Instrumentation sensors which operate on a principle of vibration of constrained actuator masses are known in the art. Unlike their gyroscopic instrumentation counterparts, they require no rotating parts. Their vibrating actuator masses may take a number of different configurations such as reed members, piezoelectric crystals, or electromagnetic members.
The principle of operation of vibratory mass instrumentation sensors is relatively simple. If the mass is vibrated or maintained in oscillation in a straight line in which it is guided by a constraint, the oscillating mass will not apply any force (other than its own weight) in a direction transverse to the guide as long as the guide maintains a constant orientation in space. However, if a guide is forced to rotate about an axis at right angles to itself, the oscillating member will apply alternating or pulsating forces to the guide member, the average magnitude of which will be proportional to the angular velocity of the forced rotation. Such forces can be measured by sensors such as piezoelectric crystals. The forces exerted by the oscillating actuator mass on the sensor crystal causes measurable electrical potential signals to be developed on the faces of the sensor crystal, which signals can be measured and calibrated to the rate of turn of the sensor instrument.
A drive circuit is required for such vibrating mass sensor instrumentation systems, for establishing and maintaining the vibrating or oscillatory state of the actuator mass at an optimum level throughout the operative period of the instrument. Generally in such instrumentation, it is most desirable to vibrate the actuator mass at its natural resonance. At any point in time such natural resonance of the mass will occur at a fixed frequency, known as the resonant frequency of the mass. When the mass is vibrated at its resonant frequency, the mass provides its maximum measurable output signal for use by the instrument's sensor. If the amplitude of the actuator mass oscillations can be maintained at a constant level throughout the period of operation of the instrument, high measurement accuracy can be maintained indefinitely by the instrument.
In general, the actuator mass driver circuits of the prior art have taken two basic configurations. The first configuration involves the use of "separate" hardware or elements for performing the "driving" function and the drive "detection" function. In such drive structures, the separate functions are required to allow control of resonance of the mass. Examples of driver circuits constructed according to this configuration are illustrated in U.S. Pat. Nos. 3,992,952 to Hutton et al (FIG. 2); 3,842,681 (FIG. 1); and 2,544,646 to Barnaby et al (FIG. 8). Each of these configurations include physical encumbrances on the sensor system in the form of specific structure arranged and configured to sense the resonant motion of the vibratory mass and to provide a sensed feedback signal for the system. Such physical encumbrances and attachments, besides requiring expensive fabrication, unduly complicate the drive circuitry.
The second actuator mass driver circuit configuration that is typically used in the art eliminates the need for the extra sensing element by using the actuator drive mass itself to provide the resonance sensing signal. An example of such a drive configuration is illustrated in U.S. Pat. No. 2,817,779 to Barnaby et al (FIG. 11) as well as in my U.S. Pat. No. 4,479,098 issued on Oct. 23, 1984 and entitled Resonance Drive Oscillator. Drive circuits of this configuration generally use a balanced bridge configuration with the actuator drive mass or element as one leg of the bridge and a dummy or compensation element (which simulates the drive actuator mass but is not a part of the physical resonance of the system) as the other leg of the bridge. The bridge is excited by a sine wave signal. Imbalances in the bridge circuit, which are intended to repesent the resonant motion, are sensed and used to maintain the excitation signal.
While the bridge configuration is adequate for most purposes it is not ideal for all applications. One problem with the bridge configuration driver circuit is that of identically "matching" the physical characteristics of the dummy bridge element with those of the actuator drive element, particularly in a manner such that the dummy and actuator mass parameters track over time and temperature. Since the frequency at which the natural resonance of the actuator mass occurs can vary considerably with changes in temperature, the actuator mass driver circuit must be able to instantaneously track such resonant frequency shifts and simultaneously drive the actuator mass at the new resonant frequency level in order to maintain the output amplitude of the actuator mass at a constant, maximum level. Any erroneous imbalance of the bridge network (as a result of parameter mismatch between the dummy and actuator drive elements) causes a phase shift in the resonance signal, which can lead to further errors in the subsequent demodulation of the objective signal of the system. The bridge imbalance as a result of such component mismatch also provides an amplitude error, which can lead to further inaccuracies in the demodulated signal.
Heretofore, the drive signal for the actuator and compensation masses of such drive circuits have been sinusoidal waveforms so as to provide a smooth oscillatory drive transition to the driven mass. The use of square-wave drive signals for the driven masses was generally considered to be avoided, since significant current/voltage spikes would be introduced into the driver circuitry, through the actuator mass, at the "step" transition drive portions of the drive signal. Such undesirable spikes are not readily eliminated by conventional filtering techniques.
With the advent of improved actuator mass structures such as monolithic crystal structures, the phase requirements of the drive system become more critcal. The physical encumbrances of the separate driver/detection configurations become too burdensome and limit the accuracy of the system. The inaccuracies of the bridge configuration resulting from mismatch between the drive element and the compensating element become more critical, and cannot always be tolerated within the accuracies of the system. In short, the drive configurations of the prior art have required one to contend with physical complexity or phase errors that cannot be tolerated for all applications.
The present invention effectively addresses and overcomes the above-mentioned deficiencies of prior art actuator mass driver circuits. The present invention provides an actuator drive circuit that neither requires the dual drive/detection circuitry nor the compensating bridge circuitry of the prior art actuator mass driver configurations. The present invention entirely eliminates the compensation elements and the need for parameter matching that lead to time and temperature tracking errors as with prior art sensor circuits. The present invention uses a square-wave drive signal, heretofore throught unsuitable for use with sensitive sensor circuitry. The drive circuitry of this invention is suitable for use with either piezoelectric or magneto-electromagnetic actuating configurations. Phase shift and amplitude errors in the resonant drive signal are minimized with the use of the present invention.