Measurement of Speed and Calibration of Tachometers Using Rotating Magnetic Field

Fast speed measurement and control signals are required for feedback control applications. In the proposed technique, a rotating magnetic field (RMF) is used to measure the speed and the variation in speed with high accuracy. In addition to a 400 Hz, three-phase input voltage circuit and an output zero-crossing detector circuit, a contact-type synchro is used as a primary transducer. A balanced three-phase ac voltage is applied to the stator windings of synchro, which produces an RMF. The frequency of electromotive force induced in the rotor circuit of synchro depends upon the speed of rotor or rotating member. The RMF revolves at a speed of 24 000 r/min, which is purposely kept very high in comparison with the speed of the rotor or rotating member. Thus, the measurement becomes very fast and the change in speed is sensed within 2.5 ms. The time response of the proposed scheme is also compared with the time response of a dc tachogenerator. A digital storage oscilloscope and a microcontroller are used for measurement of the output. The 16-b digital output of the microcontroller is made available as a feedback signal for control applications. The high accuracy of measurement of the proposed technique is also successfully used for calibration of different contact- and noncontact-type tachometers.