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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.
Air
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agent provocateur
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  • I think the term 'set points' is accurate. Instead of 'contact bounce,' I would say 'forced oscillation'. – OnStrike Dec 02 '15 at 18:46
  • 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.     – agent provocateur Dec 03 '15 at 00:33
  • 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. – jhabbott Dec 03 '15 at 21:58
  • 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. – Transistor Dec 03 '15 at 23:21
  • @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). – agent provocateur Dec 04 '15 at 20:46
  • @transistor "debouncing" inputs and switches is exactly what I am talking about from both mechanical/electro-mechanical and software perspectives. This question is about setting the set points (or implementing a delay, which is essentially the same thing from a software or controls perspective) so that this bouncing characteristic is not seen.You are describing debouncing that is implemented with software via a time delay. You can also debounce with software with setting other points as well. You can also debounce mechanically. – agent provocateur Dec 04 '15 at 20:54
  • If a keyboard switch was pressed right to the point where it registers the input, it would bounce without any debouncing designed into it in some manner. – agent provocateur Dec 04 '15 at 20:55

1 Answers1

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"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
Transistor
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  • 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. – agent provocateur Dec 04 '15 at 20:43
  • 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. – agent provocateur Dec 04 '15 at 21:09
  • 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. – agent provocateur Dec 04 '15 at 21:09
  • 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. – Transistor Dec 04 '15 at 21:25
  • 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. – agent provocateur Dec 04 '15 at 21:43
  • OK. Can anyone else clarify the original question for us? – Transistor Dec 04 '15 at 21:48
  • first sentence =) : "What is the terminology for having different/spaced-out on and off set points for a particular controlled process?" or tell me what part of that is not clear to you and I will try to clarify. – agent provocateur Dec 04 '15 at 21:58
  • I read that as a "what is the English word for ..." and think 'hysteresis' is that word. Explain more what you mean by 'set points'. – Transistor Dec 05 '15 at 14:56
  • set points or trigger points, which is supposed to be a target value where a system is triggered to act or respond in a certain way.
    like how a thermostat is triggered to turn the AC on at a certain temperature set/trigger point.     – agent provocateur Dec 07 '15 at 19:19