Browse Prior Art Database

Digital Speed Control System

IP.com Disclosure Number: IPCOM000092859D
Original Publication Date: 1967-Mar-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 53K

Publishing Venue

IBM

Related People

Carman, CD: AUTHOR [+2]

Abstract

The capstan circuit includes capstan 10 directly coupled to low-inertia DC motor 11 and photowheel 12. Tape can continuously engage capstan 10 for start, stop, and reverse operation. Photocell 13 senses the rotation of slots on wheel 12. Squaring circuit 14 develops a square wave which has a frequency directly proportional to the capstan speed and thus has a frequency period T inversely proportional to tape speed The remaining circuitry determines whether this capstan speed, as represented by the square wave, is fast or slow in relation to a timing standard. The latter compares the square wave period against a refer ence period T that is equal to the square wave period when capstan 10 is operating at the nominal velocity.

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Digital Speed Control System

The capstan circuit includes capstan 10 directly coupled to low-inertia DC motor 11 and photowheel 12. Tape can continuously engage capstan 10 for start, stop, and reverse operation. Photocell 13 senses the rotation of slots on wheel 12. Squaring circuit 14 develops a square wave which has a frequency directly proportional to the capstan speed and thus has a frequency period T inversely proportional to tape speed The remaining circuitry determines whether this capstan speed, as represented by the square wave, is fast or slow in relation to a timing standard. The latter compares the square wave period against a refer ence period T that is equal to the square wave period when capstan 10 is operating at the nominal velocity.

Reference period T is generated by a multistage reference binary counter 15 operated under crystal oscillator control. The period comparison is done by resetting counter 15 at the beginning of each photocell square wave period, and determining whether the reference period ends before or after the photocell square wave period. Waveforms A...E show the relationship between different speed squaring-circuit outputs and the reference period, designated as a full count period, T. Waveform A shows their relationship at nominal capstan speed. Waveform D shows a fast speed square wave output. Wave form E shows a slow speed wave output.

Counter 15 is reset at the beginning of each period indicated by the upward leading edge of a square wave cycle. Counter 15 begins a full count period from the reset point each time. Thus, if the tape speed is slow, counter 15 overflows for a period of time until the next square wave cycle resets it, as indicated in waveform E. However, if the tape speed is fast, counter 15 is reset before it can overflow during each counter cycle. Hence, the overflow condition, or lack of overflow, can distinguish between the fast and slow speeds of capstan 10 in relation to a fixed nominal speed.

This fast or slow capstan speed is sensed with the assistance of two-stage binary counter 21. The latter is opened by an input And that blocks any further setting inputs when counter 21 reaches a count of ten, so that it cannot recycle without being reset. Counter 21 is reset by each overflow output from counter
15. Counter 21 is maintained in reset status as long as an overflow condition persists from counter 15. Through And 30, counter 21 is incremented by the initiation of a square wave cycle sensed by a differentiating circuit.

Motor 11 is started in response to a forward-go or backward-go signal on line 28 or 27 which generates a start pulse through a differentiating circuit 19a or 19b. This start pulse sets high-power latch 23 to provide an output on lead 24. Such output causes drive circuit 26 to provide a higher than normal accelerating current to motor 11. This occurs for a period of time determined by start-reset circuitry in counter 15 and the acceleration characteristics o...