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Capstan Speed Servomechanism

IP.com Disclosure Number: IPCOM000077240D
Original Publication Date: 1972-Jun-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Frauenfelder, LS: AUTHOR [+3]

Abstract

This hybrid digital-analog speed servomechanism accurately controls the acceleration, run and deceleration modes of operation of the capstan motor of a magnetic tape unit. Motor current feedback is used during acceleration, run and during the plugging portion of deceleration. Motor voltage feedback is used during the dynamic brake portion of deceleration.

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Capstan Speed Servomechanism

This hybrid digital-analog speed servomechanism accurately controls the acceleration, run and deceleration modes of operation of the capstan motor of a magnetic tape unit. Motor current feedback is used during acceleration, run and during the plugging portion of deceleration. Motor voltage feedback is used during the dynamic brake portion of deceleration.

DC motor 10, Fig. 1, is directly connected to the capstan of a single capstan magnetic tape unit. Digital tachometer 11 provides spaced output pulses which vary in frequency with motor speed; the higher the speed, the higher the frequency. Logic network 12 receives these pulses and calculates the tachometer period by noting the number of cycles of oscillator 13 which occurs between tachometer pulses. Network 12 generates a four-bit binary number, on conductors 14-17, which represents the motor speed. This number has a binary value of 0111 when the motor speed is equal to its desired running speed. For all numbers indicating that the motor is slow the most significant bit, conductor 17, is "1". For all numbers indicating that the motor is fast this most significant bit is "0".

Variable gain digital-to-analog converter (DAC) 18 converts this binary number to an analog voltage for use in controlling motor energization. Analog integrator 19 produces an output voltage proportional to the integral of the most significant bit of the binary number.

Logic network 12 provides an active output on conductor 20 during the accelerate interval and during the decelerate interval. This active output sets the integrator output to zero and inhibits operation of the integrator during these intervals. When the integrator is first enabled, at the end of the acceleration interval, its output is zero.

The active output on conductor 20 also sets DAC 18 to its high gain mode during these two intervals. The effect of this high-gain mode during the accelerate and decelerate intervals is to increase the maximum analog output of the DAC to summing junction 21.

The output of junction 21 is applied to the input of direction control network
22. The output polarity of this network is controlled by direction line 23 and determines the motor's direction of energization. Linear power amplifier 24 energizes the motor's armature in accordance with the polarity and magnitude of the input voltage received from summing junction 25. Conductor 26 connects a voltage proportional to the motor's armature current to summing junction 25, as negative feedback, so long as rewind switch 27 is in the normal position and switch 28 is in the go position. When switch 28 is in the stop position, conductor 29 connects the motor's armature...