ROTATIONAL SPEED CONTROLLER

PURPOSE:To enable rotational speed to be controlled at the highest precision, by a method wherein the pulse of one rotational frequency per cycle is produced through an encoder, and wherein it is utilized that the encoder is not influenced by its own error if the cycle is measured, and wherein the rotational speed of a motor is controlled while calibrating the encoder signal. CONSTITUTION:The analog output 100 from an encoder is converted to a digital waveform 230 by a waveform shaping circuit 18, and the frequency is divided into the pulse signal of one rotational frequency by a 1/N frequency divider 20, and the cycle 232 per rotational frequency of a motor is measured by a cycle measuring instrument 22. Through the cycles 232, 236, 240, 244, the fluctuation time DELTAT of the motor rotation cycle is found by a processor 26. Control signals 204, 206, 208, 216, 218, 220 are gradually changed so that the time DELTAT may be shortened, and the gain of programmable gain amplifiers 9, 10, 11 and the phase shifting quantity of programmable phase shifters 12, 13, 14 are changed. As a result, rotational phase detection signal 228 cannot be influenced by the eccentricity of a slit glass plate 4.