Measuring and testing – Vibration – By mechanical waves
Reissue Patent
1999-02-18
2001-02-27
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S652000, C073S862043, C073S862046, C073S862541, C073S862637
Reissue Patent
active
RE037065
ABSTRACT:
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
z
, M
x
, M
y
) of the six force-torque components (F
x
, F
y
, F
z
, M
x
, M
y
, M
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
Bonneville Scientific Incorporated
Miller Rose M.
Trask & Britt
Williams Hezron
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