PID

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hlreed
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PID

Post by hlreed »

What does the P in PID do?
I have: Out = PE + I*i(E) + D*d(E) where E
is S1 - S2
i is integral function
d is derivative function
What is P?
Harold L. Reed
Microbes got brains
rshayes
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Re: PID

Post by rshayes »

P stands for Proportional (ie. gain without time dependence).<p>As your formula shows, the output of a PID controller is the sum of three terms. One is simply a coefficient times the input signal. The second is a different coefficient times the integral of the input signal. The third is a third coefficient times the derivative of the input signal.<p>This is usually used as a flexible method of adjusting the characteristics of a feedback loop.
Will
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Re: PID

Post by Will »

Essentially correct Stephen but the 'P' term is not, as you said the input multiplied by some factor - it is, in fact the control error i.e. the difference between the Desired value (Setpoint) and Measured Value (Process value) When the error is zero then it's integral is also zero so the output does not change at that time the first derivative (rate of change of error) is also zero. The reason is is referred to as the 'Proportional Term' is that it repreents the magnitude of error, expressed as a percentage of fullscale, required to cause a full traverse of the output/control device. Thus, if a controller had the POB set at 20% and controlling with zero error at the setpoint with the control device/valve/output at 50% open then it would require just 1 =/- 10% error (MV) change to fully open or close the control device. At such a (20%) setting the gain would be the reciprocal of the PB fractional value i.e. 1/0.2 = 5.0
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Will
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Re: PID

Post by Will »

My clumsy fingers again - the =/- means ' a +/- 10% error to . . .
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lynca
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Re: PID

Post by lynca »

here are a few good articles on the topic<p>http://www.barello.net/Papers/Motion_Control/index.htm <p>http://www.ctc-control.com/customer/ele ... /adjus.asp<p>[ May 11, 2004: Message edited by: lynca ]</p>
Will
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Re: PID

Post by Will »

Harold,
Were you satisfied with the answers you got ?
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hlreed
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Re: PID

Post by hlreed »

Yes, thank you all. I got all I needed to know.
Harold L. Reed
Microbes got brains
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