Device and method for measuring an angular speed

The angular speed measuring device comprises: PA1 a transducer (1) intended to rotate at said angular speed; PA1 means (9) for generating a mechanical vibration of said transducer (1) in response to an excitation signal (OSC), this mechanical vibration comprising a parasite component and at least one useful component having an amplitude which is representative of said angular speed, and PA1 means for producing an electric detection signal (DET) representative of said mechanical vibration and also comprising a parasite component and at least one useful component having an amplitude which is representative of said angular speed. The device is characterized in that it further comprises processing means (23) for obtaining from said electric detection signal (DET), an analog measurement signal (S") the amplitude of which only depends on said useful component of the electric detection signal (DET), the parasite component of the electric detection signal being eliminated.

The present invention concerns an angular speed measuring device 
comprising: 
a transducer intended to rotate at said angular speed; 
means for generating a mechanical vibration of said transducer in response 
to an excitation signal, said mechanical vibration comprising a parasite 
component, and at least one useful component having an amplitude which is 
representative of said angular speed; 
means for producing an electric detection signal representative of said 
mechanical vibration and also comprising a parasite component and at least 
one useful component having an amplitude which is representative of said 
angular speed. 
The invention further concerns a method of processing excitation and 
detection signals for facilitating the measurement of the useful component 
of the detection signal. 
The invention applies in particular to the determination of the useful 
component of the electric detection signal provided by a tuning fork 
gyrometer in response to an excitation signal, due to the Coriolis force 
acting on the tuning fork. 
Tuning fork gyrometers have a tendency of imposing themselves for the 
determination of the angular speed of a moving object, due to their small 
dimensions and their low costs. Complex and expensive gyroscopes have been 
used for a long time, in particular in aeroplanes or missiles, where they 
are used to follow the orientation of the aeroplane or the missile with 
respect to a fixed reference. 
But, with possibilities of miniaturisation connected to the tuning fork 
gyroscope, new applications are contemplated, in particular in the field 
of automobiles, where they may be integrated into anti-skidding devices or 
devices for equilibrium correction. 
An angular speed measuring device using a quartz tuning fork has been 
described in detail in the patent EP-B-0 515 981, of the same applicant. 
In a quartz tuning fork gyrometer, the arms of the tuning fork are 
provided with electrodes for applying an excitation signal, and with 
electrodes for detecting a detection signal which corresponds to the 
response of the tuning fork during its rotation around its longitudinal 
axis. 
Certain documents of the state of the art, in particular the document EP 0 
494 588, assume that the correct operation of a tuning fork gyrometer 
depends, on the one hand, on the implementation of the excitation and the 
detection electrodes on the tuning fork, allowing to minimise the coupling 
between the excitation signal and the detection signal, and on the other 
hand, on the quality of the electronic processing means associated to the 
tuning fork so as to fully exploit the useful component, typically very 
small, due to the Coriolis force during the movement of the gyrometer. 
However, known tuning fork gyrometers are, in the present state of the art, 
penalised by the difficulty to measure the useful part of the detected 
signal corresponding to the Coriolis force, this being all the more true 
when the rotational speed of the gyrometer is low. 
The present invention has as its aim to remedy this inconvenience, by 
proposing an angular speed measuring device based on the use of the 
Coriolis force allowing to isolate and to determine with a sufficient 
resolution the useful component of the detection signal provided by the 
transducer. 
Another aim of the invention is to propose an angular speed measuring 
device of the type described here above having a higher resolution during 
the measurement of the detected useful signal. 
To this effect, the invention has as its object an angular speed measuring 
device comprising: 
a transducer intended to rotate at said angular speed; 
means for generating a mechanical vibration of said transducer in response 
to an excitation signal, said mechanical vibration comprising a parasite 
component, and at least one useful component having an amplitude which is 
representative of said angular speed, 
means for producing an electric detection signal representative of said 
mechanical vibration and also comprising a parasite component and at least 
one useful component having an amplitude which is representative of said 
angular speed, 
characterized in that the device comprises processing means for obtaining 
from said electric detection signal, an analog measurement signal the 
amplitude of which only depend on the useful component of the electric 
detection signal, the parasite component of the electric detection signal 
being eliminated, said processing means comprising means for mixing the 
detection signal and the excitation signal and comprising phase-shifters 
which are connected to the inputs of said means for mixing, and capable of 
rendering the analog measurement signal independent of an initial 
phase-shift between the excitation signal and the detection signal, in 
such a way that the amplitude of the resulting measurement signal presents 
a value which is proportional to the angular speed to be measured. 
Advantageously, these analog processing means of the signal comprise a 
low-pass filter, connected at the output of the mixer, and having a 
cut-off frequency which is chosen to eliminate the components of the mixed 
signal which have a frequency superior to the frequency of the excitation 
signal. Furthermore, amplifiers and all-pass filters used as 
phase-shifters are connected following the mixer, for eliminating the 
dependency of the measurement signal with respect to the initial 
phase-shift between the excitation signal and the detection signal, in 
such a way that the amplitude of the resulting output signal presents a 
value which is proportional to the angular speed to be measured. 
Preferably, the processing means comprise means for respectively mixing the 
excitation signal and the detection signal with a reference signal, and 
which are associated to low-pass filter means and to phase-shifting means, 
in such a way so as to produce an output signal the amplitude of which is 
a function of the angular speed and of the amplitude of the reference 
signal, thereby being independent of the parasite component of the 
detection signal. In such a case, the reference signal is chosen with an 
amplitude sufficiently large, which is capable of increasing the 
resolution of the measurement signal of the angular speed.

Reference will now be made to FIG. 1. In this figure an example of a tuning 
fork 1 of the type used in gyrometers has been represented. Tuning fork 1 
is represented in a longitudinal cross-section in FIG. 1A comprising 
principally a base 3 fixedly connected to two arms 5, 7, the assembly 
being manufactured of a quartz piezoelectric material. As has been 
represented by a transversal cross-section view according to FIG. 1B, each 
arm 5, 7 comprises electrodes 9, 11. The excitation arm (E) 5 comprises 
excitation electrodes 9, only one of the four electrodes represented 
having been referenced, allowing the application of an electric signal 
.+-. V which allows to excite, and as a consequence allows to mechanically 
vibrate the arms 5, 7 of tuning fork 1 in a first plane, as indicated by 
the arrows 13. The detection arm (D) 7 comprises detection electrodes 11, 
only one of the four electrodes represented having been referenced, which 
allow to transform the mechanical vibrations of the detection arm into an 
electric detected signal. 
According to the theory of tuning fork gyrometers, an angular rotational 
movement of tuning fork 1 around its longitudinal axis 15 during which an 
excitation signal is applied to the excitation electrode (E) 9, generates 
a Coriolis force perpendicular to the excitation, and as a consequence a 
vibration of at least the detection arm (D) 7 in a plane perpendicular to 
the plane corresponding to the excitation vibration, as indicated by the 
arrow 17. 
This mechanical vibration is transformed by the piezoelectric quartz of 
tuning fork 1 into an electric signal which is detected by the detection 
electrodes 7 of tuning fork 1. 
In the present invention the problem of positioning the excitation 
electrodes 9 and the detection electrodes 11 on the arms of tuning fork 1 
so as to generate an optimum electric response of the tuning fork will not 
be discussed here, this problem having been discussed and solved in the 
document cited belonging to the state of the art. On the contrary, the 
present invention is concerned with a method of electronically processing 
the electric excitation and detection signals, and concerns the means 
which are associated with this, so as to isolate and measure the useful 
component of the detection signal, i.e. the component which is due to the 
Coriolis force, and this with a resolution and a speed which are 
sufficient so as to allow the use of the tuning fork in new applications. 
To this effect, reference will be made to FIG. 2, in which is represented 
in a schematical way a tuning fork 1 corresponding to that one of FIG. 1, 
associated to an electronic processing circuit of the excitation and of 
the detection signal. 
In a manner which is known as such, the excitation electrodes 9 of tuning 
fork 1 are integrated into a resonance circuit which is schematised by the 
loop 10 and which is supplied with a continuous current by an amplifier 
21, and where the detection signal DET is detected at the terminals of the 
detection electrodes 11. 
As a simplification of the considerations which will follow, it will be 
assumed that the excitation signal (OSC) is sinusoidal, and that the 
detection signal (DET) corresponds to the superposition of a parasite 
component and of a useful component corresponding to the Coriolis force, 
phase-shifted about an angle .phi..sub.0 with respect to the excitation 
signal. Under these conditions, the signals OSC and DET satisfy the 
following equations: 
EQU OSC=A.multidot.sin (.omega..sub.0 t+.phi..sub.0), (1) 
in which A is the amplitude of the excitation signal, .omega..sub.0 is its 
pulsation and .phi..sub.0 is its initial phase-shift with respect to the 
detection signal, and: 
EQU DET=B.multidot.sin (.omega..sub.0 t)+C.multidot.cos (.omega..sub.0 t),(2) 
in which the first term represents a parasite signal caused by the 
capacitive mechanical coupling between the arms of the tuning fork and the 
second term represents the useful signal which is caused by the Coriolis 
force, its amplitude C being the amplitude to be measured, and which is 
proportional to the rotational speed .OMEGA. of the tuning fork. 
It should further be noted that the phase-shift .phi..sub.0 between the 
excitation signal OSC and the detection signal DET is constant for a given 
tuning fork, and is typically around 56.degree. for a tuning fork such as 
the one represented in FIG. 1. 
The detection signal DET appears as a phase modulated signal, which can be 
defined according to the following equation: 
##EQU1## 
In practice, it has been noted that the amplitude C of the useful component 
of the detection signal is much smaller, for rotational speeds .OMEGA. 
which are around 50.degree./s, than the amplitude B of the parasite 
coupling component, so that the ratio C/B is typically around 1/50, which 
corresponds to a phase-shift angle .psi. which is very small, around 
1.degree., and which is difficult to measure and to be used for 
determining the rotational speed .OMEGA. of tuning fork 1. 
As a consequence, the principle of the present invention intends to provide 
an analog processing of the signals OSC and DET, so as to allow to extract 
more easily the amplitude C of the useful component of the detected 
signal. 
According to a first embodiment of the invention, as shown schematically in 
FIG. 2 by block 23, this analog processing consists in bringing the 
signals OSC and DET to an analog multiplier. The resulting signal S (t) 
corresponds to the following relationship (4): 
EQU S(t)=OSC.multidot.DET=A.multidot.sin (.omega..sub.0 
t+.psi..sub.0)!.multidot.B.multidot.sin (.omega..sub.0 t)+C.multidot.cos 
(.omega..sub.0 t)! (4) 
so that: 
EQU S(t)=A.multidot.B/2cos .psi.0-cos (2.omega.0t+.psi.0)!+A.multidot.C/2sin 
(2.omega.0t+.psi.0)+sin .psi.0!. 
According to the invention, the mixing of the signals OSC and DET is 
followed by an all-pass filter comprising filters having an appropriate 
cut-off frequency so as to eliminate the multiple values of .omega..sub.0. 
If the pulsation .omega..sub.0 is chosen in such a way to correspond to a 
frequency of 8 kHz for example, it will suffice to filter the components 
of S (t) at 16 kHz. What remains is a signal S' (t) according to 
relationship (4) here above: 
EQU S'(t)=A.multidot.B/2.multidot.cos .psi..sub.0 +A.multidot.C/2.multidot.sin 
.psi..sub.0. (5) 
Preferably, the signals OSC and DET are phase-shifted before their mixing, 
with the aid of all-pass filter phase-shifters allowing to fix their 
phase-shift at .pi./2. When OSC and DET are thus in quadrature, their 
mixing and filtering results in a signal S"(t) which satisfies the 
relationship (6):S"(t)=A.multidot.C/2, the amplitude of which is 
proportional to the amplitude C of the useful component, and which is 
representative of the angular speed .OMEGA. (t) which is the one to be 
measured. 
As a result, the speed may be the determined thanks to the analog 
processing described here above, solely by way of the amplitude A of the 
excitation signal OSC and the amplitude C of the useful component of the 
detection signal DET, but independently of the parasite component of the 
detection signal. 
Reference will be made to FIG. 3 in which is shown a schematical 
representation of an example of an embodiment of an electronic device 
capable of generating the analog signal S" (t), such as defined here 
above, from a signal OSC=V.sub.1 (t) et DET=V.sub.2 (t). 
The electronic device of FIG. 3 comprises in principal an oscillator stage 
25 which supplies tuning fork 1 with an excitation signal OSC, and an 
analog processing stage 27, which uses the detection signal DET and the 
excitation signal OSC for producing an output signal S" (t) allowing to 
measure the angular speed of the tuning fork. 
Oscillator stage 25 comprises, in a known manner, an amplifier 29 the 
output of which is connected to the input of an all-pass filter 31. A 
feedback loop comprising a rectifier 33 in series with a current 
integrator regulator 35 using a reference current source 37 (REFI) 
measures the amplitude of excitation signal OSC and acts on the amplitude 
A.sub.0 of the amplifier 29 for maintaining constant this amplitude and 
for regulating it to a given value, thus allowing to stabilise the level 
of the excitation signal OSC provided by the oscillator stage. A 
resistance R1 connected between the input of rectifier 33 and the mass, 
allows to obtain an indication of the current which flows through tuning 
fork 1. 
The analog processing stage 27 comprises amplifiers 38, 39 respectively of 
the excitation signal OSC and of the detection signal DET. The outputs of 
these amplifiers are connected respectively to phase-shifting means 41 and 
43 in particular realised by way of all-pass filters, allowing to put into 
quadrature the amplified signals OSC and DET. The outputs of 
phase-shifters 41, 43 are connected to the input of an analog mixer 45, 
which provides at its output a mixed signal corresponding to the equation 
(4) here above. This signal is transmitted to a low-pass filter 47 which 
eliminates the frequencies which are a multiple of the frequency of the 
excitation signal, and which provides a signal S" (t) corresponding to 
equation (6) here above. 
Preferably, for increasing the resolution .DELTA..OMEGA. (.degree./S) of 
the device, the invention allows the use of a double mixing technique of 
the excitation signal OSC and the detection signal DET with a third 
reference signal REF having a greater amplitude. 
This preferred embodiment of the invention is shown schematically in FIG. 
4, in which only the processing stage of the signal OSC and DET has been 
represented. The signal OSC=V.sub.1 (t) is mixed with the signal 
REF=V.sub.3 (t) in a mixer 49, the resulting signal being indicated by 
U.sub.1 (t). Further, the signal DET=V.sub.2 (t) is also mixed in a mixer 
51 with a reference signal, in particular with the same signal REF=V.sub.3 
(t), the resulting signal being indicated by U.sub.2 (t). The signals 
U.sub.1 (t) and U.sub.2 (t) are then processed by amplifier means 53, 55 
and are phase-shifted by phase-shifting means realised by a way of 
all-pass filters 57, 59, in a way similar to that described in relation to 
FIG. 3, by corresponding analog processing circuits. The signals coming 
from all-pass filters 57, 59 are then transmitted to a new multiplier 61, 
the output of which is transmitted to a low-pass filter 63 providing a 
measurement signal S" (t) of the angular speed of tuning fork 1. 
By choosing a reference signal V.sub.3 (t) according to the following 
relationship (7): 
EQU V.sub.3 (t)=R.multidot.cos (.omega.+.DELTA..omega.)t!, (7) 
and by conserving for the signal OSC and DET the preceding relationships, 
the following is obtained: 
EQU U.sub.1 (t)=A.multidot.R/2.multidot.cos (.DELTA..omega.t-.phi.)(8) 
and 
EQU U.sub.2 (t)=B.multidot.R/2 .multidot.cos 
.DELTA..omega.t+C.multidot.R/2.multidot.sin .DELTA..omega.t(9) 
Then, after filtering and phase-shifting as explained here above, and after 
mixing in the mixer 61, S" (t) is obtained which corresponds to the 
relationship (10): 
EQU S"(t)=A'.multidot.C'/2, (10) 
in which A'=A.multidot.R/2 and C'=C.multidot.R/2, so that: 
EQU S"(t)=(A.multidot.C/2).multidot.(R.sup.2 /4) (11) 
It results from this that the measurement signal S" (t) of the angular 
rotational speed of tuning fork 1 is, due to the technique of the double 
mixer as describes here above, always proportional to the amplitude C of 
the useful component of the detection signal, but which is further 
multiplied by a term (R.sup.2 /4) with respect to the embodiment without 
the double mixing. Thus, it suffices to choose a reference signal having 
an amplitude R which is superior to 2 Volts so as to increase the 
amplitude of the measurement signal S" (t). As a consequence, by choosing 
a reference signal REF having a sufficiently large amplitude, it is 
possible to increase as desired the resolution of the measurement of the 
angular speed. 
Furthermore, the device according to the invention may advantageously be 
used in combination with a digital circuit of an angular speed measuring 
device such as described in the European patent EP-B-0 515 981 cited here 
above. As such, when the signals V.sub.1 (t) and V.sub.2 (t) have been 
mixed with the reference signal V.sub.3 (t) so as to obtain the signal S" 
(t) corresponding to the relationship (10), instead of passing through an 
analog processing of the signal such as described here above, the output 
signal is passed through a counter so as to digitalise this signal. As has 
been explained in the patent cited here above, the device uses a reference 
clock signal for sampling the measurement signal of the angular speed. The 
resolution of the device is determined by the relationship between the 
frequency of the measurement signal of the angular speed and the frequency 
of the clock signal. The lower the frequency of the measurement signal 
with respect to the frequency of the clock signal, the higher the 
resolution. 
However, the frequency of the clock signal is fixed, for example at 20 MHz, 
and may not be easily changed without implying an increased consumption by 
the digital circuit. Thus, it is preferable to be able to lower the 
frequency of the measurement signal of the angular speed. Now then, the 
device according to the invention can do this by way of reference signal 
REF. In fact, as the signal S" (t) has a frequency which is proportional 
to (.omega..DELTA..omega.), .DELTA..omega. may be chosen close to .omega. 
in such a way that the frequency of the measurement signal will be lower. 
For example, if the frequency of the oscillation signal OSC is around 8 
KHz, the frequency of the reference signal REF will be chosen around 7 Khz 
so as to obtain a frequency of the signal S" (t) which will be around 1 
kHz. Thus, the resolution will be increased by a factor 8 with respect to 
the resolution which may be obtained by the device described in the patent 
EP-B-0 515 981 cited here before. 
From this is follows that the angular speed measuring device according to 
the invention reaches its aims by allowing, on the one hand, to provide an 
output signal which only depends on the useful component of the detection 
signal, and which allows, on the other hand, either to increase the 
amplitude of the output signal or to lower the frequency of the output 
signal so as to increase the resolution of the angular speed measurement.