PID Code

Steve_D · Feb 6, 2009

Guys, no, Im not a uni student wanting to get my homework done but I also don't like to waste time (and refining) reinventing the wheel. So if any one can point me to a block of ladder code that does a basic PID loop (D not necessary)that would be great. The RTU I am programming has very limited capabilities and PID was an "option" but when pressed for it they admitted they didn't actually have one.
Its just to regulate a pump speed at a flow rate so it doesn't have to be too flash.
Thanks in advance.

widelto · Feb 6, 2009

Here you are:
This is my approach, in this case the pump has fixed speed 1750 rpm, flow is controlled by a control valve ( salida valvula), flow is measured thru an endress&hauser coriolis flowmeter ( flujo aceite), SP is 18000 kg per hour.
My P, I nad D values are : 0.21, 3.1 and 0 respectively.

In your case values may be different.

Steve Bailey · Feb 6, 2009

In my opinion you would be better off scrapping the PLC that doesn't suit your needs and replacing it with one that does. Unless you place a very low value on your time, the cost to write and debug ladder logic to act like a PID function block could easily exceed the the cost of hardware replacement.

Andybr · Feb 6, 2009

Steve.
Even if your PLC doesn't do PID the drive might. I would check that there is not a built in PI or PID function in the drive you could use.

Peter Nachtwey · Feb 6, 2009

There is all sorts of good stuff here

ftp://ftp.deltacompsys.com/public/PID/
if you have a ML1500 or SLC.

danw · Feb 6, 2009

There are dozens of commercial, stand alone, single loop PID controllers, even a DIN rail mount version or two that could be used.

TConnolly · Feb 6, 2009

Steve_D said:
Guys, no, Im not a uni student wanting to get my homework done but I also don't like to waste time (and refining) reinventing the wheel. So if any one can point me to a block of ladder code that does a basic PID loop (D not necessary)that would be great. The RTU I am programming has very limited capabilities and PID was an "option" but when pressed for it they admitted they didn't actually have one.
Its just to regulate a pump speed at a flow rate so it doesn't have to be too flash.
Thanks in advance.

First, to answer your question, see this thread: http://forums.mrplc.com/index.php?showtopic=12208

It contains a PID I wrote in C many many years ago for an embedded controller. Its crude and I would do it much differently now, but it may help you to understand what you need to do to construct one of your own.

Second, how much is the customer willing to spend to pay you to program a PID from scratch in an inadequate controller -vs- spending for a controller with the PID instruction built in?

AJZ · Feb 7, 2009

Some basic PID code from the web:
From http://en.wikipedia.org/wiki/PID_controller#Pseudocode:
previous_error = 0
integral = 0
start:
error = setpoint - actual_position
integral = integral + error*dt
derivative = (error - previous_error)/dt
output = Kp*error + Ki*integral + Kd*derivative
previous_error = error
wait(dt)
goto start

and from http://www.embedded.com/2000/0010/0010feat3.htm:

typedef struct
{
double dState; // Last position input
double iState; // Integrator state
double iMax, iMin;
// Maximum and minimum allowable integrator state
double iGain, // integral gain
pGain, // proportional gain
dGain; // derivative gain
} SPid;
double UpdatePID(SPid * pid, double error, double position)
{
double pTerm,
dTerm, iTerm;
pTerm = pid->pGain * error;
// calculate the proportional term
// calculate the integral state with appropriate limiting
pid->iState += error;
if (pid->iState > pid->iMax) pid->iState = pid->iMax;
else if (pid->iState
<
pid->iMin) pid->iState = pid->iMin;
iTerm = pid->iGain * iState; // calculate the integral term
dTerm = pid->dGain * (position - pid->dState);
pid->dState = position;
return pTerm + iTerm - dTerm;
}

Steve_D · Feb 10, 2009

Thanks Guys, unfortunately the device is a Radio Tranmission Unit that had very limited functionality, but thats what the client wants.
I appreciate all your help, thanks again!

PID Code

Steve_D

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widelto

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Steve Bailey

Lifetime Supporting Member + Moderator

Andybr

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Peter Nachtwey

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danw

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TConnolly

Lifetime Supporting Member

AJZ

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Steve_D

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