DE10101443B4 - Method for determining a sectional image of an object structure - Google Patents

Abstract

Method for determining a sectional image of an object structure by means of an optoelectronic device with a transmitter emitting transmit-light beams, a receiver receiving receive light beams, which has a row-shaped arrangement of N receive elements, and an evaluation unit for evaluating the output signals A _n (n = 1, ... N) the receiving elements, wherein the object structure is illuminated by means of the transmitted light beams and the receiving light beams reflected back at this generating an image of the object structure on the receiving elements of the receiver, characterized in that for determining the object structure, the output signals A _n with two thresholds S ₁ , S ₂ with S ₂ ≥ S ₁ for distinguishing exposed receiving elements (10) with output signals A _n > S ₂ and unexposed receiving elements (10) with output signals A _n <S ₁ , and that the object structure has at least one edge of a sheet (3), on which a part of the transmitted light beams (6) is aligned, wherein by the reflected from the edge of the sheet (3) received light beams (8) a number N _b of successive exposed receiving elements A _n .

Description

The The invention relates to a method according to the preamble of the claim 1.

The The method in question serves to determine a sectional image an object structure by means of an optoelectronic device.

For this purpose used Devices have a transmitting light beam emitting transmitter on, wherein the transmitted light beams for illuminating an object structure be used. At a distance next to the transmitter is a received light rays receiving recipient. The recipient consists of a line-shaped Arrangement of receiving elements. The object structure is determined by the back reflection the transmitted light rays are imaged on the receiver, wherein the Receiving light beams at the outputs of the receiving elements output signals generate in the form of a contrast pattern, which is the object structure equivalent.

The DE 199 09 518 A1 relates to a method for detecting the position of a stacked flat material, in particular a paper stack at the feeder of a printing machine, wherein at least one light beam directed to the stack side, reflected from there and evaluated as a measure of the situation. The at least one light beam is directed both to the stack side and to an area above the stack top edge. The areas with and without reflection through the stack side are used to determine the position of the stack top edge. To carry out the method, a distance sensor is used which emits at least one light beam.

The DE 100 34 072 A1 relates to an arrangement for measuring the height of a sheet stack with optical measuring means. In this case, a Beleuchtungsvorrich device is provided, the light falls obliquely from below into the environment of an upper stack edge. A spatially resolving photoelectric receiver is provided for the light reflected from the vicinity of the upper stack edge.

The DE 38 25 295 A1 relates to a device for detecting the position of an edge, in particular a paper edge in a printing press. A line of light emitting elements emitted light is provided an opto-electrical converter. The edge is feasible between the line and the opto-electrical converter. The light-emitting elements are driven pulsating one after the other, synchronously interrogating the output voltage of the opto-electrical converter.

The DE 41 15 841 A1 relates to a device for the optical control of objects, the device having a sensor for receiving image signals and optionally illumination means for illuminating the object to be controlled. The device has a plurality of image guides which can be positioned independently of one another with their object-side ends and which are connected with their ends remote from the object to one or more optoelectronic sensors.

Of the Invention is based on the object, a method of the initially so-called type so that with this a safe and reliable sheet control can be carried out on a sheet-processing machine.

to solution This object, the features of claim 1 are provided. advantageous embodiments and appropriate training The invention are described in the subclaims.

The basic idea of the invention is to arrange an optoelectronic device laterally offset from the sheets to be detected in such a way that the edge of a sheet can be detected with it. The edge of a sheet or possibly the edges of several sheets are thus imaged on the receiver. In this case, the portion of the transmitted light beams reflected back from each edge of an arc is reflected back to a predetermined number of receiving elements in accordance with the position and the thickness of the sheet. Due to the amount of light reflected back on these receiving elements, their output signals are above a threshold value S ₂ , so that they are exposed. The transmitted light rays passing laterally in the arc are not or only to a small extent reflected back to the receiver, so that the corresponding receiving elements remain unexposed, that is to say the output signals A _n are below a threshold value S ₁ (S ₁ <S ₂ ). In the limiting case, the two threshold values can also coincide, so that the evaluation of the output signals takes place with a threshold value S = S ₁ = S ₂ .

By the evaluation of the consequences of consecutive exposed receiving elements is a reliable one Detection of the bows possible, being the length such a consequence, a measure of the thickness of a Bow supplies.

In contrast to known optical methods for arc detection, the surface of a sheet for its detection in incident or transmitted light method is not exposed to the transmitted light beams. Rather, the arc detection is carried out by lateral transmitted light radiation in the form of a Thickness measurement of the respective bow.

Thereby is achieved that the optical measurement for Boigernfassung largely independently from printing on a sheet. Also, the thickness measurement according to the invention the bows largely independently from the material properties of the bows. So sheets of sheet metal, Cards, slides or other materials can be detected safely. After all is advantageous in that the lateral arrangement of the device Sheet processing machines without design effort on several Make the machine, especially at the feed table done can be.

One An essential advantage of the method according to the invention is that in a sheet-processing machine also sheets then are certainly detectable, if these not only as single sheets but also as multiple sheets more than one above the other lie. It can with the optoelectronic invention Device the individual bows just then detected safely in a multiple arc arrangement when, between these bows different due to the inevitable flapping movements of the bows interspaces are present, which arise through air cushion between the sheets.

A Such a situation is particularly given when with the Optoelectronic device transporting single and multiple sheets on one Feed table of a printing press is controlled. The bows are lying then possibly in multiple sheets on the feed table, where between the individual sheets small interspaces remain due to air cushions.

By lateral arrangement of the optoelectronic device are the single edges of the arches and the top edge of the feed table detected. According to the number and the locations of the edges are followed by the receiver following certain lengths of registered consecutive exposed receiving elements. Between the individual episodes of illuminated reception elements are sequences of successive unexposed receiving elements registered. These unexposed receiving elements are thereby get that parts of the transmitted light rays into the spaces between the Bows are guided and thereby no longer or only to a limited extent be returned to the recipient.

By the evaluation of the consequences of exposed and unexposed receiving elements the number and the thicknesses of the ones lying on the feed table can be Capture bows.

Conveniently, in a training phase, the acquisition of a single sheet, whose thickness is stored as a reference value. This can be done in the following work phase with great certainty by comparison A distinction is made between the current measured values and the reference value of single sheets of multiple sheets become.

Advantageous becomes in the or in another learning phase also the upper edge of the feed table taught as reference value. By referencing the in the work phase measured values on the position of the upper edge the feed table is then a position determination of the sheet edges possible.

The The invention will be explained below with reference to the drawings. It demonstrate:

1 : Cross-section through a feed table of a sheet-processing machine with a laterally mounted on the feed table optoelectronic device for detecting sheets on the feed table.

• a) Draufsicht auf die Anordnung gemäß 1 mit verschiedenen Montageorten der optoelektronischen Vorrichtung.
• b) Seitenansicht der Anordnung gemäß 2a.

2 :

• a) top view of the arrangement according to 1 with different mounting locations of the optoelectronic device.
• b) side view of the arrangement according to 2a ,

• a) leerem Anlegetisch,
• b) einem auf dem Anlegetisch aufliegendem Bogen,
• c) mehreren auf dem Anlegetisch aufliegenden Bögen.

3 : Output signals of the receiving elements of the receiver of the optoelectronic device at

• a) empty feed table,
• b) a bow resting on the feed table,
• c) several sheets resting on the feed table.

1 schematically shows an optoelectronic device 1 that sit sideways at a feed table 2 a sheet-processing machine, in particular a printing press is arranged. On the feed table 2 If necessary, there is a bow 3 on, in particular, can on the feed table 2 , as in 1 shown on top of the feed table 2 also several bows 3 rest on each other.

The bows 3 be in the longitudinal direction of the feed table 2 that is perpendicular to the in 1 illustrated transverse axis of the feed table 2 promoted. When transporting the bows 3 exercise these flapping movements, which can be caused in particular by high transport speeds. That's why the bows are lying 3 not close to each other or on top of the feed table 2 on. Rather, they remain between the bows 3 Interspaces, by arches 3 to bow 3 vary can. The extent of the gaps may be from a fraction to a multiple of an arc. These spaces are shown schematically in 1 shown.

The optoelectronic device 1 Can be used without great constructive effort and in particular with almost no constructive interference in the sheet processing machine in the 1 be shown position shown. Appropriately, in the field of optoelectronic device 1 a depression 4 at the feed table 2 attached, which opens at the top. This creates the in 1 illustrated paragraph on the side wall of the feed table 2 on which the optoelectronic device 1 is aligned.

2 shows different mounting possibilities of the optoelectronic device 1 on the sheet processing machine. It shows 1 the transport of bows 3 during faultless operation of the sheet processing machine. From a pile of bows 3 becomes the topmost sheet 3 individually peeled off and then over the feed table 2 transported. The optoelectronic device 1 For example, in the in 2 shown installation positions on the feed table 2 be mounted, depending on the mounting position on the height of the device 1 in error-free case a double or single sheet is present. In addition, the device can 1 be arranged on the upper edge of the stack. With a device 1 at the feed table 2 is checked whether a faultless transport of a single or double sheet is present, or whether an error in the sheet promotion a different number of sheets 3 is present. With a device mounted on the stack 1 can be checked if the bows 3 be removed individually from the stack.

In the 1 illustrated optoelectronic device 1 is in a housing 5 integrated, which is integrated on a bracket, not shown. The optoelectronic device 1 has a transmitted light rays 6 emissive transmitter 7 and a receiving light beam 8th receiving recipient 9 on, which are arranged at a distance next to each other. The transmitter 7 is for example formed by a light emitting diode. The recipient 9 consists of a line-shaped arrangement of receiving elements 10 , Preferably, the receiver consists 9 from a CCD line element with typically 100 to 1000 closely adjacent receiving elements 10 , The longitudinal axis of the receiver 9 , in whose extension the transmitter 7 is perpendicular to the surface of the feed table 2 ,

The transmitter 7 is a transmission optics 11 for beam shaping of the transmitted light beams 6 downstream. The receiver 9 is a receiving optics 12 for focusing the received light beams 8th upstream. For the suppression of extraneous light influence is the receiving optics 12 a filter 13 upstream. Between the filter 13 and the receiving optics 12 is a blind 14 arranged.

Through the aperture 14 is achieved that the bows 3 in a relatively large distance range, the so-called depth of field, can be detected.

The transmitter 7 is preferably operated in the pulse mode, so that the transmitted light beams 6 be emitted in the form of light pulses with a predetermined pulse-pause ratio. In principle, an arrangement of several transmitters 7 be provided.

The transmitter 7 and the receiver 9 are to an evaluation unit 15 connected, which is formed by a microcontroller, an ASIC or the like. The evaluation unit 15 takes over the control of the transmitter 7 as well as the evaluation of the at the outputs of the receiving elements 10 pending output signals A _n (N = 1, ... N).

As in 1 are shown, the transmitted light beams 6 on the upper edge of the feed table 2 as well as the lateral edges of the arches lying thereon 3 aligned. These from the transmission beams 6 Illuminated object structure is due to the back reflection of the received light beams 8th at the sheet edges and the edge of the feed table 2 on the receiving elements 10 of the receiver 9 displayed. So that the individual bows 3 are detectable, is the resolution of the receiver 9 greater than the thickness of a bow 3 , The thickness of a bow 3 is typically about 70 microns in paper sheets, while the resolution of the receiver 9 typically at about 10 microns.

The optoelectronic device 1 is at a fixed distance from the top edge of the feed table 2 and at approximately constant distance to the edges of the sheets 3 whose layers are transported over the feed table 2 can arrange something, so that the object structure completely from the transmitted light rays 6 is illuminated.

The evaluation of the output signals A _n is described below with reference to the diagrams according to FIG 3 explained. In the diagrams, the output signals A _{n are} the reception elements numbered n = 1,... N 10 Recipient 9 applied. How out 3 As can be seen, the output signals A _{n of} the receiving elements 10 with two threshold values S ₁ , S ₂ , advantageously S ₂ > S ₁ . In principle, an evaluation with a threshold value S ₁ = S ₂ = S is possible.

An illuminated reception element 10 occurs when the incident on this amount of light of the received light beams 8th is so large that the corresponding output signal A _{n is} above S ₂ .

In contrast, there is an unexposed reception element 10 when its output signal A _{n is} below S ₁ .

In 3 Several diagrams of different measurements are shown. The feed table shown above the diagrams 2 , in particular its upper edge, whose position is denoted by 0, and the optionally lying thereon designated A - D positions of the sheets lying thereon 3 show the reference to the respectively scanned object structure.

During a training phase, a referencing of the optoelectronic device takes place first 1 on the upper edge of the feed table 2 , For this purpose, the feed table 2 with the transmitted light rays 6 illuminated, in which case no bows 3 on the feed table 2 rest, that is the in 3 illustrated feed table 2 will be without the bends in positions A - D 3 measured.

The corresponding measurement results are in 3a shown. The on the feed table 2 incident part of the transmitted light rays 6 gets from there to the receiver 9 reflected back. The output signals A _{n of} the receiving elements illuminated in this way 10 are above the threshold S ₂ . Because the surface of the feed table 2 is formed homogeneously, form these receiving elements 10 a complete sequence of exposed receiving elements.

The at the feed table 2 Passed part of the transmitted light rays 6 does not apply to the recipient 9 , so that in addition a complete sequence of unexposed receiving elements 10 is obtained.

To check if the transmitted light rays 6 correctly on the upper edge of the feed table 2 are aligned, the consequences of exposed and unexposed receiving elements 10 with limits N _{a, 1} and N _{a, 0} compared. The limit values N _{a, 1} and N _{a, 0} are in the evaluation unit 15 saved as a parameter. These limits represent a number of exposed or unexposed receiving elements 10 during the measurement of the feed table 2 at least must be maintained so that the top of the investment table 2 is recognized as recognized.

How out 3a is evident, both for the sequence of exposed receiving elements 10 the limit value N _{a, 1} exceeded as well as for the sequence of unexposed receiving elements 10 the limit value N _{a, 0} .

Another criterion for the detection of the upper edge must be the transition of the exposed to the unexposed receiving elements 10 done with a predetermined minimum slope. This is in the evaluation unit 15 as another parameter as maximum number of receiving elements 10 , within which the transition of the exposed to the unexposed receiving elements 10 must be done, the value N _{a, min} stored. Is the highest receiving element 10 the in 3a illustrated sequence of exposed receiving elements 10 less than N _{a, min} receive elements 10 from the lowest of the sequence of unexposed receiving elements 10 removed, the top edge of the feed table applies 2 as recorded. This evaluation is based on the consideration that between the consequences of the exposed and unexposed receiving elements 10 a transition area with receiving elements 10 may lie whose output signals A _n between S ₁ and S ₂ . The position n _{0 of} the upper edge of the feed table 2 preferably results from the average of the positions of the highest exposed and the lowest unexposed receiving element 10 ,

By reference to the thus determined position n _{0 of} the upper edge, the thicknesses and positions of the on the feed table can be described below 2 lying bows 3 be recorded quantitatively.

3b shows the balance of the device 1 on a single on the feed table 2 in the position A lying bow 3 , This adjustment is expediently carried out in the same or in a further learning phase in which a single arc 3 on the feed table 2 through the detection range of the optoelectronic device 1 is encouraged.

How out 3b can be seen, in this measurement again the position n _{0 of} the upper edge of the feed table 2 received as a measured value. According to the position A of the bow 3 on the feed table 2 is also at the receiving elements 10 between n ₁ and n ₂ receive a signal peak.

This signal peak is caused by the fact that the transmitted light beams 6 pointing to the edge of the bow 3 meet, on the receiving elements 10 be reflected back between n ₁ and n ₂ , so that in this area output signals with A _n > S ₂ are obtained.

The both sides of the edge of the bow 3 extending transmitted light beams 6 get into the space between feed table 2 and bow 3 or are passed laterally on this. This results in both sides of the signal peak between n ₁ and n ₂ areas of unexposed output signals A _n , which are below S ₁ . At closing is again the top edge of the feed table 2 detected.

The number N _b of exposed receiving elements 10 between n ₁ and n ₂ provides by reference to the position n _{0 of} the top edge of a measure of the position and thickness of the sheet 3 ,

The number N _b is the reference value N _{b, ref} in the evaluation unit 15 stored. In the working phase following the training phase, within which the sheet control by means of the optoelectronic device 1 takes place, the currently determined measured values of the arc detection are compared with the reference value N _{b, ref} , whereby a distinction between single and multiple sheets is determined. Is a current measured value N with the reference value N _{b, ref} with a predetermined accuracy, preferably in the evaluation unit 15 stored tolerance limits is defined match, so there is a single sheet.

Such during the work phase data acquisition is in 3c exemplified. In the 3c obtained measured values correspond to those in 3 illustrated case, in which a total of four sheets 3 in positions A, B, C and D on the feed table 2 rest.

The output signals A _n according to 3c again show the upper edge of the feed table lying at n ₀ 2 , According to the positions of the bows 3 become with the receiving elements 10 n n between ₁ and _2, n ₃ and n _4, n ₅ and n ₆ and n ₇ and n ₈ a total of four signal peaks obtained from seamless sequences of exposed receiving elements 10 are formed. The widths N ₁ , N ₂ , N ₃ , N _{4 of} the signal peaks represented by the relationships N ₁ = n ₂ -n ₁ , N ₂ = n ₄ -n ₃ , N ₃ = n ₆ -n ₅ and N ₄ = n ₈ - n _{7 are} defined, provide a measure of the thicknesses of each detected arcs 3 ,

According to the width of the spaces between the surfaces of the sheets 3 or of the feed table 2 be between the signal peaks consequences of unexposed receiving elements 10 obtained, where the number N _{z of} the receiving elements 10 in such a sequence of unexposed receiving elements 10 provides a measure of the width of such a gap. The lengths of these sequences are in the evaluation unit 15 with a stored there limit value N _{z, min} compared. If the currently determined number N _{z is} greater than N _{z, min} , then such a gap is considered detected.

At the in 3c a total of four signal peaks are obtained, which are at the top of the feed table 2 connect. Between these signal _maxima are four sequences N _z1 , N _z2 , N _z3 , N _z4 of unexposed receiving _elements 10 receive.

The sequences N _z1 , N _z2 , N _z4 are on the order of the widths of the signal _peaks and are greater than the limit value N _{z, min} , so that these sequences are classified as _gaps . _By contrast, the sequence N _{z3 is} below the limit value N _{z, min} .

Consequently, in the evaluation unit 15 the signal peaks between n ₁ and n ₂ on the one hand and n ₇ and n ₈ as separate sequences of exposed receiving elements 10 detected, each lying between two recognized as safe spaces.

The lengths N ₁ and N _{4 of} these sequences are compared with the reference value N _{b, ref} , in which case there is sufficient agreement, so that these sequences are classified as single arcs, the lengths of the sequences N ₁ and N _{4 being} a measure of provide the width of each single bow.

_By contrast, the length N _{z3 is} the third sequence of unexposed receiving _elements 10 less than the limit value N _{z, min} . Consequently, the signal peaks between n ₃ and n ₄ and n ₅ and n _{6 are} not resolved as separate signal peaks, but in the evaluation unit 15 is evaluated as a common signal peak lying between n ₃ and n ₆ . The width of this signal _peak , which is essentially defined by the relationship N ₂ + N _z3 + N ₃ , is significantly greater than the reference value N _{b, ref} , so that the signal peak between n ₃ and n ₆ is classified as a multiple _arc due to this comparison ,

Thus, the evaluation of the output signals A _n according to 3c a single resolution of the bows 3 in positions A and D as well as a double sheet recognition of the sheets 3 in positions B and C.

Due to environmental influences such as humidity, temperature or material fluctuations of different batches of sheets to be detected 3 For example, the value for the thickness of a single arc may drift. To compensate for this, the stored reference value N _{b, ref can} be tracked to the current measured values by means of an I-controller.

Due to movements of the bows 3 transverse to the conveying direction, the distance of the sheet edges to the optoelectronic device 1 be temporally changeable. This causes a drift of the amplitude values of the output signals A _n , preferably in the evaluation unit 15 are also led with. Alternatively, the drift of the amplitude values can also be compensated by tracking the threshold values S ₁ , S ₂ .

1

Optoelectronic contraption

2

feed table

3

bow

4

deepening

5

casing

6

Transmitted light beams

7

transmitter

8th

Receiving light rays

9

receiver

10

receiving elements

11

transmission optics

12

receiving optics

13

filter

14

cover

15

evaluation

Claims (15)

Method for determining a sectional image of an object structure by means of an optoelectronic device with a transmitter emitting transmit-light beams, a receiver receiving receive light beams, which has a row-shaped arrangement of N receive elements, and an evaluation unit for evaluating the output signals A _n (n = 1, ... N) the receiving elements, wherein the object structure is illuminated by means of the transmitted light beams and the receiving light beams reflected back at this generating an image of the object structure on the receiving elements of the receiver, characterized in that for determining the object structure, the output signals A _n with two thresholds S ₁ , S ₂ with S ₂ ≥ S ₁ for distinguishing exposed receiving elements ( 10 ) with output signals A _n > S ₂ and unexposed receiving elements ( 10 ) are evaluated with output signals A _n <S ₁ , and that the object structure at least one edge of an arc ( 3 ), to which a part of the transmitted light beams ( 6 ), whereby through the from the edge of the sheet ( 3 ) reflected back receive light beams ( 8th ) a number N _b of successive exposed receiving elements A _n ( 10 ), the number N _{b of} the exposed receiving elements ( 10 ) a measure of the thickness of the sheet ( 3 ). Method according to claim 1, characterized in that during a training phase a single sheet ( 3 ) in the beam path of the transmitted light beams ( 6 ) and that the number N _b of successive receiving elements ( 10 ) with output signals A _n > S ₂ as reference value N _{b, ref} in the evaluation unit ( 15 ) and that for the detection of at least one sheet ( 3 ) during a working phase subsequent to the training phase the thereby registered output signals A _n continuously on the presence of sequences of output signals A _{n of} consecutively exposed receiving elements ( 10 ), one sheet ( 3 ) is recognized as recognized if the length N _{b of} a sequence with a predetermined accuracy matches the reference value N _{b, ref} . Method according to claim 2, characterized in that a sheet ( 3 ) is recognized as if the length N _{b of} a sequence within in the evaluation unit ( 15 ) stored tolerance limits with the reference value N _{b, ref} matches. Method according to one of claims 2 or 3, characterized in that by comparing the current length N _b with the reference value N _{b, ref} single and multiple sheets are distinguishable. Method according to one of claims 1 to 4, characterized in that the transmitted light beams ( 6 ) with part of their beam diameter on an upper edge of a feed table ( 2 ) are aligned on a printing machine, wherein on the adjoining the upper edge support surface of the feed table ( 2 ) at least one sheet ( 3 ) rests, which with the edge of the sheet ( 3 ) forms the object structure. A method according to claim 5, characterized in that on the support surface of the feed table ( 2 ) a pile of sheets ( 3 ), which is part of the object structure. Method according to one of claims 1 to 6, characterized in that a part of the transmitted light beams ( 6 ) in spaces between the sheets ( 3 ) and / or the support surface is guided, wherein the height of the threshold value S ₁ is selected such that the surfaces of the arcs bounding the interstices (FIG. 3 ) and / or from the support surface as received light beams ( 8th ) on receiving elements ( 10 ) Recipient ( 9 ) reflected transmitted light beams ( 6 ) in these receiving elements ( 10 ) Generate output signals A _n , which are below the threshold value S ₁ . Method according to claim 7, characterized in that a space between two sheets ( 3 ) is recognized as being if a number N _z of consecutive, unexposed receiving elements ( 10 ) which is greater than or equal to a limit value N _{z, min} , the number N _{z of} the unexposed receiving elements ( 10 ) provides a measure of the width of a gap. Method according to one of claims 5 to 8, characterized in that for the referencing of the optoelectronic device during one or the teach-in phase, the position of the edge of the feed table ( 2 ) is determined. A method according to claim 9, characterized in that on the feed table ( 2 ) incident part of the transmitted light beams ( 6 ) to the recipient ( 9 ), whereby at least a number N _{a, 1} of successive exposed receiving elements ( 10 ) obtained by the at the feed table ( 2 ) guided portion of the transmitted light beams ( 6 ) at least a number N _{a, 0} of consecutive unexposed receiving elements ( 10 ) and that the transition region of the exposed to the unexposed receiving elements ( 10 ) the position n _{0 of} the upper edge of the feed table ( 2 ) Are defined. A method according to claim 10, characterized in that the position n _{0 of} the upper edge of the feed table ( 2 ) is recognized as recognized, if the last of the N _{a, 1} exposed receiving elements ( 10 ) and the N _{a, 0} unexposed receiving elements ( 10 ), which adjoin the transition region, less than N _{a, min} receive elements ( 10 ) are separated from each other. A method according to claim 11, characterized in that the center of the transition region defines the position n _{0 of} the upper edge. Method according to one of claims 9 to 12, characterized in that by reference to the position n _{0 of} the upper edge of the positions of the on the feed table ( 2 ) lying sheets ( 3 ) are determinable. Method according to one of claims 2 to 13, characterized in that to compensate for environmental influences of the reference value N _{b, ref} by means of an I-controller, the current values of N _{b is} tracked. Method according to one of claims 1 to 14, characterized in that the threshold values S ₁ and S ₂ for the compensation of drifts in the amplitudes of the output signals A _{n are} variable.