HOMEBREW Digest #5913 Sun 11 March 2012

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  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. Return to table of contents
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