EP0523101A1

EP0523101A1 - Accelerometer

Info

Publication number: EP0523101A1
Application number: EP19910906825
Authority: EP
Inventors: Micheal Andrew Kellett
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-04-03
Filing date: 1991-03-22
Publication date: 1993-01-20
Also published as: WO1991015774A1; GB9007483D0; GB9106345D0; AU7577591A; GB2243451B; GB2243451A

Abstract

Accéléromètre comprenant un corps principal (2), des moyens réagissant à l'accélération (1) montés à partir du corps principal de façon à de déplacer en un seul sens (A-B) ou autour d'un seul axe lorsqu'on applique une accélération à l'accéléromètre, des moyens de détection (8, 9, 10) servant à détecter l'intensité et le sens de chacun des mouvements des moyens réagissant à l'accélération par rapport au corps principal et à produire une sortie caractéristique dudit mouvement; des moyens servant à produire des signaux numériques (12, 15) à partir de la sortie du détecteur; des moyens (12, 18) de calcul de signaux de commande à partir des signaux numériques, s'utilisant pour actionner des moyens (5, 6) d'application d'une force aux moyens de réaction (1) pour contrer ledit mouvement et des moyens de production de signaux de sortie caractéristiques des forces appliquées.Accelerometer comprising a main body (2), means responsive to acceleration (1) mounted from the main body so as to move in one direction (AB) or around a single axis when applying acceleration at the accelerometer, detection means (8, 9, 10) for detecting the intensity and the direction of each movement of the means reacting to the acceleration with respect to the main body and for producing an output characteristic of said movement; means for producing digital signals (12, 15) from the output of the detector; means (12, 18) for calculating control signals from the digital signals, used to actuate means (5, 6) for applying a force to the reaction means (1) to counter said movement and means for producing output signals characteristic of the applied forces.

Description

Accelerometer

FIELD OF THE INVENTION

This invention relates to accelerometers and to systems utilising such accelerometers, and more particularly to servo accelerometers

BACKGROUND OF THE INVENTION

Accelerometers are known and can conveniently be regarded as basically incorporating an element/object that is mounted with respect to a support/main body in such manner that the object/element is displaceable in single direction relative to the main body, that is the element/object has a single degree of displacement freedom together with means to apply force in either direction of the single degree of freedom.

Servo accelerometers have been utilised for a variety of applications in situations in which it is required to measure the acceleration direction and/or magnitude of the ^• acceleration.

In general, the operational set-up or application of such accelerometers has been such as to ensure that during the measurement procedures associated with the detection and or measurement of the acceleration giving rise to a particular deflection of the element/object it is conventionally required that the deflection of the object/element should be counterbalanced by being should be restrained by the restoring force to a location close to its initial rest positionreturned to its initial rest position and maintained in such position.

OBJECT OF THE INVENTION

It is an object of the invention to provide servo accelerometers which at least reduce some of the problems met during the practical use of servo accelerometers.

STATEMENTS OF THE INVENTION

Broadly, according to a first aspect of the invention there is provided an accelerometer comprising a main body, acceleration responsive means mounted from the main body in such manner as to be movable on application to the accelerometer of acceleration along a single axis or about a single axis, sensor means for sensing the magnitude and sense of any such movement of the acceleration responsive means relative to the main body and for providing output characteristic of the such movement; means for producing from ther sensor output digital signals; means for deriving from the digital signals control signals which are utilisable to oprate means for applying force to the responsive means to counteract said movement; and means for producing output signals characteristic of the applied forces.

BRIEF DESCRIPTON OF THE DRAWINGS

For a better understanding of the invention and to show how to carry the same into effect reference will now be made to the accompanying drawings in which;

Figure 1 is a schematic representation of an accelerometer incorporating the concepts of the present invention;

Figure 2 is a perspective view illustrating the main components of a practical embodiment of the accelerometer shown in Figure 1; and

Figure 3 is a block diagram indicating in general terms a control circuit for controlling the accelerometer of Figures 1 and 2.

Before considering in detail the accelerometers of the invention it is convenient to recount some of the major features of a servo accelerometer. The general principle upon which a servo accelerometer operates is to mount an object/element in such manner that it is constrained to move along one direction only, i.e., the element/object is constrained to a single degree of freedom of movement which is the sensitive axis of the accelerometer.

In order to form a workable accelerometer it is necessary also to provide means for sensing the displacement of the element/object relative to the body of the accelerometer arising from imposed acceleration; means for producing and applying a restoring/balancing force along the sensitive axis between the element/object and the body of the accelerometer to return the object/element to its undisturbed position; and a control system to producing the restoring force of the appropriate size and direction.

In general, if an imposed acceleration is applied to the body of the accelerometer, along its working axis, the element/object will move relative to the body of the accelerometer. A measuring/control system senses this relative displacement and as mentioned produces a force that acts upon the element/object in such direction as to reduce the relative displacement arising from the acceleration. If this restoring force is just sufficient to prevent substantial further relative movement of the element/object and the body of the accelerometer , the magnitude of the restoring force is the product of the mass of the element/object and the imposed acceleration. Since the mass is readily known the value of the imposed acceleration value may be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to Figure 1 in which an element/object is a beam 1 (represented in the Figure 1 by a single line) which is hinged or otherwise mounted by hinges 1A with respect to an accelerometer main body 2 (represented in the Figure 1 by a cross in a circle) for displacement along the direction indicated by the double ended arrow AB. The beam will, depending upon the sense of the imposed acceleration forces causing the beam to be moved relative to the main body 2 either in the sense A to B or alternatively in the sense B to A.

The support of the beam 1 is such that any displacements are restricted to the direction A to B or B to A.

With the arrangement as so far described the sensitive axis of the accelerometer is along the line AB. In practice, provided that the displacement of the beam from a rest position along a direction extending perpendicular to the line AB and passing through the pivot/hinge axis is small compared with its length any errors -introduced into the measuring/control system as a result of the degree of freedom being rotational rather than purely linear will also be small .

In the accelerometer shown, the arrangements for producing the restoring forces include a motor assembly 3 comprising a magnet unit 4 including a core 5 mounted to the accelerometer body 2 and an associated coil 6 carried by the beam 1 and dimensioned as to be freely axially displaceable with respect to the core 5.

Current flow in the coil 6 will produce a magnetic field which interacts with the core magnetic field to produce corresponding axial force on the coil relative to the core.

The instantaneous position of the beam 1 with respect to the main boy is sensed by a photoelectric detection assembly 7 including a light emitter/transmitter 8, a light detector 9 for receiving light emitted by the emitter/transmitter 8 and a variable attenuation light transmission arrangement 10 producing a controlled variation of the amount of light that can pass from the emitter/transmitter 8 to the detector 9 in relation to the instantaneous displacement of the beam. In the Figure 1 the arrangement 10 is shown in the form of vane.

In particular, in the embodiment of the drawings, the light emitter/transmitter 8 includes an infra-red emitting diode and the detector 9 a light sensitive diode.

The vane 10 which is carried by the beam 1 extends generally at right angles to the longitudinal direction of the beam, and is so formed that it is able variably to attenuate the intensity of light passing therethrough according to its instantaneous location between the transmitter/emitter 8 and detector 9.

The attenuation is arranged progressively to increase along its length in a direction transverse to the length of the beam 1. Thus, when the vane moves in one direction (i.e., A to B) the attenuation effect is in one sense i.e, increasing, and when the vane moves in the reverse direction the attenuation effect is in the opposite sense i.e. decreasing. With this arrangement the sense and extent of the attenuation change are indicative of the magnitude and direction of beam displacement relative to the main body as a result of imposed acceleration forces.

A particular embodiment of a vane 10 includes a length of clear plastics strip of such thickness as to maintain lengthwise stiffness. The variable light attenuation capability is produced by photographically forming a pattern of dots on the strip, the dots pattern having a progressively varying density of distribution such that the light attenuation is a function of the density of distribution of the dots on the vane 10.

In use, as the amount of light reaching the detector 9 i.e., a photo-transistor from the emitter/transmitter 8 by way of the vane varies the current in its collector will be correspondingly varied. The difference between the resulting voltage at the collector and a reference voltage (a practical value for the reference voltage being 2.5 Volts) which is characteristic of the mechanical centre position of the beam. The mechanical centre position can conveniently be regarded as that position where one wishes the beam to be in the absense of imposed acceleration, and is usually midway between the end stops 11. In practice, also it is the place where the coil/magnet unit 4 is designed to provide linear response. In addition at the mechanical centre the angle of the beam 1 is exactly 90 degrees of arc to the intended axis of the whole assembly.

Any displacement of the beam in either direction as a result of an imposed acceleration force will displace the vane 10 and thus vary the light intensity at the detector 9 which latter will produce an output signal which is related to light intensity detected and thus the extent of beam deflection whilst the sense of the change in such intensity is indicative of beam movement direction.

This output signal is utilised to produce a voltage that is applied to the coil 6. The energisation of the coil 6 produces a force which interacts with the core 5 such as to move the beam in such direction and of such magnitude as to return the beam 1 to balance the effect of the imposed acceleration, and constraining the beam to its mechanical centre.

It will be appreciated that other methods of detecting beam movement can be employed. For example, use of the variation of a capacitance; utilisation of the Hall Effect; and changes in the inductance of the coil 6 etc.

Figure 2 schematically illustrates a practical embodiment of the accelerometer of the invention.

Those components which have been mentioned in relation to Figure 1 are identified by the same reference numerals and in view of the inherent simplicity of the embodiment shown additional discription of these components is not thought necessary. Additional to the components already mentioned the embodiment of Figure 2 incorporates stop means 11 for limiting the extent of travel of the beam in either of its directions of displacement A to B and B to A. lit is readily possible to use alternative methods to the plastics hinges 1A for mounting the beam. For example, ball races, pins in bores etc could be used.

Referring now to Figure 3 in conjunction with Figures 1 and 2 the control circuit of Figure 3 includes a microprocessor 12 which is utilised operationally to inter-relate a number of factors as will now be considered. Since the output from the light responsive detector 9 is of a relatively low magnitude it is amplified by amplifier 13, the degree of amplification effected will be as found necessary to produce an adequate signal on an output line 14. Since the output signal from the detector 9 is analogue it is converted to digital form by an analogue to digital converter 15 which is shown as being incorporated in the microprocessor. In practice, the analogue signal is converted, into an eight bit digital form.

_Q On receipt of the eight bit digital signal which is characteristic of beam deflection and the direction of the deflection the microprocessor 12 operates using a simple form of control algorithm for the purposes of beam deflection control by simply being arranged to respond to 5 beam displacement and to produce as output a corresponding correcting current i.e., the control system is arranged to adjust, in response to beam displacement only, the drive i.e., current flow, to a driver circuit 17 for the beam position control coil 6 in such sense and direction as to _Q produce the requisite magnetic field in the coil 6 that reacts with the core 5 in such sense and of such magnitude as to return the beam towards its undeflected position.

The microprocessor 12 is arranged to not only produce a succession of voltage pulses for application drive circuit 5 to the coil 6 but also incorporates facilities for adjusting the pulse width (as indicated at 18) by generating a series of pulses of constant frequency and variable width. The coil driver 19 converts these voltage pulses to current pulses in the coil 6. The 0 microprocessor 12 generates pulses in either direction, thereby to adjust the magnitude of the pulses and thus the restoring current for successive pulses by suitable variation of the pulse width of the digital signals. The repetition rate of the pulses is arranged to be sufficiently fast compared with the mechanical time constants of the beam so that for practical purposes the force applied to the beam by the coil 6 can be considered to be continuous and of magnitude proportional to the pulse width, the variation of which the microprocessor 12 uses to control the force applied to the beam 1 by variation of pulse width.

It has been found that a consequence of the application of a restoring force that is directly proportional only to the displacement of the beam and thus the vane 10 from the mechanical centre position thereof operational problems arise in relation to the time factors required to stabilise the resetting of the beam. Such operational problems involve the detection of the sense and magnitude of beam movement; the production of the pulses, variation of the currents through the coil and the effects of factors such as mentioned on the control loop of the control system.

Thus, if a restoring force directly proportional to beam movement were to be applied to the beam 1, the beam requires a lengthy period of time in order to stabilise and to come to rest at its new position (that arising from the acceleration forces and the application of the control current to the coil 6). This time period would be a function of the extent of the mechanical damping of the accelerometer. Thus, in practice, a system in which the correcting force is simply proportional to beam displacement is inherently unstable and thus is no practical use.

The motion of the beam may be much better controlled by introducing a velocity term into the control algorithim so that the resulting control current can be expressed by the following equation:- I = (Kl x d) + (K2 x v)

In this equation Kl and K2 are constants of the system and would be determined by suitable calibration of the accelerometer of the invention; 'd' the beam displacement relative to the main body 2; 'v' the velocity and 'I¹ the current.

More complex control functions including other derivatives and integral functions of beam displacement could be incorporated into the beam position restoration control arrangements.

Since, the control system of the present invention is digital in character the linear function represented by the equation involves a succession of measurements and calculations, The microprocessor 12 makes a beam position measurement at the start of a time interval T and a calculation is effected according to the equation to determine the appropriate pulse width and to ensure that the pulse is delivered to the driver circuit 17 in time for the pulse to be completed by at the end of the timing interval T. In other words the end of the pulse is set by the end time of the timing interval T and the time of the start of the pulse within the timing interval T is set according to the result of the measurement and calculation. If desired, the pulse width could be substituted for the term I in the equation.

In relation to servo accelerometers that are particularly required to measure accelerations of the order of lg (9,81 m/s/s) full scale certain physical restraints are found to apply.

Firstly the maximum drive force from the coil will be of the order of that able to apply 2g acceleration to the beam. In practice, the provision of more power for the purposes of control of beam position will be found to be inefficient or expensive in relation to the capability of the remainder of the system.

The approximate linear range of the coil/core system can be arranged to be of the order of + or - 1mm., for a beam of length 20mm., from the hinge 1A. In practice, the use of wider linear coil displacements have been found to involve disproporationate expense in the production of the accelerometer and its associated control system.

This restriction on the extent of displacement is also reinforced by the need to limit the maximum operational displacement or motion of the beam to reduce linearity errors caused by the fact that the beam is in reality rotating about the axis.

In practice, the time 'T' is essentially controlled by the cost of the control system. The shorter the time interval 'T' the higher the cost of the control system. For a given maximum displacement of the beam, frequency of the pulses and the maximum beam displacement range there is a maximum time interval 'T' in which the transducer will function correctly and a minimum sampling range.

In accordance with the concepts of the invention and with a view to minimising costs an eight bit analogue to digital convertor has been utilised in preference to a nine or higher bit convertor.

An eight bit convertor arranged so that the limit stop to limit stop displacement of the beam 1 is about 3mm in total and gives a change in the converter count of 200. It is important to ensure that the convertor is still functioning within the limits of its operational range when the beam is located at either of its end stops 11. This is ensured by allowing the convertor to have a counting range of 200 from limit stop to limit stop. This results, ^"in practice, in a count of +- 66 for +- 1mm of beam motion.

The time interval 'T' is set to 2ms, this being a value possible with a conveniently available eight bit microprocessor type. With these figures the resolution of current control is one part in 767 with the components used in a practical realisation of the invention such as shown in Figures 2 and 3.

If, in response to a constant applied acceleration, the restoring force applied to the beam exactly balances the acceleration forces so that the beam is at rest, this being a conventional mode of operation with analogue systems, there could only be 66 control steps between 0 and lg giving a very poor resolution of +-approx 6 bits.

In accordance with the concepts of the invention it has been found that it is highly advantageous not to allow the beam to reach the fully balanced condition. In fact it has been found that the quantisation of both displacement detector 9 and the coil driver 17 as well as the discrete time sampling of the control system can, . in conjunction with suitably selected control constants, even to the extent of deliberately introducing beam disturbing components into the control algorithm, ensure that the beam can never move to an in-balance condition but is caused always to oscillate at a small (a few counts) amplitude all of the time and in so doing avoid problems that can arise from beam movement time lags arising from, for example, mechanical inertias of the system. In the embodiment of the drawings the frequency of oscillation is set at about 50Hz. With this arrangement the beam is effectively held in balance on average over several sampling periods.

With this arrangement provided that the amplitude of beam oscillation is not so great as to cause errors due to non-linearity the ultimate accuracy of the control system is determined by the ratio of the sampling time to the reading time.

In the embodiment under discussion digital filter arrangements are embedded in the control system software with a cut-off frequency of about 15Hz, thus allowing for an increase in resolution of times 500/15=33 or about five bits whereby the effective resolution of the complete transducer system is thus increased to H—11 bits. Even though an eight bit converter is being used.

Thus by averaging several readings and by ensuring that the beam waggles the effective resolution can be increased beyond that available from an eight bit analogue to digital converter.

A particular problem arising with the detection of the beam as illustrated in the embodiment of Figure 2 is that if an output of 2.5 Volts is obtained at mechanical centre position at room temperature then the beam position for 2.5 volts at the extremes of operating temperature might be several millimetres from the mechanical centre arising from temperature changes in the characteristics of the optical system.

In order to correct for this situation temperature sensing arrangements are utilised for enabling a self calibration function which serves to adjust the current in the ϋhotoemitter 8 ir. order to reduce the effects of teraperature change.

This calibration can be under the control of the microprocessor or under separate control.

Thus the control system incorporates temperature sensing arrangements 16 which are intended to initiate compensation for temperature drift of the optical beam position sensor, the vane 10, and the emitter detector combination. Compensation is effected by driving the beam to each of its beam deflection limit stops i.e., towards B and towards A and measuring the voltage output from the detector at each limit stop. The control system is arranged to adjust the current to the emitter 8 so that the output of the phototransistor 9 will be 2.5 Volts with the beam at the mechanical centre.

The compensation can be initiated when external parameters/factors change i.e., temperature change detected by the microprocessor 1; when the accelerometer is part of a larger system with temperature compension such other systems can create the demand for the compensation or at specified time intervals.

If desired, the temperature calibration i.e., the variation of temperature sensitivity with temperature changes can be measured and stored in the manner disclosed in our copending British Patent Application ELECTRONIC CONTROLLER UNIT No 8823409.1

It should be noted that whilst the above description has been specifically directed to linear accelerations and displacements the concepts of the invention can be applied to accelerometers involving non-linear i.e., angular accelerations and thus displacements/movements which are for example rotational, or combinations of rotational and linear displacements/movements.

In addition, the optical system could take alternative forms for example, the vane can be replaced by a shutter arrangement using parallel light between emitter and detector.

Claims

1. An accelerometer, comprising a main body, acceleration responsive means mounted from the main body in such manner as to be movable on application to the accelerometer of acceleration along a single direction or about a single axis, sensor means for sensing the magnitude and sense of any such movement of the acceleration responsive means relative to the main body and for providing output characteristic of the such movement; means for producing from the sensor output digital signals; means for deriving from the digital signals control signals which are utilisable to operate means for applying force to the responsive means to counteract said movement, and means for producing output signals characteristic of the applied forces.

2. An accelerometer as claimed in claim 1, and wherein the means for deriving said control signals from the digital signals includes means for successively noting the magnitude and sense of said movement and computing therefrom successive control and output values within predetermined successive time intervals; and means for deriving from the control values pulsed signals which are used within said time intervals to operate the means for applying force to the responsive means.

3. An accelerometer as claimed in claim 2, and wherein means are provided for enabling calibration of the sensor means in such manner that the operation range thereof is maintained within predetermined limits of movement in either sense with respect to said single direction or single axis.

4. An accelerometer as claimed in claim 3, and wherein said calibration is arranged to be effected by inducing a forced movement of the responsive means to each of its limits of its movement range by means of calibration control signals applied to the force application means; measuring for each limit position the outputs of the sensor means; and adjusting the control of the sensor means so that the sensor means is set to its required operating range.

5. An accelerometer as claimed in claim 2,3 or 4, and comprising means for averaging the sensor output over a plurality of pulse time intervals, the arrangement being such that the resolution of the resulting output signal is increased beyond that of the output from the sensor means.

6. An accelerometer as claimed in claim 3,4 or 5, and wherein the control values are derived substantially according to the following equation Modulus I = (Kl x d) + (K2 x v) where Kl and K2 are constants arising from the structure of the accelerometer, 'd' movement of the imposed acceleration responsive means, the velocity of the imposed acceleration responsive • means and I the counteracting force applied.

7. An accelerometer as claimed in 2,3,4,5 or 6, and wherein means are provided for the deliberate introduction into the input to the means for applying forces to the responsive means signals which prevent the responsive means from attaining a fully balanced condition in the with respect to acceleration forces acting on the main body on the main body, the arrangement also being such that the responsive means is arranged always to be in oscillation within a movement range considerably narrower than its acceleration responsive movement range.

8. An accelerometer as claimed in any of claims 1 to 7 , and wherein the induced acceleration responsive means is a a beam or the like hinged or otherwise mounted to the accelerometer main body for movement relative to a single direction or single axis, and wherein the means for producing the counteracting forces are magnetic, and wherein the intantaneous position of the beam with respect to its rest position is sensed by a photoelectric detection assembly.

9. An accelerometer as claimed in claim 8, and wherein the sensor means includes photoelectric detection assembly including a light emitter/transmitter, a light detector for receiving light emitted by the transmitter and an variable attenuation light transmission arrangement for producing a controlled variation of the amount of light that can pass from the emitter to the detector in relation to the instantaneous displacement of the beam.

10. An accelerometer as claimed in claim 9, and wherein the light attenuation means comprises a vane providing a graded light transmission capability in the direction of deflection of the beam.

11. An accelerometer as claimed in 10, and wherein the vane includes a length of clear plastics strip of such thickness as to maintain lengthwise stiffness, and whereim the variable light attenuation capabiity is attained by forming a pattern of dots on the strip, the dots pattern having a progressively varying density of distribution, the arrangement being such that light attenuation is a function of the density of distribution of the dots on the vane. -19-

12, An accelerometer as claimed in claim 11, and wherein the light emitter/transmitter includes an infra-red emitting diode and the detector is a phototransistor the arrangement being such that as the amount of light reaching the detector from the emitter/transmitter varies the current in the collector of the phototransistor will vary the arrangement being such that any difference between this collector voltage and a reference voltage is characteristic of the instantaneous setting of the vane.

13. An accelerometer as claimed in any of claims 4 to 12 as appendent to claim 3, and including temperature sensing arrangements arranged to initiate calibration for temperature changes compensate for temperature drift of the sensor means in relation to temperature changes.