GB2061502A

GB2061502A - A Sensor for Detecting Rotational Movement

Info

Publication number: GB2061502A
Application number: GB7936270A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1979-10-19
Filing date: 1979-10-19
Publication date: 1981-05-13

Abstract

A sensor for detecting rotational movement about an axis comprises a resonator in the form of a cup 1 to which radial vibrations are applied by means of transducers 10 and 11. When the resonator rotates radial vibrations are detected by transducers 14 and 15. The configuration illustrated renders the sensor insensitive to linear motion. <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 referred to in the article "Development of an accurate tuningfork gyroscope" by G.H. Hunt and A.E.W. Hobbs, Proc. Instn. Mech. Engrs. 1964-65, vol. 1 79 Pt 3E p. 129 et. seq.It is, however, important that a sensor for detecting rotational movement is insensitive to linear or translational movement which could gove rise to an erroneous indication of rotational movement.

The present invention seeks to provide a sensor for detecting rotational movement, which utilises a vibrating element and in which the aforementioned requirement can be satisfied.

According to this invention, a sensor for detecting rotational movement about an axis includes a body which is symmetrically shaped and positioned about said axis, means for applying radial vibrations to said body in the direction of a plane which is parallel to and passes through said axis, and means for detecting the presence of torsional vibrations induced in said body and which are indicative of rotation of said body about said axis.

Preferably the frequency of the applied radial vibrations is such as to cause resonance of said body. The resonance may be at the fundamental resonance frequency of the body or at a harmonic thereof. The manufacture and construction of the sensor is likely to be eased if the fundamental frequency or a very low harmonic thereof is chosen.

Preferably said body is in the form of a thin walled hollow cylinder. When said radial vibrations are applied to it at a resonance frequency whilst the body is not rotating about said axis, the radial movement of the wall of the cylinder is a maximum at anti-nodes (an anti-node corresponding to the position at which the radial vibrations are applied) and zero at the nodal positions. A torsional vibration (i.e. a rotational vibration in a direction about said axis) caused by rotation of the body about said axis entails the presence of radial vibrations at these nodal positions. These radial vibrations, of course, combine with the originally applied radial vibrations and modify the resultant standing wave pattern which is apparent as periodic physical displacements of the cylinder wall.

Preferably the torsional vibrations are detected by monitoring the change in the radial vibrations which occur when said body rotates about said axis. Preferably again the amplitude of the radial vibrations is monitored at a point on said body at or near a position which corresponds to a node in the absence of rotational movement of the body about said axis. Because of the difficulty of mounting a transducer exactly at a nodal position, it is more convenient to provide a pair of transducers with their two outputs combined together in such a manner as to provide a zero output in the absence of torsional vibrations.

Conveniently, however, they are still located as near as possible to a nodal position, so that the amplitude of two radial vibrations is very small.

Thus any error in exactly cancelling out the radial vibrations results in only a very small residual signal, which can be neglected as compared with the required torsional vibration. The sensitivity with which the torsional vibrations can be detected and measured largely determines the minimum rate of rotation, which can be detected using the sensor in accordance with the present invention.

Preferably transducers are provided at antinodal positions so as to detect the magnitude of the applied radial vibrations. By adjusting the frequency of the applied radial vibrations and monitoring the resultant amplitude of the resonance induced, the sensor can be operated at or near its most sensitive resonance frequency.

Conveniently, the body is in the form of a cup, in which one end of the cylinder is closed by means of a relatively massive base. Preferably the base is securely mounted at the point on which said axis passes through it. In this case the transducers by means of which the vibrations are applied to the body, and the resulting vibrations measured are located at the end of the cylinder which is remote from the relatively massive base.

The invention is further described by way of example with reference to the accompanying drawings in which, Figure 1 illustrates in a diagrammatic form part of a rotation sensor in accordance with the present invention, Figure 2 is an explanatory diagram, and Figure 3 shows a sensor in a schematic form.

Referring to Figure 1 ,there is shown therein a cup shaped resonator 1 , which is symmetrically shaped and mounted about an axis 2. The resonator 1 consists of a cylindrical portion 3, which is formed of relatively thin uniformly thick walls, one end of the cylinder being closed by means of a relatively thick base plate 4. The resonator 1 is accurately machined from a single solid block of material so that it is perfectly symmetrical about the axis 2. The resonator 1 is mounted on a base 5 by means of a support 6, which is secured to the base plate 4 and is coaxial with the axis 2.

A number of vibration transducers are mounted near the open end of the resonator 1 and their circumferential positions can best be seen from Figure 3. A pair of piezo electrical transducers 10 and 11 are mounted on diametrically opposite sides of the cylindrical wall 3, both transducers 10 and 11 lie in a single flat plane which passes through and is parallel with the axis 2. In operation, radial vibrations are applied to the cylindrical wall 3 by means of these two transducers at a frequency which produce anti-nodes at the transducers themselves and at points on the circumference of the cylindricalwall mid-way between the transducers 10 and 11.

Further transducers 12 and 13 are mounted at these anti-nodal points.

Whilst the sensor is not rotating about the axis 2, only radial vibrations with modes at points 30, 31, 32 and 33 (see Figure 2) are present in the walls of the cylinder 3, but when rotation about the axis 2 takes place, torsional vibrations are set up which appear as radial vibrations at these points 30, 31, 32 and 33. The principle behind the generation of the torsional vibrations is indicated in Figure 2, in which the nature of the deformation of the cylindrical wall whilst it is vibrating is shown. The solid line and the broken line represent the opposite extremes of the distortion to which the circular cross-section of the cylindrical wall is subject.If it is assumed that at resonance frequency the radial vibration has an amplitude 1o sin wt, then the normalised amplitude of the induced torsional motion is: QT Ic sin wit 2 where Q is a constant angular rotation rate and T is the time-constant of decay of the torsional vibration. The effect of the torsional vibration is to produce a secondary radial vibration at points 30 to 33 in Figure 2. The amplitude of this vibration is:

and it will be seen that this is proportional to the rate of rotation Q. In order to measure this secondary radial vibration to produce a rotational rate signal, transducers are used which reject all of the signal resulting directly from the primary radial vibration applied by transducers 10 and 11.

This is done in principle by accurately positioning pick-off transducers on an anode of the primary vibration. In principle these nodes lie at points 30 to 33. In practice, the positioning accuracy needed to provide adequate rejection of the primary vibration is very great. Consequently in the practical arrangement shown in Figure 3, two transducers 14 and 1 5 are used. These are positioned symmetrically about the nodal point 32 and they are connected together so as to cancel out any signals which stem only from the applied radial vibration.

Referring in more detail to Figure 3, a driving frequency signal which is applied to transducers 10 and 11 is obtained from an osciilator which forms part of a phase locked loop 16. The resulting radial vibrations are detected by means of the pick-up transducers 12 and 13 and fed back to the input of the phase lock loop 1 6. The overall effect of the loop 16 is to produce an output frequency, which corresponds to the resonance value of the resonator 1. The loop 16 also includes a conventional automatic gain control facility which is provided so as to stabilise the amplitude of the vibrations, since excessive amplitude levels could impair the sensitivity of the sensor.

The presence of the automatic gain control facility enables the calibration factor of the sensor to be stablilised. The output signal level is proportional to the amplitude of the primary vibration, so that a fixed stable primary vibration amplitude is required to produce an output signal whose level is related to the rotation rate of the sensor in a specific manner.

Torsional vibrations present in the resonator 1 when it rotates about the axis 2, are detected in the form of radial vibrations by means of the transducers 14 and 1 5. These two transducers are coupled via separately controllable attenuators 1 7 and 18 to a common amplifier 19.

The amplifier 19 is arranged to provide a fixed gain. A feedback resistor 20 is provided for this purpose, and the relative values of attenuators 1 7 and 18 are initially adjusted, whilst the resonator 1 is stationary so as to remove the effect of applied radial vibrations from the transducers 14 and 1 5. The output of the amplifier 1 9 is passed via a buffer stage 21 to an output 22 and is also passed directly via a further amplifier 23 to two variable attenuators 24 and 25. These are coupled to two more transducers 26 and 27, which are mounted diametrically opposite transducers 14 and 15. These transducers 26 and 27 receive a damping signal which is balanced by means of the variable attenuators 24 and 25. By means of the damping signal the transducers 26 and 27 vibrate in opposition to transducers 14 and 15.The degree of electrical damping provided determines the time constant T and enables it to be held at a stable pre-set value. This determines the sensitivity of the rotation sensor.

The vibrating component of the sensor comprises the resonator 1. A high degree of concentricity is required between the inner and outer surfaces of the cylinder to ensure that the resonator vibrates at the same frequency in the primary and secondary vibration directions.

Concentricity is also required to ensure that torsional vibrations cannot be induced by linear motion of the base 5. Ideally the transducers should be of small size and should be carefully matched to ensure that the balance of the resonator is not disturbed. Conveniently small piezo electrical vibration transducers can be used.

In setting up the sensor, the outputs of the transducers 14 and 15 are first adjusted by means of the adjustable attenuators 1 7 and 18 to give maximum rejection of the applied radial vibrations. This is followed by adjustment of the damping feedback loop, which includes amplifiers 19 and 23, to give the required sensitivity. The variable attenuators 24 and 25 are also adjusted to balance the transducers 26 and 27.

Typically the value of the frequencies used will be in the audio range 1 to 10 kHz and it is envisaged that the sensor will be able to respond to very low rates of rotation, significantly less than 0.10 per second.

Claims

1. A sensor for detecting rotational movement about an axis including a body which is symmetrically shaped and positioned about said axis, means for applying radial vibrations to said body in the direction of a plane which is parallel to and passes through said axis, and means for detecting the presence of torsional vibrations induced in said body and which are indicative of rotation of said body about said axis.

2. A sensor as claimed in claim 1 and wherein the frequncy of the applied radial vibrations is such as to cause resonance of said body.

3. A sensor as claimed in claim 1 or claim 2 and wherein said body is in the form of a thin walled hollow cylinder.

4. A sensor as claimed in any of the preceding claims and wherein the torsional vibrations are detected by monitoring the change in the radial vibrations which occur when said body rotates about said axis.

5. A sensor as claimed in claim 4 and wherein the amplifitude of the radial vibrations is monitored at a point in said body at or near a position which corresponds to a node in the absence of rotational movement of the body about the said axis.

6. A sensor as claimed in claim 5 and wherein two transducers are mounted near to said anode with their two outputs combined together in such a manner as to provide a zero ouput in the absence of torsional vibrations.

7. A sensor as claimed in any of the preceding claims and wherein vibration transducers are provided at anti-nodal positions of the applied radial vibrations so as to determine and control the magnitude of these vibrations.

8. A sensor as claimed in any of claims 3 to 7 and wherein said body is in the form of a cup, in which one end of the cylinder is closed by means of a relatively massive base.

9. A sensor for detecting rotational movement about an axis substantially as illustrated in and described with reference to the accompanying drawings.

9. A sensor as claimed in claim 8 and wherein the base is securely mounted at the point on which said axis passes through it.

10. A sensor for detecting rotational movement about an axis substantially as illustrated in and described with reference to the accompanying drawings.

New Claims or Amendments to Claims filed on 23 September 1980 Superseded claims 1 to 10 New or Amended Claims:

1. A sensor for detecting rotational movement about an axis including a thin walled hollow cylinder which is symmetrically shaped and positioned about said axis, piezo electric transducer means for applying radial vibrations to said cylinder in the direction of a plane which is parallel to and passes through said axis, and means for detecting the presence of torsional vibrations induced in said cylinder and which are indicative of rotation of said cylinder about said axis.

2. A sensor as claimed in claim 1 and wherein the frequncy of the applied radial vibrations is such as to cause resonance of said cylinder.

3. A sensor as claimed in any of the preceding claims and wherein the torsional vibrations are detected by monitoring the change in the radial vibrations which occur when said cylinder rotates about said axis.

4. A sensor as claimed in claim 3 and wherein the amplitude of the radial vibrations is monitored at a point in said cylinder at or near a position which corresponds to a node in the absence of rotational movement of the cylinder about said axis.

5. A sensor as claimed in claim 4 and wherein two piezo electric transducers are mounted near to said node with their two outputs combined together in such a manner as to provide a zero output in the absence of torsional vibrations.

6. A sensor as claimed in any of the preceding claims and wherein additional piezo electric transducers means are provided at anti-nodal positions of the applied radial vibrations so as to determine and control the magnitude of these vibrations.

7. A sensor as claimed in any of the preceding claims and wherein said cylinder is closed at one end by means of a relatively massive base.

8. A sensor as claimed in claim 7, and wherein the base is securely mounted at the point on which said axis passes through it.