Gyroscope

Abstract

A vibrating structure gyroscope includes a permanent magnet, a structure arranged in a magnetic field of the permanent magnet and arranged to vibrate under stimulation from at least one primary drive electrode and a drive system that includes: one primary drive electrode arranged at least one primary sense electrode arranged to sense motion in the vibrating structure and a drive control loop controlling the primary drive electrode dependent on the primary sense electrode. The structure also includes a compensation unit arranged to receive a signal from the drive system representative of a gain in the drive control loop and arranged to output a scale factor correction based on that signal. As the magnet degrades (e.g. naturally over time as the material ages), the magnetic field weakens. To compensate for this, the primary drive control loop will automatically increase the gain.

Claims

1. A vibrating structure gyroscope, comprising: a permanent magnet; a vibrating structure arranged in a magnetic field of the permanent magnet and arranged to vibrate under stimulation from at least one primary drive electrode; a drive system arranged to vibrate the vibrating structure at a resonance frequency, the drive system comprising: the at least one primary drive electrode arranged to induce motion in the vibrating structure, at least one primary sense electrode arranged to sense motion in the vibrating structure; and a drive control loop controlling the primary drive electrode dependent on the primary sense electrode; and a compensation unit arranged to receive a signal from the drive system representative of a gain in the drive control loop and arranged to output a scale factor correction, for application to a rate signal of the gyroscope based on the signal.

2. The vibrating structure gyroscope as claimed in claim 1, wherein the signal from the drive system comprises one or more of: the amplitude of the drive signal for the primary drive electrode; the amplitude of the signal from the primary sense electrode; and the gain of the drive control loop.

3. The vibrating structure gyroscope as claimed in claim 1, wherein the compensation unit is arranged to output the scale factor correction based on the signal from the drive system and a stored reference value, wherein the stored reference value is a value of a signal from the drive system obtained during a calibration procedure.

4. A vibrating structure gyroscope as claimed in claim 3, wherein the compensation unit is arranged to output the scale factor correction further based on a known relationship between a signal level of the signal from the drive system, magnetic field strength and scale factor error.

5. The vibrating structure gyroscope as claimed in claim 1, wherein the compensation unit includes a lookup table that is arranged to provide a scale factor correction value according to the input signal from the drive system.

6. The vibrating structure gyroscope as claimed in claim 1, wherein the compensation unit is arranged to receive a temperature signal and to output the scale factor correction based on both the signal from the drive system and the temperature signal.

7. The vibrating structure gyroscope as claimed in claim 6, wherein the compensation unit includes a lookup table that is arranged to provide a scale factor correction value according to both the signal from the drive system and the temperature signal.

8. The vibrating structure gyroscope as claimed in claim 1, further comprising: a sensing system arranged to sense the vibrations of the vibrating structure and arranged to output an angular rate signal based on the sensed vibrations; wherein the vibrating structure gyroscope is arranged to apply the scale factor correction to the angular rate signal to provide an output of the vibrating structure gyroscope.

9. A method of calibrating a gyroscope, comprising: providing a gyroscope as claimed in claim 1; evaluating a strength of a signal from the drive system in a test environment while the gyroscope is not rotating; and storing in the compensation unit information based on the evaluation of the drive system that allows determination of the scale factor correction from the signal from the drive system.

10. The method of as claimed in claim 9, further comprising storing in the compensation unit information on a relationship between the strength of the signal from the drive system and the magnetic field of the permanent magnet.

11. The method as claimed in claim 9, wherein evaluating comprises evaluating the strength of the signal from the drive system across a range of temperatures.

12. The method as claimed in claim 9, wherein storing comprises storing the information in a lookup table.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which:

(2) FIG. 1 shows an example arrangement of an inductive vibrating structure gyroscope; and

(3) FIG. 2 shows an example of a gyroscope with scale factor compensation.

(4) Referring to FIG. 1, an inductive vibrating ring gyroscope 1 is shown. A ring shaped resonator 10 is attached to a support frame 12 by flexible support legs (not shown) that extend from an outer circumference of the resonator 10 to the support frame 12 and which allow the resonator 10 to vibrate in primary mode and a secondary mode of oscillation. The support frame 12 is mounted to a glass pedestal 14 which, in turn, is mounted on a glass substrate 16.

(5) A magnet assembly 18 comprises a lower pole piece 20, an upper pole piece 24 and a permanent magnet 22 which is located between the lower pole piece 20 and the upper pole piece 24. The lower pole piece is mounted to the substrate 16 underneath the resonator 10 while the upper pole piece 24 is formed as a cap, the rim of which is formed above the resonator 10. A magnetic field generated by permanent magnet 22 is directed through the resonator 10.

(6) FIG. 2 shows an inductive vibrating structure gyroscope 30 together with its control and detection systems. The physical structure of the resonator 10 and magnet assembly 18 of the gyroscope 30 may be as shown in FIG. 1.

(7) A drive system 31 is arranged to provide a drive signal to the primary drive electrode P.sub.D (in practice this may be a diametrically opposed pair of electrodes). A pick-off signal is generated by primary pick-off electrode P.sub.P (the primary sense electrode) which is situated at a position 90 degrees round the resonator ring 10 from the drive electrode P.sub.D. The pick-off signal is amplified by amplifier 32 and is provided to Voltage Controlled Oscillator/Phase Lock Loop circuit 33 which adjusts the phase and frequency of the signal to lock onto the resonant frequency of the resonator 10 so as to maintain the primary mode of oscillation. The adjusted signal is provided through amplifier 35 to the primary drive electrode P.sub.D to maintain the resonance. The pick-off signal is also provided in parallel to Automatic Gain Control (AGC) circuit 34 which adjusts the gain of amplifier 35 to ensure that amplitude of resonance is maintained.

(8) In the example of FIG. 2 the AGC 34 receives the pick-off signal (actually the amplified version of the pick-off signal output from amplifier 32) and compares it against a threshold. If the magnitude of the pick-off signal is lower than the threshold, it increases the gain of amplifier 35, while if the magnitude of the pick-off signal is greater than the threshold, it decreases the gain of amplifier 35. This changes the magnitude of the drive signal which in turn changes the amplitude of the resonator's oscillations which in turn changes the amplitude of the pick-off signal. Thus the primary drive control loop (comprising the amplifier 32, VCO/PLL 33, AGC 34 and amplifier 35) constantly adjusts the signals so as to maintain the resonator 10 in resonance and at the correct amplitude of motion.

(9) The gain of AGC 34 is also provided as an output which is provided to compensation unit 36. Compensation unit 36 outputs a scale factor correction 37 based on the input from AGC 34.

(10) In some examples, compensation unit 36 may calculate the scale factor correction 37 solely on the basis of the input from AGC 34 and stored information (such as a formula and known parameter values). In other examples, the compensation unit may additionally take into account a reference value Ref which is acquired and stored during a calibration procedure and which indicates the AGC gain that was required at the time of calibration (and therefore represents the state of the magnet at that time). In some other examples the compensation unit 36 may perform a lookup of the current gain value from AGC 34 in a lookup table 38 which has been pre-calculated and stored within compensation unit 36 at the time of calibration. The lookup table has pre-calculated scale factor corrections that correspond to certain magnet strengths and which will therefore correspond to certain gain levels of the AGC 34.

(11) The scale factor correction 37 is applied to the rate signal at 39 to provide a corrected rate output (“Rate” in FIG. 2) of the gyroscope 30.

(12) The rate signal is acquired via secondary pick-off electrode S.sub.P and amplifier 40.

(13) The output from the secondary pick-off contains both the ‘Real’ and ‘Quadrature’ components in the observed signal, which are orthogonal in phase and determined through the frequency input from the primary loop. The ‘Real’ component provides the desired gyroscope output of actual rate applied. The ‘Quad’ component is generated through imperfections in the system that cause energy to be coupled into the secondary motion and this quadrature (i.e. 90°) component does not contribute to the rate output.

(14) In an open-loop example, the output of amplifier 40 is passed through a demodulator 42 to extract the Real component and this is used as the rate output (to be corrected by scale factor correction 37 at 39). In closed-loop examples such as shown in FIG. 2, the output of amplifier 40 is also passed through a demodulator 43 to extract the Quad component. The Real and Quad components are recombined and used to generate a secondary drive signal via amplifier 41 which is applied to secondary drive electrode SD so as to null the secondary mode motion of the resonator 10. The magnitude of the Real part of the signal required to null this motion is then used as the rate output (to be corrected by scale factor correction 37 at 39).

(15) It will be appreciated that the AGC 34 will also compensate for other operating conditions such as temperature variations. To take account of this, the compensation unit 36 may also have a temperature input from temperature sensor 50. In such examples, the formula or lookup table 38 stored in compensation unit 36 also takes account of temperature. For example the lookup table 38 could have entries for a number of different gain levels, and for each gain level, could provide a scale factor compensation output for each of a plurality of temperatures.

(16) According to this system, the scale factor of the gyroscope can be corrected throughout the lifetime of the product as the magnet ages and/or degrades. This may be achieved as follows:

(17) Firstly, during manufacture the gyro primary drive level is characterized over temperature. This may be done on a test rig in a well-controlled environment and with the gyroscope in known states of rotation (e.g. stationary or rotating at a known angular rate).

(18) Secondly, during use the gyroscope measures the required primary drive level for the gyro (e.g. directly or via the gain of the AGC) and compares against the information obtained during calibration.

(19) Thirdly, the gyroscope uses this data and the comparison to correct for the variation in scale factor due to the aging of the magnet since manufacture (or since calibration). An increase in primary drive correlates to a reduction in magnet strength and therefore a positive increase in scale factor.

(20) In order to allow for the calibration information (temperature and drive level) to be stored in the gyroscope, the gyroscope may be provided with a data transfer interface. This may take the form of a bi-direction communication interface which also allows for output of the gyroscope data during use.

(21) This scale factor compensation scheme is particularly suited to high performance gyroscopes. The scale factor improvement will be dependent on the particular gyroscope design, but by way of example, in one existing gyroscope with a 20 year life time and a magnet degradation of 100 ppm per year, the existing scheme will have a constant scale factor correction that is off by around 1000 ppm at the start and end of the product's life. The correction provided by this disclosure can completely or almost completely cancel out this contributor to scale factor and thus results in a scale factor improvement of up to 1000 ppm compared with the existing system.

(22) In one particular example a SmCo magnet was found to have an aging coefficient of around 0.0532 ppm per hour when operated at a relatively high temperature of 165 degrees C. (high temperatures are known to increase the aging effect in magnets). Over a 20 year product life this equates to over 9300 ppm, i.e. almost 1 percent. This degradation creates a change in scale factor that is significant for high performance gyroscopes and the present disclosure provides a way to track and compensate for this change in scale factor.

(23) It will be appreciated that there are other sources of scale factor error that may also be compensated separately from or in addition to this process.