PID Controllers—Some Further Discussion
by Peter Raeth

Ray Zeune, a Senior Staff Engineer with Delta Design, wrote to say that he wanted to use a microcontroller evaluation board to get a better feel for PID controllers. He asks if there is any software available. After searching for several hours, the pickings were slim in the public-domain arena. There is a ton of expensive commercial software available. Just go to Google. Enter "software" in the all box and "PID control" in the exact phrase box. Then set the domain filter to don't and .edu. That will bring up the commercial software. You can also find a lot of code that is "public domain," but requires the purchase of other software.

Since the question was asked, we should take the opportunity to explore PID controllers a bit further. We can look at the software that is available and bring out some additional information that will extend the discussion we had the last time we got together.

PID Routines for MC68HC11K4 and MC68HC11N4 Microcontrollers

From Motorola Semiconductor, written by James W. Gray, this technical note provides full working code. "While an expensive DSP-class processor is the correct choice for the most demanding applications, several members of the M68HC11 family have the speed and resources to control multiple channels. This note provides two working examples of PID control-loop software. The first example, written primarily in C, shows a PID algorithm in a straightforward way using floating-point math. Key features of the C environment are covered for readers who are more used to assembly language. The second example implements a PID algorithm in assembly language. It uses the MC68HC11N4 on-chip math coprocessor to speed up arithmetic operations. Both examples are complete and ready to run on a Motorola M68HC11EVS evaluation board. External interfacing is identical for both examples—an 8-bit analog to digital converter is used for input, and an 8-bit PWM waveform is output. Because the code in both examples carries more than 16 bits of precision, and because both processors support 16-bit PWM, only minor changes are needed to increase precision." This is the only public-domain software I could find. If others find additional code, we can post links here during a follow-up.

PID Without the Math

According to the posting, "Author David St. Clair retired after 40 years of practice in the field of instrumentation and control in the process industries (8 years with Eastman Kodak and 32 years with DuPont). He took, in 1947, what he understood to be the first college course offered in the theory of feedback control, a chance event at MIT that started his career in the field. He arguably has applied the scientific method to solving control problems in the process industries longer than anyone, or at least that was probably true when he retired in 1987. He had been explaining the concepts to the non-specialist for most of that time. He relishes this opportunity to spread the word to a larger audience. His booklet was originally issued in 1983 as an internal report in the DuPont Company to help engineers and technicians, who have no special training in feedback control, understand the basic considerations and limitations. It handily broke all records at the DuPont Company for number of requested copies (over 1200) when issued. That report was released to the public and published in 1990. It subsequently sold almost 17,000 copies. In 1992 it was expanded for DuPont to about twice the size. This report also broke distribution records in the Company, with over 2500 copies requested when issued. The part unique to DuPont was deleted and more was added to make the current Second Edition." Considering the software (no source) that comes with the book, and the experience of the author, the $US30 price is a pittance.

The PID Control Algorithm—How it Works and How to Tune It

From this booklet's author, John Shaw, of Process Control Solutions, we hear that "Proper understanding of controller tuning requires understanding of the PID algorithm. The PID algorithm comprises three types of responses: Proportional, Integral, and Derivative. The first part of this book will examine the PID control algorithm, as typically implemented in industry. Each of the three elements will be discussed in detail. The book will then discuss common process responses, including dead time, process lags, and their combinations. The amount that each of these elements contribute to the control must be adjusted to correspond to the process being controlled. This adjustment is known as "tuning" the control loop. An incorrectly tuned loop will not function correctly, providing less than the desired performance. We will first look at how to determine if a loop is properly tuned and then cover some of the typical methods of tuning loops." He also makes available an Excel workbook that simulates PID controllers. Together, the booklet and simulator sell for $US55. Quite a reasonable price. He also posts a code fragment that illustrates the basic idea of PID controllers and links to tutorial material.

Comparison of PID Control Algorithms

ExperTune posts considerable information on controllers in general. One article had a rather disturbing introduction: "In practice, manufacturers of controllers don't adhere to any industry-wide standards for PID algorithms. Different manufacturers and vendors use different PID algorithms and sometimes have several algorithms available within their own product lines. Just as there are no industry standards for PID controllers that companies adhere to, nomenclature and action for similar modes varies. Some manufacturers call the Proportional Band the Proportional Gain. Manufacturers interchange names and units for integral or reset action. In this article, integral action is defined in time/repeat and reset in repeat/time. One is the reciprocal of the other. The action of either reset or integral can be reversed depending on the manufacturer's units." Last week we mentioned that there are subtle variations in how PID is implemented. This article will open your eyes further to that reality. Be sure to peruse the other material on this site and sign up for their free training disk. You can spend many good learning hours here.

Hunting for PID Setpoints

Writing for CircuitCellar, George Martin answers the question "If I have a pressure control (PD), and it continuously hunts for the setpoint, would adding some integral (reset) fix this problem? What parameters would work?"

He recommends the PID controller as a potential solution. "The Proportional component of the PID loop gets you to the goal, the integral component takes out any offsets, and the derivative component helps control the overshoot and ringing."

Reading Fuel Levels

Also for CircuitCellar, George Martin responds to a question on getting an accurate reading from a fuel-level sensor in spite of splashing in the tank. He casts this problem for PID controllers. It is a good read.

From Plant Data to Process Control

Here is a commercial book written by Liuping Wang and William Cluett, and published by Taylor & Francis. Of all the large-press offerings, this one appears to be the least theoretical while taking aim at practical applications. "This is a professional-level book aimed at process engineers in industry and researchers into systems and control engineering. It looks at theoretical advances in the area of process engineering and a wide variety of implementations. It also introduces new ideas, techniques, and algorithms to the areas of process identification and process control. Theoretical advances in these areas and a wide variety of applications are examined."

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Flying Robotic Insect Slated to Explore Mars If you want to read about a far-out control problem, read this EE Times article by R. Colin Johnson. "Motivated by the notion that the Mars landscape may prove easier to navigate by air than with ground-based rovers, NASA is backing a research project to build toy-sized flying robots, modeled on the entomology of insects, that can hover like helicopters. Patented as 'entomopters,' the robots are on the drawing board of University of Missouri professor Kakkattukuzhy Isaac."

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