Triaxial normal and shear force sensor

Abstract

A triaxial force sensor using a hemispherical target supported by a compliant element such as a spring or an elastomer supported by a rigid support member. The sensor includes a plurality of ultrasonic transducers disposed in a plane at equal intervals about the target and vertically and laterally offset from the target. The transducers are oriented at an oblique angle to the plane, and aimed at the target in its rest position. The target is displaced by sufficient force applied to elastically deform the compliant element, which displacement alters the transit times of ultrasonic signals from the transducers which are reflected from the target. If at least three sensor units are employed non-colinearly, the six force-torque components, Fx, Fy, Fz, Mx, My, Mz, can be determined from the pulse transit times, the speed-of-sound in the medium or media between the transducers and the target, the deformation response of the compliant element, and the known geometry and spacing of the transducers. Pairs of transducers may be rotationally offset from each other to determine different force-torque components. A plurality of sensors as described may be employed together in a multi-sensor array. An alternative embodiment employing both the amplitude and the transit time of an ultrasonic pulse is also disclosed.

What is claimed is:  
     
       1. A sensor for use in determining shear and normal components of a force applied thereto, comprising: 
       a substantially rigid support member carrying a compliant structure;  
       at least one target for reflecting ultrasonic waves supported by said compliant structure;  
       at least two ultrasonic transducers aimed at said target, each of said transducers adapted to emit and detect ultrasonic signals, said transducers being laterally and vertically offset from said at least one target; and  
       a substantially acoustically-transparent, substantially acoustically-nonrefractive medium disposed between said transducers and said at least one target.  
     
     
       2. The sensor of claim  1 , wherein said at least two transducers are located in a common plane. 
     
     
       3. The sensor of claim  1 , wherein said at least two transducers comprise four transducers located at 90° intervals around said target. 
     
     
       4. The sensor of claim  1 , wherein said target presents a constant-radius, arcuate, reflective surface to said transducers. 
     
     
       5. The sensor of claim  1 , wherein said target is at least partially embedded in said compliant structure. 
     
     
       6. The sensor of claim  5 , wherein said compliant structure comprises an elastomer. 
     
     
       7. The sensor of claim  6 , wherein said elastomer is selected from a group comprising urethanes, silicone rubbers, neoprene rubbers, natural rubbers, plastics, and gels. 
     
     
       8. The sensor of claim  1 , wherein said compliant structure comprises a metal spring structure. 
     
     
       9. The sensor of claim  1 , wherein said compliant structure comprises a combined metal and elastomer spring structure. 
     
     
       10. The sensor of claim  1 , wherein said at least two transducers are embedded in an elastomer layer, said layer comprising a portion of said substantially acoustically-transparent, substantially acoustically-nonrefractive medium. 
     
     
       11. The sensor of claim  1 , further comprising a displaceable medium disposed between said at least two transducers and said target, said displaceable medium comprising a portion of said substantially acoustically-transparent, substantially acoustically-nonrefractive medium. 
     
     
       12. The sensor of claim  1 , wherein said transducers are selected from a group comprising PVDF films and ceramics. 
     
     
       13. The sensor of claim  1 , wherein said substantially rigid support member includes an aperture therein, said compliant structure is supported by said support member along at least a portion of the periphery of said aperture, and said target is suspended within said aperture by said compliant structure. 
     
     
       14. The sensor of claim  1 , wherein said substantially rigid support member includes an aperture therein, said aperture is symmetric in shape, and said target is supported in the center of said aperture. 
     
     
       15. The sensor of claim  1 , wherein said at least two transducers are located in a common plane, at least one of said at least two transducers is oriented to emit said ultrasonic signals perpendicular to said common plane, and wherein said sensor further includes angle-changing structure for aiming said perpendicularly emitted ultrasonic signals at said target by changing the angle of said emitted ultrasonic signals after emission from said at least one oriented transducer and for changing the angle of said ultrasonic waves after reflection from said target to the perpendicular to said common plane for detection by said at least one oriented transducer. 
     
     
       16. The sensor of claim  15 , wherein said angle-changing structure comprises an acoustic prism. 
     
     
       17. The sensor of claim  15 , wherein said angle-changing structure comprises an acoustic reflector. 
     
     
       18. The sensor of claim  1 , wherein at least one of said transducers is aimed at said target through an acoustic prism placed therebetween. 
     
     
       19. The sensor of claim  1 , wherein at least one of said transducers is aimed at said target via an acoustic reflector. 
     
     
       20. A multi-sensor array for use in determining shear and normal components of a force applied thereto, comprising: 
       a substantially rigid support member carrying a plurality of laterally separated compliant structures located at predetermined intervals;  
       a plurality of targets for reflecting ultrasonic waves, each of said targets being supported by one of said compliant structures;  
       each of said targets having associated therewith at least a pair of ultrasonic transducers adapted to emit and receive ultrasonic signals, said transducers of each pair being aimed at said target with which said transducers are associated and horizontally and vertically offset therefrom; and  
       a substantially acoustically-transparent, substantially acoustically-nonrefractive medium disposed between said transducers and said targets.  
     
     
       21. The multi-sensor array of claim  20 , wherein said targets are located at substantially equal intervals and in a common plane. 
     
     
       22. The multi-sensor array of claim  20 , wherein said transducers are located in a common plane. 
     
     
       23. The multi-sensor array of claim  20 , wherein said at least a pair of transducers comprises four transducers located at 90° intervals around each of said targets. 
     
     
       24. The multi-sensor array of claim  20 , wherein said targets each present a constant-radius, arcuate, reflective surface to the transducers associated therewith. 
     
     
       25. The multi-sensor array of claim  20 , wherein at least some of said targets are embedded in said compliant structures. 
     
     
       26. The multi-sensor array of claim  25 , wherein said compliant structures comprise an elastomer. 
     
     
       27. The multi-sensor array of claim  26 , wherein said elastomer is selected from a group comprising urethanes, silicone rubbers, neoprene rubbers, natural rubbers, plastics and gels. 
     
     
       28. The multi-sensor array of claim  20 , wherein said compliant structures comprise metal spring structures. 
     
     
       29. The multi-sensor array of claim  20 , wherein said compliant structures comprise combined metal and elastomer spring structures. 
     
     
       30. The multi-sensor array of claim  20 , wherein said transducers are embedded in an elastomer layer, said layer comprising at least a portion of said substantially acoustically-transparent, substantially acoustically-nonrefractive medium. 
     
     
       31. The multi-sensor array of claim  20 , further comprising a displaceable medium disposed between said transducers and said targets, said displacable medium comprising a portion of said substantially acoustically-transparent, substantially acoustically-nonrefractive medium. 
     
     
       32. The multi-sensor array of claim  20 , wherein said transducers are selected from a group comprising PVDF films and ceramics. 
     
     
       33. The multi-sensor array of claim  20 , wherein said substantially rigid support member includes a plurality of apertures therein, each of said compliant structures is supported by said support member along at least a portion of the periphery of one of said apertures, and said targets are suspended within said apertures by said compliant structures. 
     
     
       34. The multi-sensor array of claim  20 , wherein said substantially rigid support member includes a plurality of apertures therein, said apertures are symmetric in shape, and said targets are supported in the centers of said apertures. 
     
     
       35. The multi-sensor array of claim  20 , wherein said transducers are located in a common plane, at least one of said transducers is oriented to emit said ultrasonic signals perpendicular to said common plane, and further including structure for aiming said perpendicularly emitted ultrasonic signals at the target with which said at least one oriented transducer is associated by changing the angle of said perpendicularly emitted ultrasonic signals after emission and for changing the angle of said ultrasonic waves after reflection from said target to the perpendicular to said common plane for detection by said at least one oriented transducer. 
     
     
       36. The multi-sensor array of claim  35 , wherein at least one of said angle-changing structures comprises an acoustic prism. 
     
     
       37. The multi-sensor array of claim  35 , wherein at least one of said angle-changing structures comprises an acoustic reflector. 
     
     
       38. A sensor for use in determining shear and normal components of a force applied thereto, comprising: 
       
         a substantially rigid support member carrying a compliant structure;  
       
       
         at least one target for reflecting ultrasonic waves supported by said compliant structure;  
       
       
         at least two ultrasonic transducers spaced from and aimed at said at least one target, said transducers being adapted to emit or detect ultrasonic signals; and  
       
         a substantially acoustically - transparent, substantially acoustically - nonrefractive medium disposed between said transducers and said at least one target.    
     
     
       39. The sensor of claim  38 , wherein said at least two transducers are located substantially in a common plane.  
     
     
       40. The sensor of claim  38 , wherein said at least two transducers comprise four transducers located at  90 ° intervals around said at least one target.  
     
     
       41. The sensor of claim  38 , wherein said at least one target presents a constant- radius, arcuate, reflective surface to said at least two transducers.    
     
     
       42. The sensor of claim  38 , wherein said at least one target is at least partially embedded in said compliant structure.  
     
     
       43. The sensor of claim  42 , wherein said compliant structure comprises an elastomer.  
     
     
       44. The sensor of claim  43 , wherein said elastomer is selected from a group comprising urethanes, silicone rubbers, neoprene rubbers, natural rubbers, plastics, and gels.  
     
     
       45. The sensor of claim  38 , wherein said compliant structure comprises a metal spring structure.  
     
     
       46. The sensor of claim  38 , wherein said compliant structure comprises a combined metal and elastomer spring structure.  
     
     
       47. The sensor of claim  38 , wherein said at least two transducers are at least partially embedded in an elastomer layer, said elastomer layer comprising a portion of said substantially acoustically- transparent, substantially acoustically - nonrefractive medium.    
     
     
       48. The sensor of claim  47 , further comprising a displaceable medium disposed between said elastomer layer and said at least one target, said displaceable medium comprising a portion of said substantially acoustically- transparent, substantially acoustically - nonrefractive medium.    
     
     
       49. The sensor of claim  38 , wherein said at least two transducers are selected from a group comprising PVDF films and ceramics.  
     
     
       50. The sensor of claim  38 , wherein said substantially rigid support member includes an aperture therein, said compliant structure is supported by said support member along at least a portion of a periphery of said aperture, and said at least one target is suspended within said aperture by said compliant structure.  
     
     
       51. The sensor of claim  38 , wherein said substantially rigid support member includes an aperture therein, said aperture is symmetric in shape, and said at least one target is supported in the center of said aperture.  
     
     
       52. The sensor of claim  38 , wherein said at least two transducers are located substantially in a common plane, said at least two transducers are oriented to emit or detect said ultrasonic signals perpendicular to said common plane, and wherein said sensor further includes angle- changing structure for aiming said perpendicularly emitted ultrasonic signals at said at least one target by changing an angle of said perpendicularly emitted ultrasonic signals after emission from a transducer and for changing an angle of said ultrasonic waves after reflection from said at least one target to perpendicular to said common plane for detection by a transducer.    
     
     
       53. The sensor of claim  52 , wherein said angle- changing structure comprises an acoustic prism.    
     
     
       54. The sensor of claim  52 , wherein said angle- changing structure comprises an acoustic reflector.    
     
     
       55. A multi- sensor array for use in determining shear and normal components of a force applied thereto, comprising:    
       
         a substantially rigid support member carrying a plurality of laterally separated compliant structures located at predetermined intervals;  
       
       
         a plurality of targets for reflecting ultrasonic waves, each of said targets being supported by one of said compliant structures;  
       
       
         each of said targets having associated therewith at least two ultrasonic transducers, said transducers being adapted to emit or receive ultrasonic signals, said at least two transducers associated with each target being spaced from and aimed at said target; and  
       
         a substantially acoustically - transparent, substantially acoustically - nonrefractive medium disposed between said transducers and said targets.    
     
     
       56. The multi- sensor array of claim  55 , wherein said targets are located at substantially equal intervals, and substantially in a common plane.    
     
     
       57. The multi- sensor array of claim  55 , wherein said transducers are located substantially in a common plane.    
     
     
       58. The multi- sensor array of claim  55 , wherein said at least two transducers comprise four transducers located at  90 ° intervals around each of said targets.    
     
     
       59. The multi- sensor array of claim  55 , wherein said targets each present a constant - radius, arcuate, reflective surface to the at least two transducers associated therewith.    
     
     
       60. The multi- sensor array of claim  55 , wherein at least some of said targets are at least partially embedded in said compliant structure.    
     
     
       61. The multi- sensor array of claim  60 , wherein said compliant structure comprises an elastomer.    
     
     
       62. The multi- sensor array of claim  61 , wherein said elastomer is selected from a group comprising urethanes, silicone rubbers, neoprene rubbers, natural rubbers, plastics and gels.    
     
     
       63. The multi- sensor array of claim  55 , wherein said compliant structures comprise metal spring structures.    
     
     
       64. The multi- sensor array of claim  55 , wherein said compliant structures comprise combined metal and elastomer spring structures.    
     
     
       65. The multi- sensor array of claim  55 , wherein said transducers are at least partially embedded in an elastomer layer, said elastomer layer comprising at least a portion of said substantially acoustically - transparent, substantially acoustically - nonrefractive medium.    
     
     
       66. The sensor of claim  65 , further comprising a displaceable medium disposed between said elastomer layer and said targets, said displaceable medium comprising a portion of said substantially acoustically- transparent, substantially acoustically - nonrefractive medium.    
     
     
       67. The multi- sensor array of claim  55 , wherein said transducers are selected from a group comprising PVDF films and ceramics.    
     
     
       68. The multi- sensor array of claim  55 , wherein said substantially rigid support member includes a plurality of apertures therein, each of said compliant structures is supported by said support member along at least a portion of a periphery of one of said apertures, and said targets are suspended within said apertures by said compliant structures.    
     
     
       69. The multi- sensor array of claim  55 , wherein said substantially rigid support member includes a plurality of apertures therein, said apertures are symmetric in shape, and said targets are supported in the centers of said apertures.    
     
     
       70. The multi- sensor array of claim  55 , wherein said transducers are located substantially in a common plane, said transducers are oriented to emit or detect said ultrasonic signals perpendicular to said common plane, and further including angle - changing structures for aiming said perpendicularly emitted ultrasonic signals at each target with which oriented transducers are associated by changing an angle of said perpendicularly emitted ultrasonic signals after emission from a transducer and for changing an angle of said ultrasonic waves after reflection from each of said targets to perpendicular to said common plane for detection by a transducer.    
     
     
       71. The multi- sensor array of claim  70 , wherein at least some of said angle - changing structures comprise acoustic prisms.    
     
     
       72. The multi- sensor array of claim  70 , wherein at least some of said angle - changing structures comprise acoustic reflectors.    
     
     
       73. A method of determining shear and normal components of a force, comprising: 
         positioning at least one acoustically - reflective target;    
       
         providing a plurality of ultrasonic transducers spaced from said at least one target and aiming said transducers of said plurality at said at least one target from;  
       
       
         compliantly supporting said at least one target in an initial, reference position;  
       
         disposing at least one substantially acoustically - transparent medium between each of said transducers and said at least one target;    
       
         applying said force to said at least one target to displace said at least one target from said initial position;  
       
       
         emitting ultrasonic signals from transducers of said plurality toward said at least one target while said at least one target is being displaced from said initial position for reflection from said at least one target and timing transit times between emission of said signals and return of signals reflected from said at least one target to transducers of said plurality; and  
       
       
         calculating shear and normal components of said force from said transit times.  
       
     
     
       74. The method of claim  73 , further including changing a direction of said emitted signals between emission and reflection and changing direction of said reflected signals between reflection and return.  
     
     
       75. The method of claim  74 , wherein said changing said direction of said emitted and reflected signals is effected by reflection.  
     
     
       76. The method of claim  74 , wherein said changing said direction of said emitted and reflected signals is effected by refraction.