| Published: June 3, 2010 – 5:20 pm
Yesterday, I was sorry to learn of the death of one of my dear college professors, Richard (Dick) Phelan. He was a good man, who lived a good life. His ideas will, one day, change the world. He certainly changed me and for that, I will be forever grateful.
I have mentioned Professor Phelan often to my engineering colleagues, and well, to anyone whoâ€™ll listen. He specialized in a niche area of mechanical or electrical engineering generally called â€ścontrol theory.â€ť
Professor Phelan realized that the way most engineers and scientists go about controlling things like the temperature of an oven or a houseâ€“ a thermostat, or the speed of a carâ€“ cruise control, is not nearly as good as it could be. And the reason is elegant and mathematically simple.
Thanks for reading this far, if you did. I will go on about Dickâ€™s big idea in the next few paragraphs. I hope you get that he had an idea that could change the world and allow us to do so much more with less. Richard Phelan meant a lot to me.
Dick Phelan and Pseudo Derivative Feedback (PDF)
If youâ€™re into this kind of thing, you know that most controllers work best with an integrator in the forward loop. For most of us, the classic example is sticking your hand in the shower to get a signal (hot, cold, cool, or warm) that your nerves, brain, and muscles use to set it just right. You adjust a faucet knob and wait. Your brain integrates the signal; you subtract what you feel from a suitable proportion or fraction of what you want, and eventuallyâ€“Â Ahhâ€¦
An old trick, and a seemingly reasonable one, is to put a differentiator in the feedback loop. That way, the feedback signal gets stronger (uh, bigger), with not just the change, but with the rate of change. All good.
The scheme I described above is Proportional+Integral+ Derivative (PID) control. It is everywhere. I mean everywhere. Even the most sophisticated satellite attitude controller systems are PID. Oh yes, one can analyze the Laplace Transform heck out of those things, what with their spindly struts and drilled-out, mass-saving, trusses, busses, and brackets. But, when it comes down to it, often, very often, our colleagues abandon all the extra analysis and just optimize a few PID variables.
Hereâ€™s what Dick Phelan realized, pointed out, and fixed. With PID, weâ€™re integrating a signal we just differentiated. Any transducer noise or vibration often makes the feedback signal too big, or just wacky.
Instead of PID, try this: Feedback a signal in two places. On a control diagram, itâ€™s two summing junctions. Feed the signal back upstream of the integrator, and then, with a separate proportionality constant, downstream of the integrator. The signal downstream of the integrator acts as though we differentiated. But, there is no, or much less, noise.
Dick called his scheme Pseudo Derivative Feedback, or PDF. Iâ€™ve seen others refer to the same thing with an I in front of the acronym to indicate integration. Thatâ€™s not bad, but in tribute to Professor Phelan, I prefer PDF.
Now, Dick Phelan was a bit of character. Although one of his earlier textbooks gives the reader a solid understanding of Laplace transforms, later in life, he didnâ€™t think much of them. Consider these words from his textbook: Feedback and Control Systems, Cornell University Press, 1977:
â€śThe apparent justifications for past use of Laplace transforms were (a) it was a good way to keep students occupied in an intellectually stimulating way for quite a period of time, and (b) it established a jargon for the fraternity while effectively sophisticatingâ€“ in the true dictionary definition of the wordâ€“ a basically simple field of study into one that was awe-inspiring to the unknowingâ€¦â€ť
Well, control theory can get complicated, but itâ€™s the key to our future. We have to find ways to not just use less, as traditional environmentalists wanted us to do. Instead we have to find ways to be more efficient, much more efficient. We need to find ways to do more with less. PDF will help us engineers and designers do that. Thanks Professor Phelan. Letâ€™s change the world.