Skip navigation
Electronic Design

Bob's Mailbox

PID Controllers And The $50 Trillion Process Control System (Comments From A Process Control Engineer, R.A. Pease)

Alan Greenspan has admitted that he screwed up and had a bad model for the economy. He claims he misunderstood what was going to happen. What did Spice suggest for him to do? I coulda told you that Greenspan was not doing a good job on his PID controller.

He waited too long to start decreasing the interest rates, and then he decreased them too slowly. I noticed that at the time! Then, by leaving the interest rate at 1% for too long, he got the ARMs to start out too low. And then when the rates went up, the subprime mortgage holders got whip-sawed.

This is exactly how you make a limit-cycle oscillator! In other words, Mr. Greenspan did not have enough D (derivative) term in his controller, and he failed to anticipate new problems. And he had too much gain in the I (integral) path. I can do this any day, on my bench—but I don’t destroy a nation’s economy.

No, I don’t want to take over Greenspan’s job. I don’t want that job. But I could still do it less badly.


More Homemade Heaters

Hi Bob,

Some years ago, my feet were getting cold and I had a bunch of old stove elements from a local scrap yard. I siliconed them to a steel plate (I would just clamp them down. /rap) and used a waterbed heater thermostat. It looks deadly but it’s only about 60 W. The elements are for 230 V and used in series. It’s strong enough to stand on. (Buying four 50-W resistors would probably cost more. /rap) By the way, my electricity use was 53 kWh last month. I’ve been concentrating on conservation.

–S. Premena

Hi, Premena,

Good man! Keep conserving! An average of 80 Watt hours per hour is quite good!


Hopping On Hybrids

Hi Bob,

I saw the video of you getting a test drive in the Tesla vehicle ("Video: Riding Shotgun In Tesla's Roadster"). We’ve been doing hybrid vehicles here now for around 10 years. Our speciality is city buses (New York, San Francisco, Toronto, London), but we’ve also looked into delivery trucks and some military vehicles. They certainly don’t have the acceleration of the Tesla! The savings are pretty significant: around 30% savings in fuel and over 50% improvement in pollutants. There should be more savings yet when the new lithium battery is introduced.

–Don Rumrill

Hi, Don,

Yeah, but who wants to pay that much money for low ROI “savings”? And we have some fuel-cell buses in Santa Clara County, and they must be much worse for ROI. I am sure you know that even better than I do.


Getting More From Gas

Hi Bob,

Road and Track did a couple of articles about a year ago about maximizing your fuel economy. Among the counterintuitive results: First, accelerate aggressively to the speed limit and then go promptly to constant speed because gasoline engines are not efficient at part load. Full load and near idle are the most efficient modes, so it makes sense to operate near those levels.

I think they recommended shifting up at relatively low rpms, even though you are using a lot of throttle. (That’s what I do and what I recommend. I accelerate on torque at gentle speeds and then shift up. /rap) Also, assuming that you are driving an automatic non-hybrid (I do not own any automatic or “slush box” cars. I rarely drive one. /rap) and don’t want to wear out your conventional starter motor, don’t pop the car into neutral and coast to a stop with the engine running.

(I do not believe that. Most slush-box cars do let the engine rev down to an idle. They do not keep the revs up, unless you have downshifted to 2 or L. In my car, turning off the key is definitely less wasteful than keeping it in gear or keeping it idling. It is less wasteful on a hybrid. /rap)

I’ve gotten pretty good at looking a block or two ahead and timing many of the traffic signals in my area. (Exactly. Great way to save gas. /rap) It’s as much a habit now as automatically holding my breath as I approach the doorways of Manhattan office buildings to avoid inhaling smoke.

–Bruce Hyman

Hi, Bruce,

Our rules say smokers must stay at least 25 feet from doors. Thanks for writing.


Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.