Force sensitive scale and dual load sensor cell for use therewith

• US 5,837,946 A
• Filed: 06/16/1995
• Issued: 11/17/1998
• Est. Priority Date: 06/16/1995
• Status: Expired due to Term

• Current Assignee: Avery Weigh-Tronix LLC (Illinois Tool Works Inc.)
• Original Assignee: Weigh-Tronix, Inc. (Illinois Tool Works Inc.)
• Inventors: Johnson, Thomas H., Fruechte, Kevin M., Justice, Gene L., Younger, Roger A.
• Primary Examiner(s): Spyrou, Cassandra C.
• Assistant Examiner(s): MAI, ANH T
• Application Number:
  US08/491,034
• Time in Patent Office: 1,250 Days
• Field of Search: 177/136, 177/139, 177/211
• US Class Current: 177/136
• CPC Class Codes: G01G 19/083 lift truck scale
• CPC Class Codes: G01G 3/1404 combined with means to connect the strain gauges on electrical bridges
• CPC Class Codes: G01G 3/1414 Arrangements for correcting or for compensating for unwanted effects
• CPC Class Codes: G01L 1/2225 the direction being perpendicular to the central axis
• CPC Class Codes: G01L 1/2262 involving simple electrical bridges
• CPC Class Codes: G01L 1/2281 for temperature variations

Abstract

A force sensitive scale and dual sensor load cells for use therewith isolate, measure, and reject out residual rejected effects from the output of the scale. By isolating residual rejected effects, the overall accuracy of the scale may be improved, and the scale may be made substantially insensitive to the position of an object on the scale. In addition, where a scale includes multiple load cells coupled in parallel to a load receiving member and spaced apart in the direction of the force vector, the outputs of the load cells may be effectively scaled to cancel out frictional effects inherent in the scale.

Claims

1. A load cell, comprising:

(a) a load cell body;

(b) a first force sensor positioned on the load cell body to sense a load force applied to the load cell body along a first force vector, and provide a first output representative of the load force applied to the load cell;

(c) a second force sensor positioned on the load cell body to sense non-load forces applied to the load cell body along a second force vector, and provide a second output representative of forces other than a load force applied to the load cell; and

(d) means, coupled to receive the first and second outputs, for adjusting the first output based on the second output to reject residual rejected effects due to the non-load forces and generating a third output representative of a load force applied to the load cell.

2. The load cell of claim 1, wherein the load cell body includes a deformable member having a narrowed portion with opposing planar side surfaces, and wherein the first sensor is disposed on at least one of the side surfaces.

3. The load cell of claim 2, wherein the deformable member further includes opposing top and bottom surfaces oriented generally orthogonal to the side surfaces, and wherein the second sensor is disposed on at least one of the top and bottom surfaces.

4. The load cell of claim 2, wherein the deformable member is disposed between a threaded shank and a circular flange oriented generally orthogonal to a longitudinal axis of the shank.

5. The load cell of claim 1, wherein the first force sensor includes four force transducers electrically coupled in a wheatstone bridge circuit, two of the force transducers being oriented on the load cell body to be placed in tension in response to a force applied along the first force vector, and two of the force transducers being oriented on the load cell body to be placed in compression in response to a force applied along the first force vector.

6. The load cell of claim 5, wherein the second force sensor includes four force transducers electrically coupled in a wheatstone bridge circuit, two of the force transducers being oriented on the load cell body to be placed in tension in response to a force applied along the second force vector, and two of the force transducers being oriented on the load cell body to be placed in compression in response to a force applied along the second force vector.

7. The load cell of claim 6, wherein the force transducers in the first and second sensors each comprise a strain gauge.

8. The load cell of claim 7, wherein the first and second outputs of the first and second sensors are coupled in parallel, and wherein the rejection means comprises at least one resistor, coupled to the second output of the second sensor, for scaling the second output relative to the first output.

9. The load cell of claim 8, wherein the resistor is a variable resistor.

10. The load cell of claim 8, wherein the first and second sensors respectively include first and second inputs, each providing an excitation signal for driving the respective sensor, and wherein the rejection means further includes input scaling means for scaling the relative magnitudes of the excitation signals provided to the first and second sensors through the first and second inputs.

11. The load cell of claim 10, further comprising temperature compensating means for compensating the first sensor for the effects of temperature, the temperature compensating means including at least one thermistor coupled to the first input of the first sensor.

12. The load cell of claim 1, wherein the rejection means includes scaling means, responsive to the second output, for scaling the first output to reject residual rejected effects therefrom and thereby provide the third output.

13. The load cell of claim 12, wherein the first and second sensors respectively include first and second inputs for receiving an excitation voltage, wherein the first and second sensors each comprise a resistive network which varies in resistance in response to the applied force along the respective force vector, and wherein the first and second outputs of the first and second sensors are coupled in parallel.

14. The load cell of claim 13, wherein the scaling means comprises at least one resistor, coupled to one of the first and second outputs, for scaling the resistance across the one of the first and second outputs relative to the other of the first and second outputs.

15. The load cell of claim 13, wherein the scaling means comprises input scaling means for scaling the relative magnitudes of the first and second inputs driving the first and second sensors.

16. A method of sensing force with a load cell having a load cell body and first and second force sensors positioned thereon to sense forces applied to the load cell body respectively along first and second force vectors, the method comprising the steps of:

(a) sensing a load force applied to the load cell body along the first force vector with the first force sensor and generating a first output representative of the load force applied to the load cell body;

(b) sensing non-load forces applied to the load cell body along the second force vector with the second force sensor and generating a second output representative of forces other than a load force applied to the load cell; and

(c) scaling the first output in response to the second output to reject residual rejected effects from the first output to generate a load force signal.

17. A scale, comprising:

(a) a base;

(b) a force receiving member adapted to receive an object to be weighed;

(c) a plurality of load cells coupled between the force receiving member and the base to deflect in response to a force applied to the force receiving member by the object, each load cell having a first output primarily responsive to at least a portion of the force applied by the object, and a second output primarily responsive to residual rejected effects sensed by the load cell; and

(d) load cell driving means for providing a force signal representative of the force applied to the force receiving member by the object, the driving means including;

(1) means, coupled to the first and second outputs of each load cell, for adjusting the first output of each load cell based on the second output for each load cell to reject residual rejected effects;

(2) cornering means for adjusting the relative sensitivities of the first outputs of the load cells such that the sum of the first outputs is insensitive to the position of the object on the force receiving member; and

(3) summing means for summing the first outputs of the load cells to provide the force signal.

18. The scale of claim 17, wherein the base and the force receiving member respectively include first and second vertically oriented plates, and wherein the plurality of load cells include first and second load cells laterally spaced from one another and extending substantially perpendicular to the first and second plates.

19. The scale of claim 18, wherein the force receiving member further includes a pair of forks mounted to the second plate which extend outwardly from the second plate generally perpendicular to an outer surface thereof.

20. The scale of claim 19, wherein the first plate includes at least one hook for securing the first plate to a crossbar on a lift truck;

whereby the scale is a lift truck scale.

21. The scale of claim 19, wherein the plurality of load cells further includes third and fourth load cells respectively aligned in vertical columns with the first and second load cells.

22. The scale of claim 19, further comprising at least first and second flexures respectively aligned in vertical columns with the first and second load cells.

23. The scale of claim 22, wherein the first and second plates each include two laterally spaced vertical columns of at least two mounting apertures, and wherein the load cells and the flexures have identical mounting hardware;

whereby the load cells and the flexures are interchangeably mountable in each mounting aperture.

24. The scale of claim 17, wherein the first and second outputs of each load cell are generated by first and second force sensors, respectively positioned on the load cell to sense first and second forces applied to the load cell along first and second force vectors, each force sensor including four force transducers electrically coupled in a wheatstone bridge circuit, two of the force transducers being oriented to be placed in tension in response to a force applied along the respective force vector, and two of the force transducers being oriented to be placed in compression in response to the force applied along the respective force vector.

25. The scale of claim 24, wherein the force transducers include strain gauges.

26. The scale of claim 24, wherein the first and second outputs of the first and second sensors are coupled in parallel, and wherein the rejection means comprises at least one resistor, coupled to the second output of the second sensor, for scaling the second output relative to the first output.

27. The scale of claim 24, wherein the plurality of load cells include a top left, a top right, a bottom left, and a bottom right load cell spaced apart from one another in a generally vertical plane.

28. The scale of claim 27, wherein the first and second sensors respectively include first and second inputs, each providing an excitation signal for driving the respective sensor, and wherein the rejection means includes input scaling means for scaling the relative magnitudes of the excitation signals provided to the first and second sensors through the first and second inputs.

29. The scale of claim 28, wherein the first and second inputs of the first and second sensors are coupled to a common excitation signal, and wherein the input scaling means includes first and second top/bottom trim potentiometers coupled to the common excitation signal, the first top/bottom trim potentiometer coupled between the second inputs to the top left and bottom left load cells, and the second top/bottom trim potentiometer coupled between the second inputs to the top right and bottom right load cells so that residual rejected effects are rejected by adjusting the first and second trim potentiometers.

30. The scale of claim 29, wherein the cornering means includes excitation scaling means for scaling the excitation signals to the first inputs of the load cells relative to one another.

31. The scale of claim 30, wherein the excitation scaling means includes a left/right balance potentiometer coupled to the common excitation signal and first and second top/bottom balance potentiometers each coupled to the left/right balance potentiometer, the first top/bottom balance potentiometer coupled between the first inputs to the top left and bottom left load cells, and the second top/bottom balance potentiometer coupled between the first inputs to the top right and bottom right load cells;

whereby the scale is cornered by adjusting the left/right and top/bottom balance potentiometers.

32. The scale of claim 31, further comprising zeroing means, coupled across the common excitation signal, for zeroing the force signal when no external force is applied to the force receiving member, the zeroing means comprising a potentiometer driven by the force signal.

33. The scale of claim 17, wherein the rejection means includes scaling means, responsive to the second output of each load cell, for scaling the first output of each load cell to reject residual rejected effects therefrom.

34. The scale of claim 33, wherein each load cell includes a sensor for generating the first output, the sensor including an input for receiving an excitation voltage, wherein each sensor comprises a resistive network which varies in resistance in response primarily to the force applied by the object, and wherein the cornering means includes excitation scaling means for scaling the relative magnitudes of the inputs to the sensors on the plurality of load cells.

35. The scale of claim 17, wherein the summing means comprises means for coupling the first outputs of the load cells to a common output signal.

36. The scale of claim 17, further comprising tilt adjusting means for adjusting the force signal to compensate for tilt in the force receiving member, the tilt adjusting means including first and second tilt sensors.

37. The scale of claim 36, wherein the tilt adjusting means generates a tilt adjusted signal from the force signal by applying the equation:

##EQU19## where W is the tilt adjusted signal, S is the force signal, φ

₁ is the first tilt angle, φ

₂ is the second tilt angle, δ

₁ and δ

₂ are phase shift constants, and A is a sensitivity factor constant.

38. A method of weighing an object on a scale including a plurality of load cells coupled between a force receiving member and a base to deflect in response to a force applied to the force receiving member by the object, each load cell having a first output primarily responsive to at least a portion of the force applied by the object, and a second output primarily responsive to residual rejected effects sensed by the load cell, the method comprising the steps of:

(a) scaling the first output of each load cell in response to the second output of the load cell to reject residual rejected effects from the first output of each load cell;

(b) adjusting the relative sensitivities of the first outputs of the load cells such that the sum of the first outputs is substantially insensitive to the position of the object on the force receiving member; and

(c) summing the first outputs of the load cells to provide the force signal.