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Algorithm for Implementing PID Control for Servo Systems on Microprocessor

IP.com Disclosure Number: IPCOM000051279D
Original Publication Date: 1981-Jan-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 32K

Publishing Venue

IBM

Related People

Polk, DR: AUTHOR

Abstract

A high performance servo system is provided which uses a combination of proportional, integral, and derivative control, commonly referred to as PID control. The PID control function is the sum of three terms, one which is proportional to velocity error, one which is proportional to the integral of velocity error, and one which is proportional to the derivative of velocity error. Fast velocity response to changes in input commands and accurate steady-state velocity tracking are achieved with this type of control. The system is shown in the figure.

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Algorithm for Implementing PID Control for Servo Systems on Microprocessor

A high performance servo system is provided which uses a combination of proportional, integral, and derivative control, commonly referred to as PID control. The PID control function is the sum of three terms, one which is proportional to velocity error, one which is proportional to the integral of velocity error, and one which is proportional to the derivative of velocity error. Fast velocity response to changes in input commands and accurate steady-state velocity tracking are achieved with this type of control. The system is shown in the figure.

Implementation of PID control is usually accomplished with analog circuits using RC networks and active filters. An analog tachometer provides a feedback signal which is proportional to motor velocity. When the input velocity command is generated by discrete logic or by a microprocessor, a digital-to-analog (D/A) conversion is required to provide an analog input voltage to the PID controller. In many cases the driver device used to provide power to the motor operates as a switching circuit and is better suited to digital input data. Thus, an analog-to- digital (A/D) conversion at the PID controller output is often required before the data is acceptable to the driver. These data conversion processes, analog circuits, and analog tachometer significantly increase the complexity and cost of the servo system.

Complexity and cost of the servo system can be significantly reduced by implementing PID control directly in a microprocessor as a software function. The PID control function shown in the figure consists of three terms: (1) proportional, (2) integral, and (3) derivative.

The first term is a gain term and may be implemented directly in the microprocessor by multiplying the velocity error by the coefficient A. This coefficient is scaled so that it is a power of two which permits the multiply to be carried out in minimum time by simply shifting the data right if A is less than one or left if A is greater than one.

The last two terms are filtering functions which require micro processor memory for implementation. To minimize memory requirements and processing time, a difference equation form of implementation is used. This form is derived by tak...