PID loops

I am a project Engineer from india
i would like to known how to find PID loops in process
if i had one VFD (variable speed drive ) how to control speed using
5 or 6 PID loops

Regards,
Sandeep Patil

Sorry, i don't understand your question.

What do you want to do ?
Do you have to apply a frequentie convertor with a PID control?
Or do you have to figure out how a frequentie control works with an existing PID?

Jurgen

PID Loop

I have 5 parameters
& one VFD
1. Suction pressure
2. case pressure
3. discharge pressure
4. discharge temp
5. motor amps

so based on these loops is it possible to control VFD speed

regards
sandeep patil

OK, this info is more usefull.

The way I would look at the problem.
Use only one PID for the output to the frequency convertor (only use the PID if you need it, you probably do but anyway).

Determin what all the input variables have as function to the output.
In other words work out an formula with all the variables inside that you mentioned. And keep in mind what you are going to use as setpoint for the whole regulation. Wich variabel do you want to keep as a setpoint.
If you want to make all the variables as a setpoint you will get a hell of an equation in the end, but still possible. The key here is to determen the influens, interaction between the inputvariables. Try to put them in a formula to each other. Then try to combine all the formulas and simplify the hole bunch of them.

At this point it would be nice if you post the result of your effort. I bet a lot of the guy's (ladies) would love to help you out in the formula department.

When you have the end result you will have to put it in a PLC. Tell us what kind of PLC you have a we micht be able to give you some advise how to implement the nice result of the formula's

Good luck !

Jurgen

I have done something similar, only on a gas transmission line. One valve with
Control of downstream pressure, backpressure, flow control, upstream under pressure.
I had 4 PID loops, then used a low signal select to control valve. Another approach would be to have different selectable modes.

PID is a single-input, single output (SISO) controller.

You can always try to provide a single input based on some combination of your input variables, but I wouldn't recommend it as JvdV does, since if two of these parameters change in opposite directions by the same amount after scaling, it's possible that no change in the output will be implemented.

The single output can be scaled/split and applied to multiple output devices, but in your case, you only have one: speed. Mickey gives a good example of multiple PID loops functioning to control a single output device using a low select function, but a single PID loop couldn't accomodate more than one input varaiable, so four were used (one for each of the four input variables).

Let me ask you this: since the VFD is controlling a motor (I'm assuming), what is the load of the motor and what is it that you are actually trying to control? Do you really need to control all five of those parameters you mentioned at the same time?

loop control

Without knowing exactly what you want to accomplish...

You can "cascade" loops..which can be very effective. Which menas taking any one variable into a loop.. using the output of this loop to control another loop (i.e reversed), using one loop as a setpoint for another loop whilst using the derived setpoint of another loop as the PV for the same loop.. like using formulas.. but you can tune them online !!
Think about it.. !!

regards
Richard