GB2286052A

GB2286052A - Weighing machine with correction for the influence of moisture

Info

Publication number: GB2286052A
Application number: GB9501186A
Authority: GB
Inventors: Matthias Eger; Michael Fiedler; Stefanie Meyer; Norbert Schnell
Original assignee: Sartorius AG
Current assignee: Sartorius AG
Priority date: 1994-01-29
Filing date: 1995-01-20
Publication date: 1995-08-02
Anticipated expiration: 2015-01-20
Also published as: FR2715727A1; DE9418210U1; FR2715727B1; GB9501186D0; CH689285A5; DE4402655C1; GB2286052B

Abstract

For a balance having a measured value pickup which converts the weight of the goods to weighed into an electric signal, and having a moisture pickup which influences the output signal from the measurement converter in such a manner that the moisture dependency of the measurement converter is corrected, it is proposed that a strain gauge (4) is introduced as moisture pickup and that this strain gauge (4) is bonded to a curved surface (15). …<??>This gives a correction, which is good value and reliable, of the moisture coefficient of the measurement converter, both an analogue electric correction circuit and a computational correction in a microprocessor being possible. … <IMAGE> …

Description

A ' 2286052 WEIGHING MACHINE WITH CORRECTION FOR THE INFLUENCE OF MOISTURE

The present invention relates to a weighing machine with correction for the influence of moisture.

In DE 31 06 534 C2 there is described a weighing machine with a transducer which converts the weight of an object being weighed into an electrical signal and with a moisture pick-up which so influences the output signal of the transducer that correction is made for the moisture dependence of the transducer. The moisture pick-up provides an analog electrical output signal and a analog-todigital converter is connected downstream thereof. The digitalised signal of the transducer and the digitalised signal of the moisture pick-up are fed to the microprocessor, which corrects the signal of the transducer so that the influence of moisture is eliminated from the final result. This, arrangement, however requires a moisture pick-up with electrical output signal, an analog-to- digital converter for the signal of the moisture pick-up, and hardware and software prerequisites in the microprocessor for the digital moisture correction.

There is thus a need for a simple means of moisture detection, which may also offer the possibility of performing moisture correction either by analog electrical circuits or through digital computing operations. The analog electrical moisture correction is A & of merit,, notwithstanding the avail abi 1 ity of cheap digital component groups, for the reason that the moisture correction can be performed directly at the transducer and an easier exchange of the transducer without changes in the microprocessor is thereby possible. This applies not only in the case of service, but also for the replacement of a transducer without moisture correction by a transducer with moisture correction.

According to the present invention there is provided a weighing machine with a measurement transducer which converts the weight of a load being weighed into an electrical signal and with a moisture detector which so influences the output signal of the transducer that correction is made for the moisture dependence of the transducer, wherein at least one strain gauge is used as the moisture detector and is glued onto a curved surface.

Strain gauges are normally used to measure the material strain. They can also be used in measurement transducers of weighing machines to convert the weight-dependent material strain in a spring body into an electrical signal. The moisture dependence of these strain gauges is generally known, but is always regarded as a disturbing effect in the transducers and is therefore kept as low as possible. In the case of the weighing machine of the present invention, this effect is actually exploited and is made significantly greater by gluing onto a curved surface. The moisture influence in the strain gauge comes into being from, for example, swelling of a synthetic material foil underneath the actual expansion- sensitive layer and due to the swelling of an adhesive At ' substance between the synthetic material foil and the generally metallic substrate. Consequently, the spacing between the expansion-sensitive layer and the substrate increases and the expansion-sensitive layer, in the case of a convexly curved substrate, is expanded more by reason of the curvature of the substrate surface, since the expansion-sensitive layer is repositioned outwardly into a region of greater circumference. In the case of concavely curved substrate, the expansion-sensitive layer is correspondingly compressed. The greater the curvature of the substrate surface, the greater the effect. The application of strain gauges to slightly concave thin regions for expansion measurement is known in the case of strain gauge measurement transducers for weighing machines. In the present invention, however, the strain gauge is not specifically glued onto a thin zone and does not serve for measurement of the material expansion in the thin zone. It is preferably glued onto a solid region which is largely free of expansion or contraction caused by the load being weighed. In the case of a weighing machine embodying the invention, the material expansion of the substrate is thus not transmitted to the strain gauge and made measurable, but rather a change in length of the expansion-sensitive layer is achieved with a constant substrate only by the swelling of the synthetic matrial foil and the adhesive substance.

The strain gauge for moisture correction may be glued onto a curved surface transducer, or may be a component of a part secured to the transducer, such as by screws.

A.

Expediently, a spring body with a strain gauge and a Wheatstone bridge circuit is used as the transducer and the strain gauge for moisture correction is used along the bridge diagonal as load resistance of the output signal or is used as input resistance in the feed of the supply voltage. Alternatively, the strain gauge for moisture correction may be used to vary the amplification factor of an amplifier or may be used in parallel with one of the bridge branches.

If the transducer operates on the principle of the electromagnetic force compensation, the strain gauge for moisture correction may be part of a resistance network which converts the coil current into a measurement voltage, or may be used to influence the zero point of the measurement transducer.

In one embodiment, the change in resistance of the strain gauge for moisture correction is digitalised.

If so desired, the strain gauge for moisture correction may additionally have a temperature coefficient so that it corrects for the moisture coefficient of the transducer as well as for its temperature coefficient. The gauge is preferably glued to a material, for which it is not teMDerature-compensated.

Embodiments of the present invention will now be more particularly described with reference to the accompanying drawings, in which:

Fig. 1 is a schematical view of a strain gauge attached to a concave surface in a weighing machine embodying the invention; m d L Fig. 2 is a schematic sectional view of a strain gauge attached to a convex surface in a weighing machine embodying the invention; Fig. 3 is a schematic sectional view of a weighing machine embodying the invention, with four strain gauges for measurement of weight and one strain gauge for moisture correction; Fig. 4 is a first electrical circuit diagram for the strain gauge of Fig. 3; Fig. 5 is a second electrical circuit diagram for the strain gauge of Fig. 3; Fig. 6 is a schematic sectional view of a weighing machine embodying the invention, with one strain gauge for moisture correction in an analog electrical correction circuit; is a schematic sectional view of a weighing machine embodying the invention, with two strain gauges for moisture correction in a digital correction circuit; Fig. 8 is a schematic sectional view of a form of strain gauge which is fastenable by screws; and Fig. 9 is a schematic sectional view of a second form of strain gauge, which is arranged on a threaded member.

Fig. 7 A 1 Referring now to the drawings there is shown in Fig. 1 a strain gauge which serves for moisture correction in an electronic weighing machine embodying the invention. A concave surface is oroduced in a solid substrate 1 by means of a groove 15, which is, for example, milled into the substrate, and a strain gauge is glued onto the surface. The gauge consists of an insulating carrier foil 3, an expansion-sensitive electrically conductive layer 4 and a covering 1 ayer 5. Electrical connections 14 are provided at the expansion sensitive layer. The gauge is glued to the substrate by an adhesive layer 2. The gauge is temperature-compensated in such a manner that its resistance does not change in the case of changes in temperature and thereby caused expansion of the substrate and the gauge. Since the substrate 1 with the groove 15 is very solid and/or the location of the substrate is not exposed to any external forces, no expansion or compression is transmitted from the substrate to the gauge. The resistance of the gauge is thus dependent neither on temperature nor on load. Rather, the resistance of the gauge changes only in dependence on moisture: for increasing air humidity, the carrier f oi 1 3 and the adhes i ve 1 ayer 2 take up water and swel 1. This change in thickness together with the concave surface results in the expansion-sensitive layer 4 being compressed and thereby lowering its electrical resistance. According to the ranges of curvature of the groove 15 and according to the moisture dependence of the thickness of the carrier foil and the adhesive layer, the moisture dependent change in resistance can be up to 0.1%, which is a value which, in the normal use of strain gauges, is reached only on maximum loading.

9 1 Ad 1 A strain gauge on a convex surface 17 is shown in Fig. 2. The substrate 16 can be, for example, a round rod. An adhesive layer 21. a carrier foil 3', an expansion-sensitive layer 4' with connections 14' and a cover layer 5' correspond with the similarly designated layers in Fig. 1. On swelling of the adhesive layer 21 and the carrier foil 31, the expansion- sensitive layer 41 is in this case expanded because of the different curvature, and its resistance correspondingly increases for increasing moisture.

The use of the moisture-sensitive strain gauge in conjunction with a measurement transducer which has the form of a spring body and detects load.-dependent change by means of strain gauges is shown in Figs. 3 and 4. Fig. 3 shows a section through the spring body and Fig. 4 the associated electrical circuit. The end part 23 of the spring body that is fixed to the machine structure is fastened on a base plate 20 and the other end part 22 of the spring body carries a weighing pan 21. An upper guide link 24 and a lower guide link 25 are formed by way of two bores 26 and 27 and a section connecting them. The links are thus connected to the end parts 22 and 23 by four thin zones, which carry four strain gauges 6, 7, 8 and 9. Under loading, the gauges 6 and 9 are expanded and the gauges 7 and 8 are compressed. The gauges 6 to 9 are connected into a Wheatstone bridge circuit (Fig. 4), which is supplied from a voltage source U 0 The output voltage of the circuit is fed to an amplifier 10, digitalised by an anal og-to-di gi tal converter 11, processed in a microprocessor 12 and indicated in digital display 13. The parts of the measurement transducer and the associated measurement circuit are generally known and therefore have been described only briefly.

Ad h For moisture correction of the transducer, a strain gauge as shown in Fig. 2 can be provided. Such a strain gauge, designated 28, is glued onto a part 29 with a convex surface. The part 29 is secured firmly by a screw 30 to the end part 23 of the transducer fixed to the base plate 20. The strain gauge 28 is incorporated into the circuit in the feed of the supply voltage for the Wheatstone bridge circuit. When the gauge 28 is expanded on the convex surface with increasing moisture and its electrical resistance rises, the feed voltage across the Wheatstone bridge falls and the sensitivity becomes less. Consequently, the original positive moisture coefficient of the measurement transducer is corrected for in the case of proper dimensioning of the resistance change. The magnitude of the moisture correction can be chosen simply by the curvature of the surface of the part 29 and by the electrical resistance of the strain gauge 28 in relation to the electrical resistance of the other strain gauges 6 to 9. A fine adjustment is possible in known manner through connecting a (high) resistance in parallel with the strain gauge 28.

An alternative circuit arrangement for the same measurement transducer is shown in Figs. 3 and 5. Like parts as in Fig. 4 are denoted by the same reference numerals. The moisture-correcting strain gauge 38 is in this case mounted on the concave inward surface 33 of the bore 26 at a distance from the thin zones.

Consequently, the gauge 38 is compressed in the case of increasing moisture and lowers its electrical resistance. If, as shown in Fig.

5, the gauge is to function as a moisture-dependent load resistance, A k is then a lowering in resistance leads to a lowering of the bridge output voltage and thus to a correction of the original positive moisture coefficient of the transducer. A fine compensation is possible in this case by an adjustable input resistance in front of the strain gauge 38.

Through choice of the form of circuit according to Fig. 4 or Fig. 5 and choice of a convex or concave surface to which the strain gauge is glued, the sign of the moisture correction can thus be chosen as desired and matched to the sign of the moisture coefficient of the transducer to be corrected. The magnitude of the correction can be determined by way of the radius of curvature of the surface to which the gauge is glued and by way of the electrical resistance of the gauge. An adjustment is possible by parallel or series resistances.

Instead of the circuits of Figs. 4 and 5, other circuits are, of course, possible. Thus, for example, a moisture-dependent voltage divider can be formed from the moisture-dependent strain gauge and a fixed resistor, or the moisture-dependent strain gauge can be incorporated into the feedback of an operational amplifier as an amplification-determining resistance.

A particular advantage of the described correction in a transducer which itself utilises strain gauges is that the moisture gauge can be constructed in the same manner and consists of the same material as each measurement transducer gauge. Consequently, the behaviour as a function of time in the case of a (sudden) change in moisture is also the same (equal time constants) and the moisture d h correction is also effective during a change in moisture. The same applies to the possible temperature dependence of the moisture influence as well as the possible influence of other substances, for example solvents, which have a swelling effect on the measurement transducer strain gauges and on the moisture strain gauge.

Moreover, the moisture correction can be combined in advantageous manner with temperature compensation. For this purpose, for example only the strain gauge 28 in Figs. 3 and 4 need have a corresponding temperature coefficient. If the temperature coefficient of the measurement transducers 22 to 25 is caused by the temperature coefficient of the modulus of elasticity of the material of the measurement transducer, it is generally positive. This positive temperature coefficient is corrected for by a positive temperature coefficient of the strain gauge 28.This positive temperature coefficient can be achieved by, for example, a corresponding choice of the alloy of the gauge 28, or more simply by, for example, a gauge which is temperature-compensated for steel as the substrate and is glued to a material of greater coefficient of thermal expansion, for example aluminium. In the case of measurement transducers, the moisture coefficient and the temperature coefficient of which fluctuate only slightly within a series and for which thus no individual adustment is necessary, the moisture coefficient can be brought approximately to zero by a single gauge 28 through choice of the correct curvature and the temperature coefficient can be brought approximately to zero by choice of the correct material of the part 29. If, thereagainst, an individual is i d 1 adjustment of the moisture coefficient and of the temperature coefficient is necessary, it is more expedient to connect the moisture-dependent strain gauge 28 and a separate temperaturedependent resistance in series into the feed of the supply voltage to the Wheatstone bridge circuit in order to enable an individual adjustment through parallel resistances.

A measurement transducer operating on the principle of electromagnetic force compensation with a strain gauge for moisture correction is shown in Fig. 6. The transducer consists of a system carrier 41, which is fixed to the machine structure and at which a load pick-up 42 is fastened to be movable in vertical direction by way of two guide links 44 and 45 with hinge points 46. The load pick-up 42 in its upper part carries a load pan 43 for the reception of the load to be weighed and transmits the force corresponding to the mass of the load by way of a coupling element 49 to a load arm of a transmission lever 47. The lever 47 is mounted to the system carrier 41 by a cross spring joint 48. A coil body with a coil 51 is fastened to a compensating arm of the lever 47. The coil 51 is disposed in an air gap of a permanent magnet system 50 and produces a compensating force. The magnitude of the compensating current is so regulated by a position sensor 56 and a regulating amplifier 54 that equilibrium prevails between the weight of the load and the electromagnetically produced compensating force. The compensating current produces a measurement voltage, which is fed to an analog to-digital converter 57, across a measuring resistance 55. The digitalised result is taken over by a digital signal -processing unit A & 58 and displayed digitally in a display 59. The parts described so far of the transducer are generally known and therefore explained only briefly.

In addition, the transducer in Fig. 6 has a strain gauge 52, which is glued onto the surface of a convex part 53 and connected electrically in parallel with the measuring resistance 55. A variable series resistance 61 is provided for adjustment. The moisture correction then functions in the manner as already described. For increasing moisture, the gauge 52 is expanded and increases its electrical resistance. Consequently, the electrical resistance of the parallel connection of the measuring resistance 55 and the resistances 52 and 61 increases and the voltage across the anal og-to- digital converter 57 increases for equal current with a is coil 51, whereby an assumed negative moisture coefficient of the transducer is corrected.

If the transducer has a positive moisture coefficient, the moisture strain gauge can be glued onto a concave surface, for example at the location 60 of the system carrier 41, and the electrical circuit can remain the same.

Fig. 7 shows a circuit variant for moisture correction with a measurement transducer operating on the principle of electromagnetic force compensation. Like parts as in Fig. 6 are denoted by the same reference numerals. A first moisture strain gauge 70 is glued to a concave part 60 of the system carrier 41 and a second moisture strain gauge 71 is glued to a convex part 601. They can thus change their electric resistances in opposite manner under the influence of A 1 moisture and can be connected together with two fixed resistances 72 and 73 into a Wheatstone bridge circuit, the output voltage of which can be digitalised in known manner by way of an amplifier 74 and an anal og-to- di gital converter 75 and fed to a microprocessor 58 for digital correction. The use of two gauges on oppositely curved surfaces in conjunction with the Wheatstone bridge circuit has the advantage that a possibly present residual temperature coefficient of the moisture strain gauges has no influence on the moisture correction.

The moisture strain gauges can either by glued directly onto a convexly or concavely curved region of the transducer or can be glued onto a special part which if, for example, attached by a screw to the transducer. This second possibility is of interest, in particular, for subsequent equipping of transducers and for the case that matching of the moisture correction to the moisture coefficient of the transducer is to take place in coarse steps by strain gauges on differently curved surfaces and as fine adjustment by series or parallel resistors. An appropriate correction part can then be selected, fastened and connected according to the measured moisture coefficient of the transducer. Such parts, which are securable by screws, are illustrated in Figs. 8 and 9.

The part 80, which is securable by a screw, shown in Fig. 8 is similar to a cable clip. It comprises a curved portion 82 and a planar portion 83 with a hole 81 for fastening. A strain gauge for moisture correction can be glued either to the outer convex surface 82' (gauge 84) or to the inner concave surface W' (gauge 85), or At.

14 - both gauges 84 and 85 can be glued in place if a Wheatstone bridge circuit such as in Fig. 7 is to be provided. The part 80 can easily be produced as a punched and bent part.

Fig. 9 shows an alternative part 90 securable by screwing.

The part 90 consists of a cylindrical rod which, in its lower portion 91, carries a thread so that it can be screwed into a threaded hole in the transducer. A strain gauge 92 for moisture correction is glued onto the upper, thread-free portion of the rod.

To facilitate screwing-in, the part 90 has a slot 93 at the upper end.

In the preceding description, circuits for the correction of moisture coefficients of the sensitivity have been discussed. If so desired, a moisture coefficient of the zero point of a transducer can be corrected by such a strain gauge applied to a curved surface.

For this purpose, the moisture-dependent strain gauge need be connected only between one pole of the supply voltage U 0 and an input of the amplifier 10 in the Wheatstone bridge circuit according to Figs. 4 and 5; thus, it lies in parallel with one of the measurement strain gauges 6 to 9. To obtai n the symmetry of the bridge circuit, a fixed resistance must then be additionally connected in parallel with the strain gauge in the neighbouring bridge circuit. In the circuit according to Fig. 6, a moisture dependent additional current must be connected by way of the measuring resistance 55. In the circuit according to Fig. 7, the computing program of the microprocessor can, of course, correct for the sensitivity as well as for the zero point. Details of these circuits can be readily designed by the appropriate expert.

At L Equally, the expert can readily design different circuits for the analog- to-digital conversion. For example, the moisturedependent strain gauge can be incorporated as a frequencydetermining resistance into a resistance-capacitance oscillatory.

This results in an analog frequency signal, which can easily be digitalised in the microprocessor.

Equally, the combination, which has been explained for the strain gauge measurement transducer, of moisture correction and temperature correction can be used for weighing machines operating on the principle of electromagnetic force compensation or on other principles.

The gluing of the moisture-dependent strain gauge to a curved surface, whether concave, convex or differently curved, can be effected by any suitable medium or method which secures the strainregistering part of the gauge in place and allows that part to respond to swelling or contracting action occurring between the part and the curved surface.

A 1 - 16

Claims

1. A weighing machine provided with a measurement transducer for converting the weight of a load weighed by the machine into an electrical signal and moisture detecting means for influencing the output signal of the transducer to provide correction for the moisture dependence of the transducer, the moisture detecting means comprising at least one strain gauge glued to a curved surface.

2. A weighing machine as claimed in claim 1, wherein the curved surface is convex.

3. A weighing machine as claimed in claim 1, wherein the curved surface is concave.

4. A weighing machine as claimed in any one of the preceding claims, wherein the curved surface is on the transducer.

5. A weighing machine as claimed in any one of claims 1 to 3, wherein the curved surface is provided on a part secured to the transducer by securing means.

6. A weighing machine as claimed in any one of the preceding claims, wherein the transducer comorises a spring body with strain measuring means and a Wheatstone bridge circuit, and said at least one strain gauge is arranged in a diagonal of the bridge to provide a load resistance of the transducer output signal.

49 1

7. A weighing machine as claimed in any one of claims 1 to 5, wherein the transducer comprises a spring body with strain measuring means and a Wheatstone bridge circuit, and said at least one strain gauge is arranged to provide an input resistance in the feed of supply voltage to the circuit.

8. A weighing machine as claimed in any one of claims 1 to 5, wherein the transducer comprises a spring body with strain measuring means and a Wheatstone bridge circuit, and said at least one strain gauge is arranged to vary the amplification factor of an amplifier connected to the circuit.

9. A weighing machine as claimed in any one of claims 1 to 5, wherein the transducer comprises a spring body with strain measuring means and a Wheatstone bridge circuit, and said at least one strain gauge is connected in parallel with one of the branches of the bridge.

10. A weighing machine as claimed in any one of the claims 1 to 5, wherein the transducer operates on the principle of electromagnetic force compensation and said at least one strain gauge is part of resistance means for converting a coil current of the transducer into a measurement voltage.

11. A weighing machine as claimed in any one of the claims 1 to 1, wherein the transducer operates on the principle of electromagnetic force compensation and said at least one strain gauge is arranged to influence the zero point of the transducer.

A h

12. A weighing machine as claimed in any one of claims 1 to 5, comprising means for converting an analog signal indicative of change in resistance of said at least one strain gauge into a digital value.

13. A weighing machine as claimed in any one of the preceding claims, wherein said at least one strain gauge has a temperature coefficient to additionally enable correction for the temperature coefficient of the transducer.

14. A weighing machine as claimed in claim 13, wherein said at least one strain gauge is glued to a material which is without compensation for temperature.

15. A weighing machine as claimed, in any one of the preceding claims, wherein said at least one strain gauge comprises a layer of expansion-sensitive material and a layer of synthetic material, the synthetic material layer being glued to the curved surface by an adhesive substance.

16. A weighing machine as claimed in claim 15, wherein boith the layer of synthetic material and the adhesive substance are swellable by moisture.

17. A weighing machine substantially as hereinbefore described with reference to Figs. 1 and 2.

1 A 1

18. A weighing machine substantially as hereinbefore described with reference to Figs. 1 and 3 of the accompanying drawings.

19. A weighing machine as claimed in claim 1 and substantially as hereinbefore described with reference to Fig. 4 of the accompanying drawings.

20. A weighing machine as claimed in claim 1 and substantially as hereinbefore described with reference to Fig. 5 of the accompanying drawings.

21. A weighing machine substantially as hereinbefore described with reference to Fig. 6 of the accompanying drawings.

22. A weighing machine substantially as hereinbefore described with reference to Fig. 7 of the accompanying drawings.

23. A weighing machine substantially as hereinbefore described with reference to Fig. 8 of the accompanying drawings.

24. A weighing machine substantially as hereinbefore described with reference to Fig. 9 of the accompanying drawings.