Techniques to measure small angular displacement have been developed in conjunction with the marine horsepower meters. In this paper, the author reviews briefly the history of the devices for torque and horsepower measurement and presents a new digital technique for the measurement of micro-angular displacements and torque.
Angular displacements (torsional displacements in case of the torque meter) are converted to the phase difference of the two sinusoidal voltages generated by a device which consists of two sets of an inner-teeth gear and an outer-teeth gear. The gears are mounted on the rotor and the stator respectively. The phase difference is converted to the number of pulses by the use of the gate I and a clock pulse generator, as is commonly done in a counter-type AD converter.
The pulse train representing the phase angle appears periodically at every gear pitch. The feature of the author's device is that the periodical pulse train is fed into the gate II to be counted during the specified time interval
t. This double-gate system normalizes the phase shift time by the period of appearance of the pulse train, making the output insensitive to the variation of the shaft speed. The specified time
t can also be normalized by the period of the clock pulse so that the output is insensitive also to the frequency variation of the clock pulse generator.
The paper shows a detailed discussion on the error of the device. The most important is that the double-gate system achieves a self-compensation of the quantization error in gate I by the asynchronization error in gate I. The self-compensation comes from the averaging operation in gate II. The experiment has shown that the gear pitch error and the rotor speed fluctuation may be utilized advantageously in the self compensation even if asynchronization is non-existent in gate I. This paper also shows various applications of the device such as those in digital dynamometers for engine testing, precision torque control systems and manifold pressure guage.
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