Abstract:
An optical displacement sensor is provided, in which an optical fiber is disposed between a light source and a light receiving means so that light emitted from the light source is conducted therethrough so as to be duly received by the light receiving means whereby a beam diameter can be controlled and a uniform intensity distribution of emitted light can be ensured without providing a pinhole aperture. Also, a six-axis force sensor incorporating such an optical displacement sensor is provided.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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.  
         [0003]     2. Description of the Related Art  
         [0004]     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 the external force received at the action section is measured according to an output signal from the optical displacement sensor.  
         [0005]     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, and which in combination enable measurement of a six-axis direction displacement. The optical displacement sensor 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 center 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.  
         [0006]      FIG. 1  is a plan view of a main body of a conventional six-axis optical force sensor  101  as disclosed in, for example, Japanese Patent Application Laid-Open No. H03-245028. The six-axis force sensor  101  is basically composed of the aforementioned main body shaped cylindrical, and top and bottom lids which are not shown in the figure. Referring to  FIG. 1 , 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 elastically deformed for an appropriate displacement amount corresponding to a force to be measured 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 .  
         [0007]     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 displacement sensor  109 .  
         [0008]      FIG. 2  is an explanatory perspective view of the 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.  
         [0009]     As described above with reference to  FIGS. 1 and 2 , the conventional six-axis optical force sensor  101  comprises: the frame  105  which includes elastic spoke sections  104  structured so as to be elastically deformed by an applied force to be measured; and three of the optical displacement sensors  109  each of which consists of the optical sensor  108  adapted to detect the displacement according to the deformation, and the light source  106 .  
         [0010]     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.  
         [0011]     In the optical displacement sensor disclosed in the aforementioned Japanese Patent Application Laid-Open No. H03-245028, light emitted from an LED passes through a pinhole aperture provided at the front face of the LED, propagates diffusedly and impinges on an optical sensor as described above. The pinhole aperture operates to ensure a uniform intensity distribution of light emitted as well as control the diameter of a light beam. Since electrodes and wires are usually disposed toward a light emitting face of an LED tip, the light emitted from the LED is apt to incur a non-uniform intensity distribution as a whole. This is one reason the pinhole aperture adapted to ensure a uniform light intensity distribution is provided as described in the aforementioned Japanese Patent Application Laid-Open No. H03-245028. The pinhole aperture is positioned at an appropriate part of the light emitted from the LED, where a uniform light intensity distribution is secured.  
         [0012]     Such a pinhole aperture, however, requires a high accuracy of processing, and therefore invites an increased cost as well as an increased number of components. Also, such a pinhole aperture structure inevitably reduces the amount of light to impinge on an optical sensor, and in order to compensate for reduction in the amount of light to impinge on an optical sensor, an increased current must be supplied to the LED thus inviting increased electric power consumption. This increased electric power consumption leads to an increase of heat generation, which has influence on the amount of light emitted from the LED therefore resulting in deteriorating measurement accuracy. And, in connection with the increased electric power consumption, since a conventional six-axis force sensor has three light sources (see  FIG. 1 ), the problem of increase in electric power consumption is crucial.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention has been made in view of the above problems, and it is an object of the present invention to provide a six-axis force sensor, in which the diameter of a light beam can be controlled and a uniform intensity distribution of emitted light can be secured without providing a pinhole aperture structure.  
         [0014]     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 which is disposed at the other one thereof not having the light source, and which receives 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 center axis of the light emitted from the light source; and an optical fiber which is disposed between the light source and the light receiving means in such a manner as to keep its relative position steady with respect to the light source, and which conducts the light emitted from the light source so that the light can be received by the light receiving means. Consequently, a beam diameter can be duly controlled, and non-uniformity of intensity distribution of light emitted from the light source can be alleviated while the light travels through the optical fiber, thus eliminating the necessity of a pinhole aperture. Also, since light exiting out from the optical fiber has a smaller diffusing angle ( 12  degrees, for example) than light emitted from the light source such as an LED ( 120  degrees, for example), a light beam is allowed to impinge on the light receiving face of the light receiving means with a minute diameter (the distance between the light exit end of the optical fiber and the light receiving face of the light receiving means is set to about 0.5 mm), whereby the ratio of the output variation of the light receiving means to the displacement (change in reception position) amount of the light beam is increased, thus enhancing precision in detecting displacement.  
         [0015]     In the first aspect of the present invention, a lens to condense the light emitted from the light source on an entrance facet of the optical fiber may be provided between the light source and the optical fiber. Consequently, the light emitted from the light source can be used effectively, thus contributing to reduction in power consumption.  
         [0016]     In the first aspect of the present invention, the optical fiber may be a single-mode fiber. Consequently, when an optical fiber having a small diameter (for example, 10 μm) is used, a planar light source such as an LED (usually having an emission diameter of 330 μm or larger) can work as a pseudo-point light source.  
         [0017]     According to a second aspect of the present invention, an external force detecting device includes at least one optical displacement sensor structured as recited in the first aspect, in which an external force applied to the measurement object is detected based on a signal of measurement results by the optical displacement sensor. Consequently, the external force detecting device has the above-described advantages that are gained by the optical displacement sensor according to the first aspect of the present invention.  
         [0018]     In the second aspect of the present invention, a plurality of optical displacement sensors may be provided such that the two-axis directions with respect to which displacement is measured differ among the optical displacement sensors, and the plurality of optical displacement sensors may share one light source in common, with one optical fiber branching into a number equal to a number of light receiving means. Consequently, the number of light sources can be reduced to one for provision of a plurality of optical displacement sensors, which means reduction in power consumption as well as a decreased number of components.  
         [0019]     According to the present invention, since the optical fiber has a beam divergence angle (for example, 12 degrees) smaller than that of LED (for example, 120 degrees) thus allowing the light beam to be received at the light receiving face of a PD assembly with a minute diameter (the distance between the light emitting end of the optical and the light receiving face of the PD assembly is set to about 0.5 mm), the output variation ratio of the PD assemble with respect to the displacement amount (travel distance) of the light beam is increased resulting in an enhanced precision of displacement detection. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a top plan view of a main body of a conventional six-axis force sensor;  
         [0021]      FIG. 2  is an explanatory perspective view of a conventional optical displacement sensor shown in  FIG. 1 ;  
         [0022]      FIG. 3  is a perspective view of a six-axis force sensor according to a first embodiment of the present invention;  
         [0023]      FIG. 4  is a top plan view of a main body of the six-axis force sensor of  FIG. 3 ;  
         [0024]      FIG. 5  is an explanatory perspective view of one optical displacement sensor shown in  FIG. 4 ;  
         [0025]      FIG. 6  is a plan view of a light receiving face of a PD assembly shown in  FIG. 5 ;  
         [0026]      FIG. 7  is a graph showing a relation between change in position (travel distance) of a light beam at the light receiving face of the PD assembly and variation ratio of output by the PD assembly when the diameter of the light beam at the light receiving face of the PD assembly is changed;  
         [0027]      FIG. 8  is a perspective view of a six-axis force sensor according to a second embodiment of the present invention;  
         [0028]      FIG. 9  is a top plan view of a main body of the six-axis force sensor of  FIG. 6 ; and  
         [0029]      FIG. 10  is an explanatory perspective view of optical displacement sensors shown in  FIG. 7 ;  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     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.  
         [0031]     One embodiment of the present invention will hereinafter be described with reference FIGS.  3  to  7 . Referring first to  FIG. 3 , a six-axis force sensor  20  according to a first embodiment is structurally composed of a cylindrical main body  21   a,  a disk-like top lid  21   b,  and a disk-like bottom lid (not seen). Referring now to  FIG. 4 , the main body  21   a  is constituted basically by a frame  25 , which integrally includes: a cylindrical support section  22 ; an action section  23  disposed centrally inside the support section  22  and adapted to receive an external force; and three elastic spoke sections  24  crookedly structured so as to readily provide elastic deformation in all directions and supportably connecting the action section  23  to the support section  22 . The frame  25  is made of a single piece of an aluminum alloy material and shaped by cutting and electric discharge machining. The support section  22  and the action section  23  are fixed respectively to two components to which a measurement force is applied, and when the applied force acts on the six-axis force sensor  20  structured as described above, micro-displacements with respect to three-axis directions and micro-rotations with respect to rotational directions thereabout are generated between the support section  22  and the action section  23 .  
         [0032]     Referring again to  FIG. 4 , the support section  22  has three light sources (LED&#39;s, for example)  2  disposed at its inner circumference at 120 degree intervals (i.e. at an equi-angular distance), and three lenses  3  and three optical fibers  4  are arranged at 120 degree intervals (i.e. at an equi-angular distance) at positions corresponding to the three light sources  2 , respectively. The lens  3  may be, for example, an aspheric plastic lens. The optical fiber  4  is preferably put with the light source  2  and the lens  3  in an integral structure in order to keep its relative position steady with respect thereto. Meanwhile, the action section  23  has three optical sensors (light receiving elements: PD assemblies, for example)  1  disposed at 120 degree intervals (i.e. at an equi-angular distance) corresponding to the three optical fibers  4 , respectively. Each of the optical sensors  1 , the light sources  2 , the lenses  3 , and the optical fibers  4  constitute an optical displacement sensor  29 . One end (light outlet) of the optical fiber  4  is positioned to oppose the optical sensor  1 , and light emitted from the light source  2  is condensed by the lens  3 , impinges on the other end (light entrance) of the optical fiber  4 , travels therethrough, exits out from the light outlet thereof, and irradiates the center of the light receiving face of the optical sensor  1 .  
         [0033]     Referring to  FIG. 5 , each optical displacement sensor  29  according to the first embodiment comprises: a PD assembly, that is a light receiving means as the optical sensor  1 ; an LED, that is a light emitting element as the light source  2 ; the lens  3  to condense light emitted from the LED  2 ; and the optical fiber  4 , into which the light condensed by the lens  3  is introduced, and from which the light introduced exits out as a light beam  5  so as to irradiate the center of the light receiving face of the PD assembly  1 . The distance between the light outlet of the optical fiber  4  and the light receiving face of the PD assembly  1  is set to, for example, about 0.5 mm.  
         [0034]     In the optical displacement sensor  29 , the PD assembly  1  is disposed at one of a reference object and a measurement object, and the LED  2  is disposed at the other one thereof at which the PD assembly  1  is not disposed, wherein light emitted from the LED  2  is received by the PD assembly  1  via the lens  3  and the optical fiber  4  as described above, and according to the state of the light received by the PD assembly, the displacement of the measurement object relative to the reference object can be measured with respect to two-axis direction in a surface perpendicular to the center axis of the light exiting out from the optical fiber  4 . This operation is common to another embodiment to be described later with reference to  FIGS. 8, 9  and  10 .  
         [0035]      FIG. 5  shows that the light receiving face of the PD assembly  1  consists of four sections. This will be further described by referring to  FIG. 6 . As shown in  FIG. 6 , the PD assembly  1  comprises four PD&#39;s  1   a  to  1   d,  and the light beam  5  (see  FIG. 5 ) impinges on the PD&#39;s  1   a  to  1   d.  It is preferable that the center axis of the light beam  5  be perpendicular to the light receiving face of the PD assembly and be positioned at the center of the four PD&#39;s  1   a  to  1   d.    
         [0036]     A relation between the diameter of the light beam  5  and the variation of an output of the PD&#39;s  1   a  to  1   d  will be described with reference to  FIG. 7 . In  FIG. 7 , the horizontal axis represents the travel distance of the light beam  5 , and the vertical axis represents the variation ratio of the output. Specifically, the travel distance is defined by the light beam  5  traveling in the horizontal direction (in  FIG. 7 ) on the light receiving face of the PD assembly  1 , and the variation ratio of the output is defined by a formula: {(A+D)−(B+C)}/(A+B+C+D)×100% where A, B, C, and D are light intensities detected by the PD&#39;s  1   a,    1   b,    1   c  and  1   d,  respectively.  FIG. 7  shows five measurement results with the diameter of the light beam  5  set at 600 μm, 400 μm, 200 μm, 100 μm, and 50 μm, respectively.  
         [0037]     As seen from  FIG. 7 , with a smaller diameter of the light beam  5 , the output varies more sharply in response to a given amount of travel distance, namely, change in position, of the light beam  5 , thus indicating that the light beam  5  with a smaller diameter works more effectively. The diameter of the light beam  5  can be reduced by setting a small diameter on the optical fiber  4  (for example, a single-mode fiber having a diameter of 10 μm). Thus, the LED  2  as a planar light source is adapted to work as a pseudo-point light source thereby realizing a reduced diameter. Also, since light exiting out from the optical fiber  4  has a smaller diffusing angle (12 degrees, for example) than light emitted from the LED  2  ( 120  degrees, for example), a light beam is allowed to impinge on the light receiving face of the PD assembly  1  with a minute diameter (the distance between the light exit end of the optical fiber  4  and the light receiving face of the PD assembly  1  is set to about 0.5 mm).  
         [0038]     Another embodiment of the present invention will be described with reference to FIGS.  8  to  10 . Referring first to  FIG. 8 , a six-axis force sensor  30  according to a second embodiment is structurally composed of a cylindrical main body  31   a,  a disk-like top lid  31   b,  and a disk-like bottom lid (not seen). Referring now to  FIG. 9 , the main body  31   a  is constituted basically by a frame  35 , which integrally includes: a cylindrical support section  32 ; an action section  33  disposed centrally inside the support section  32  and adapted to receive an external force; and three elastic spoke sections  34  crookedly structured so as to readily provide elastic deformation in all directions and supportably connecting the action section  33  to the support section  32 . The frame  35  is made of a single piece of an aluminum alloy material and shaped by cutting and electric discharge machining. The support section  32  and the action section  33  are fixed respectively to two components to which a measurement force is applied, and when the applied force acts on the six-axis force sensor  30  structured as described above, micro-displacements with respect to three-axis directions and micro-rotations with respect to rotational directions thereabout are generated between the support section  32  and the action section  33 .  
         [0039]     Referring again to  FIG. 9 , one light source (an LED, for example)  2  is disposed at an arbitrary position of the inner circumference of the support section  22 , and one lens  3  and one optical fiber  6  are arranged at a position corresponding to the light source  2 . The lens  3  may be, for example, an aspheric plastic lens. The optical fiber  6  is trifurcated so as to have one light entrance, and three light outlets preferably set with the light source  2  and the lens  3  in an integral structure. Light condensed by the lens  3  impinges on the light entrance of the optical fiber  6 , travels therethrough, and exits out from the three light outlets. The three light outlets are arranged at 120 degree intervals (i.e. at an equi-angular distance). Meanwhile, the action section  33  has three optical sensors (light receiving elements: PD assemblies, for example)  15 ,  16  and  17  disposed at 120 degree intervals (i.e. at an equi-angular distance) corresponding respectively to the three light outlets of the trifurcated optical fiber  6 . The optical sensors  15 ,  16  and  17 , the light source  2 , the lens  3 , and the optical fiber  6  constitute a triple optical displacement sensor  39 . The three light outlets of the optical fiber  6  are positioned to oppose the optical sensors  15 ,  16  and  17 , respectively, and light emitted from the light source  2  is condensed by the lens  3 , impinges on the light entrance of the optical fiber  6 , travels therethrough, then branches into three ways, each exiting out from each of the three light outlets thereof so as to irradiate the center of the light receiving face of each of the optical sensors  15 ,  16  and  17 .  
         [0040]     Referring to  FIG. 10 , the optical displacement sensor  39  according to the second embodiment comprises: three PD assemblies, that are light receiving means as the optical sensors  15 ,  16  and  17 ; an LED, that is a light emitting element as the light source  2 ; the lens  3  to condense light emitted from the LED  2 ; and the trifurcated optical fiber  6  having one light entrance and three light outlets, wherein the light condensed by the lens  3  is introduced from the light entrance, and branches into three ways, and the branched lights exit out from respective light outlets as light beams  7 ,  8  and  9  so as to irradiate the centers of the light receiving faces of the PD assemblies  15 ,  16  and  17 . The distance between the light outlets of the optical fiber  6  and the respective light receiving faces of the PD assemblies  15 ,  16  and  17  is set to, for example, about 0.5 mm.  
         [0041]     In the second embodiment described above, a further advantage is provided that only one light source, together with one lens, is required rather than three.  
         [0042]     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.