having contacts respectively connected in series with the rotor windings of the unloader and conveyor power synchro receivers 27 and 29. The interlocks 67 and 69 are individually controlled by Fine Position Error Detectors 71 and 73. The error detectors '71 and 73 have inputs which are individually connected to the position monitor output lines 63 and 65 respectively, and each produces a control signal in response to the signal applied to its input exceeding a predetermined magnitude. The error detectors 71 and 73 are adjusted so that they will respond and actuate their associated interlocks 67 and 69 when the actual positions of the unloader 17 and the conveyor 19 relative to the position of the slide 13 differ by more than a predetermined amount from their prescribed positions in their respective cycles.

It will be helpful to observe at this point that the rotors of the synchro transmitter 23 and the synchro receivers 27 and 29 are so adjusted in setting up the system that, when the respective transfer mechanisms, i.e. the unloader 17 and the conveyor 19, are in proper operative position relative to the slide 13, the positions of the windings of the power synchro receivers 27 and 29 relative to their stator windings will be substantially the same as the position of the rotor windings of the power synchro transmitter 23 relative to its stator windings. Consequently, the signals on the monitor output lines 63 and 65 are representative not only of the positions of the various transfer mechanisms such as the unloader 17 and the conveyor 19 relative to the position of the press slide 13, but are also representative of the positions of the rotor `windings of the respective power synchro receivers 27 and 29 relative to the position of the rotor windings of the power synchro transmitter 23. Similarly, when the signals on the monitor output lines 63 and 65 drop below that amount which causes the error detectors 71 and 73 to maintain the contacts of their associated interlocks 67 and 69 open, not only are the unloader and the conveyor 17 and 19 in proper operative position relative to the slide, 1but also the rotor windings of the unloader power synchro receiver 27 and the conveyor power synchro receiver 29 are in proper position relative to their stator windings for connecting them to the rotor windings of the power synchro transmitter 23 without causing the ow of excessive rotor current between them.

An additional point worth observing is that, because of the speed reducing drive trains 47, 51, and 53, the press slide drive shaft 14, the unloader shaft 18, and the conveyor drive shaft 20 rotate at a fraction of the rate at which the rotors of the power synchros 23, 27, and 29 turn. Thus, with a :1 speed reduction ratio, for each degree that the conveyor drive shaft rotates, its associated power synchro receiver 29 turns 30. Consequently, if the error detectors 71 and 73 are adjusted to respond to a give number of degrees of difference be tween the position of the press slide drive shaft 14 and the positions of the unloader and conveyor drive shafts 18 and 20, respectively, they will also respond to a pre- Y determined number of degrees of difference between the position of the rotor of power synchro transmitter 23 and the positions of the unloader and conveyor power synchro receivers 27 and 29 respectively, but the latter predetermined differences to which the error detectors 71 and 73 will respond are greater by a factor corresponding to the speed reduction factor of the speed reducing drive train. For example, if the position error detectors 71 and 73 are set to produce a control signal when the position error between the drive shaft 14 and the drive shafts 18 and 20 respectively exceed 2, the same position error detectors are also responsive to the positions of the rotors of the unloader and conveyor power synchro receivers 27 and 29 differing by more than 30 from the position of the rotor of the power synchro transmitter 23.

In accordance with a safety feature of the invention, means are also provided to disable any power synchro receiver which fails to stay in synchronism with the power synchro transmitter. To this end, there is proveded a second plurality of switching means, and means for opening each of them in response to the signal produced by a respective one of the position monitors 55, 57 and 59, 61 reaching a second and larger predetermined value. Referring still to FIG. 3, the second plurality of switching means preferably include sets of interlocks 75 and 77 having contacts respectively connected in series with the stator windings of the conveyor power synchro receiver 29 and the unloader power synchro receiver 27. The safety interlocks 75 and 77 are individually controlled by a pair of coarse position error detectors whose inputs are individually connected to the monitor output lines 63 and 65 respectively. The error detectors 79 and 81 are set to apply a control signal to the safety interlocks 75 and 77 so as to interrupt the flow of current to their associated stator windings as soon as the magnitude of the signal appearing at the position monitor output lines 63 and 65 exceeds a value which is larger than that required to open the contacts of the interlocks 67 and 69. Consequently, if, due to a malfunction or other cause, one of the power synchro receivers should fall out of step with the power synchro transmitter 23 beyond a predetermined angle, operation of the synchro receiver which has fallen out of step is promptly interrupted. It will be recognized, of course, that instead of disabling the particular synchro receiver which has fallen out of step it may be preferable to shut down the entire press. Indeed, in the specific embodiment disclosed herein, in addition to disabling an out-of-step receiver, the press drive 44 is also disengaged, and hence the press slide 13 is brought to a stop. The reason for shutting off power from a synchro receiver which has lost synchronism with the rest of the system is that such a synchro receiver has a tendency to motorize, that is, to run up to nearly synchronous speed. It is quite important to prevent this from occurring since the system of synchro transmitter and receivers is designed to operate at a fraction of synchronous speed so that any synchro receiver which is allowed to run up to near synchronous speed would probably cause serious damage to the transfer mechanism which it drives.

To assist the operator of the press in bringing the slide in synchronism with the various transfer mechanisms, a set of visual position indicators are also provided. Thus there is provided a visual press position indicator 83 cornprising an indicating synchro transmitter 85 whose rotor is mechanically coupled to the press slide drive shaft 14- and an indicating synchro receiver 87 having a set of stator windings connected to corresponding windings of the synchro transmitter 85 and having a rotor winding which in a manner well known to those skilled in the art is connected together with the rotor winding of the synchro transmitter 85 to a source of alternating current. A dial or a pointer is mounted on the rotor shaft of the synchro receiver 87 and gives the operator an indication at all times of the position of the press slide 13 in its operating cycle. Similarly, to indicate the positions of the unloader 17 and the conveyor 19, an additional set of visual unloader and conveyor position indicators 8-9 is provided, including, for displaying the position of the unloader, a synchro transmitter 91 and a synchro receiver 93 and, for the conveyor 19, a synchro transmitter and a synchro receiver 97. The synchro pairs 91, 93 and 95, 97 are electrically interconnected in the same manner as the synchro pair 85, 87. The rotor shafts of the indicating synchro transmitters 91 and 95 are individually coupled mechanically to the drive shafts 18 and 20 of the unloader 17 and the conveyor 19 respectively, thus displaying their positions at all times to the operator.

To set up the press, the operator first drives the press slide 13 in an inching mode up briefly energizing the clutch control 45a for brief periods. AS the press slide is thus driven in short increments through its cycle, it successively reaches proper operative positions relative to the positions of the transfer mechanisms such as the unloader 27 and the conveyor 29. As these positions are reached, the contacts of the tie-in interlocks 67 and 69 successively close, permitting the rotor windings of the respective power synchro receivers 27 and 29 to be connected to the corresponding winding of the synchro transmitter 23. This might be performed automatically, but, to have fuller control, in the preferred embodiment of the invention an additional switch must be closed by the operator when he observes (by means of indicators not shown in FIG. 3) that conditions are proper for tying in a particular one of the power synchro receivers.

DETAILED DESCRIPTION OF AN EXEMPLARY SYSTEM FOR SYNCHRONOUSLY DRIVING UNITS OF A PRESS An exemplary synchronous drive system for an automatic press, generally shown in block form in FIG. 3, is illustrated in detail in FIGS. 4er-4e. Before proceeding with the detailed description of the system, the location of the various blocks of FIG. 3 will be indicated on FIGS. 4a-e in order to help orient the reader. The power synchro transmitter 23 and the unloader and conveyor power synchro receivers 27 and 29 are shown in FIG. 4a, along with power control relay contacts MR11-MR71 for controlling connection of the stator and rotor windings of the power synchros. Also shown in FIG. 4a` is the clutch control circuit 45a for permitting inching" of the press slide 13.

The power control relays MR1-MR7, whose contacts appear in FIG. 4a, are shown stacked at the` right of FIG. 4b. The coils of the power control relays MR1 MR7 are energized through a group of contacts shown -to their left in FIG. 4b, most of which are carried by a set of mode selector relays SR2-SR7 shown in FIG. 4c. The selector relays SR2-SR7 are in turn selectively energized by means of a mode selector switch SSZ, also in FIG. 4c, so as to condition a desired group of the power control relays MR1-MR7 of FIG. 4b for operation. The several settings of the switch SS2 permit the control systern to operate in several different modes in each of which a different set of the power control relays MRI- 9 MR7 is conditioned for operation. FIG. 4c also shows the visual position indicators 83 and 89.

FIGS. 4d and 4e illustrate in detail two pairs of error detectors 71, 79 and 73, 81 corresponding to those shown in block form in FIG. 3. Each of the detectors shown in FIGS. 4d and 4e actuates a relay having a Contact in series with the circuits shown in FIGS. 4b and 4c which in turn control the connection of the windings of certain ones of power synchros shown in FIG. 4a.

With this brief orientation, the exemplary system shown in FIGS. 4a-4e may now be considered in more detail.

F1. The power synchro connections Referring first to FIG. 4a, one set of windings of each of the power synchros 23, 27, and 29 is connected to a source of polyphase power through power control relay contacts MR11, MR21, and MR31 respectively. As explained earlier, in the preferred embodiment, it is the stator windings which are connected to the power source, and the power synchros are three-phase wound rotor induction motors energized from a three-phase power supply through lines 98, 99, and 100 connected to the output terminals L1, L2, and L3 of the three-phase supply. Means are also provided for selectively interconnecting the other set rof windings of the synchros which in this case will be the rotor windings. To this end, power control relay contacts MR41 are connected between the rotor windings of power synchro transmitter 23 and the unloader power synchro receiver 27, and a second set of power control relay contacts MR6t1 is connected between the rotor windings of the synchro transmitter 23 and the corresponding windings of the Conveyor power synchro receiver 29. To limit the flow of current through the rotor windings when the contacts MR41 are closed, a

set of three current limiting resistors 101, 103, and 105 is connected in series with them, and a second set of such resistors 106, 107, and 10'8 is connected in series with the contacts MR61 to limit current through the rotor windings of the power synchros 23 and 29.

By means of a first volt meter 109 connected across one of the ycontacts MR41, the difference between the voltages induced in the rotor windings of the synchro transmitter 23 and the synchro receiver 27 by the rotating magnetic fields generated by currents through their respective stator windings may be observed. As apparent in Section D, the voltage difference is representative of the difference between the positions of the rotor windings of the power synchro devices relative to their respective stator windings. A second volt meter 11'1 is connected across one of the contacts MR61 so that with those contacts open, the deflection of the volt meter l11\1 will be indicative of the difference between the positions of the rotors of the power synchro transmitter 23 and the conveyor power synchro receiver 29 relative to their respective stator windings. The indications provided by the volt meters 109 and 111 are used during starting up of the press in a manner to be explained in a subsequent section.

As an optional feature, means are also provided for connecting Aa bank of star-connected resistors 113, 114, 1'15 to the rotor windings of either one of the power synchro receivers 27 and 29 so as to allow them to operate independently of the power synchro transmitter 23 as motors. For this purpose, a set of power control relay contacts MR51 is connected in series between the resistors 113-115 and the rotor windings of the power synchro receiver 27, and a second set of power control relay contacts MR71 is connected between the resistors 113-115 and the rotor windings of the conveyor power synchro receiver 29.

The foregoing paragraphs described briey the power connections to the rotor and stator windings of the power synchros 23, 27, and 29. It will be understood, of course, that only two power synchro receivers have Ibeen shown,

CII

to simplify description and that any number of additional power synchro receivers, driving additional transfer mechanisms or other machine elements may be added within the power limitations of the power synchro transmitter 23. Indeed more than one synchro transmitter may be mechanically coupled to the slide 13, each such transmitter having a family of synchro followers.

Turning now to the control system for the synchronous drive, power is supplied thereto through a power transformer 116 having a primary winding 117 whose terminals L1, L2 are connected to the correspondingly labeled terminals of the three-phase power source through lines 118, 119, respectively. The output of the transformer appears across a secondary winding 120 connected to a pair of bus lines X1 and X2 which are shown as running along the edges of FIGS. 4a-e and from which power is derived by all components of the control system. In a typical installation, the voltage across lines X1, X2 iS 118 volts.

F2. The pressing slide drive control `Control over the `driving of the press slide 13 and of the power synchro transmitter 23 is provided by the clutch control lcircuit a. It is shown to have two modes of operation, which may be selected by `the mode selector switch SSL As illustrated in FIG. 5, the selector switch has three positions. `In the first position, the clutch 45 is de-energized and the slide 13 is at rest. With the switch SS1 in its second position, the press drive is placed in an inching mode. In particular, the clutch 45 iS engaged under the control of a solenoid operated clutch valve SV1 (FIG. 4a) whose coil SV1A is connected across the bus lines X1, X2 through an inch pressure operated switch 121, normally open contacts SS11 of the switch SSI, several other normally closed contacts whose significance will appear subsequently, and through a pressto-open emergency cut-off switch 122. Thus when operation in the Inch mode is desired, switch SSI is placed in its second or Inch position in which its contacts SS1 are closed. The operator then depresses the Inch switch 121 and, so long as he maintains the switch depressed, the clutch valve SV1 remains energized and open, the clutch remains engaged, and the press drive -43 is coupled to the press slide 13.

`If continuous operation is desired, the switch SSrl is placed in its third position, in which only its normally open contacts SS12 are closed. The latter contacts bypass `both the Inch switch 121 and the contacts SSll, causing the solenoid valve SV1 to be continuously energized and the press slide 13 to be continuously driven.

F3. Mode selector control Great liexibility in the operation of the synchronous drive is permitted by the mode selector switch SS'Z (FIG. 4c). As indicated in the table of FIG. `6 and as apparent from FIG. 4c, as the switch SSZ is placed in its successive positions, of which there are six, successive ones of its five normally open contacts SS22-8826 are closed. Each of the contacts is connected in series with the coil of a respective one of the control relays SR2-S117 across the bus lines X1, X2 with the exception of the contact 8R24 which is connected in series with the windings of 4both of the lcontrol relays SR4 and SRS.

With the selector switch in its first position, all of the switch contacts are open and the synchronous drive system is inoperative. With the selector switch in its second and third positions, one or the other of the power synchro receivers 27 and 29 may be energized along with the power synchro transmitter 23, thus permitting the operator to tie in one or the other of the synchro receivers, but not both, to the power synchro transmitter 23.

Both of the power synchro receivers 27 and 29 may be tied to the power synchro transmitter 23 when the Switch S82 is in its fourth position. Finally, in the fifth and sixth positions of the selector switch SSZ, one or the other of the power synchro receivers 27 and 29 may be connected to the bank of resistors 113-115 so as to cause them to operate independently of the power synchro transmitter 23 as motors capable of jogging the transfer mechanisms which they drive into synchronism with the slide 13 in a manner which is opposite to that contemplated when the selector switch is in its other positions.

F4. Mode selector in unloader (second) position Assume first that the selector switch SSZ is in its second position, wherein only the unloader 17 is to be synchronized with the slide 13 and wherein contacts SS22 of the switch are closed. Power is applied to the coil of the relay SR2 through the closed contacts S822, causing the selector relay to pull in and close its normally open contacts SR21 SR23.` Contacts 8R21 complete a circuit through normally closed contacts UC22 and CC22 and through the coil of the power control relay MRI, causing it to pull in its normally open contacts MR11 and to connect stator windings of the power synchro transmitter 23 to the three phase power lines 98-100. The functions of the normally closed contacts UC22 and CC22, and those of all other normally closed contacts, will be explained subsequently, it being sutiicient to note at t'his point that they serve as part of the system interlocks to which reference has already been made.

Contacts SR22, also closed when the selector switch is in its second position, complete a circuit across the bus lines X1 and X2 and through normally closed contacts UC23 through the coil of power control relay MR2, causing it to close its contacts MR21 and to complete a path for power from the power lines 98-100 to the stator windings of the unloader power synchro receiver 27.

Finally, the contacts SR23 are connected to an unloader tie control circuit generally identified by the reference numeral 123. The unloader tie-in circuit 123 includes means whereby the rotor windings of the unloader synchro receiver 27 may be connected to the corresponding windings of the power synchro transmitter 23 and also includes means which are responsive to monitoring devices to prevent the rotor windings from being connected to one another if the rotor positions of the synchros 23 and 27 differ by more than a predetermined amount. Its principal components are the unloader tie control relay CR7, the power control relay MR4, both in FIG. 4b, and the unloader tie disabling control relay UF which appears in FIG. 4d. The control relay UF is energized in response to a control signal produced by the Unloader Fine Position Error Detector 73, whenever the positions of the press slide 13 and the unloader 17, as measured at their drive shafts 14 and 18, differ by more than a permissible amount.

To energize the control relay UF, its coil is connected across the secondary winding 125 of a power transformer 126, a half-wave rectifying diode 127, and the anodecathode circuit of a silicon-controlled rectifier 128. The primary winding 124 of the transformer 126 is connected through lines 129, 130, to supply lines X2 and YS respectively. The latter is connected to the supply line X1 through relay contacts SR21, SR31, and R41 (FIG. 4b). Consequently, half-wave rectified power is made available to the transformer 126 when the selector switch SS2 is in its second, third, and fourth positions, and such current is driven through the coil of control relay UF when the silicon controlled rectifier 128 in series therewith receives a control signal at its input, i.e., across its anode and control electrode. The control signal is produced by the Unloader Fine Position Error Detector 73 whose output, appearing across the secondary winding 243 of the output transformer 221 is applied to the input 0f the rectifier 128.

The coil of the unloader control relay CR7 is connected across the supply lines X1 and X2 through the normally open contacts SR23, which are closed when the selector switch is in its second position, through the normally closed relay contacts UC24, the normally closed contacts UF1 of the unloader tie disabling control relay UF, and through an unloader tie initiating switch 135. Consequently, if the press drive 21 has been operated in the Inch mode so as to bring the press slide 13 in a proper operative position relative to the unloader 17 and so as to bring the rotor windings of the power synchros 23 and 27 in proper positions relative to their respective stators, depressing ofthe unloader tie initiating switch will cause the relay CR7 to energize and to close its contacts CR71- CR75.

To apprise the operator that conditions are proper for depressing the unloader tie initiating switch 135, a green light 139 is connected across the supply lines X1, X2 through the contacts UF1, UC24, and SR23 so that when the control relay UF is released due to the absence of an error indicating signal from its associated Fine Error Detector 73, the light will glow. When the operator closes the tie initiating switch 135, and thereby energizes the control relay `CR7, it seals itself in through its contacts CR73 which are connected so as to bypass the relay contacts UF 1 and the switch 135. Additionally, through its contacts CH71, the energized relay CR7 completes a circuit between the lines X1 and X2 through the coil SVZA of a solenoid operated air valve SVZ associated with the unloader 17. So long as the air valve SVZ is closed, the unloader mechanism is inoperative. This is an optional safety feature designed to insure that the unloader mechanism will be mechanically tied to the rest of the system only if the unloader drive shaft 18 is in proper operative position relative to the slide drive shaft 14.

Closing of contacts CR72 of the control relay CR7 completes the circuit yby which the coil of power control relay MR4 is connected across the supply lines X1, X2, causing the relay to close its contacts MR41 and to connect the rotor windings of the unloader power synchro receiver 27 to the corresponding windings of the power synchro transmitter 23. Contacts CR74 of the control relay `CR7 bypass the relay contacts UF1 and permit the green light 139 to glow even if the relay UF should be energized and its contacts should open.

In accordance with a further feature of the invention, means are also provided for taking preventive action and in particular for decoupling the press drive 43 and for disabling the unloader power synchro receiver 27 in response to a signal from the Unloader Coarse Position Error Detector 81 (FIG. 4d) in the event that the relative positions of the unloader 17 and t'he slide 21 in their respective cycles should differ by more than a predetermined permissible amount.

The principal components of an exemplary safety circuit, generally identified by the reference numeral 141, for carrying out this feature of the invention are the first unloader disabling control relay UC1 shown in FIG. 4d and energized in response to a control signal from the Coarse Error Detector 81, a silicon controlled rectifier 144 connected in series with the coil of the control relay UC1, and a second unloader disabling control relay UCZ shown in FIG. 4b.

The silicon controlled rectifier 144 and the coil of control relay UC1, which are in series with one another, are together connected in parallel with the coil of control relay UF and its associated silicon controlled rectifier 128 so that power is made available to the coil of control relay UC1 under the same circumstances as those during which power is applied to the coil of the control relay UF. To cause current to flow through the coil of the control relay UC1 in response to a control signal produced by the Unloader Coarse Position Error Detector 81, the control electrode and the cathode of the silicon controlled rectifier 144 are connected across the output of the detector 81 appearing at the secondary winding 243 of the output transformer 221.

The first unloader disabling control relay UC1 carries a set of normally open contacts UC11 and the coil of the second unloader disabling control relay UC2 is connected across the supply lines X1, X2 through the relay contacts UC11 and through the contacts CR75 of the unloader tie control relay CR7. Consequently, the safety circuit 141 is conditioned for operation by energization of the control relay CR7 and its second control relay UC2 becomes energized in response to energization of its first control relay UC-l when a suiiicient unloader position vs. slide position error is detected by its associated Unloader Coarse Position Error Detector `81. The amount of error sufficient to cause the disabling control relays UCI and UC2 to be energized will depend upon the inherent stability of the synchronous drive system and upon the speed reduction between the power synchros and the press units to which they are coupled. In a particular version of the system herein described which has been built, the Coarse Error Detector 81 has been set to energize the relay UC1 when the drive shafts 14 and 18 of the press slide 13 and the unloader 17 were more than 4 from synchronous position, i.e., when the positions of the rotor windings of the power synchros 23 and 27 relative to their respective stator windings differed by more than 60.

The relay UC2 has six contacts. Normally open contacts UC25 are connected across contacts UC11 and CR75, and serve to lock in the relay UC2 even if one of those contacts should subsequently open. Normally closed contacts UC21 are in series with the circuit used to energize solenoid SV1A of the clutch valve SVI. Consequently, one result of excessive error between the positions of the unloader 17 and the slide 13 is that the press drive 43 is effectively disconnected both from the press slide 13 and from the synchro transmitter 21 so that both of them are stopped. Prompt stoppage of the press slide 13 is particularly desirable, since it might crash down on the disabled transfer mechanism were it to continue to cycle.

Normally closed contacts UC22 are connected in series with the coil of power control relay MR1 so that a second, optional, result of excessive error in the position of the unloader 17 is that power is immediately cut oli from the stator windings of the power synchro transmitter 23.

Normally closed contacts UC23 of the unloader control relay UC2 are in series with the coil of power control relay MR2 so that as a further result, power is also cut off from the stator windings of the unloader power synchro receiver 27. Finally, through normally open contacts UC26, a red warning light 14S is energized from lines X1, X2 so as to apprise the operator of the reason for the press shut-down.

In the foregoing paragraphs, operation of the synchronous drive system was described with the mode selector switch SS2 in its second position. It was seen that with the selector switch in its second position, the operator is limited to tying only the unloader 27 to the slide 13. It was also seen that the system includes means for preventing interconnection of the windings of the power synchro transmitter 23 and the unloader power synchro receiver 27 unless the rotor positions of the power synchros were within a predetermined number of degrees of one another and means were also described for decoupling the press drive 43 from the press slide 13 and for de-energizing the unloader power synchro receiver 27 if the unloader was detected to have fallen out of synchronous position relative to the press slide 13 beyond a predetermined limit. It will now be shown that similar means are provided for controlling and monitoring the operation of the conveyor 29 when the selector switch SS2 is in its third position.

F5. Mode selector switch SS2 in slide-conveyor (third) position With the selector switch SS2 in its third position, its normally open contacts SS23 complete a circuit across lines X1 and X2 through the coil of control relay SR3, causing that control relay to pull in its normally open contacts R31, SR32, and SR33. Through contacts SR31,

power control relay MRI is energized and power is applied to the stator windings of the power synchro transmitter 23. Through contacts SR32, power is applied from the lines X1 and X2 through normally closed contacts CC23 to the coil of power control relay MR3, causing three phase power to be applied to the stator windings of the conveyor power synchro receiver 29. Finally, through contacts SR33, connected in series with the coil of power control relay MR6 through normally closed contacts CC24 and normally open contacts CR81 power is applied to the coils of the power control relay MRG from supply lines X1 and X2, so as to connect the rotor windings of the conveyor power synchro receiver 29 to the corresponding windings of the power synchro transmitter 23, if the contacts CR81 are closed. The latter contacts are part of a conveyor tie-in circuit, generally identified by the reference numeral 140.

Similar to the unloader tie-in circuit 123, the conveyor tie-in circuit includes means for connecting the rotor windings of the conveyor power synchro receiver 29 to the corresponding windings of the power synchro transmitter 23 and also includes means for preventing such an interconnection from taking place if the drive shaft positions of the press slide 13 and the conveyor 19 and the rotor positions of the synchros 23 and 29 differ by more than a predetermined amount from their synchronous positions. Its principal components are the power control relay MR6, the conveyor tie control relay CRS, the conveyor tie disabling control relay CF shown in FIG. 4e, and a silicon controlled rectifier 146 connected in series with the coil of the control relay CF.

Through lines YS and Y6 the series circuit comprising the coil of the control relay CF and the silicon controlled rectifier 146 is connected in parallel with the corresponding circuits comprising the coil of relay UF and the silicon controlled rectifier 128 and the coil of relay UCI and the silicon controlled rectifier 144. Consequently, power is made available to the circuit comprising the coil of the control relay CF and the silicon controlled rectifier 146 under the sarne circumstances as those in which power is made available to the other two control relay circuits.

In carrying out the invention, the control relay CF is energized in response to a control signal produced by the conveyor Fine Position Error Detector 71 and to this end the input of the silicon controlled rectifier 146, comprising its control electrode and its cathode, is connected to the output of the detector 71 appearing at the secondary winding 243a of its output transformer 221a.

To let the operator know when the relative positions of the press slide drive shaft 14 and conveyor drive shaft 20 are proper for the conveyor power synchro receiver 29 to be energized, i.e. when the relay CF has become energized, a second green light 147 is connected between the lines X1 and X2 through normally closed contacts CF1, normally closed contacts CC24, and the normally open contacts SR33 of the control relay SR3. The contacts CF1 are carried by the conveyor tie disabling control relay OF. So long as a position error indicative control signal is produced by the conveyor Fine Position Error Detector 71, contacts CF1 are held open and the light 147 is not energized. As soon, however, as the error between the drive shaft positions of the press slide 13 and the conveyor 19 falls below about 2 or whatever other error the conveyor Fine Position Error Detector 71 is set for, the relay CF becomes de-energized, its contacts CF1 close, and the light 147 glows.

Also connected through the contacts CF1 and across lines X1, X2 is the series circuit comprising the conveyor tie initiating switch 149, the coil of the conveyor tie-incircuit control relay CRS, normally closed contacts CC24, and selector relay contacts SR33. When the green light 147 glows and in response thereto the operator depresses the conveyor tie initiating switch 149, the tie-in control relay CRS is energized and, through its contacts CR82 which are connected to bypass the conveyor tie-in switch 149 and the normally closed relay contacts CF1, the relay locks itself in.

In addition to its normally open contacts CR81, -which when closed supply power to the coil of power control relay MR6, the conveyor tie-in control relay CR8 also carries an additional set of normally open contacts CR84 (FIG. 4c) which form a part of a safety circuit, generally identified by the reference numeral 151, for de-energizing the stator windings of the conveyor synchro receiver 29 in case of excessive position error.

The conveyor safety circuit 151 is similar to the unloader safety circuit 141 described in connection with the unloader 27. Operative to cut off power from the conveyor power synchro receiver 29 in response to a control signal from the coarse error monitor 79, the conveyor safety circuit 151 includes as its principal components the control relay CC2 whose coil is connected across the supply lines X1, X2 through contacts CR84 and normally open contacts CC11, the conveyor disabling control relay CCI (FIG. 4e) which carries the normally open contacts CC11, and a silicon controlled rectifier 152 connected in series with the coil of the control relay CC1. Power is made available to the series circuit comprising the relay coil and the silicon controlled rectifier 152 when the switch SS2 is in its second, third, and fourth positions by connecting the circuit in parallel with the series connected combination of the coil of control relay CF and the silicon controlled rectifier 146.

In accordance with the invention, the conveyor safety circuit 151 is made responsive to the conveyor Coarse Position Error Detector 79 and to this end, there is applied to the input of the silicon controlled rectifier 152 the control signal which is produced by the detector 79 and which appears at the secondary winding 243a of the` output transformer 221a. The conveyor Coarse Position Error Detector 79 may be identical to the unloader Course Position Error Detector 81 and is operative to render the silicon controlled rectifier 152 conductive and to cause current to ow through the coil of control relay CCI when the position of the rotor of the conveyor power synchro receiver 29 differs by more than a predetermined amount from the position of the rotor of the power synchro transmitter 23. Consequently, in a Imanner similar to that described with reference to the safety circuit 141, the conveyor safety circuit 151 is conditioned for operation when, in response to seeing the conveyor in tie position green light 147, the operator depresses the conveyor tie initiating switch 149 and ties the conveyor power synchro receiver 29 to the power synchro transmitter 23. If subsequent to this tie-in the synchro receiver 29 falls out of step by an excessive amount, the contacts C11 close, relay CC2 is energized, and its contacts CC21 through CC26 close.

Energization of the relay `CC2 has an effect upon the system similar to that which follows energization of the control relay UC2 associated with the unloader safety circuit 141. Normally closed contacts UC21, in series with coil SV1A of the clutch valve SV1 open, causing the clutch to be disengaged and the slide drive 43 to be decoupled from the slide 21. Power control relay MRI is de-energized by the opening of normally closed contacts CC22 which are in series with the coil, causing power to be cut off from the stator windings of the power synchro transmitter 23. Power is also cut off from the stator windings of the conveyor power synchro receiver 29 due to de-energization of the power control relay MR3 whose coil is in series with the normally closed contacts `CC23 of the safety control relay CC2. Normally closed contacts CC24, in series with the coil of power control relay MR-6, de-energize that relay and cause the rotor windings of the conveyor power synchro receiver 29 to be disconnected from the corresponding windings of the power synchro transmitter 23. Contacts CC bypass the normally open contacts CC11 and CR84, thus insuring that 4the safety relay CC2 remains energized even though one of those contacts should be opened. Finally, through normally open contacts CC26, energization of the safety relay CC2 causes power to be applied to a second red light 153 -which is appropriately identified on a control board (see FIG. 7) to warn the operator of the cause for the shut-down of the press.

To summarize briefly, it has been shown that in the second and third positions of the selector switch SS2, means are rendered operative in the system for controlling the tying of the unloader power synchro receiver 27 and the conveyor power synchro receiver 29 respectively to the power synchro transmitter 23 and for automatically shutting down the press and deaenergizing the respective power synchro receivers in the event that an excessive position error appears between the slide 13 and a respective one of the unloader 17 and the conveyor 19. In the following few paragraphs it will be shown that instead of individually operating one or the other of the unloader power synchro receiver 27 with its associated control and safety circuitry and the conveyor power synchro receiver 29 with its associated power control and safety circuitry, they may both be operated by placing the selector switch SS2 in its fourth position.

F6. Mode selector switch SS2 in press, unload, conveyor (fourth) position When the selector switch SS2 is placed in its fourth position, its contacts SS24 are closed, completing a path from supply line X1 through selector relays SR4 and SRS to the other supply line X2. Energization of the control relays SR4 and SRS activates all of the control circuits which are otherwise actuated with the selector switch SS2 in its second and third positions. Specifically, the control relay SR4 carries four normally open sets of contacts SR41-8R44. Relay contacts SR41 are connected in parallel with relay contacts SR21 and SR31 and are operative to apply power to the coil of power control relay MRI. Relay contacts SR42, connected in parallel with relay contacts 8R22, serve to apply power to the coil of power control relay MR2. Through relay contacts SR43, which are connected in parallel with relay contacts 8R32, power is applied to the coil of power control relay MR3, and the power control relay MR4 is energized through relay contacts SR44, connected in parallel with relay contacts 8R23. Finally, the control relay SRS carries a single set of contacts 8R51, connected in parallel with relay contacts SR33 and operative to supply power to the coil of power control relay MR6. Thus it is seen that, collectively, the contacts of control relays SR4 and SRS close exactly the same circuits which are collectively closed by the contacts of control relays SR2 and SR3, respectively energized when the selector switch SS2 is in its second and third positions. It follows therefore that all of the operations permitted separately by the second and third settings of the selector switches S2 are available to the operator when he sets the selector switch in its fourth position.

In a subsequent section, operation of the control system with the selector switch in its fourth position will be described, since the control system would be used in this mode when the synchronous drive system is started under ordinary circumstances. Before doing so, however, those portions of the control system which are actuated with the selector switch SS2 in its fifth and sixth positions will be described.

F7. Mode selector switch SS2 in its unloader jog and conveyor jog (fifth and sixth) positions As previously indicated, the normal method used by an operator to bring the press slide 13 into proper operative position relative to the unloader 17 and the conveyor 19, and any other transfer mechanisms which the system might drive, is to operate the press slide drive 21 in an inching mode until the slide 13 has been brought into proper operative position with the several transfer mechanisms successively. This is the preferred method for practicing the present invention. However, an alternative method and apparatus for carrying out such a method are made available when the selector switch SS2 is in its fifth and sixth positions. According to this method, the final line-up between the slide 13 and a transfer mechanism is achieved by intermittently operating the power synchro receiver coupled to the transfer mechanism as a motor, independently from the power synchro transmitter 23. As will become apparent, in positions tive and six of the switch SSZ, jogging is performed only after the position of the particular transfer mechanism relative to the slide 13 is within the limits established by the Coarse Position Error Detectors. Only then does the operator have the option of moving the transfer mechanisms to achieve closer position agreement as an alternative to doing so by jogging the slide by means of the Inch switch 131.

In its fifth position the selector switch SS2 closes its contacts SS25 and through them energizes control relay SR6 causing it to close its normally open contacts 5R61- SR62. These contacts form part of a circuit whose principal components are the power control relays MR2 and MRS and the bank of resistors 113-115 and which is operative to cause the rotor of the unloader power synchro receiver 27 to be moved in small measured increments independently from the power synchro transmitter 23 until the transfer mechanism which it drives is brought into proper operative position relative to the slide 13. In particular, the coil of power control relay MR2 is connected across supply lines X1, X2 through a series circuit which includes the relay contacts SR61, the emergency interrupt switch 122, line 155, pressure operated normally open unloader jog control switch 157, line 159, and normally closed relay contacts UC23 of the unloader safety circuit 141.

Through its contacts 8R62, the selector relay SR6 also serves to connect the coil of power control relay MRS across the lines X1, X2. Accordingly, when the selector switch SS2 is in its fifth position, the control relay MRS is continuously energized and its contacts MRSI connect the resistor bank 113-115 to the rotor windings of the unloader power synchro receiver 27. Furthermore, the line YS is not energized, hence the relay UC2 cannot be actuated so that the relay contacts UC23 are closed, and the operator can, by intermittently depressing the unloader jog control switch 157, cause the power control relay MR2 to be intermittently energized and, through its contacts MR21, power to be intermittently applied to the stator windings of the synchro receiver 27. With its rotor windings interconnected through the resistors 113-115, the synchro receiver 27, which it will be recalled in the preferred embodiment of the invention is a polyphase electric motor, operates as a motor, currents being induced to circulate between the rotor windings through the resistors by the rotating flux field produced by the current driven through its stator windings. Thus, by intermittently depressing the unloader jog switch 157, the operator may jog the unloader 17 into proper operative position relative to the slide 13 while the slide itself is stationary.

Means are also provided for causing the conveyor power synchro receiver 29 to be moved intermittently so as to bring the conveyor into proper operative position relative to the slide 13. Means for performing this function are actuated by placing the selector switch SS2 in its sixth position and are comprised of the selector relay SR7 and the power control relays MR3 and MR7 as well as the bank of resistors 113-115. Selector relay SR7 is connected across the supply lines X1 and X2 through switch contacts SS26 which become closed when the selector SS2 is placed in its sixth position. Contacts SR71 of the selector relay SR7 are connected in series circuit with a conveyor jog control switch 161, normally closed contacts CC2`3, and the coil of power control relay MR3 between the supply lines X1 and X2. Additionally, through contacts SR72 of the selector relay SR7,

the coil of power control relay MR7 is connected across the lines X1, X2. Consequently, with the selector switch SS2 in its sixth position, the resistors 113-115 are connected through relay contacts MR71 to the rotor windings of the conveyor power synchro receiver 29 and, provided that the position of the conveyor 19 relative to the slide 13 is within limits so that the normally closed contacts CC23 in series with the conveyor jog control switch 161 are closed, the operator may, by depressing the switch, cause the conveyor power synchro receiver 29 to jog the conveyor into sufficiently close position relative to the slide 13 to permit tying of the conveyor power synchro receiver 29 to be initiated.

F8. The fine and coarse position error detectors The provision of means for producing suitable control signals when the positions of the various transfer mechanisms driven by the synchronous drive system relative to the position of the slide 13 differ by more than predetermined amounts forms an important part of the invention. Also important is the provision of detecting means which are capable of distinguishing between position errors of the above type, but of different size so that, depending on the size of the detected error, either preventive or corrective action may be undertaken by the system automatically. Thus, in accordance with a specific feature of the present invention, a plurality of control means are provided, each for producing a control signal when the position of a given transfer mechanism relative to the position of the slide 13 exceeds a first predetermined limit, and for producing a second control signal when the position of the same transfer unit differs by a second and larger predetermined amount from the position of the slide. It will be recognized by those skilled in the art that many types of position responsive controls could be employed to provide the requisite control signals in the system embodying the present invention. For sake of completeness, a particularly sensitive and satisfactory circuit for producing such signals will now be described with reference to FIGS. 4d and 4e.

In a preferred embodiment of the invention, each control circuit includes two main parts: a position monitor and a pair of position error detectors. The position monitor produces a signal representative of the position difference between the slide 13 and the unloader 17 and is formed of the synchro transmitter 59 and the synchro receiver 61. As described in Section E, the synchros 59 and 6&1 are mechanically coupled to the drive shafts 14 and 18 of the press slide and the unloader 17 respectively.

The stator windings (not shown) of the synchros 59 and 61 are electrically interconnected and the rotor winding of the transmitting synchro 59 is energized by alternating current derived from a center tapped coil I171 connected across supply line X2 and supply line Y4, the latter being connected through line 173 to supply line X1 either through relay contacts 8R23- or through relay contacts SR44 (FIG. 4b) when the selector switch SSZ is in its second or fourth position. The output of the position monitor formed of the synchros 59, 61 appears at the terminals 61a, 61b of the rotor winding of the unloader synchro receiver 61. Provided the synchros 59 and `|51 are properly aligned so that their rotors are in the same positions relative to their respective stators when the unloader 117 is exactly in the right positionrelative to the slide 13, there will appear a voltage at the terminals 61a, 61b whose magnitude and phase relative to the phase of the alternating voltage across lines X2, Y4 is indicative of the direction and magnitude of the difference between the positions of the slide 13 and the unloader 17.

As an optional feature, an indicating circuit may be provided to display to the operator the amount of position error represented by the signal appearing at the output terminals of the'unloader synchro 61. Such a circuit is shown in FIG. 4d and is generally identified by the reference numeral 175. It includes a voltmeter 177 having a center null position and connected across the output terminals 179, 181 of an A.C. bridge whose arms are comprised of the series connected resistors 183 and 185 and of the two halves of the center tapped secondary winding 187 of a transformer 189 and rectifying diodes 191, 192 connected in series therewith. Connected b.,- tween the junction point of the resistors 183` and 185 and between the center tap of the transformer secondary winding 187 is the secondary winding 193 of a supply transformer 195 having a primary winding 197 connected across the supply lines X2, Y4.

So long as no voltage is imposed upon the transformer primary winding 190 by the unloader synchro 61, equal amounts of pulsating current are driven by the transformer 195 through the top and bottom halves of the transformer secondary winding 189 and through the diodes 191 and 192. Consequently, the voltages at output points 179 and 181 of the bridge are equal, the net output of the bridge is zero, and the meter 177 reads zero. If, however, there is a position error between the drive shafts of the slide 13 and the unloader 17, an A C. current is driven through the primary winding 190 of the transformer 1187 and induces a current that is in phase with the current flowing in either the top or the bottom half of the transformer secondary winding 189, depending upon the direction of the error between the shafts. Therefore, the net current driven through the resistors 183 and 185 will differ, causing the bridge to produce a net output and the meter 177 to indicate the magnitude and the polarity of the error signal produced by the unloader synchro 61.

Turning now to a consideration of the Unloader Fine Position Error Detector 73, it includes an input stage for converting the A.C. signal produced at the outputs of the unloader synchro 61 into a pulsating D C. signal suitable for controlling the remaining stages of the detecting circuit. The A.C. to D.C. converting input stage includes an input transformer 199 having a primary winding 201 shunted by a transient suppressing capacitor 203 and connected across the outputs 61a, 61b of the unloader synchro receiver 61. The secondary winding 205 of the transformer 199 is connected across the input terminals of a standard full-wave rectifier bridge 207. Connected across the output terminals of the full-wave rectifier 207 is a filtering capacitor 209 and a Zener diode 210 so that there appears across the diode 210 a filtered D.C. which is proportional to the A.C. produced at the outputs of the synchros 61, but which is limited to a predetermined magnitude to protect subsequent portions of the circuit.

The second stage of the Unloader Fine Position Error Detector 73 is a triggered oscillator which includes a silicon controlled rectifier 2111 and a transistor 215 to amplify the D.C. voltage across the Zener diode 210 and to initiate the ow of current through the rectifier 211 in response to a relatively small input signal. The output circuit of the rectifier 211 connected between its anode and its cathode includes a diode 215 poled in the same direction as the rectifier 211, a capacitor 217, and the primary winding 219 of an output transformer 221, shunted by a tuning capacitor `223 and connected between the anode of the rectifier 211 and the capacitor 217.

Voltage regulated power for driving current through the silicon controlled rectifier 211 is provided by a second full wave rectifier bridge 22S which is supplied with alternating current through lines 227 and 229 from supply lines X1, X2. To maintain the voltage produced at the output terminals of the bridge at a steady level, a second Zener diode 231 is connected across them through a voltage dropping resistor 233. Through a first bus line 235 the negative terminal of the second Zener diode 231 is connected to the corresponding terminal of the first Zener diode 210 so that the voltages appearing at the positive terminals of the Zener diodes 210- and 231 appear against the same reference potential.

The base-emitter (input) junction of transistor 213 is connected across the Zener diode 210 while the emittercollector (output) circuit of the transistor is connected through a load resistor 237 between the negative terminals of the Zener diodes 210 and 231 and the cathode of diode 215 in the output circuit of the silicon controlled rectifier 211. Finally, a biasing voltage divider, comprising a pair of series connected resistors 239, 241 is connected across the Zener diode 231 which in the exemplary circuit may have an 18 volt reverse breakdown rating. Conveniently, the resistors are of equal value and their junction point is connected to the control electrode of the silicon controlled rectifier 211 so that it is maintained at +9 volts relative to the negative terminals of the Zener diodes 21-0 and 231.

In the absence of a signal at the outputs of the unloader synchro 61, the voltage across the Zener diode 210, and therefore across the base-emitter junction of the transistor 213 is zero, and the transistor is cut off. The collector-emitter circuit of the transistor 213 is connected in series through the resistor 237, the diode 215, the anode-cathode circuit of the silicon controlled rectifier 211, the primary winding 219 of the transformer 221 and line 236 across the Zener diode 231. Because of the high open circuit impedance of the transistor 213 a large part of the voltage produced at the outputs of Zener diode 231 appears across the transistor so that the cathode of the silicon controlled rectifier 211 is maintained at or above the voltage at which its control electrode is held by the voltage dividing resistors 239 and 241 and the rectifier 211 is cut off.

If, on the other hand, a signal is produced at the unloader synchro outputs 61a and 61b, a forward bias is applied to the base-emitter junction of the transistor 213, causing it to draw current from the rectifier bridge 225 through line 236, transformer primary winding 219, silicon controlled rectifier 211, diode 215, and resistor 237. As a result, the voltage at the cathode of the rectifier 211 is pulled to a level which is negative relative to the +9 volts at `which its control electrode is held by the resistors 239, 241, and the rectifier 211 is biased into conduction. This conduction is oscillatory, the frequency of oscillations being determined by the values of the capacitors 217, 223 and of the transformer 221. A frequency of about 1000 c.p.s. has been found suitable.

Briefly summarizing therefore, it is seen that the detector 73 includes an A.C. to D.C. converting input stage, and a triggered oscillator output stage.

The output of the detector 73 appears on the secondary winding 243 of the transformer 221, shunted by a transient suppressing capacitor 245, and is applied across the anode and the control electrode of the silicon controlled rectifier 128, which it will be recalled forms the input of the unloader tie control circuit 123.

The Unloader Coarse Position Error Detector 81 may be constructed identically with the Unloader Fine Position Error Detector 73 and will therefore not be discussed in detail. It will be sufiicient to note that, to render the Coarse Error Detecting circuit 61 less sensitive than the Fine Error Detecting circuit 73, it is connected to the output terminals 61a, 61h of the unloader synchro 61 through a pair of series connected lattenuating resistors 259 and 261, the latter of which is variable so as to permit adjustment of the sensitivity of the Coarse Error Detector 81. The output of the Coarse Error Detector 81 appears across the secondary winding of an output transformer 221' and is applied to the silicon controlled rectifier 144 of the unloader safety circuit 141.

In the foregoing paragraphs of this section, the contro] means associated with the unloader 17 and shown in FIG. 4d was described in detail. The control means associated with the conveyor 19 maybe identical and is so shown in FIG. 4e. The details of the control means serving the conveyor 19 will not be repeated, but a brief indication of its principal parts will be given for sake of completeness.

-As the circuit of FIG. 4d, so the circuit of FIG. 4e cornprises a position monitor formed of a pair of synchros 55 and 57 and a pair of position error detectors 71 and 79. An output appears at the secondary winding of the output transformer 221a of the circuit 71 when the relative positions of the press slide 13 and the conveyor 19 differ by more than a predetermined amount, and this output is applied to the input of the silicon controlled rectier 146 of the conveyor tie-in control circuit 145.

When the relative positions of the press slide 13 and the conveyor 19 differ by a predetermined amount, `a control signal is produced at the secondary winding of the output transformer 221a of the Coarse Position Error Detector 79` and this signal is applied to the cathode and the control electrode of the silicon controlled rectifier 152 which forms the input of the conveyor safety circuit 151.

F9. Starting up the synchronous drive system To `aid the operator in starting up the system, its controls and indicators are mounted in a convenient cluster upon an instrument panel 265 substantially as shown in FIG. 7. To indicate the respective positions of the press slide 13, the unloader 17 and the conveyor 19, the indicating synchro receivers 87, 93, and l97 (FIGS. 3 and 4c) are mounted on the instrument panel 265 and carry suitable dials 267, 269, and 271 upon their respective rotor shafts.

Mounted on the panel 265 below the synchro dials 267-271 are the volt meters 109 and 111 (FIG. 4a) which indicate zero when the rotors of the unloader power synchro receiver 27 and the conveyor power synchro receiver 29 are in synchronous position relative to the rotor of the power synchro transmitter 23 and whose pointers are defiected in proportion to the amounts by 'which the positions of the respective power synchro receivers 27 and 29 differ from that of the power synchro transmitter 23.

Mounted to the right of the unloader power synchro receiver position indicator meter 109 is the unloader position error indicating meter 177 (FIG. 4d) and similarly mounted to the right of the conveyor power synchro receiver position indicator meter 111 is the conveyor fine position indicating meter 177 (FIG. 4e).

The meters 177 and 177 complement the meters 109 `and 111. The latter pair indicate a null each time the rotors of their respective power synchro receivers are brought in synchronous position with the rotor of the power synchro transmitter. In the exemplary embodiment, this occurs thirty times for each complete rotation of the drive shafts 18 and 20 relative to the press slide drive shaft 14, and the meters 109 and 111 each register twentynine nulls which are of no interest to the operator. Only one of the rotor nulls of the respective synchro receivers 27 and 29 coincides with the alignment of the respective drive shafts 18 1and 20, with the press slide drive shaft 14. The meters 177 and 177 serve to show when this occurs. v

Located below the row of indicating meters are a set of warning lights 139 and 145 for the unloader (FIG. 4b) and 147, 153 for the conveyor (FIGS. 4b and 4c).

In the bottom row are the control switches, consisting of the unloader jog switch 157, the unloader tie initiating switch 135, the press inch control switch 121, the conveyor tie initiating switch 149, the conveyor jog switch 161, and the emergency stop power interrupt switch 122, all of which are shown in FIGS. 4a and 4b of the detailed circuit diagram.

Finally, completing the instrument panel 265 are the mode selector switches SSI and SS2.

Turning now to the manner in which the synchronous drive system is started, let it be assumed first that the mode selector switch SS2 is in its fourth position 'wherein the control circuits associated with the unload power synchro receiver 27 and the conveyor synchro receiver 29 are both operative. The operator will begin by pressing intermittently on the press inch switch 121 while 0bserving the dials 267-271, after having placed the drive mode selector switch SSl in its Inch position. With each depression of the press inch switch 121, the slide 13 will be moved slightly, and this movement will be indicated by the dial 267. The operator has an option of synchronizing either the unloader or the conveyor first. It is reasonable to assume that he will synchronize whichever of the units the press slide is brought into synchronism with. Let that unit be assumed to be the unloader 17. As the press slide is brought gradually closer in position to that of the unloader 17, the pointers of both of the position indicating meters 109 and 177 'will gradually drop to zero. The operator now knows that the press inch control switch 121 should be pressed only lightly lest the slide overtravel its proper position relative to the unloader 17. When the drive shaft 14 of the press slide 13 is within approximately two degrees of the drive shaft 18 of the unloader l17, the green light 139 glows, indicating that conditions are appropriate for tying the unloader power synchro receiver 27 to the power synchro transmitter 23. Upon seeing this indication, the operator depresses the unloader tie initiating switch 135, and the rotor windings of the unloader power synchro receiver 27 and of the power synchro transmitter 23 are automatically interconnected.

Once the unloader synchro receiver 27 has been tied to the power synchro transmitter 23, the operator will again press the inch switch 121 which will now have the effect of not only advancing the press slide 13 but also the unloader 17 driven by its power synchro receiver 27. The operator now returns his attention to the dials 267 and 269 which will turn in unison and to the dial 2'7I1. When the -rst two dials indicate that the press slide 13 and the unloader 17 have been brought close to the proper operative position relative to the conveyor 19, the operator looks at the meters 111 and 177. The synchro position error indicating meter 111 will by now have dropped almost to zero and the pointer of the conveyor position error indicating meter 177 will begin to drop toward its center null position. When the press slide 13 and the unloader 17 are in proper position for tying of the conveyor power synchro receiver 29 to the power synchro transmitter 23, the green light 147 glows and in response thereto, the operator depresses the conveyor tie initiating switch 149. This automatically causes the conveyor power synchro receiver rotor 'windings to be connected to the corresponding windings of the power synchro transmitter 23 and, assuming that the unloader 17 and the conveyor `19 are the only transfer mechanisms which are to be tied to the. power press slide 13, the start up operation of the synchronous drive system is complete. The selector switch SSl may now be turned to its third position wherein the press slide drive 21 operates in a continuous mode.

As an alternative to the method outlined in the foregoing paragraphs, the operator may reduce the difference between the positions of the press slide 13 and the particular transfer mechanisms 17 and 19 with which he is concerned by operating the power synchro receiver associated with the particular transfer unit as a motor independently of the power synchro transmitter 23. For example, the operator may place the selector switch SS2 in its yfth o r unloader jog position, and reduce the difference between the positions of the slide 13 and the unloader 17 by briefly depressing the unloader jog control switch y157. Provided that the position of the slide 13 relative to that of the unloader 17 is in the direction in which the unloader is driven by its associated power synchro receiver 27 when it operates as a motor, as described in Section G7 previously, depressing the unloader jog control switch 157 will reduce the error sufficiently so that the green light 139 will glow, indicating that the unloader tying-in may be accomplished. In a similar 23 manner, with the selector switch SSZ in the sixth position, alignment of the conveyor '.19 with the slide 13 may be brought about by briefly depressing the conveyor jog switch 161.

G. CONCLUSION `It may be seen from the foregoing that there has been brought to the art a drive system whereby various machine elements, and in particular transfer units of a power press, may be driven in synchronism with one another. The system disclosed herein is rugged, simple in construction, and in operation, insuring not only immediate shut down of the press in the event of instability, but also insuring that components of the system which might cause harm are energized only under conditions which tend to contribute to maximum system stability.

I claim as my invention:

1. In an automatic press having a slide coupled to and cyclically moved by a drive, and transfer mechanisms coupled to and cyclically driven by individual drive shafts, an improved synchronous drive for said transfer mechanisms comprising in combination (a) a iirst three phase induction motor having a rotor coupled to said slide through a speed changing drive train so as to make n revolutions for each cycle of said slide,

(b) additional three phase induction motors, one for each of said transfer mechanisms, each having a rotor individually coupled to the drive shaft of its associated transfer mechanism through a speed changing drive train so as to cause each transfer mechanism to complete one cycle for each n revolutions of its associated motor,

(c) a set of stator windings and a set of rotor windings on each of said motors, the rotor of each of said additional motors being so positioned relative to the respective transfer mechanism to which it is coupled that its rotor windings are brought into a synchronous positions with the rotor windings of said rst motor when said respective transfer mechanism is in its normal operative position in its cycle relative to the position of said slide in its cycle.

(d) means for connecting the stator windings of each of said motors to a common three-phase power source,

(e) means for individually connecting the rotor windings of said additional motors to the rotor windings of said iirst motor,

(f) a plurality of position monitors, each mechanically coupled to said slide and to a respective one of said transfer mechanisms for producing an electrical signal whose magnitude is indicative of the magnitude of the deviation from normal of the position of a respective one of said transfer mechanisms relative to the position of said slide in their respective cycles,

(g) a first plurality of switch means, each set to open in response to the signal produced by a respective one of said position monitors reaching a irst predetermined value and individually connected between the rotor Iwindings of said rst motor and the rotor windings of a respective one of said additional motors, and

(h) a second plurality of switching means each set to open in response to the signal produced by a respective one of said position monitors reaching a second and larger predetermined value, and individually connected between said stator winding connecting means and the stator windings of a respective one of said additional motors.

2. In an automatic press having a cyclically reciprocable slide and several transfer mechanisms, each coupled to individual drive shafts, an improved system for driving said transfer mechanisms in synchronism with said slide comprising in combination (a) a plurality of polyphase wound rotor induction motors, each having rst and second sets of relatively movable polyphase windings,

(b) 4means for mechanically coupling the rotor of one of said motors to the drive shaft of said slide and additional means for mechanically coupling the rotors of the other of said motors individually to the drive shafts of said transfer mechanisms so that when the respective transfer mechanisms are in proper operative position relative to the slide, the relative positions of said sets of windings of the respective other motors are substantially the same as the relative position of the windings of said one motor,

(c) means for connecting the rst set of windings of said motors to a common polyphase power source,

(d) drive means mechanically coupled to the drive shaft of said slide,

(e) control means for selectively causing said drive means to operate either in small increments until the slide is successively brought into proper operative position .relative to each of said transfer mechanisms, or continuously, and

(f) means for successively connecting the second set of windings of respective ones of said other motors to the second set of windings of said one motor at the instants when the slide is brought into proper operative position relative to the respective transfer mechanisms driven by those motors.

3. The system of claim 2 further characterized by a plurality of means each for preventing the second set of windings of a respective one of said other motors from being connected to the corresponding windings of said one of said motors in response to the position of the transfer mechanism coupled to said respective one of said other motors relative to the position of said slide differing by more than a rst predetermined amount from its proper operative position.

4. The combination of claim 3 further characterized by a plurality of means, each for causing the rst set of windings of a respective one of said motors to be disconnected from said common polyphase power source in response to the position of the transfer mechanism coupled to said respective one of said motors differing from its proper operative position relative to the position of said slide by a second, larger predetermined amount.

References Cited UNITED STATES PATENTS 2,246,333 6/ 1941 Wickerham B18-42X ORIS L. RADER, Primary Examiner A. G. COLLI-NS, Assistant Examiner U.S. C1. X.R. 318-44

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