PID control

I am using Micrologix1200 of AB. We are usng PID loops to control the firing angle of Voltage Rectifier Circuit.I tried to tune the PID, but not successfull. I want some clarification: What is the significance of Integral (Ti)? It is the average of Error ? How it is calculated by PLC system?

The way I think of the integral term is that it "looks backward" and corrects the output based on hte error that has accumulated over time. Mathematicaly it is the sum of the errors divided by the time constant.

Old fellers like me started out referring to the integral term as "reset" because it resets the output to get back to the control error. The integral term eliminates the droop that you get from proportional control alone.

If you search this site for Peter Natchaway you will find some great tips for tuning PID loops.

Tom Jenkins said:

Mathematicaly it is the sum of the errors divided by the time constant.

Click to expand...

Actually, Tom, it is the sum of the errors MULTIPLIED by the time constant.
§ Error dt = € Error delta(t)

Since the time constant is contant (), what the integral term represents is how much affect you want to give the sum of ALL the errors, ever since the PID block became active.

The I term is what keeps the valve (or whichever device) open when there is zero error. It "remembers" all the time that there was an error, and summed up all those values. It may not be adding any more to that sum (since error = 0), but the previous summation isn't going away.

It is entirely possible that hkpindia's situation isn't appropriate for PID control, and that's why he's having problems tuning.