WO2012113822A1

WO2012113822A1 - Method and apparatus for measuring force

Info

Publication number: WO2012113822A1
Application number: PCT/EP2012/052998
Authority: WO
Inventors: Daniel Schwind
Original assignee: Gtm Gassmann Testing And Metrology Gmbh
Priority date: 2011-02-22
Filing date: 2012-02-22
Publication date: 2012-08-30
Also published as: DE102011000869A1; DE102011000869B4

Abstract

In a method for measuring force with a force measuring apparatus (1) which has a two-armed lever (2) pivotably mounted in an eccentric manner, wherein a load application joint is respectively arranged on each lever arm (4, 8) and is in the form of a flexible joint spring (3, 6, 9), the deformation of which can be measured using deformation measuring devices, wherein a predefinable load acts on a first lever arm (4) and the lever (2) is changed to a force measuring state by exerting a tensile force or a compressive force on a second lever arm (8) and the tensile or compressive force required for this purpose is determined, an axial force acting on at least one joint spring (3, 6, 9) is detected in this joint spring (3, 6, 9) and a predefinable portion of the axial force determined is taken into account for predefining the force measuring state. A compensation tensile or compressive force is determined on the basis of the axial force determined and is additionally taken into account in the case of an electronically controlled or regulated predefinition of the force measuring state. In a preceding method step, the force measuring apparatus (1) is calibrated and values determined in the process are determined for an additionally required compensation tensile or compressive force and are stored in a retrievable manner in a storage device.

Description

Method and device for force measurement

The invention relates to a method for force measurement with a force measuring device, which is an off-center

having pivotally mounted two-armed lever, wherein on each lever arm in each case a load engagement joint is arranged, which is designed as a flexible hinge spring whose

Deformation can be measured by means of a deformation measuring device, wherein a predeterminable on a first lever arm

Stress acts and by exercising a train or

Compressive force placed on a second lever arm of the lever in a force measuring state and the required tensile or compressive force is determined.

Such force measuring devices, which are described for example in DE 39 27 475 C2, are used inter alia for the calibration of force sensors and for the calibration of

Load cells used. On the off-center pivot

mounted lever is attached to the first, usually longer lever arm a weight whose weight is known as accurately as possible. At the second, usually more than 5 to 10 times shorter lever arm via a force transducer, the force-transmitting with the second

Lever arm is connected, a tensile or compressive force exerted on the second lever arm. By over the

Force transducer exerted tensile or compressive force, the lever is brought into a force measurement state. In practice, the lever is balanced in order to achieve the force measurement state, so that the of the Compensating weight on the first lever arm and the forces exerted on the second lever arm via the force transducer moments and the sum total of all acting on the lever moments is as low as possible or zero. Depending on what is assumed to be known

Lever arm length can then be determined by the force transducer to achieve the Kraftmesszustands required tensile or compressive force with great accuracy and a

Calibration, calibration or adjustment of the force transducer or the load cell are performed.

It is already known from practice that with increasing load on the lever arms in particular the longer lever arm is slightly deformed or bent, so that a

Shortening of the effective lever arm length occurs. Since the effective lever arm length is used to determine the tensile or compressive forces required to achieve the force measurement condition, inadequate deflection of the lever arm results in a systematic measurement error that limits the accuracy of the measurements. To the by a

Deflection of the lever arm caused to reduce measurement uncertainty, it is known from practice that

For example, the pivot points of the hinge springs

different heights are adjusted and a change in the pivot points is mechanically compensated or recorded and taken into account for the evaluation. The one for this

However, required effort is considerable. It has been shown that with increasing

Load the axial force exerted on the joint springs, which have as little influence as possible on the measurement results should, often on the deformation measuring devices overstates or affects the measurement result, disturbing or superimposed, so that seemingly moments on the lever

be exercised, which in turn affects the measurement result or the best possible accuracy of the measurement results.

Since the above-described disturbances often have a non-linear dependence on the load exerted on the lever, a considerable effort is required

necessary to minimize this disturbing influence and to improve the accuracy. In practice, a measurement uncertainty in the order of about 5xlCT ^{4 is} achieved with great effort in such force measuring devices. It would be desirable if the measurement uncertainty could be further reduced.

It is therefore regarded as an object of the present invention to reduce the measurement uncertainty and to improve the accuracy of a force measurement with the simplest possible method.

This object is achieved with a

generic method solved in that in a hinge spring acting on this hinge spring

Axialkraft determined and a predetermined proportion of

determined axial force for the specification of the

Force measurement state is taken into account.

The axial force is that component of a force acting on the hinge spring force acting in the axial direction of the hinge spring on the hinge spring and no lateral deflection of the hinge spring causes, but only a tensile force on the

Performs hinge spring. It has been shown that by a correction, in the knowledge or depending on the

actually performed on a lever arm or on the corresponding hinge spring axial force is applied, the measurement accuracy improved by about a factor of 10 and the measurement uncertainty can be reduced to a range of about 5xlCT ⁵ to 7xlCT ⁵ . The axial force can be measured in a joint spring in a simple manner and without major design effort. Since only one share, preferably a small proportion of

determined axial force for the specification of the

Force measurement is taken into account, the

Measurement uncertainty in the determination of the axial force only to a correspondingly low proportion in the entire

Measurement uncertainty. A very high accuracy is therefore not required for the measurement of the axial force, since their contribution to the overall measurement uncertainty is very low.

By determining the force acting on the lever or respectively on the lever arms axial force can be caused by the axial force disturbing influences on a deflection of the lever arm as well as a simple way

Crosstalk in the measurement of the conditional by the attacking moments deformation of the hinge springs can be compensated in a simple manner or

Getting corrected. An exact knowledge of the individual

Effects of the attacking axial forces that would increase or ultimately dominate the measurement uncertainty without a corresponding correction is not required. Likewise, no structurally complex compensation or correction methods are required in which, for example, the deflection of a lever arm or a displacement of the pivot points of the joint springs or a consequent shortening of the effective

Lever arm length recorded and taken into account for the evaluation.

It is envisaged that, depending on the

determined axial force that for the specification of the

Force measurement state exerted tensile or compressive force is adjusted. While without taking into account the axial force of the desired force measurement state assumes that all the forces acting on the lever moments in the sum of zero, can be in the knowledge of the axial force exerted on the second lever arm via the force transducer tensile or compressive force

be changed in order to compensate for the shortening of the effective Hebelarmlänge caused by the axial force in this way. This compensation can already take place during the measurement, so that after completion of the measurements no extensive correction and evaluation of the measured data is required.

It has been shown that due to the greater influence of the longer lever arm, it may be appropriate that the axial force is determined and taken into account for the specification of the force measurement state, which on the

Articulated spring acts on the longer lever arm. Since in a force measurement state in which the forces acting on the lever moments should compensate as possible, acting on the short lever arm axial force accordingly the inverse aspect ratio of the two lever arms is greater than on the long lever arm, a larger axial force can be measured with a lower measurement uncertainty in this case on the short lever arm. For this reason, it may also be advantageous that the one

Axialkraft determined and for the specification of

Force measurement is taken into account on the

Articulated spring acts on the shorter lever arm. According to an advantageous embodiment of

According to the invention, it is provided that, depending on the determined axial force, a compensation tension force or pressure force is determined and, in the case of an electronic one

controlled or regulated specification of the force measurement state is additionally taken into account. Since the specification of the

Force measurement state during a measurement process regularly controlled electronically and the

Axial force in a simple and cost-effective manner

can be detected electronically or digitally or converted into corresponding electronic signals, can be a function of the measured axial force

realized correction of the force measurement in a very simple manner. In particular, already in operation force measuring devices can be retrofitted with an extremely low design effort, so that the determination of the axial force and the implementation of a required depending on the determined axial force correction the measurement uncertainty can be significantly reduced or reduced by about a factor of 10 , Conveniently, in a previous

Process step carried out a calibration of the force measuring device and thereby determined values for an additionally required Kompensationszug- or -druckkraft determined and stored in a memory device retrievable. If force measurements with the force measuring device are carried out after completion of this calibration, then

Knowing the force acting on a lever arm axial force an assigned value for the Kompensationszug- or - pressure force retrieved and for the specification of the

Force measurement state are taken into account. A time delay in the evaluation of the measurement result can be excluded. It will not

additional effort while performing a

Force measurement required.

The invention also relates to a method for calibrating a force measuring device with a two-armed lever mounted eccentrically pivotable. According to the invention

provided that to compensate for a load-related shortening of the lever arm length of several load cells in a load range with the lowest possible

Uncertainty of the force measuring device are calibrated one after the other and then repeatedly arranged a different number of calibrated force transducers parallel to each other on the second lever arm and measured by specifying the associated Kraftmesszustands to determine for different loads each required for the compensation correction value. This procedure can also be called

"Self-Calibration" can be referred to as the

Measurement uncertainty of the force measuring device with Increasing load or axial force would increase over a wide measuring range can be improved without a complex comparison with already-calibrated force measuring devices or with force transducers

is required, which have already been calibrated over the entire measuring range.

The invention also relates to a device for

Implementation of the above-described method for force measurement, which has a two-armed lever mounted eccentrically pivotable, wherein a load engagement joint is arranged on each lever arm, which is designed as a flexible hinge spring, the deformation of which is measurable by means of a deformation measuring device. To the

To improve measurement accuracy of such a force measuring device is inventively provided that the

Joint spring a spring bar with a horizontal and perpendicular to the lever arm aligned first

Has a rejuvenation area in which a

Deformation measuring device is arranged, and spaced therefrom an axial force measuring device is arranged on the hinge spring. By the perpendicular to the lever arm

aligned tapering area, the pivot point in the hinge spring is limited to the rejuvenation area, whereby the effect of the load-related shortening of the effective lever arm length and the associated

Relocation of the fulcrum are better known and consequently can also have a small effect on the measurement uncertainty. Due to the spaced arrangement of

Axial force measuring device of this rejuvenation area will be a mutual influence between the

Deformation measuring device in the first rejuvenation area and the Axialkraftmesseinrichtung which is spaced therefrom, largely excluded.

To improve the accuracy of Axialkraftmessung is provided that the hinge spring a horizontally and parallel to the lever arm aligned second

Has rejuvenation area in which the

Axialkraftmesseinrichtung is arranged. By a suitable embodiment of the respective tapering regions can be achieved that both the torques generated by the respective load as well as in the

Joint spring attacking axial force can be measured largely independently and with great accuracy. An elaborate evaluation of the respective

Measurement signals and especially complex corrections or compensation evaluations are not required.

Such hinge springs can be produced inexpensively and with little effort from a spring bar. According to an advantageous embodiment of

Erfindungsgedankenens is provided that the device of an electronic control device and a

Memory device for storing Kompensationszug- and -druckkraftwerten. The correction required as a function of the respectively measured axial force can take place via the electronic control device in real time and without appreciable delay in the performance of a force measuring procedure. When using such a force measuring device and for carrying out the method described above are no

special knowledge and no additional measures necessary. Embodiments of the inventive concept will be explained in more detail on the basis of exemplary embodiments, which are illustrated in the drawing. It shows:

Fig. 1 is a schematic representation of a device for force measurement, which has an off-center pivotally mounted two-armed lever, Fig. 2 is a hinge spring with two tapering areas, the deformation by means of several

Deformation measuring equipment is measurable,

3 shows a schematic representation of a diagram of an experimentally determined measurement uncertainty without or with an electronic correction as a function of the measured axial force,

4 is a schematic representation of a force transducer which can be calibrated in the force measuring device shown by way of example in FIG. 1, and FIG

Fig. 5 is a schematic representation of an arrangement of three force transducers, which are arranged side by side and can be loaded simultaneously and uniformly with a compressive force via a connecting element.

One shown in Fig. 1 schematically

Force measuring device 1 has a two-armed lever 2 which is pivotally mounted on an eccentrically arranged first hinge spring 3. At a longer first lever arm 4 is located at the outer end 5, a second flexible joint spring 6. On the second hinge spring 6 a plurality of interconnected weights 7 are attached. The weight pieces 7 exert on the second hinge spring 6 a first moment on the pivotally mounted lever 2.

At a shorter second lever arm 8, a loading frame 10 is fixed via a third hinge spring 9, which is vertically displaceable in a base frame 11. In the base frame 11, a cross member 12 is also guided vertically displaceable. Between the loading frame 10 and the traverse 12 is a force transducer 13. About the traverse 12 can on the force transducer 13 a

Compressive force are exerted, wherein on the load cell 13 and the load frame 10, a second moment is generated, which acts on the lever 2 and the by the

7 counteracts weight pieces acting on the first lever arm 4 first moment.

By a suitable displacement of the traverse 12 can be exerted on the force transducer 13 exactly that pressure force that is required so that the forces acting on the lever 2 moments in the sum are zero. About the known weight of the weights 7 and the

Knowledge of the geometric relationships of the first

Lever arm 4 and the second lever arm 8 and the three

Joint springs 3, 6 and 9 can determine the amount of force exerted on the force transducer 13 compressive force and calibrate the force transducer 13 in this way or calibrate. In order to specify the fulcrum of the joint springs 3, 6 and 9 as precisely as possible, the three hinge springs 3, 6 and 9 each have a first tapering region 14, the horizontally and perpendicular to the lever 2, and the

associated lever arm 4, 8 is aligned. In this first tapering region 14 are each a plurality of strain gauges 15 which are interconnected via an evaluation circuit, not shown, such that a via the hinge springs 3, 6 and 9 on the lever. 2

applied bending moment can be measured that leads to a corresponding deformation of the relevant hinge spring 3, 6 or 9.

To the accuracy in the implementation of such

To improve force measurements is according to the invention

provided that on at least one of the three hinge springs 3, 6, 9 which is transmitted via the relevant hinge spring 3, 6 or 9 transmitted to the lever 2 axial force. With increasing axial force deforms due to the force exerted on the lever 2 bending moments of the lever 2, so that the effective length of the first lever arm 4 and the second lever arm 8 changes and measured in the three hinge springs 3, 6 and 9 deformation no longer actually transmitted to the lever 2 bending moments.

In order to be able to measure the axial force, the third hinge spring 9 arranged on the second lever arm 8 has a second tapering region 16, which is aligned parallel to the second lever arm 8 or to the lever 2. In the second tapering region 16 are also strain gauges 17, which enforced by a suitable, not shown evaluation circuit by the force acting on the third hinge spring 9 axial force

Detecting deformation of the third hinge spring and thereby the Can measure axial force. The third hinge spring 9 is shown enlarged in Fig. 2 for illustrative purposes.

In order to make the axial force-dependent correction, the axial force acting on the third joint spring is measured during a measuring process and experimentally during a preceding calibration process

determined proportion of the axial force for the specification of the force measurement state taken into account. This is achieved in a simple manner that the on the lever. 2

acting bending moments are no longer regulated in the sum to zero or specified by a corresponding displacement of the traverse 12 and thereby exerted on the load cell 13 pressure force, but an additional axialkraftabhängiges correction torque

added and the total of the bending moments and the correction torque is minimized.

Based on the following equations and considerations, an approach should be illustrated, which is the

Axialkraftkorrektur invention can be based on. If the measuring device in the

Equilibrium and remain motionless and can still be regarded as a closed system, all attacking moments are in equilibrium and add up to a total moment of zero:

where m is the mass of the weights 7, g the locally acting gravitational acceleration, p _a the air density and Denoting the mass density, where Ii the length of the first lever arm 4 and I ₂ denotes the length of the second lever arm 8, wherein M _b denotes the sum of the bending forces acting on the hinge springs 3, 6, 9 bending moments and wherein the F on the second lever arm exercised train or

Compressive force and AF a systematic error for the

Force measurement referred to, for example, caused by an unavoidable lever deflection or by a crosstalk of the axial forces on the bending moments of the hinge springs 3, 6, 9.

Is this equation changed and after the

dissolved power F, the results of the

following context:

The force F to be measured with the force measuring device 1 results as the sum of individual contributions, the first contribution consisting exclusively of precisely measurable variables such as the air density p _a and the mass density p _m . So far, it has been assumed in the methods known from the prior art that the total sum of the bending forces acting on the joint springs 3, 6, 9, must be adjusted to zero and must be attempted by constructive measures, the

systematic measurement error AF to be as low as possible. With the approach according to the invention instead

electronically determines a correction force F _kor r and into the Equalization control of the bending moments so coupled, so that an amended total sum of the bending moments M ' _b , which includes the correction generated by the correction force F _kor r correction torque Mfcorr = Ffcorr' I2, detected and controlled to zero. Consequently, the following applies for the changed total sum of the bending moments M ' _b : M' _b = M _b + M _corr = M _b + F _corr · 1 ₂

When the correction force F _corr is chosen so that the correction force F _corr approximately the systematic

Error AF equals that with the

coupled correction force adjusted changed

Total sum of the bending moments M ' _b and the systematic error AF largely from, so that applies:

M'b / 1 ₂ = -AF.

In practical terms, in the correction process, precisely such a deformation of the hinge springs is caused that the moment for the deformation of the hinge springs the

corresponds to the systematic force error detected.

As a result, the method can only be used when using joint springs whose bending moments are detected, and not e.g. when using pans and cutting

is applicable. The electronic correction value F _cor r can be represented by a polynomial of higher order and calculated from the measured axial force. This is the measurement error the axial force only with a very small proportion as

Contribution to the measurement uncertainty.

FIG. 3 schematically shows, as a solid line 18, an experimentally determined relative measurement uncertainty of the measuring device 1 shown in FIG. 1 as a function of the axial force exerted via the third joint spring 9. The relative measurement uncertainty increases to a little more than 3xl0 ^{~ 4} up to the maximum applied axial force of 1000 kN. Used in measurements with the

Force measuring device 1 described above

Axialkraftkorrektur performed, so remains the determined after the correction and as a dashed line 19th

shown relative measurement uncertainty substantially constant and at a value less than 5xl0 ^{~ 5} . The Axialkraftkorrektur described above causes the consequence of a very significant reduction in the measurement uncertainty.

To those depending on the particular attacking

To determine axial force required axial force correction, a self-calibration of the measuring device. 1

be performed. A plurality of identical force sensors 13, one of which is shown by way of example in FIG. 4, are in the dead load range of the force measuring device 1 or in a measuring range with the smallest possible

Measurement uncertainty calibrated. Subsequently, more force transducers 13 are successively arranged side by side in the loading frame 10 and via a connecting element 20 shown by way of example in FIG

connected to each other, that on each load cell 13 an equal share of the crosshead 12 on the

Loading frame 10 exerts pressure applied. The Force measuring device 1 is brought by a suitable displacement of the traverse 12 in the force measurement state and determines the correction torque for the relevant axial force.

Claims

Patent claims

1. Method for measuring force with a

Force measuring device (1) having an eccentrically pivotally mounted two-armed lever (2), wherein on each lever arm (4, 8) in each case a load engagement joint is arranged, which is designed as a flexible hinge spring (6, 9) whose deformation is measurable by means of deformation measuring means in which a predeterminable load acts on a first lever arm (4) and is brought into a force measuring state by exerting a tensile or compressive force on a second lever arm (8) of the lever (2) and the tensile or compressive force required for this is determined, characterized in that at least one hinge spring (3, 6, 9) acts on this hinge spring (3, 6, 9)

Axialkraft recorded and a specifiable proportion of

determined axial force for a specification of

Force measurement state is taken into account, in

Depending on the determined axial force adapted to specify the force measurement state tensile or compressive force is adjusted.

2. The method according to claim 1, characterized in that that axial force is determined and taken into account for the specification of the Kraftmesszustands on the

Articulated spring (6) acts on the longer (4) lever arm.

3. The method according to any one of the preceding claims 1 or 2, characterized in that a Kompensationszug- or in dependence on the determined axial force

-force determined and an electronic

controlled or regulated specification of the force measurement state is additionally taken into account.

4. The method according to any one of the preceding claims, characterized in that in a preceding

Process step performed a calibration of the force measuring device (1) and thereby determined values for an additionally required Kompensationszug- or -druckkraft determined and retrievable in a memory device

get saved.

5. A method for calibrating a force measuring device with an off-center pivotally mounted two-armed lever, wherein each load arm is arranged a load engagement joint, which is designed as a flexible hinge spring whose deformation is measurable by means of Verformungsmesseinrichtungen and wherein at least one hinge spring acting on the hinge spring axial force is detectable, characterized in that to compensate for a

load-related shortening of the lever arm length of several load cells (13) in a load range with the lowest possible measurement uncertainty

Force measuring device (1) are calibrated and then several times each a different number of

calibrated load cells (13) are arranged parallel to each other on the second lever arm (8) and measured in order to determine a correction value required for the compensation for a plurality of different loads.

6. A force measuring device with a method according to one of claims 1 to 4, which is an off-center

pivotally mounted two-armed lever, wherein on each lever arm in each case a Lastangriffsgelenk is arranged, which is designed as a flexible hinge spring whose deformation is measurable by means of a deformation measuring device, characterized in that the hinge spring (3, 6, 9) a spring bar with a horizontal and perpendicular to the lever arm (4, 8) aligned first tapering region (14), in which a deformation measuring device is arranged, and spaced therefrom a

Axialkraftmesseinrichtung on the hinge spring (3, 6, 9) is arranged.

7. The device according to claim 6, characterized in that the hinge spring (3, 6, 9) has a parallel to the lever arm (4, 8) aligned second tapering region (16) in which the Axialkraftmesseinrichtung is arranged.

8. Apparatus according to claim 6 or claim 7, characterized in that the device is an electronic control device and a memory device for

Storage of Kompensationszug- and -druckkraftwerten.