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Digital Tachometer for a Pulse Width Modulation Speed Control

IP.com Disclosure Number: IPCOM000075392D
Original Publication Date: 1971-Sep-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Keidl, SD: AUTHOR

Abstract

A DC servomotor 10 is powered in a closed-loop system using a digital timing device 11 to provide a pulse width modulated voltage proportional to velocity. The digital tachometer 11 provides an output pulse width proportional to the period of a pulse output emitter signal. The tachometer input 12 may be from any pickup 13 which provides an output period proportional to velocity, such as an optical disc or magnetic reluctance pickup.

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Digital Tachometer for a Pulse Width Modulation Speed Control

A DC servomotor 10 is powered in a closed-loop system using a digital timing device 11 to provide a pulse width modulated voltage proportional to velocity. The digital tachometer 11 provides an output pulse width proportional to the period of a pulse output emitter signal. The tachometer input 12 may be from any pickup 13 which provides an output period proportional to velocity, such as an optical disc or magnetic reluctance pickup.

The ideal application is an infinite-gain system wherein the velocity error is zero. However, in reliable systems, stability criteria will cause the system to oscillate. In the infinite-gain system, the pulse signal from a speed-sensing emitter is first shaped by a single-shot (SS) circuit to yield a fixed pulse width signal with period proportional to velocity. The down transition of each SS pulse starts a time-out (T01) or delay circuit which yields an up transition after the end of the time-out period. For an infinite-gain system this period, plus the single- shot pulse period, will equal the desired emitter output pulse period at operating speed. A second time-out (T02) circuit is connected to the output of the first. Initiation of this time out period occurs on the reset of the first time-out T01 by the rising edge of the SS output. For an infinite-gain system, the period of this second time-out is made slightly longer than the period of the first time-out plus the SS time. For infinite gain the first time-out T01 must reset (be ready to complete time out again) in less than the SS time. The second time-out T02 must reset in zero time.

Below speed, as seen in Fig. 3, T1 is greater than T3. Time-out T01 is able to complete its time out every single-shot output period. Therefore, T02 can never complete its time out period, since at the end of T3 it is always reset and its output remains down. Full motor voltage is applied to the DC servomotor. As soon as speed is reached and exceeded (T1 = T3) time-out 1 does not complete a full time out, but time-out 2 does for the first time and remains timed out as long as the servomotor is above speed. The output of T02 now remains up resulting in no voltage to the motor. Therefore, the motor and load friction decelerate the motor until time-out 1 can completely time out again. Thus the infinite-gain system is unstable and will oscillate around the run velocity, the amplitude of oscillation in most cases being too large to make the system useful.

To linearize the system and make it stable, the loop gain is reduced by designing the tachometer 11 to have a linear range; that is, between a speed W1 which is below run speed and a speed W2 which is above run speed, the tachometer has an output pulse train voltage with an average value inversely proportional to velocity. Since the motor 10 responds to the average value of the applied pulse voltage, if the period is short compared to the mechanical system ti...