Patent Publication Number: US-2021181177-A1

Title: Sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority from Japanese Patent Application Serial No. 2019-227654 (filed on Dec. 17, 2019), the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a sensor. 
     BACKGROUND 
     A mechanical device such as a speed reducer is housed in a housing filled with a lubricating oil in order to prevent the mechanical parts such as gears from being damaged. If the mechanical parts are worn out during operation of the mechanical device, abrasion powder (for example, a conductive substance such as iron powder) is mixed into the lubricating oil. The abrasion powder is, for example, of a conductive substance such as iron powder. As the mechanical parts are increasingly worn out and enter a wear-out failure period, which is defined in a failure rate curve (a bathtub curve), an increased amount of abrasion powder is mixed into the lubricating oil. For this reason, a sensor for sensing the amount of the abrasion powder in the lubricating oil allows for accurate preventive maintenance of the mechanical parts. 
     Such a sensor is known from and disclosed in, for example, Japanese Patent Application Publication No. 2005-331324 (“the &#39;324 Publication”). The disclosed sensor includes a cup-shaped electrode arranged around the outer periphery of a permanent magnet and an electrode including a plurality of rod-shaped conductors arranged next to each other in the circumferential direction such that the rod-shaped conductors oppose the cup-shaped electrode. This sensor is configured to check how much the oil is contaminated by detecting the amount of the metal powder that can cause a short cut between the electrodes. 
     The present applicant has also filed a patent application for the sensor disclosed in Japanese Patent Application Publication No. 2019-128311 (“the &#39;311 Publication”). 
     The configurations disclosed in the &#39;324 Publication, however, have the following disadvantages. Since the electrodes need to be arranged next to each other in the circumferential direction, the size reduction can not be accomplished beyond a certain level when the detectable amount of the metal powder is taken into consideration. Accordingly, the sensor disclosed in the &#39;324 Publication is so large that it may not be accommodated within a small-volume oil pan. In recent years, in particular, there is a demand for smaller speed reducers and the like. This results in a demand for compact sensors suitable for smaller speed reducers and the like. At the same time, sensors are required to assure reliable operation and accurate failure prediction and detection. There is a demand for sensors satisfying all of these properties. 
     In addition, the sensor disclosed in the &#39;311 Publication has a gap for detection formed between the electrodes in the radial direction. In order to prevent initial abrasion powder from causing the sensor to erroneously operate, the size of the gap can not be reduced. This creates a desire to realize a further smaller size. 
     SUMMARY 
     The present invention attempts to fulfill the objective of providing a sensor that can achieve a reduced size and high operational reliability. 
     An aspect of the present invention provides a sensor including a first electrode, a second electrode arranged relative to the first electrode with a gap being provided therebetween, a sensor body having the first and second electrodes arranged therein, and a catching portion arranged in the gap, where the catching portion has an outer peripheral surface made of an insulating material. Here, electric connection is established along the outer peripheral surface via conductive particles gathering to the catching portion. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the gap for detecting the conductive particles extends along the outer peripheral surface of the attracting portion. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. In addition, since the direction of the detection can be configured to extend along the outer peripheral surface, the sensor can be reduced in size without compromising the detection sensitivity when compared with the case where a plurality of axially extending gaps for detection are arranged next to each other in the circumferential direction of the sensor. 
     A sensor relating to one aspect of the present invention includes a first electrode, a second electrode spaced away from the first electrode with a gap being provided therebetween in a circumferential direction, and an attracting portion arranged in the gap, where the attracting portion has an outer peripheral surface made of an insulating material. Here, the first electrode, the attracting portion and the second electrode are arranged next to each other in the circumferential direction, and conductive particles are attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the circumferential direction between the first electrode and the second electrode, resulting in a change in electrical resistance between the first electrode and the second electrode. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the gap for detecting the conductive particles extends along the outer peripheral surface of the attracting portion in the circumferential direction. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. In addition, since the direction of the detection can be configured to extend in the circumferential direction along the outer peripheral surface of the attracting portion, the sensor can be reduced in size without compromising the detection sensitivity when compared with the case where a plurality of axially extending gaps for detection are arranged next to each other in the circumferential direction of the sensor. 
     The sensor relating to one aspect of the present invention may include a third electrode spaced away from the first and second electrodes with a gap being provided therebetween in the circumferential direction, and an attracting portion arranged in the gap, where the attracting portion has an outer peripheral surface made of an insulating material. Conductive particles may be attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the circumferential direction between the first electrode and the third electrode, resulting in a change in electrical resistance between the first electrode and the third electrode. 
     The sensor relating to one aspect of the present invention may include a fourth electrode spaced away from each of the first, second and third electrodes with a gap being provided therebetween in the circumferential direction, and an attracting portion arranged in the gap, where the attracting portion has an outer peripheral surface made of an insulating material. Here, conductive particles may be attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the circumferential direction between the first electrode and the fourth electrode, resulting in a change in electrical resistance between the first electrode and the fourth electrode. 
     In the sensor relating to one aspect of the present invention, outer peripheral surfaces of the first and second electrodes may form a side surface of a columnar body. 
     In the sensor relating to one aspect of the present invention, the first and second electrodes may be magnets and arranged such that magnetic flux lines extend radially outward from outer peripheral surfaces of the first and second electrodes. 
     A sensor relating to one aspect of the present invention includes a cylindrical sensor body, magnets in the sensor body, where the magnets divide the sensor body into four portions in a circumferential direction, and an attracting portion arranged such that the attracting portion fills gaps between the magnets in the sensor body. Here, the attracting portion protrudes radially outward beyond outer peripheral surfaces of the magnets, the magnets are magnetized in a radial direction, circumferentially adjacent ones of the magnets are magnetized oppositely, and the magnets serve as electrodes, and conductive particles are attracted to an outer peripheral surface of the attracting portion, so that a short circuit is caused in the circumferential direction between the magnets, resulting in a change in electrical resistance between the magnets serving as the electrodes. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the gaps for detecting the conductive particles extend along the outer peripheral surface of the attracting portion in the circumferential direction. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. In addition, since the direction of the detection can be configured to extend in the circumferential direction along the outer peripheral surface of the attracting portion, the sensor can be reduced in size without compromising the detection sensitivity when compared with the case where a plurality of axially extending gaps for detection are arranged next to each other in the circumferential direction of the sensor. 
     A sensor relating to one aspect of the present invention includes a first electrode, a second electrode and a third electrode. Here, a first catching portion is arranged between the first electrode and the second electrode, a second catching portion is arranged between the second electrode and the third electrode, and the first, second and third electrodes are arranged next to each other in an axial direction of a sensor body. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the gaps for detecting the conductive particles extend along the outer peripheral surfaces of the catching portions and are arranged next to each other in the axial direction. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. In addition, since the direction of the detection can be configured to extend in the axial direction along the outer peripheral surfaces of the attracting portions, the sensor can be reduced in size without compromising the detection sensitivity when compared with the case where a plurality of axially extending gaps for detection are arranged next to each other in the circumferential direction of the sensor. 
     A sensor relating to one aspect of the present invention includes a first electrode, a second electrode spaced away from the first electrode with a gap being provided therebetween in an axial direction, and an attracting portion arranged in the gap, where the attracting portion has an outer peripheral surface made of an insulating material. Here, the first electrode, the attracting portion and the second electrode are stacked on each other in the axial direction, and conductive particles are attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the second electrode, resulting in a change in electrical resistance between the first electrode and the second electrode. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the gap for detecting the conductive particles extends along the outer peripheral surface of the attracting portion in the axial direction. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. In addition, since the direction of the detection can be configured to extend in the axial direction along the outer peripheral surface of the attracting portion, the sensor can be reduced in size without compromising the detection sensitivity when compared with the case where a plurality of axially extending gaps for detection are arranged next to each other in the circumferential direction of the sensor. 
     The sensor relating to one aspect of the present invention may include a third electrode spaced away from the first and second electrodes with a gap being provided therebetween in the axial direction, and an attracting portion arranged in the gap, where the attracting portion has an outer peripheral surface made of an insulating material. Here, the first, second and third electrodes and the attracting portions may be stacked on each other in the axial direction, and conductive particles may be attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the third electrode, resulting in a change in electrical resistance between the first electrode and the third electrode. 
     The sensor relating to one aspect of the present invention may include a fourth electrode spaced away from each of the first, second and third electrodes with a gap being provided therebetween in the axial direction, and an attracting portion arranged in the gap, where the attracting portion has an outer peripheral surface made of an insulating material. Here, the first to fourth electrodes and the attracting portions may be stacked on each other in the axial direction, and conductive particles may be attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the fourth electrode, resulting in a change in electrical resistance between the first electrode and the fourth electrode. 
     In the sensor relating to one aspect of the present invention, outer peripheral surfaces of the first and second electrodes may form a side surface of a columnar body. 
     In the sensor relating to one aspect of the present invention, the first and second electrodes may be magnets and arranged such that magnetic flux lines extend radially outward from outer peripheral surfaces. 
     A sensor relating to one aspect of the present invention includes a cylindrical sensor body, magnets in the sensor body, the magnets dividing the sensor body into three portions in an axial direction, and attracting portions arranged such that the attracting portions fill gaps in the axial direction between the magnets in the sensor body. Here, the magnets and the attracting portions are stacked on each other in the axial direction, the attracting portions protrude radially outward beyond outer peripheral surfaces of the magnets, the magnets are radially magnetized, axially adjacent ones of the magnets are oppositely magnetized, the magnets serve as electrodes, and conductive particles are attracted to outer peripheral surfaces of the attracting portions, so that a short circuit is caused in the axial direction between the magnets, resulting in a change in electrical resistance between the magnets serving as the electrodes. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the gaps for detecting the conductive particles extend along the outer peripheral surface of the attracting portion and arranged next to each other in the axial direction. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. In addition, since the direction of the detection can be configured to extend in the axial direction along the outer peripheral surfaces of the attracting portions, the sensor can be reduced in size without compromising the detection sensitivity when compared with the case where a plurality of axially extending gaps for detection are arranged next to each other in the circumferential direction of the sensor. 
     A sensor relating to one aspect of the present invention includes a first electrode, a second electrode spaced away from the first electrode with a gap being provided therebetween in an axial direction, and a catching portion arranged in the gap, where the catching portion has an outer peripheral surface made of an insulating material. Here, the catching portion includes a plurality of catching portions having different axial lengths. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the first electrode, the attracting portion and the second electrode spaced away from each other in the axial direction and stacked on each other form a gap for detecting conductive particles, which extends along the outer peripheral surface of the attracting portion in the axial direction. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. Furthermore, the attraction of the conductive particles is tuned by using a plurality of catching portions having different lengths in the axial direction. With this configuration, even when a large amount of conductive particles is attracted, the detection sensitivity of the sensor can be adjusted depending on the attraction of the conductive particles, so that the sensing can be reliably performed. In particular, when the sensor is placed in a large-size speed reducer or the like and a large amount of conductive particles is thus produced during the initial stage, the sensor can be configured such that the attraction of the conductive particles is limited or the sensing scheme is changed if a large amount of conductive particles is attracted. This enables the sensor to reliably perform the sensing. 
     A sensor relating to one aspect of the present invention includes a first electrode, a second electrode spaced away from the first electrode with a gap being provided therebetween in an axial direction, and an attracting portion arranged in the gap, where the attracting portion has an outer peripheral surface made of an insulating material. Here, the first electrode, the attracting portion and the second electrode are stacked on each other in the axial direction, and conductive particles are attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the second electrode, resulting in a change in electrical resistance between the first electrode and the second electrode. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the first electrode, the attracting portion and the second electrode spaced away from each other in the axial direction and stacked on each other form a gap for detecting conductive particles, which extends along the outer peripheral surface of the attracting portion in the axial direction. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. 
     The sensor relating to one aspect of the present invention may include a third electrode spaced away from the first electrode with a gap being provided therebetween in the axial direction and spaced away from the second electrode with a gap being provided therebetween in the axial direction, and attracting portions arranged respectively in the gaps, where the attracting portions have an outer peripheral surface made of an insulating material. The first electrode, the attracting portion and the third electrode may be stacked on each other in the axial direction, and conductive particles may be attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the third electrode, resulting in a change in electrical resistance between the first electrode and the third electrode. 
     The sensor relating to one aspect of the present invention may include a fourth electrode spaced away from the first electrode with a gap being provided therebetween in the axial direction and spaced away from the second and third electrodes with a gap being provided therebetween in the axial direction, and attracting portions arranged in the gaps, where the attracting portions have an outer peripheral surface made of an insulating material. Conductive particles may be attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the fourth electrode, resulting in a change in electrical resistance between the first electrode and the fourth electrode. 
     In the sensor relating to one aspect of the present invention, the second, fourth and third electrodes may be spaced away from each other in the circumferential direction, and attracting portions may be arranged in gaps between the electrodes. 
     In the sensor relating to one aspect of the present invention, outer peripheral surfaces of the first and second electrodes may form a side surface of a columnar body. 
     In the sensor relating to one aspect of the present invention, a magnet may be positioned closer in the axial direction to the first electrode at least than to the second electrode, and the magnet may be arranged to form a magnetic flux line extending in the axial direction. 
     In the sensor relating to one aspect of the present invention, the first electrode may be a magnet. 
     A sensor relating to one aspect of the present invention includes a cylindrical sensor body. The sensor body includes a first electrode, an attracting portion and a second electrode stacked on each other in an axial direction, and a magnet magnetized in the axial direction. Conductive particles are attracted to an outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the outer peripheral surface of the second electrode, resulting in a change in electrical resistance between the first electrode and the second electrode. With such configurations, the above-mentioned objective is accomplished. 
     According to the sensor relating to one aspect of the present invention, the first electrode, the attracting portion and the second electrode spaced away from each other in the axial direction and stacked on each other form a gap for detecting conductive particles, which extends along the outer peripheral surface of the attracting portion in the axial direction. In this way, when compared with the case where the gap for detecting the conductive particles extends in the radial direction of the end surface of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. 
     In the sensor relating to one aspect of the present invention, the second electrode may be divided into portions in a circumferential direction of the sensor body. 
     Advantageous Effects 
     A sensor relating to one aspect of the present invention can produce the following effects. In the sensor body shaped like a columnar body, the gap for detection extending along the outer peripheral surface is positioned in an outermost manner. In this way, the sensing can be still performed accurately and the sensor can be reduced in size without compromising the sensitivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing one example of a mechanical device including a sensor relating to a first embodiment of the present invention. 
         FIG. 2  is a perspective view showing the sensor relating to the first embodiment of the present invention. 
         FIG. 3  is a sectional view showing the magnetic flux line direction in the sensor relating to the first embodiment of the present invention. 
         FIG. 4  shows how the electrodes are arranged in the sensor relating to the first embodiment of the present invention. 
         FIG. 5  is a sectional view showing how detection is performed in the sensor relating to the first embodiment of the present invention. 
         FIGS. 6A and 6B  show another example of the magnets included in the sensor relating to the first embodiment of the present invention. 
         FIG. 7  is a perspective view showing another example of the sensor relating to the first embodiment of the present invention. 
         FIG. 8  is a perspective view showing a sensor relating to a second embodiment of the present invention. 
         FIG. 9  is a sectional view showing the magnetic flux line direction in the sensor relating to the second embodiment of the present invention. 
         FIG. 10  shows a section orthogonal to the axial direction in order to illustrate another example of the sensor relating to the second embodiment of the present invention. 
         FIG. 11  shows a section along the axial direction in order to illustrate another example of the sensor relating to the second embodiment of the present invention. 
         FIG. 12  is a sectional view taken along the axial direction and showing a sensor relating to a third embodiment of the present invention. 
         FIG. 13  shows magnets included in the sensor relating to the third embodiment of the present invention. 
         FIG. 14  shows how the electrodes are arranged in the sensor relating to the third embodiment of the present invention. 
         FIG. 15  is a sectional view showing how detection is performed in the sensor relating to the third embodiment of the present invention. 
         FIG. 16  shows a section along the axial direction in order to illustrate another example of the sensor relating to the third embodiment of the present invention. 
         FIG. 17  is a perspective view showing a sensor relating to a fourth embodiment of the present invention. 
         FIG. 18  shows a section along the axial direction in order to illustrate another example of the sensor relating to the fourth embodiment of the present invention. 
         FIG. 19  is an exploded perspective view showing another example of a sensor relating to a fifth embodiment of the present invention. 
         FIG. 20  is an exploded perspective view showing another example of the sensor relating to the fifth embodiment of the present invention. 
         FIG. 21  shows a section orthogonal to the axial direction in order to illustrate another example of the sensor relating to the fifth embodiment of the present invention. 
         FIG. 22  is a sectional view taken along the axial direction and showing how the electrodes are arranged in a sensor relating to a sixth embodiment of the present invention. 
         FIG. 23  is a sectional view taken along the axial direction and showing a sensor relating to a seventh embodiment of the present invention. 
         FIG. 24  shows magnets included in the sensor relating to the seventh embodiment of the present invention. 
         FIG. 25  is a sectional view taken along the axial direction and showing a sensor relating to an eighth embodiment of the present invention. 
         FIG. 26  shows how the electrodes are arranged in the sensor relating to the eighth embodiment of the present invention. 
         FIG. 27  shows a section along the axial direction in order to illustrate another example of a sensor relating to a ninth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes a sensor relating to a first embodiment of the present invention with reference to the drawings.  FIG. 1  is a sectional view showing one example of a mechanism including the sensor relating to the first embodiment of the present invention. In  FIG. 1 , the reference numeral  1  indicates the mechanism. The constituents common to more than one drawing are denoted by the same reference signs throughout the drawings. It should be noted that components in the drawings are not necessarily drawn to scale for the sake of convenience of description. 
     The mechanism  1  relating to the first embodiment is, for example, a mobile part such as a robot arm. The mechanism  1  includes a speed reducer  2 , a flange  3  provided on the input side, a servomotor  4 , and a device A 1  provided on the output side, as shown in  FIG. 1 . 
     The speed reducer  2  includes a casing  2   a  mounted to the flange  3 , an input shaft  2   c  connected to an output shaft  2   b  of the servomotor  4 , and an output shaft  2   d  connected to the output-side device A 1 . The input shaft  2   c  and the output shaft  2   d  are supported such that it is capable of rotating about an axis AX relative to the casing  2   a . The output from the servomotor  4  is input to the speed reducer  2  via the input shaft  2   c , reduced in speed by the speed reducer  2 , and then transmitted to the output-side device A 1  via the output shaft  2   d . Thus, the output-side device A 1  and the flange  3  are capable of rotating relative to each other. 
     The flange  3  is a tubular member and houses therein at least a portion of the speed reducer  2 . The servomotor  4  is mounted to the flange  3 . An opening of the flange  3  at one end thereof in the direction along the axis AX is closed by the speed reducer  2 , and an opening of the flange  3  at the other end thereof is closed by the servomotor  4 . Thus, the flange  3  has a tightly closed hollow portion (a space S) formed therein. The space S contains therein a lubricating oil, so that the flange  3  also serves as an oil bath. 
     The casing  2   a  of the speed reducer  2  houses therein a gear mechanism, for example. The space within the casing  2   a  communicates with the space S within the flange  3 . As the speed reducer  2  operates, the gear mechanism in the casing  2   a  rotates, which subsequently causes the lubricating oil to circulate between the space in the casing  2   a  and the space S in the flange  3 . As the lubricating oil circulates, conductive particles mp (see  FIG. 5 ) such as abrasion powder (conductive abrasion powder) produced in the speed reducer  2  moves into the space S in the flange  3 . 
     In the space S, a sensor  10  is installed for sensing the amount of the conductive particles mp contained in the lubricating oil. The sensor  10  is fixed onto the flange  3  via, for example, a support member  2   e . The sensor  10  uses magnets to gather the conductive particles (iron powder) mp, which are contained in the lubricating oil, between paired electrodes and uses a change in electrical resistance between the paired electrodes to sense the amount of the conductive particles mp in the lubricating oil. The sensor  10  may be alternatively positioned, for example, inside the casing  2   a  but can be at any position in the mechanism  1  as long as the position is within the space containing therein the lubricating oil. 
     Next, with reference to  FIGS. 2 to 4 , a detailed description is given of the sensor relating to the present embodiment.  FIG. 2  is a perspective view showing the sensor relating to the present embodiment.  FIG. 3  is an end view showing the sensor relating to the present embodiment.  FIG. 4  shows how the electrodes are arranged in the sensor relating to the present embodiment. In  FIG. 2 , the reference numeral  10  denotes the sensor. 
     As shown in  FIG. 2 , the sensor  10  has a substantially cylindrical outer shape (shaped like a columnar body) around an axial line axd. The sensor  10  includes a first electrode  11 , a second electrode  12 , a third electrode  13 , a fourth electrode  14  and an attracting portion (catching portion)  15 . 
     An end surface  10   a  of the sensor  10  has a substantially circular shape extending in the direction orthogonal to the axial line axd. The first, second, third and fourth electrodes  11 ,  12 ,  13  and  14  each have a substantially fan-shaped sectional shape with a quadrant, when seen through the end surface  10   a  of the sensor  10 . The first, second, third and fourth electrodes  11 ,  12 ,  13  and  14  all have substantially the same shape. 
     The first, second, third and fourth electrodes  11 ,  12 ,  13  and  14  are arranged such that they are rotated around the axial line axd. Accordingly, the first to fourth electrodes  11  to  14  are symmetrically arranged around the axial line axd. The electrodes  11  to  14  are arranged in the order of the first electrode  11 , the second electrode  12 , the fourth electrode  14 , and the third electrode  13 , clockwise in the circumferential direction rtd when seen through the end surface  10   a.    
     The first, second, fourth and third electrodes  11 ,  12 ,  14  and  13  are all at the same position in the direction along the axial line axd. The first, second, fourth and third electrodes  11 ,  12 ,  14  and  13  are arranged such that their respective outer peripheral surfaces  11   a  to  14   a  form the same or flush cylindrical surface. 
     The first, second, third and fourth electrodes  11 ,  12 ,  13  and  14  all have the same length in the direction along the axial line axd. The first, second, third and fourth electrodes  11 ,  12 ,  13  and  14  are spaced away from each other in the direction extending in the end surface  10   a . Gaps G 1  to G 4  are formed between the electrodes  11  to  14 . 
     The gap G 1  denotes the spacing distance in the circumferential direction rtd between the first electrode  11  and the second electrode  12 . The gap G 2  denotes the spacing distance in the circumferential direction rtd between the first electrode  11  and the third electrode  13 . The gap G 3  denotes the spacing distance in the circumferential direction rtd between the second electrode  12  and the fourth electrode  14 . The gap G 4  denotes the spacing distance in the circumferential direction rtd between the third electrode  13  and the fourth electrode  14 . 
     The first, second, third and fourth electrodes  11 ,  12 ,  13  and  14  are all magnets. The magnets are, for example, permanent magnets. The first, second, third and fourth electrodes  11 ,  12 ,  13  and  14  are all magnetized in the radial direction of the sensor  10 . 
     The first electrode  11  and the fourth electrode  14 , which are symmetrically positioned with respect to the axial line axd, are magnetized such that their respective outer peripheral surfaces  11   a  and  14   a  are like poles. The second electrode  12  and the third electrode  13 , which are symmetrically positioned with respect to the axial line axd, are magnetized such that their respective outer peripheral surfaces  12   a  and  13   a  are like poles. Here, the electrodes  11  to  14  are magnetized such that the adjacent ones of the electrodes  11  to  14  have different polarities. 
     For example, as shown in  FIG. 3 , the second electrode  12  and the third electrode  13  are magnetized such that their respective outer peripheral surfaces  12   a  and  13   a  are the N pole. The first electrode  11  and the fourth electrode  14  are magnetized such that their respective outer peripheral surfaces  11   a  and  14   a  are the S pole. The direction of magnetization can be reversed in all of the electrodes  11  to  14 . 
     Arranged in the above described manner, the electrodes  11  to  14 , which are formed by magnets, are attracted to each other by their magnetic force. As a result, the electrodes  11  to  14  can be secured onto the cross-shaped attracting portion  15  without the use of an adhesive portion such as an adhesive agent. 
     Between the electrodes  11  to  14 , the attracting portion  15  is arranged. The attracting portion  15  is made of an insulating non-magnetic material, for example, resin. The attracting portion  15  has a section shaped like a cross when seen through the end surface  10   a  along the axial line axd. 
     The attracting portion  15  has an attracting protrusion  15 G 1  filling the gap G 1  between the first electrode  11  and the second electrode  12  and protruding radially outward beyond the outer peripheral surfaces  11   a ,  12   a . The attracting portion  15  has an attracting protrusion  15 G 2  filling the gap G 2  between the first electrode  11  and the third electrode  13  and protruding radially outward beyond the outer peripheral surfaces  11   a ,  13   a.    
     The attracting portion  15  has an attracting protrusion  15 G 3  filling the gap G 3  between the second electrode  12  and the fourth electrode  14  and protruding radially outward beyond the outer peripheral surfaces  12   a ,  14   a . The attracting portion  15  has an attracting protrusion  15 G 4  filling the gap G 4  between the third electrode  13  and the fourth electrode  14  and protruding radially outward beyond the outer peripheral surfaces  13   a ,  14   a.    
     The attracting protrusions  15 G 1  to  15 G 4  are formed such that they all have the same protruding height in the radial direction. Alternatively, the attracting protrusions  15 G 1  to  15 G 4  may be formed such that they have any, for example, different protruding heights in the radial direction. By adjusting the protruding heights of the attracting protrusions  15 G 1  to  15 G 4  in the radial direction, the sensitivity of the detection, described below, can be tuned. 
     The size, in the circumferential direction rtd, of the gaps G 1  to G 4  between the electrodes  11  to  14  is larger than the size of the conductive substance that can be contained in the lubricating oil. For example, the conductive substance has a size of approximately 1.0 μm to 100 μm. The gaps G 1  to G 4  are preferably arranged at such intervals that no short circuit is created by the iron powder resulting from initial wear period. The gaps G 1  to G 4  all have the same size in the circumferential direction rtd. 
     Between the electrodes  11  to  14 , which are magnets, magnetic flux lines run radially outside the attracting protrusions  15 G 1  to  15 G 4  to connect together the electrodes  11  to  14 , as shown in  FIG. 3 . Between the first electrode  11  and the second electrode  12 , magnetic flux lines start from the outer peripheral surface  12   a  of the second electrode  12 , which is the N pole, run radially outside the attracting protrusion  15 G 1 , and proceed toward the outer peripheral surface  11   a  of the first electrode  11 , which is the S pole. Between the first electrode  11  and the third electrode  13 , magnetic flux lines start from the outer peripheral surface  13   a  of the third electrode  13 , which is the N pole, run radially outside the attracting protrusion  15 G 2 , and proceed toward the outer peripheral surface  11   a  of the first electrode  11 , which is the S pole. 
     Between the fourth electrode  14  and the second electrode  12 , magnetic flux lines start from the outer peripheral surface  12   a  of the second electrode  12 , which is the N pole, run radially outside the attracting protrusion  15 G 3 , and proceed toward the outer peripheral surface  14   a  of the fourth electrode  14 , which is the S pole. Between the fourth electrode  14  and the third electrode  13 , magnetic flux lines start from the outer peripheral surface  13   a  of the third electrode  13 , which is the N pole, run radially outside the attracting protrusion  15 G 4 , and proceed toward the outer peripheral surface  14   a  of the fourth electrode  14 , which is the S pole. 
     The electrodes  11  to  14  can have a non-magnetic conductor portion at an end thereof opposite the end surface  10   a . If such is the case, in the electrodes  11  to  14 , the magnet and the conductor portion are in contact with each other so that the electrodes  11  to  14  are electrically conductive.  FIG. 2  shows the conductor portion  12   b  of the second electrode  12 . 
     Output lines are connected to the first, second, third and fourth electrodes  11 ,  12 ,  13  and  14 . The first, second, third and fourth electrodes  11 ,  12 ,  13  and  14  are respectively electrically connected to a sensing unit  5  (see  FIG. 1 ) via the output lines. The electrodes  11  to  14  are insulated from each other. As shown in  FIG. 4 , the first electrode  11  and one of the other electrodes  12  to  14  form a pair of electrodes, and the attracting portion  15  arranged between the paired electrodes constitutes a single detecting unit together with the pair of electrodes. In  FIG. 4 , the electrode pairs constituting the detecting units are denoted by assigning the sign “+” to the output line of the first electrode  11  and “−” to the output lines of the other electrodes  12  to  14 . 
       FIG. 5  is a sectional view showing how detection is performed in the sensor relating to the present embodiment. In the present embodiment, the sensor  10  includes three detecting units, corresponding to the second, third and fourth electrodes  12 ,  13  and  14 . There are no particular limitations on the number of the electrodes  12  to  14  and the number of the detecting units. Since the electrodes  11  to  14  of the sensor  10 , which are magnets, produce magnetic flux lines between the paired ones of the electrodes  11  to  14 , the conductive particles (abrasion powder) mp contained in the lubricating oil are attracted to the attracting portion  15 , as shown in  FIG. 5 . Since the magnetic flux lines formed by the electrodes  11  to  14  start from the position immediately close to the attracting portion  15  and run radially, the conductive particles mp can be attracted highly efficiently. If the conductive particles mp are gathered in the vicinity of the attracting portion  15  in this manner, the detecting units experience a change in electrical resistance. While no conductive particles (abrasion powder) mp are attracted, the detecting units may exhibit the same electrical resistance. 
     In the present embodiment, the detecting units corresponding to the second and third electrodes  12  and  13  are connected to each other in parallel. Between the first electrode  11  and the second electrode  12 , voltage is applied by the same voltage source. Between the first electrode  11  and the third electrode  13 , voltage is applied by the same voltage source. If the conductive particles mp are gathered in the vicinity of the attracting protrusion  15 G 1 , the detecting unit corresponding to the second electrode  12  experiences a change in electrical resistance. If the conductive particles mp are gathered in the vicinity of the attracting protrusion  15 G 2 , the detecting unit corresponding to the third electrode  13  experiences a change in electrical resistance. 
     The sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  11  and the second electrode  12 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusion  15 G 1 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of the attracting protrusion  15 G 1 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  11  and the second electrode  12  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     Likewise, the sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  11  and the third electrode  13 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusion  15 G 2 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of the attracting protrusion  15 G 3 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  11  and the third electrode  13  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     Between the first electrode  11  and the fourth electrode  14 , voltage is applied by the same voltage source. If the conductive particles mp are gathered in the vicinity of both of the attracting protrusions  15 G 1  and  15 G 3 , the detecting unit corresponding to the fourth electrode  14  experiences a change in electrical resistance. Alternatively, if the conductive substance is gathered in the vicinity of both of the attracting protrusions  15 G 2  and  15 G 4 , the detecting unit corresponding to the fourth electrode  14  experiences a change in electrical resistance. 
     The sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  11  and the fourth electrode  14 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusions  15 G 1  and  15 G 3 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of both of the attracting protrusions  15 G 1  and  15 G 3 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  11  and the fourth electrode  14  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . This prediction is not made possible until the conductive particles mp are gathered in the vicinity of both of the attracting protrusions  15 G 1  and  15 G 3 . 
     Likewise, the sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusions  15 G 2  and  15 G 4 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of both of the attracting protrusions  15 G 2  and  15 G 4 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  11  and the fourth electrode  14  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . This prediction is not made possible until the conductive particles mp are gathered in the vicinity of both of the attracting protrusions  15 G 2  and  15 G 4 . 
     As described above, the change in electrical resistance experienced by the detecting unit corresponding to the second electrode  12  and the detecting unit corresponding to the third electrode  13  is used to detect a change in electrical resistance in one attracting protrusion, which is selected from the attracting protrusions  15 G 1  and  15 G 2 . On the other hand, the change in electrical resistance experienced by the detecting unit corresponding to the fourth electrode  14  is used to detect a change in electrical resistance in two attracting protrusions, which are either the attracting protrusions  15 G 1  and  15 G 3 , or the attracting protrusions  15 G 2  and  15 G 4 . This means that the plurality of detecting units are capable of sensing different conditions. In other words, the detecting units are configured to detect a change in electrical resistance in two stages. The detecting units can perform the sensing in two stages and two systems. Accordingly, the reliability of the failure prediction can be improved. 
     The sensing unit  5  outputs a signal when a designated one of the detecting units experiences a change in electrical resistance. For example, the sensing unit  5  may be configured to output a signal to a higher-level control device such as a manipulator when two or more of the detecting units experience a drop in electrical resistance, or configured to output a signal when all of the detecting units experience a drop in electrical resistance. 
     The drop in electrical resistance may be indicated by an ON signal and an OFF signal corresponding to electrical disconnection and connection. The sensing unit  5  may sense two states of electrical disconnection and connection (hereinafter, may be referred to as “perform digital sensing”). The sensing unit  5  may be connected to a higher-level control device (not shown) such as a manipulator in a wired or wireless manner. The higher-level control device may be configured to, upon reception of a signal from the sensing unit  5 , issue an alert for demanding maintenance of, for example, the speed reducer  2  with a predetermined notifying device (for example, a display or voice output device). 
     As described above, the sensor  10  of the present embodiment includes the plurality of detecting units, and the sensing unit  5  outputs a signal when designated one or more of the detecting units experience a drop in electrical resistance. In this way, the sensing unit  5  can be configured to output no signal when just one of the detecting units experiences a change in electrical resistance caused by initial abrasion powder but the other detecting units do not experience a change in electrical resistance. Accordingly, the sensor can be prevented from operating unexpectedly. Furthermore, in the sensor  10 , the sensing unit  5  can be configured to output a signal under a designated condition. Therefore, the single sensor  10  can be configured to output a signal in a timely and optimal manner for individual users, who have different requests for failure prediction timing. 
     While no conductive particles mp are attracted, the detecting units can exhibit the same electrical resistance. This can lower the voltage to be applied to the sensor  10 . The detecting units are connected in parallel to each other. This can lower the voltage applied between the paired electrodes in each detecting unit. The sensor  10  is preferably positioned such that the first electrode  11 , which forms the detecting unit with every one of the other electrodes  12  to  14 , deals with a large amount of conductive particles mp. 
     According to the sensor  10  relating to the present embodiment, the gaps G 1  to G 4  for detecting the conductive particles mp are arranged next to each other in the circumferential direction rtd along the outer peripheral surfaces  11   a  to  14   a . In this way, when compared with the case where such gaps for detecting conductive particles are arranged next to each other in the radial direction on the end surface  10   a  of the sensor  10 , the sensor  10  can be reduced in size without compromising the detection sensitivity. In addition, the direction of the detection performed by the detecting units can be configured to extend in the circumferential direction rtd along the outer peripheral surfaces  11   a  to  14   a . Accordingly, when compared with the case where a plurality of detection gaps extending in the axial line axd direction are arranged next to each other in the circumferential direction of the sensor, the sensor  10  can be reduced in size without compromising the detection sensitivity. Furthermore, since the magnetic flux lines produced by the electrodes  11  to  14 , which are magnets, start from the position immediately close to the attracting portion  15  and run radially, the conductive particles mp can be attracted highly efficiently. Thus, the size reduction can not result in lower attraction efficiency. Furthermore, the sensor  10  of the present embodiment can be constituted by a reduced number of parts, assembled easily and manufactured at a reduced cost. 
       FIGS. 6A and 6B  show another example of the magnets included in the sensor relating to the present embodiment. In the sensor  10  relating to the present embodiment, the magnets forming the electrodes  11  to  14  have a section shaped like a fan. As shown in  FIGS. 6A and 6B , however, the magnets forming the electrodes  11  to  14  can have a section shaped like a circumferential quarter of a ring. In this case, all of the electrodes  11  to  14  also have a direction of magnetization extending in the radial direction of the sensor  10 . The second electrode  12  and the third electrode  13  are magnetized such that their respective outer peripheral surfaces  12   a  and  13   a  are like poles. The first electrode  11  and the fourth electrode  14  are magnetized such that their respective outer peripheral surfaces  11   a  and  14   a  are like poles. Here, the electrodes  11  to  14  are magnetized such that the adjacent ones of the electrodes  11  to  14  are different poles. 
     Arranged in the above-described manner, the electrodes  11  to  14 , which are formed by magnets, are attracted to each other by their magnetic force. As a result, the electrodes  11  to  14  can be secured onto the cross-shaped attracting portion  15  without the use of an adhesive portion such as an adhesive agent. 
       FIG. 7  is a perspective view showing another example of the sensor relating to the present embodiment. In the sensor  10  relating to the present embodiment, the output lines extending from the electrodes  11  to  14  can be formed by using flexible substrates  11   f  to  14   f , as shown in  FIG. 7 . 
     The output line connected to the first electrode  11  is formed by the flexible substrate  11   f , which is interposed between the first electrode  11  and the attracting portion  15 . A portion of the flexible substrate  11   f  that is in contact with the first electrode  11  is electrically conductive as, for example, coating has been removed and thus in electrical communication with the first electrode  11 . In addition, since the electrodes  11  to  14 , which are formed by magnets, are attracted to each other due to their magnetic force, the flexible substrate  11   f  is secured while being sandwiched between the first electrode  11  and the attracting portion  15 . 
     As a result, the flexible substrate  11   f  can be fixedly connected to the first electrode  11  without the use of an adhesive agent or the like and with the electrical connection being maintained. In the same manner, in the sensor  10  relating to the present embodiment, the flexible substrate  12   f  is fixedly connected to the second electrode  12 , the flexible substrate  13   f  is fixedly connected to the third electrode  13 , and the flexible substrate  14   f  is fixedly connected to the fourth electrode  14 . 
       FIG. 7  shows that the flexible substrate  11   f  is interposed between the first electrode  11  and the attracting protrusion  15 G 2 , but the flexible substrate  11   f  can be alternatively interposed between the first electrode  11  and the attracting protrusion  15  G 1 . 
     The following describes a sensor relating to a second embodiment of the present invention with reference to the drawings.  FIG. 8  is a perspective view showing the sensor relating to the present embodiment.  FIG. 9  is an end view showing the sensor relating to the present embodiment. In  FIG. 8 , the reference numeral  20  denotes the sensor. The second embodiment is different from the above-described first embodiment in terms of the outer shape of the sensor and magnets. The constituents of the mechanism  1  illustrated in  FIG. 1  are not described here. 
     As shown in  FIG. 8 , the sensor  20  has a substantially prismatic outer shape (shaped like a column) around an axial line axd. The sensor  20  includes a first electrode  21 , a second electrode  22 , a third electrode  23 , a fourth electrode  24  and an attracting portion (catching portion)  25 . 
     An end surface  20   a  of the sensor  20  has a substantially rectangular shape extending in the direction orthogonal to the axial line axd. The first, second, third and fourth electrodes  21 ,  22 ,  23  and  24  form side surfaces of a prism. The first, second, third and fourth electrodes  21 ,  22 ,  23  and  24  have a section substantially shaped like a rectangle, when seen through the end surface  20   a  of the sensor  20 . The electrodes  21  to  24  are shaped like a rectangular flat plate. The first, second, third and fourth electrodes  21 ,  22 ,  23  and  24  all have substantially the same shape. 
     The first, second, third and fourth electrodes  21 ,  22 ,  23  and  24  are arranged such that they are rotated around the axial line axd. Accordingly, the first to fourth electrodes  21  to  24  are symmetrically arranged around the axial line axd. The electrodes  21  to  24  are arranged in the order of the first electrode  21 , the second electrode  22 , the fourth electrode  24 , and the third electrode  23 , clockwise in the circumferential direction rtd when seen through the end surface  20   a.    
     The first, second, fourth and third electrodes  21 ,  22 ,  24  and  23  are all at the same position in the direction along the axial line axd. The first, second, fourth and third electrodes  21 ,  22 ,  24  and  23  are arranged such that their respective outer peripheral surfaces  21   a  to  24   a  are at the same distance from the axial line axd and form the sides of square section. 
     The first, second, third and fourth electrodes  21 ,  22 ,  23  and  24  all have the same length in the direction along the axial line axd. The first, second, third and fourth electrodes  21 ,  22 ,  23  and  24  are spaced away from each other in the direction extending in the end surface  20   a . Gaps G 1  to G 4  are formed between the electrodes  21  to  24 . 
     The gap G 1  denotes the spacing distance in the circumferential direction rtd between the first electrode  21  and the second electrode  22 . The gap G 2  denotes the spacing distance in the circumferential direction rtd between the first electrode  21  and the third electrode  23 . The gap G 3  denotes the spacing distance in the circumferential direction rtd between the second electrode  22  and the fourth electrode  24 . The gap G 4  denotes the spacing distance in the circumferential direction rtd between the third electrode  23  and the fourth electrode  24 . 
     The first, second, third and fourth electrodes  21 ,  22 ,  23  and  24  are all magnets. The magnets are, for example, permanent magnets. In all of the first, second, third and fourth electrodes  21 ,  22 ,  23  and  24 , the direction of magnetization extends in the radial direction of the sensor  20 . In other words, the electrodes  21  to  24  are thin plate-shaped magnets magnetized such that the front surface, which is the principal surface of the plate, and the back surface are different poles. 
     The first electrode  21  and the fourth electrode  24 , which are symmetrically positioned with respect to the axial line axd and oppose each other, are magnetized such that their respective outer peripheral surfaces  21   a  and  24   a  are like poles. The second electrode  22  and the third electrode  23 , which are symmetrically positioned with respect to the axial line axd and oppose each other, are magnetized such that their respective outer peripheral surfaces  22   a  and  23   a  are like poles. Here, the electrodes  21  to  24  are magnetized such that the adjacent ones of the electrodes  21  to  24  are different poles. 
     For example, as shown in  FIG. 9 , the second electrode  22  and the third electrode  23  are magnetized such that their respective outer peripheral surfaces  22   a  and  23   a  are N poles. The first electrode  21  and the fourth electrode  24  are magnetized such that their respective outer peripheral surfaces  21   a  and  24   a  are S poles. The direction of magnetization can be reversed in all of the electrodes  21  to  24 . 
     Arranged in the above-described manner, the electrodes  21  to  24 , which are magnets, are arranged such that the N-pole surfaces are opposed and parallel to each other and the S-pole surfaces are opposed and parallel to each other. Accordingly, the electrodes  21  to  24  are attracted to each other by their magnetic force. As a result, the electrodes  21  to  24 , which are magnets, can be secured onto a center portion  25   c  of the cross-shaped attracting portion  25  without the use of an adhesive portion such as an adhesive agent. 
     Between the electrodes  21  to  24 , the attracting portion  25  is arranged. The attracting portion  25  is made of an insulating non-magnetic material, for example, resin. The attracting portion  25  has a section shaped like a cross when seen through the end surface  20   a  along the axial line axd, and, in the attracting portion  25 , the center portion  25   c  is larger than the radial end. Here, the center portion  25   c  is shaped like a prism. The center portion  25   c  fills the space between the electrodes  21  to  24 . 
     The attracting portion  25  has an attracting protrusion  25 G 1  filling the gap G 1  between the first electrode  21  and the second electrode  22  and protruding radially outward beyond the outer peripheral surfaces  21   a ,  22   a . The attracting portion  25  has an attracting protrusion  25 G 2  filling the gap G 2  between the first electrode  21  and the third electrode  23  and protruding radially outward beyond the outer peripheral surfaces  21   a ,  23   a.    
     The attracting portion  25  has an attracting protrusion  25 G 3  filling the gap G 3  between the second electrode  22  and the fourth electrode  24  and protruding radially outward beyond the outer peripheral surfaces  22   a ,  24   a . The attracting portion  25  has an attracting protrusion  25 G 4  filling the gap G 4  between the third electrode  23  and the fourth electrode  24  and protruding radially outward beyond the outer peripheral surfaces  23   a ,  24   a.    
     When seen in the section transverse the direction along the axial line axd, the attracting protrusions  25 G 1  to  25 G 4  are shaped as radially outward extensions of the diagonals of the rectangle (square) formed by the electrodes  21  to  24 . The attracting protrusions  25 G 1  to  25 G 4  are formed such that they all have the same protruding height in the radial direction. Alternatively, the attracting protrusions  25 G 1  to  25 G 4  may be formed such that they have any, for example, different protruding heights in the radial direction. By adjusting the protruding heights of the attracting protrusions  25 G 1  to  25 G 4  in the radial direction, the sensitivity of the detection, described below, can be tuned. 
     The size, in the circumferential direction rtd, of the gaps G 1  to G 4  between the electrodes  21  to  24  is larger than the size of the conductive substance that can be contained in the lubricating oil. For example, the conductive substance has a size of approximately 1.0 μm to 100 μm. The gaps G 1  to G 4  are preferably arranged at such intervals that no short circuit is created by the iron powder resulting from initial wear period. The gaps G 1  to G 4  all have the same size in the circumferential direction rtd. 
     Between the electrodes  21  to  24 , which are magnets, magnetic flux lines run radially outside the attracting protrusions  25 G 1  to  25 G 4  to connect the electrodes  21  to  24 , as shown in  FIG. 9 . Between the first electrode  21  and the second electrode  22 , magnetic flux lines start from the outer peripheral surface  22   a  of the second electrode  22 , which is the N pole, run radially outside the attracting protrusion  25 G 1 , and proceed toward the outer peripheral surface  21   a  of the first electrode  21 , which is the S pole. Between the first electrode  21  and the third electrode  23 , magnetic flux lines start from the outer peripheral surface  23   a  of the third electrode  23 , which is the N pole, run radially outside the attracting protrusion  25 G 2 , and proceed toward the outer peripheral surface  21   a  of the first electrode  21 , which is the S pole. 
     Between the fourth electrode  24  and the second electrode  22 , magnetic flux lines start from the outer peripheral surface  22   a  of the second electrode  22 , which is the N pole, run radially outside the attracting protrusion  25 G 3 , and proceed toward the outer peripheral surface  24   a  of the fourth electrode  24 , which is the S pole. Between the fourth electrode  24  and the third electrode  23 , magnetic flux lines start from the outer peripheral surface  23   a  of the third electrode  23 , which is the N pole, run radially outside the attracting protrusion  25 G 4 , and proceed toward the outer peripheral surface  24   a  of the fourth electrode  24 , which is the S pole. 
     The electrodes  21  to  24  do not need to be in contact with the attracting protrusions  25 G 1  to  25 G 4 . As shown in  FIG. 9 , as moving outward radially from the center portion  25   c , the spacing distance between the electrodes  21  to  24  and the attracting protrusions  25 G 1  to  25 G 4  may increase. The electrodes  21  to  24  can have a non-magnetic conductor portion at a position opposite the end surface  20   a . If such is the case, in the electrodes  21  to  24 , the magnet and the conductor portion are in contact with each other so that the electrodes  21  to  24  are electrically conductive. 
     Output lines are connected to the first, second, third and fourth electrodes  21 ,  22 ,  23  and  24 . The first, second, third and fourth electrodes  21 ,  22 ,  23  and  24  are respectively electrically connected to the sensing unit  5  (see  FIG. 1  as mentioned in the first embodiment) via the output lines. The electrodes  21  to  24  are insulated from each other. The first electrode  21  and one of the other electrodes  22  to  24  form a pair of electrodes, and the attracting portion  25  arranged between the paired electrodes constitutes a single detecting unit together with the pair of electrodes. 
     In the present embodiment, the sensor  20  includes three detecting units, corresponding to the second, third and fourth electrodes  22 ,  23  and  24 . There are no particular limitations on the number of the electrodes  22  to  24  and the number of the detecting units. Since the electrodes  21  to  24  of the sensor  20 , which are magnets, produce magnetic flux lines between the paired ones of the electrodes  21  to  24 , the abrasion powder mp (see  FIG. 5  mentioned in the first embodiment) contained in the lubricating oil are attracted by the attracting portion  25 . If the conductive particles mp are gathered in the vicinity of the attracting portion  25  in this manner, the detecting units experience a change in electrical resistance. While no conductive particles (abrasion powder) mp are attracted, the detecting units may all exhibit the same electrical resistance. 
     Through the output lines connected to the electrodes  22  to  24 , the detecting units are electrically connected to the sensing unit  5 . In the present embodiment, the detecting units corresponding to the second and third electrodes  22  and  23  are connected to each other in parallel. Between the first electrode  21  and the second electrode  22 , voltage is applied by the same voltage source. Between the first electrode  21  and the third electrode  23 , voltage is applied by the same voltage source. If the conductive particles mp are gathered in the vicinity of the attracting protrusion  25 G 1 , the detecting unit corresponding to the second electrode  22  experiences a change in electrical resistance. If the conductive particles mp are gathered in the vicinity of the attracting protrusion  25 G 2 , the detecting unit corresponding to the third electrode  23  experiences a change in electrical resistance. 
     The sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  21  and the second electrode  22 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusion  25 G 1 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of the attracting protrusion  25 G 1 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  21  and the second electrode  22  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     Likewise, the sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  21  and the third electrode  23 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusion  25 G 2 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of the attracting protrusion  25 G 3 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  21  and the third electrode  23  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     Between the first electrode  21  and the fourth electrode  24 , voltage is applied by the same voltage source. If the conductive particles mp are gathered in the vicinity of both of the attracting protrusions  25 G 1  and  25 G 3 , the detecting unit corresponding to the fourth electrode  24  experiences a change in electrical resistance. If the conductive substance is gathered in the vicinity of both of the attracting protrusions  25 G 2  and  25 G 4 , the detecting unit corresponding to the fourth electrode  24  experiences a change in electrical resistance. 
     The sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  21  and the fourth electrode  24 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusions  25 G 1  and  25 G 3 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of both of the attracting protrusions  25 G 1  and  25 G 3 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  21  and the fourth electrode  24  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . This sensing is not made possible until the conductive particles mp are gathered in the vicinity of both of the attracting protrusions  25 G 1  and  25 G 3 . 
     Likewise, the sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusions  25 G 2  and  25 G 4 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of both of the attracting protrusions  25 G 2  and  25 G 4 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  21  and the fourth electrode  24  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . This sensing is not made possible until the conductive particles mp are gathered in the vicinity of both of the attracting protrusions  25 G 2  and  25 G 4 . 
     As described above, the change in electrical resistance experienced by the detecting unit corresponding to the second electrode  22  and the detecting unit corresponding to the third electrode  23  is used to detect a change in electrical resistance in one attracting protrusion, which is selected from the attracting protrusions  25 G 1  and  25 G 2 . On the other hand, the change in electrical resistance experienced by the detecting unit corresponding to the fourth electrode  24  is used to detect the change in electrical resistance in two attracting protrusions, which are either the attracting protrusions  25 G 1  and  25 G 3  or attracting protrusions  25 G 2  and  25 G 4 . This means that the plurality of detecting units are capable of sensing different conditions. In other words, the detecting units are configured to detect a change in electrical resistance in two stages. The detecting units can perform the sensing in two stages and two systems. Accordingly, the reliability of the failure prediction can be improved. 
     The sensing unit  5  outputs a signal when a designated one of the detecting units experiences a change in electrical resistance. For example, the sensing unit  5  may be configured to output a signal to a higher-level control device such as a manipulator when two of the detecting units, more specifically, the detecting unit corresponding to the second electrode  22  and the detecting unit corresponding to the third electrode  23  experience a drop in electrical resistance. The sensing unit  5  may be alternatively configured to output a signal when all of the detecting units including the detecting unit corresponding to the fourth electrode  24  experience a drop in electrical resistance. 
     The drop in electrical resistance may be indicated by an ON signal and an OFF signal corresponding to electrical disconnection and connection. The sensing unit  5  may sense two states of electrical disconnection and connection (hereinafter, may be referred to as “perform digital sensing”). The sensing unit  5  may be connected to a higher-level control device (not shown) such as a manipulator in a wired or wireless manner. The higher-level control device may be configured to, upon reception of a signal from the sensing unit  5 , issue an alert for demanding maintenance of, for example, the speed reducer  2  with a predetermined notifying device (for example, a display or voice output device). 
     As described above, the sensor  20  of the present embodiment includes the plurality of detecting units, and the sensing unit  5  outputs a signal when designated one or more of detecting units experience a drop in electrical resistance. In this way, the sensing unit  5  can be configured to output no signal when just one of the detecting units experiences a change in electrical resistance caused by initial abrasion powder but the other detecting units do not experience a change in electrical resistance. Accordingly, the sensor can be prevented from operating unexpectedly. Furthermore, in the sensor  20 , the sensing unit  5  can be configured to output a signal under a designated condition. Therefore, the single sensor  20  can be configured to output a signal in a timely and optimal manner for individual users, who have different requests for failure prediction timing. 
     While no conductive particles mp are attracted, the detecting units can exhibit the same electrical resistance. This can lower the voltage to be applied to the sensor  20 . The detecting units are connected in parallel to each other. This can lower the voltage applied between the paired electrodes in each detecting unit. The sensor  20  is preferably positioned such that the first electrode  21 , which forms the detecting unit with all of the other electrodes  22  to  24 , deals with a large amount of conductive particles mp. 
     According to the sensor  20  relating to the present embodiment, the gaps G 1  to G 4  for detecting the conductive particles mp are arranged next to each other in the circumferential direction rtd along the outer peripheral surfaces  21   a  to  24   a . In this way, when compared with the case where such gaps for detecting conductive particles are arranged next to each other in the radial direction on the end surface  20   a  of the sensor  20 , the sensor  20  can be reduced in size without compromising the detection sensitivity. In addition, the direction of the detection performed by the detecting units can be configured to extend in the circumferential direction rtd along the outer peripheral surfaces  21   a  to  24   a . Accordingly, when compared with the case where a plurality of detection gaps extending in the axial line axd are arranged next to each other in the circumferential direction of the sensor, the sensor  20  can be reduced in size without compromising the detection sensitivity. Since the magnetic flux lines produced by the electrodes  21  to  24 , which are magnets, start from the position immediately close to the attracting portion  25  and run radially, the conductive particles mp can be attracted highly efficiently. Thus, the size reduction can not result in lower attraction efficiency. As a result, the size reduction can carry on. Furthermore, the sensor  20  of the present embodiment can be constituted by a reduced number of parts, assembled easily and manufactured at a reduced cost as reasonable plate-shaped magnets are employed. 
       FIG. 10  shows a section orthogonal to the axial direction in order to illustrate another example of the sensor relating to the present embodiment. In the sensor  20  relating to the present embodiment, the magnets forming the electrodes  21  to  24  are partly spaced away from the attracting protrusions  25 G 1  to  25 G 4 . As shown in  FIG. 10 , however, grooves  25   m  may be provided in the bases of the attracting protrusions  25 G 1  to  25 G 4  and the center portion  25   c  for receiving the magnets forming the electrodes  21  to  24 . In this case, the grooves  25   m  preferably do not cover the outer peripheral surfaces  21   a  to  24   a  of the electrodes  21  to  24 . 
     Irrespective of whether there are the grooves  25   m  or not, the same amount of conductive particles mp can preferably gather, which is determined by the total length in the circumferential direction rtd of the attracting protrusions  25 G 1  to  25 G 4 , namely, in the example case of the attracting protrusion  25 G 1 , the surface distance from the adjacent outer peripheral surface  21   a  to the outer peripheral surface  22   a . In this way, the detection sensitivity of each detecting unit can be designated independent from whether there are the grooves  25   m . At the same time, irrespective of whether there are the grooves  25   m  or not, the size in the circumferential direction rtd of the gaps G 1  to G 4  preferably remains the same. 
     In the present example, the sensor  20  can be easily assembled by inserting into the grooves  25   m  the electrodes  21  to  24  along the axial line axd direction from the end surface  20   a  side. With this configuration, the electrodes  21  to  24  can be more rigidly secured than when the electrodes  21  to  24  are secured onto the attracting portion  25  only through the magnetic force of the electrodes  21  to  24 . 
       FIG. 11  shows a section along the axial direction in order to illustrate another example of the sensor relating to the present embodiment. In the sensor  20  relating to the present embodiment, the electrodes  21  to  24  and the attracting portion  25  are flush with each other on the end surface  20   a . As shown in  FIG. 11 , however, an end surface  20   d  can be formed as an inclined surface the central portion of which protrudes outward in the axial line axd direction and electrodes  26   a  to  26   d , which are magnets, can be provided. In this case, the electrodes  26   a  to  26   d  and electrodes  21  to  24  are also arranged such that adjacent ones of the magnets are different poles. 
     With such configurations, in the end surface  20   a , which is a plane orthogonal to the axial line axd, weak magnetic flux lines leaking from the magnets forming the electrodes  21  to  24  can form further detecting units together with the strong magnetic flux line produced by the electrodes  26   a  to  26   d . In this way, the reliability and sensitivity of the detection performed by the sensor  20  can be set at a predetermined level. 
     In the sensor  20  relating to the present embodiment, the output lines extending from the electrodes  21  to  24  can be also formed by using the flexible substrates  11   f  to  14   f , as shown in  FIG. 7  mentioned in the first embodiment. In this case, the electrodes  21  to  24 , which are magnets, are arranged such that the N-pole surface is opposed and parallel to the S-pole surface. Accordingly, the flexible substrates  11   f  to  14   f  are pressed against the attracting portion  25  with stronger force than in the first embodiment. This can in turn allow the flexible substrates  11   f  to  14   f  to be more rigidly secured and contribute to maintain excellent contact. 
     The following describes a sensor relating to a third embodiment of the present invention with reference to the drawings.  FIG. 12  is a sectional view taken along the axial direction and showing the sensor relating to the third embodiment of the present invention.  FIG. 13  is used to illustrate the magnets of the sensor relating to the present embodiment. In  FIG. 12 , the reference numeral  30  denotes the sensor. The third embodiment is different from the above-described first embodiment in terms of the configurations of the sensor. The constituents of the mechanism  1  illustrated in  FIG. 1  are not described here. 
     As shown in  FIG. 12 , the sensor  30  has a substantially cylindrical outer shape around an axial line axd. The sensor  30  includes a first electrode  31 , a second electrode  32 , a third electrode  33 , and an attracting portion (catching portion)  35 . 
     A front end  30   a  of the sensor  30  is substantially shaped like a hemisphere. The first, second and third electrodes  31 ,  32  and  33  are plate members having a circular outline centered around the axial line axd when seen in the axial line axd direction. The first, second and third electrodes  31 ,  32  and  33  all have substantially the same outline and substantially the same thickness. The second electrode  32 , the first electrode  31  and the third electrode  33  are stacked in the axial line axd direction in the stated order from the front end  30   a  toward the base end along the axial line axd direction. 
     The second electrode  32 , first electrode  31  and third electrode  33  are arranged next to each other in the direction along the axial line axd and parallel to each other concentrically. The second electrode  32 , first electrode  31  and third electrode  33  are arranged such that their respective outer peripheral surfaces  31   a  to  33   a  form the same or flush cylindrical surface. The second electrode  32 , first electrode  31  and third electrode  33  all have the same radial size in the direction orthogonal to the axial line axd. 
     The second electrode  32 , first electrode  31  and third electrode  33  are spaced away from each other in the direction along the axial line axd. Gaps G 1  and G 2  are formed between the electrodes  31  to  33 . The gap G 1  denotes the spacing distance in the direction along the axial line axd between the first electrode  31  and the second electrode  32 . The gap G 2  denotes the spacing distance in the direction along the axial line axd between the first electrode  31  and the third electrode  33 . 
     The first, second and third electrodes  31 ,  32  and  33  are all magnets. The magnets are, for example, permanent magnets. In all of the first, second and third electrodes  31 ,  32  and  33 , the direction of magnetization extends in the radial direction of the sensor  30 . The first electrode  31  is magnetized such that the halves of the radially outer peripheral surface  31   a  are different poles. Likewise, the second and third electrodes  32  and  33  are both magnetized such that the halves of their respective outer peripheral surfaces  32   a  and  33   a  are different poles. 
     The poles of the outer peripheral surface  31   a  of the first electrode  31  are opposite to the poles of the outer peripheral surfaces  32   a  and  33   a  of the second and third electrodes  32  and  33 . In other words, the electrodes  31 ,  32 ,  33 , which are adjacent to each other along the axial line axd, are magnetized such that different poles are adjacent. That is to say, the first electrode  31  is a magnet magnetized in the same manner as the second and third electrodes  32  and  33  but rotated around the axial line axd by 180°. 
     For example, as shown in  FIG. 12 , the first electrode  31  is magnetized such that the lower half of the outer peripheral surface  31   a  is the N pole. The second and third electrodes  32  and  33  are magnetized such that the upper half of their respective outer peripheral surfaces  32   a  and  33   a  is the N pole. The direction of magnetization can be reversed in all of the electrodes  31  to  33 . 
     Arranged in the above-described manner, the electrodes  31  to  33 , which are magnets, are attracted to each other in the axial line axd direction by their magnetic force. As a result, the electrodes  31  to  33  can be arranged to overlap each other in the axial line axd direction and secured onto the attracting portion  25  without the use of an adhesive portion such as an adhesive agent. 
     Between the electrodes  31  to  33 , the attracting portion  35  is arranged. The attracting portion  35  is made of an insulating non-magnetic material, for example, resin. The attracting portion  35  is a plate-shaped member having a circular outline when seen along the axial line axd from the side of the front end  30   a.    
     The attracting portion  35  has an attracting protrusion  35 G 1  filling the gap G 1  between the first electrode  31  and the second electrode  32  and protruding radially outward beyond the outer peripheral surfaces  31   a ,  32   a . The attracting portion  35  has an attracting protrusion  35 G 2  filling the gap G 2  between the first electrode  31  and the third electrode  33  and protruding radially outward beyond the outer peripheral surfaces  31   a ,  33   a.    
     The attracting protrusions  35 G 1  and  35 G 2  are formed such that they both have the same protruding height beyond the outer peripheral surfaces  31   a  to  33   a  in the radial direction. Alternatively, the attracting protrusions  35 G 1 ,  35 G 2  may be formed such that they have any, for example, different protruding heights in the radial direction. By adjusting the protruding heights of the attracting protrusions  35 G 1  and  35 G 2  in the radial direction, the sensitivity of the detection, described below, can be tuned. 
     The thickness, in the axial line axd direction, of the gaps G 1  and G 2  between the electrodes  31  to  33  is larger than the size of the conductive substance that can be contained in the lubricating oil. For example, the conductive substance has a size of approximately 1.0 μm to 100 μm. The gaps G 1  and G 2  are preferably arranged at such intervals that no short circuit is created by the iron powder resulting from initial wear period. The gaps G 1  and G 2  have the same size in the direction along the axial line axd. 
     Between the electrodes  31  to  33 , which are magnets, magnetic flux lines run radially outside the attracting protrusions  35 G 1 ,  35 G 2  to connect the electrodes  31  to  33  in the direction along the axial line axd, as shown in  FIG. 12 . As shown in the lower part of  FIG. 12 , between the first electrode  31  and the second electrode  32 , magnetic flux lines start from the outer peripheral surface  31   a  of the first electrode  31 , which is the N pole, run radially outside the attracting protrusion  35 G 1 , and proceed toward the outer peripheral surface  32   a  of the second electrode  32 , which is the S pole. At the same time, as shown in the upper part of  FIG. 12 , between the first electrode  31  and the second electrode  32 , magnetic flux lines start from the outer peripheral surface  32   a  of the second electrode  32 , which is the N pole, run radially outside the attracting protrusion  35 G 1 , and proceed toward the outer peripheral surface  31   a  of the first electrode  31 , which is the S pole. 
     As shown in the lower part of  FIG. 12 , between the first electrode  31  and the third electrode  33 , magnetic flux lines start from the outer peripheral surface  31   a  of the first electrode  31 , which is the N pole, run radially outside the attracting protrusion  35 G 2 , and proceed toward the outer peripheral surface  33   a  of the third electrode  33 , which is the S pole. At the same time, as shown in the upper part of  FIG. 12 , between the first electrode  31  and the third electrode  33 , magnetic flux lines start from the outer peripheral surface  33   a  of the third electrode  33 , which is the N pole, run radially outside the attracting protrusion  35 G 2 , and proceed toward the outer peripheral surface  31   a  of the first electrode  31 , which is the S pole. 
     Outside the second electrode  32  in the direction along the axial line axd, in other words, on the front end  30   a , a front end portion  35   a  made of the same material as the attracting portion  35  is formed. The front end portion  35   a  has the same outline as the attracting protrusions  35 G 1 ,  35 G 2  when seen in the direction along the axial line axd. The front end portion  35   a  has a spherical surface. Outside the third electrode  33  in the direction along the axial line axd, in other words, near the base of the sensor  30 , a base end portion  35   b  made of the same material as the attracting portion  35  is formed. The base end portion  35   b  has the same outline as the attracting protrusions  35 G 1 ,  35 G 2  when seen in the direction along the axial line axd. The base end portion  35   b  has the same thickness as the attracting protrusions  35 G 1 ,  35 G 2  in the direction along the axial line axd. 
     The base end portion  35   b , the third electrode  33 , the attracting protrusion  35 G 2 , the first electrode  31 , the attracting protrusion  35 G 1  and the second electrode  32 , which are stacked on each other along the axial line axd, all have a center hole through which a fastening member  38  (in the illustrated embodiment, a screw) is inserted and centered around the axial line axd. The fastening member  38  is inserted through the center holes centered around the axial line axd, so that the electrodes  31  to  33 , the attracting protrusions  35 G 1 ,  35 G 2 , the front end portion  35   a  and the base end portion  35   b  are secured to each other. A tube  37  is provided radially outside the screw  38  and surrounds the screw  38 . The tube  37  contributes to keep the electrodes  31  to  33  and the screw  38  insulated from each other and to secure the electrodes  31  to  33  and attracting protrusions  35 G 1 ,  35 G 2  relative to the front and base end portions  35   a ,  35   b  at predetermined positions in the radial direction. 
     Output lines are connected to the first, second and third electrodes  31 ,  32  and  33 . The first, second and third electrodes  31 ,  32  and  33  are respectively electrically connected to the sensing unit  5  (see  FIG. 1 ) via the output lines. The electrodes  31  to  33  are insulated from each other. The first electrode  31  and one of the other electrodes  32  to  33  form a pair of electrodes, and the attracting portion  35  arranged between the paired electrodes constitutes a single detecting unit together with the pair of electrodes. In  FIG. 14 , the electrode pairs constituting the detecting units are denoted by assigning the sign “+” to the output line of the first electrode  31  and “−” to the output lines of the other electrodes  32 ,  33 . 
       FIG. 15  is a sectional view showing how detection is performed in the sensor relating to the present embodiment. In the present embodiment, the sensor  30  includes two detecting units, corresponding to the second and third electrodes  32 ,  23 . There are no particular limitations on the number of the electrodes  32  to  33  and the number of the detecting units. Since the electrodes  31  to  33  of the sensor  30 , which are magnets, produce magnetic flux lines between the paired ones of the electrodes  31  to  33 , the conductive particles (abrasion powder) mp contained in the lubricating oil are attracted by the attracting portion  35 , as shown in  FIG. 15 . If the conductive particles mp are gathered in the vicinity of the attracting portion  35  in this manner, the detecting units experience a change in electrical resistance. While no conductive particles (abrasion powder) mp are attracted, the detecting units may exhibit the same electrical resistance. 
     In the present embodiment, the detecting units corresponding to the second and third electrodes  32  and  33  are connected to each other in parallel. Between the first electrode  31  and the second electrode  32 , voltage is applied by the same voltage source. Between the first electrode  31  and the third electrode  33 , voltage is applied by the same voltage source. If the conductive particles mp are gathered in the vicinity of the attracting protrusion  35 G 1 , the detecting unit corresponding to the second electrode  32  experiences a change in electrical resistance. If the conductive particles mp are gathered in the vicinity of the attracting protrusion  35 G 2 , the detecting unit corresponding to the third electrode  33  experiences a change in electrical resistance. 
     The sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  31  and the second electrode  32 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusion  35 G 1 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of the attracting protrusion  35 G 1 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  31  and the second electrode  32  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     Likewise, the sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  31  and the third electrode  33 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusion  35 G 2 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of the attracting protrusion  35 G 2 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  31  and the third electrode  33  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     As described above, the change in electrical resistance experienced by the detecting unit corresponding to the second electrode  32  and the detecting unit corresponding to the third electrode  33  is used to detect a change in electrical resistance in one attracting protrusion, which is selected from the attracting protrusions  35 G 1  and  35 G 2 . This means that the plurality of detecting units are capable of sensing different conditions. In other words, the detecting units are configured to detect a change in electrical resistance in two systems. 
     The sensing unit  5  outputs a signal when designated one or more of the detecting units experiences a change in electrical resistance. For example, the sensing unit  5  may be configured to output a signal to a higher-level control device such as a manipulator when two or more of the detecting units experience a drop in electrical resistance, or configured to output a signal when all of the detecting units experience a drop in electrical resistance. 
     The drop in electrical resistance may be indicated by an ON signal and an OFF signal corresponding to electrical disconnection and connection. The sensing unit  5  may sense two states of electrical disconnection and connection (hereinafter, may be referred to as “perform digital sensing”). The sensing unit  5  may be connected to a higher-level control device (not shown) such as a manipulator in a wired or wireless manner. The higher-level control device may be configured to, upon reception of a signal from the sensing unit  5 , issue an alert for demanding maintenance of, for example, the speed reducer  2  with a predetermined notifying device (for example, a display or voice output device). 
     As described above, the sensor  30  of the present embodiment includes the plurality of detecting units, and the sensing unit  5  outputs a signal when designated one or more of the detecting units experience a drop in electrical resistance. In this way, the sensing unit  5  can be configured to output no signal when just one of the detecting units experiences a change in electrical resistance caused by initial abrasion powder but the other detecting units do not experience a change in electrical resistance. Accordingly, the sensor can be prevented from operating unexpectedly. Furthermore, in the sensor  30 , the sensing unit  5  can be configured to output a signal under a designated condition. Therefore, the single sensor  30  can be configured to output a signal in a timely and optimal manner for individual users, who have different requests for failure prediction timing. 
     While no conductive particles mp are attracted, the detecting units can exhibit the same electrical resistance. This can lower the voltage to be applied to the sensor  30 . The detecting units are connected in parallel to each other. This can lower the voltage applied between the paired electrodes in each detecting unit. The sensor  30  is preferably positioned such that the first electrode  31 , which forms the detecting unit with every one of the other electrodes  32 ,  33 , deals with a large amount of conductive particles mp. 
     According to the sensor  30  relating to the present embodiment, the gaps G 1 , G 2  for detecting the conductive particles mp extend along the outer peripheral surfaces  31   a  to  33   a  and are arranged next to each other in the direction along the axial line axd. In this way, when compared with the case where such gaps for detecting conductive particles are arranged next to each other in the radial direction on the end surface of the sensor  30 , the sensor  30  can be reduced in size without compromising the detection sensitivity. In addition, the detecting units extending along the outer peripheral surfaces  31   a  to  33   a  can be adjacent to each other in the direction along the axial line axd and the direction of the detection can be configured to extend in the direction along the axial line axd. Accordingly, when compared with the case where a plurality of detection gaps extending in the axial line axd direction are arranged next to each other in the circumferential direction of the sensor, the sensor can be reduced in size without compromising the detection sensitivity. Since the magnets are exposed on the surface of the sensor, the magnets also serve as the electrodes  31  to  33 , and the magnetic flux lines start from the position immediately close to the attracting portion  35  and run radially, the conductive particles mp can be attracted highly efficiently. Thus, the size reduction can not result in lower attraction efficiency. Furthermore, the sensor  30  of the present embodiment can be constituted by a reduced number of parts, assembled easily and manufactured at a reduced cost. 
       FIG. 16  shows another example of the sensor relating to the present embodiment. In the sensor  30  relating to the present embodiment, the detecting units corresponding to the electrodes  31  to  33  constitute two parallel systems. As shown in  FIG. 16 , however, the detecting units can alternatively form two stages. 
     Specifically, the second electrode  32  near the front end  30   a  and the first electrode  31  positioned at the center in the axial line axd direction constitute a first-stage detecting unit. The second electrode  32  near the front end  30   a  and the third electrode  33  at the opposite position in the axial line axd direction constitute a second-stage detecting unit. In  FIG. 16 , the electrode pairs constituting the detecting units are denoted by assigning the sign “+” to the output line of the second electrode  32  and “−” to the output lines of the other electrodes  31 ,  33 . 
     In this example, the sensing unit  5  is configured to sense a change in electrical resistance between the second electrode  32  and the first electrode  31 . If the conductive particles mp contained in the lubricating oil in the mechanism  1  are gathered in the vicinity of the attracting protrusion  15 G 1 , this causes a drop in electrical resistance (or a short circuit) between the second electrode  32  and the first electrode  31  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     In the present example, the sensing unit  5  predicts a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of the attracting protrusions  35 G 1  and  35 G 2 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of both of the attracting protrusions  35 G 1  and  35 G 2 , this causes a drop in electrical resistance (or a short circuit) between the second electrode  32  and the third electrode  33  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such the change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . This sensing is not made possible until the conductive particles mp are gathered in the vicinity of both of the attracting protrusions  35 G 1  and  35 G 2 . 
     As described above, the change in electrical resistance experienced by the detecting unit corresponding to the first electrode  31  is used to detect a change in electrical resistance in one attracting protrusion, which is the attracting protrusion  35 G 1 . On the other hand, the change in electrical resistance experienced by the detecting unit corresponding to the third electrode  33  is used to detect the change in electrical resistance in two attracting protrusions, which are the attracting protrusions  35 G 1  and  35 G 2 . This means that the plurality of detecting units are capable of sensing different conditions. In other words, the sensing units are configured to detect a change in electrical resistance in two stages. Accordingly, the reliability of the failure prediction can be improved. 
     For the second electrode  32 , the first electrode  31  and the third electrode  33 , which are arranged next to each other in the axial line axd direction,  FIG. 14  shows the detecting unit configuration “−” “+” “−” and  FIG. 16  shows “+” “−” “−.” It is, however, possible to provide for detecting unit configuration “−” “−” “+.” 
     The following describes a sensor relating to a fourth embodiment of the present invention with reference to the drawings.  FIG. 17  is a perspective view showing the sensor relating to the fourth embodiment. The fourth embodiment is different from the above-described third embodiment in terms of the shape of the attracting portion. The common constituents are assigned with the same reference numerals and are not described here. 
     In the sensor  30  relating to the present embodiment, as shown in  FIG. 17 , the attracting portion  35  covers a circumferential portion of the electrodes  31  to  33 . More specifically, the attracting protrusions  35 G 1 ,  35 G 2 , which serve as the gaps G 1 , G 2 , correspond to approximately half in the circumferential direction of the electrodes  31  to  33 , and the remaining circumferential portion of the electrodes  31  to  33  is covered with the same resin as the resin forming the attracting portion  35 . In other words, the attracting portion  35  includes a substantially cylindrical casing  35 B. The casing  35 B has window portions  35 Ba to  35 Bc exposing an approximately half of the outer peripheral surfaces  31   a  to  33   a  of the electrodes  31  to  33 . 
     The window portions  35 Ba to  35 Bc are positioned at the same position in the circumferential direction rtd when the sensor  30  is seen in the axial line axd direction. The electrodes  31  to  33 , which are magnets, have a region in which the N pole abuts the S pole. The casing  35 B having the window portions  35 Ba to  35 Bc can cover such boundary regions, which are shown in  FIG. 13  on the left and right sides. This can prevent erroneous detection that can be attributed to the magnetic flux lines that run circumferentially behind the electrodes  31  to  33  and do not interact with the gaps G 1 , G 2 . 
     The casing  35 B can be provided with a screw portion  35 Bg having a larger diameter, which is positioned next to the base end portion  35   b  in the axial line axd direction. For example, the screw portion  35 Bg enables the sensor  30  to be secured onto the flange  3  without the use of the support member  2   e  (see  FIG. 1 ). The screw portion  35 Bg is concentrically arranged with the casing  35 B and extends in the axial line axd direction from the casing  35 B. The screw portion  35 Bg and the casing  35 B can be integrally shaped. 
     The sensor  30  relating to the present embodiment can produce the same effects as the above-described third embodiment. 
       FIG. 18  shows a section along the axial line direction in order to illustrate another example of the sensor relating to the present embodiment. In the sensor  30  relating to the present embodiment, the casing  35 B is a single-piece member. As shown in  FIG. 18 , however, the casing  35 B may include two separate members divided by a plane parallel to the axial line axd. 
     In the present example, an upper casing  35 C has the window portions  35 Ba to  35 Bc, and a lower casing  35 D has a line passage  35 Df in communication with the outer peripheral surfaces  31   a ,  32   a ,  33   a  and extending in the direction along the axial line axd. The line passage  35 Df is open at the base end portion  35   b  side and closed at the front end  30   a  side. 
     In the present example, the output lines are flexible substrates  31   f ,  32   f ,  33   f.    
     The output line connected to the first electrode  31  is formed by the flexible substrate  31   f  and interposed between the first electrode  31  and the attracting portion  35 . A portion of the flexible substrate  31   f  that is in contact with the first electrode  31  is electrically conductive as, for example, coating has been removed and thus in electrical communication with the first electrode  31 . The electrodes  31  to  33 , which are formed by magnets, are attracted to each other due to their magnetic force. Accordingly, the flexible substrate  31   f  is fixed while being sandwiched between the first electrode  31  and the attracting portion  35 . 
     As a result, the flexible substrate  31   f  can be fixedly connected to the first electrode  31  without the use of an adhesive agent or the like and with electrical connection being maintained. In the same manner, in the sensor  30  relating to the present embodiment, the flexible substrate  32   f  is fixedly connected to the second electrode  32 , and the flexible substrate  33   f  is fixedly connected to the third electrode  33 . The flexible substrates  31   f ,  32   f  and  33   f  are connected to the sensing unit  5  via the internal space within the line passage  35 Df. 
     The present example can produce the same effects as the above-described embodiments. 
     The following describes a sensor relating to a fifth embodiment of the present invention with reference to the drawings.  FIG. 19  is an exploded perspective view showing the sensor relating to the fifth embodiment. The fifth embodiment is different from the above-described fourth embodiment in terms of the shape of the magnets. The common constituents are assigned with the same reference numerals and are not described here. 
     In the sensor  30  relating to the present embodiment, the first, second and third electrodes  31 ,  32  and  33  have a substantially rectangular outline when seen in the direction along the axial line axd. The electrodes  31  to  33  are shaped like a rectangular flat plate. The first, second and third electrodes  31 ,  32  and  33  all have substantially the same shape. 
     In the sensor  30  relating to the present embodiment, the direction of magnetization in the electrodes  31  to  33  and the positioning of the electrodes  31  to  33  relative to the attracting portion  35 , which is the casing  35 B, are substantially the same as in the above-described third embodiment. Specifically, the first, second and third electrodes  31 ,  32  and  33  are rectangular plate members centered around the axial line axd. The first electrode  31 , second electrode  32  and third electrode  33  are stacked in the axial line axd direction in the order of the second electrode  32 , first electrode  31  and third electrode  33  from the front end  30   a  toward the base end along the axial line axd direction. 
     The second electrode  32 , first electrode  31  and third electrode  33  are parallel to each other, arranged concentrically and next to each other in the direction along the axial line axd. The second electrode  32 , first electrode  31  and third electrode  33  are arranged such that the four sides of their respective outer peripheral surfaces  31   a  to  33   a  form a flush surface of the same prism. The second electrode  32 , first electrode  31  and third electrode  33  have the same radial size (the length of one of the sides of the rectangle, or the length of the diagonal of the rectangle) in the direction orthogonal to the axial line axd. 
     In all of the first, second and third electrodes  31 ,  32  and  33 , which are magnets, the direction of magnetization extends in the radial direction of the sensor  30 . The first electrode  31  has an outer peripheral surfaces  31   a  radially bounding the outer periphery and divided into four end surfaces. The first electrode  31  is magnetized such that each pair of end surfaces opposing with respect to the axial line axd includes different poles. The second and third electrodes  32  and  33  have an outer peripheral surface  32   a ,  33   a  divided into four end surfaces. The second and third electrodes  32  and  33  are also magnetized such that each pair of end surfaces opposing with respect to the axial line axd includes different poles. 
     The poles in the outer peripheral surface  31   a  of the first electrode  31  are opposite to the poles in the outer peripheral surfaces  32   a  and  33   a  of the second and third electrodes  32  and  33 . In other words, the electrodes  31 ,  32 ,  33 , which are adjacent to each other along the axial line axd, are magnetized such that different poles are adjacent. That is to say, the first electrode  31  is a magnet magnetized in the same manner as the second and third electrodes  32  and  33  but rotated around the axial line axd by 180°. 
     For example, as shown in  FIG. 19 , the first electrode  31  is magnetized such that the lower end surface of the outer peripheral surface  31   a  is the N pole. The second and third electrodes  32  and  33  are magnetized such that the upper end surface of their respective outer peripheral surfaces  32   a  and  33   a  is the N pole. The direction of magnetization can be reversed in all of the electrodes  31  to  33 . 
     Arranged in the above-described manner, the electrodes  31  to  33 , which are magnets, are attracted to each other in the axial line axd direction through their magnetic force. As a result, the electrodes  31  to  33  can be arranged to overlap each other in the axial line axd direction and secured onto the attracting portion  35  without the use of an adhesive portion such as an adhesive agent. Furthermore, since the electrodes  31  to  33  have a rectangular outline, the direction of magnetization can be easily fixed when the electrodes  31  to  33  are attached to the casing  35 B. 
     In the present embodiment, the third electrode  33 , the first electrode  31  and the second electrode  32 , which are stacked on each other along the axial line axd, all similarly have a center hole through which a fastening member  38  (in the illustrated embodiment, a screw) is inserted and centered around the axial line axd. 
     This embodiment can also produce the same effects as the above-described embodiments. 
       FIG. 20  is an exploded perspective view showing another example of the sensor relating to the present embodiment. In the sensor  30  relating to the present embodiment, the casing  35 B has a substantially cylindrical outer shape. As shown in  FIG. 2 , however, the casing  35 B can alternatively have a substantially prismatic outer shape. In the present example, the electrodes  31  to  33  can be positioned relative to the casing  35 B such that the sides of the electrodes  31  to  33 , which have a rectangular outline, can be parallel to the sides of the casing  35 B, which has a substantially prismatic shape. In this way, the direction of magnetization can be more easily fixed when the electrodes  31  to  33  are attached to the casing  35 B. 
     The present example can also produce the same effects as the above-described embodiments. 
       FIG. 21  is an exploded perspective view showing another example of the sensor relating to the present embodiment. In the preceding description of the sensor  30  relating to the present embodiment, the casing  35 B is a single-piece member. As shown in  FIG. 21 , however, the casing  35 B may include two separate members divided by a plane parallel to the axial line axd. In the present example, the electrodes  31  to  33 , which are magnets, are shaped like a flat plate having a rectangular outline, the line passage  35 Df is provided, and the output lines are the flexible substrates  31   f ,  32   f ,  33   f.    
     In the present example, the upper casing  35 C is provided with the window portions  35 Ba to  35 Bc. The window portions  35 Ba to  35 Bc have a slightly smaller circumferential size than the electrodes  31  to  33 , as shown in  FIG. 21 . The line passage  35 Df is formed in the lower casing  35 D such that the line passage  35 Df is in communication with the outer peripheral surfaces  31   a ,  32   a ,  33   a  and extends along the axial line axd. The line passage  35 Df is open at the base end portion  35   b  side and closed at the front end  30   a  side. 
     In the present example, the output line connected to the first electrode  31  is also formed by the flexible substrate  31   f , which is interposed between the first electrode  31  and the attracting portion  35 . A portion of the flexible substrate  31   f  that is in contact with the first electrode  31  is electrically conductive as, for example, coating has been removed and thus in electrical communication with the first electrode  31 . The electrodes  31  to  33 , which are formed by magnets, are attracted to each other due to their magnetic force. Accordingly, the flexible substrate  31   f  is secured while being sandwiched between the first electrode  31  and the attracting portion  35 . 
     As a result, the flexible substrate  31   f  can be fixedly connected to the first electrode  31  without the use of an adhesive agent or the like and with the electrical connection being maintained. In the same manner, in the sensor  30  relating to the present example, the flexible substrate  32   f  is fixedly connected to the second electrode  32 , and the flexible substrate  33   f  is fixedly connected to the third electrode  33 . The flexible substrates  31   f ,  32   f  and  33   f  are connected to the sensing unit  5  via the internal space within the line passage  35 Df. 
     In the present example, the lower casing  35 D can be assembled by inserting the electrodes  31  to  33  into the upper casing  35 C and interposing the flexible substrates  31   f ,  32   f  and  33   f , which have been inserted into the line passage  35 Df. In this way, the assembling can be more easily completed. 
     The present example can also produce the same effects as the above-described embodiments. 
     The following describes a sixth embodiment of the sensor relating to the present invention with reference to the drawings.  FIG. 22  is a sectional view taken along the axial line and showing how the electrodes are arranged in the sensor relating to the present embodiment. The sixth embodiment is different from the above-described third embodiment in terms of the number of the electrodes. The common constituents are assigned with the same reference numerals and are not described here. 
     The sensor  30  relating to the present embodiment has, in addition to the electrodes  31  to  33 , an attracting protrusion  35 G 3  forming a gap G 3  and a fourth electrode  34  between the third electrode and the base end portion  35   b . The electrodes  31  to  34  are stacked on each other in the axial line axd direction in the order of the second electrode  32 , the first electrode  31 , the third electrode  33  and the fourth electrode  34  from the front end  30   a  toward the base end portion  35   b  along the axial line axd direction. 
     In the present embodiment, the shape of the fourth electrode  34  and the positioning of the fourth electrode  34  relative to the gap G 3  and the attracting protrusion  35 G 3  are similar to the shape of the first electrode  31  and the positioning of the first electrode  31  relative to the attracting protrusions  35 G 1 ,  35 G 2  and the gaps G 1 , G 2 . In the present embodiment, the direction of magnetization in the fourth electrode  34  is the same as in the first electrode  31  and opposite to that in the adjacent third electrode  33 . 
     In the sensor  30  relating to the present embodiment, the detecting units corresponding to the electrodes  31  to  34  constitute two parallel systems. As shown in  FIG. 22 , the detecting units can form two stages. 
     Specifically, the second electrode  32  near the front end  30   a  and the first electrode  31  positioned at the center in the axial line axd direction constitute a first-stage detecting unit. The second electrode  32  near the front end  30   a  and the third electrode  33  at the opposite position in the axial line axd direction constitute a parallel detecting unit. Furthermore, the first electrode  31  and the fourth electrode  34  constitute a second-stage detecting unit. In  FIG. 22 , the electrode pairs constituting the detecting units are denoted by assigning the sign “+” to the output line of the first electrode  31  and “−” to the output lines of the other electrodes  32  to  34 . 
     In the present embodiment, the sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  31  and the second electrode  32 . If the conductive particles mp contained in the lubricating oil in the mechanism  1  are gathered in the vicinity of the attracting protrusion  15 G 1 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  31  and the second electrode  32  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     In the present embodiment, the sensing unit  5  is configured to sense a change in electrical resistance between the second electrode  32  and the third electrode  33 . If the conductive particles mp contained in the lubricating oil in the mechanism  1  are gathered in the vicinity of the attracting protrusion  35 G 2 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  31  and the third electrode  33  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     In the present embodiment, the sensing unit  5  predicts a failure of the parts constituting the mechanism  1  also based on, for example, a change in electrical resistance caused by the gathering of the conductive particles mp in the vicinity of both of the attracting protrusions  35 G 1  and  35 G 3 . If the conductive particles mp contained in the lubricating oil are gathered in the vicinity of both of the attracting protrusions  35 G 1  and  35 G 3 , this causes a drop in electrical resistance (or a short circuit) between the second electrode  32  and the fourth electrode  34  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . This sensing is not made possible until the conductive particles mp are gathered in the vicinity of both of the attracting protrusions  35 G 1  and  35 G 3 . 
     As described above, the change in electrical resistance experienced by the detecting unit corresponding to the first electrode  31  is used to detect a change in electrical resistance in one attracting protrusion, which is the attracting protrusion  35 G 1 . Likewise, the change in electrical resistance experienced by the detecting unit corresponding to the third electrode  33  is used to detect a change in electrical resistance in one attracting protrusion, which is the attracting protrusion  35 G 2 . On the other hand, the change in electrical resistance experienced by the detecting unit corresponding to the fourth electrode  34  is used to detect a change in electrical resistance in two attracting protrusions, which are the attracting protrusions  35 G 1  and  35 G 3 . This means that the plurality of detecting units are capable of sensing different conditions. In other words, the detecting units are configured to detect a change in electrical resistance in two stages. Accordingly, the reliability of the failure prediction can be improved. 
     For the second electrode  32 , first electrode  31 , third electrode  33  and fourth electrode  34 , which are arranged next to each other in the axial line axd direction,  FIG. 22  shows the detecting unit configuration “−” “+” “−” “−.” It is, however, possible to provide for detecting unit configuration “−” “−” “+” “−.” Furthermore, detecting unit configurations “+” “−” “−” “−” and “−” “−” “−” “+” are also realizable, in which case a change in electrical resistance can be detected in three stages. 
     This embodiment can also produce the same effects as the above-described embodiments. 
     The following describes a sensor relating to a seventh embodiment of the present invention with reference to the drawings.  FIG. 23  is a sectional view taken along the axial direction and showing the sensor relating to the seventh embodiment of the present invention.  FIG. 24  is used to illustrate the magnets of the sensor relating to the present embodiment. In  FIG. 23 , the reference numeral  40  denotes the sensor. The seventh embodiment is different from the above-described third embodiment in terms of the configurations of the sensor. The constituents of the mechanism  1  illustrated in  FIG. 1  are not described here. 
     As shown in  FIG. 23 , the sensor  40  relating to the present embodiment has a substantially cylindrical outer shape around an axial line axd. The sensor  40  includes a first electrode  41 , a second electrode  42 , a fastening member  48 , and an attracting portion (catching portion)  45 . 
     In the sensor  40 , the first electrode  41 , second electrode  42  and attracting portion  45  are plate members having a circular outline centered around the axial line axd when seen in the axial line axd direction. The first electrode  41 , attracting portion  45  and second electrode  42  all have substantially the same outline and substantially the same thickness. The first electrode  41 , attracting portion  45  and second electrode  42  are stacked on each other in the axial line axd direction in the order of the first electrode  41 , attracting portion  45  and second electrode  42  from an upper end surface  40   a  along the axial line axd direction. 
     The first electrode  41 , attracting portion  45  and second electrode  42  are arranged next to each other in the direction along the axial line axd and parallel to each other concentrically. The first and second electrodes  41 ,  42  are arranged such that their respective outer peripheral surfaces  41   a ,  42   a  form the same or flush cylindrical surface. An electrode plate  41   b , which serves as, for example, a washer, is arranged on the first electrode  41  on the upper end surface  40   a  side. 
     The first and second electrodes  41  and  42  are spaced away from each other in the direction along the axial line axd. A gap G 1  is formed between the first electrode  41  and the second electrode  42 . The first electrode  41  is a magnet. The magnet is a permanent magnet. The first electrode  41  is magnetized in the direction along the axial line axd, as shown in  FIG. 24 . The second electrode  42  is made of an electrically conductive magnetic material such as, for example, iron, ferrite, or silicon steel. 
     The attracting portion  45  is shaped like a circular plate having a larger radial size than the first and second electrodes  41  and  42 . The attracting portion  45  has an attracting protrusion  45 G 1  filling the gap G 1  between the first electrode  41  and the second electrode  42  and protruding radially outward beyond the outer peripheral surfaces  41   a ,  42   a . The thickness, in the axial line axd direction, of the gap G 1  between the first electrode  41  and the second electrode  42  is larger than the size of the conductive substance that can be contained in the lubricating oil. For example, the conductive substance has a size of approximately 1.0 μm to 100 μm. The thickness of the gap G 1  is preferably determined such that no short circuit is created by the iron powder resulting from initial wear period. In the case where the gap G 1  is formed of a plurality of members, the members have the same size in the direction along the axial line axd. 
     The first electrode  41 , attracting portion  45  and second electrode  42  all have therein a through hole, through which the fastening member  48  (in the illustrated embodiment, a bolt) is inserted. The fastening member  48  is inserted through the through holes, so that the first electrode  41 , attracting portion  45  and second electrode  42  are secured to each other. The bolt (fastening member)  48  is fastened by a nut  48   a . A tube  47  is provided radially outside the bolt (fastening member)  48  and surrounds the bolt  48 . The tube  47  contributes to keep the first and second electrodes  41  and  42  and the bolt  48  insulated from each other and to secure the first and second electrodes  41  and  42  and the attracting protrusion  45 G 1  relative to each other at predetermined positions in the radial direction. The present embodiment can be practiced without the tube  47 . 
     Between the second electrode  42  and the first electrode  41 , which is a magnet, magnetic flux lines run radially outside the attracting protrusion  45 G 1  to connect the second and first electrodes  42  and  41  in the direction along the axial line axd, as shown in  FIG. 23 . 
     Output lines are connected to the first and second electrodes  41  and  42 . The first and second electrodes  41  and  42  are respectively electrically connected to the sensing unit  5  (see  FIG. 1 ) via the output lines. The first and second electrodes  41  and  42  are insulated from each other. The first electrode  41  and the second electrode  42  form a pair of electrodes, and the attracting portion  45  arranged between the paired electrodes constitutes a single detecting unit together with the pair of electrodes. 
     The sensing unit  5  is configured to sense a change in electrical resistance between the first electrode  41  and the second electrode  42 . The sensing unit  5  includes a sensor drive circuit for predicting a failure of the parts constituting the mechanism  1  based on, for example, a change in electrical resistance caused in the axial line axd direction by the gathering of the conductive substance in the vicinity of the attracting portion  45 . If the conductive substance contained in the lubricating oil is gathered in the vicinity of the attracting portion  45 , this causes a drop in electrical resistance (or a short circuit) between the first electrode  41  and the second electrode  42  to which voltage is being applied, resulting in a change in output level of the output lines. The sensing unit  5  senses such a change in electrical resistance, thereby predicting a failure of the parts constituting the mechanism  1 . 
     The drop in electrical resistance may be indicated by an ON signal and an OFF signal corresponding to electrical disconnection and connection. The sensing unit  5  may sense two states of electrical disconnection and connection (hereinafter, may be referred to as “perform digital sensing”). The sensing unit  5  may be connected to a higher-level control device (not shown) such as a manipulator in a wired or wireless manner. The higher-level control device may be configured to, upon reception of a signal from the sensing unit  5 , issue an alert for demanding maintenance of, for example, the speed reducer  2  with a predetermined notifying device (for example, a display or voice output device). 
     According to the sensor  40  relating to the present embodiment, the gap G 1  for detecting the conductive particles mp extends along the outer peripheral surfaces  41   a  and  42   a  and in the direction along the axial line axd. In this way, when compared with the case where such a gap for detecting conductive particles extends in the radial direction on the end surface of the sensor  40 , the sensor  40  can be reduced in size without compromising the detection sensitivity. Since the magnet is exposed on the surface of the sensor and serves also as the first electrode  41  and the magnetic flux lines are formed radially outside the attracting portion  45 , the conductive particles mp can be attracted highly efficiently. Thus, the size reduction can not result in lower attraction efficiency. The detection sensitivity can be determined by the size in the direction along the axial line axd. Accordingly, an increase in radial size can be prevented even if measures are taken to prevent initial abrasion powder from causing erroneous operation. Furthermore, the sensor  40  of the present embodiment can be constituted by a reduced number of parts, assembled easily and manufactured at a reduced cost. 
     The following describes a sensor relating to an eighth embodiment of the present invention with reference to the drawings.  FIG. 25  is a sectional view taken along the axial direction and showing the sensor relating to the eighth embodiment of the present invention.  FIG. 26  shows how the electrodes are arranged in the sensor relating to the present embodiment. The eighth embodiment is different from the above-described seventh embodiment in terms of the configurations of the second electrode and casing. Except for these different features, the common constituents are assigned with the same reference numerals and are not described here. 
     The sensor  40  relating to the eighth embodiment is configured to sense the amount of the conductive substance contained in the lubricating oil, similarly to the sensor  40  relating to the seventh embodiment described above. As shown in  FIG. 25 , the sensor  40  has a substantially cylindrical outer shape and includes a plurality of detecting units and a sensing unit  5  configured to output a signal when the detecting units experience a change in electrical resistance. 
     More specifically, the sensor  40  includes a first electrode  41 , a plurality of second electrodes  42 , and an attracting portion  45  disposed between the first electrode  41  and the second electrodes  42 . The second electrodes  42  are insulated from each other by a casing  46  and the like. The first electrode  41  and one of the second electrodes  42  form a pair of electrodes, and the attracting portion  45  arranged between the paired electrodes constitutes a single detecting unit together with the pair of electrodes. 
     The casing  46  receives therein the four second electrodes  42 A,  42 B,  42 C and  42 D, which are divided from each other in the circumferential direction, so that their positions can be fixed, and insulates the four second electrodes  42 A,  42 B,  42 C,  42 D from each other, as shown in  FIG. 26 . The casing  46  can be made of the same resin as the attracting portion  45 . 
     In the embodiment illustrated, the sensor  40  includes the four second electrodes  42 A,  42 B,  42 C, and  42 D, which are divided from each other in the circumferential direction, so that four detecting units are formed. There are no particular limitations on the number of the second electrodes  42  and the number of the detecting units. The magnet serving as the first electrode  41  in the sensor  40  produces magnetic flux lines between the first electrode  41  and a paired one of the second electrodes  42 A,  42 B,  42 C,  42 D. Thus, the conductive substance contained in the lubricating oil is attracted to the attracting portion  45 . The first electrode  41  and each one of the second electrodes  42 A,  42 B,  42 C,  42 D correspond to a detecting unit positioned on the side surface of the sensor body. 
     If the conductive substance is gathered in the vicinity of the attracting portion  45  in this manner, the detecting units experience a change in electrical resistance. While no conductive particles are attracted, the detecting units exhibit the same electrical resistance. The direction of the detection performed by each of the detecting units is determined by the gap G 1  and extends along the axial line axd. 
     The first electrode  41  and the second electrodes  42 A.  42 B,  42 C,  42 D are respectively connected to output lines as shown in  FIG. 26 , and each detecting unit is electrically connected to the sensing unit  5  via a corresponding one of the output lines. In this embodiment, the detecting units are connected in parallel to each other, and voltage is applied between the first electrode  41  and each of the second electrodes  42 A,  42 B,  42 C,  42 D by the same voltage source. The sensing unit  5  outputs a signal when a designated number of the detecting units experience a change in electrical resistance. For example, the sensing unit  5  may be configured to output a signal to a higher-level control device such as a manipulator when two or more of the detecting units experience a drop in electrical resistance, or configured to output a signal when all of the detecting units experience a drop in electrical resistance. 
     As described above, the sensor  40  includes the plurality of detecting units, and the sensing unit  5  outputs a signal when a designated number of the detecting units experience a drop in electrical resistance. In this way, the sensing unit  5  can be configured to output no signal when just one of the detecting units experiences a change in electrical resistance caused by a large-diameter conductive piece. Furthermore, in the sensor  40 , the sensing unit  5  can be configured to output a signal under a designated condition. Therefore, the single sensor  40  can be configured to output a signal in a timely and optimal manner for individual users, who have different requests for failure prediction timing. 
     While no conductive particles are attracted, the detecting units all exhibit the same electrical resistance. This can lower the voltage to be applied to the sensor  40 . The detecting units are connected in parallel to each other. This can lower the voltage applied between the paired electrodes in each detecting unit. 
     This embodiment can also produce the same effects as the above-described embodiments. 
     The following describes a sensor relating to a ninth embodiment of the present invention with reference to the drawings.  FIG. 27  is a sectional view taken along the axial direction and showing the sensor relating to the ninth embodiment of the present invention. The ninth embodiment is different from the above-described seventh and eighth embodiments in terms of the attracting portion and magnet. Except for these different features, the common constituents are assigned with the same reference numerals and are not described here. 
     In the present embodiment, a magnet  49  is buried inside the attracting portion  45  and the outer peripheral surface of the magnet  49  is not externally exposed. The magnet  49  is in contact, in the direction along the axial line axd, with a surface of the first electrode  41  that faces the second electrode  42 . Here, the first electrode  41  is a conductor shaped like a circular plate. The magnet  49  is preferably close to the first and second electrodes  41  and  42 . The magnet  49  may be in contact with the second electrode  42 . A casing  46   b  is arranged on the first electrode  41  on the upper end surface  40   a  side. The casing  46   b  covers a surface of the first electrode  41  that faces the upper end surface  40   a.    
     The first electrode  41 , magnet  49 , attracting portion  45 , second electrode  42  and casing  46  are stacked on each other in the direction along the axial line axd. The first and second electrodes  41  and  42  have substantially the same radial size, so that the outer peripheral surfaces  41   a  and  42   a  form a flush cylindrical surface. The magnet  49  has a smaller radial size than the first and second electrodes  41  and  42 . The attracting portion  45  has an attracting protrusion  45 G 1  having a larger radial size than the first and second electrodes  41  and  42 . 
     The attracting portion (insulator)  45  is, for example, made of an insulating non-magnetic material such as a resin. The magnet  49  produces magnetic flux lines between the first electrode  41  and the second electrode  42 . Thus, the conductive substance contained in the lubricating oil is gathered to the vicinity of the attracting protrusion  45 G 1 . Note that the term “detection region” denotes the region within which the lubricating oil circulates. 
     In the sensor  40  relating to the present embodiment, a sensing plane denotes the surface of the attracting protrusion  45 G 1  that protrudes radially outward beyond the cylinder connecting the outer peripheral surface  41   a  of the first electrode  41  and the outer peripheral surface  42   a  of the second electrode  42 , which are substantially flush with each other. In other words, on the sensing plane, conductive abrasion powder is attracted between the first electrode  41  and the second electrode  42  by the magnetic flux lines, so that the first electrode  41  and the second electrode  42  are electrically connected. This causes a change in resistance between the first electrode  41  and the second electrode  42 , which is to be detected. Note that the outer peripheral surface  41   a  of the first electrode  41  may not need to be flush with the outer peripheral surface  42   a  of the second electrode  42 . 
     As the creepage distance between the first electrode  41  and the second electrode  42  increases, a larger amount of conductive abrasion powder can be attracted before the resistance between the first electrode  41  and the second electrode  42  drops to a threshold value or before a short circuit occurs. As the creepage distance between the first electrode  41  and the second electrode  42  decreases, a smaller amount of conductive abrasion powder can be attracted before the resistance between the first electrode  41  and the second electrode  42  drops to a threshold value or before a short circuit occurs. 
     The creepage distance between the first electrode  41  and the second electrode  42 , which determines the detection sensitivity of the detecting units, depends on how much the attracting protrusion  45 G 1  of the attracting portion  45  protrudes. In other words, the creepage distance between the first electrode  41  and the second electrode  42 , between which a short circuit may occur when conductive particles are gathered, can be changed by increasing or decreasing the protrusion height of the attracting protrusion  45 G 1  in the radial direction and the thickness of the attracting protrusion  45 G 1  in the direction along the axial line axd. The thickness of the attracting protrusion  45 G 1  in the direction along the axial line axd depends on the gap G 1  and the thickness of the magnet  49 . 
     The sensor  40  relating to the present embodiment has a group of attracting portions  45  that are different in protrusion height and thickness. The sensor  40  relating to the present embodiment can be assembled with a selected one of the attracting portions  45 . In other words, the group of attracting portions  45  that are different from each other in thickness (the size in the axial line axd direction) and/or radial protrusion height serves as a sensitivity adjusting unit. In this way, the creepage distance between the first electrode  41  and the second electrode  42  can be selected from among a plurality of values by making a selection in the sensitivity adjusting unit. Having the sensitivity adjusting unit, the sensor  40  relating to the present embodiment is capable of setting the sensitivity at a predetermined level. 
     Speed reducers of different models (sizes) produce different amounts of iron powder (abrasion powder) during initial wear period. In the case of large speed reducers, a large amount of iron powder is produced by initial wear, and such iron powder may fill the electrical gap in the sensor between the electrodes  41  and  42 . If such is the case, the sensor may react and erroneously operate. Therefore, the electrical gap in the sensor needs to be determined considering the model of the speed reducer, but this requirement may disadvantageously result in a larger sensor size in the diameter direction. To address this issue, the sensor  40  relating to the present embodiment has a sensitivity adjusting unit, which includes attracting portions  45  with different sizes. This configuration produces the same effects as the enlargement of the sensor in the diameter direction and thus allows the sensor  40  to maintain the size. 
     This embodiment can also produce the same effects as the above-described embodiments. 
     Modified Example 1 
     Furthermore, the sensor according to a modified example 1 relating to the above-described embodiment of the present invention may include the following configurations. 
     The sensor according to the modified example 1 includes:
         a first electrode;   a second electrode spaced away from the first electrode with a gap being provided therebetween in an axial direction; and   an attracting portion arranged in the gap, the attracting portion having an outer peripheral surface made of an insulating material,   wherein the first electrode, the attracting portion and the second electrode are stacked on each other in the axial direction,   wherein conductive particles are attracted to the outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the second electrode, resulting in a change in electrical resistance between the first electrode and the second electrode.   wherein a magnet is positioned closer in the axial direction to the first electrode at least than to the second electrode, and   wherein the magnet is arranged to form a magnetic flux line extending in the axial direction.       

     In the sensor according to the modified example 1, the first electrode is a magnet. 
     Modified Example 2 
     Furthermore, the sensor according to a modified example 2 relating to the above-described embodiment of the present invention may include the following configurations. 
     The sensor according to the modified example 2 includes:
         a cylindrical sensor body,   wherein the sensor body includes:   a first electrode, an attracting portion, and a second electrode stacked on each other in an axial direction; and   a magnet magnetized in the axial direction, and   wherein conductive particles are attracted to an outer peripheral surface of the attracting portion, so that a short circuit is caused in the axial direction between the first electrode and the outer peripheral surface of the second electrode, resulting in a change in electrical resistance between the first electrode and the second electrode.       

     In the sensor according to the modified example 2, the second electrode is divided into portions in a circumferential direction of the sensor body.