Browse Prior Art Database

Position Control Servomechanism

IP.com Disclosure Number: IPCOM000080254D
Original Publication Date: 1973-Nov-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 4 page(s) / 43K

Publishing Venue

IBM

Related People

Leggate, JW: AUTHOR

Abstract

This digital servomechanism controls motor acceleration and deceleration by the use of a counter, which measures the motor's movement as a command step from one position to another is executed. In order to linearize the motor's acceleration and deceleration, and to increase the system gain at slow-motor speeds, the feedback pulses provided by a digital tachometer are divided by an increasingly larger number as a function of the distance from the starting point (for acceleration), and as a function of the distance from the stopping point (for deceleration).

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Position Control Servomechanism

This digital servomechanism controls motor acceleration and deceleration by the use of a counter, which measures the motor's movement as a command step from one position to another is executed. In order to linearize the motor's acceleration and deceleration, and to increase the system gain at slow-motor speeds, the feedback pulses provided by a digital tachometer are divided by an increasingly larger number as a function of the distance from the starting point (for acceleration), and as a function of the distance from the stopping point (for deceleration).

Motor 10, Fig. 1, is energized by power amplifier 11. The motor directly drives load 12 and digital tachometer 13. This tachometer generates an output pulse on conductor 14 for each incremental unit of motor rotation. For example, the tachometer may provide 500 to 1,000 pulses for one motor revolution.

The power amplifier 11 is controlled by digital-to-analog converter (DAC) 15. The DAC and the power amplifier represent a saturating element in the closed- loop position servo. That is, motor energization is a linear function of the DAC contents only for short step distances. For longer step distances, the motor executes the major portion of the step at steady-state speed while the DAC 15 holds the power amplifier 11 saturated.

When a command is received to move the load 12 from one position to another, counter 16 is reset to zero. Conductor 17 also becomes active and network 18 detects that the count within the counter 16 is such, that no tachometer pulse division is being performed by count divider network 19. As a result, network 18 controls OR 20, by way of conductor 21, to fill a selected number of low-order DAC bits. As a result, the motor 10 is energized and execution of the requested step begins. The number of active low-order bits necessary is dependent upon the frictional characteristics of the system and must be sufficient to start motor rotation.

As the motor rotates, the tachometer pulses pass through count divide network 19 and the counter 16 increments. This increasing count effects an increase in motor energization by way of the DAC 15. As motor movement continues away from the starting point, identified as 22 in Fig. 2, the contents of the counter 16 are supplied to the count divide network 19 by way of conductor 23. The effect of this network is to linearize motor acceleration and to increase system gain when the load is adjacent starting point 22 and stopping point 24, Fig. 2. When the load is immediately adjacent these two points, the counter 16 responds to each tachometer pulse. As the load 12 departs from these two points by a greater distance, the tachometer pulses are divided by an increasingly larger number.

When the motor 10 is near the starting position, every tachometer pulse increases the motor's energizing voltage. If this process were to continue, motor acceleration would build up to an unacceptable magnitude. In order...