PID Loop system

karivelm

Member
Join Date
Nov 2006
Location
Quebec
Posts
66
I have a system that has 2 position linear transducers(4 to 20 ma)& 2 proportionnal valves. The system is used for position indexation.

Example 100% position desired : both indexers should move from position 0 to 100% at the same time.

One indexer is the master the other is the slave. The setpoints will be direct values such as 100,50 and 0% .

I Currently have the problem where the master is always in front of the slave. I am aiming to have only a 1.5% position between both but it goes up to 13% sometime. The slave is always behind the master.

Does anybody have a suggestion !!!

PID.jpg
 
Am I missing something?

If you want the slave to ALWAYS follow the master, can't it just be as simple as using the same PID CV output that the master uses massaged (if necessary) and applied to the second valve. You did say that they should move in unison so why the second tranducer and loop to begin with?
 
Because there is one proportionnal valve for each Indexer (master & slave). The system is designed to move anode buts, the weight is not evenly distributed between the master & slave.

Master & slave each have a proportionnal valve to control the movement. The goal is to have them index from 0-100% but without allowing a position difference of more than 1.5 %
 
You'll win me over yet, Peter.

As more time goes by I believe more strongly that just because you can do something doesn't mean you should. This seems to be one of those cases.

An RMC75 would have made this a whole lot easier.

Keith
 
I did not do the engineering but I am stuck doing the programming. We started to test the system this week actually. I we then noticed that the slave did not seem to respond exactly how we needed.

I just wanted to know if anybody had an idea so that the slave could catch up to the master
 
karivelm said:
I just wanted to know if anybody had an idea so that the slave could catch up to the master
Certainly, but by the time you get it going in a PLC it would be cheaper to buy the controller. We have a target generator that generates target or set point as a function of time. This is very smooth and you change the velocity, acceleration and deceleration using parameters. Both axes follow the target positions.

PIDs will not produce an output unless there is an error.
A good control system will use feed forwards to predict the required output as a function of velocity, acceleration and jerk. This will reduce the following error significantly.

If you don't get paid by the hour I strongly suggest you do what will get you running in the shortest time. Contact the distributors I suggested in your November post and get a demonstation or call Delta Computer Systems 360-254-8688.
 

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