Six axis force sensor employing multiple shear strain gages

• US 5,490,427 A
• Filed: 10/17/1994
• Issued: 02/13/1996
• Est. Priority Date: 10/17/1994
• Status: Expired due to Fees

• Current Assignee: Fanuc USA Corporation
• Original Assignee: Fanuc USA Corporation
• Inventors: Akeel, Hadi A., Yee, Albert G.
• Primary Examiner(s): Chilcot, Richard
• Assistant Examiner(s): DOMBROSKE, GEORGE M
• Application Number:
  US08/324,414
• Time in Patent Office: 484 Days
• Field of Search: 73/763, 73/767, 73/768, 73/774, 73/781, 73/815, 73/819, 73/855, 73/866.5
• US Class Current: 73/767
• CPC Class Codes: G01L 5/162 of piezoresistors
• CPC Class Codes: G01L 5/1627 of strain gauges using piezoresistors G01L5/162

Abstract

A force sensor is disclosed in which strain is measured exclusively by shear strain gages mounted on L or T-shaped legs. Each such leg has at least two arms which are orthogonal to one another. That is, the arms of the L or T each have a lengthwise or longitudinal axis, and two of these longitudinal axes are oriented at approximately 90° with respect to one another. Each leg also has at least two shear strain gages mounted on orthogonal arms to measure forces in orthogonal directions. Three or more of these L or T shaped legs are integrated in a generally cylindrical and hollow body defining the force sensor. The shear strain gage outputs from the arms of this structure are sent to a digital signal processor which determines the magnitude and direction of any force/moment applied to the cylindrical body. Further, the shear strain gages are mounted in pockets sized to hold the strain gages and provide increased sensitivity to shear strain.

Claims

1. A force sensor for measuring forces interacting between bodies connected to opposite sides of the force sensor, on a sensor body structure in at least two dimensions, the force sensor comprising:

an L-shaped leg integrated in the sensor body and having a first arm and a second arm each having a direction substantially perpendicular to that of the other arm, each arm also having a generally planar surface;

a first shear strain gage mounted on the first arm of the L-shaped leg such that the first strain gage primarily measures shear forces which are both parallel to the plane of the first arm and perpendicular to the direction of the first arm;

a second shear strain gage mounted to the second arm of the L-shaped leg such that the second strain gage primarily measures shear forces which are both parallel to the plane of the second arm and perpendicular to the direction of the second arm.

2. The force sensor of claim 1 further comprising a first open section in the first arm of the L-shaped leg, the first open section having a generally U-shaped crosssection including a floor against which the first shear strain gage is mounted.

3. The force sensor of claim 2 further comprising a second open section in the second arm of the L-shaped leg, the second open section having a generally Ushaped cross-section including a floor against which the second shear strain gage is mounted.

4. The force sensor of claim 1 wherein the force sensor includes at least three L-shaped legs each having first and second shear strain gages mounted thereon, wherein the at least three L-shaped legs are arranged in the structure such that the shear strain gages collectively provide information identifying forces acting on the structure along three mutually orthogonal axes and identifying moments about the three axes.

5. The force sensor of claim 4 wherein the structure includes three Lshaped legs and wherein the first and second arms of each L-shaped leg are not coplanar.

6. The force sensor of claim 4 wherein the structure includes four Lshaped legs and wherein the first and second arms of each L-shaped leg are coplanar.

7. The force sensor of claim 6 wherein the sensor body is generally hollow and cylindrical and has an axis parallel to the generally cylindrical surface, and wherein the four L-shaped legs are integrated in the cylindrical surface of the sensor body at locations separated from one another by about 90°

.

8. force sensor of claim 7 wherein the four L-shaped legs are oriented on the cylindrical sensor body such that for each L-shaped leg, the first shear strain gage primarily responds to forces acting laterally on the cylindrical surface and the second shear strain gage primarily responds to forces acting in a direction parallel to a principle axis of the cylindrical sensor body.

9. The force sensor of claim 1 wherein the L-shaped leg forms part of a T-shaped leg integrated in the sensor body.

10. The force sensor of claim 9 wherein the T-shaped leg includes three shear strain gages.

11. A method of determining the magnitude and direction of forces applied to a generally hollow and cylindrical force sensor having axial and transverse axes and having three or more legs each of which has at least a first shear strain gage oriented to detect primarily force components in the axial direction and a second shear strain gage oriented to detect primarily force components along one of the transverse axes, the method comprising the following steps:

monitoring shear strain detected by the first strain gages and caused primarily by forces acting in the axial direction and/or moments about one of the transverse axes;

monitoring shear strain detected by the second strain gages and caused primarily by forces acting along one or more transverse axes and/or moments about the axial axis;

processing the shear strains detected by the first and second strain gages to determine the magnitude and direction of any forces and/or moments applied to the force sensor.

12. The method of claim 11 wherein the steps of monitoring shear strain monitor shear strain occurring in pockets in which the strain gages are mounted.

13. The method of claim 11 further comprising a step of adjusting the position of a tool connected to the force sensor in response to the forces determined in the step of processing.

14. The method of claim 11 wherein the step of processing is conducted at least in part by a digital processor located within the cylindrical force sensor.

15. The method of claim 14 wherein the digital processor determines the complete magnitude and direction of any forces and/or moments applied to the force sensor.

16. A force sensor for measuring forces and moments in six degrees of freedom, the force sensor comprising:

a hollow generally cylindrical force sensor body having an axis;

three or more L-shaped legs arranged on the force sensor body;

at least two pockets provided in each L-shaped leg;

a plurality of shear strain gages mounted in the pockets of L-shaped legs, wherein in each leg at least a first shear strain gage is oriented to respond to forces primarily parallel to the sensor body axis and at least a second shear strain gage is oriented to respond to primarily transverse forces which are not parallel to the sensor body axis.

17. The force sensor of claim 16 wherein the force sensor body is a monolithic structure.

18. The force sensor of claim 16 further comprising a digital processor coupled to the shear strain gages, wherein the digital signal processor is programmed to determine from output signals of the plurality of shear strain gages magnitudes and directions of forces and/or moments applied to the force sensor body.

19. The force sensor of claim 16 comprising four L-shaped legs on the force sensor body, each L-shaped leg being angularly spaced from two other adjacent Lshaped legs by about 90°

.

20. The force sensor of claim 16 wherein the L-shaped legs are spaced from one another by apertures in the sensor body.

21. The force sensor of claim 16 wherein the L-shaped leg is part of a Tshaped leg which is connected to the force sensor body at three ends.

22. The force sensor of claim 16 further comprising a robot arm connected to said force sensor body.

23. A force sensor for measuring forces interacting between bodies connected to opposite sides of the force sensor, on a sensor body structure in at least two dimensions, the force sensor comprising:

an L-shaped leg integrated in the sensor body and having a first arm and a second arm mutually integrally connected to form the L-shaped leg and each arm having a direction substantially perpendicular to that of the other arm, said first and second arms being connected in series between support regions respectively associated to said opposite sides of the force sensor;

a first shear strain gage mounted on the first arm of the L-shaped leg such that the first strain gage primarily measures shear forces which are perpendicular to the direction of the first arm;

a second shear strain gage mounted to the second arm of the L-shaped leg such that the second strain gage primarily measures shear forces which are perpendicular to the direction of the second arm.

24. A method of determining the magnitude and direction of forces applied to a generally hollow and cylindrical force sensor having axial and transverse axes and having three or more Lshaped legs each of which has a first arm and a second arm mutually integrally connected such that each arm has a direction substantially perpendicular to that of the other arm and each L-shaped leg having at least a first shear strain gage mounted on the first arm and oriented to detect primarily force components in the axial direction and a second shear strain gage mounted on the second arm and oriented to detect primarily force components along one of the transverse axes, the method comprising the following steps:

monitoring shear strain detected by the first strain gages and caused primarily by forces acting in the axial direction and/or moments about one of the transverse axes;

monitoring shear strain detected by the second strain gages and caused primarily by forces acting along one or more transverse axes and/or moments about the axial axis;

processing the shear strains detected by the first and second strain gages to determine the magnitude and direction of any forces and/or moments applied to the force sensor.

25. A force sensor for measuring forces and moments in six degrees of freedom, the force sensor comprising:

a hollow generally cylindrical force sensor body having an axis and mutually oppositely arranged first and second sides;

a first support region and a second support region each respectively associated to a respective one of said opposite sides;

three or more L-shaped legs arranged on the force sensor body, each L-shaped leg having a first arm and a second arm mutually integrally connected such that each arm has a direction substantially perpendicular to that of the other arm, and said first and second arms being connected in series between said first and second support regions;

at least two pockets provided in each L-shaped leg;

a plurality of shear strain gages mounted in the pockets of L-shaped legs, wherein in each leg at least a first shear strain gage is oriented to respond to forces primarily parallel to the sensor body axis and at least a second shear strain gage is oriented to respond to primarily transverse forces which are not parallel to the sensor body axis.