How to implement discrete PID function in GE fanuc 90-30 Series CPU?

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Thread Starter

Chirag Shah

We are in need to implement Dicerete PID function in flow control loop. We have the GE fanuc's 90-30 seires CPU (Hot standby) & remote I/Os over genius bus.

Please provide us the method to implement the digtial PID function for the same.

Chirag Shah
 
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Tomy Zacharia

Dear Chirag,

By discrete PID function, do you mean PWM (pulse width modulation) output also known as on-off control or you just want to have a working PID block for control of some process?

Tomy Zacharia
 
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Dear Tomy,

Thanks for your reply. I in need to control the Pressure loop with two digital output for controlling of the pressure regulator solenoids using GE Fanuc 90-30 PLC. As per my knowledge PWM will solve my problem.

Thanks
Chirag Shah
 
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Tomy Zacharia

Dear Chirag,

First of all my apologies for a very late answer.

My assumption is that you have two solenoids, one increases control output namely pressure and the other decreases the control output.

In the PLC PID instruction there will be a value called bias. By default it is zero. Change the value to midscale i.e. 16000 counts. Then using a "greater than" instruction check for CV greater than 16000. If so then trigger solenoid 1. If the CV is less than 16000 then trigger solenoid 2.

Do post back your results.

Tomy Zacharia
 
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Dear. Tomy.

this is going to be interesting since this is a very old posting... I am currently in the same position as Chirag was. 6 years ago.

i just read your post. and isn't that just a comparison? (1600)<Pressure>(16000) then there's no need for a complicated PID right?

but since i am kinda stuck in the same problem in my case a valve positioner. couldnt it be controlled with a time base logic. or better yet combining the comparison + timebase logic.
something like a duty cycle.

for example as the PV gets further away from the SP the valve should stay more time open or closed which ever the case might be.

and as the PV gets closer to the SP the less time it needs to stay open or close.

wouldn't it be better? or maybe not worth the effort?
 
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