GB2113842A - A sensor for detecting rotational movement - Google Patents

Abstract

A sensor for detecting rotational movement about two perpendicular axes comprises in one embodiment two wheels 1, 2 mounted on a common hub and set into torsional vibrations in mutual anti-phase about a Z axis. Rotation of the wheels about the X and Y axes is detected by monitoring the flexural vibrations which are present in the plane of the two wheels. Although in principle, one wheel is sufficient, two are provided so as to make the sensor insensitive to externally applied vibrations. <IMAGE>

Description

SPECIFICATION A sensor for detecting rotational movement This invention relates to a sensor for detecting rotational movement. Conventionally such a sensor can consist of gyroscopes in which inertial bodies rotate at high speed about a fixed axis.

Devices of this kind are relatively complex and expensive, and because they contain moving parts which are subject to frictional wear their reliability is not always sufficiently good. An alternative form of sensor has been proposed in which a vibrating element is used as the rotation sensitive component.

A sensor of this kind is described in our earlier U.K. patent application 7936270 (published under number G.B. 2061 502A), but it is capable of detecting rotational movement about only a single axis. It is desirable to be able to detect rotational movement about two angularly off-set axes and the present invention seeks to provide such a sensor which utilises a vibrating element.

According to this invention, a sensor for detecting rotational movement about two angularly off-set axes includes a body lying in the plane of the two axes and means for inducing in it torsional vibration about a third mutually perpendicular axis, and means for detecting oscillatory vibrations resulting from flexure of said 'body about each of said two off-set axes when it is rotated about an axis lying in said plane.

Preferably two similar bodies are provided which are mounted side by side on said third mutually perpendicular axis, and means are provided for inducing the torsional vibrations in each body in mutual anti-phase. This is to say, each body vibrates torsionally in anti-phase with respect to the other body.

Preferably each body is a wheel having a rim which is secured to a hub on said third axis by spokes. Conveniently each wheel has two pairs of spokes, each pair lying in the direction of respective ones of two mutually perpendicular axes.

The invention is further described by way of example with reference to the accompanying drawings, in which Figures 1 and 2 show plan and elevation views of a sensor in accordance with the invention, Figure 3 is an explanatory diagram and Figures 4 and 5 show electrical circuits forming part of the sensor.

Referring to Figures 1 and 2, there is shown therein a sensor device which employs a vibrating structure to provide output signals which are proportional to the angular rotation rates about two co-ordinate axes simultaneously. The provision of the two axes outputs from a single sensor avoids the need to provide two separate sensors and thereby enables the sensor to be particularly small and compact.

The sensor consists of two wheels 1, 2 which comprise relatively massive rims 25 and 26 which are mounted on a common hub 3 by means of two pairs of spokes 21,22 and 23,24.The two wheels are of identical dimensions and weights and the two pairs of spokes are aligned with each other and are spaced equally around the rim so as to be at right angles to each other. A small spiggot 4 is attached to the centre of the hub 3 on the common axis Z of the two wheels and is used to mount the whole sensor on an external structure, for example, by means of a single screw on the axis of the spiggot.

The two wheels 1,2 are able to vibrate in a torsional mode about their common Z axis and they are arranged to vibrate in opposite directions and with equal amplitudes. In this way the net torque on the central hub 3 is zero so that no vibration is coupled to the mounting. This enables the operation of the sensor to be substantially independent of any external vibrations coupled to it via the mounting spiggot 4.

Torsional vibrations are induced in the wheels 2 by means of piezoelectric ceramic transducers 4, 5, 6 and 7, which are mounted on the sides of the spokes as shown in the drawings. These transducers are of conventional form, such that when a voltage is applied to each transducer a bending movement is induced in each of the spokes which rotates the rims 25, 26 by a small amount with respect to the central hub 3. By positioning the transducers on one wheel 1 on the opposite sides of the spokes to that of the other wheel 2, the two wheels vibrate in anti-phase, or alternatively to achieve the same effect, voltages of opposite polarity can be applied to the transducers associated with the different wheels 1 or 2.The torsional vibration is maintained at the required frequency and amplitude by applying a suitable a.c. voltage at the resonance frequency of the wheels, and the way in which the correct voltage is obtained is described subsequently with reference to Figure 4.

The two wheels are free to vibrate in a flexural mode as is illustrated diagrammatically in Figure 3. Flexure, or bending, of the spokes causes motion either in the X, Z plane or the Y, Z plane. If the two wheels flex in opposite directions with equal amplitudes, the resulting bending moment at the hub 3 is zero and consequently the flexural vibration frequencies are independent of the mounting conditions and are unaffected by external vibrations. The amplitudes of the flexural vibrations in the X, Z and Y, Z planes are monitored by further piezoelectric transducers 8, 9, 10 and 11. The transducers 8, 9 monitor flexural vibration in the Y, Z plane and transducers 1 0, 11 monitor flexural vibrations in the X, Z plane.

The thickness and width of the spokes 21, 22, 23 and 24 are adjusted so that the flexural vibration frequencies are identical to the torsional vibration frequency of the two wheels 1, 2.

The sensor is able to detect rotation about the X axis and about the Y axis. When the sensor is rotated bodily about the Y axis, a flexural component of motion is induced in the Y, Z plane, which causes a flexural vibration in this plane having an amplitude proportional to the rate of rotation. This flexural vibration causes an a.c.

signal to be produced at the transducers 8 and 9 and the amplitude of this signal is a measure of the rate of rotation about the Y axis. Also the phase of the signal produced at transducers 8 and 9 when measured relative to a signal proportional to the torsional displacement of the wheels 1, 2 has a value which reverses when the direction of angular rotation about the Y axis reverses, so a measurement of this phase difference provides an indication of the direction of angular motion. This .is subsequently described in more detail with reference to Figure 5. Similarly, rotation of the sensor about the X axis causes the transducers 10 and 11 to generate an a.c. signal in an analogous manner.

Although the two sets of sensors 8, 9 and 1 0, 11 are mounted on mutually perpendicular axes this is not essential, since if the two axes are angularly off-set by a different angle, it is still possible to resolve the transducer outputs into perpendicular directions. However, the arrangement shown is clearly more convenient, since the outputs of the transducers 8, 9, 10, 11 provides the information directly concerning the two mutually perpendicular directions X, Y.

The wheels can be machined from a single piece of metal, for example, stainless steel, with the piezoelectric transducers being either soldered or brazed to the metal surface. The electrical connections to the transducers can be made by means of fine wires or by means of printed tracks on an insulating base mounted on the stainless steel wheel.

The way in which the two wheels are maintained in flexural vibration at their natural resonance frequency is explained with reference to Figure 4. A.c. signals are applied to the transducers 4 and 6 on the wheel 1 and transducers 4' and 6' on the second wheel 2, so as to maintain the required torsional vibration. The actual resulting torsional vibration is monitored by the transducers 5, 7 and 5', 7'. The outputs from the two transducers 5 and 7 are combined and fed via a common amplifier 12. Similarly, the outputs from the transducers 5' and 7' are combined, but are fed through an inverting amplifier 12' having the same gain as that of amplifier 12. The signal inversion is provided as the two wheels 1 and 2 vibrate in anti-phase.The combined signal is fed to one input of a phase detector 13, where it is compared in phase with the output of a variable oscillator 1 5. Any difference between the two input signals of the comparator results in an error signal proportional to the phase difference and this is fed via an integrator 14 (which takes the form of a low pass filter) and is applied to the frequency control input of the oscillator 1 5. The closed loop comprising the phase comparator 13, the integrator 14 and the oscillator 1 5 is operative to stabilise the frequency of the oscillator 1 5 at a value corresponding to the natural torsionai resonance frequency of the two wheels.The output of the oscillator 1 5 is fed directly to the transducers 4 and 6 and via a further inverting circuit 20 to the other transducers 4' and 6'. The rims of the wheels 1,2 themselves serve to complete a feedback loop between the driving transducers 4 and 6 and the monitoring transducers 5 and 7 so as to ensure that the torsional vibration occurs at the natural resonance frequency. The inverting amplifier 20 is provided merely to ensure that the two wheels vibrate in anti-phase.

In order to produce an electrical output proportional to the angular rate of rotation, the output of the transducers 8, 9 and 10, 11 are monitored. It is necessary to provide means for damping the amplitude of the flexural vibrations so that the output of the transducers can be calibrated so that the amplitude is truly representative of a rotation rate and so that the output can respond to changes in angular rotation rate with sufficient speed. Figure 5 shows the circuit which produces an output indicative of rotation about the Y axis. A similar circuit for the transducers 10, 11 produces an exactly analogous output for rotation about the X axis, but is not illustrated.

Referring to Figure 5 the actual value of rotation rate about the Y axis is obtained from transducer 8 which feeds its output via an amplifier 1 6 to a synchronous demodulator which comprises transformers 1 7 and 18, diode rectifiers 27, 28, 29, 30, resistors 31,32, 33, 34, integrator 35, and a voltage indicator 36. The output of the amplifier 16 is applied to the other transducer 9 in such a sense as to oppose the vibration which is being sensed by transducer 8. This arrangement acts as a negative feedback circuit so as to provide the necessary damping for the flexural vibration.

The output from transducer 8 is an a.c. signal which has an amplitude proportional to the angular rotation rate about the Y axis, and a phase relative to the torsional displacement of wheels 1, 2 which reverses when the direction of angular rotation reverses. This is converted by the synchronous demodulator into a d.c. voltage proportional to the angular rotation rate and with a polarity which reverses when the direction of angular rotation reverses. The synchronous demodulator shown in Figure 5 consists of a transformer 1 7 having a secondary winding which is connected across opposite points of a ring of four semiconductor diodes 27, 28, 29, 30, each of which has a resistor 31, 32, 33, 34 of the same value in series with it. The secondary winding of the other transformer 1 8 is connected across the two remaining points of the diode ring. The primary winding of transformer 1 8 is driven from the output of amplifiers 1 2 and 12' which provide a signal proportional to the torsional displacement of the wheels 1, 2. By this means opposite pairs of diodes are switched on during alternate half cycles of the input waveform applied to transfrmer 1 8.

The voltage developed between the centre taps 37, 38 of the secondary windings of the transformers 1 7, 1 8 is a full-wave rectified version of the input voltage applied to transformer 1 7, having a positive polarity if the two input waveforms to transformer 1 7, 18 are in-phase and a negative polarity if they are out-of-phase. The voltage displayed on the voltage indicator 36, after being smoothed by the integrator 35, is therefore proportional to the magnitude and direction of the angular rotation about the Y axis.

As mentioned previously an additional arrangement similar to that shown in Figure 5 produces a signal representative of the angular rotation rate about the X axis, and by combining the X axis signal and Y axis signal in a suitable manner so as to preserve the sense of the rotation, the actual resultant axis of rotation can be determined together with the rate of rotation about such an axis.

Claims (10)

1. A sensor for detecting rotation about two angularly offset axes including a body lying in the plane of the two axes and means for inducing in it torsional vibration about a third mutually perpendicular axis, and means for detecting oscillatory vibrations resulting from flexure of said body about each of said two off-set axes when it is rotated about an axis lying in said plane.

2. A sensor as claimed in claim 1 and wherein two similar bodies are provided which are mounted side by side on said third mutually perpendicular axis, and means are provided for inducing the torsional vibrations in each body in mutual anti-phase.

3. A sensor as claimed in claim 2 and wherein each body is a wheel having a rim which is secured to a hub on said third axis by spokes.

4. A sensor as claimed in claim 3 and wherein each wheel has two pair of spokes, each pair lying in the direction of respective ones of two mutually perpendicular axes.

5. A sensor as claimed in claim 4 and wherein the means for inducing torsional vibration in each wheel comprises piezo-electric transducers mounted on the side of each spoke so as to produce circumferential motion of the rim of the wheel with respect to the central hub.

6. A sensor as claimed in claim 4 and wherein the means for detecting the oscillatory vibrations resulting in said flexure comprises piezoelectric transducers mounted on a face of each spoke.

7. A sensor as claimed in any of said claims and including means for maintaining said torsional vibrations at the natural resonance frequency of said bodies.

8. A sensor as claimed in any of the preceding claims and including means for comparing the phase of said oscillatory vibrations with the phase of said torsional vibration to determine the direction of rotation about each of said axes.

9. A sensor as claimed in any of the preceding claims and including means for damping the amplitude of said oscillatory vibrations.

10. A sensor for detecting rotation about two angularly offset axes substantially as illustrated in and described with reference to Figures 1 and 2 of the accompanying drawings.