Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Patent
1990-08-24
1991-11-05
Budd, Mark O.
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
310328, 310331, 356350, H01L 4108, F16F 118
Patent
active
050633210
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a torsional vibration drive having the features of the preamble of claim 1.
Ring laser gyros are able to measure rotational velocities with great accuracy. It is known, however, that without special countermeasures ring laser gyros are unable to measure very small rotational velocities since the frequencies of the two oppositely rotating lightwaves, which differ due to inertial rotation, are contracted into a common intermediate frequency (pulling effect of coupled oscillators, the so-called lock-in effect). Although it is possible to employ a number of laser gyro specific design criteria in order to influence the magnitude of this dead zone which extends around the zero point of rotational velocity, it remains so large, even with the best possible configuration of the relevant parameters, that the rotational velocity measuring error is unacceptable in almost all practical applications, for example, in position control and, most of all, in navigation systems.
One method employed to avoid this error is known as "dithering". Here, the laser gyro is caused to perform uniform torsional vibrations about an axis perpendicular to the plane of the beam path.
The time during which frequency synchronization occurs at the points where the laser gyro reverses its torsional vibrations, is determined by the maximum amplitude and the frequency of the vibrations and can thus be minimized. The ultimately remaining respective measuring error of the laser gyro is thus reduced significantly. Torsional vibration must not produce any structural deformation of the basic gyro body, such as, for example, periodic expansion, compression or bending, since, due to periodic or non-periodic loss modulation of the optic resonator, this leads to false measuring behavior of the laser gyro.
In prior art torsional vibrators, a small diameter spoke wheel was employed whose hub was fastened to the support system to be measured and whose outer cylindrical rim fits into an opening in the center of the gyro block. The entire gyro block was thus supported by the torsional vibrator and both together then constituted a spring-mass system which was excited at its resonant frequency by a magnetic or piezoelectric drive.
Many different modifications of this basic solution are known. The present invention is based on torsional vibrators which have spokes of uniform cross section, to which piezoceramic plates are applied and which thus constitute multilayer flexural elements. If an electrical voltage is applied, a torque is generated in the spokes resulting in a torque which acts on the wheel rim. Customarily the excitation voltage corresponds to the frequency of the resonant frequency of the system.
It is very difficult to increase the operating frequency of these systems and simultaneously keep the vibration amplitude constant. This is so because of a fundamental characteristic of piezoelectric resonant systems which is that the maximum resonant amplitude of a transducer is inversely proportional to the resonant frequency.
Increases in vibration amplitude at a fixed frequency are limited by the tensile strength of the piezoceramic material which is significantly less than that of the metal spokes to which it is applied. If the piezoceramic member is displaced into a region of low stress, this only reduces its effectiveness as a drive. In the past, these and other factors were obstacles to improvements in the configuration of piezo-driven torsional vibration devices in laser gyros.
The following considerations which preceded the invention restrict the simplicity of design of a torsional vibrator.
1. Normally, the mass of the basic laser body is very large and the space allocated to the vibratory mechanism is very small. This requires the torsional vibrator to be made of a material which has a modulus of elasticity that is comparable to that of steel. Usually, steels are employed which additionally have a low coefficient of expansion.
2. The rotation axis must be kept stable. This requires the torsional vibrator to be extremely
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Budd Mark O.
Teldix GmbH
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