GB2217849A

GB2217849A - Position sensor

Info

Publication number: GB2217849A
Application number: GB8904012A
Authority: GB
Inventors: David Geoffrey Searle
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1988-04-22
Filing date: 1989-02-22
Publication date: 1989-11-01
Anticipated expiration: 2009-02-22
Also published as: GB2217849B; GB8904012D0; GB8809506D0

Abstract

A homopolar magnetic field, which is asymmetric with respect to the axis of a rotor 6, is produced by a stator-mounted exciting coil 5 of a sensor for indicating the position of the rotor 6 in an electric motor. A further coil eg single loop 7 on the rotor 6 modifies the field of the exciting coil 5. This modification varies in dependence on the angular position of the rotor 6. A resulting output signal is produced in a sensing coil 10 carried by the stator. A second sensing coil 10' displaced 90 DEG from the first may be provided. <IMAGE>

Description

Position Sensor This invention relates to position sensors, and more especially, though not exclusively, to a form of position sensor for providing an indication of the position of the rotor of a D.C.

brushless motor. Such motors may be employed, for example, for actuating the arms and grab mechanisms of manipulators. In use of such manipulators it is necessary for an indication of the position of the arm joints and grab mechanisms to be accurately relayed to a control system remote from the arm.

An object of the invention is to provide a position sensor suitable for use with a D.C. brushless motor employed for such a purpose. However, the invention may also find application for sensing the position of the rotors of D.C. brushless motors, as well as other forms of motors designed for a variety of different purposes where accurate information concerning the position of the motor rotor, or equipment operated thereby, is required. The invention also incorporates within its scope electric motors incorporating such position sensors.

According to the invention a position sensor for providing an indication of the position of the rotor of an electric motor comprises a first coil carried by the motor stator and energisable by the passage of an alternating current therethrough to produce a homopolar magnetic field which is asymmetric with respect to the rotor axis, a second coil carried by the rotor for influencing the magnetic field produced by the first coil, in a manner such that the flux pattern of the magnetic field is modified in dependence upon the angular position of the rotor, and at least one sensing coil carried by the stator in a position such as to produce output signal which is dependent upon the modified flux pattern.

Preferably the coil carried by the rotor is in the form of a complete loop having end portions of semicircular shape disposed close to the rotor surface, the stator sensing coil similarly having semicircular end portions disposed close to the end portions of the rotor coil. The field produced within the rotor coil will almost exactly oppose that part of the homopolar field through the coil so that no resultant flux is produced, whereas outside the coil the field will be strengthened. Accordingly, when the rotor coil is immediately adjacent the sensing coil the output from the latter will be a minimum, whereas when it is immediately opposite the sensing coil the output signal from the latter will be at a maximum.

Preferably two such sensing coils are employed, displaced 90e from each other. Then there will be a unique relationship between the output signals from the two sensing coils at each rotor position.

The first coil of the position sensor, which constitutes the exciting coil, is preferably disposed around the end of the stator, approximately in the same plane as the end portions of the rotor coil and sensing coil. The exciting coil may, for example, be wound around the stator end windings using the end windings as a support, although provided the flux encircles the respective ends of the rotor and sensing coils its position is not critical.

In some cases the exciting coil is provided by series connected windings at opposite ends of the stator, each winding preferably being disposed around the end portions of the rotor and sensing coils at the respective end of the stator.

The advantages of a position sensor in accordance with the invention are that all the coils of the sensor can be disposed within the space envelope of the motor with which it is associated, it does not require any mechanical coupling when setting up, it is capable of providing an absolute rotor position indication after a supply failure and reconnection, and the output signal is capable of processing to provide commutation commands. In addition it is capable of operation in the environment of, and up to the same temperatures as the motor, and is resistant to radiation, so that it is particularly suitable for use in motors driving manipulator arms in hazardous environments for providing an indication of the positions of the arms.

The invention will be further explained by describing by way of example, with reference to Figures 1 to 4 of the accompanying schematic drawings, one form of position sensor for indicating the position of the rotor of an electric motor, and a modification thereof.

In the drawings Figures 1 and 2, represents an exploded perspective view and an axial section, in diagrammatic form, of a brushless D.C. electric motor incorporating a rotor position indicator in accordance with the invention.

The motor, which is shown without its casing, comprises a stator 1 having a main stator winding 2 set in axially directed slots in the stator core 3, in the usual way, with overhanging windings 4.

The stator core also carries an exciting coil 5, forming part of the position sensor, and comprising a series of turns at each end of the stator, disposed around the end windings 4 of the main stator winding 2, the two sections of the exciting coil 5 being connected in series with each other.

The motor rotor 6 is of convenitonal construction for a brushless D.C. motor but carries, in addition, a coil 7 in the form of a complete loop set into the surface of the rotor core, having parallel side regions 8 spaced 1800 from each other joined by semicircular end regions 9 which, in the assembled motor, lie approximately in the transverse planes containing the respective turns of the exciting coil 5.

The stator also carries two further coils 10, 10' each comprising a series of turns having parallel side regions set in respective slots within the stator core and spaced 1800 from each other, and semicircular end regions 12, 12' which also lie approximately in the transverse planes containing the end regions 9 of the rotor coil 7 and the respective turns of the exciting coil 5.

The two coils 10, 10' are 90 from each other around the stator axis.

Then the passage of a cyclically varying electric current through the exciting coil 5 to produce an alternative homopolar magnetic field causes a voltage signal to be induced in the further coils 10, 10', which constitute the sensing coils of the sensor, the output from which coils are fed to a suitable indicator or control equipment.

The magnetic flux produced is modified in dependence -upon the position of the rotor coil 7 to produce an output from the sensing coil 10 as indicated diagrammatically at 20 in Figure 3 which assumes that the coil has zero resistance. Thus when the rotor coil 7 is adjacent to this sensing coil the flux through the sensing coil would be zero if the rotor coil was fully effective at eliminating flux through itself. At this rotor position therefore there would be no voltage in the sensing coil. As the rotor turns (in either direction) the flux outside of the rotor coil would progressively link with the sensing coil to produce a voltage signal in the coil which would be directly proportional to rotor positional displacement from the zero voltage position.This would continue until the rotor coil was opposite the position of the sensing coil when a maximum induced voltage would appear in the sensing coil.

Because the coil 10 will in reality have some resistance there will be some small amount of flux through the rotor coil. The one disadvantage of this sensing field is that it will induce a voltage in the driving winding coils because there is a net flux across the stator/rotor gap, but as this voltage will be very small compared to the driving voltage this will in general be acceptable although where necessary it may be filtered out.

It will be observed that in the arrangement just described the output signal from the single coil 10 will vary in an identical manner as the rotor is moved on either side of the maximum or minimum output signal position with the rotor coil opposite or adjacent to the sensing coil 10. Accordingly by arranging for the sensing coil 10' to be offset by 90' from the coil 10 the output from the sensing coil 10' will be similar to that of the coil 10 but displaced by 90 from it as indicated by the broken line 20' in Figure 3.

There is then a unique relationship between the signals from the coils 10, 10' at each rotor position. This is shown in Figure 4 which illustrates the traces obtained from the two sensing coils 10, 10' on an oscilloscope screen at four positions of the rotor. In Figure 4a the top trace is derived from the sensing coil 10 when the rotor is adjacent to it, Figures 4b, 4c and 4d showing the traces obtained when the rotor is rotated through 90, 180 and 2700. The lower trace illustrates the output from the second sensing coil 10', the maximum signal being obtained as shown in Figure 4b when the rotor is displaced 90e from its initial position.

In a modification, not shown, the exciting coil 5 has turns at one end of the stator only.

In both embodiments it is important that the rotor coil 7 be set close to the rotor surface at its ends, so that the magnetic flux can fully penetrate the coil.

The path of the homopolar flux is not, of course, confined to the stator and rotor cores as a return path has to be provided.

As is usual with homopolar fields the flux must therefore split in the rotor and leave via the two shaft ends, then pass through the motor end plates and motor frame and then enter the back of the stator core.

The most satisfactory wire size and number of turns for the exciting and sensing coils for any particular application of the invention will depend upon the motor size and use to which it is to be put, but may readily be found by trial for each case.

Claims

1. A position sensor for providing an indication of the position of the rotor of an electric motor comprising a first coil carried by the motor stator and energisable by the passage of an alternating current therethrough to produce a homopolar magnetic field which is asymmetric with respect to the rotor axis, a second coil carried by the rotor for influencing the magnetic field produced by the first coil, in a manner such that the flux pattern of the magnetic field is modified in dependence upon the angular position of the rotor, and at least one sensing coil carried by the stator in a position such as to produce output signal which is dependent upon the modified flux pattern.

2. A position sensor according to Claim 1 wherein the coil carried by the rotor is in the form of a complete loop having end portions of semicircular shape disposed close to the rotor surface, and the stator sensing coil similarly has semicircular end portions disposed close to the end portions of the rotor coil.

3. A position-sensor according to Claim 2 incorporating two said sensing coils displaced 90 from each other about the axis of the stator.

4. A position sensor according to any preceding claim wherein said first stator coil is disposed around the end of the stator, approximately in the same plane as the end portions of the rotor coil and sensing coil.

5. A position sensor according to Claim 4 wherein said first stator coil is wound around the stator end windings.

6. A position sensor according to Claim 4 or 5 wherein said first stator coil is provided by series connected windings at opposite ends of the stator, each winding being disposed around the end portions of the rotor and sensing coils at the respective end of the stator.

7. An electric motor incorporating a position sensor according to any preceding claim.

8. An electric motor according to Claim 7 which is a D.C.

brushless motor.

9. A brushless D.C. electric motor incorporating a rotor position indicator substantially as shown in and arranged to operate substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.