Load cell

• US 4,657,097 A
• Filed: 10/15/1985
• Issued: 04/14/1987
• Est. Priority Date: 02/13/1984
• Status: Expired due to Term

• Current Assignee: Mettler-Toledo Incorporated (Mettler-Toledo International Inc.)
• Original Assignee: Reliance Electric Company (ABB Limited)
• Inventors: Griffen, Neil C.
• Primary Examiner(s): Goldberg, E. A.
• Assistant Examiner(s): Foster, Patrick W.
• Application Number:
  US06/786,719
• Time in Patent Office: 546 Days
• Field of Search: 177/211, 177/229, 73/1 B, 73/862.65, 73/862.66, 73/767
• US Class Current: 177/211
• CPC Class Codes: G01L 1/2243 the supports being parallelogram-shaped
• CPC Class Codes: G01L 1/2262 involving simple electrical bridges
• CPC Class Codes: G01L 1/2268 Arrangements for correcting or for compensating unwanted effects

Abstract

Load cells incorporating one or more gaged beams as the load sensing elements are compensated for off-center loading in the longitudinal and/or lateral directions. The load cell may also include one or more flexure arms extending parallel to the beam. The relative position of the gages and the neutral axis of the beam is made such that the gages produce a correctible response to off-set loading by positioning the gages and/or physically altering the beam. Preferably, conventional or special strain gages are mounted upon the beam(s) to produce a correctible response by the gages by rotating and/or displacing them with respect to the longitudinal axis of the beam. Resistors are then connected to certain of the strain gages to compensate for off-center loading. The strain gages are advantageously placed all on one side of a beam but may be placed on opposite sides thereof.

Claims

1. A moment-insensitive load cell comprising a single beam member having a longitudinal and a lateral axis, said beam member having only first and second pairs of strain gauges mounted thereon, all of said gauges being mounted on the same horizontal surface of said beam member, the gauges of each pair being spaced apart longitudinally and sensing tensile and compressive strains, respectively, in said beam, the positions of said first pair of strain gauges with respect to the neutral axis of the beam being such that the gauges produce a correctable output response to loads applied off-center on said beam, said first and second pairs of strain gauges being connected in a bridge circuit, means for applying a load on said beam member between longitudinally spaced gauges, and a resistor connected in circuit with at least one of said strain gauges to make the output of said bridge circuit substantially independent of the position along at least one axis of said beam of a load applied thereto.

2. A load cell as claimed in claim 1 wherein each of the strain gages of said first pair is mounted on said beam with at least a portion of its strain sensing elements at an angle to the longitudinal axis of the beam.

3. A load cell as claimed in claim 1 wherein each of the strain gages of said first pair is displaced from the longitudinal axis of the beam.

4. A load cell as claimed in claim 1 wherein material is removed from one side of said beam to cause said gages to produce a correctible output response to off-center loading.

5. A load cell as claimed in claim 1 including a base, a load receiving arm spaced from said base, said beam being connected between said load receiving arm and said base, and at least one flexure arm extending generally parallel to said beam and interconnecting said load receiving arm and base.

6. A method of making a moment-insensitive compensated load cell comprising the steps of providing a single beam member having longitudinal and lateral axes, mounting first and second pairs of strain gauges on the same horizontal surface of said beam with the gauges of each pair spaced apart longitudinally, providing load receiving means for applying a load on said beam member between the longitudinally spaced gauges, adjusting the relative positions of the first pair of gauges and the neutral axis of the beam so that the gauges produce a correctible response to loads applied off-center on said beam, connecting the first and second pairs of gauges in a bridge circuit, placing test compensating resistors in circuit with at least some of the gauges, determining the response of the bridge circuit to loads applied off-center on said beam between the longitudinally spaced gauges, using said response and the values of the test compensating resistors to calculate the values of resistors to be connected in circuit with said gauges to compensate for the effects of off-center loading, and connecting resistors of the proper value in circuit with the gauges to make the output of the bridge circuit substantially independent of the position of a load along at least one axis of the beam member.

7. A load cell comprising two substantially parallel beams having longitudinal and lateral axes, means rigidly connecting one end of said beams together, means rigidly connecting the other end of the beams together, a pair of strain gauges mounted on a horizontal surface of each of the beams to detect bending strains therein, at least one strain gauge on each beam having at least a portion of its strain sensing elements oriented at an angle to the longitudinal axis of the beam, means connecting the gauges in a bridge circuit, and a resistor connected in circuit with at least one of the gauges to make the output of the bridge circuit substantially independent of the position of a load along at least one of said axes.

8. A load cell as claimed in claim 7 wherein at least one strain gage on each beam is displaced from the longitudinal axis of the beam.

9. A method of compensating a load cell having two parallel beam members with longitudinal and lateral axes, comprising the steps of mounting a pair of strain gauges on a horizontal surface of each beam to detect bending strains therein and so that at least one gauge on each beam has at least a portion of its strain sensing elements oriented at an angle to the longitudinal axis of the beam, connecting the gauges in a bridge circuit, placing test compensating resistors in circuit with said gauges, determining the response of said bridge circuit to loads applied off-center on said load cell, using said response and the values of the test compensating resistors to calculate the values of resistors to be connected in circuit with said gauges to compensate for the effects of off-center loading, and connecting resistors of the proper value in circuit with said gauges to make the output of said bridge circuit substantially independent of the position of a load along at least one axis of the beam members.

10. A load cell comprising at least one elongated beam, having a longitudinal and a lateral axis, a pair of strain gages on said beam, said beam having material removed from one side thereof to cause said gages to produce a correctible response to an off-set load on said beam, and a resistor connected in circuit with at least one of said gages to make the output of the gages substantially independent of the position along at least one axis of the beam of a load applied thereto.

11. A load cell as claimed in claim 10 including a second beam substantially parallel to the first beam, said beams being rigidly connected together at each end, a pair of strain gages on said second beam, means connecting all of the strain gages in a bridge circuit, each of said beams having material removed from a side thereof to cause said bridge to produce a correctible response to an off-set load on said load cell.

16. A load cell as claimed in claim 10 including a base, a load receiving arm spaced from said base, said beam being connected between said load receiving arm and said base, and at least one flexure arm extending generally parallel to said beam and interconnecting said load receiving arm and base.

12. A load cell comprising an elongated beam having a longitudinal and a lateral axis, strain gages mounted on a vertical surface of said beam to detect shear strains therein resulting from a load applied to said beam, means connecting said strain gages in an electrical bridge circuit, said gages being positioned with respect to the neutral axis of the beam to increase the sensitivity of the gages to loads applied off center on said beam, and a resistor connected in circuit with at least one of said gages to make the output of the bridge circuit substantially independent of the position along at least one axis of the beam of a load applied thereto.

13. A load cell as claimed in claim 12 wherein each of said strain gages is mounted on said beam with at least a portion of its strain sensing elements rotated at a small angle from the shear strain detecting position to increase the sensitivity of the gages to loads applied off center on the beam.

14. A load cell as claimed in claim 12 wherein each of said strain gages is displaced vertically from the neutral axis of the beam to increase the sensitivity of the gages to loads applied off center on the beam.

15. A load cell as claimed in claim 12 wherein material is removed from one side of said beam to cause said increase in the sensitivity of the gages to loads applied off center on the beam.

17. A method of compensating a shear beam load cell including an elongated beam having a longitudinal and a lateral axis, strain gages mounted on a vertical surface of said beam to detect shear strains therein resulting from a load applied to said beam, the strain gages being connected in an electrical bridge circuit, comprising the steps of positioning said strain gages with respect to the neutral axis of the beam to increase the sensitivity of the gages to loads applied off center on the beam, placing test compensating resistors in circuit with said gages, determining the differences in output readings of said bridge circuit for loads applied at different positions on said load cell, using said differences and the values of the test compensating resistors to calculate the values of resistors to be connected in circuit with said gages to substantially eliminate said differences, and connecting a resistor of the proper value in circuit with at least one of said gages to make the output of the bridge circuit substantially independent of the position of a load along at least one of said longitudinal and lateral axes.