Measuring and testing – Speed – velocity – or acceleration – Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
2002-09-17
2003-11-25
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Angular rate using gyroscopic or coriolis effect
C073S504130
Reexamination Certificate
active
06651499
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a control system for a vibrating structure gyroscope particularly, but not exclusively, suitable for use with a silicon micro-machined vibrating structure gyroscope.
Vibrating structure gyroscopes are known using a variety of different mechanical vibratory structures. These include beans, tuning forks, cylinders, hemispherical shells and rings made from ceramic, metal or silicon. A common feature in these known systems is that they are required to maintain a resonance carrier mode oscillation at a natural frequency determined by the mechanical vibratory structure. This provides the linear momentum which produces Coriolis force when the gyroscope is rotated around the appropriate axis.
Advances in micro-machining technology have made it possible to produce vibrating structure gyroscopes from silicon in high volumes and at low cost. Such gyroscopes are being developed for automotive applications such as vehicle dynamic control systems and for car navigation. The performance characteristics of these micro-machined gyroscopes are tailored to meet automotive requirements with the maximum specified rate range typically being plus or minus 100° per second.
Such micro-machined vibrating structure gyroscopes are inherently rugged and of low cost which makes them attractive for use in other more demanding applications such as for aircraft navigation or for guided munitions. These latter applications typically require the gyroscope to operate over a significantly wider range of rotation rates. Whilst it is possible to extend the rate range capability of gyroscopes developed for automotive applications this will typically result in degradation of other key performance parameters such as noise and bias.
Conventional vibrating structure gyroscopes having a planar ring vibrating structure made of metal or silicon or having a cylindrical vibrating structure give good overall performance. Planar ring vibrating structures are typically driven in Cos
2
&thgr; vibration modes as shown schematically in
FIGS. 1
a
and
1
b
of the accompanying drawings. One mode, having radial anti-nodes aligned along axes P, as shown in
FIG. 1
a
, is excited as the primary mode. When the gyroscope is rotated around the axis normal to the plane of the ring Coriolis forces F
c
are developed which couple energy into the secondary mode, whose radial anti-nodes are aligned along axes S, as shown in
FIG. 1
b
. The magnitude of the force is given by:
F
c
=2 mv&OHgr;
app
(1)
where m is the modal mass, v is the effective velocity and &OHgr;
app
is the applied rotation rate. The primary mode vibration amplitude typically is maintained at a fixed level. This also maintains the velocity, v, at a fixed level and hence ensures that the developed Coriolis forces are directly proportional to the rotation rate, &OHgr;
app
. The amplitude of secondary mode motion induced by these Coriolis forces may conventionally be enhanced by accurately matching the resonant frequencies of the primary and secondary modes. The motion is then amplified by the Q (measure of the relation between stored energy and the rate of dissipation of energy) of the secondary mode giving enhanced vibrating structure gyroscope sensitivity. When operating in this open loop mode the sensitivity (scalefactor) of the gyroscope will be dependent on the Q of the secondary mode which may vary significantly over the operating temperature range. This dependence may be eliminated by operating the gyroscope in a force feedback (closed loop) mode. In this mode the induced secondary mode motion is actively nulled with the applied force being directly proportional to the rotation rate.
A typical conventional closed loop control system for a vibrating structure gyroscope is shown schematically in
FIG. 2
of the accompanying drawings. This conventional control system basically consists of two independent loops namely a primary loop
1
between a primary pick-off means
2
which acts as a motion detector output from the vibrating planar ring structure
3
and a primary drive means
4
which acts as a forcing input creating vibration in the structure
3
. A secondary loop
5
is provided between a secondary pick-off means
6
and a secondary drive means
7
.
The output signal
8
from the primary pick-off means
2
is amplified by an amplifier
9
and demodulated by demodulators
10
and
11
. The demodulated signal from the demodulator
10
is passed first to a phase locked loop
12
which compares the relative phases of the primary pick-off and primary drive signals at the means
2
and
4
and adjusts the frequency of a voltage control oscillator
13
to maintain a 90° phase shift between the applied drive at means
4
and the resonator motion of the structure
3
. This maintains the motion of the structure
3
at the resonance maximum. The demodulated output from the demodulator
11
is supplied to an automatic gain control loop
14
which compares the level of the output signal from the primary pick-off means
2
in the automatic gain control loop
14
to a fixed reference level V
o
. This signal V
o
is applied at
15
to a voltage adder
16
and the output voltages therefrom supplied to the automatic gain control loop
14
. The output voltage, from the automatic gain control loop
14
is remodulated at remodulator
17
at the frequency supplied by the voltage controlled oscillator
13
and then fed via an amplifier
18
to the primary drive means
4
. The primary drive voltage level is adjusted in order to maintain a fixed signal level, and hence amplitude of motion, at the primary pick-off means
2
.
The secondary loop
5
is such that the signal received from the secondary pick-off means
6
is amplified by amplifier
19
and demodulated by demodulators
20
and
21
to separate real and quadrature components of the rate induced motion. The real component is that which is in-phase with the primary mode motion. The quadrature component is an error term which arises due to the mode frequencies not being precisely matched. The demodulated baseband signal received from the demodulator
20
is filtered by a quadrature loop filter
22
and the demodulated baseband signal received from the demodulator
21
is filtered by a real loop filter
23
to achieve the required system performance in respect of bandwidth and noise. The signal received from the loop filter
22
is remodulated at remodulator
24
and passed to a voltage adder
25
where it is summed with the signal received from the loop filter
23
after remodulation by remodulator
26
. The summed output signal from the voltage adder
25
is applied to the secondary drive means
7
via an amplifier
27
to maintain a null at the secondary pick-off unit
6
. The real baseband signal SD (real), which is the output signal from the real loop filter
23
is taken off before remodulation at the remodulator
26
, scaled and filtered at output filter
28
to produce the rate output signal
29
from the system. The real baseband signal SD (real) is directly proportional to the real secondary drive applied to the vibrating structure
3
.
For this mode of operation the rate output &OHgr;
out,
is given by:
Ω
out
=
k
SD
(
real
)
g
ppo
g
sd
V
o
w
(
2
)
where V
o
is the fixed primary mode amplitude reference voltage set level, w is the primary mode resonance frequency, k is a constant including the modal mass and modal coupling coefficient, g
ppo
is the primary mode pick-off gain and g
sd
is the secondary mode driver gain.
For a gyroscope operating in this conventional closed loop mode, the minimum detectable rotation rate that can be resolved is determined by the sensitivity of the secondary mode pick-off means
6
. This is determined by the electronic noise of the secondary pick-off amplifier
19
. For a fixed pick-off gain, the only way to enhance the resolution of the vibrating structure gyroscope is to increase the secondary mode motion generated by a given applied rate, that is to increase the in loop scalef
Fell Christopher P
Townsend Kevin
BAE Systems plc
Moller Richard A.
Nixon & Vanderhye P.C.
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