Force detector using resistance elements

Abstract: There is disclosed a force detector for detecting forces exerted in axial directions or moments exerted about respective axes using the origin (P) as a working point in an XYZ three-dimensional coordinate system. On the upper surface of the substrate (10), four bridge portions (11 to 14) are formed along the positive and negative directions of respective coordinate axes, and resistance elements (R1 to R24) having a property such that an electric resistance varies due to a mechanical deformation are arranged on predetermined symmetric positions of respective bridge portions. Bridge circuits are constructed by these resistance elements, and detection of a force is made by bridge voltage produced in the bridge circuit. A strain generative body (20) is used in order to transmit an external force to the substrate. This strain generative body is provided with a first flexible portion (23) having a sufficient flexibility with respect to the Z-axis direction and a second flexible portion (24) having a sufficient flexibility with respect to the direction perpendicular to the Z-axis, thus permitting detection sensitivities with respect to respective axes to be made uniform.

Description: This invention relates to a force detector, and more particularly to a force detector for detecting forces applied or exerted in respective axial directions and moments applied or exerted about respective axes in an XYZ three-dimensional coordinate system wherein the origin is used as a working point. Generally, devices for detecting a force exerted on a working point are constructed to detect a stress-strain produced by the application of this force to thereby indirectly make a force detection. The detection of a stress-strain is carried out by providing sensors such as strain gauges at respective portions of a strain generative body which produces a strain-stress by the application of a force, and measuring changes in resistance values, etc. of these detectors. Recently, there has been proposed a technique to arrange, on a semiconductor substrate, resistance elements having the property of the piezo resistance effect that an electric resistance varies by mechanical deformation to detect a force from changes in the resistance values of these resistance elements. This technique intends to cause a mechanical deformation on the semiconductor substrate by the application of force, thus to electrically detect changes in the resistance values of the resistance elements produced by such a mechanical deformation. For example, there is disclosed, in the International Application based on the Patent Cooperation Treaty No. PCT/JP88/394 (corresponding to WO-A-88/08521 and to EP-A-311 695), a technique to form a plurality of resistance elements at predetermined positions along the X-axis and Y-axis directions on a substrate extending on the XY-plane and assemble these resistance elements into a peculiar bridge circuit, thereby making it possible to detect forces applied or exerted in respective axial directions and moments exerted about respective axes as changes in the bridge voltage. However, the force detector disclosed in the above-described International Application has the following problems. (1) In order to allow a mechanical deformation to be easily produced on a substrate on the basis of an applied force, several portions of the substrate are hollowed out to form a first bridge portion along the X-axis direction and a second bridge portion along the Y-axis direction. Where an attempt is made to bond this substrate to a strain generative body comprised of metal, etc., an approach is employed to mount a bonding agent or adhesive on the surface of the stain generative body to melt this bonding agent under the condition of a high temperature to mount the substrate thereon and make a bonding between the strain generative body and the substrate. However, since the substrate includes hollowed out portions therein, the molten bonding agent permeates through the hollowed portions to appear on the substrate surface. The bonding agent which has appeared on the substrate surface in this way exerts a bad influence on various wiring implemented on the substrate surface. This is not favorable. (2) Fluctuation in the bridge porion is apt to occur. As a result, there are instances where correct measured values cannot be obtained. (3) Detection of six components of forces in three axial directions of XYZ and moments about three axes thereof can be made, but there are instances where any other component may interfere with a measured value of a component subject to measurement, failing to obtain a correct measured value. (4) At the time of detecting forces or moments of six components, there occurs a considerable difference between the detection sensitivities of respective components. In actual terms, the detection sensitivity of Fx or Fy becomes equal to about one tenth of the detection sensitivity of Fz. Namely, there is a decuple difference between the detection voltage value when a force is applied in the Z-axis and that when the same force as above is applied in the X-axis or Y-axis. For this reason, it is difficult to precisely process respective output signals. The object of this invention is to provide a force detector capable of providing substantially uniform sensitivities in all directions. This object can be attained by the force detector having the features of claim 1 or claim 2. In the force detector according to claim 1, the first and second flexible portions are formed in the strain generative body. Since the first flexible portion has a sufficient flexibility with respect to the Z-axis direction, when a force in the Z-axis direction is applied to the working portion, a mechanical deformation in the Z-axis direction is sufficiently transmitted to the resistance element by bending of the first flexible portion. Further, since the second flexible portion has a sufficient flexibility with respect to the direction perpendicular to the Z-axis, when a force in the X-axis or Y-axis direction is applied to the working portion, a mechanical deformation in the X-axis or Y-axis direction is sufficiently transmitted to the resistance element by bending of the second flexible portion. Eventualy, substantially uniform detection sensitivities can be provided with respect to any one of components in X-axis, Y-axis and Z-axis directions. Thus, the above-described problem (4) can be solved. Also in the force detector according to claim 2, substantially uniform detection sensitivities are provided with respect to the components of respective directions. Thus, the above-described problem (4) can be solved. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side cross sectional view of a force detector according to an embodiment of this invention, FIG. 2 is a top view of the force detector shown in FIG. 1, FIG. 3 is a side cross sectional view cut along the cutting line 20-20 of the force detector shown in FIG. 2, FIG. 4 is a top view of a strain generative body used in the force detector shown in FIG. 1, FIG. 5 is a side cross sectional view showing the state where a force Fx in the X-axis direction is applied to the force detector shown in FIG. 1, FIG. 6 is a side cross sectional view showing the state where a force Fz in the Z-axis direction is applied to the force detector shown in FIG. 1, FIG. 7 is a side cross sectional view showing the state where a moment My about the Y-axis is applied to the force detector shown in FIG. 1, FIG. 8 is a top view showing the state where a moment Mz about the Z-axis is applied to the force detector shown in FIG. 1, FIG. 9 is a side cross sectional view of a force detector according to another embodiment of this invention, FIG. 10 is a side cross sectional view showing the state where a force Fx in the X-axis direction is applied to the force detector shown in FIG. 26, FIG. 11 is a top view showing the state where a force Fx in the X-axis direction is applied to the force detector shown in FIG. 26, FIGS. 12 and 13 are a side cross sectional view and a top view of a force detector according to a further embodiment of this invention, respectively, FIGS. 14 and 15 are side cross sectional views of a force detector according to a still further embodiment of this invention. FIGS. 16 and 17 are a side cross sectional view and a top view of a force detector according to a still more further embodiment of this invention, respectively, FIGS. 18 and 19 are a side cross sectional view and a top view of a force detector according to a further different embodiment of this invention, respectively, FIGS. 20 and 21 are a side cross sectional view and a top view of a force detector according to a still further different embodiment of this invention, respectively, FIGS. 22 and 23 are cross sectional views showing an embodiment according to this invention using a strain gauge as a resistance element, FIG. 24 is a top view of a strain generative body having through-holes, FIGS. 25 is a cross sectional view (only cross section is illustrated) of a force detector using a strain generative body shown in FIG. 24, and FIGS. 26 and 27 are top views of force detectors using a strain generative body having through-holes. FIG. 1 is a side cross sectional view of a force detector according to an embodiment of this invention, and FIG. 19 is a top view of the above detector. This detector is roughly composed of two components of a semiconductor substrate 10 and a strain generative body 20. An actual product requires, in addition to the above components, elements such as a cover for protecting the upper surface of the semiconductor substrate 10, a housing for accommodating the detector, and an electrical wiring, etc. However, the description of these attachments will be omitted for convenience. A working point P is defined at the central portion on the upper surface of the semiconductor substrate 10, and an XYZ three-dimensional coordinate system is defined as shown using the working point P as the origin. Thus, the upper surface of the semiconductor substrate 10 is just included in the XY plane. The cross sectional view cut along the X-axis of the detector shown in FIG. 2 corresponds to the side cross sectional view of FIG. 1. For further clarifying the structure of the semiconductor substrate 10, the side cross sectional view cut along the cutting line 20-20 of the detector shown in FIG. 2 is shown in FIG. 3. In addition, for further clarifying the structure of the strain generative body 20, the top view of only the strain generative body 20 is shown in FIG. 4. The structure of the strain generative body 20 will be readily understood by making a reference to the side cross sectional view of FIG. 3 and the top view of FIG. 4. The entirety of the strain generative body is substantially disk-shaped. The central working portion 21 is projected downwardly, and the force detection is actually made by applying an external force to the lower end of the working portion 21. The upper end of the working portion 21 is bonded to the lower surface of the semiconductor substrate 10. On the other hand, the fixed portion 22 therearound is ordinarily fixed to a housing for accommodating the detector. The upper surface of the fixed portion 22 is fixed to the lower surface of the semiconductor subtrate 10. Between the working portion 21 and the fixed portion 22, an annular flat plate portion 23 and a tubular portion 24 are provided. The annular flat plate portion 23 is formed as a portion in the form of washer. This portion 23 extends along the XY plane and is thin in thickness, and is provided at the position surrounding the periphery of the Z-axis. The tubular portion 24 is cylindrical about the Z-axis and is thin in thickness. Since the above-mentioned tubular portion 24 is formed, an annular groove 25 is formed on the side of the upper surface of the strain generative body 20. As stated above, the strain generative body 20 is composed of working portion 21, annular flat plate portion 23, tubular portion 24, and fixed portion 22 from the center toward the periphery in order recited. These respective portions are not discrete parts, but serve as a portion of the strain generative body integrally constructed. It is preferable that Kovar (registered trademark, an alloy of iron, cobalt nickel) is used as the material of the strain generative body 20. When the fixed portion 22 of the strain generative body 20 is fixed to apply an external force to the working point 21, there occurs bending in the annular flat plate portion 23 and the tubular portion 24. As a result, the strain generative body itself is deformed. Such a deformation is transmitted to the semiconductor substrate 10. Thus, a force corresponding to this deformation is detected. This detector can detect forces in three axial directions and moments about three axes with respect to the working point P. The role of the strain generative body 20 is to transmit a force applied to the lower end of the working portion 21 to the working point P to produce displacements in three axial direction and rotational strains about three axes. At this time, when easinesses of such displacements or strains is different with respect to all axes, it results that detection sensitivities would he different with respect to all axes. An employment of the strain generative body of the above-described structure permits easinesses of displacements or strains with respect to respective axes to be substantially uniform. The reason thereof will be described below. FIG. 5 is a side cross sectional view showing how the strain generative body 20 is deformed when a force Fx in the X-axis direction is applied to the working point P. Such a deformation is obtained by applying a force as shown to the working portion 21. In this case, the tubular portion 24 is bent as shown, so the strain generative body 20 is mechanically deformed. This deformation is transmitted to the semiconductor substrate 10. Since the tubular portion 24 is formed thin in thickness, it can be easily bent in the X-axis direction. Accordingly, this detector exhibits a sufficient detection sensitivity with respect to the force Fx. It is to be noted that when a force Fy in the Y-axis direction is applied, entirely the same action as stated above is carried out. Thus, this detector exhibits a sufficient detection sensitivity with respect to the force Fy. FIG. 6 is a side cross sectional view showing how the strain generative body 20 is deformed when a force Fz in the Z-axis direction is applied to the working point P. Such a deformation is obtained by applying a force as shown to the working portion 21. In this case, the annular flat plate portion 23 is bent as shown, so the strain generative body 20 is mechanically deformed. Thus, this deformation is transmitted to the semiconductor substrate 10. Since the annular flat plate portion 23 is formed thin in thickness, it can be easily bent in the Z-axis direction. Accordingly, this detector exhibits a sufficient detection sensitivity with respect to the force Fz. FIG. 7 is a top view showing how the strain generative body 20 and the semiconductor substrate 10 are deformed when a moment about the Y-axis is applied to the working portion P. Such a deformation is obtained by applying a force F as shown to the bottom portion of the working portion 21. In this case, the annular flat plate portion 23 is bent as shown, so the strain generative body 20 is mechanically deformed. This deformation is transmitted to the semiconductor substrate 10. Since the annular flat plate portion 23 is formed thin in thickness, it can be easily bent in such a direction. Accordingly, this detector exhibits a sufficient detection sensitivity with respect to the moment My. In addition, when a moment Mx about the X-axis is applied, the action is entirely the same as stated above. Thus, the detector has a sufficient detection sensitivity also with respect to the moment Mx. FIG. 8 is a top view showing how the strain generative body 20 and the semiconductor substrate 10 are deformed when a moment Mz about the Z-axis is applied to the working point P. Such a deformation is obtained by applying a force to twist the working portion 21 about the Z-axis. In this case, the tubular portion 24 is bent in a twisting direction, so the strain generative body 20 is mechanically deformed. This deformation is transmitted to the semiconductor substrate 10. Since the tubular portion 24 is formed thin in thickness, it can be easily bent in such a twisting direction. Accordingly, this detector exhibits a sufficient detection sensitivity with respect to a moment Mz. In short, since the strain generative body 20 shown here is provided with the annular flat plate portion 23 as a first flexible portion having a sufficient flexibility with respect to the Z-axis direction and the tubular portion 24 as a second flexible portion having a sufficient flexibility with respect to the direction perpendicular to the Z-axis, even if a force in any axial direction is applied, or a moment about any axis is applied, the strain generative body can be sufficiently deformed. Accordingly, with this force detector, substantially uniform detection sensitivity can be provided with respect to respective axes. Other embodiments of the strain generative body The cross sectional view of a force detector according to another embodiment of this invention is shown in FIG. 9. A strain generative body 20 of this detector is provided with two wall portions 24a and 24b. This embodiment differs from the above-described embodiment in this respect. For this reason, two kinds of grooves 25a and 25b are formed accordingly. FIGS. 10 and 11 are a side cross sectional view and a top view showing the state of deformation when a force Fx in the X-axis direction is applied, respectively. As shown in FIG. 11 the strain generative body 20 has four through-holes of quadrant instead of grooves to form bridge portions. Therefore wall portions 24a and 24b do not form a whole annular tube. Since two wall portions 24a and 24b are both bent, deformation is more easily produced. By this deformation, the portion indicated by the mark "+" is expanded, while the portion indicated by the mark "-" is contracted. The most conspicuous positions of the expansion/contraction are the position immediately above the grooves. Accordingly, it is preferable for improving the sensitivity to form resistance elements R immediately above the grooves 25a and 25b, respectively. The embodiment showing the side cross sectional view in FIG. 12 and the top view in FIG. 13 is the same as the embodiment shown in FIG. 9 in the structure of the strain generative body 20, but differs from the latter in that twenty four sets of resistance element groups are arranged over respective annular grooves, thus to provide fourty eight sets of resistance element groups in total. Thus, force detection can be made with a still more higher precision. The embodiment showing the side cross sectional view in FIGS. 14 and 15 is characterized in that two annular flat plate portions 23a and 23b are provided. The embodiment showing the side cross sectional view in FIG. 16 and the top view in FIG. 17 is characterized in that there are included two tubular portions 24a and 24b to arrange all the resistance element groups on the annular grooves 25 formed therebetween. The embodiment showing the side cross sectional view in FIG. 18 and the top view in FIG. 19 is characterized in that all the resistance element groups are arranged over the annular groove formed in the vicinity of the working portion 21. The embodiment showing the side cross sectional view in FIG. 20 and the top view in FIG. 21 is characterized in that twenty four sets of resistance element groups are arranged over the annular groove 25 and different twenty four sets of resistance element groups are arranged on the annular flat plate portion 23. It is to be noted that in FIGS. 1 to 21, reference symbol R does not indicate a single resistance element, but it indicates a group of resistance elements. While resistance elements are formed on the semiconductor substrate in all the above-described embodiments, this invention is not limited to force detectors using such substrates. This invention is also applicable to, e.g., a detector using a strain gauge as the resistance element. The embodiment showing the side cross sectional view in FIG. 22 is characterized in that strain gauges S are directly stuck on the upper surface of the strain generative body 20 of the same structure as that of the embodiment shown in FIG. 1. The arrangement of a plurality of strain gauges S and the bridge circuit configuration are those of the embodiment shown in FIG. 1. Further, the embodiment showing the side cross sectional view in FIG. 23 is characterized in that a flat plate 90 is bonded on the upper surface of the strain generative body 20 of the same structure as that of the embodiment shown in FIG. 16 and strain gauges S are stuck on the upper surface of the flat plate 90. The arrangement of a plurality of strain gauges S and the bridge circuit configuration are the same as those of the embodiment shown in FIG. 16. In such detectors using the strain gauge S as the resistance element, it is unnecessary to use the semiconductor substrate. For the material of the strain generative body 20 or the flat plate 90, most typical metal may be satisfactorily used because it is unnecessary to take matching with the semiconductor substrate into consideration. In this embodiment, high strength aluminum is used as the material of the strain generative body 20 and the flat plate 90, and the both members are brazed to each other. For sticking strain gauges S onto these members, a bonding agent of phenol system or epoxy system may be used. When insulator is used for the flat plate 90, it is possible to form a poly-silicon layer having the property of the piezo resistance effect or a micro-crystal silicon layer on the surface of the plate 90 with a desired pattern to form resistance elements using a semiconductor processing technique. An embodiment shown in FIGS. 24 and 25 (only cross, section is illustrated) is exactly the same as the embodiment shown in FIGS. 1 to 4 except that the strain generative body 20 comprises four quadrant through-holes 101 to 104 to form four bridge portions 201 to 204. FIG. 24 shows only the strain generative body. The cross section cut along the X-axis is identical with FIG. 1, where the substrate is also shown. An embodiment shown in FIG. 26 is exactly the same as the embodiment shown in FIG. 17 and an embodiment shown in FIG. 27 is exactly the same as the embodiment shown in FIG. 19, except that the strain generative body 20 comprises four quadrant through-holes 105 to 108 and 109 to 112, respectively. As described above, the force detector according to this invention is constructed so that the strain generative body is provided with a first flexible portion having a sufficient flexibility with respect to the Z-axis direction and a second flexible portion having a sufficient flexibility with respect to the direction perpendicular to the Z-axis. Accordingly, even if a force in any axial direction is applied, or even if a moment about any axis is applied, the strain generative body can be sufficiently deformed. Thus, substantially uniform detection sensitivity can be provided with respect to respective axes.

Claims: A force detector comprising: a resistance element (R) having a property such that an electric resistance varies due to a mechanical deformation and formed on a predetermined plane, and a strain generative body (20) for allowing the resistance element to produce a mechanical deformation, wherein when an origin (P) is taken on said plane to define an XYZ three-dimensional coordinate system so that said plane is included in an XY plane of said three-dimensional coordinate system, the force detector is operative to detect a force exerted in a coordinate axis direction or a moment exerted about a coordinate axis using said origin as a working point, characterized in that said strain generative body (20) comprises: a working portion (21) positioned below said origin and transmitting an applied force to said origin, a fixed portion (22) for fixing the periphery of said plane, a first flexible portion (23) formed between said working portion and said fixed portion and having a sufficient flexibility with respect to a Z-axis direction, and a second flexible portion (24) formed between said working portion and said fixed portion and having a sufficient flexibility with respect to a direction perpendicular to said Z-axis, wherein the two flexible portions (23,24) are dimensioned such that substantially uniform detection sensitivities are achieved with respect to force components in all directions. A force detector comprising: a substrate (10) including a resistance element (R) on an upper surface thereof, said resistance element having a property such that an electric resistance varies due to a mechanical deformation, and a strain generative body (20) connected to a lower surface of said substrate in order to produce a mechanical deformation in said substrate, wherein when an origin (P) is taken on the upper surface of said substrate to define an XYZ three-dimensional coordinate system so that the upper surface of said substrate is included in an XY plane of said three-dimensional coordinate system, said force detector is operative to detect a force exerted in a coordinate axis direction or a moment exerted about a coordinate axis using said origin as a working point, characterized in that said strain generative body comprises: a working portion (21) connected at a position below said origin of said substrate and transmitting an applied force to said origin, a fixed portion (22) for fixing the periphery of said substrate, a first flexible portion (23) formed between said working portion and said fixed portion and having a sufficient flexibility with respect to a Z-axis direction, and a second flexible portion (24) formed between said working portion and said fixed portion and having a sufficient flexibility with respect to a direction perpendicular to said Z-axis, wherein the two flexible portions (23,24) are dimensioned such that substantially uniform detection sensitivities are achieved with respect to force components in all directions. A force detector as set forth in claim 1, wherein said first flexible portion is consituted by an annular flat plate portion (23) provided so as to surround the periphery of said Z-axis, said annular flat plate portion extending along a plane substantially perpendicular to said Z-axis and having a small thickness in said Z-axis direction. A force detector as set forth in claim 1 or 2, wherein said second flexible portion is constituted by a tubular portion (24) provided so as to surround the periphery of said Z-axis, said tubular portion extending along said Z-axis and having a small thickness in a direction perpendicular to the Z-axis. A force detector as set forth in claim 1, 2 or 3, wherein holes (101-104) are formed through said strain generative body (20) so as to form bridge portions (201-204 with a part between said holes.

Inventors: OKADA KAZUHIRO, OKADA, KAZUHIRO
Applicants: WAKO KK, WACOH CORPORATION
Application year: 1990
Main classification: G01L1
IPC codes: G01L5, G01L1