GB2252845A

GB2252845A - Electro-mechanical position servo means

Info

Publication number: GB2252845A
Application number: GB8509907A
Authority: GB
Inventors: Nicolas Laurence Brignall
Original assignee: GEC Ferranti Defence Systems Ltd; Ferranti PLC
Current assignee: Ferranti International PLC; Leonardo UK Ltd
Priority date: 1985-04-17
Filing date: 1985-04-17
Publication date: 1992-08-19
Anticipated expiration: 2005-04-17
Also published as: GB2252845B

Abstract

An electro-mechanical position servo means includes a motor 10 driving a load 11 about a load axis 19, the position of the load being determined by a gyroscope arrangement 26 which provides feedback signals to close the servo loop at motor driver 12. The gyroscope 26 comprises a rate integrating gyro 13 including load angle pick-off 22 and torquer coil 21'. The pick-off output is fed back by way of amplifier 41 to torquer coil input 27 to form a gyro feedback loop, the inherent integration effected by torquer coil causing the pick-off signal, and gyroscope output signal, at 27 to be in the form of a rate or velocity signal. Two servo feedback paths are formed, a velocity feedback path 28 feeding directly to a summing junction 30 before the motor driver 12 and a position feedback path 31, 35 to the summing junction 30 including an integrator 33 of the gyroscope signals and an input summing junction 32 by which position demand signals are input at 36 as a function of position rate of change. Instead of a rate integrating gyro 13 with its gyro feedback loop the gyroscope 26 may comprise a rate gyro. <IMAGE>

Description

ELECTRO-MECHANICAL POSITION SERVO MEANS This invention relates to electro-mechanical position servo means for controlling the angular position of a motor-driven rotatable load and is particularly, but not exclusively, concerned with such positional control in apparatus for stabilising the position of such a load against movement of a body by which the load is carried.

Gyroscopes are commonly used for stabilisation applications in which the orientation of an equipment is to be isolated from the effect of vibration or angular movement of the supporting body, such as a vehicle, or part of the equipment, such as a mirror or radar aerial thereof, is stabilised so as to remove such disturbances. Equipment which employs such stabilisation includes electro-optic systems (TV, thermal imager, laser designator or rangefinder), radar systems, inertial navigation and weapon systems. The equipment may be mounted on ground vehicles, ships, aircraft, helicopters or on satellites in space.

The gyroscopes used give an electrical output representing either the rate at which the gyroscope is rotating (rate gyroscopes), or the total angle through which it has rotated (rate integrating gyroscope). A signal demodulation is usually necessary to convert the raw signal into a DC voltage representing rate or angle. Rate integrating gyroscopes are provided with a torquer input by means of which the gyro gimbal, and hence the reference direction, may be rotated with respect to inertial space.

Such stabilisation systems normally provide stabilisation about two orthogonal axes. Throughout this specification only the apparatus associated with one such axis is shown for convenience.

Examples of currently known position stabilisation emplcying rate-integrating gyros and rate gyros are shown in Figures 1 and 2 respectively.

Referring to Figure 1 which comprises parts 1(a) and l(b), Figure 1(a) is a schematic diagram of a exposition servo means 10 as used in stabilisation apparatus. A motor 11 is coupled to rotationaily drive a load 11 about a load axis. The motor will normally be of a direct drive type for high performance systems and is driven by a motor driver comprising, in effect, a power amplifier which supplies a driving current to the motor which in turn produces an output torque, and hence load acceleration, proportional to the current.The load is mechanically coupled to a rate-integrating gyro 13, which may conveniently be carried by the load, having an input axis aligned with the load axis and an orthogonal output axis about which consequential rotation is measured by pick-off means (not shown) to produce a gyro signal which is demodulated in demodulator 14. The gyro signal is processed by passage through a compensation circuit 15, the compensated signal being then applied to the motor driver 12. The system is steered by applying a current at input 16 to the gyro torquer coil arrangement, producing a torque about the output axis and a consequential error signal in the servo loop which rotation of the motor to the demanded position eliminates.

Figure l(b) shows the corresponding electro-mechanical block diagram. The mechanical components assocated with the motor and load are shown generally within chain lines 17 and the components of the load within broken lines 18. The electro-mechanical components of the gyro 13 are shown within the dotted lines. Those elements already identified in Figure 1(a) are given like reference numerals.

The motor 10 generates torque about load axis 19 and therefore angular acceleration of the load 11 which is integrated twice mechanically to give angular position of the load axis. An internal feedback path 20 is shown by the back emf generated in the motor proportional to its angular velocity. The gyro 13 has a torquer coil arrangement 21 by which the system is steered and includes a pick-off 22 which produces an output signal proportional to the angular position of load axis 19. Depending on the nature of the pick-off, the pick-off signal is demodulated at 14 and applied by way of compensation cicruit 15 to the motor driver 12.

Within the known system the double integration of the load acceleration causes the system to be inherently unstable and it is necessary to use phase lead circuits in the compensation circuit 15 to stabilise the system. However there are a number of disadvantages associated with such phase lead.

Firstly, the servo is only conditionally stable for a particular servo gain, becoming unsuitable for increased or decreased gain; secondly, a phase lead circuit causes increase in overall circuit noise because its gain rises with frequency; thirdly, the step response always shows a substantial overshoot making rapid changes of load position difficult; and fourthly, recovery from large signals, which'may lead to saturation of the system amplifiers, may be poor.

Figure l(b) also shows two further inputs at 23 and 24. External influences can disturb the stabilisation, such as external disturbance torques applied as shown at 23 and rotation of the mounting as shown applied at 24 to the internal velocity feedback loop 20.

The angular deflection due to such disturbance inputs is described by the servo compliance which can be calculated from servo parameter values. The stabilisation performance is optimised in known systems by minimising the compliance at the expense of meeting some other response requirements to steering inputs.

An alternative method of stabilising the servo positioning loop is to use a tacho or other device to provide a gimbal rotation rate signal. This has the disadvantage that the tacho feedback loop attempts to control the system with respect to its mount whereas the gyro attempts to control the system with respect to inertial space and the stabilisation provided is active over a limited frequency range.

Instead of a rate integrating gyro 13 as shown in Figures 1(a) and l(b) a rate gyro may be used as shown in corresponding schematic and block diagrams of Figs. 2(a) and 2(b). The essential differences are that in the absence of a torquer coil arrangement in the gyro 13' the steering signal on line 16' is injected to the servo loop at junction 16", and control is over angular rate not angular position of the load.

The loop however, is inherently stable. To recover angular position control, the compensation is frequently of proportional plus integral form, although by considering the servo compliance as a function of frequency it can be shown that the compliance is increased, and stabilisation poor, over a wide band of frequencies around the servo bandwidth. The bandwidth of the control is limited because a conventional rate gyro processes a mechanical resonance and the resulting phase shifts make it necessary to restrict the bandwidth to a fraction of the resonant frequency.

It is an object of the present invention to provide a position controlled servo system including a motor driven rotatable load and a feedback control means incorporating an electrical-mechanical gyroscope which is inherently stable and mitigates the disadvantages to known systems cause by stabilisation compensation circuits.

According to a first aspect of the present invention an electro-mechanical position servo means includes a drive motor responsive to current from a motor driver to produce angular acceleration of a load about a load axis proportional to said current, angular velocity and angular position of the accelerated load about said load axis representing first and second stages respectively of integration of the load acceleration, and a feedback control means including an electro-mechanical gyroscope arrangement coupled to the load and responsive to the instantaneous angular position thereof to produce an electrical gyroscope arrangement output signal proportional to the rate of change of angular position of the load about said load axis, a velocity feedback path operable to feed said gyroscope arrangment output signal to a motor summing junction at the input to the motor drive and a position feedback path operable to feed said gyroscope arrangement output signal by way of an integration stage to the motor summing junction, the position feedback path including a demand input summing junction to which a demand signal is applied for combination with the signal in the displacement feedback path.

According to a second aspect of the present invention a motion-stabilised mirror system includes a mirror mounted for rotation about two orthogonal axes, with respect to a body by which it is carried, by electro magnetic positioning means as defined by the preceding paragraph associated with each of said two axes.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1(a) is a schematic diagram of a known position servo system including a motor driven load and a feedback loop, including a rate integrating gyro and a feedback signal compensation circuit, and illustrating the input. for position steering signals, Figure l(b) is an electro-mechanical block diagram of the system of Figure 1, Figures 2(a) and 2(b) are schematic and block diagrams respectively of a position servo system similar to Figure 1 but incorporating a rate gyro and illustrating the input for position steering signals therewith, Figure 3 is an electro-mechanical block diagram of a position servo system according to the present invention and including a rate integrating gyro, and Figure 4 is an electro-mechanical block diagram of a position servo system, also according to the present invention, but including a rate gyro.

Referring to Figure 3 it will be seen that many of the component parts correspond to those shown and described above with reference to Figure l(b) and will be given like reference numerals.

The position servo system according to the present invention includes drive motor 10 responsive to current from a motor driver 12 to produce angular acceleration of a load 11 about a load axis 19, which acceleration is proportional to the current. Angular velocity and angular position of the load about the load axis 19 represent first and second stages respectively of integration of the load acceleration. The position servo system also includes feedback control means shown generally at 25 including an electromagnetic gyroscope arrangement 26 coupled to the load. The gyroscope arrangement 26 is responsive to the instantaneous angular position of the load to produce an electrical gyroscope arrangement output signal on line 27 proportional to the rate of change of angular position of the load about the load axis, that is, its angular velocity.

The feedback control means also includes a velocity feedback path by way of line 28 which connects line 27 to one input of a motor summing junction 29, the output 30 of which summing junction is fed to the motor driver 12.

The feedback control means further includes a position feedback path comprising line 31 which connects the line 27 by way of an input summing junction 32 to an integration stage 33. The output of the integration stage is fed by way of a gain adjusting element 34, for example, an amplifier, and line 35 to the second input of the motor summing junction 29.

The input summing junction 32 is arranged to receive also as an input a line 36 by which load position steering signals are applied to the system.

The gyroscope arrangement 26 includes a rate integrating gyro 13 as described above. The rate integrating gyro is of conventional electro mechanical design having a gyro wheel mounted for rotation about a spin axis which is orthogonal to mutually orthogonal input and output axis. The gyro wheel is mounted in a gimbal frame which is pivotable about the output axis and the input axis is aligned with the load axis 19, the gyro being rotated about this axis with load 11. The gyro includes a torquer coil arrangement 21' responsive to an electrical signal applied thereto by way of a torquing input 37 to apply a torque about the output axis. The torquer coil arrangement effects an integration operation on the applied signal, indicated by integrator element 38.A mechanical summing junction exists, as shown at 39, between the torque produced about the output axis, due to gyro rotation about the input, or load, axis, and the steering torque produced about the output axis, due to the torquer coil. Pick-off means 22 is coupled to the output axis and determines the angular position of the gimbal frame about this axis as a result of these summed torques.

The pick-off means produces a pick-off signal which is demodulated (if necessary) by demodulator 14 and applied on pick-off output line 27' to line 27. The gyroscope arrangement 26 also includes a gyro feedback path comprising a line 40, which connects to the pick-off output line 27' to the torquing input 37 of the gyro by way of a gain element 41, e.g. an amplifier.

It will be appreciated that although the pick-off of a rate-integrating gyro normally provides a pick-off signal which is a measure of gimbal angular position about the output axis the closing of the gimbal feedback loop including the integrating element 38 of the torquer arrangement means that the pick-off signal appearing on line 27' is in fact a function of the rate of change of angular position, and this comprises the gyroscope arrangement output signal on line 27.

In known systems of the type shown in Figure l(b), wherein the rate integrating gyro is steered by means of an input signal to the torquer arrangement, such signal, to effect a positional displacement, is conventionally applied as a rotation rate to counter the integrating effect of the torquer arrangement.

In accordance with this convention the servo position steering signal may also be applied still as a rate of change of position on line 36 to input summing junction 32.

The gain element 34 shown in the position feedback path may be an amplifier or attenuator in order to regulate the relative values between signals in the position and velocity feedback paths. If desired the, or an additional, gain element may be included in the velocity feedback path.

Furthermore, the input summing junction 32 is located to receive the gyroscope arrangement output signal, in the form of a rate signal, and the servo position steering signal also in the form of a rate signal. It will be appreciated that the servo position steering signal may be provided as a position signal to a summing junction 32' (shown by broken lines) instead of 32, and located after the integration device 33.

The system described is inherently stable throughout a wide frequency range and it is not necessary to provide elaborate compensation circuits with their undesirable side effects. It is possible however to incorporate one or more compensation circuits in the feedback loop for example as shown at 42 at the output of the gyroscope arrangement. The purpose of such a compensation circuit is to add desirable properties to the servo system as a matter of choice rather than remove disadvantages due to instability as a necessity.

The present invention may also be implemented employing a rate gyro as shown in the block diagram of Figure 4.

This arrangement is similar to that shown in Figure 3 except that the gyroscope arrangement 26 is now provided simply by a rate gyro 13' which produces its gyroscope arrangement signal as a function of rate of change of position.

The rate gyro may be replaced by one of the servoed rate gyroscopes, such as the dynamically tuned gyro (DTG) or flex-gyro.

The position servo means described above in general terms is particularly suited for use with a motion stabilised mirror which is mounted for rotation about two orthogonal rotation axes with respect to a body by which it is carried, such as a vehicle. The position servo means as described is then associated with each of the two rotation axes, preferably with the gyro input axis of each gyro aligned with the gyro output axis of the other.

Claims

1. Electro-mechanical position servo means including a drive motor responsive to current from a motor driver to produce angular acceleration of a load about a load axis proportional to said current, angular velocity and angular position of the accelerated load about said load axis representing first and second stages resprectively of integration of the load acceleration, and a feedback control means including an electro-mechanical gyroscope arrangement coupled to the load and responsive to the instantaneous angular position thereof to produce an electrical gyroscope arrangement output signal proportional to the rate of change of angular position of the load about said load axis, a velocity feedback path operable to feed said gyroscope arrangement output signal to a motor summing junction at the input to the motor drive and a position feedback path operable to feed said gyroscope arrangement output signal by way of an integration stage to the motor summing junction, the position feedback path including a demand input summing junction to which a demand signal is applied for combination with the signal in the displacement feedback path.

2. Electro-mechanical position servo means as claimed in claim 1 in which at least one of the velocity or position feedback paths includes a gain control element operable to vary the relative amplitudes of the signals of said paths applied to the motor summing junction.

3. Electro-mechanical position servo means as claimed in claim 1 or claim 2 in which the demand input summing junction is located in the position feedback loop at the input to the integration stage, the input summing junction being arranged to receive the gyroscope arrangement output signal and a demand input signal which is proportional to a rate change of position of the load.

4. Electro-mechanical position servo means as claimed in any one of claims 1 to 3 in which the gyroscope arrangement is a rate gyro.

5. Electro-mechanical position servo means as claimed in any one of claims 1 to 3 in which the gyroscope arrangement comprises a rate-integrating gyro having a gimbal frame responsive to rotation of the gyro about an input axis aligned with the load axis to rotate relative to the gyro about an orthogonal output axis, a torquer coil arrangement responsive to a signal applied thereto by way of a torquing input to apply a torque about the gyro output axis, said torquer coil effecting an integration operation on the applied signal, pick-off means operable to produce a pick-off signal proportional to the angular displacement of the gyro gimbal frame about said gyro output axis and a gyro feedback path operable to apply the pick-off signal by way of a gain element to the torquing input, the pick-off signal also comprising the gyroscope arrangement output signal representing the rate of change of position about the load axis.

6. Electro-mechanical position servo means as claimed in claim 5 including feedback compensation means to which the pick-off signal is applied prior to its application to the velocity, position and gyro feedback paths.

7. Electro-mechanical position servo neans substantially as herein described with reference to and as shown by, the accompanying drawings.

8. A motion-stabilised mirror system including a mirror mounted for rotation about two orthogonal axes, with respect to a body by which it is carried; by electro-mechanical positioning means as claimed in any one of the preceding claims associated with each of said two axes.

8. A motion-stabilised mirror system including a mirror mounted for rotation about two orthogonal axes, with respect to a body by which it is carried, by electro-mechanical positioning means as claimed in any one of the preceding claims associated with each of said two axes.

Amendments to the claims have been filed as follows 1; Electro-mechanical position servo means including a drive motor responsive to current from a motor driver to produce angular acceleration of a load about a load axis proportional to said current, angular velocity and angular position of the accelerated load about said load axis representing first and second stages respectively of integration of the load acceleration; and a feedback control means including an electro-mechanical gyroscope arrangement coupled to the load and responsive to the instantaneous angular position thereof to produce an electrical gyroscope arrangement output signal proportional to the rate of change of angular position of the load about said load axis; a velocity feedback path operable to feed said gyroscope arrangement output signal to a motor summing junction at the input to the motor drive and a position feedback path operable to feed said gyroscope arrangement output signal by way of an integration stage to the motor summing junction, the position feedback path including a demand input summing junction to which a demand signal is applied for combination with the signal in the displacement feedback path.

5. Electro-mechanical position servo means as claimed in any one of claims 1 to 3 in which the gyroscope arrangement comprises a rate-integrating gyro having a gimbal frame responsive to rotation of the gyro about an input axis aligned with the load axis to rotate relative to the gyro about an orthogonal output axis; a torquer coil arrangement responsive to a signal applied thereto by way of a torquing input to apply a torque about the gyro output axis, said torquer coil effecting an integration operation on the applied signal, pick-off means operable to produce a pick-off signal proportional to the angular displacement of the gyro gimbal frame about said gyro output axis and a gyro feedback path operable to apply the pick-off signal by way of a gain element to the torquing input, the pick-off signal also comprising the gyroscope arrangement output signal representing the rate of change of position about the load axis.

6; Electro-mechanical position servo means as claimed in claim 5 including feedback compensation means to which the pick-off signal is applied prior to its application to the velocity; position and gyro feedback paths.

7. Electro-mechanical position servo means substantially as herein described with reference to and as shown by, Figure 3 or Figure 4 of the accompanying drawings.