Diagnostic method for a weighing cell

Abstract

A force-measuring device (1) with a parallelogram linkage has a measurement transducer coupled to it. A coil (25) of the transducer has guided mobility in a magnet system (27) and can carry an electric current (24). A position sensor (21) detects the deflection of the coil (25) from a balanced position relative to the magnet system when a load is placed on the force-measuring device. The electric current (24) flowing through the coil (25), by way of the interaction between the coil and the magnet system, returns the coil and the movable parallel leg to the balanced position. A system-characterizing means (29) is established in a processor unit (26). The system-characterizing means and an unchangeable system reference means (30) are compared to determine the functionality of the device. The functionality is verified by the magnitudes of the electric current and the deflection of the coil from its balanced position.

Claims

1. A method for verifying a functionality of a force-measuring device which works according to the principle of electromagnetic force compensation, the force measuring device comprising a stationary parallel leg, a movable parallel leg which receives the load of a weighing object, two parallel guides that connect the stationary and movable parallel legs, a measurement transducer that is coupled to the movable parallel leg through a force-transmitting connection and that comprises a coil that is arranged with guided mobility in a magnet system and that can carry an electric current, a position sensor, arranged to detect a deflection of the coil from a balanced position relative to the magnet system, the deflection occurring as a result of placing a load on the movable parallel leg, such that an electric current flows through the coil by way of the electromagnetic force acting between the coil and the magnet system to return to, or to maintain in, the balanced position the coil and the movable parallel leg, which is connected to either the coil or the magnet system, and a regulating unit for regulating the amperage of the electric current in response to the position sensor signal such that the coil returns to the balanced position, the method comprising the steps of: establishing, by means of a processor unit, at least one system-characterizing resource of the force-measuring device, and comparing at least one unchangeable system reference resource, which is stored in a persistent memory file of the processor unit to the system-characterizing resource, the comparison resulting in a determination of the functionality of the force-measuring device; and using a magnitude of the amperage and a magnitude of the deflection of the coil from the balanced position to verify the determined functionality, wherein each of the at least one system-characterizing resource and the at least one system reference resource establishes a relationship between the amperage of the electric current and the magnitude of the coil deflection.

2. The method of claim 1, wherein: one of the at least one system reference resource represents the functionality of the force-measuring device at the time of an initial adjustment, that is, adjustment that: took place during production or close to the completion of the force-measuring device; or reflects the condition of a fault-free functionality of the force-measuring device.

3. The method of claim 1, wherein: either the at least one system-characterizing resource, the at least one system reference resource, or both, includes a system table that lists the respective values of the weight force of the applied load associated with different magnitudes of the deflection of the coil from its balanced position and with different amperages of the electric current, and/or a system function with at least one parameter and with at least the magnitude of the deflection of the coil and the amperage of the electric current as input quantities.

4. The method of claim 3, wherein: the at least one parameter of the system function is stored as a parameter table, wherein the at least one parameter of the system function is load-dependent.

5. The method of claim 3, further comprising the step of: determining the values of the system table, the at least one parameter of the system function, or both, by at least one of the steps of: varying the deflection of the coil and measuring, at essentially the same time, the amperage of the electric current caused by the deflection of the coil; and varying the amperage of the electric current and measuring, at essentially the same time, the deflection of the coil caused by the amperage of the electric current.

6. The method of claim 5, wherein: the step of determining the values of the system table, the at least one parameter of the system function, or both, is conducted both with and without a weight being applied to the movable parallel leg, wherein the weight is either an externally handled weight or a weight set in place internally by a handling mechanism.

7. The method of claim 1, wherein: each at least one system reference resource is established either individually for each force-measuring device or generically for a given type of force-measuring device.

8. The method of claim 1, wherein: the result obtained from the comparing steps is used to do at least one of the following: to investigate a fracture, tear, or deformation of a pivot of the parallel-guiding mechanism; to investigate a position change of the coil relative to an original position thereof; and to investigate a position change of the position sensor relative to an original position thereof, wherein the respective original positions are associated with the condition of the force-measuring device in which the system reference resource was established.

9. The method of claim 1, further comprising the steps of: establishing a trend line of the functionality, based on the currently determined system-characterizing resource and the previously established system-characterizing means; and predicting the future functionality, based on the established trend line.

10. The method of claim 4, further comprising the step of: determining the values of the system table, the at least one parameter of the system function, or both, by at least one of the steps of: varying the deflection of the coil and measuring, at essentially the same time, the amperage of the electric current caused by the deflection of the coil; and varying the amperage of the electric current and measuring, at essentially the same time, the deflection of the coil caused by the amperage of the electric current.

11. The method of claim 10, wherein: the step of determining the values of the system table, the at least one parameter of the system function, or both, is conducted both with and without a weight being applied to the movable parallel leg, wherein the weight is either an externally handled weight or a weight set in place internally by a handling mechanism.

12. The method of claim 9, wherein the future functionality is predicted for the time remaining until the next service of the force-measuring system.

13. A gravimetric force-measuring device, working according to the principle of electromagnetic force compensation, comprising: a processor configured with instructions, stored on a non-transitory, computer-readable medium, to perform the method of claim 1.

14. A non-transitory, computer-readable medium with instructions stored thereon, that when executed by a processor that is a part of a force-measuring device operating according to the principle of electromagnetic force compensation, perform the steps of claim 1, by using, as input values, at least the amperage of the electric current and the magnitude of the deflection of the coil from a balanced position thereof, to issue a signal for initiating an action of the force-measuring device.

15. The computer-readable medium of claim 14, wherein: the instructions include a step of recalling, from a persistent memory file of the processor, a system reference resource and at least one system-characterizing resource.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject of the invention will be explained in the following through examples of preferred embodiments which are illustrated in the attached drawings, wherein:

(2) FIG. 1 schematically represents a sectional view of a force-measuring cell of a top-loading force-measuring device designed as a lever system, seen from the side;

(3) FIG. 2 illustrates a monolithically constructed force-measuring cell designed as a lever system, seen from the side;

(4) FIG. 3 illustrates a force-measuring cell designed as a direct-measuring system;

(5) FIG. 4 shows a block diagram through which the flow of functions in a force-measuring device according to the invention is described;

(6) FIG. 5 shows a block diagram through which the life cycle of a force-measuring device according to the invention is described;

(7) FIG. 6 represents a position/current graph over the entire deflection range of the coil, with the system functions S.sub.E1, S.sub.E2 and S.sub.E3 of a system reference means and an ideal system function A;

(8) FIG. 7 represents a position/current graph over the entire deflection range of the coil, with the system functions S.sub.K1, S.sub.K2 and S.sub.K3 of a system-characterizing means of a force-measuring device in which the flexure pivots, elastic joints or diaphragm springs have been damaged;

(9) FIG. 8 represents a section of the position/current graph of FIG. 6 with the system functions S.sub.K1, S.sub.K2 and S.sub.K3 of a system-characterizing means of a force-measuring device in which the flexure pivots, elastic joints or diaphragm springs have been damaged, showing possible threshold values for the system functions;

(10) FIG. 9 represents a position/current graph over the entire deflection range of the coil, with a system function S.sub.K of a system-characterizing means of a force-measuring device exhibiting a shift in the position measurement in comparison to a system function S.sub.E of a system reference means; and

(11) FIG. 10 represents a position/current graph over the entire deflection range of the coil, with a system function S.sub.K of a system-characterizing means of a force-measuring device in which the coil has shifted from its centered position in the magnet system in comparison to a system function S.sub.E of a system reference means.

DETAILED DESCRIPTION

(12) In the following description, features that have identical functions and similar configurations are identified by the same reference symbols.

(13) FIG. 1 schematically illustrates a force-measuring cell 10 of a force-measuring device 1 designed as a lever system, seen from the side in a sectional view. By way of the stationary parallel leg 11, the force-measuring device 1 stands on a supporting structure. The load to be measured is placed on a weighing pan 15 that rests on the movable parallel leg 12 which is connected to the stationary parallel leg 11 by way of two parallel guides 14. The parallel guides 14 are connected by way of flexure pivots 16 to the movable parallel leg 12 and to the stationary parallel leg 11. Flexure pivots 16 define an axis of rotation and in the directions transverse to the axis of rotation, they behave as practically rigid force-transmitting elements. The force-measuring device does not necessarily have to be configured with the weighing pan on top as shown here. It can likewise be realized with a hanging pan, in most cases with a hanger mechanism. The coupling 13 transmits the weight force to the first lever arm 18 of the balance beam 17 which is supported by a fulcrum. Attached to the other end of the balance beam 17, i.e. to the outer end of the second lever arm 19, is the measurement transducer 20 which counteracts the lever-reduced weight force of the weighing load with a compensation force. The measurement transducer 20 shown here is shown as a current-conducting coil 25 which is immersed with guided mobility in a magnet system 27. When the compensation force generated by the measurement transducer 20 at the second lever arm 19 corresponds to the weight force of the load at the first lever arm 18, the balance beam 17 is in equilibrium and thus in the balanced position. This balanced position is monitored by a position sensor 21.

(14) When a mass or a force is applied to the weighing pan 15, the movable parallel leg 12 is deflected downward, guided in parallel motion by the parallel guides 14. The balance beam 17, which is connected to the movable parallel leg 12 through the coupling 13, transmits this movement with a defined reduction ratio to the other end of the balance beam 17 which faces towards the measurement transducer 20. The position sensor 21 generates a position signal 22 corresponding to the deflection of the coil 25 from the balanced position. This position signal 22 is sent as input signal to a position-controller unit which regulates an electric current 24 through the coil 25 in such a way that, due to the resultant compensation force, the coil 25 and the balance beam 17 to which the coil is connected are returned to the balanced position. In the stationary state of the regulation (when the coil 25 has returned to its balanced position), the magnitude of the electric current 24 flowing through in the coil represents a measure for the quantity that is to be determined, i.e. the mass or force acting on the movable parallel leg 12. The magnitude of the electric current 24 is measured by means of a processor unit 26 (see FIG. 3), and the result is subsequently presented on a display as the measurement value.

(15) FIG. 2 illustrates a possible configuration of a force-measuring cell 10 of a force-measuring device 1 of monolithic construction, seen from the side in a sectional view. The stationary parallel leg 11, the movable parallel leg 12, the coupling 13, the parallel guides 14 and the balance beam 17 are integrally connected to each other, made out of one homogeneous block of material. All of these elements are formed out of a metal block through suitable manufacturing techniques to separate them, for example chip-removing machining processes, cutting, or spark erosion. The flexure pivots 16, the fulcrum of the balance beam and the connecting ends of the coupling 13 are formed as thin material bridges, wherein the material thickness of the flexure pivots 16 is adapted to the capacity range of the force-measuring device 1, so that the material bridges of the flexure pivots 16 are designed stronger for larger weighing capacities.

(16) FIG. 3 illustrates a possible embodiment of a force-measuring cell 100 designed as a direct-measuring system. The stationary parallel leg 111 rests on the supporting ground structure. The movable parallel leg 112 which serves to receive the load is connected to a force-transmitting rod 117 and is guided by parallel guides 114 which in this example are configured as diaphragm springs. In this embodiment, the measurement transducer 120 is arranged at the lower end of the force-transmitting rod 117, wherein the coil is shown connected to the movable parallel leg 112 and the magnet system 127 is arranged at the stationary parallel leg 111. Further realizations of this concept would be possible by arranging the measurement transducer 120 in the area between the parallel guides 114 and/or by switching places in the arrangement of the magnet system 127 and coil 125.

(17) The thinner a material bridge of a material connection or a spring pivot or diaphragm is designed, the more susceptible a force-measuring cell 10, 100 will be to damage resulting from shocks directed at the movable parallel leg 12, 112, impact forces from being dropped or set down abruptly, or other exposures to excessive stress. As a consequence, the material bridges, flexure pivots, or diaphragm springs can become bent out of shape, cracked or even entirely broken or torn apart.

(18) The position sensor 21 and the measurement transducer 20, 120 are likewise susceptible to shocks, excessive stress conditions and/or collision impact forces. These components of a force-measuring device 1 are adjusted in their positions and alignments during the manufacturing process or close to the time of completion of a force-measuring device 1. A subsequent adjustment of the force-measuring device 1 is therefore always referenced against the position in which the force-measuring device was originally set up and aligned. If a position of a component deviates from the original position, it will cause an error in the weighing result that is not noticeable to the user.

(19) In spite of damage to the force-measuring cell 10, 100, be it in a material connection, a flexure pivot, a diaphragm spring or from a dislocation of the coil or the position-measuring device relative to a position in which the force-measuring device 1 was calibrated, it may still be possible to perform what appears to be a valid weighing, since a damage of the aforementioned kind is not recognizable by currently available means. In spite of the damage, the force-measuring device 1 delivers a weighing result, albeit an incorrect one, as the force-measuring device appears to be unimpaired and to work in an error-free manner.

(20) In the following, the flow of functions in the operation of a force-measuring device 1 is described in more detail by following the block diagram of FIG. 4. A load placed on the weighing pan 15 exerts a force F on the movable parallel leg 12, 112. The balance beam 17 with the coil 25 connected to it, or the magnet system 27 connected to the balance beam 17, or the force-transmitting rod 117 with the coil 125 connected to it, or the magnet system 127 connected to the force-transmitting rod 117 is displaced from its balanced position, i.e. they move into a different position. The new position x is detected by the position sensor 21 and sent as a position signal 22 to a position controller unit 23. In response to the position signal 22, the position controller unit 23, in most cases a PID controller, continuously determines the magnitude of the coil current 24 that is needed to return the system to its balanced position. As a result of the electrical coil current 24, the coil 25, 125 causes a magnetic field and, through interaction with the magnet system 27, 127, generates a compensating force which returns the balance beam 17 or the force-transmitting rod 117 with the coil 25, 125 to the balanced position. The same cycle continuously repeats itself in the sense of a closed-loop regulation to maintain the system in its balanced position. This control loop regulates the deflection of the balance beam 17 or the force-transmitting rod 117 dynamically, i.e. several times a second, for example in the range of 500 Hz to 10 kHz.

(21) Since the electric current 24 represents a direct measure for the compensating force, the electric current 24 is measured and the result is used by the processor unit to calculate the weight force as the value to be displayed. Additional factors that enter into the calculation of the displayed value by the processor unit 26 include for example the ambient temperature, the temperature of the magnet, as well as time-dependent dynamic effects.

(22) The method of verifying the functionality according to the invention is distinguished by the feature that, in addition to the magnitude of the electric current 24, the processor unit uses the position signal 22 of the position sensor 21, i.e. the magnitude of the deflection of the coil 25, 125 from its balanced position, for the assessment of functionality. This is indicated in FIG. 4 by the broken line. Instead of the position signal 22, it is also possible to provide input signals to the processor unit 26 which contain the same information regarding the position x, i.e. the position of the coil 25, 125 within the magnet system 27, 127. This is indicated by the dash-dotted line in FIG. 4. This information could for example be sent to the processor unit 26 in the form of a position signal 22 from a second, additional sensor 28, for example an acceleration sensor, a velocity sensor, or an angular or linear position sensor.

(23) Accordingly, the force-measuring device 1 is able to use the amount of the deflection of the coil 25, 125 from its balanced position in the verification of the functionality, and thus to take into account if there is damage for example to the magnet system 27, 127, the position-measuring system, or in particular the flexure pivots, the elastic joints or the diaphragm springs of the parallel guides 14, 114 or the lever-reduction system.

(24) The system reference means 30 which was established in the course of an adjustment of the force-measuring device 1 (see the description of FIG. 5) is stored in the processor unit 26. The system reference means 30 is stored in a persistent memory file, i.e. a non-volatile memory, and can only be overwritten in a new adjustment of the force-measuring device 1. This is indicated in FIG. 4 by the symbol of the closed padlock. In contrast, the system-characterizing means 29 can be updated during operation of the force-measuring device 1, for example after a verification of functionality has taken place. However, the no longer current system-characterizing means does not have to be erased but can remain stored in the processor unit 26, for example in order to establish a trend history of the functionality of a force-measuring device 1.

(25) The term calibration refers to the process of determining a deviation between the measurement value and the true value of the measured quantity under given measurement conditions without making a correction. On the other hand, if a correction is made, the process is called adjustment. For example, when a balance is adjusted manually by trained personnel fine-tuning its functions with the appropriate tools, or semi-automatically by the user placing an external or built-in reference weight on the load receiver, or automatically if the balance is equipped with an adjustment mechanism including a reference weight, the deviation is corrected.

(26) In the form of a block diagram, FIG. 5 illustrates the life cycle of a force-measuring device 1. It begins with the production of all parts and components and their assembly into a force-measuring device 1. Normally as a next step or close to the time of completion of the force-measuring device 1, a calibration as well as an adjustment of the components and parts of the force-measuring device 1 takes place. This includes that a system reference means 30 is established which is representative of the condition of faultless functionality. This system reference means 30 is stored in a persistent memory file in the processor unit 26. The system reference means 30 can be stored in the form of a system table or a system function with at least one parameter. In addition, further adjustment settings of the force-measuring device 1 can be stored in the processor unit 26.

(27) A system reference means 30 can be established individually for each force-measuring device 1 or generically for a given type of force-measuring device 1. For a generically determined system reference means, an arithmetic mean value can be established based on previously determined reference means and can then be used for all force-measuring devices 1 of the same type.

(28) The force-measuring device 1 can now be delivered to the user and put into operation. For specialized force-measuring devices 1 which are adapted to particular operating conditions such as for example the permanent presence of a pre-load, the determination of the system reference means can be deferred to the time of installation at the user location. Force-measuring devices according to the current state of the art have to be checked at certain time intervals by the manufacturer to verify their function and to satisfy the accuracy requirements of national regulations, calibration requirements and/or industrial standards. This can be done on location by a service technician or at the facility of the manufacturer of the force-measuring device 1.

(29) If damage is detected in the functionality check by the manufacturer and if the cost of repair does not appear worthwhile, the force-measuring device 1 is taken out of operation. If the functionality check has a positive result, the force-measuring device 1 can continue to stay in use for the next time interval. If the force-measuring device is connected to a communications network, a report can be sent automatically to the manufacturer. This keeps the manufacturer informed on the functional status of the force-measuring device, so that appropriate steps can be taken if necessary.

(30) As a third possibility in a case where a damaged condition has been identified, the components that have become defective or whose functionality is deficient can be exchanged. This needs to be followed by a new adjustment of the components and parts of the force-measuring device 1 by the manufacturer. In the process, a new system reference means 30 is determined and stored in the processor unit 26 in a persistent memory file, with the previous system reference means 30 being overwritten. In addition, other adjustment settings can be stored in the processor unit 26 at the same time. Subsequently, the force-measuring device 1 is released to be put into operation again by the user.

(31) The method according to the invention, which is illustrated in FIG. 5 by the broken lines, is inserted into this life cycle before the periodic maintenance service is performed by the manufacturer, in order to verify during the time interval between two maintenance services that the force-measuring device 1 is functioning properly. One can thus ascertain already prior to an actual maintenance service whether the device functions properly or its functionality is compromised. This prevents the use of the force-measuring device 1 in case of a latent defect where the force-measuring device 1 appears to the user to be undamaged so that it seemingly functions in a faultless manner. On the other hand, the time interval between two maintenance services could also be extended as long as the functionality test shows a positive result, whereby the user saves the cost of downtime associated with taking the force-measuring device 1 out of operation for servicing.

(32) In each cycle of the method according to the invention, the at least one system reference means 30 is compared to at least one system-characterizing means 29, and based on the comparison the functionality of the force-measuring device 1 is determined. Both the system-characterizing means 29 as well as the system reference means 30 establish a relationship between the magnitude of the electric current 24 and the magnitude of the deflection of the coil 25, 125 from its balanced position and can exist in the form of numerical values in a system table or a system function with at least one parameter. The system reference means 30 which is stored in a persistent memory file of the processor unit 26 represents the faultless condition of the force-measuring device 1, while the system-characterizing means 29 represents the actual functional condition of the force-measuring device 1 at the current time.

(33) Prior to the comparison of the system-characterizing means 29 against the system reference means 30, the system-characterizing means 29 has to be established in the form of numerical values in a system table or system function with at least one parameter. The determination of the values of the system table or system function with at least one parameter as a step that precedes the comparison of the system-characterizing means 29 against the system reference means 30, as well as the determination of the values of a system table or the system function with at least one parameter for the system reference means which occurs at the time the adjustment can be performed in different ways.

(34) In the following, possible ways to determine the values of a system table as well as a system function with at least one parameter are demonstrated. The determination is made advantageously during the production process of the force-measuring device 1, in particular during the adjustment phase. As a first possibility, the determination is made by varying the deflection of the coil 25, 125 and essentially at the same time measuring the amount of electric current 24 associated with the deflection of the coil 25. Alternatively, as a second possibility, the determination is made by varying the amount of electric current 24 and essentially at the same time measuring the deflection of the coil 25, 125 corresponding to the of magnitude of the electric current 24. In order to be able to vary the deflection of the coil 25, 125 or the magnitude of the electric current 24, the processor unit can set a target position or a target current by means of two interface connections to the position controller unit 23.

(35) The two procedures that have been described above for the determination of the values of a system table and/or a system function with at least one parameter can also be supplemented by performing them again with the additional step of placing a calibration weight on the load receiver. Based on a measurement with the calibration weight and a measurement without the calibration weight, a separate respective system reference means 30 and/or system-characterizing means 29 can be established for the magnet system 27, 127, for the parallel guides 14, 114 and their flexure pivot 16 or elastic joints, the position sensor 21 and/or the lever transmission ratio.

(36) Based on the system-characterizing means 29 and the previously determined system-characterizing means 29, it is possible to establish a trend history of the functionality, which allows the remaining operating time until the next maintenance service of the force-measuring device to be predicted.

(37) If the force-measuring device 1 is equipped with an internal calibration weight which can be coupled to the movable parallel leg 12, 112 and which is activated when a calibration is required, the force-measuring device 1 can, depending on the application, perform one or more of the aforementioned possible procedures for the determination of the values of a system table and/or the system function with at least one parameter, either under the control of a menu or autonomously.

(38) After the functionality has been verified, the processor unit 26 can release the force-measuring device 1 for operation, lock the force-measuring device 1, or issue a warning to the user, or generally initiate an action of the force-measuring device 1. A warning can include the information that a maintenance service is imminent or that the functionality is impaired.

(39) FIGS. 6 to 10 illustrate different system functions of force-measuring devices 1 in the form of position/current graphs. The system function establishes a relationship between the magnitude of the electric current 24 and the magnitude of the deflection of the coil 25, 125 from its balanced position. This is a possible form of representation of a system-characterizing means 29 or a system reference means 30. The plus and minus 100% marks on the horizontal axis define the out-of-balance deflection of the coil 25, 125 in the magnet system 27, 127.

(40) FIG. 6 shows a system function A of an idealized force-measuring device 1 with an ideal behavior of the flexure pivots 16 or elastic joints and of the magnet system 27, 127 drawn as a straight line, which means that a deflection of the coil 25, 125 from its balanced position causes a linear change of the electric current 24. In contrast to the system function A, the system functions S.sub.E1, S.sub.E3 and S.sub.E3 reflect the real behavior of the magnet system 27, 127 and also represent a system reference means 30.

(41) If the force-measuring device 1 is subjected to an excessive stress load, caused for example by hitting the weighing pan 15 or dropping the device on the ground, the system functions will change. FIG. 7 represents a system-characterizing means 29 with the system functions S.sub.K1, S.sub.K2, S.sub.K3 of a force-measuring device 1 in which at least one flexure pivot 16 or an elastic joint or a diaphragm spring has been damaged by an excessive stress load. Clearly evident are the irregular shapes of the system function graphs, specifically the deviations of the system functions at the deflected position P. Such symptoms of a damaged flexure pivot 16 or elastic joint or of a damaged diaphragm spring can take on very different forms. For example as in the system function S.sub.K2, there can be a strong deviation of the coil current 24 within a narrow range of the deflection variable, or as in the system function S.sub.K3, there can be a small deviation of the coil current 24 occurring over a wide range of the deflection variable.

(42) FIG. 8 is an enlarged representation of the rectangular portion of FIG. 7 that is framed with broken lines. A comparison between a system reference means 30 and a system-characterizing means 29 entails a quantitative characterization of the deviation as well as a qualitative characterization based on a threshold value. The threshold value can for example be defined as a tolerance band whose boundaries may not be crossed or, as shown for the system function SK3, a limit value can be set for the area enclosed between the system reference means 30 and the system-characterizing means 29, or the two possibilities can be combined. The tolerance band does not need to have the same width over the entire deflection range. The width can also be dependent on the deflection variable, so that a narrower tolerance can be defined for a deviation of the system function of a system-characterizing means 29 in the vicinity of the balanced position. This makes it possible to ensure that the most different symptoms in the system function can be quantified and their significance can be qualified.

(43) FIG. 9 demonstrates the effect that a dislocation of the position sensor 21 has on a system function S.sub.K of a system-characterizing means 29. If the position sensor 21 is no longer in the same position in which the adjustment was performed, the system function S.sub.K of the system-characterizing means 29 in the position/current graph will be shifted horizontally relative to the system function S.sub.E of the system reference means 30. Thus, a deviation can be detected easily and quickly.

(44) FIG. 10 shows how a dislocation of the coil 25, 125 from its centered position in the magnet system 27, 127 affects the system-characterizing means 29. Analogous to the example of FIG. 9, the dislocation of the coil results in a system function S.sub.K which is shifted in the vertical direction of the graph in relation to the system function S.sub.E of the system reference means 30, which means that in all deflected positions of the coil 25, 125 a stronger current 24 will flow through the coil 25, 125 than in the adjusted condition. In both cases, i.e. with the dislocation of the coil 25, 125 from its centered position in the magnet system 27, 127 and with the dislocation of the position sensor 21, appropriate tolerances or threshold values need to be defined, preferably with narrower tolerances in the vicinity of the balanced position.

(45) It has been found that a system reference means 30 and a system-characterizing means 29 are dependent on the mass of the weighing load. In other words, the values of the system table and/or the parameters of the system function of a system reference means 30 or of a system-characterizing means 29 are valid for a specific magnitude of the applied load. This is illustrated in FIGS. 6. 7 and 8 through the different system functions S.sub.E1, S.sub.E2, S.sub.E3 and S.sub.K1, S.sub.K2, S.sub.K3. The larger the force that is acting on the movable parallel leg 12, 112, the stronger will be the curvature of a system function in the position/current graph. The graphs of the system functions SE3 and SK3 are curved upwards. This curvature results from a change in the direction of the force in the measurement transducer 20, 120, which occurs for example in push-pull systems. The system reference means 30 and the system-characterizing means 29 therefore include at least one system table and/or a system function with the corresponding parameters, the system table and/or the system function is used to verify the functionality when the applied force on which it is based agrees most closely with the actually applied load, or else an interpolation is made between two values of the system table or between two system functions.

(46) Although the invention has been described by presenting several examples of specific embodiments, it is considered evident that numerous further variants could be created based on the teachings of the present invention, for example by combining features of the individual embodiments with each other and/or by interchanging individual functional units between the embodiments.