U.S. patent number RE37,065 [Application Number 09/252,941] was granted by the patent office on 2001-02-27 for triaxial normal and shear force sensor.
This patent grant is currently assigned to Bonneville Scientific Incorporated. Invention is credited to Allen R. Grahn.
United States Patent |
RE37,065 |
Grahn |
February 27, 2001 |
**Please see images for:
( Certificate of Correction ) ** |
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, F.sub.x, F.sub.y, F.sub.z, M.sub.x,
M.sub.y, M.sub.z, 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.
Inventors: |
Grahn; Allen R. (Salt Lake
City, UT) |
Assignee: |
Bonneville Scientific
Incorporated (Salt Lake City, UT)
|
Family
ID: |
26986805 |
Appl.
No.: |
09/252,941 |
Filed: |
February 18, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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329465 |
Oct 26, 1994 |
5553500 |
|
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Reissue of: |
504224 |
Jul 19, 1995 |
05604314 |
Feb 18, 1997 |
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Current U.S.
Class: |
73/628; 73/652;
73/862.043; 73/862.046; 73/862.541; 73/862.637 |
Current CPC
Class: |
G01L
1/255 (20130101); G01L 5/173 (20200101); G01L
5/228 (20130101); G01B 17/04 (20130101); G01N
2291/02827 (20130101) |
Current International
Class: |
G01L
5/22 (20060101); G01B 15/00 (20060101); G01L
5/16 (20060101); G01B 15/06 (20060101); G01L
1/25 (20060101); G01H 005/16 (); G01H 005/00 () |
Field of
Search: |
;73/627,628,644,652,862.041,862.042,862.043,862.046,862.05,862.541,862.637 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Bauer, F., Piezoelectric and Pyroelectric Polymers, Polymers as
Synthetic Metals Conference, London, May 1983. .
Begej, S., Fingertip-Shaped Tactile Sensor with Shear Force-Sensing
Capability, NSF-88-50, Abstracts of Phase I SBIR Awards, NSF, 1988.
.
Cutkosky, M.R., et al., Skin Material for Robotic Fingers, IEEE
International Conference on Robotics and Automation, Mar. 1987.
.
Grahn, A.R., et al., Six Component Robotic-Force Torque sensor,
Final Report 178347, 1987. .
Grupen, R.A. et al., A Survey of General Purpose Manipulation,
International Journal of Robotics Research, vol. 8, No. 1, Feb.
1989. .
Hackwood, S. et al., A Torque-Sensitive Tactile Array for Robotics,
International Journal of Robotics Research, vol. 2, No. 2, Summer
1983. .
Harmon, L.D., Robotic Taction for Industrial Assembly, Report No.
1, Dept. of Biomed. Eng. Case Western Univ., Oct. 1982. .
Howe, R.D., et al., Review of Robotic Tactile Sensing, to be
published, 1989. .
Jacobsen, S.C., et al., Tactile Sensing System Design Issues in
Machine Manipulation, IEEE Robot Conf. Proc., 1987. .
Nicholls, H.R., et al., A Survey of Robot Tactile Sensing
Technology, International Journal of Robotics Research, vol. 8, No.
3, Jun. 1989. .
Siegel, D.M. et al., A Capacitive Based Tactile Sensor, SPIE vol.
579 Intelligent Robots and Computer Vision, 1985. .
Sigel, D.M., et al., Contact Sensors for Dexerous Robotic Hands, MS
thesis, MIT, 1986. .
Sinden, F.W., et al., A Planar Capacitive Force Sensor with Six
Degrees of Freedom, IEEE Robot Conf. Proc., 1986..
|
Primary Examiner: Williams; Hezron
Assistant Examiner: Miller; Rose M.
Attorney, Agent or Firm: Trask Britt
Parent Case Text
This patent application is a continuation-in-part of patent
application Ser. No. 08/329,465 filed Oct. 6, 1994, now U.S. Pat.
No. 5,553,300, issued Sep. 10, 1996.
Claims
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.degree. 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.degree. 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..Iadd.
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. .Iaddend..Iadd.
39. The sensor of claim 38, wherein said at least two transducers
are located substantially in a common plane. .Iaddend..Iadd.
40. The sensor of claim 38, wherein said at least two transducers
comprise four transducers located at 90.degree. intervals around
said at least one target. .Iaddend..Iadd.
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. .Iaddend..Iadd.
42. The sensor of claim 38, wherein said at least one target is at
least partially embedded in said compliant structure.
.Iaddend..Iadd.
43. The sensor of claim 42, wherein said compliant structure
comprises an elastomer. .Iaddend..Iadd.
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. .Iaddend..Iadd.
45. The sensor of claim 38, wherein said compliant structure
comprises a metal spring structure. .Iaddend..Iadd.
46. The sensor of claim 38, wherein said compliant structure
comprises a combined metal and elastomer spring structure.
.Iaddend..Iadd.
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. .Iaddend..Iadd.
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. .Iaddend..Iadd.
49. The sensor of claim 38, wherein said at least two transducers
are selected from a group comprising PVDF films and ceramics.
.Iaddend..Iadd.
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. .Iaddend..Iadd.
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. .Iaddend..Iadd.
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.
.Iaddend..Iadd.
53. The sensor of claim 52, wherein said angle-changing structure
comprises an acoustic prism. .Iaddend..Iadd.
54. The sensor of claim 52, wherein said angle-changing structure
comprises an acoustic reflector. .Iaddend..Iadd.
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. .Iaddend..Iadd.
56. The multi-sensor array of claim 55, wherein said targets are
located at substantially equal intervals, and substantially in a
common plane. .Iaddend..Iadd.
57. The multi-sensor array of claim 55, wherein said transducers
are located substantially in a common plane. .Iaddend..Iadd.
58. The multi-sensor array of claim 55, wherein said at least two
transducers comprise four transducers located at 90.degree.
intervals around each of said targets. .Iaddend..Iadd.
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. .Iaddend..Iadd.
60. The multi-sensor array of claim 55, wherein at least some of
said targets are at least partially embedded in said compliant
structure. .Iaddend..Iadd.
61. The multi-sensor array of claim 60, wherein said compliant
structure comprises an elastomer. .Iaddend..Iadd.
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.
.Iaddend..Iadd.
63. The multi-sensor array of claim 55, wherein said compliant
structures comprise metal spring structures. .Iaddend..Iadd.
64. The multi-sensor array of claim 55, wherein said compliant
structures comprise combined metal and elastomer spring structures.
.Iaddend..Iadd.
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. .Iaddend..Iadd.
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. .Iaddend..Iadd.
67. The multi-sensor array of claim 55, wherein said transducers
are selected from a group comprising PVDF films and ceramics.
.Iaddend..Iadd.
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. .Iaddend..Iadd.
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. .Iaddend..Iadd.
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.
.Iaddend..Iadd.
71. The multi-sensor array of claim 70, wherein at least some of
said angle-changing structures comprise acoustic prisms.
.Iaddend..Iadd.
72. The multi-sensor array of claim 70, wherein at least some of
said angle-changing structures comprise acoustic reflectors.
.Iaddend..Iadd.
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. .Iaddend..Iadd.
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. .Iaddend..Iadd.
75. The method of claim 74, wherein said changing said direction of
said emitted and reflected signals is effected by reflection.
.Iaddend..Iadd.
76. The method of claim 74, wherein said changing said direction of
said emitted and reflected signals is effected by refraction.
.Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention relates generally to force sensors, and more
specifically to an ultrasonic sensor for the measurement of normal
and shear forces.
2. State of the Art.
With rare exception, tactile or contact-type sensors in the art
respond to normal forces only. From the measurement of normal force
distribution, three (F.sub.z, M.sub.x, M.sub.y) of the six
force-torque components (F.sub.x, F.sub.y, F.sub.z, M.sub.x,
M.sub.y, M.sub.z) can be computed. These three components are the
normal force and the two orthogonal torques in the plane of the
sensor. Normal-force sensing is adequate for tasks involving object
or feature identification, determining object location with respect
to the sensor, and under some circumstances, estimating impending
slip from the normal force and knowledge of the coefficient of
friction between the object and the sensor surface.
However, for certain applications, such a limited sensing
capability is inadequate. Examples of such applications include,
without limitation, grasping and manipulation by a robot hand;
measurement of forces generated by an object such as a tire, shoe,
boot or ski moving over the sensor; determination of pressure
points, forces and movements of bodily extremities with respect to
footwear such as athletic shoes, boots, and ski boots as well as
sporting (golf clubs, tennis rackets, baseball bats) and industrial
(hand tools, grips for electrically-powered tools) implements;
determination of balance and gait analysis for athletic training
and medical treatment and rehabilitation; use in a joystick, cursor
control or other position-dependent control devices; and for
accelerometers.
There have been several attempts to develop arrays of triaxial
force sensors or full six-axis tactile sensors. For example,
tactile array elements have been composed of magnetic dipoles
embedded in an elastomer, the position and orientation of which
were detected by magneto-resistive sensors. However, only one- or
two-element sensors have been fabricated to prove feasibility of
the concept. Another approach has employed sensors using emitters
(charge or magnetic) embedded in a compliant layer. Emitter
position is measured by an array of field-effect transistors or
Hall-effect devices fabricated on a silicon substrate. Prototype
sensors of this design were found to be highly sensitive to
external fields.
A capacitance-based approach has also been attempted, but
implemented only with respect to normal-force sensing. An existing,
optically-based tactile sensor may have been modified to
incorporate shear sensing capabilities. Presumably, the technique
being investigated is the position monitoring of optical targets
embedded in a substrate. However, such a design does not lend
itself to incorporation into necessarily compact sensors as used in
robot end-effectors, due, among other consideration, to the
presence of a relatively large, stiff bundle of optical fibers
exiting the sensor.
A miniature force-torque sensor has been developed by the assignee
of the present invention. This sensor was intended for mounting on
the gripping surfaces of robot end-effectors. The sensor consists
of an elastomeric spring element joining two rigid parallel plates,
one of which is mounted to the end-effector. Ultrasonic pulse-echo
ranging through the elastomer is used to detect fine movements of
one plate relative to the other. The sensor is compliant, the
degree thereof as well as the sensitivity and load range of the
sensor being alterable by changing the elastomer composition. The
six force-torque components may be calculated from the transit
times and specifically times-of-flight (TOF) of a plurality of
differently-aimed pulse-echo signals as one plate is deflected with
respect to the other under application of force. A further
description of the aforementioned sensor appears in U.S. Pat. No.
4,704,909, assigned to the assignee of the present invention, and
incorporated herein by reference.
Other force sensors developed by the assignee of the present
invention, which sensors employ pulse-echo ranging, are U.S. Pat.
Nos. 4,964,302 and 5,209,126, assigned to the assignee of the
present invention and incorporated herein by reference. The sensors
disclosed in these two patents do not, however, have triaxial force
component determination capability.
SUMMARY OF THE INVENTION
The sensor of the present invention provides a highly accurate,
robust and relatively inexpensive sensor, in comparison to prior
art sensors known to the inventors. In its preferred embodiments,
the sensor employs transit time of reflected ultrasonic pulses to
determine three force components. The sensor may be used singly, or
in arrays incorporating a plurality of basic sensor units.
A preferred embodiment of the basic sensor unit of the present
invention comprises a target suspended above laterally- and
vertically-offset ultrasonic transducers, each having an emitting
and receiving capability. The target is preferably of spherical or
hemispherical shape; if the latter, the flat portion of the
hemisphere is oriented parallel to the plane in which the
transducers are located, with the arcuate portion of the hemisphere
facing the plane. The transducers are aimed at the target and thus
emit signals at an oblique angle to the transducer plane. The
target is preferably embedded in a compliant material, such as an
elastomer layer, which extends at least partially between the
target and the transducers. Forces applied to the surface of the
elastomer layer above the target distort the elastomer and may move
the target both vertically and horizontally with respect to its
original position. Target position is measured by ultrasonic
echo-ranging; that is, one measures transit time of the
obliquely-oriented ultrasonic pulses which pass from each
transducer through the elastomer, impinge upon the target and
reflect back to that transducer. From the transit time measurement
and knowledge of the speed-of-sound within the elastomer, the
distance from the transducer to the target can be calculated. Since
a plurality of transducers are disposed about and aimed at the
target, target movement results in a plurality of different transit
times, from which force components can be calculated using the
known compressibility characteristics of the compliant layer. At
least three, and preferably four, transducers are aimed at each
target for triaxial force determination.
The basic sensor unit may also be employed in a joystick or cursor
control device, or as an accelerometer. In the latter case, a
second group of transducers may be placed over the target in
contraposition to the first set, if desired, for the contemplated
application.
If desired, a plurality of basic sensor units may be arranged in a
planar sensor array, the term "planar" being used herein to denote
not only a sheet-like array extending in a linear plane, but also
such an array which is concave, convex, or otherwise arcuate or
non-linear in configuration, as required by the particular
application.
Sensor accuracy may be enhanced with minimum time skew by pulsing
each transducer in rapid succession before the echo of the
preceding pulse has returned to the transducer. The time lag or
difference of the second and successive pulses in a pulse burst
after the first pulse is subtracted from the transit time of that
pulse. The resulting, lag-compensated transit times of the pulses
in a burst are then averaged.
If an array is formed, the scan rate to effect continuous scanning
of all targets in the sensor array may be enhanced by rapidly
pulsing transducer columns in succession before the pulses from the
previously-pulsed columns have reflected and returned to the
transducers of those columns.
An alternative transit time measurement technique, in lieu of
pulsing an ultrasonic signal toward the target, is to generate a
continuous oscillatory signal or several cycles of continuous
signal and to measure the phase shift between the outgoing and
returning (reflected) signal. Hence, the term "transit time
measurement" as used herein is intended to encompass such
measurement by phase shift determination.
In development of the invention in the form of an array of basic
sensor units, it has been determined that particular structural
features of an array may present advantages in terms of the ability
of an array to withstand relatively large forces and to measure
such forces in terms of normal and shear force components in an
accurate manner, as well as ease of initial fabrication and repair
or replacement of array components. Such features include the use
of a rigid planar support member, such as a metal plate, to support
a plurality of targets (such as in a 4.times.4 or 8.times.8 target
array), each target being associated with an individual biasing
element and each target/biasing element combination being disposed
in an individual aperture extending through the support member,
which overlies the transducers of the array and aligns each target
with its associated transducers. Another advantageous feature for
fabrication of an array is the horizontal or planar placement of
the transducers, aimed upwardly and perpendicular to the transducer
plane, in combination with the use of acoustically refractive
elements or "prisms" to reorient the ultrasonic waves of the
transducers of each basic sensor unit at desired oblique angles
toward the target. Such a design greatly simplifies construction of
the array, and may result in a sturdier structure than is possible
with some other embodiments of the invention. Reflectors may also
be employed in lieu of refractive elements. Further, directly-aimed
transducers may be employed in combination with refractively- or
reflectively-aimed transducers (or the latter two with each other)
in an array or in a basic sensor unit, to accommodate physical
design constraints or a particular sensor or array topography.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are schematic side elevations of an illustrative
transducer and target arrangement for two-dimensional monitoring of
target position;
FIG. 2 is a schematic top elevation of a target and four-transducer
configuration as may be employed in a basic sensor unit;
FIG. 3 is a schematic top elevation of a target and
three-transducer configuration as may be employed in a basic sensor
unit;
FIG. 4 is a partial section side elevation of a preferred
configuration for a four-transducer basic sensor unit;
FIG. 5 is a schematic exploded side elevation of a preferred
configuration for a single-level, multi-sensor array, FIG. 5A is a
depiction of an alternative layering arrangement for target
placement and transducer protection, FIG. 5B is a section taken
along lines 5--5 in FIG. 5, FIG. 5C is a top elevation of a
ridgeline of the lower substrate of the array, and FIG. 5D is a top
elevation of a portion of the segmented upper substrate of the
array;
FIG. 6 is a schematic top elevation of the single-level,
multi-sensor array of FIG. 5;
FIG. 7A is a schematic top elevation of an upper transducer level
of an alternative multi-level, multi-sensor array embodiment of the
invention, FIG. 7B is a schematic side elevation of the upper
transducer level depicted in FIG. 7A, FIG. 7C is a schematic side
elevation (rotated 90.degree. about the vertical with respect to
FIG. 7B) of a lower transducer level, of the multi-level
multi-sensor array, and FIG. 7D is a schematic of the assembled
multi-level, multi-sensor array;
FIG. 8 is a wiring schematic of an array of transducers which may
be employed with either the preferred or alternative embodiments of
the multi-sensor array of the present invention;
FIG. 9 is a timing schematic illustrating a technique of pulsing a
transducer or column in a burst of closely time-spaced pulses to
obtain an average value with minimum time skew;
FIG. 10 is a timing schematic illustrating a technique of pulsing
transducer columns in a multi-sensor array in rapid succession to
effect continuous scanning of target positions;
FIG. 11 is a perspective view of a plurality of basic sensor units
incorporated in a multi-sensor array which includes a reinforced
contact layer to accommodate high forces without degradation;
FIG. 12 is a schematic of the processing circuitry employed to
convert pulse transit times from transducers aimed at a target into
the actual target position;
FIG. 13 is a schematic side elevation of the sensor unit of the
present invention employed in a joystick;
FIGS. 14A and 14B are schematics of the sensor unit of the present
invention specifically adapted for use as an accelerometer;
FIG. 15 is a schematic of a spring-supported target version of the
invention;
FIG. 16 is a simplified schematic of sensor geometry for
mathematical purposes;
FIG. 17 is a schematic, partial sectional side elevation of yet
another preferred configuration for a basic sensor unit;
FIGS. 18 and 19 are, respectively, a side sectional elevation and a
bottom elevation of a rigid support member suitable for use in a
multi-sensor unit array according to the present invention;
FIG. 20 is an enlarged side sectional elevation of a portion of the
support member of FIGS. 18 and 19;
FIG. 21 is a top elevation of three sensor unit targets supported
by biasing elements in a rigid support member of a multi-sensor
array;
FIG. 22 is a partial sectional side elevation of a preferred target
and biasing element configuration suitable for use with a rigid
support member in a single- or multi-sensor array;
FIGS. 23-25 are partial sectional side elevations of alternative
target, biasing element and support member configurations according
to the present invention;
FIGS. 26-36 comprise additional side elevations of alternative
metal and combined metal/elastomer target biasing elements suitable
for use in the present invention;
FIG. 37 is a schematic partial sectional side elevation of a
preferred transducer arrangement employing acoustic prisms to
refract ultrasound waves travelling to and from targets;
FIG. 38 is a schematic depiction of the refraction of sound waves
travelling between a transducer and a target through an acoustic
prism and adjacent transmitting medium; and
FIG. 39 is a schematic partial sectional side elevation of an
alternative transducer arrangement employing acoustic reflectors to
reorient ultrasound waves travelling to and from targets.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The force measuring technique for the sensor of the present
invention is, in its simplest form, based upon ultrasonic
pulse-echo ranging between a movable target and a transducer,
although, as noted above, phase-shift measurement of an oscillatory
signal may also be employed to determine signal transit time. The
target is embedded in a deformable medium having known
sound-transmission attributes, the medium extending between the
target and the transducer. It is currently preferred that this
deformable medium comprise an elastomer, and for purposes of
convenience, the term "elastomer" will be employed in this
discussion, although it will be understood that other materials,
such as gels, rubber compounds, plastics, liquid-filled bags or
balloons, etc., may be employed. Alternatively, a spring or springs
such as coil, leaf, belleville or other spring configuration
supporting a target may be employed in combination with a
sound-transmitting medium to convey the ultrasonic signals. The
target moves when the supporting medium is distorted or compressed
by sufficient force. The distance between the target and the
transducer is determined from the time it takes an ultrasonic
signal to traverse the intervening medium and return. From this
time interval measurement, hereinafter "transit time", knowledge of
the speed-of-sound in the medium and the medium's modulus (i.e.,
the stress required to produce a particular degree of compression),
the forces compressing the medium can be calculated.
Two basic principles are involved in pulse-echo distance
measurement. First, the speed-of-sound, c, in the medium
(elastomer) is known so that its thickness between the target and
the transducer can be determined from the two-way transit time, t,
of the pulse, by: ##EQU1##
The second principle is that the interface between the target and
the elastomer must reflect the pulse. Therefore, it is imperative
that the target have an acoustic impedance which differs
significantly from that of the elastomer.
The time required for an ultrasonic pulse to make a round trip
between the transducer and the target is given by equation (1)
above. For 3 mm of silicon rubber, the time of flight (transit
time) of the pulse is about 6 microseconds. Therefore, if the
expected maximum force compresses the rubber to 60% of its original
thickness (1.2 mm compression) and it is required to resolve this
force to one part in 200 (corresponding to a distance resolution of
6 microns), then the ultrasonic pulse transit time must be resolved
to within 12 ns.
Polyvinylidene Fluoride (PVDF) is preferably used for the
transducer materials in the sensor of the invention. The material
has a low mechanical Q, low acoustic cross coupling between
adjacent array elements, and simplifies array fabrication. PVDF is
a thin-film polymer material which is demonstrated to be five to
ten times more piezoelectric than crystalline quartz when stretched
and poled. Other polymers which offer a piezoelectric capability
may also be employed. It should be understood, however, that still
other transducer materials, such as ceramics, may be used in
appropriate circumstances such as in high-temperature
environments.
A great degree of freedom is available in the choice of an
appropriate elastomer for target support and signal transmission.
The primary function of the elastomer is to act as a linear spring.
Stated another way, the elastomer compresses in direct proportion
to the amount of force applied. Ideally, the elastomer's force
versus compression characteristics should be linear so that a
simple proportionality constant can be used for force or pressure
calculations.
The use of an elastomer-embedded target provides a number of
benefits. For example, the exposed surface of the elastomer layer
containing the target can sustain limited wear and damage without
degradation of sensor performance. In addition, the elastomer layer
above the reflectors may be made so that it can be replaced when it
becomes damaged or contaminated. The exposed surface can be easily
textured to aid in grasping or to reduce the normally high
coefficient of friction (if rubber is used) to prevent sticking.
Finally, sludge, mud or other contaminants on the sensor pad
surface, or the manipulation of rubber objects, would not affect
sensor performance.
FIGS. 1A and 1B depict a simplified transducer and target
arrangement 50 according to the present invention for determining
target position in two dimensions. Ultrasonic transducers 52,
located in a common plane 54, are aimed at the arcuate surface 56
of hemispherical target 60. Transducers 52 are both vertically and
laterally offset from target 60, so that ultrasonic pulses travel
to and from target 60 through medium 62, typically an elastomer, at
an oblique angle. With this configuration, the normal force and one
of two shear-force components can be measured. The sum of the
transit times from the transducers, t.sub.1 +t.sub.2, is
proportional to the normal force component, F.sub.z. The difference
in transit times, t.sub.2 -t.sub.1, is proportional to the shear
force component, F.sub.x or F.sub.y.
The proportionality constants inferred above depend upon the
speed-of-sound in the compliant, acoustically transparent medium 62
located between the transducers 52 and the target 60 (in order to
convert the time interval measurement into distance), the
appropriate elastomer stiffness constant, the geometry describing
the positions of the transducers and target, and target
geometry.
As shown in FIG. 1, when no force or pressure is applied to the
contact surface 64 of the compliant medium (elastomer) in which the
target is embedded, the transit times t.sub.1 and t.sub.2 are
equal. When a force F is applied to contact surface 64, the medium
62 distorts and compresses in the direction of the force vector,
and target 60 is displaced in proportion to the level of force F
applied. Transit times t.sub.1 and t.sub.2 then differ, and from
this difference the target location may be calculated. For
measuring a second shear force component, another pair of
transducers 52, as shown in FIG. 2, may be positioned at right
angles to the first pair. If desired, only three transducers 52, as
shown in FIG. 3, may be disposed about a target 60 at 120.degree.
intervals for measurement of the three force components. Such an
arrangement minimizes the number of transducers required, but may
not be desirable, as it complicates the otherwise straightforward
mathematics involved in calculating the target position, and it is
difficult to fabricate multi-sensor arrays using this
arrangement.
The angled ultrasonic pulses (i.e., angle .crclbar. in FIGS. 1A and
1B) can be obtained by mounting the transducer material on angled
facets, mounting the transducer material on a flat surface and
using reflectors to reflect the pulses at the desired angle, or
mounting the transducer material on a flat surface and using wedges
of a suitable material like prisms to refract the pulses at the
desired angle. The first alternative is preferred, due to the bulk
added to the sensor by the other two alternatives, and, in the
third alternative, the severe constraints placed on the acoustic
properties of the refracting wedge material. For arrays with a
large number of basic sensor units, the PVDF transducer material
for a large number of transducers should be installed as a single
sheet in a single operation rather than employing single, discrete
transducer elements or strips of such elements. This approach
renders multi-sensor arrays much more economical, as well as
ensuring more accurate transducer placement.
FIG. 4 depicts a preferred physical configuration for a
four-transducer basic sensor unit 50 in accordance with the present
invention. The four PVDF film transducers are mounted to a
substrate 70 with an inner wall 72 set at a 45.degree. angle.
Target 60 is embedded in a compliant medium 62 which extends
between the target 60 and the transducers, filling the void
therebetween. The upper surface 64 of medium 62 provides a contact
surface for application of a force, F. Wires or printed conductors,
shown in broken lines at 74 (preferably the latter, and molded into
or onto substrate 70 at the time of its fabrication) extend through
substrate 70 to the lower surface 76 thereof, where they
communicate with conductors of the electronics 78 of electronics
module 80 disposed underneath substrate 70. Electronics module 80
then communicates with a host processing unit such as a personal
computer (PC) via connector 82 and a suitable interface board. FIG.
12 depicts a schematic of the circuitry of the electronics module
80 and interface board with a sensor unit of array 100, wherein the
transducers 52 are selected and pulsed, the ultrasonic echo signals
are amplified and detected, and the corresponding transit times are
measured. The detected pulse transit times are converted to force
component values by the PC using the known speed-of-sound and
compliancy characteristics of the medium supporting the target
60.
FIGS. 5, 5A-5D and 6 depict a preferred embodiment of a
single-level, multi-sensor array 100 of the present invention. For
the sake of clarity, reference numerals previously employed are
used again to identify the same elements. In array 100, a plurality
of hemispherical targets 60 (nine in this instance, for exemplary
purposes only, and not by way of limitation) are disposed above
transducers 52, each target 60 having four transducers 52 aimed at
it, preferably from a common distance and angle. If a wide angle
(e.g., 25.degree. to the sensor plane) is employed, the sensor unit
may be made thinner, but shear force sensing capability is somewhat
diminished. If a closer angle is employed (e.g., 60.degree. to the
sensor plane), greater shear force sensitivity results. Targets 60
are embedded in a compliant medium 62, which provides a contact
surface 64 for the application of a force, F. A first plurality of
rows of transducers 52 are carried by perforated substrate 102, and
a second plurality of rows of transducers 52 running at a
90.degree. angle to the first plurality is carried by ridged
substrate 104.
As can be seen by viewing FIGS. 5, 5A, 5C and 5D together, segments
106 of perforated substrate 102 are received in valleys 108 of
ridged substrate 104. When the two substrates are assembled,
windows 110, the widest portions of which lie along ridgelines 112,
provide transducers 52 of ridged substrate 104 with a clear field
of fire at targets 60. Compliant layer 62, with targets 60 molded
therein, includes protrusions 66 which are received in each recess
flanked by four transducers 52 when perforated and ridged
substrates 102 and 104, respectively, are assembled.
As shown in FIG. 5A, targets 60 may be embedded in a separate
compliant layer 62, which is easily removable and replaceable for
repair purposes and to provide a ready means for altering the
compliancy of the support for targets 60 to accommodate differing
anticipated force ranges. In this case, substrate 102 is molded so
as to have transducer 52 embedded therein as shown and to provide a
continuous, planar upper support surface 114 for compliant layer
62. This arrangement also offers better protection for the
transducers than the arrangement of FIG. 5.
As shown in FIGS. 5 and 6, transducers 52 are activated in columns
by transmitting conductors T to emit ultrasonic pulses which are
then reflected from targets 60 and received by the same transducers
52 from which they are emitted, the received signals being output
from transducers 52 via receiving conductors R. This arrangement is
depicted and described in more detail with respect to FIG. 8.
It will be readily understood that the medium extending between
targets 60 and transducers 52 must be sufficiently acoustically
transparent and nonrefractive for the ultrasonic pulses to travel
therethrough without excessive attenuation and via a direct and
consistent path. The term "medium", of course, is not limited to
single-component mediums, but may comprise multiple layers. Unless
the ultrasound signals contact the interface of two medium
components at about 90.degree., it is desirable that the
components' indices of refraction be substantially similar.
An alternative multi-level, multi-sensor array embodiment 200 of
the invention is depicted in FIGS. 7A through 7D. Upper transducer
level 202 includes an acoustically-transparent substrate 203 having
a plurality of ridges 204 which, in three dimensions, would extend
outwardly from the plane of the drawing, the ridges each having two
sides 206 disposed at the same angle .crclbar.. PVDF transducers 52
are located at predetermined intervals along the ridges, as shown
in FIG. 7A. Each of a plurality of targets 60 is located between a
pair of transducers 52 aimed at that target. As shown in FIG. 7B,
the targets are located "above" the transducers 52 in upper level
202, the term "above" being relevant only insofar as the drawing is
concerned, it being understood that the sensors and sensor arrays
of the present invention may be used in any orientation.
In one arrangement, the ridges 204 may project into the compliant,
acoustically-transparent, nonrefractive elastomer layer 208 in
which the targets 60 are embedded. It may be preferred for some
applications to fully embed the ridges 204 and transducers 52 in
one layer of elastomer or, alternatively, in a substantially
noncompressible but acoustically transparent material layer 210,
such as high durometer urethane compound to better protect the
transducers, and to locate the targets in a separate compliant
layer 212 thereabove. This also permits easy replacement of the
targets and compliant upper layer 212 in the event of damage while
the transducers remain unaffected.
FIG. 7C depicts the lower level 214 of multi-sensor array 200.
Lower level 214 is similar to upper level 202 and includes an
acoustically-transparent substrate 215 including ridges 216, but
ridges 216 in lower level 214 are truncated at their tops,
providing fiat upper surfaces 218 flanked by angled side surfaces
220 on which transducers 52 are mounted. Truncation of the ridges
reduces the depth of the assembled sensor array 200, which is
desirable for most applications. The transducers of lower level 214
transmit their pulses through the acoustically transparent material
222 overlying the lower level transducers 52, then through the
upper substrate 203 and one or more higher layers such as 208, 210
and 212, depending upon the design employed, to reach targets 60
and reflect therefrom back to the lower level transducers.
As shown in FIG. 7D, when assembled, sensor array 200 includes
upper level 202 and lower level 214 at right or 90.degree. angles,
relative to the vertical. This rotational offset of the upper and
lower levels 202 and 214 permits, as shown, the pulses from the
transducers 52 of both upper and lower levels 202 and 214 to reach
the targets 60 without interference. It is also noteworthy, again
with reference to FIG. 7D, that the transducers 52 of lower level
214 are aimed at targets 60 which are farther laterally distant,
permitting equal angles for transducer orientation in both levels.
Finally, also as shown in FIG. 7D, it may be desirable to segment
the upper elastomer layer containing the targets 60 into discrete
blocks 230, each block being free to move relatively independently
of the others. Such a configuration can reduce hysteresis and help
prevent the embedded targets 60 from separating from the elastomer.
Also, in some high-force applications, it may be desirable to
reinforce the exposed elastomer pad surface 232 (see FIG. 11) with,
for example, steel, Kevlar, nylon or other cord material 234 such
as is employed in vehicle tires, or even a metal plate or non-woven
mesh. It may also be desirable to coat or cover the pad surface 232
with a more wear-resistant material or one having a different
coefficient of friction with respect to that of the element
contemplated to apply force F to it, so as to enhance or reduce
friction between the pad surface 232 and the contacting
element.
With respect to the multi-layer embodiment 200, it again is
important to emphasize that the material between the lower level of
transducers 52 and the target 60 must be both sufficiently
acoustically transparent and nonrefractive so as to avoid undue
signal attenuation. In other words, the reflection coefficient for
the interfaces between material 222 and substrate 203 and between
the material 210 and the elastomer layer 212 should be close to
zero. Stated another way, the acoustic impedance of the materials
on either side of these interfaces should be approximately equal.
Therefore, the product of material density times speed-of-sound
needs to be about the same for both materials.
Urethane materials appear to be most promising for fulfillment of
these requirements. Urethanes are tough, abrasion resistant and
have high tear strength. They are also easy to bond to and readily
pass ultrasonic waves. Furthermore, urethane compounds are
available which can be formulated to have a wide range of
harnesses, from approximately 15 Shore A to 75 Shore D durometer.
In many applications, the first would be more than soft enough for
the sensor pad or elastomer layer in which the targets 60 are
embedded. The latter would be almost rigid and thus entirely strong
enough for the substrate material and protective layers overlying
the transducers mounted on the substrates. Since all compounds are
urethanes, their product of density and speed-of-sound can be made
relatively close. Of course, it is contemplated that other
materials such as natural rubbers, silicone rubbers, neoprene,
butyl rubbers, etc., may have equal utility for certain
applications. Since the speed-of-sound through silicone rubbers is
about 1/3 less than through urethanes, better resolution may be
obtained via use of the former. In addition, softer silicone
compounds than urethanes are currently available, making silicones
more desirable for some applications.
It should also be noted that ultrasonic pulses from lower level
transducers may pass through the PVDF layers of the upper level, as
well as through the substrate material itself. This presents little
difficulty, as the PVDF material is almost acoustically transparent
for this application; its acoustic impedance is close to that of
urethane. Furthermore, it is very thin, relative to the acoustic
wavelength employed, and the metallization on the PVDF has little
effect on attenuation since it is very thin.
FIG. 8 schematically depicts, from above, a wiring circuit which
may be employed in either a single or multi-level, multi-sensor
array for measuring the normal force F.sub.z and the F.sub.y shear
force component. To prevent time skew in the F.sub.y measurement,
one transmitting column such as T.sub.1 would be pulsed and echoes
would be received on the two receiving rows associated with the
same site, such as R.sub.1 and R.sub.2, or R.sub.3 and R.sub.4, or
R.sub.5 and R.sub.6. That is, there would be two parallel
receiving/detecting channels associated with each target 60
associated with transmitting column T.sub.1. Another, similar array
structure contains the PVDF transducers and conductors for
measuring F.sub.x, the other structure being, as previously noted,
rotated 90.degree. with respect to the first.
FIG. 9 is an exemplary timing schematic for a technique permitting
averaging of TOF measurements from and back to a single transducer
52 (at the intersection of one column and one row, as shown in FIG.
8), with minimal time skew. In the drawing figure, three excitation
pulses are applied to the same transmitting column in rapid
succession. The three echoes, I, II and II, produced by reflection
of these pulses from a target 60 are detected in rapid succession.
The pulses are emitted at 2 .mu.s intervals, as shown. When echo II
is detected, 2 .mu.s is subtracted from its overall TOF value of 12
.mu.s (12 .mu.s-2 .mu.s=10 .mu.s). Similarly, 4 .mu.s is subtracted
from the overall TOF for echo III. The resulting TOF values for all
three echoes are then averaged. Using such a "rapid-fire" pulsing
technique, it takes only 14 .mu.s, and not 30 .mu.s, to make the
three measurements. The above timing intervals and TOF figures are
not meant to be accurate, but merely illustrative of the
technique.
FIG. 10 is an exemplary timing schematic for a technique of pulsing
transmitting columns T (see FIG. 8) in rapid succession to increase
the scan rate of the sensor array. Column a is pulsed, then b, then
c, at 1 .mu.s intervals. As shown, it would take only 13 .mu.s to
receive three echoes. The alternative of pulsing a column and
waiting until the echo is detected before pulsing another column
would take 11 .mu.s per measurement, or 33 .mu.s for detecting
three echoes. Again, the stated timing intervals and TOF figures
are not meant to be accurate, but merely illustrative of the
technique.
To complete the sensor system according to the present invention, a
few additional processing circuitry components are required.
Specifically, an electronics module is employed to select and
excite the array elements and multiplex the high-gain receiving
amplifier. The module may be located at the sensor site or remotely
therefrom. The module is also connected to a custom interface board
in an IBM-compatible personal computer or other host processing
unit. FIG. 12 is a schematic of the processing circuitry with the
electronics module connected to a basic sensor unit or multi-sensor
array, and to the interface board in the host processing unit.
FIG. 13 depicts a basic sensor unit 50 modified into a
position-dependent control device 300 having four transducers 52
which has been modified by suspending target 60 from a joystick
shaft 302 which is mounted to a universal or other flexible joint
304, providing the ability for a user to grasp a handle 306 at the
upper end of the joystick shaft 302 and move it in any direction.
Thus, the position of the joystick shaft 302 may easily be related
to the transit times of the ultrasonic signals emanating from the
transducers 52 through a sound-transmitting medium 62 interposed
between target 60 and the transducers. This modification of the
invention has ready applicability in joysticks for vehicle control,
including aircraft, and for computer applications including cursor
control and video games, as well as for commercial and industrial
applications wherein the position of a control or sensing member is
desired to be ascertained. Of course, the joystick shaft 302 may be
spring-biased to return to a central or other desired position when
no force is applied.
FIG. 14A depicts the basic sensor unit 50 modified by positioning
of the transducers 52 in a vertical orientation, with target 60 in
the middle of the transducer group. Such an arrangement may be
readily used as an accelerometer for forces in the plane 54 of the
transducers 52, as movement and time of movement of target 60 in
the surrounding compliant medium 62 responsive to acceleration or
deceleration is easily measured. Such a modification of the
invention may easily be used in a motor vehicle as a trigger for
the deployment of airbags, particularly due to the recent
development and emphasis on side-impact airbags by several
manufacturers. Of course, if only fore-and-aft acceleration and
deceleration are desired to be measured, a sensor unit employing
only two transducers would suffice.
FIG. 14B depicts the basic sensor unit 50 augmented by the addition
of four more transducers 52, arranged as the original four, but
disposed above target 60. This modification of the invention is
particularly suited for use as a triaxial accelerometer, such as
are employed in aircraft and missile guidance systems, in test
equipment for crash and other tests where acceleration and
deceleration data is desired, and to control adjustable vehicle
suspensions. Movement and time of movement of target 60 suspended
within compliant medium 62 in any direction responsive to
acceleration and deceleration forces is easily and accurately
ascertained.
FIG. 15 shows an embodiment of the sensor of the invention wherein
target 60 is suspended on a spring 308 such as a coil spring, and a
sound-transmitting liquid or gel medium 62 is interposed between
transducers 52 and target 60. Of course, other spring types, such
as belleville or leaf, might be employed, and it is contemplated
that a plurality of springs might be used to support target 60 from
below, from the side, and from above.
Following is a description of illustrative mathematics employed to
determine the position (and therefore the forces) on a target 60
using the sensing geometry illustrated in FIGS. 1A and 1B, wherein
one pair of transducers 52 is employed, the transducers being
oriented at an exemplary angle .crclbar. of 45.degree. to the
horizontal plane in which the transducers are located. For purposes
of clarity, this geometry has been reproduced in much-simplified
form in FIG. 16, with additional annotations as referred to below.
The exact equations as set forth below would be straightforward to
implement and would quickly run on a PC-class computer.
FIG. 16 shows the sensing geometry for one pair of ultrasonic
transducers 52. These transducers would measure normal force and
one component of shear force in the plane of the transducers. The
equations for the other shear force component (at right angles to
the first) would be similar. The approach taken is that the
time-of-flight measurement (TOF) times the speed-of-sound+2 plus
the radius of the spherical target gives distances S.sub.1 and
S.sub.2. Straight lines (1.sub.1 and 1.sub.2) are drawn at these
distances, parallel to the respective ultrasonic transducers
(1.sub.3 and 1.sub.4). These two lines (1.sub.1 and 1.sub.2)
intersect at a point (x, y) on the figure. This intersection point
is the location of the diametrical center of the spherical or
hemispherical target. The following should be noted:
1) The shear force is related to x, which is a function of s.sub.1
-s.sub.2.
2) The normal force is related to y, which is a function of s.sub.1
-s.sub.2.
3) The ##EQU2##
factors are due to the transducers being oriented at
45.degree..
4) The actual forces are proportional to the change in TOF when no
forces are applied vs when forces are applied (i.e., t.sub.10
-t.sub.1 and t.sub.20 -t.sub.2).
5) Adding the other two ultrasonic transducers does not affect the
equations. It just adds two more equations of the same form.
6) Each pair of transducers gives a value for a shear force and the
normal force. To improve accuracy, the two normal force values
would be averaged.
Now:
##EQU3##
When c=the speed-of-sound in the elastomer ##EQU7##
For forces, our calculations .DELTA.x and .DELTA.y
.thrfore..DELTA.x=K.sub.1 {(t.sub.10 -t.sub.1)-(t.sub.20
-t.sub.2)}=shear force
.DELTA.y=K.sub.2 {(t.sub.10 -t.sub.1)+(t.sub.20 -t.sub.2)}=normal
force
Where t.sub.10 is (TOF).sub.1, when no forces are applied, t.sub.20
is (TOF).sub.2 when no forces are applied, t.sub.1 and t.sub.2 are
(TOF).sub.1 and (TOF).sub.2, respectively, when a force F is
applied and K.sub.1 and K.sub.2 are constants, which include the
speed-of-sound in the elastomer, rubber stiffness, and the factor.
##EQU8##
Using four transducers spaced at 90.degree. intervals about a
spherical or hemispherical target, all transducers being angled at
45.degree. to the target, the mathematical equations giving the
shear force in terms of TOF are quite simple. First of all, the x,
y, z coordinates of the center of the reflector are: ##EQU9##
where t.sub.1, t.sub.2, t.sub.3 and t.sub.4 are the TOF's
associated with pulses from each of the four transducers, and c is
the speed-of-sound in the material disposed between the transducers
and the target. Note, the above equations are only strictly true
for a point target or reflector; for a real reflector target, a
constant offset value has to be added. However, this offset cancels
out of the equation when .DELTA.x and .DELTA.y are calculated. The
forces corresponding to the change in position of the target are
simply the change in coordinate value (from no force to force being
applied to the sensor) times the appropriate rubber stiffness
parameter (shear stiffness for F.sub.x and F.sub.y, compressive
stiffness for F.sub.z, and constants such as ##EQU10##
due to the geometry of the sensor). As is apparent from the
equations immediately above, normal force F.sub.z can be obtained
by averaging all four time-of-flight measurements from one sensor
unit. The averaging process will increase the accuracy of the
calculated value of F.sub.z. Further, since differential time
intervals are used in calculating the value of the two shear force
components, the calculation can be made independent of temperature
effects on the elastomer. The primary effect of temperature on the
sensor is to cause the elastomer to expand and the speed-of-sound
to decrease with an increase in temperature. Secondarily, elastomer
stiffness also decreases somewhat with increasing temperature. If
the above shear force equations are modified slightly by dividing
the time difference by the sum of the two time intervals (e.g.,
(t.sub.1 -t.sub.3)/(t.sub.1 +t.sub.3)), then the effects of
temperature on rubber thickness and speed-of-sound are
eliminated.
While not set forth in detail, it will be readily understood that,
through use of at least three basic sensor units 50 (either three-
or four-transducer configuration) in a common plane and
non-colinearly arranged, the three torque components M.sub.x,
M.sub.y and M.sub.z may be calculated from the difference in the
force components at the three sensor unit sites.
Referring now to FIGS. 17 through 39 of the drawings, additional
preferred and alternative embodiments of the sensor of the present
invention are depicted as basic sensor units and as multi-unit
sensor arrays.
FIG. 17 schematically depicts basic sensor unit 400, which is
combined with other, like units 400 to define a multi-unit sensor
array as subsequently described. Sensor unit 400 includes rigid
support member 402, which may comprise steel, aluminum, bronze or
other suitable metal, or a rigid, substantially incompressible
nonmetallic material. The degree of rigidity required is naturally
dependent upon the forces to which sensor unit 400 is subjected.
Support member 402, as shown, may comprise part of a larger support
member for an array of sensor units, or may comprise a
free-standing support structure, as desired. Support member 402
includes an aperture 404 therethrough, aperture 404 preferably
being of a counterbore configuration with upwardly-facing shoulder
406 extending substantially continuously about bore wall 408.
Aperture 404 may be of square, circular or other suitable
cross-sectional shape.
Target 60, which in this instance comprises a bullet-shaped steel
pin with a hemispherical lower end, is suspended in aperture 404 by
a biasing element 410, which may comprise any suitable elastomer,
spring or other biasing structure, as will be described hereafter
in more detail. In FIG. 17, biasing element 410 comprises an
elastomeric ring, for exemplary purposes only. If target 60 is
employed with an elastomeric biasing element, it may include a
flange, transversely-extending protrusions or other structure as
shown to engage the elastomer body and prevent slippage when force
is applied to the sensor unit 400. Biasing element 410 is supported
about its periphery against normal forces by shoulder 406, and
against shear forces by the upper portion 412 of bore wall 408.
Support member 402 is oriented over angled transducers 52 aimed at
the lower end of target 60.
As with previously-described variations of the invention, it is
preferable to employ four transducers with each target, although
three may suffice. Transducers 52 are preferably bedded or potted
in a hard, substantially noncompliant silicone rubber layer 414 for
protection against mechanically or chemically-caused damage. A much
softer, very compliant silicone rubber or gel mass 416 is disposed
in cavity 418 between target 60 and layer 414 to facilitate
unrefracted acoustic transmission between transducers 52 and the
lower face of target 60. Mass 416 provides no support to target 60
against either shear or normal forces, and need not be bonded to
layer 414. At least a portion of cavity 418 remains empty to
accommodate free downward movement of target 60 and biasing element
410. It is also preferable that cavity 418 be vented as at 420 to
prevent trapped, compressed air from augmenting the design biasing
characteristics of biasing element 410. While support member 402
may rest on silicone rubber layer 414, it is preferable that it be
independently supported and suspended above layer 414 and
transducers 52 for greater stiffness and maximum accuracy.
FIGS. 18 and 19 depict a rigid, planar support member 502 for a
4.times.4 array 500 of basic sensor units 400. Four columns and
four rows of square apertures 402 having shoulders 406 are
depicted, although arrays with more or fewer sensor units 400, or
different numbers of sensor units 400 in the rows and columns are
also contemplated, as are arrays with sensor units 400 disposed in
radial, circular or other patterns as desired. Planar support
member 502 includes a plurality of fastener bores 504 for securing
member 502 to an underlying rigid substrate (not shown) such as a
floor, platform, roadbed or other structure. As shown in FIGS. 18
and 19, transducer cavity 506 is machined or otherwise formed in
the bottom of member 502 to accommodate an array of transducers 52
to be associated and aligned with targets 60 and biasing elements
410, one target/biasing element assembly being disposed in each
aperture 402. It will be appreciated that support member 502 will
rigidly and precisely suspend targets 60 via their individual
biasing elements 410 above the transducer array at stable,
repeatable rest positions. Further, high lateral or shear forces
may be accommodated using this design. While extremely high loading
forces may result in flexing cross-talk between adjacent sensor
units 400 with this design, the use of a metal plate having
predictable elastic deformation characteristics permits electronic
compensation for such phenomena during signal processing. In
addition, use of a multi-apertured support member 502 facilitates
repair and replacement of targets and biasing members, as well as
permitting ready access to the underlying transducer array.
FIG. 20 depicts a plurality of adjacent apertures 404 in support
member 502, showing borewalls 408, shoulders 406 and various styles
of vents 420. FIG. 21 depicts three targets 560 of yet another
configuration, supported in three apertures 404 in support member
502 by biasing elements 510, also of a different configuration from
those previously described. FIG. 22 comprises a partial side
sectional elevation of target 560 supported by element 510 in an
aperture 404.
Target 560 includes a lower hemispherical portion 562 which
comprises the actual target location toward which ultrasonic waves
are projected by transducers 52. Medial square plate portion 564
extends laterally beyond lower portion 562 to define target support
flange 566, and upwardly to define target support surface 568.
Plate portion 564 is surmounted by upper frustoconical portion 570
to facilitate point loading of target 560 by the element (shoe,
tire, etc.) applying a force to the sensor array and reduce any
tendency of the target to "rock" due to off-center loading. If
desired, a center vertical bore 572 may be formed in target 560 for
calibration purposes.
Biasing element 510 comprises an upper, larger elastomer square 512
and a lower, smaller elastomer square 514 which are preferably
integrally formed and interconnected along line 516 by a small web
of material. Target 560 is placed within biasing element 510 so
that support flange 566 rests upon lower square 514 and lower
portion 562 protrudes therethrough, while support surface 568 is
surrounded by upper ring 512. If desired, target 560 may be molded
into biasing element 510 during fabrication of the latter. Venting
of ring aperture 574 is desirable to prevent possible air trapping
and compression, as previously noted with respect to cavity
418.
It will be appreciated that, unlike the configuration of FIG. 17,
wherein the biasing element 410 is placed in shear during the
application of a normal force N to the sensor unit 400 and in
compression during the application of a shear force S, cooperative
configurations of target 560 and biasing element 510 ensure that
both normal forces N and shear forces S will respectively place
lower ring 514 and upper ring 512 in compression. Thus, in the
latter case, the biasing response to both normal and shear forces
will be more predictable, and close to the same if both rings are
of the same cross section as shown, and made of the same elastomer.
It is also contemplated that rings 512 and 514 may intentionally be
made of different cross-sectional size or configuration, or of
elastomers with different durometer ratings, such as two different
polyurethanes, in order to custom-tailor the sensor unit's response
to applied forces. Similarly, elastomer durometer ratings ranging
from very hard to very soft are easily achievable with silicone
rubbers, polyurethanes and other elastomers known in the art, so
that a sensor may be caused to respond with the sensitivity desired
for a particular range of loading. Thus, the same basic sensor
array might be employed with one selection of biasing elements 510
to accurately determine forces applied by the foot of a person, and
with another selection of elements 510 to determine loads applied
by a motor vehicle. Similarly, the upper portions 570 of targets
560 may be configured in various manners to provide greater or
lesser frictional engagement with the intended force-applying
element, and may be coated or plated with different materials to
enhance or reduce friction.
FIGS. 23 through 25 show additional alternative configurations and
arrangements of targets 60, support members 402 and elastomeric
biasing elements 410 which are configured to place the biasing
elements in compression under both normal and shear loading.
However, it is believed that these arrangements may be subject to
hysteresis problems due to measurable internal shear within the
bodies of elements 410.
As noted above, biasing elements 410 may comprise metallic springs
or spring-like elements, or combinations of elements, both metallic
and non-metallic. FIGS. 26-36 depict numerous such alternative
variations of target and biasing element assemblies, each of which
may comprise a preferred arrangement for a particular force
measurement function.
FIGS. 26-33 depict the use of metal biasing elements 410 of various
configurations, it being understood that resilient or "springy"
materials such as hard plastic or fiberglass, or composite
materials such as laminated carbon fibers, may also be employed
where desired in lieu of spring steel or other metal. The
particular material employed to achieve the spring effect in FIGS.
26-33 is limited only by one's ability to fabricate the spring
element from a particular material. FIGS. 34 and 35, unlike FIGS.
26-33, employ the resiliency of a compressible material such as a
urethane or rubber to respond to both shear and normal forces, the
metallic component of the assembly providing a target 60 and a
linkage between the target 60, the compressible elements 410 and
the supporting structure, and may only incidentally provide some
biasing effect. The structure of FIG. 36 employs both metal and
resilient elastomer active biasing elements. A spring steel plate
410a is used to respond in flexure to normal forces N, while an
elastomer layer or elements 410b is used to provide a bias against
shear forces S. As shown, the target 60 may be affixed to plate
410a via a screw or rivet, as desired. Plate 410a may be flat, but
is preferably of bowed configuration as shown, and may include
features of the biasing elements 410 of FIGS. 26-33. The use of an
elastomeric element placed in shear (rather than compression) as a
biasing element has demonstrated a significant reduction in
hysteresis in testing (on the order of 50%), and may be considered
as a preferred structural implementation of the present invention
for some applications.
It is noted that the arrangements of FIGS. 34 and 35, which employ
a rigid shell supported from below and laterally by rubber or other
suitable elastomer, may be employed without a rigid support member
such as 402 or 502, as they might rest on and be constrained by
protrusions in an underlying hard rubber layer such as 414. It is
further noted that the arrangement of FIG. 35 may be subject to
cross-talk between adjacent sensor units 400 in an array 500, due
to the presence of foam rubber in the inter-protrusion valleys
between the rigid shells.
Yet another feature of the invention which has particular utility
in the formation of a sensor array 500 is the disposition of
transducers 52 in a horizontal position parallel to the plane of
the array 500, and the use of acoustically refractive elements or
prisms to reorient the ultrasonic waves directed to and reflected
from targets 60. FIG. 37 schematically depicts such an arrangement
as employed with a rigid support member 502, targets 60 and biasing
elements 410 being disposed in apertures 402. Transducers 52, of
the aforementioned PVDF film, lie in a horizontal plane 600 and are
aimed vertically upward. Acoustic prisms 602, each of nylon,
polyethylene or other suitable acoustically refractive material
which is highly transparent to ultrasound, are placed over each
transducer 52 to refract transduceremitted sound waves 610. Prisms
602 each include a flat lower surface 604 disposed over a
transducer 52, and an angled surface 606 disposed at a selected
angle to the horizontal. Each prism 602 refracts the emitted sound
waves from a vertical or perpendicular orientation to the
transducer plane to an angle perpendicular to angled surface 606
and refracts the returning sound waves reflected from targets 60
back to a vertical orientation and onto the emitting transducer 52,
as shown. This configuration permits more inexpensive fabrication
of transducer arrays with more precise transducer placement, and
also permits closer spacing of basic sensor units so that more of
same may be placed per unit area within an array.
Sound waves emitted by transducers 52 and refracted by prisms 602
are preferably conducted between prism faces 606 and targets 60
through a very soft or compliant layer or masses 608 of silicone
rubber, which may even comprise an unset gel rather than a cohesive
mass. As previously noted, silicone rubber is a desirable acoustic
coupling material due to the relatively low speed of sound c
therethrough, on the order of 1,000 meters per second (m/s), which
provides enhanced resolution. By way of comparison, c for urethanes
ranges from about 1,500 to 1,800 m/s, for polyethylene c is about
2,000 m/s, and for nylon c is about 2,600 m/s. As noted previously,
it is undesirable for the transmission mass or layer 608 to provide
any support for targets 60, as such support would have to be
factored in to the force measurements attributable to differences
in wave travel time responsive to change in target position under
force. It will be appreciated that a fluid coupling medium, such as
silicone oil, may also be employed in lieu of solid or gel
couplants, with appropriate containment structure.
In selecting appropriate materials for prisms 602 and coupling
layer or masses 608, the speed of sound c.sub.1 in the prism
material should exceed that (c.sub.2) in the mass material.
Further, the mass density .rho..sub.1 of the prism material should
be less than that (.rho..sub.2) of the mass or layer material. FIG.
38 of the drawings depicts a transducer 52 aimed into an acoustic
prism 602 with a couplant mass 608 interposed between prism surface
606 and target 60. With reference to the angular relationships
shown in FIG. 38, for minimum reflectivity as acoustic wave 610
travels between transducer 52 and target 60,
where Z.sub.1 equals the acoustic impedance of the prism material
602 (c.sub.1.times..rho..sub.1) and Z.sub.2 equals the acoustic
impedance of the couplant material 608 (c.sub.2.times..rho..sub.2),
.crclbar..sub.i is the sound wave angle of incidence from
transducer 52 through prism 602 with respect to a normal to the
line of intersection between prism and couplant material, and
.crclbar..sub.t is the sound wave angle of transmission through the
couplant 608 with respect to the normal line. It will be
appreciated that, while exact equality in the above relationship is
desirable, as a practical matter, such is difficult to achieve.
Yet another variation of the invention is illustrated in FIG. 39 of
the drawing wherein, as in FIG. 37, transducers 52 are placed in a
plane 600 and aimed perpendicularly upward with respect thereto.
However, in lieu of acoustic prisms 602, acoustic reflectors 620 of
metal or metallized plastic are employed to reflect vertical sound
waves 610 toward targets 60 and return sound waves back to
transducers 52. Any suitable metal or metal coating may be
employed, such as steel, aluminum, brass or other ferrous and
non-ferrous metals. A grid of such reflectors 620 with highly
accurate reflector locations and angles may be molded from plastic
and then metallized using techniques well-known in the art. As with
the embodiment of FIG. 38, a highly acoustic transmissive layer or
masses 608 are employed between transducers 52 and targets 60.
In fabrication of basic sensor units or arrays to accommodate
particular space limitations or configurations or sensor or array
topography, it may be desirable or even necessary to combine
directly-aimed transducers with transducers aimed through acoustic
prisms or by acoustic reflectors or prism-aimed transducers with
reflector-aimed transducers. With appropriate mathematic processing
to accommodate different distances and speeds of sound through
materials, such an arrangement is within the ability of one of
ordinary skill in the art and is contemplated as within the scope
of the invention.
While the present invention has been described in terms of the
illustrated embodiments, those of ordinary skill in the art will
readily understand and appreciate that it is not so limited. Many
additions, deletions and modifications to the embodiments
illustrated and described herein are possible without departing
from the scope of the invention as hereinafter claimed.
* * * * *