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Using a Fixed-Point Digital Signal Processor as a PID Controller

Jerry Murphree, Brent Brzezinski, Joey K. Parker The University of Alabama

Abstract

The digital signal processor (DSP) is a tool that has become available for control engineers in recent years. Proportional – integral – derivative (PID) control loops have been standard tools for decades. Combining these two tools provides a flexible and powerful demonstration of control system concepts for undergraduate mechanical engineering students.

Undergraduate controls courses have traditionally focused on mathematical analysis, including transfer functions, block diagram manipulation, root locus, frequency response (Bode plots), etc. In recent years a number of computer simulation packages have been developed for control system design. However, there is a continuing need to develop prototype systems to enhance the students’ conceptual understanding of difficult materials.

The control system described in this paper uses a custom fabricated circuit board built around the Texas Instruments TMS320F243 digital signal processor with all necessary signal conditioning. The DSP control system is accessed through a custom LabVIEWÔ program interface, so students do not need to know any details of the DSP system operation. PID control gains and the sampling time are entered by the student and sent to the DSP through this interface. The DSP system also returns measured output information for plotting and offline analysis. A ball-screw driven by a small DC motor provides a prototype mechanical system. Some details of the linear actuator system, the PID control algorithm, and its implementation on the DSP are provided.

Introduction

Single chip digital signal processors (DSPs) have been available since the mid 1980’s. DSPs are highly specialized microprocessors dedicated to fast, real-time computations. One common characteristic of the DSP is the “multiply and accumulate” instruction, or MAC. This instruction multiplies two values and stores the results in the accumulator in a single clock cycle. This operation is particularly beneficial in the computation of numerical algorithms such as digital filters. DSPs have found widespread use in modern telecommunications equipment, such as cell phones and computer modems1. A more recent application for digital signal processors is in control. Control laws can also be cast in the same numerical algorithms that are easily implemented on a DSP. Manufacturers have also begun to add on-chip peripherals (analog-to- digital converters, digital-to-analog converters, timers, counters, pulse-width modulation, etc.) to conventional DSPs that make them particularly attractive to the control engineer 2.

Proportional--integral--derivative (PID) controllers have been the backbone of motion and process control for decades. Most of the classic undergraduate controls textbooks3,4 include material on the design and utilization of this powerful concept. An excellent tutorial on PID

Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright Ó 2002, American Society for Engineering Education

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Murphree, J., & Brzezinski, B., & Parker, J. (2002, June), Using A Fixed Point Digital Signal Processor As A Pid Controller Paper presented at 2002 Annual Conference, Montreal, Canada. 10.18260/1-2--10427