Measuring and testing – Dynamometers – Responsive to multiple loads or load components
Reexamination Certificate
2003-01-10
2004-11-30
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Dynamometers
Responsive to multiple loads or load components
Reexamination Certificate
active
06823744
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a six-axis force sensor, and particularly to a small and highly precise six-axis force sensor in which strain resistance devices fabricated using semiconductor manufacturing process technology are used and which can detect six force and moment components and be utilized as a force-sensing sensor of a robot or the like.
BACKGROUND OF THE INVENTION
Modern automatic machines such as machine tools and robots, in the course of their work or operation, perform work in which they apply forces to workpieces, or are themselves subject to actions of forces from outside. Consequently, it is necessary for machine tools and robots to detect forces and moments acting on them from outside and to perform control corresponding to these forces and moments. For control corresponding to external forces and moments to be carried out properly, it is necessary for the forces and moments acting from outside to be detected exactly.
In this connection, various types of multi-axis force sensors to be used as force-sensing sensors and man-machine interfaces have been proposed. Generally, force sensors can be classified, according to the detection method that they use, as either elastic-type force sensors, which detect a deformation proportional to a force, or force-balance-type force sensors, which measure a force by balancing it with a known force. As a principle structure, force sensors generally have multiple strain resistance devices provided on a distorting body part which deforms elastically in correspondence with external forces.
With this structure, when an external force acts on a distorting body part of the multi-axis force sensor, electrical signals corresponding to degrees of deformation of the distorting body part are outputted from the strain resistance devices. On the basis of these electrical signals it is possible to detect two or more force or moment components acting on the distorting body part.
To keep up with size reductions of devices equipped with multi-axis force sensors, size reductions of multi-axis force sensors themselves are sought. Accordingly, there is an increasing need for multi-axis force sensors which have good sensitivity and high precision while being small.
A typical multi-axis force sensor is the six-axis force sensor. The six-axis force sensor is a force sensor of the elastic type mentioned above, and has multiple strain resistance devices on a distorting body part. The six-axis force sensor divides an external force into axial stress components (forces Fx, Fy, Fz) in the axis directions and torque components (torques Mx, My, Mz) about the axis directions of three orthogonal coordinate axes (an X-axis, a Y-axis, a Z-axis), and detects it as six axis components.
A first example of a multi-axis force sensor in related art is the ‘Multiple Force Component Load Cell’ disclosed in JP-B-S.63-61609 (Publication date Nov. 29, 1988; corresponding U.S. Pat. No. 4,448,083). This document discloses a six-axis force sensor. This six-axis force sensor has a construction wherein multiple strain gauges are affixed to a distorting body having a solid (three-dimensional) structure. This sensor is a six-component force sensor and has a structure such that mutual interference arising among the six force components detected is reduced. The six-axis force sensor is made up of a central force-receiving part, a fixed annular part around this, and between these, four T-shaped connecting parts equally spaced around the axis of the force-receiving part. The strain gauges are affixed to low-rigidity portions of the beams of the T-shaped connecting parts.
With this structure wherein strain gauges are affixed to a distorting body, size reduction is limited; manufacturing reproducibility is poor and dispersion arises among units; and also problems such as peeling of the affixing layer arise due to repeated shock stresses and thermal stresses. When peeling of the affixing layer occurs, the measuring precision deteriorates. Alignment deviation also causes the measuring precision to deteriorate. The problem arises that it is difficult to make the mounting positions accurate enough to ensure good detection accuracy.
A second example of a multi-axis force sensor in related art is the ‘Two-or-more-Component Force-Detecting Device’ disclosed in Japanese Patent Publication No. 2746298 (Published May 6, 1998, corresponding U.S. Pat. No. 4,884,083). In the multi-axis force sensor disclosed in this document, multiple strain resistance devices are fabricated on a substrate using semiconductor manufacturing process technology, and a strain gauge element is assembled integrally to a distorting body part. The substrate is made up of a peripheral part and a central part. According to this document, the problems of the multi-axis force sensor of the first related art mentioned above can be resolved, the precision of the fabrication process can be raised, the reproducibility of fabrication can be made good, and the multi-axis force sensor can be reduced in size. However, with this sensor, there is a high probability of mutual interference arising among the six axis components detected.
A third example of a multi-axis force sensor of related art is the ‘Contact Force Sensor’ disclosed in JP-B-H.7-93445. In this contact force sensor also, piezoelectric sensors made by forming resistance devices on one side of an annular structural body made of a semiconductor are used, and semiconductor manufacturing process technology is utilized.
Of the multi-axis force sensors of these first through third examples of related art, whereas in the first multi-axis force sensor strain resistance devices (strain gauges) are affixed as external elements, in the second and third multi-axis force sensors, strain resistance devices are formed integrally on a semiconductor substrate by utilizing semiconductor device manufacturing process technology. The second and third multi-axis force sensors have the merit that they make it possible to resolve the problems associated with the first multi-axis force sensor.
However, related art multi-axis force sensors fabricated using semiconductor device manufacturing process technology have had the characteristic structurally that, when an attempt is made to detect a force or moment on each of three orthogonal axes, the whole substrate distorts isotropically in correspondence with the applied force or moment, and have also had the problem that the disposition of the multiple strain resistance devices on the substrate is not optimal and an external force acting on the distorting body part cannot be separated into components with good precision.
That is, in six-axis force sensors, there has been the problem that for example when an external force is applied so that only an axial stress component Fx arises, stresses arise and outputs are produced in connection with components other than Fx, which should properly be 0. There has been the problem that it is difficult to separate an external force applied from an unknown direction into individual components with good precision. The electrical signal components outputted from the resistance devices corresponding to the respective axis components superpose onto the other axes, and the measuring sensitivity of the axis components of force or moment decreases.
The problem of not being able to separate the axis components (forces and moments) of an external force acting on the distorting body part in a six-axis force sensor is known as the problem of ‘other axis interference’. This problem of other axis interference is one which cannot be ignored from the point of view of realizing a practical six-axis force sensor.
The problem of other axis interference in a six-axis force sensor will now be explained more specifically, from a mathematical point of view, using equations.
In a six-axis force sensor, as mentioned above, as six axis components pertaining respectively to an X-axis, a Y-axis and a Z-axis, forces Fx, Fy and Fz and moments Mx, My and Mz are detected. The six-axis force sensor outputs six signals Sig
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Hirabayashi Yusuke
Ohsato Takeshi
Birch & Stewart Kolasch & Birch, LLP
Honda Giken Kogyo Kabushiki Kaisha
Miller T
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