3
$\begingroup$

What is the terminology for having different/spaced-out on and off set points for a particular controlled process?

When a system does not have the set points set appropriately, it could exhibit something like contact bounce.
This is not limited to any particular kind of system. It can be for a electro-mechanical system with physical contacts and switches, or it can be for a software system. "Debouncing" can be done in a variety of ways, such as physical distances, time delays, introducing some kind of hysteresis behavior, etc.

E.g.

A thermostat controls the AC in a house. It is set to turn on at 90 degrees and turn off at 70 degrees. When the thermostat reads 90 degrees, it turns the AC on, and when the thermostat reads 70 degrees, it turns the AC off.

Bad Examples

If this system did not have the correct set points, it would exhibit something akin to contact bounce:

  • If the thermostat is set to switch on/off at 80 degrees, when the thermostat reads 80 degrees, it turns the AC on, but could bounce back and forth between on and off while the temperature varies slightly from 79-81 degrees.
  • Similarly, if the thermostat is set to switch on at 80 degrees and off at 79 degrees, when the thermostat reads 80 degrees, it will turn the AC on, and as the AC cools the temperature down from 80 to 79, it will turn the AC off. This will occur relatively rapidly but not as rapidly as the example given above.
$\endgroup$
7
  • $\begingroup$ I think the term 'set points' is accurate. Instead of 'contact bounce,' I would say 'forced oscillation'. $\endgroup$
    – OnStrike
    Dec 2, 2015 at 18:46
  • $\begingroup$ it doesnt necessarily have to be "forced" in every case. but yeah contact bounce is more of a electrical term. I thought i learned the terminology for this during school but i dont remember. $\endgroup$ Dec 3, 2015 at 0:33
  • $\begingroup$ I think 'bounce' is a fairly good term used for this kind of thing. And to de-bounce it is to apply whatever measures to prevent it if such behaviour is undesirable. De-bouncing examples: a range or threshold, a timer, a rolling average, a 'latching' mechanism. $\endgroup$
    – jhabbott
    Dec 3, 2015 at 21:58
  • $\begingroup$ Contact bounce is different. It is, as the name suggests, when contacts bounce on a single closing action and is primarily related to the mechanical construction of the switch. In many software applications the keyboard / switch inputs have to be 'debounced' to prevent multiple keystrokes. This is usually done by only registering a keypress if it has been seen continuously on for a time, say, 10 ms. This question is about preventing contact chatter when the variable (temperature in this case) is just on the setpoint. Many loads would not tolerate rapid switching very well. $\endgroup$
    – Transistor
    Dec 3, 2015 at 23:21
  • $\begingroup$ @jhabbott I agree that bounce is a fairly good term to describe the behavior we are trying to avoid, but I would like to know what the actual spaced-out set points is called (if such terminology exists for it). $\endgroup$ Dec 4, 2015 at 20:46

1 Answer 1

1
$\begingroup$

"Hysteresis".

In your mechanical thermostat air-con example the bimetallic strip is designed to toggle closed when set point is exceeded. The toggle action gives a quick closing of the contacts and minimises arcing. The toggle action requires the bimetallic strip to reduce in temperature by a degree or two before switching back on again. (20 degrees in your example would be too much. You want the hysteresis to be small enough that you don't experience much fluctuation in temperature but not so small that it cycles the air-con too frequently.)

In a software system the hysteresis can be more easily adjusted. The code for such a device might look like this pseudo-code:

setpoint = 80
hysteresis = 0.8
if actual >= setpoint: output on                 // start cooling
if actual < (setpoint - hysteresis): output off  // stop cooling
$\endgroup$
9
  • $\begingroup$ I disagree with Hysteresis for this case. For me, hysteresis is more a characterization of how a system's setting or output differs based on current and previous inputs and time. Applying hysteresis to my AC example would show that the thermostat reads differing values depending on what temperature it was previously reading and how quickly the temperature changes. Ex. thermostat reads 80deg, and the temperature of the room increases at a quick rate, it may then read 85deg, but if the same magnitude of temperature change was applied at a different rate of speed, it may then read 84deg instead. $\endgroup$ Dec 4, 2015 at 20:43
  • $\begingroup$ The thermostat should always display the correct temperature and the controlled processes should work without bouncing. With hysteresis just in the thermostat reading, similar to the example given in my comment above, bouncing can be mitigated, but the thermostat would display an incorrect reading. $\endgroup$ Dec 4, 2015 at 21:09
  • $\begingroup$ I think that the behavior of hysteresis can be USED by a mechanical or control system to minimize what I would currently call "contact bounce", but "hysteresis" itself is not what I would call the setting of the correct set points to avoid "contact bounce"-like characteristics. Hopefully others can confirm or reject this for me. $\endgroup$ Dec 4, 2015 at 21:09
  • $\begingroup$ We seem to be disagreeing over terminology. Can we agree on these: Contact bounce The switch bounces between on and off several times before settling. Contact bounce (also called chatter) is a common problem with mechanical switches and relays. Switch and relay contacts are usually made of springy metals. Hysteresis: Often, ... added to an electronic circuit to prevent unwanted rapid switching. ... used to compensate for contact bounce in switches, or noise in an electrical signal. $\endgroup$
    – Transistor
    Dec 4, 2015 at 21:25
  • $\begingroup$ Hmm.. I agree with your Contact Bounce definition since its literally describing the bouncing of the physical contacts in mechanical/electro-mechanical components like switches. I do not know if there is another term for something like "Contact Bounce" behavior in respect to set points for a controlled system/process (like the AC turn on/off set points example). Your description of hysteresis however seems to describe just one way that hysteresis is utilized in the electrical engineering realm. To me, hysteresis is a phenomena that occurs in many systems/components, not a method of debouncing. $\endgroup$ Dec 4, 2015 at 21:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.