Browse Prior Art Database

Digital Servo System

IP.com Disclosure Number: IPCOM000086969D
Original Publication Date: 1976-Nov-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 4 page(s) / 109K

Publishing Venue

IBM

Related People

Davis, JB: AUTHOR

Abstract

The timing truth table and schematic block diagram of a digital servo system, which generates correction pulses at a rate inversely proportional to the magnitude of the difference between the actual and desired positions, are shown in the drawings.

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Digital Servo System

The timing truth table and schematic block diagram of a digital servo system, which generates correction pulses at a rate inversely proportional to the magnitude of the difference between the actual and desired positions, are shown in the drawings.

The servo is particularly suited for operating the mechanism described in "Computer Controlled Measuring Table," by R. L. Buckingham, J. B. Davis and
H. H. Herd, IBM Technical Disclosure Bulletin, Vol. 15, No. 4, September 1972, pp. 1324-1325.

The servo is operated by a sequence of digital program signals initiated whenever the positional sensing mechanism (such as an interferometer) generates a measurement of actual position. The prototype unit uses a 9- decade laser interferometer display and provides correction signals when the error is continued to the low order 3 decades. The system uses 13 program steps and associated strobes, as shown in Fig. 1. Fig. 2 shows the basic logic of the servo system.

Information from the position sensing unit is loaded periodically (200Hz) into a 9-decade register 1. Information as to desired or "target" position is entered into 9-decade register 2 from an external source such as a computer system. The start pulse is generated each time register 1 is loaded. The decades are selected by selectors 3 and 4, according to the pattern shown in Fig. 3, decade 8 being the most significant.

During program steps 0-8, the two registers are compared decade by decade in the BCD COMPARE unit 6. The outputs therefrom indicate whether the position data is equal to, greater than, or less than the target data. These signals are sampled by strobe 0-8 in gates 12, 13 and 14, respectively. An additional gate signal, PROGRAM 0-5, is applied to gate 12 to test for an inequality (the equal signal inverted by 9) in the high order 6 decades, which indicates an error too large to be corrected by the servo. The condition is stored in latch 15 which will prevent the generation of correction pulses and issue a range error signal. Latches 16 and 17 are turned on by "greater than" and "less than" conditions and are cross-connected to gates 13 and 14 so that the first inequality encountered will establish the state of the complete comparison. These latches also control gates 27 and 28 to st...