Abstract:
A shaft position detection device for detecting a certain position on a shaft comprising: a movable shaft including a detection portion to be detected, a characteristic of which is different from the other portions; a position detection coil to which the movable shaft is freely inserted; an inductance detecting means for detecting changing of inductance between a case of the movable shaft being in the position detection coil and a case of the movable shaft being out of the position detection coil; in which a position of the detection portion in response to the changing of the inductance. The difference of the magnetic characteristic is defined by a cross section area of the shaft, winding a magnetic tape, forming a magnetic layer.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a shaft position detection sensor which decreases erroneous detections due to grease coated on a shaft. The shaft position detection sensor magnetically detects a position on the shaft which is constructed in such a manner that a permeability or a cross section area of a detection portion to be detected is different from the other portions. 
     In the case that a position on a ball screw is detected in a movement mechanism using a ball screw and a ball nut, a position detection sensor as described later has been known. An optical sensor is provided on the ball nut while a portion where a reflectance rate being different from other portions is provided on a certain portion of the ball screw, so that the strength of the reflection light is detected. 
     However, in such position sensor optically detecting the position, it is necessary to place the detecting unit extremely close to the ball screw. Therefore, the optical detection unit may be dirty due to grease coated on a surface of the ball screw. As described above, detection accuracy may be deteriorated. 
     SUMMARY OF THE INVENTION 
     In order to solve the above mentioned problems, the present invention provides a shaft position sensor which magnetically detects a position on a shaft which is constructed in a manner that a permeability of a detection portion to be detected is different from the other portions or a shaft which is made of a magnetic material and is structured in a manner that a cross section area of a detection portion to be detected is different from that of the other portions. The shaft position sensor comprises a detection coil into which the shaft is inserted, and a detection unit which detects a difference between an inductance detected when a detection portion to be detected is in the detection coil and an inductance when a detection portion is out of the detection coil. 
     According to the invention, the shaft is constructed in a manner that a permeability or a cross section area of a detection portion to be detected is different from that of the other portions, so that the position on the shaft is detected by detecting the inductance of the detection coil. Therefore, the sensor is not influenced due to the grease coated on the shaft. Accordingly, the detection accuracy can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a basic structure of a shaft position detection sensor of the present invention; 
     FIGS. 2A to 2C show a detection portion formed on a shaft, FIG. 2A shows a cross section area being changed at a detection portion, FIG. 2B shows a magnetic tape wound on the shaft, and FIG. 2C shows a magnetic layer formed on the shaft; 
     FIG. 3 shows a cross section of a coil and a detection portion where the shaft position detection sensor of the invention is applied to detect a position on a ball screw; 
     FIG. 4 is a perspective view of the coil housing of FIG. 3; 
     FIG. 5 is an enlarged cross section of a primary portion showing a storage condition of coil; 
     FIG. 6, comprised of FIGS. 6A and 6B is an explanation diagram for a surface effect at edge portion of the shaft; 
     FIG. 7 is a block diagram showing a structure of the detection portion of the present invention; and 
     FIG. 8 is a time chart showing an operation of detection unit as shown in FIG. 7. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A shaft position detection sensor of the present invention will be described with reference to the drawings. 
     With reference to FIGS. 1 and 2, the detection principle of the shaft detection sensor of the invention is explained. 
     FIG. 1 shows a basic structure of the shaft position sensor 1. A shaft has a detection portion 2a to be detected the magnetic characteristic of which is different from the other portions. For example, the followings arrangements are selectable. First, as shown in FIG. 2A, a cross section area of the detection portion 2a is smaller than that of the other portions so that step portions 2a and 2b are formed. Second, as shown in FIG. 2B, a magnetic tape 2c is wound at the detection portion 2a. Third, as shown in FIG. 2C, a magnetic layer 2d is formed at the detection portion 2a by deposition or sputtering so that the magnetic characteristic of the detection portion 2a is made different from the other portion. The detection portion 2a is not limited at one portion. The detection portion 2a may be provided at plural portions along the shaft 2. 
     Reference numeral 3 denotes a detection coil. The shaft 2 is inserted into the detection coil 3. The shaft 2 is straightly moved relative to the detection coil 3 indicated by arrow A as shown in FIG. 1. An inductance difference is detected between when a detection portion 2a is in the detection coil 3 and a case of a detection portion 2a is out of detection coil 3. In FIG. 1, the detection coil 3 is fixed, and the shaft is movable. Needless to say, the relationship between the detection coil 3 and the shaft 2 may be reversed, that is, the detection coil 3 is movable and the shaft 2 is fixed. 
     Reference numeral 4 is a resonance portion which uses the detection coil 3 as an inductance element. The output of the resonance portion 4 is applied to an alternating component detection portion 5 at a subsequent stage. The alternating component detection portion 5 detects an alternating component from the signal applied from the resonance portion 4. An output signal of the alternating component detection portion 5 is applied to a signal period detection portion 6 at a subsequent stage. The difference between the inductance of the detection portion 2a and that of the other portions is detected as a change in period of the signal detected by the signal period detection portion. 
     An output of the signal period detection portion 6 is applied to a binarize portion 7 at subsequent stage in order to convert it into a binary signal which represents whether the detection portion 2a of the shaft 2 is detected. 
     As described above, according to the invention, because the shaft position is magnetically detected, the decline of the detection accuracy due to the grease coated on the shaft 2 is prevented in comparison with the optical detection. 
     FIGS. 3 to 8 show an embodiment of the present invention. The embodiment relates to the position detection of the ball screw to detect a stroke end or a home position. A cross section area of the ball screw is varied as shown in FIG. 2A. 
     FIG. 3 shows a partial view of the ball screw and a coil portion. 
     In the drawing, reference numeral 8 is a ball screw, outer surface of which a spiral lead groove 8a of about 1 mm depth is formed. A detection portion 9 to be detected is formed on the ball screw 8. A cross section area is smaller than that of the other portions so that step portions 10 are formed. Namely, a shallow groove 11 is formed on the ball screw 8 in the depth d (about 0.15 mm). This shallow grove 11 is designed not to affect the engagement of the ball screw 8 and ball nut. The width W of the shallow groove is made greater than the thickness of the coil housing. 
     A coil 12 is accommodated in a coil housing 13. As shown in FIG. 4, the coil housing 13 is composed of a mounting plate 14 made of Ni--Fe system material and a cylindrical coil accommodating part 15 which is provided on the mounting plate 14. An insertion aperture 16 is formed at a center of the coil accommodating part 15 allowing the ball screw 8 to penetrate thereinto. The coil housing 13 is fixed to the ball nut or a fixing bracket in a manner that the mounting plate 14 is attached by screws or the like. 
     The coil accommodating portion 15 is composed of an outer case 15a having L-shaped cross section and an inner bobbin 15b having U-shaped cross section as shown in FIG. 5. 
     The outer case 15a is made of Ni--Fe system material such as permalloy in order to reduce influence of external magnetic flux or temperature and to concentrate the sensing magnetic flux. 
     The inner bobbin 15b made of synthetic resin is provided to protect the coil 12 from the disperse of grease from the ball screw 8. 
     The coil 12 is wound around the inner bobbin 15b and is disposed in a space defined between the outer case 15a and the inner bobbin 15b. The number of windings of the coil 12 is about 50 to 100 times, and the diameter of coil is about 0.05 to 0.1 mm. As seen in FIG. 4, the ends of the coil 12 are lead out from a hole 17 which is formed on an outer circumferential surface of the coil accommodating part 15 at a position close to the mounting plate 14. 
     In this embodiment of the invention, the following can be employed as a detection principle. 
     (a) In case of energization frequency for coil 12 being low, the inductance is varied in proportion to the cross section area of the ball screw 8. Namely, the difference between the cross section area of the detection portion 9 and that of the other portions is detected as a change of the inductance. 
     (b) In case of energization frequency being high, it is well known as a surface effect that if an edge is formed on the surface of the shaft, then the magnetic flux is concentrated to the edge. Therefore, in case of energization frequency for coil 12 being high, the magnetic flux is concentrated to the edge of the step portion 10 so that the inductance will be varied due to this edge. 
     If the energization frequency for coil 12 is in a range from 500K to 1 M Hz, the magnetic flux tend to pass the surface of the shaft as shown in FIG. 6A. If an edge exist on the shaft, the magnetic flux is concentrated on the edge as shown in FIG. 6B. Therefore, when the detection portion 9 passes the coil 12, the inductance will raise several percent. 
     One can choose from the above mentioned two ways in consideration of the depth of the groove 11, S/N ratio of the signal, and stability of a sensitivity. Either way may be used even if a spiral lead groove 8a is formed on the ball screw 8 because the cross section area of the ball screw 8 is constant irrespective of the rotational angle. Therefore, it is possible to decrease the detection error of the inductance. 
     FIG. 7 shows a specific example of the detection circuit. FIG. 8 shows waveform diagrams of the corresponding part of FIG. 7. 
     Reference numeral 19 is a resonance circuit which uses the coil 12 as an inductance circuit. For example, a modified Wien-bridge oscillation circuit is used. An output signal S19 is reversed to the detection signal S12 of the coil 12 in relationship of the leading and trailing edges. In the drawings, reference &#34;T&#34; represents a period and &#34;ΔT&#34; represents a variation of the period in response to changing of the inductance. 
     The output signal S19 of the resonance circuit 19 is sent to a buffer 20 to be wave-shaped, and then sent to a counter 21 as a clock signal. 
     The counter divides the output signal 20 into 1/n (n≧2, n being integral number) so that detection accuracy of the inductance change is amplified to n times. (See period n×(T+ΔT)). Accordingly, the change of inductance is made easy to detect. 
     An output signal S21 of the counter 21 is subsequently sent to a mono-multi-vibrator 22 and a retriggerable mono-multi-vibrator 23 to measure a period of the signal. 
     The mono-multi-vibrator 22 is designed to have an adjustable reference time constant generated by an external resistor and a capacitor so that an output pulse whose pulse width is in response to the inductance change is obtained with the signal width of the reference time constant responsive to the trailing timing of the output signal S21 of the counter 21. Accordingly, a threshold level for inductance is predetermined so that the reference time constant is determined to be zero pulse width if the inductance is lower than the predetermined threshold level. In FIG. 8, the width ΔU of the signal S22 is varied in response to the change of the inductance. For example, the threshold level is determined as an intermediate value between the inductance of the detection portion of the ball screw and that of the other portions. 
     The retriggerable mono-multi-vibrator 23 receives an output pulse from the mono-multi-vibrator 22 so that the reference time constant (see RC&#39; in FIG. 8) is set by an external resistor and a capacitor in a manner that the vibrator 23 outputs &#34;0&#34; in case of the pulse width being zero while outputs &#34;1&#34; in case of the pulse width being not zero. (It should be noted that it is negative logic.) Then, the binarized signal is sent to an open-collector transistor 24 to obtain a detection signal. 
     As described above, according to the invention, the detection portion of which a permeability is different from that of the other portions in the shaft is detected by the inductance change in response to whether the detection portion is located in the detection coil. Further, the detection portion having a cross section area different from that of the other portions in the shaft made of magnetic material causes the inductance change in response to whether the detection portion is located in the detection coil. Accordingly, the detection accuracy is prevented from lowering due to grease coated on the shaft. 
     Moreover, according to the invention, a magnetic tape may be wound on the shaft or a magnetic layer may be formed to define the detection portion. That is, it is possible to form a detection portion after the shaft has been produced. 
     Still further, according to the invention, one can choose to detect the inductance change in proportion to the cross section area of the shaft or the inductance change due to an edge between the detection portion and the other portions, according to the energization frequency applied to the detection coil. 
     Still further, according to the invention, the alternating component of the oscillation signal of the resonance portion, which uses the detection coil as an inductance element, is detected. Then, the period of the alternating component is detected and is binarized so that the position of the detection portion is detected in a relatively simple manner. 
     Still further, the invention is applied to the ball screw of which a cross section area is constant even if a lead groove is formed on the surface. Therefore, it is possible to decrease the unevenness of the detection accuracy.