Inductive position sensor

Abstract

A detector to measure the displacement of relatively moveable bodies along an axis comprising: a resonant electrical intermediate device further comprising an inductor, whose width varies along the displacement axis, and a capacitor in electrical series which co-operates with an antenna comprising transmit and receive windings whose mutual inductance varies according to the position of the electrical intermediate device relative to the antenna.

1 . An inductive position detector comprising a first inductive device and a second inductive device which defines a measurement path and comprises at least two receive windings, wherein:
 relative movement of the first and second inductive devices changes the position of the first inductive device on the measurement path;   the first inductive device comprises a substantially laminar passive resonant circuit formed by an inductive winding in series with a capacitor, with the inductive winding forming a loop whose extent in a direction transverse to the measurement path varies periodically;   the receive windings of the second inductive device are each formed by at least a respective outward conductive winding in series with a respective return conductive winding;   the outward winding and the return winding of each receive winding cross at least three times to form at least four inductive loops where adjacent loops have opposite current flow, in use, and the transverse distance of each outward winding to the measurement path varies periodically with distance along the measurement path; and   the second inductive device is configured to induce an alternating current in the passive resonant circuit, in use, such that the induced alternating current in the passive resonant circuit induces an alternating signal in each receive winding by virtue of the mutual inductance of the receive windings and the passive resonant circuit, which signals are indicative of the position of the first inductive device on the measurement path.   
   
   
       2 . An inductive position detector as claimed in  claim 1 , wherein the transverse distance of the return winding of each receive winding to the measurement path varies periodically with distance along the measurement path. 
   
   
       3 . An inductive position detector as claimed in  claim 1 , wherein the area defined by each of the said inductive loops is substantially equal. 
   
   
       4 . An inductive position detector as claimed in  claim 1 , comprising a plurality of first inductive devices, each comprising a respective resonant circuit, with each resonant circuit having a different resonant frequency. 
   
   
       5 . An inductive position detector as claimed in  claim 1 , wherein at least one receive winding of the second inductive device is supplied, in use, with a pulsed alternating current which induces an alternating current in the passive resonant circuit, such that the passive resonant circuit induces an alternating current in the receive windings of the second inductive device in the off periods of the pulsed alternating current. 
   
   
       6 . An inductive position detector as claimed in  claim 1 , wherein the second inductive device comprises a transmit winding arranged to induce an alternating current in the passive resonant circuit. 
   
   
       7 . An inductive position detector as claimed in  claim 6 , wherein the transmit winding has a substantially constant width along the measurement path. 
   
   
       8 . An inductive position detector as claimed in  claim 1 , wherein the shape of the periodic variation of the transverse distance of the outward winding to the measurement path with distance along the measurement path is rectangular, sinusoidal, triangular, hexagonal or polygonal. 
   
   
       9 . An inductive position detector as claimed in  claim 1 , wherein the period of the variation of the extent of the inductive winding of the first inductive device in the direction transverse to the measurement path is the same as that for the variation of the transverse distance of the outward winding of each receive winding. 
   
   
       10 . An inductive position detector as claimed in  claim 1 , wherein the period of the periodic variation for each outward conductive winding is the same and the second receive winding is displaced relative to the first receive winding by a fixed distance substantially equal to an integer multiple of one quarter of the period. 
   
   
       11 . An inductive position detector as claimed in  claim 1 , wherein the period of the periodic variation for each outward conductive winding is different. 
   
   
       12 . An inductive position detector as claimed in  claim 11 , wherein the difference in the periods of the two outward windings is such that the combination of the distance of each outward winding from the measurement path at a given position along the measurement path uniquely identifies that position along the measurement path. 
   
   
       13 . An inductive position detector as claimed in  claim 1 , wherein the two outward windings are spaced in a direction transverse to the measurement path. 
   
   
       14 . An inductive position detector as claimed in  claim 1  further comprising a signal processor configured to generate a speed signal by processing the induced signal in at least one of the receive windings and further configured to generate a position signal by comparison of the induced signal in the two receive windings. 
   
   
       15 . An inductive position detector as claimed in  claim 1 , wherein the windings are provided as conductive tracks on a printed circuit board. 
   
   
       16 . An inductive position detector as claimed in  claim 1 , wherein the windings of the second inductive device are provided on a substantially laminar substrate having two opposed faces and for each receive winding the outward conductive winding is provided on one of said faces and the return conductive winding is provided on the other of said faces. 
   
   
       17 . An inductive position detector as claimed in  claim 1 , wherein the measurement path is linear. 
   
   
       18 . An inductive position detector as claimed in  claim 1 , wherein the measurement path is curved, particularly circular. 
   
   
       19 . A method of temperature compensation in a detector as claimed in  claim 1 , the method comprising measuring the resistance of a winding of the second inductive device. 
   
   
       20 . A method of measuring torque on a rotating shaft, the method comprising:
 providing an inductive position detector as claimed in  claim 1 ;   mounting the first inductive device about the shaft for rotation therewith;   mounting the second inductive device on the shaft for rotation therewith such that measurement path follows the circumferential direction of the shaft;   detecting the induced signal in the second inductive device by means of inductive coupling;   determining relative movement of the first and second inductive devices indicative of a torque on the shaft.