GB2128752A

GB2128752A - Electromagnetic angular position sensor

Info

Publication number: GB2128752A
Application number: GB08327997A
Authority: GB
Inventors: Andrew William Temple
Original assignee: Brookes and Gatehouse Ltd
Current assignee: Navico UK Ltd
Priority date: 1982-10-20
Filing date: 1983-10-19
Publication date: 1984-05-02
Also published as: GB8327997D0; GB2128752B

Abstract

An angular position sensor comprises a generator coil 10 driven by an oscillator 20 to provide an alternating magnetic field which is symmetrical about an axis. A conductive annular rotor 16 of, e.g. copper, or other material such that it distorts the magnetic field around it, is arranged so that it is asymetrical about its axis of rotation and lies outside the equatorial plane of the magnetic field. As the rotor 16 rotates about its axis, the emf induced in two sensing coils 12 and 14 disposed within the magnetic field varies in dependence upon the angular position of the rotor 16 relative to the axis of the undistorted magnetic field. In addition to being used to monitor the rotation of, for example, a rotating shaft, the rotor can alternatively be coupled to a two-axis joystick controller. The sensor can be used in radio direction-finding (Fig. 7, not shown) by connecting the sensing coils to aerials and moving the rotor 16 until a null output is received from coil 10. <IMAGE>

Description

SPECIFICATION Angular position sensor The present invention relates to electromagnetic devices for determining angular position, for example, the angular position of a rotary shaft or joystick or of an incoming electromagnetic wave at a receiver.

A number of conventional devices for determining angular position incorporate AC synchros. However, these have the disadvantage that their construction is relatively complex, requiring a rotating electromagnetic coil and sliding contacts. Consequently, such devices tend to be unreliable and, due to the fact that the moving and stationary parts are mechanically coupled, are not suited to applications where robustness and low frictional torque are desirable, for example, in determining incident wind direction on board ship.

Other known devices for determining the angular position of a rotating body use optical or capacitive sensing techniques. However, such devices are adversely affected by environments with a high moisture content such as are found on board ship and, as they cannot be adequately sealed against the entry of moisture due to the low torque requirement, are not, therefore, suited to marine applications.

These difficulties have been overcome, to a certain extent, by using potentiometric sensing devices. However, such devices can only be made using specialist techniques and their manufacture frequently requires heavy investment in specialized plant.

The invention provides an angular position sensor as defined in the appended claims to which reference should now be made.

As the magnetic coupling and hence the coil output are dependent on the angular position of the body relative to the axis of symmetry of the undistorted magnetic field, the device can be utilised in two different ways.

Either the magnetic field may be held stationary, while the body is rotated with the object whose angular position is to be monitored or the body may be held stationary and the field rotated. As a result, the device is versatile and can fulfil a number of different functions.

The body may be mounted for rotation with a member whose rotational axis coincides with the axis of symmetry of the undistorted magnetic field and in which the means for generating the magnetic field is a fixed coil; the emf induced in the or each sensing coil being related to the angular position of the body about its axis of rotation.

With this arrangement, the device can replace conventional AC synchros with the advantage that its construction is simple and does not require a rotating coil or sliding contacts. Because there is no mechanical coupling between the moving and stationary parts of the device, it is particularly well-suited to applications requiring low frictional torque.

Due to its relatively simple construction, the angular position sensor of the invention is robust and can be manufactured cheaply and without specialized machinery.

Alternatively, the body may be mounted in a gimbal so as to be movable with a joystick about a fixed centre, the means for generating the magnetic field being a fixed coil; the emf induced in the or each sensing coil being related to a neutral position in which the body is disposed in the plane of the generator coil.

Joystick controls of this kind are important in the fields of, for example, computer aided design and industrial robots and the joystick control of the invention is particularly advantageous in that it is almost completely free of backlash and static friction. It is also capable of being sealed completely without involving a great deal of additional expense.

Embodiments of the invention will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1 shows an angular position sensor in accordance with the invention; Figure 2 shows the undistorted magnetic field generated in the device of Fig. 1; Figure 3 shows the device of Fig. 1 modified to indicate the angular position of a rotary shaft; Figure 4 shows the effect of the magnetic rotor on the magnetic field of Fig. 2; Figure 5 shows the device of Fig. 1 modified to act as a two-axis joystick controller; Figure 6 shows the effect of an electricallyconducting rotor on the magnetic field direction of Fig. 2; and Figure 7 shows the device of Fig. 1 modified to indicate the incident direction of an electromagnetic signal.

The angular position sensor shown in Fig. 1 comprises an electromagnetic coil 10, for generating an alternating magnetic field, a pair of sensing coils 1 2 and 14, and a body or rotor 1 6 in the form of an inclined annulus.

An oscillator 20 is connected to drive the circular generator coil 10 so that it produces an alternating symmetrical magnetic field as is shown in section in Fig. 2. The oscillator frequency is chosen to minimise radio interference. Suitable frequencies range from 500 KHZ to several MHZ. A frequency of 1.6 MHZ is sufficiently low to allow simple circuit techniques to apply but is high enough to allow low-value and, hence, small-size components to be used in the electronic circuits associated with the device.

The sensing coils 12 and 14 are identical to one another and are arranged so that the planes containing the coils 1 2 and 14 are perpendicular to one another and to the plane containing the generator coil 10. With this arrangement, the axis of symmetry 18 of the magnetic field produced by the generator coil 10 lies within the planes bounded by the coils 1 2 and 14 and, in the absence of the rotor 16, no emf is induced in either sensing coil 1 2 or 14 by the alternating magnetic field.

The rotor 1 6 takes the form of a planar body, in this instance, an annulus. The annular rotor may be of either magnetic of electrically-conductive material but it is preferred to use an electrically-conductive rotor for ease of manufacture. The rotor 1 6 of Figs. 1 and 6 is formed, for example, by a ring cut from a piece of copper tubing at an angle to its axis.

Fig. 3 shows how the angular position sensor of the invention can be modified to monitor the angular position of a rotary shaft 28 (indicated in dotted lines in Fig. 3) using a magnetic rotor 1 7 of ferrite or iron powder.

The rotor 1 7 is mounted on the end of the shaft 28 by means of a support 30 so that it is inclined to the plane of the generator coil 10. The shaft 28 is positioned so that its longitudinal axis coincides with the axis of symmetry of the undistorted magnetic field.

The rotor 1 7 is of ferromagnetic material and thus provides a magnetic path of lower reluctance than the surrounding air. As a result, the magnetic field is distorted around the rotor 1 7 as shown in Fig. 4. The support 30 is of non-magnetic material so that it does not itself cause any distortion of the magnetic field.

As the shaft 28 rotates, the rotor 1 7 and, hence, the axis 18" of the distorted magnetic field rotate with it. The electromagnetic coupling between the magnetic field and the sensing coils gives rise to an output signal from each of the coils 1 2 and 14 which varies as the shaft 28 rotates.

If the physical parameters of the device are suitably adjusted, the outputs of the sensing coils 1 2 and 14 are sinusoidal and may be made proportional to the sine and cosine, respectively, of the angle through which the rotor 1 7 has rotated from the position shown in Fig. 3. In this way, the outputs of the sensing coils 1 2 and 14 give a unique indication of all angular positions from 0" to 359 of the rotor 1 7 relative to the axis 1 8 of the undistorted field and, hence, relative to its axis of rotation.

As the rotor 1 7 performs a single revolution the phase of the output signal in each sensing coil 1 2 or 1 4 reverses. The sensing coil outputs are, therefore, applied to a pair of synchronous rectifiers 22 which compare the phase of each sensing coil output signal with the phase of the output of the oscillator 20 and produce a DC signal of the correct polarity and of a magnitude corresponding to the amplitude of the sensing coil output. The DC signals are then amplified and passed through correcting circuits 24 to the output terminals 26 of the device.Since errors in the output of the sensing coils 1 2 and 14 occur if the coils 10, 1 2 and 1 4 are not precisely positioned relative to one another, the correcting circuits 24 are included to provide electrical correction of these errors and, thus, avoid the need for a complex movable coil assembly. The correcting circuits 24 include conventional means for compensating the zero offsets which arise when the sensing and generator coils are not precisely orthogonal and for adjusting the output to take into account the errors in the output due to a lack of orthogonality between the sensing coils 1 2 and 14 and for an unbalanced output between them.

The correcting circuits 24 enable the device to detect angular positions to an accuracy of better than 1'.

Alternatively, the output of the device may be adjusted to take into account errors due to lack of orthogonality by means of a small movable toroidal or cylindrical ferrite core mounted within the device with its central axis of symmetry coincident with the axis of the undistorted magnetic field.

As there is no mechanical connection between the shaft 28 and the sensing coils 1 2 and 14, the device can be used to monitor continuous rotation of the shaft 28 in either direction.

As mentioned above, the lack of frictional contact between the rotor 1 7 and the stationary parts of the device renders the device particularly well-suited to applications where low torque is desirable as is the case in determining incident wind direction. The device can be adapted to fulfil this function by mounting a suitable vane on the shaft 28 so that the shaft is caused to rotate until it is aligned with the wind direction. The output of the sensing coils 1 2 and 14 then provides a direct and continuous indication of the incident wind direction.

Fig. 5 shows how the device of Fig. 1 can be adapted to function as a joystick controller.

The annular rotor 1 6 is mounted in conventional gimbal rings 50 and 52. The rotor 1 6 is secured to the inner ring 50 by means of pivot mountings 54 so that it is pivotable about a diameter and the inner ring 50 is secured to the outer ring 52 in a similar manner by means of pivot mountings 56. The pivot axes of the rotor 1 6 and of the inner ring 50 are orthogonal so that the rotor 1 6 can be tilted and pivoted about a central axis.

The gimbal rings may be replaced by any other known gimbal or ball-joint mounting which provides a suitable degree of movement.

The rotor 1 6 is secured to a joystick 58 by means of a mounting fork 60. When the joystick 58 is in a central, neutral position the central axis of the annular rotor 1 6 coincides with the axis of symmetry of the magnetic field produced by the generator coil 10. In this position, the rotor 1 6 does not have any effect on the direction of the magnetic field and no emf is induced in the sensing coils 12 and 14.

If the joystick 58 is moved out of the neutral position, the rotor 1 6 tilts out of the plane of the generator coil 10 (for example, as shown in Fig. 5). Eddy current effects in the electrically-conductive rotor 1 6 cause the magnetic field around the rotor 1 6 to be distorted and the axis of the field to tilt to the position 18' as shown in Fig. 6.

As a result, the axis of symmetry of the magnetic field is no longer within the plane of the coil 14. The tilted magnetic field has a component in a direction perpendicular to the plane of the sensing coil 14 and the alternating magnetic field induces a corresponding emf in the coil 1 4. As the rotor 1 6 rotates around the axis 18, the skewed axis 18' rotates with it, so that unless the rotor 1 6 is symmetrical about one or other of the sensing coils 1 2 or 14, the alternating field induces emfs in both sensing coils 1 2 and 14.The magnitude of the sensing coil outputs provides an indication of the inclination of the rotor 1 6 to the plane of the generator coil 10 and the ratio of the two output signals provides an indication of the angular position of the rotor about the undistorted magnetic field axis.

The controller can easily be adapted to produce a three-phase output so that it is compatible with other three-phase equipment by providing three sensing coils at 120"to one another instead of the two orthogonal sensing coils described above. Alternatively, and more cheaply, the two-phase output of the joystick controller shown in the drawings can be converted externally to a three-phase signal using conventional matrixing circuitry.

Methods of applying a correction factor similar to those described in relation to the angular position sensor of Fig. 3 may be utilised when the device is adapted for use as a joystick controller.

The device shown in Fig. 7 is modified to monitor the direction of an incoming electromagnetic wave. The geometrical arrangement of the coils 10, 12 and 14 is the same as the arrangement shown in Fig. 1 but the functions of the coils are reversed. The output signal from each one of a pair of crossed loop aerials 40 is amplified and applied to one of the coils 1 2 and 14. The aerials 40 are perpendicular to one another so that the output of each aerial 40 represents the vector component, in one of two perpendicular directions, of an incoming electromagnetic wave 42. When the device is in use, each of the coils 1 2 and 14 generates an alternating magnetic field of a magnitude proportional to one component of the incoming wave 42. The direction of the resultant magnetic field is, therefore, related to that of the incoming wave.The axis of symmetry of the magnetic field is in the plane of the coil 10, which in the device of Fig. 7 acts as the sensing coil, and, in the absence of the inclined magnetic rotor no emf is induced in the coil 10 by the alternating field. When the inclined rotor is added, the magnetic field is distorted as described in connection with Fig. 4 so that it has a component in a direction perpendicular to the plane of coil 10. As in the device shown in Fig. 1, the output of the sensing coil is related to the angular position of the rotor relative to the magnetic field.

The position of the rotor is adjusted until the output of the coil 10 reaches a null. The angular position of the rotor in the null position then provides an indication of the direction of the incoming wave. When the device is used for direction finding in this way, the coupling efficiency can be improved by providing the device with a fixed internal ferrite core.

In order to render the angular position sensor of the invention even more compact, the rotor may be mounted inside the coil assembly rather than outside it. However, this arrangement is less convenient as it may be necessary to split the sensing coils to allow the shaft to be connected to the rotor.

As is apparent from the description above, an angular position sensor in accordance with the invention is capable of fulfilling a variety of functions and, due to its relatively simple construction, can be compact and robust, and may be manufactured cheaply and by conventional methods.

Claims

1. An angular position sensor comprising means for generating an alternating magnetic field which is symmetrical about an axis, a rotatable body, lying outside the equatorial plane the magnetic field and asymmetrical about its axis of rotation, and one or more sensing coils disposed within the magnetic field; the body being of material such that it distorts the magnetic field surrounding it so that, in use, the emf induced in the or each sensing coil by the alternating magnetic field is dependent on the angular position of the body relative to the axis of the undistorted magnetic field.

2. An angular position sensor according to claim 1 wherein the or each sensing coil is arranged so that the plane containing the or each sensing coil is parallel to the axis of the undistorted field.

3. An angular position sensor comprising a generator coil drivable to produce an alternating magnetic field symmetrical about the generator coil axis, one or more sensing coils so arranged that their axes are orthogonal to the generator coil axis, and a body of material capable of distorting a magnetic field, the body being disposed outside the planes containing both the generator and sensing coils and rotatable about the axis of one of the generator or sensing coils; the electromagnetic coupling between the generator and sensing coils being dependent on the angular position of the body about its axis of rotation.

4. An angular position sensor comprising means for generating an alternating field symmetrical about an axis; a sensing coil disposed in said field; and a body of material capable of distorting a magnetic field; the body lying outside the equatorial plane of the field and outside the plane containing the sensing coil; the electromagnetic interaction between the sensing coil and the field being dependent on the positions of the sensing coil and the body relative to one another.

5. An angular position sensor according to any preceding claim in which the body is mounted for rotation with a member whose rotational axis coincides with the axis of symmetry of the undistorted magnetic field, and in which the means for generating the magnetic field is a fixed coil; the emf induced in the or each sensing coil being related to the angular position of the said member about its rotational axis.

6. An angular position sensor according to any of claims 1 to 4 in which the body is mounted in a gimbal so as to be movable with a joystick about a fixed centre and in which the means for generating the magnetic field is a fixed coil; the emf induced in the or each sensing coil being related to the position of the joystick relative to a neutral position in which the body is disposed in the plane of the generator coil.

7. An angular position sensor according to any preceding claim having two sensing coils so disposed that their axes are perpendicular to one another.

8. An angular position sensor according to any preceding claim in which the emf induced in the or each sensing coil is proportional to the sine of the angle through which the body has rotated from the position in which it is symmetrical about a plane parallel to the axis of the said coil.

9. An angular position sensor according to claim 1 including a receiver adapted to receive an electromagnetic signal and to produce two output signals, each of which represents the vector component of the incoming signal in one of two inclined directions; the means for generating the magnetic field including two coils whose axes are inclined to one another at the same angle as the said inclined directions, each coil being driven by one of the output signals from the receiver so that the direction of the generated field and, hence, the emf induced in the or each sensing coil, is related to the direction from which the incoming signal approaches the receiver.

10. An angular position sensor according to any preceding claim in which the body is of magnetic material.

11. An angular position sensor according to claim 10, in which the body is ferrite.

1 2. An angular position sensor according to any preceding claim in which the body is of electrically-conductive material.

1 3. An angular position sensor substantially as hereinbefore described with reference to Figs. 1, 3 and 4 or to Figs. 1, 5 and 6 or to Fig. 7 of the drawings.

14. A wind direction sensor in accordane with any of claims 1 to 7.

1 5. Apparatus for sensing the angular position of a rotatable member; comprising a generator coil drivable to provide an alternating magnetic field symmetrical about a central axis of said generator coil and arranged so that the central axis coincides with the axis of rotation of the rotatable member; a sensing coil arranged so that its central axis is orthogonal to that of the generator coil; and a body of material capable of distorting a magnetic field and coupled for rotation with the rotatable member the body lying outside the planes containing the generator and sensing coils so that the electromagnetic coupling between the generator and sensing coils is dependent on the angular position of the rotatable member about its axis of rotation.

16. Joystick control means comprising; a generator coil drivable to provide an alternating magnetic field symmetrical about a central axis of the generator coil; a sensing coil arranged so that its central axis is orthogonal to that of the generator coil; a body of material capable of distorting a magnetic field; gimbal mounting means for mounting the body, and joystick means coupled to the body whereby the body is movable about a fixed centre from a neutral position in which the body is disposed in the plane of the generator coil; the electromagnetic coupling between the generator and sensing coils being depependent on the position of the joystick means relative to the neutral position.