Measuring and testing – Speed – velocity – or acceleration – Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
1999-11-05
2002-03-19
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
active
06357296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of vibratory rotation sensors such as, for example, hemispherical resonator gyroscopes (“HRG”s), and more particularly to a vibratory sensor where unwanted secondary harmonics are eliminated using a virtual node/antinode.
2. Description of Related Art
Vibratory sensors employ a resonating member to pick up rotation of the sensor based on the effect of Coriolis force on the resonating member. One type of vibratory sensor, used herein for example purposes only, is referred to as a hemispherical resonator gyroscope. It is to be understood that while the present discussion uses the HRG to illustrate the benefits and operation of the present invention, the present invention's application extends to any vibratory sensor which measures oscillations of a resonating member.
Hemispherical resonator gyroscopes are known in the art for measuring an angular rate of a body about a predetermined axis. HRGs are of critical importance in space applications, such as the orienting of satellites and space vehicles. HRGs are reliable and have a long active life, making the gyro especially suited for this purpose. The gyros are typically comprised of a forcer electrode assembly, a hemispherical thin-walled quartz shell, and a pick-off electrode assembly joined together with a rare-earth metal such as indium. The unit is housed in a vacuum chamber with electrical feeds to communicate voltage signals from the gyro to a microprocessor for interpretation. The general operation of the gyroscope is discussed in the Letters Patent to Loper, Jr. et al., U.S. Pat. No. 4,951,508, which is fully incorporated herein by reference.
The hemispherical resonator
10
is a bell-shaped thin walled structure with a rim that can be made to deform from a circular profile to an elliptical profile when subjected to certain external electrical fields. The resonator is supported by an integral stem which itself is supported by the housing for the pick-off and forcer electrodes. By applying a cyclical forcing voltage, a standing wave pattern can be established in the resonator. To establish the standing wave, the hemispherical resonator is initially biased at a voltage of known magnitude, and then a varying electrical field is applied at the forcer electrodes. If the forcer electrodes apply the appropriate varying electrical field at angular intervals of 90 degrees, the resonator will flexure in a standing wave such as that shown in FIG.
1
.
The primary harmonic resonating wave has four nodes a,b,c,d and four antinodes e,f,g,h around the perimeter of the resonator, alternating and equal spaced forty-five degrees apart. Nodes are points on the standing wave where displacement is a minimum, and antinodes are points on the standing wave where displacement is a maximum. Operation of the HRG requires precise tracking of the standing wave movement, which in turn requires that the location of the nodes and antinodes be accurately determined.
It is a physical property of the gyroscope that if an unrestrained resonator is rotated about an axis normal to the page (see FIG.
2
), the standing wave will precess in an opposite direction to the original rotation due to Coriolis force. Moreover, the amount of the angular precess will be 0.3 times the angular displacement of the resonator, where 0.3 is a geometric property of the resonator's hemispherical shape and holds constant for any rotation angle and any rotation rate. For example, if the resonator of
FIG. 1
is rotated ninety degrees in the counter-clockwise direction, as indicated by the angular displacement of the notch
20
, the standing wave will precess twenty-seven degrees clockwise as shown in FIG.
2
. In this manner when an HRG is rotated about its primary axis, by measuring the change in the angular position of the standing wave information about the rotation of the HRG can be determined.
The position of the standing wave both before and after the rotation of the gyroscope is determined by the pick-off electrodes positioned about the external annular component of the housing. By measuring the capacitance across the gap formed between the pick-off electrodes and the resonator, the distance across the gap can be accurately determined. This information is processed by a microprocessor in a manner such that the exact position of the standing wave is determined. By measuring the change in position of the standing wave, the rotation of the gyro can readily be determined.
HRGs operate in one of two modes—whole angle mode and force rebalance mode. In whole angle mode, the standing wave is allowed to precess unhindered under the influence of the Coriolis force caused by the rotation of the gyro as just described. The instantaneous position of the standing wave is evaluated by computing the arctangent of the ratio of the amplitude of the two pickoff signals. In the whole angle mode the gyro's dynamic range is limited solely by the resolution and processing of the pick-off signal estimation.
In the force rebalance mode, the standing wave is constrained such that it does not precess under the influence of the Coriolis force, and the magnitude of the restraining force is used to calculate the rotation rate of the gyro. In this mode, an additional forcing signal is included which holds the standing wave at a fixed azimuthal location. The amount of force necessary to maintain the standing wave fixed is proportional to the input rotational rate. For force rebalance gyros, the case-oriented control and readout processing is eliminated, and the output noise performance can be optimized because the dynamic range requirements of the pick-off signal estimation are greatly reduced.
In the force rebalance mode, four separate control mechanisms are necessary. The first control mechanism is the phase-lock loop, which is necessary to track the natural frequency and phase of the high Q resonance. This loop provides a timing reference for the other readout and drive mechanisms. The second control mechanism is the amplitude control loop, which establishes and maintains the required stable standing wave amplitude. The third control mechanism is the quadrature control loop, which is used to eliminate the small frequency mismatch between the two principal axes of flexure. The final control mechanism is the rate control loop, which is attributed only to the force rebalance mode, and holds the standing wave in a fixed position while measuring the inertial rate directly through the applied closed-loop forcing.
The phase-lock, amplitude control, and quadrature loops are required for both whole angle mode and force rebalance mode. The amplitude control and the phase-lock loops maintain the flexing amplitude and timing reference, respectively. These processes are associated with the antinode axis pickoffs and forcers. The nominal flexing amplitude defines the stored momentum and the rate scale factor in the HRG in the force rebalance mode. The phase-locked loop is necessary to track the free-running oscillation of the resonator so that the demodulation and drive functions can be synchronized to the narrowband resonance. The quadrature and rate control loops use independent forcers to drive the nodal pickoff amplitude components to zero. The quadrature control loop suppresses the quadrature-mode vibration which develops because of the small frequency mismatch between the two axes.
The rate control loop uses the rate drive to null the in-phase nodal amplitude component, i.e., standing wave deflection.
FIG. 3
is a representation of a rate control loop, where the box
30
represents a model of the HRG mechanics. The model includes a scale factor K which converts volts to an electrostatic force that cancels the Coriolis force due to an inertial input, and the resultant difference force to dynamic response P(s) of the in-phase nodal amplitude y
i
. The difference force includes a thermal noise component &OHgr;
TN
as well as a bias component &OHgr;
B
. The input from the HRG pickoff is amplified
40
a
Baker John C.
Johnson Gregory M.
Zaida Daniel T.
Chapman John E.
Litton Systems Inc.
Price and Gess
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