Abstract:
An optical displacement sensor comprises: a light source; a light receiving means adapted to receive light emitted from the light source; and a light diffracting element disposed between the light source and the light receiving means. The light receiving means include a first light receiving element group centrally located, and a second light receiving element group constituted by two separate clusters disposed so as to sandwich the first light receiving element group, and the light diffracting element functions to diffract the light emitted from the light source along one direction of the two-axis directions into a zero-order beam to be received at the first light receiving element group, and higher-order beams received at the second light receiving element group.

Description:
This application claims priority from Japanese Application No. 2003-316478, filed Sep. 9, 2003 (incorporated by reference herein). 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical displacement sensor and an external force detecting device, and particularly to an optical displacement sensor which detects relative displacement between a reference object and a measurement object based on displacement of a light reception position, and further to an external force detecting device which detects an external force applied to the measurement object based on a signal outputted from the optical displacement sensor. 
   2. Description of the Related Art 
   An external force detecting device, such as a six-axis optical force sensor, is conventionally known, in which a displacement amount of an action section to receive an external force, namely a measurement object, relative to a support section to support the action section, namely a reference object, is detected by an optical displacement sensor, and an external force applied to the action section is measured according to an output signal from the optical displacement sensor. 
   For example, a six-axis optical force sensor comprises optical displacement sensors to measure a six-axis direction displacement, based on which a six-axis force is calculated. Specifically, such a six-axis optical force sensor comprises three optical displacement sensors, each of which uses an optical sensor unit and is capable of measuring a two-axis (X and Y) direction displacement, thereby enabling measurement of a six-axis direction displacement. The optical sensor unit comprises a light emitting diode (LED) as a light source and a photodiode (PD) assembly as a light receiving element, such that the LED opposes the PD assembly with their respective optical central axes aligned to each other. The PD assembly is composed of four PD&#39;s and receives light emitted from the LED at its center area equally shared by the four PD&#39;s, whereby displacement of light receiving position at the PD assembly, that is to say relative positional displacement between a component attached to the LED and a component attached to the PD assembly can be detected in the optical displacement sensor. In the six-axis optical force sensor, a six-axis force applied between the component attached to the LED and the component attached to the PD assembly is measured according to an output signal from each of the optical displacement sensors. 
     FIG. 1  is a plan view of a conventional six-axis force sensor as disclosed in Japanese Patent Application Laid-Open No. H03-245028. A six-axis force sensor  101  shown in  FIG. 1  is structurally composed of a cylindrical main body, and top and bottom lids (not shown). The main body is constituted basically by a frame  105 , which integrally includes: a cylindrical support section  102 ; an action section  103  located centrally inside the support section  102  and adapted to receive an external force; and three elastic spoke sections  104  crookedly structured so as to be duly deformed elastically in all directions and supportably connecting the action section  103  to the support section  102 . The frame  105  is made of a single piece of an aluminum alloy material and shaped by cutting and electric discharge machining. The support section  102  and the action section  103  are fixedly attached respectively to two components to which a measurement force is applied, and when a force applied acts on the six-axis force sensor  101  structured as described above, a micro-displacement with respect to three-axis direction and a micro-rotation with respect to three-axis rotational direction are generated between the support section  102  and the action section  103 . 
   The six-axis force sensor  101  further includes three light sources  106  disposed at the inner circumference of the support section  102  at 120 degree intervals (i.e. at an equi-angular distance), and three optical sensors (light receiving elements)  108  disposed at the action section  103  at 120 degree intervals (i.e. at an equi-angular distance) so as to oppose respective three light sources  106  with mutual optical axes aligned to each other. Each optical sensor  108  and each light source  106  disposed opposite to the optical sensor  108  make up an optical sensor unit (optical displacement sensor)  109 . 
     FIG. 2  is an explanatory perspective view of the optical sensor unit (optical displacement sensor)  109  of  FIG. 1 . As shown in  FIG. 2 , each of the optical sensors  108  is constituted by a PD assembly composed of four PD&#39;s  108   a . The light sources  106  disposed so as to oppose respective optical sensors  108  are each constituted by an infrared high-intensity LED with a pinhole aperture provided at its front face, and light emitted from the LED  106  and passing through the pinhole aperture propagates diffusedly and impinges on the center portion of the optical sensor  108  so as to be substantially equally irradiated on all the four PD&#39;s  108   a . If the support section  102  and the action section  103  are displaced relative to each other by an external force, then the light emitted from the LED  106  is irradiated unequally on the four PD&#39;s  108   a , and light amounts received at respective four PD&#39;s  108   a  are measured for calculation of relative displacements with respect to X- and Y-axis directions. And, the six-axis force sensor  101  calculates forces with respect to six-axis directions according to the above-calculated relative displacements, and a signal is outputted therefrom. 
   However, the aforementioned conventional optical displacement sensor, and the aforementioned six-axis force sensor, i.e., external force detecting device, incorporating the conventional optical displacement sensor has the following problems when respective optical axes of the LED  106  as the light source, and PD assembly  108  to receive light emitted from the LED  106  are aligned to each other. Specifically, in the alignment work, while positional adjustment with respect to the X- and Y-axis directions is easy, rotational adjustment about a Z-axis perpendicular to the X- and Y-axes (to precisely bring the cross-shaped boundary formed by the four PD&#39;s  108   a  in line with the X- and Y-axes) is very difficult 
   Conventionally, the rotational adjustment has to be carried out such that an LED and a PD assembly are tentatively arranged, and misalignment in the rotational direction about the Z-axis is checked and corrected based on a signal from the PD assembly, which is gained by causing an action section at which either the LED or the PD assembly is disposed to be displaced in the X- and Y-axis directions. This involves a lot of works, and requires an immense amount of time and effort, especially when displacement amount is large. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in light of the above problem, and it is an object of the present invention to provide an optical displacement sensor, and an external force detecting device, in which a position of a light receiving means relative to a light source with respect to a rotational direction about an optical axis of light emitted from the light source can be adjusted easily in a reduced time. 
   In order to achieve the above object, according to a first aspect of the present invention, an optical displacement sensor comprises: a light source disposed at one of a reference object and a measurement object; a light receiving means disposed at the other one thereof at which the light source is not disposed, and adapted to receive light emitted from the light source thereby measuring displacement of the measurement object relative to the reference object with respect to two-axis directions in a plane perpendicular to an optical axis of the light emitted from the light source; and a light diffracting element disposed between the light source and the light receiving means. The light receiving means includes a first light receiving element group centrally located, and a second light receiving element group constituted by two separate clusters disposed so as to sandwich the first light receiving element group, and the light diffracting element functions to diffract the light emitted from the light source into a zero-order beam, and higher-order beams along one direction of the two-axis directions, such that the zero-order beam is received at the first light receiving element group, and the higher-order beams are received at the second light receiving element group. 
   According to a second aspect of the present invention, an optical displacement sensor comprises: a light source disposed at one of a reference object and a measurement object; and a light receiving means disposed at the other one thereof at which the light source is not disposed, and adapted to receive light emitted from the light source thereby measuring displacement of the measurement object relative to the reference object with respect to two-axis directions in a plane perpendicular to an optical axis of the light emitted from the light source. The light emitted from the light source, at least when received at the light receiving means, has its intensity distribution shaped in an oval configuration with two symmetry axes oriented so as to match the two-axis directions, respectively, with respect to which the displacement of the measurement object relative to the reference object is measured. 
   According to a third aspect of the present invention, an optical displacement sensor comprises: a light source disposed at one of a reference object and a measurement object; a reflector member disposed at the other one thereof at which the light source is not disposed; a light receiving means disposed at the one object at which the light source is disposed, and adapted to receive light which is emitted from the light source and which impinges on the reflector member to be reflected backward, thereby measuring displacement of the measurement object relative to the reference object with respect to two-axis directions in a plane perpendicular to an optical axis of the light emitted from the light source,; and a light diffracting element disposed at an optical path from the light source to the light receiving means via the reflector member. The light receiving means includes a first light receiving element group centrally located, and a second light receiving element group constituted by two separate clusters disposed so as to sandwich the first light receiving element group, and the light diffracting element functions to diffract the light emitted from the light source into a zero-order beam, and higher-order beams along one direction of the two-axis directions, such that the zero-order beam is received at the first light receiving element group, and the higher-order beams are received at the second light receiving element group. 
   According to a fourth aspect of the present invention, an optical displacement sensor comprises: a light source disposed at one of a reference object and a measurement object; a reflector member disposed at the other one thereof at which the light source is not disposed; and a light receiving means disposed at the one object at which the light source is disposed, and adapted to receive light which is emitted from the light source and which impinges on the reflector member to be reflected backward, thereby measuring displacement of the measurement object relative to the reference object with respect to two-axis directions in a plane perpendicular to an optical axis of the light emitted from the light source. The light emitted from the light source, at least when received at the light receiving means, has its intensity distribution shaped in an oval configuration with two symmetry axes oriented so as to match the two-axis directions, respectively, with respect to which the displacement of the measurement object relative to the reference object is measured. 
   In the second or fourth aspect of the present invention, the light source may be constituted by a light emitting diode (LED), and a cylindrical lens may be disposed between the LED and the light receiving means whereby the light emitted from the LED has its intensity distribution turned into the oval configuration with two symmetry axes. 
   In the first to fourth aspects of the present invention, the light receiving means may be structured to be rotatable about the first light receiving element group. 
   According to a fifth aspect of the present invention, there is provided a method of adjusting such an optical displacement sensor as structured according to the first aspect of the present invention. The method comprises a step of adjusting a position of the light receiving means relative to the light source with respect to a rotational direction about the optical axis of the light emitted from the light source based on a reception state of the higher-order beams at the second light receiving element group. Consequently, the optical displacement sensor can be adjusted without troublesome work in a reduced time. 
   According to a sixth aspect of the present invention, there is provided a method of adjusting such an optical displacement sensor as structured according to the second aspect of the present invention. The method comprises a step of adjusting a position of the light receiving means relative to the light source with respect to a rotational direction about the optical axis of the light emitted from the light source based on a reception state of the light at the light receiving means. Consequently, the optical displacement sensor can be adjusted easily without provision of the light diffracting element and the second light receiving element group used in the first aspect. 
   According to a seventh aspect of the present invention, there is provided a method of adjusting such an optical displacement sensor as structured to the third aspect of the present invention. The method comprises a step of adjusting a position of the light receiving means relative to the light source with respect to a rotational direction about the optical axis of the light emitted from the light source based on a reception state of the higher-order beams at the second light receiving element group. Consequently, the optical displacement sensor structured to include the reflector member can be also adjusted without troublesome work in a reduced time. 
   According to an eighth aspect of the present invention, there is provide a method of adjusting such an optical displacement sensor as structured according to the fourth aspect of the present invention. The method comprises a step of adjusting a position of the light receiving means relative to the light source with respect to a rotational direction about the optical axis of the light emitted from the light source based on a reception state of the light at the light receiving means. Consequently, the optical displacement sensor provided with the reflector member can be adjusted easily without provision of the light diffracting element and the second light receiving element group used in the third aspect. 
   According to a ninth aspect of the present invention, an external force detecting device is provided which incorporates an optical displacement sensor structured according to any one of the aforementioned first to fourth aspects, and an external force applied to a measurement object is detected based on a signal containing a measurement result by the optical displacement sensor. In the ninth aspect of the present invention, the optical displacement sensor may be provided in a plural number, and a plurality of optical displacement sensors may detect respective displacements with respect to two-axis directions different from one another. Since the external force detecting device incorporates the optical displacement sensors according to the present invention, the adjustment work about the device can be performed easily in a reduced time. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a conventional six-axis force sensor; 
       FIG. 2  is an explanatory perspective view of a conventional optical sensor unit (optical displacement sensor) shown in  FIG. 1 ; 
       FIG. 3  is an explanatory perspective view of a structure of an optical displacement sensor according to a first embodiment of the present invention; 
       FIG. 4  is a plan view of a light receiving face of a PD assembly shown in  FIG. 3 ; 
       FIGS. 5A to 5F  are explanatory views of adjustment in a rotational direction about a Z-axis performed based on positional relation between the light receiving face of the PD assembly of  FIG. 4  and respective beams (zero-order beam, +first-order beam, and −first-order beam), wherein  FIG. 5A to 5C  show positional relation examples, and  FIG. 5D to 5F  show adjustment directions about the Z-axis in respective positional relations shown by  FIGS. 5A to 5C ; 
       FIG. 6  is a plan view of a light receiving face of another PD assembly different from that shown in  FIG. 4 ; 
       FIG. 7  is an explanatory perspective view of a structure of an optical displacement sensor according to a second embodiment of the present invention; 
       FIG. 8  is a plan view of a light receiving face of a PD assembly shown in  FIG. 7 ; 
       FIGS. 9A to 9F  are explanatory views of adjustment in a rotational direction about a Z-axis performed based on positional relation between the light receiving face of the PD assembly of  FIG. 8  and respective beams (zero-order beam, +first-order beam, and −first-order beam), wherein  FIG. 9A to 9C  show positional relation examples, and  FIG. 9D to 9F  show adjustment direction about the Z-axis in respective positional relations shown by  FIGS. 9A to 9C ; and 
       FIG. 10  is an explanatory perspective view of a structure of an optical displacement sensor according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings. In the embodiments described below, an optical displacement sensor according to the present invention is applied to such a six-axis optical force sensor as shown in  FIG. 1 , but the present invention is not limited to application to an external force detecting device for detecting six-axis force. 
   A first embodiment of the present invention will be described with reference to  FIGS. 3 to 6 . Referring first to  FIG. 3 , an optical displacement sensor comprises: a photodiode (PD) assembly  1  as a light receiving means; a light emitting diode (LED)  2  which is a light emitting element as a light source; a light diffracting element  3  to disperse one beam into three beams; and a lens  4  to shape and condense beams. 
   In the optical displacement sensor shown in  FIG. 3 , the PD assembly  1  is mounted at one of a reference object and a measurement object, and the LED  2  is mounted at the other one thereof, at which the PD assembly  1  is not mounted, wherein light emitted from the LED  2  is received at the PD assembly  1 , and the positional displacement of the measurement object relative to the reference object with respect to two-axis directions in a plane perpendicular to the optical axis of the light emitted from the LED  2  is measured on the basis of the state of light reception at the PD assembly  1 . This applies to optical displacement sensors according to other embodiments of the present invention shown in  FIGS. 2 and 10 . 
   The light emitted from the LED  2  is dispersed by the light diffracting element  3  into three beams. Here, the three beams are respectively referred to as; a zero-order beam that is positioned at the middle; a +first-order beam that is positioned on the right when facing the light receiving face of the PD assembly  1 ; and a −first-order beam that is positioned on the left when facing the light receiving face of the PD assembly  1 . 
   The light diffracting element  3  must be preliminarily subjected to angular adjustment so that the three beams, specifically the zero-order beam, the +first-order beam, and the −first-order beam are aligned strictly in a straight line in either an X-axis or a Y-axis direction. In the present embodiment, the +first-order beam, and the −first-order beam are used for explanation, but higher-order diffracted light beams may alternatively be used. 
   The PD assembly  1  shown in  FIG. 3  has eight PD&#39;s arranged at its light receiving face (encircled by a chain line). The PD assembly  1  is shown in detail in  FIG. 4 . Referring to  FIG. 4 , the aforementioned eight PD&#39;s ( 1   a  to  1   h ) are arranged at the light receiving face of the PD assembly  1 , and light emitted from the LED  2  falls incident on the light receiving face. The PD&#39;s  1   a ,  1   b ,  1   c  and  1   d  constitute a first light receiving element group, and the PD&#39;s  1   e ,  1   f ,  1   g  and  1   h  constitute a second light receiving element group consisting of two isolated clusters sandwiching the first light receiving element group. The optical axis of the light incident on the light receiving face is oriented perpendicular to the light receiving face, and it is preferred that the center of the zero-order beam be positioned at the center of an area occupied by the PD&#39;s  1   a  to  1   d  (the first light receiving element group), the center of the +first-order beam be positioned at the center of an area occupied by the PD&#39;s  1   e  and  1   f  (one cluster of the second light receiving element group), and that the center of the −first-order beam be positioned at the center of an area occupied by the PD&#39;s  1   g  and  1   h  (the other cluster of the second light receiving element group). In the optical displacement sensor according to the first embodiment, rotational adjustment around a Z-axis (oriented perpendicular to the light receiving face of the PD assembly  1 ) is performed based on positional relation between the above-described light receiving face of the PD assembly  1  and respective beams (the zero-order, +first-order, and −first-order beams). 
   A method of rotational adjustment about the Z-axis will be described with reference to  FIGS. 5A to 5F . Referring first to  FIGS. 5A to 5C , the zero-order, −first-order beam, and +first-order beams are denoted by  2   a ,  2   b  and  2   c , respectively. And, symbols A to H in  FIGS. 5A to 5F  denote intensities of lights received at the PD&#39;s  1   a  to  1   h , respectively. In the first embodiment, the positional relation between the PD assembly  1  and the LED  2  is adjusted with respect to rotational direction about the Z-axis based on difference between the total of the intensities F and G and the total of the intensities E and H. This adjustment is preferably performed by rotating the PD assembly  1  which is structured to be rotatable about the center of the first light receiving element group, specifically the PD&#39;s  1   a  to  1   d.    
   When the zero-order beam  2   a , the −first-order beam  2   b , and the +first-order beam  2   c  impinge on the PD assembly  1  as shown by  FIG. 5A , the total intensity of F+G minus the total intensity of E+H leaves a positive value as shown by  FIG. 5D . In such a case, the PD assembly  1  is rotated in a direction so as to cancel −θ shown in  FIG. 5A . 
   When the zero-order beam  2   a , the −first-order beam  2   b , and the +first-order beam  2   c  impinge on the PD assembly  1  as shown by  FIG. 5B , the total intensity of F+G minus the total intensity of E+H leaves a zero value as shown by  FIG. 5E . This indicates that the PD assembly  1  and the LED  2  are appropriately positioned to each other with respect to the rotational direction about the Z-axis, and the PD assembly  1  does not have to be rotated in either direction. 
   When the zero-order beam  2   a , the −first-order beam  2   b , and the +first-order beam  2   c  impinge on the PD assembly  1  as shown by  FIG. 5C , the total intensity of F+G minus the total intensity of E+H leaves a negative value as shown by  FIG. 5F . In such a case, the PD assembly  1  is rotated in a direction so as to cancel +θ shown in  FIG. 5C . 
     FIG. 6  shows another PD assembly  10  for the first embodiment, which replaces the PD assembly  1  shown in  FIG. 4 . The PD assembly  10  has PD&#39;s  10   a  to  10   d , in place of the PD&#39;s  1   a  to  1   d , which constitute a first light receiving element group, PD&#39;s  11   a  to  11   d , in place of the PD&#39;s  1   e  and  1   f , which constitute one cluster of a second light receiving element group, and PD&#39;s  12   a  to  12   d , in place of the PD&#39;s  1   g  and  1   h , which constitute another cluster of the second light receiving element group. In the embodiment, the second light receiving element group may be constituted any number of PD&#39;s provided that two clusters thereof sandwiching the first light receiving element group respectively have a plurality of PD&#39;s. The PD assembly  10  and the LED  2  can be positioned to each other with respect to rotational direction about the Z-axis following the method explained with reference to  FIGS. 5A to 5F . 
   A second embodiment of the present invention will be described with reference to  FIGS. 2 ,  8 , and  9 A to  9 F. Referring to  FIG. 7 , an optical displacement sensor according to the second embodiment comprises a PD assembly  15  as a light receiving means, and a laser source  16  as a light source. Light emitted from the laser source  16  has its light intensity distribution shaped oval in cross section with two axes of symmetry crossing at right angles, and this distribution shape is leveraged into the optical displacement sensor according to the second embodiment. 
   Referring to  FIG. 8 , the PD assembly  15  is of a conventional structure, specifically has four PD&#39;s  15   a  to  15   d  arranged at its light receiving face, and light  16   a  emitted from the laser source  16  impinges on the light receiving face of the PD assembly  15 . The optical axis of the light  16   a  is oriented perpendicular to the light receiving face, and it is preferred that the center of the light  16   a  be positioned at the center of an area occupied by the PD&#39;s  15   a  to  15   d  so that respective light intensities at the PD&#39;s  15   a  to  15   d  are equal to one another. In the optical displacement sensor according to the second embodiment, rotational adjustment about a Z-axis (oriented perpendicular to the light receiving face of the PD assembly  15 ) is performed based on positional relation between the above-described light receiving face of the PD assembly  15  and the light  16   a.    
   A method of rotational adjustment about the Z-axis will be described with reference to  FIGS. 9A to 9F , wherein symbols A to D denote intensities of lights received at the PD&#39;s  15   a  to  15   d , respectively. In the second embodiment, the positional relation of the PD assembly  15  relative to the laser source  16  with respect to rotational direction about the Z-axis is adjusted based on difference between the total of the intensities A and C and the total of the intensities B and D. This adjustment is preferably performed by rotating the PD assembly  15  which is structured to be rotatable about the center of the first light receiving element group, specifically the PD&#39;s  15   a  to  15   d.    
   When the light  16   a  impinges on the PD assembly  15  as shown by  FIG. 9A , the total intensity of A+C minus the total intensity of B+D leaves a positive value as shown by  FIG. 9D . In such a case, the PD assembly  15  is rotated in a direction so as to cancel −θ shown in  FIG. 9A . 
   When the light  16   a  impinges on the PD assembly  15  as shown by  FIG. 9B , the total intensity of A+C minus the total intensity of B+D leaves a zero value as shown by  FIG. 9E . This indicates that the PD assembly  15  and the laser  16  are appropriately positioned to each other with respect to the rotational direction about the Z-axis, and the PD assembly  15  does not have to be rotated in either direction. 
   When the light  16   a  impinges on the PD assembly  15  as shown by  FIG. 9C , the total intensity of A+C minus the total intensity of B+D leaves a negative value as shown by  FIG. 9F . In such a case, the PD assembly  15  is rotated in a direction so as to cancel +θ shown in  FIG. 9C . 
   A third embodiment of the present invention will be described with reference to  FIG. 10 . An optical displacement sensor according to the third embodiment comprises a PD assembly  15  (same as employed in the second embodiment) as a light receiving means, an LED  2  (same as employed in the first embodiment) which is a light emitting element as a light source, and a cylindrical lens  17  disposed between the PD assembly  15  and the LED  2 . 
   Light emitted from the LED  2 , which originally is not shaped oval in cross section, has its cross section modified into an oval configuration with two axes of symmetry when passing through the cylindrical lens  17 , and is received at the light receiving face of the PD assembly  15 . Since the PD assembly  15  is structured in the same way as in the second embodiment, and since the light received by the PD assembly  15  has an oval cross section like in the second embodiment, rotational adjustment about the Z-axis can be performed following the method described in the second embodiment, and an explanation thereof is omitted. 
   In the foregoing embodiments, a light receiving means is mounted at one of a reference object and a measurement object, and a light source is mounted at the other one thereof, but the present invention is not limited to this structure but may be applied to a structure which is disclosed in Japanese Patent Application No. 2003-299827 by the present inventors, filed claiming priority to Japanese Patent Application No. 2003-141421, and in which both a light receiving means and a light source are mounted together at one of a reference object and a measurement object, and a reflector is mounted at the other one thereof at which the light receiving means and the light source are not mounted, wherein light emitted from the light source is reflected backward by means of two or three reflection surfaces of the reflector and received by the light receiving means, whereby the positional displacement of the measurement object relative to the reference object with respect to two-axis directions in a plane perpendicular to the optical axis of the light emitted from the light source can be measured. Further, the present invention can be applied to any optical displacement sensor with whatever structure disposed between a light receiving means and a light source. 
   In the first embodiment of the aforementioned Japanese Patent Application No. 2003-299827, as explained with reference to  FIG. 7  therein, the direction in which the two reflection surfaces of the reflector oppose each other is set to an intermediate direction oriented so as to make a 45 degree angle to both of the two axes (X-axis and Y-axis) with respect to which displacement is to be detected by an optical sensor unit (optical displacement sensor) thereby enabling the optical sensor unit to detect displacement with respect to the two axes (X- and Y-axes). Accordingly, for example, when the second or third embodiment of the present invention, which leverages the oval-shaped intensity distribution of light emitted from the light source, is applied to the optical displacement sensor employing a reflector with two reflection surfaces as described in the first embodiment of the aforementioned Japanese Patent Application No. 2003-299827, it is preferred that the two symmetry axes of the oval configuration be arranged so as to make a 45 degree angle to the two-axis directions with respect to which displacement is measured by the optical displacement sensor. In this connection, when the second or third embodiment of the present invention is applied to the optical displacement sensor employing a reflector with three reflection surfaces as described in the second embodiment of the aforementioned Japanese Patent Application No. 2003-299827, the axis arrangement described above is not required as understood from  FIG. 11  therein. 
   The present invention can be applied to measurements of various physical quantities detectable on the basis of displacement, in addition to the above-described external force detecting devices such as six-axis force sensors. 
   While the present invention has been illustrated and explained with respect to specific embodiments thereof, it is to be understood that the present invention is by no means limited thereto but encompasses all changes and modifications that will become possible within the scope of the appended claims.