1. Field of the Invention
The present invention relates to an apparatus for use in a position sensor which senses the relative position between at least two relatively movable members. The invention has application in linear and rotary inductive position sensors.
2. Related Art
WO95/31696 discloses an inductive position sensor in which one member carries an excitation winding and a number of sensor windings mounted on a sensor printed circuit board and the other member carries a resonator, also mounted on a printed circuit board. In operation, an alternating current is applied to the excitation winding which causes an alternating magnetic field to be set up in the region of the sensor printed circuit board. This field inductively couples with the resonator when it is placed in proximity to the sensor printed circuit board, to cause the resonator to resonate. This in turn generates its own spatially patterned alternating magnetic field in the vicinity of the sensor printed circuit board. This resonator magnetic field induces alternating signals in the sensor windings whose amplitude sinusoidally varies with the relative position between the two members. A similar system is disclosed in EP 0182085 which uses a conductive screen in place of the resonator. However, the use of a conductive screen has the disadvantages that the output signal levels are much smaller and that the signal induced in the sensor windings by the conductive screen has substantially the same phase but opposite polarity to any signal generated in the sensor windings directly from the excitation winding. In contrast, with a resonator, there is a 90xc2x0 phase shift between the signal induced in the sensor windings by the resonator and any signal induced in the sensor windings by the excitation winding. Therefore, with a resonator, it is possible to employ synchronous detection to isolate the resonator signal from any excitation breakthrough.
The design of the sensor windings in both these systems is critical to the operation of the position sensor. In particular, they must be shaped so that there is the necessary sinusoidal coupling with the resonator and so that there is minimum coupling with the excitation winding. This is achieved by forming the sensor windings from a number of series connected loops arranged so that signals induced in adjacent loops by a common magnetic field oppose each other and by forming the excitation winding as one or more loops around the periphery of the sensor windings.
One aim of the present invention is to provide an alternative geometry of sensor and excitation windings.
The sensor windings and the excitation windings are preferably formed on a two-layer printed circuit board since these are cheap and easy to manufacture as compared with multi-layer circuit boards. For low cost, small size and optimum EMC performance, the electronics used to generate the excitation current and to process the signals induced in the sensor windings should be mounted on the same circuit board as the sensor windings. In the systems provided to date, these connections have been made by twisted pair flying leads, which are fiddly, expensive and can be unreliable. Connections can be made using conventional PCB track routing techniques. However, the connection tracks usually result (i) in a disturbance in the sinusoidal coupling relationship between the sensor windings and the resonator; and (ii) in the sensor windings being sensitive to the magnetic field generated by the excitation coil, both of which reduce the overall accuracy of the position sensor.
One solution to this problem is simply to use multilayer printed circuit boards. However, this approach increases cost and can reduce position sensing accuracy since several printed circuit board layers require accurate registration instead of just two.
According to one aspect, the present invention provides a transducer for use in a position encoder comprising: a first circuit having at least two loops connected in series and being arranged so that signals generated in the loops by a common electromagnetic field oppose each other; and a second circuit having at least one loop which extends along the measurement path and which is superimposed on but electrically isolated from the loops of the first circuit.
Such a transducer provides the advantage that for a given size of circuit board, the area of the loops of the first circuit can be maximised regardless of the number of turns of the second circuit.
This aspect also provides an apparatus for use in a position encoder comprising: a first plurality of conductor tracks arranged along a measurement path; a second plurality of conductor tracks arranged along the measurement path; wherein the first and second plurality of conductor tracks are superimposed on each other to define (i) a first circuit having at least two loops along the measurement path, the loops being connected in series and being arranged so that signals generated in the two loops by a common electromagnetic field oppose each other; and (ii) a second circuit having at least one loop which extends along said measurement path; characterised in that the conductor tracks of said first plurality of conductor tracks which form part of said first circuit are arranged in first and second groups which are transversely spaced relative to said measurement path; the conductor tracks of said second plurality of conductor tracks which form part of the first circuit are arranged for connecting conductor tracks in the first group with conductor tracks in the second group; and the conductor track or tracks of the second plurality of conductor tracks which form said at least one loop of said second circuit are located between said first and second groups of conductor tracks.
According to another aspect, the present invention provides a transducer for use in a position sensor, comprising: two layers of conductor tracks insulated, at least at any crossovers, by an insulator, the conductor tracks being arranged so as to define (i) a first circuit having at least two loops which extend along a measurement path, said loops being connected in series and being arranged so that signals induced in said at least two loops by a common magnetic field oppose each other; and (ii) a second circuit having at least one loop which extends along said measurement path and which is superimposed on but electrically isolated from said at least two loops of said first circuit; wherein an electromagnetic coupling area between the at least one loop of said second circuit and one or more loops of said first circuit is varied to oppose the effect of a variation coupling between the at least one loop of the second circuit and a loop of the first circuit caused by an arrangement of connection tracks to said second circuit.
Preferably, in this aspect, the second circuit comprises a multi-start spiral winding, and wherein said variation in coupling areas is provided in the vicinity of the or each start to the spiral winding.