DE3426165A1 - Dynamometer - Google Patents

Abstract

A dynamometer is proposed having a measuring element which yields resiliently under load and an arrangement for capacitively detecting the spring deflection, which arrangement comprises a fixed and a movable electrode. In this case, the measuring element (12; 46; 90; 100) consists of silicon and is provided with a thin-film electrode (34'; 56, 58; 86'; 96'), the fixed electrode (34; 54, 60; 86; 96) is arranged on a substrate (14; 42, 52; 84; 94), and the measuring element is connected to the substrate via spacers (24; 48; 90, 92; 109) in a permanent fashion to form a module. The said features permit an extremely compact design for the measuring cell of the dynamometer and thus also of the dynamometer itself. Furthermore, they permit economic series production of the measuring cell with constant quality. Application, for example, for use in scales. <IMAGE>

Description

Dynamometer

The invention relates to a dynamometer with a resilient under load flexible measuring element and with an arrangement for capacitive detection of the load-related Deflection of the measuring element, comprising a stationary and one assigned to the measuring element Electrode.

Such dynamometers are for example from the German Offenlegungsschrift 29 41 378 and the European patent application 0 017 581 known. The known Force gauges are characterized, among other things, by a relatively large space requirement especially in the vertical direction (direction of force introduction), first of all Line due to the overall height of the measuring element and the scanning Measuring cell.

The present invention arose from the object of providing a dynamometer with a compact structure, also in the vertical direction, with good measuring properties and is committed to economical series production in reproducible good quality suitable. This task is essentially then resolved when the Measuring cell corresponds to the specified conditions. Accordingly, the invention proposes before that in a dynamometer of the type mentioned, the measuring element made of silicon exists and is provided with a thin-film electrode that the stationary electrode, also as a thin-film electrode, is arranged on a substrate, and that the measuring element is permanently connected to the substrate via spacers to form an assembly is. As will become clear later on the basis of examples, it is possible to use modern techniques and materials according to this concept very compact and simple Manufacture measuring cells to be handled.

Pure silicon is primarily used as the material for the measuring element in the form of single crystals. Single crystals in (111) - or (100) orientation proved to be particularly advantageous with regard to both the Etching behavior as well as the mechanical behavior.

In a preferred embodiment of the invention, the substrate is made made of borosilicate glass, the thermal expansion of which is largely that of the measuring element is equivalent to. Another advantageous embodiment is characterized in that the substrate is also made of silicon.

The spacers are preferably made by etching the substrate or the measuring element. Another advantageous option is to to form the spacers by applied layers. The spacers point expediently in the form of circular ring sections or else are rectangular. In the latter case, for example, the anisotropic etching method is available (for silicon with orientation 100).

In a preferred embodiment, the electrodes form circular areas, each of which has a ring electrode for homogenizing the electric field (and thus to oppression are surrounded by stray capacitances.

In another useful embodiment, the electrodes are rectangular, which in some cases allows a better use of space.

In an advantageous embodiment, the measuring element is rectangular, with a central force-absorbing element via flexible bearings forming thin spots with a rigid frame is connected.

In order to avoid the effects of moisture, for example, the assembly be hermetically sealed.

In some cases it is useful if the assembly is used as a dielectric contains a non-conductive fluid. It can be, for. B. act to oil, whereby the penetration of air humidity is also avoided.

An advantageous variant is seen in that the connecting tracks for the electrodes are diffused into the measuring element or the substrate. In order to can reduce the increase, which may be disruptive when using thin-film technology will.

To eliminate certain disturbances and to increase the measurement signal Another advantageous embodiment provides that the measuring element is between two Substrates is arranged with a fixed electrode each.

In order to be independent of temperature fluctuations and other disturbances it is advisable to provide reference links. Accordingly, in a Another embodiment of the invention proposed that on a common substrate a measuring element and a reference element are arranged. A preferred embodiment this variant is characterized in that the measuring element and the reference element a Form common component, with additional spacers the electrode spacing of the reference part constant regardless of the load. Alternatively, two of the same type could be used Sensors are arranged adjacent to one another, one sensor with the measuring force is applied and the second is not.

Embodiments of the invention are described below with reference to not to scale drawings explained.

In the drawings, Figure 1 is a section through a first embodiment, along the line 1-1 in Figure 2, Figure 2 is a plan view, Figure 3 is a cross section along 3 - 3 in Figure 4, Figure 4 is a diagonal section along 4 - 4 in Figure 3, including upper part, Figure 5 is a block diagram of the circuit and evaluation, FIG. 6 shows a section through a second exemplary embodiment, FIG. 7 shows a section through a third embodiment, Figure 8 a section (along 8 - 8 in Figure 9) through a fourth embodiment, and Figure 9 is a plan view of the fourth Embodiment.

Example I (FIGS. 1 to 5) A measuring cell 10 comprises a measuring element 12 in the form of a square silicon single crystal and a substrate 14 made of glass, about twice as thick as the measuring element and also square. The substrate 14 is glued into a stable bracket 16 made of metal. The measuring force F is centered applied to the measuring element 12.

The mechanical elements that complete the dynamometer carry do not contribute to an understanding of the invention and have therefore been omitted (e.g. guidance and design of the force transducer, housing, etc.). For example, you can post Kind of the arrangement described in US Pat. No. 4,291,776.

The formation of measuring element 12 and substrate 14 is particularly clear from Figures 3 and 4. The substrate 14 has four circular ring sections 24, which have the function of spacers. The spaces 26 serve on the one hand the ventilation (pressure and temperature compensation), on the other hand the implementation of electrical Connections. The inner surface bounded by the circular ring sections 24 shows a thin electrode in the form of a ring 28 with an associated connection point 30 and connection wire 32 and a circular electrode 34 with connection point 36 and connection wire 38.

The measuring element 12 (not visible in Figure 3) has on its The surface facing the substrate 14 has the same arrangement, consisting of a circular electrode 34 'with connection point 36' and wire 38 'as well as ring electrode 28' with connection point 30 'and wire 32'. The two circular and ring electrodes 34, 34 'and 28, 28' are located exactly opposite each other, while the connection points (30, 32, 30 ', 32' and 36, 38, 36 ', 38') are offset diagonally: The opposite corner of the counterpart (Measuring element 12 or substrate 14) is due to space reasons (accessibility of the electrical Connections) removed.

It should be emphasized that although the drawings are not to scale, but are greatly enlarged. Both the measuring element 12 and the substrate 14 of the example have an area of only about one square centimeter, and their Thickness is only a few millimeters in each case. The distance between the electrodes 34, 34 ' from each other is on the order of a few micrometers.

The block diagram of Figure 5 leaves the essential elements of the electrical Recognize circuit. The measuring capacitor consisting of the circular electrodes 34, 34 ' is connected via signal lines 67, 67 'to the two inputs of an oscillator 66 (e.g. B. Type 74 LS 624 from Texas Instruments) connected. It is due to an operating voltage from Z. B.

5 volts. The two ring electrodes 28, 28 (in technical terms often called 'guard rings') are each connected to the signal lines via an isolating amplifier (1: 1) 68, 68 67, 67 '.

The two isolating amplifiers 68, 68 'have a high-impedance input and a low-resistance output. This results in extensive interference suppression of the measurement signal with regard to variable scatter capacities.

Alternatively, a differently constructed oscillator (66) could also be used be e.g. B. of the type LM 331 from National Semiconductor. In this case-would be one Put a measuring electrode and a ring electrode to earth, and it could be one of those two isolating amplifiers can be saved, since stray capacitances to earth are practical do not affect the measurement signal.

In the presence of current-controlled oscillators with input resistances of virtually zero, the ring electrodes 28, 28 'could even be dispensed with.

The pulses generated by the oscillator 66 are predetermined during Periods are counted by a counter 70 and the sums are transferred to a memory 72 and then offset in a microcomputer 74 to form the finished measurement result. About one The decoder / driver 76 controlled by the microcomputer 74 then sends the result to the Digital display 78. A clock (crystal oscillator) 80 supplies the basic frequency also for the microcomputer 74 and, via a counter 82 acting as a divider, the control signals for starting counting, resetting the counter 70 and the takeover of the pulse buzz into the memory 72 and from there into the microcomputer 74.

The measuring principle is known and therefore only needs a brief mention: The measuring element 12 compresses under load, the capacitance of the measuring capacitor (electrodes 34, 34 ') changes. From the correspondingly changed frequency at the output of the Oscillator 66 can be the acting force (in the case of a scale: the weight) determine.

Production of the assembly a) Measuring element 12 using thin-film technology (e.g. vapor deposition) are metallic layers made of a titanium-tungsten mixture as well as gold applied to a silicon single crystal. Using photolithography are made from the metal layer (thickness of the order of one micrometer) the ring and circular electrodes 28 'and 34' and the associated connection points 30 'or

36 'shaped. The connecting wires 32 ', 38' made of gold are after known as 'thermocompression bonding'.

b) Substrate 14 An alkali-containing borosilicate glass becomes less Thermal expansion used (e.g. Schott 8330 or Corning 7740). The glass comes with z. B. by cathode atomization (sputtering) applied thin metal layers made of a titanium-tungsten mixture and gold, followed by a masked photolithographic process. Then the surface is treated with hydrofluoric acid etched, it creates the spacers in the form of the circular ring sections 24. Then the mask layer remaining on the latter is removed.

This is followed by coating with the electrodes and connection points as well as the bonding of the connecting wires, as described above under a).

Both the measuring element 12 and the substrate 14 are expediently not produced in individual pieces, but produced according to the known multiple copying method, for example, based on a standard format of 4 x 4 inches, which then cuts will.

c) Manufacture of the assembly. One measuring element 12 and one substrate in each case 14 are placed on top of one another and positioned so that the electrodes are opposite one another. Then the two elements are anodically bonded (at a temperature of around 400 degrees Celsius and a voltage of around 800 volts for around 10 minutes) connected with each other.

As an alternative to the pairing of glass / silicon, both elements could also be made from Consist of silicon. This variant would have the advantage of being because of the same Material does not rely on differences in mechanical properties or in thermal behavior Have to be considerate. On the other hand, one of the elements must then be marked with a (for example sputtered) glass layer can be provided for the final anodic bonding to enable.

Alternatively, the measuring element 12 and substrate 14 could also be achieved by the thermal pressure method (thermocompression bonding ') after the connection points are through Plating with gold or another suitable metal prepared accordingly became. If the two elements 12 and 14 are made of silicon, they can also use eutectic mixtures of silicon with e.g. gold or aluminum. Depending on the circumstances or requirements, additional ones can be used in a manner known per se Layers can be useful, e.g. as an electrically insulating layer between Measuring element or substrate and electrode and / or as a protective layer against moisture over the electrodes and associated conductor tracks.

The materials used for this would be aluminum oxide or silicon dioxide, for example or silicon nitride into consideration.

Example II (Figure 6) The second example shows a differential acting dynamometer, which is constructed rotationally symmetrical.

On a console 40 is a cylindrical substrate 42 made of glass (or Silicon) attached. It has a thin, circumferential collar 44. The round one Measuring element 46 made of silicon has a closed outer rim 48 as well over several vertical openings 50. The upper end of the assembly forms a second substrate w made of glass or silicon, in the form of a distribution plate a cylinder section. Electrodes 54-60 in the form of concentric circular areas are attached to the top of the substrate 42 (54), on both sides of the Measuring element 46 (56, 58) and on the underside of the distribution plate 52 (60). The three Elements 42, 46 and 52 are in turn firmly connected to form an assembly, namely circumferentially over the contact surfaces 61.

The result is a hermetically sealed and thus z. B.

assembly impervious to moisture. The pressure equalization between the upper and lower space allow the passages 50.

If a force / eFin is directed via a force distributor 62 with an outer ring base 64, so this force leads to a concentric curvature: the rim 48 is after deflected at the bottom, the middle part of the measuring element 46 moves upwards.

Accordingly, the capacities change in the opposite direction between electrodes 54 and 56 and 58 and 60, respectively.

The two correspondingly increased or decreased output signals for example evaluated by means of a quotient formation (C1 / C2 or better C1-C2 / Cl + C2), this results in an elimination of some disturbances such as B.

Fluctuations in temperature.

The electrical circuit can correspond to that according to FIG. There are only the components for the additional capacitor electrodes 58, 60 to be added (oscillator 66 ', Counter 70 ', memory 72'). Further the work program of the microcomputer 74 would have to be adapted accordingly. Again the isolating amplifier 68, 68 'relevant part of the circuit is to be designed depends within the meaning of the explanations relating to example I, it depends on which oscillator (66, 66 ') is used and whether corresponding ring electrodes (28, 28 ') are provided (not in FIG. 6 drawn). All of the modifications mentioned here are per se the person skilled in the art familiar and therefore do not require any further explanation.

Example III (Figure 7) A round substrate 84 made of silicon has via a circular electrode 86 and a ring electrode 88. A round measuring element 90, thinner than the substrate 84 and of slightly smaller diameter, has the corresponding Counter electrodes 86 'and 88'. Two spacers 91, 92 in the form of interruptions separate circular ring sections are provided. They separate the area of the ring electrodes 88, 88 'from that of the circular electrodes 86, 86'. With central load through the measuring force F will only compress the central area within the spacers 92, the distance between the ring electrodes 88, 88 ', however, remains constant. To this A measuring sensor and a reference sensor are combined in an integral assembly.

The arrangement of the leads to the electrodes is essentially the same those according to Example I (see FIGS. 1 - 4), as well as the manufacture of the assembly. Only the spacers 91, 92 are produced differently in this case, namely by applying (sputtering) a layer of glass of the desired height (a few micrometers) through a mask.

Example IV (Figures 8 and 9) The last embodiment shows a measuring cell in a rectangular shape. A substrate 94 has a square sch Electrode 96 (it could also be round) and a rectangular one surrounding it (here: square) frame electrode 98.

A measuring element 100 with a square base has a on the bottom also square recess 102 produced by etching, in which the Electrode 96 'and the frame electrode 98' are arranged. The center 104 of the measuring link 100 has the shape of a truncated pyramid and is used to absorb the measuring force, which can be introduced punctiform or as a surface force. For the first case a recess 105 is provided in the middle of the center 104; in the second case a cap 107 may be used (see Figure 8) It is about four rectangular, Thin strips 106 acting as flexible bearings are connected to the outer rigid frame 108.

Openings 110 in the four corners ensure mutual separation the bending bearing. The circumferential foot 109 of the frame that delimits the recess 102 108 acts as a spacer.

In FIG. 9, two solder points 112, 114 with connecting wires can be seen on the right 116, 118. On the left in the picture you can see two more soldering points 120, 122 with wires 124, 126.Solder 114 belongs to the (lower) electrode 96, solder 122 to the (upper) Electrode 96 '. Correspondingly, the soldering point 112 belongs to the (lower) frame electrode 98 and the soldering point 120 to the (upper) frame electrode 98 '. The electrical connection happens between the top electrodes and the solder points on substrate 94 in that the corresponding conductor track sections 128, 130 on the sensing element 100 and are provided on the substrate 94. When assembling the assemblies through anodic bonding (according to c) in example I) after proper positioning the respective interconnecting conductor track sections 128 and 130 by 'thermocompression' bonding 'conductively connected. Then the wires (116, 118, 124, 126) can be added.

When the measuring element 100 is loaded, only the inner part compresses while the rigid frame 108 does not change position.

Correspondingly, the electrodes 96, 96 'again act as a measuring part and the electrodes 98, 98 'as a reference part.

One advantage of the rigid center 104 configuration is therein see that the electrode gap extends over the entire surface during compression changed practically the same (parallel guidance of the center 104 through the flexure bearings 106); this results in a higher measurement signal than in the case of warping (such as in Examples I to III). -For increased demands on the measurement quality it is possible to use separate sensors for individual disturbance variables (e.g. temperature, air pressure, Humidity), with their output parameters then a further one To apply the oscillator and to count the resulting frequencies and after to offset the corresponding correction formulas in the microcomputer for the corrected measured value.

It goes without saying that the signal conversion described above is only one Example is. Other methods are equally applicable, e.g. B. in the case of Examples II and III the known Wheatstone bridge circuit.

In a further embodiment of the dynamometer according to the invention, further elements are integrated into the assembly consisting of measuring element and substrate be, so z. B.

electronic components of the evaluation circuit in the form of integrated Circuits on the substrate (for which the substrate would have to be widened accordingly), which would further streamline the dynamometer manufacturing process.

To conclude, let’s come back to some essential aspects of those described Force meter pointed out: To achieve the greatest possible load-related changes in capacity (and thus large measurement signals) are precisely detectable small distances between the Electrodes desired or necessary.

These small spaces are made possible by the use of Glass and / or silicon, which are available today with a good surface quality. With the use of thin film and etching techniques, this becomes the fulfillment above Condition allows without it being more precise (and therefore more expensive) mechanical parts requirement. In this way, inexpensive series production can be realized. The good mechanical ones Properties of pure silicon in particular (e.g. with regard to low tendency to creep and the practically ideal spring characteristic in the area of interest) good measurement quality.

In the examples described, the electrodes were made by building-up techniques such as sputtering. or vapor deposition. Others could also be within the scope of the invention known techniques can be used, such as high doping of silicon substrates. All of these variants are intended to be encompassed by the term “thin-film electrodes”.

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Claims (17)

Claims 1. Dynamometer with a resiliently yielding under load ½ measuring element and with an arrangement for capacitive detection of the load-related Deflection of the measuring element, comprising a stationary and one assigned to the measuring element Electrode, characterized in that - the measuring element (12; 46; 90; 100) made of silicon and is provided with a thin-film electrode (34 '; 56, 58; 86'; 96 '), - The stationary electrode (34; 54, 60; 86; 96), also as a thin-film electrode, is arranged on a substrate (14; 42, 52; 84; 94), and - the measuring element with the substrate via spacers (24; 48; 90, 92; 109) inextricably linked to form an assembly connected is. 2. Force meter according to claim 1, characterized in that the substrate consists of a borosilicate glass, the thermal expansion of which is that of the measuring element largely corresponds. 3. Force meter according to claim 1, characterized in that the substrate also consists of silicon. 4. Force meter according to claim 1, characterized in that the Spacers are formed by etching the substrate or the measuring element. 5. Force meter according to claim 1, characterized in that the Spacers are formed by applied layers. 6. Force meter according to claim 1, characterized in that the Spacers (24) have the shape of circular ring sections. 7. Force meter according to claim 1, characterized in that t the Spacers (109) are rectangular. 8. Force meter according to claim 1, characterized in that the Electrodes (34, 34 ') form circular areas, each of which is supported by a ring electrode (28, 28 ') are surrounded. 9. Force meter according to claim 1, characterized in that the Electrodes (96, 96 ') are rectangular. 10. Force meter according to claim 1, characterized in that the Measuring element (100) is rectangular, with a central force-absorbing element (104) over Bending bearing forming thin points (106) is connected to a rigid frame (108). 11. Force meter according to claim 1, characterized in that the Assembly is hermetically sealed. 12. Force meter according to claim 1, characterized in that the Assembly contains a non-conductive fluid as a dielectric. 13. Force meter according to claim 1, characterized in that the Connection tracks for the electrodes in the measuring element or the substrate diffused are. 14. Force meter according to claim 1, characterized in that the Measuring element (46) between two substrates (42, 52) each with a stationary electrode (54, 60) is arranged. 15. Force meter according to claim 1, characterized in that on A measuring element and a reference element are arranged on a common substrate. 16. Force meter according to claim 15, characterized in that the The measuring element and the reference element share a common Form component (90), with additional spacers (92) the electrode spacing of the reference part independent of the load keep constant. 17. Force gauge according to one of the preceding claims, characterized characterized in that the assembly also includes integrated circuits as a component the evaluation circuit includes.