Electricity: measuring and testing – Magnetic – Displacement
Patent
1991-05-08
1993-05-25
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
Displacement
32420716, 32420717, 32420724, 32420725, 310168, 34087031, G01B 714, G01B 730, G01D 520
Patent
active
052143783
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to homopolar inductive displacement sensors which enable the angular displacement of a shaft or linear displacement of an element to be converted into an a.c. analogue voltage.
BACKGROUND ART
Inductive sensors having a plurality of wound poles arranged along a displacement path are described in WO 88/06716, but they are of heteropolar construction. The direction of flux at any given moment between stator and rotor alternates between successive wound poles. For example, FIGS. 1 and 2 (which correspond to FIGS. 6 and 7 of WO 88/06716) show a similar pair of devices each comprising an annular stator 21 having twelve salient poles (k to v) bearing windings of alternating polarity and extending radially inwardly towards a rotor 22. This rotor has two 180.degree. portions of different effective permeability, either because they are sections 23,24 of different materials (FIG. 2) or because one section has a screening layer of a nonmagnetic conductor (FIG. 1). In each case, the windings on the poles confronting one section 23 have higher reactances than the windings on the other poles. As the rotor turns, each pole winding experiences a change in reactance as it is passed by the interface between the rotor sections. Thus measured voltage drops change with rotation in a stepwise fashion, or smoothly if the stator slots are skewed relative to the rotor interfaces between sections 23,24 or vice versa. Monitoring analogue voltages across orthogonal pairs of taps (AC and BD) allows a plus or minus 180.degree. analogue signal representative of rotor angle to be derived.
DISCLOSURE OF THE INVENTION
In contrast, the present invention provides a homopolar inductive device for providing a signal relating to relative angular displacement. That is, there are a plurality of wound poles arranged along a displacement path, with successive poles in a homopolar relationship. Generally a device according to the invention has its poles provided by a single winding which has one or more intermediate taps by which it is divided into a plurality of serially connected coil portions. The invention makes available a method of operating such a device to obtain such a signal.
It should be noted here that a heteropolar device is one in which the direction of flux alternates between adjacent wound poles, as mentioned above. In a homopolar device, the flux direction is the same between adjacent wound poles. This is true irrespective of the return path of the flux. Thus, the homopolar arrangement can be considered to be a single divided pole.
The invention provides an inductive displacement sensor for providing electrical output data relating to displacement along a path, said sensor comprising first and second elements, which extend along the path on respective sides and confront each other across said path; said first element having a plurality of coil portions (ab,bc) disposed along the path and oriented so that their axes of polarity cross the path to the other side; said second element comprising inductance affecting means adapted to affect the inductance of an adjacent coil portion and arranged so that at any given time, only a fraction of the coil portions of the first element are affected thereby; and wherein said inductance affecting means is displaceable relative to said first element, said relative displacement effectively displacing a region of affected inductance so that it applies to a spatially varying fraction of the coil portions (ab,bc); characterised in that said coil portions (ab,bc) are connected as a homopolar series so that when current is passed through said series so that any one coil portion is polarised N-S from the path side of the first element to the other path side, all of the coil portions have this polarity; and when any one coil portion is polarised S-N, all of the coil portions have this polarity. The second element may be a ferromagnetic element that is always adjacent all of the core portions but bears a screen which screens it from at least one, but not all, coi
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Radiodetection Limited
Strecker Gerard R.
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