How do PID temp controllers implement control

  • Thread starter Engn_sam
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  • #1
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My group members and I are trying to build a snow heating mat. So far, we are using nichrome heating wires sandwiched in silicone rubber connected to an arduino that controls a solid state relay to turn on and off electric current from a wall plug. What I want to know is how to control the temperature. We decided to stick with an on and off control since it's the easiest to work out but I want to know how you implement a P or PID control. Do I need like a variable resistor or something? Or what way is the PID control supposed to vary the power output?
 

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  • #2
anorlunda
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I don't think PI or PID is justified for something like that. A simple proportional control wi a dead and will do.

You need the dead and to prevent it from cycling endlessly.
 
  • #3
billy_joule
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Here is the arduino PID library:
http://playground.arduino.cc/Code/PIDLibrary

Here is an example using the library with a relay:
http://playground.arduino.cc/Code/PIDLibraryRelayOutputExample

Background on the development of the arduino PID library:
http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/

Some background on control theory:
https://en.wikipedia.org/wiki/Control_theory

If you google "arduino sous vide" you'll get many results where a relay, heater & PID* are used together with an arduino to maintain a temperature setpoint.

*While a PID controller is used, I & D are set to zero so it's P only. like anorlunda recommends.
 
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  • #4
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I don't think PI or PID is justified for something like that. A simple proportional control wi a dead and will do.

You need the dead and to prevent it from cycling endlessly.
What's a dead and. and is it safe to use a proportional control with a solid-state relay
 
  • #6
anorlunda
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With on-off control, you need deadband to prevent endless cycling. With deadband, you can't use integral control, because that would cycle.

All you need is two statements. Let T be temperature, P set point, and D deadband.

if P-T > D then turn_on;
If T-P > D then turn_off;

D is your only tuning parameter.

Good luck.
 

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