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Contents:
Re: PID ("Devonna Dieterle")
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Date: Sat, 10 Mar 2012 07:56:49 -0500
From: "Devonna Dieterle" <djdieterle at hughes.net>
Subject: Re: PID
Error is not the proportional term. Error is the difference between
setpoint and process variable and the goal of a pid tuning loop is to
eliminate the error. Proportional gain is just what is implied a
proportional response to the error based on that terms gains setting.
Proportional gain is best used to diminish a large error. With only
proportional gain the error will oscillate around the setpoint, but never
eliminate/minimize the error. It's take the integral term which integrates
its gain value as a function of time to reduce the error beyond the ability
of proportional gain. Lacking a method to infinitely vary the output in
response to error makes the ability of the integral term to extinguish the
error not managed by proportional gain a pretty tough putt in a closed loop
system. My best guess is with an on/off output, any integral gain value
applied will wind-up in value and put the system back in oscillation. Most
pid controllers also have a filter on the PV signal before it is passed to
the pid algorithm. If this value is increased to the point that the pv
signal is effectively dampened out of it's normal responsive range the loop
can also appear to be in a steady state.
Also I need to take my wife off the hook when posting from her pc,
Jeff Dieterle
Date: Fri, 09 Mar 2012 01:23:25 -0500
From: "A. J. deLange" <ajdel at cox.net>
Subject: PID
Devonna is confusing the 'proportional band' of PID with proportional
output. These are separate things. The PID algorithm determines how much
output is required based on where the PV is in the proportional band
(P) i.e. the error, the integral of the error (I) and its differential (D).
The 'tuneset' consists of the three gains applied to each of these before
they are summed to form the output. How the output is controlled is a
separate matter. In a 4-20 ma loop full output is requested by a current of
20 ma, 0 output by a current of 4. In a 0 - 5V output, 0 volts corresponds
to a demand for 0 output and 5 volts to a demand for full output. In an
on/off output half power is signaled by logic 1 for half the output cycle
time, 70% by logic one for 70% of the cycle time and so on. This gives
infinite control just as the 4-20 loop and 0-5 volt outputs do and it can,
thus, do it for the full PID algorithm. I use full PID with on/off output
(to open and close a steam valve on a 30 second cycle) and recordings of
temperature vs time show the PV eventually settling out right on the set
point value (0 error) which would not happen if I were not active. What I am
doing is, of course, PWM. It's just that the PRI is 1/30 Hz. As the
bandwidth of the loop (determined by the thermal mass of the HLT and its
rate of heat loss) is less than 1/30 Hz the recordings are smooth.
A.J.
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End of HOMEBREW Digest #5913, 03/11/12
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