Patent GB-953610-A (GB)
Title: Motor control circuit
IPC: G11B15/46
Filed: 1962-01-09
Abstract: 953,610. Automatic speed control. MINNESOTA MINING & MANUFACTURING CO. Jan. 9, 1962 [Jan. 9,1961], No. 820/62. Heading G3R. A D.C. motor 250 has a fine speed control arrangement comprising a phase detector 20 for comparing the phase of a reference signal with that of a speed-indicating signal. A tachometer generator 35 develops an A.C. signal having a frequency related to the motor speed, and a reference signal recorded on a motor-driven magnetic tape 60 may also be used for the input indicative of speed to the detector 20. The reference signal may be provided by source 10 or crystal oscillator 31, as determined by switch 12. The reference pulses from source 10 are passed through bandpass filter 11 and switch 12 to amplifier 13 which makes the sinusoidal signal rectangular. An integrator network 15 develops a triangularly shaped wave, and variable-gain amplifier 16 has a clipped-wave output which is introduced as the reference input to the phase detector 20. When the motor is started, and detector 20 provides a high A.C. output which is applied to synchronizing relay amplifier 40 controlling contacts 41. The tachometer signal is then fed to trigger circuit 55 and binary divider circuits 56-59 associated with speed selector switch 63. This switch is ganged with switch 70 at the output of a chain connected to oscillator 31 and comprising trigger circuit 32 and binary divider circuits 33-36 coupled through emitter follower 30 to recording head 51 adjacent tape 50. This tape may be integral with tape 60, the switch 70 functioning to record the same signal frequency on the tape 50 independent of the speed of the tape. The tachometer signal, or a selected frequency division thereof, is coupled through switch 63 to limiter 25 and thence to phase detector 20, the switch 63 being set so that the frequency supplied to the limiter 25 is at a standard value when the motor is at its selected operating speed. Contacts 41 are returned to normal when the output of phase detector 20 is at a minimum due to the motor being operated at the selected speed. Amplifier 43 operates contacts 42 if a reference frequency is present on tape 60 and the tape is running at above a predetermined speed. Magnetic head 61 develops the recorded signals which are introduced through filter 53 and limiter 54 to amplifier 43 for controlling contacts 42. When the contacts 41 are released and the contacts 42 are operated, the tachometer signal is replaced by the signal from the head 61. The varying D.C. output signal from the detector 20 is applied through variable gain amplifier 22 to power amplifier 72 coupled to the motor. The above arrangements are for fine control of the motor. For coarse control, the signal from limiter 25 is introduced to lowpass filter 66 and to bandpass filter 65. The limited signal is also applied to phase detector 26 which compares the tachometer signal from limiter 25 and the output of limiter 28. At low speeds, the control signal from rectifier and emitter follower 68 to the limiter 28 provides rapid acceleration of the motor until filter 66 ceases to pass an effective signal. The limiter 28 now provides a signal to phase detector 26 which is related to the signal from bandpass filter 65. The amplifier 22 then decreases the rate of acceleration of the motor until, in the vicinity of the operating speed, the phase detector 26 is rendered ineffective, and the phase detector 20, alone, as part of the fine control arrangement, controls the power delivered to the motor. Thereafter, if the speed changes by a substantial amount, the phase detector 26 again becomes effective to return the motor quickly to its correct speed.