Measuring and testing – Instrument proving or calibrating – Speed – velocity – or acceleration
Reexamination Certificate
1999-11-30
2004-03-02
Kwok, Helen (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Speed, velocity, or acceleration
C073S504130
Reexamination Certificate
active
06698271
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for reducing bias error in a Vibrating Structure Sensor, particularly but not exclusively, suitable for use with a Vibrating Structure Gyroscope and to a Vibrating Structure Sensor.
SUMMARY OF THE INVENTION
Vibrating structure sensors such as gyroscopes may be constructed using cylindrical or planar ring structures as the vibrating element. These are typically excited into a cos
2
&thgr; resonance mode. For a perfectly symmetric vibrating structure the cos 2&thgr; mode actually exists as a degenerate pair of vibration modes at a mutual angle of 45°. The vibrations are shown schematically in
FIGS. 1A and 1B
. One of these modes (
FIG. 1A
) is excited as the carrier mode. When the structure is rotated around the axis normal to the plane of the vibrating structure Coriolis forces couple energy into the response mode (FIG.
1
B). The carrier mode vibration typically is maintained at a constant amplitude at the peak resonance frequency. When the sensor body is rotated Coriolis forces couple energy into the response mode. The amplitude of motion of the response mode is directly proportional to the applied rotation rate.
The vibrating structure may be driven into resonance by various drive means including electromagnetic, electrostatic, piezo-electric, optical or thermal expansion. The induced motion may similarly be detected by various pick-off means, including electromagnetic, piezo-electric or optical. The orientation of the drives and pick-off means around the resonant structure are shown schematically in FIG.
2
. The primary drive means
1
excites the resonant carrier motion which is detected by the means
2
which is located at 90° to the primary drive means
1
. It is usual to operate the structure with the primary pick-off means output constant to maintain a constant carrier mode amplitude. The secondary pick-off means
3
is located 135° from the primary drive means
1
and is used to detect the response mode motion. For a perfectly radially symmetric vibrating structure, there will be no response mode motion in the absence of an applied rotation. The secondary pick-off signal output will be directly proportional to the applied rotation rate. An additional secondary drive means
4
, positioned at 45° to the primary drive means
1
, may be employed to operate the sensor in a forced feedback or closed-loop mode. In this mode, the secondary pick-off output is nulled by applying a force on the secondary drive means
4
. The applied force is equal and opposite to the rotation induced Coriolis force and there is thus no resultant response mode motion.
The performance of the sensor is characterized in terms of its scale factor and bias stability over the range of operating conditions. It is generally preferable to operate the sensor in a closed-loop configuration as this gives superior scale factor performance to the open-loop configuration. This is due to the fact that, with the response mode motion nulled, its dynamic behavior does not affect the rate response so variations in the quality factor, Q, over temperature will not affect the scale factor response.
FIG. 3
shows a simplified block diagram of a conventional sensor control system operation.
In
FIG. 3
the system includes a primary drive amplifier
5
, a primary pick-off amplifier
6
, a secondary drive amplifier
7
and a secondary pick-off amplifier
8
. A primary drive input at
9
excites the carrier mode resonance and maintains a constant signal, and hence a constant amplitude of motion, at the primary pick-off output
10
for the primary resonance indicated at
11
. An applied rate, &OHgr; at
12
will thus produce a Coriolis force which couples energy from the primary carrier mode into the secondary resonance or response mode
13
. In
FIG. 3
, this coupling is represented substantially by a multiplier
14
. The force F
c
is given by:
F
c
=&OHgr;.PPO.K.&ohgr;
p
(1)
where PPO is the primary mode amplitude, &ohgr;
p
is the primary drive frequency and K is a constant. This motion is detected and amplified by the secondary pick-off amplifier
8
. In the open-loop mode, this signal amplitude is a direct measure of the applied rate. In closed-loop operation, the secondary pick-off signal output
15
is fed back to the secondary drive input
16
. The secondary drive then applies a force driving to the response mode such that the secondary pick-off output
15
is nulled. In the absence of any errors, this force will be equal and opposite to the Coriolis force and thus there will be no net response mode motion. The amplitude of the applied force is proportional to the applied rate &OHgr;.
Detailed modeling of the control loops and resonator modal behavior enables the primary error sources to be identified and quantified. The dominant source of bias error is found to arise from the misalignment angle, &egr;
r
, between the primary drive
9
and secondary pick-off
15
(i.e., deviation from 135°). The contribution of this error mechanism to the bias is given by:
Biase
∝
fϵ
r
Q
(
2
)
where f is the resonant frequency. The magnitude of this error is directly proportional to f and inversely proportional to Q. In practical sensors f is relatively stable over the operating temperature range. The Q value, however, is an inherent material property which may vary significantly over the operating temperature range, thus giving rise to a significant bias variation. In the system block diagram (FIG.
3
), this error is represented by a coupling
17
between primary and secondary channels which adds a portion of the primary mode motion into the secondary pick-off output
15
. In order to maintain the pick-off output
15
at zero, the response mode must be driven such that the motion is equal and opposite to the error being summed in. The input to the response mode resonance is no longer zero so the secondary drive is not a true representation of the applied rate &OHgr;.
In the above discussion, it has been assumed that the carrier mode and response mode frequencies are exactly matched. In practice, material anisotropies and manufacturing tolerances have given rise to some degree of mismatch in these frequencies. Techniques for bringing these frequencies into balance are known and are described, for example, in EP 0411489 B1 and GB 2272053 A. The general procedure involves bringing the modes into alignment with the drives and trimming the split between their resonance frequencies to within a specified tolerance. This is achieved by controlled adjustment of the stiffness or mass at appropriate points around the structure.
In order to optimize the gyro performance the secondary pick-off misalignment error, &egr;
r
, must be minimized. This is conveniently done as part of the vibrating structure balancing procedure and is achieved by adjusting the effective position of either the primary drive means
1
or the secondary pick-off means
3
. The balancing procedure is performed with the sensor effectively operating open loop. With the primary drive means
1
on resonance, the observed secondary pick-off output
15
will vary depending on the primary drive to carrier mode alignment angle.
The modelled secondary pick-off response, with no misalignment error, is shown in
FIG. 4
for a carrier mode frequency of 5 kHz with a frequency split of 0.2 Hz and a Q of 5000. These are typical resonator mode parameters for a known vibrating structure sensor. The response has been resolved into components which are in-phase (line
18
) and in quadrature (line
19
) with respect to the carrier mode motion. It is the in-phase component
18
which gives the rate output signal. Both in-phase
18
and quadrature
19
signals are zero when either mode is aligned to the drive. The amplitude of the signal variation with mode angle is dependent upon the level of frequency split and will tend to zero at all points for perfectly balanced modes. In practice, there will always be a residual frequency split and hence a variation in the secondary pick-off sig
Fell Christopher Paul
Hopkin Ian David
Townsend Kevin
Bae Systems, PLC.
Kenyon & Kenyon
Kwok Helen
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