How Do I Get This Crazy Thermal PID Loop to Tune?!
Most temperature controllers today have sure fire tuning algorithms that are simple to use; just press the “auto-tune” button, go get a cup of coffee, and when you get back your system is running like a champ. Of course this makes you look like a genius … when it works. But sometimes you run across a system with an older temperature control that either doesn’t have “auto-tune” or the algorithm just isn’t working for your specific system. Trying to manually “tune” your system by tweaking the three PID values (Proportional , Integral, and Derivative) can be a very humbling experience. People who have mastered this art are held in very high regard – on a par with Yoda of Star Wars fame.
One such “PID Jedi” is Owen Hensinger, now retired after 45+ years in the control business. Owen has experienced first hand every technology invented to control temperature in the industrial equipment world. I have seen him time after time walk into impossible “tuning” situations with nothing more than a screwdriver and emerge later with happy customers who think he walks on water.
You too can be a “PID Jedi” – if you know the master secret:
The ‘OWEN-Tune’ Procedure
Assume that the process is at setpoint and has the default PID values of P 45, I 60, D 15, F (input filter) 2
For thermal loops, set the input filter to a value equivalent to 3 second response time. This will be determined by the sample rate of the controller. As an example, if the controller has a sample rate of 1 sample per second, set the input filter to 3.
To set the P term
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Step
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Description
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1
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Set I to 1000
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2
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Set Control Mode to Manual
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3
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Set P to ½ of current setting
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4
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Set Control Mode to Auto
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5
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Check to see that the output is stable (look at output % not PV)
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6
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If output is stable proceed to the I term
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7
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If output is NOT stable, set Control Mode back to Manual
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8
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Increase P by 10
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9
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Set Control Mode to Auto
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10
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Check to see that the output is stable (look at output % not PV)
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11
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If output is stable proceed to Setting the I term
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12
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If output is still NOT stable, set Control mode back to manual, go back to step 8
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To set the I term
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Step
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Description
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1
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Set I to 60
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2
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Set Control Mode to Auto
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3
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Check to see that the output is stable (look at output % not PV)
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4
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If output is stable
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5
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Set I to ½ of current setting
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6
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Check to see that the output is stable (look at output % not PV)
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7
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If output is stable go to step 5
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8
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If output is NOT stable
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9
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Increase I by ½ of current setting
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10
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If output is still unstable increase I by ½ the delta from the old setting until stable
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11
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When output is stable leave Control Mode in Auto and proceed to the D term.
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To set the D term:
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Step
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Description
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1
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Set D to 1/6th of the I term above
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2
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Set the Digital Filter (Output) to 2 or 3
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It turns out it’s not that hard to be a PID Jedi. You just need to be trained by a master.
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