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Hello again

Thank you all for your recent help. I have certainly experienced a steep learning curve with Visilogic!

I require some help with using a PID function to control the putput of one of my valves.

The problem I have is that the valve is "chattering" when near to the setpoint, i.e opening and shutting very fast as the setpoint is reached. I need to slow down the operation of the output, the control does not need to be so precise - i.e control within +/- 3 mbar of the setpoint at 103 mbar, or only open again after 2 seconds of closing. I thought that I would use the PID function to sort this problem but I am really stuck.

COuld anyone please help me in very simple terms, to show me how to implement the PID into my ladder?

I am using MI 2 as a linearisation of a 4 to 20 mamp input ranged 0 to 1000 mbar. This is what I want to use as my process input.

Set point is 103 mbar

Control can be +/- 3 or 4 mbar

Would like to control Output 8 with teh PID function.

Thankyou in advance :) :)

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I'll advice first to take a look at the examples - VisiLogic > Help menu > Examples > ... Project examples > PID.

To make the control less agressive - in general you ned to increase both PB and Ti and to decrease (up to 0) Td.

Small tip - take a log of the system with PID Server (VisiLogic > Tools menu). Send your application and the log file to support@unitronics.com and I hope I'll be able to help you more specifically.

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You never actually said it, but I am taking as a matter of faith that you are actually using a modulating valve and not a simple on/off valve. Your description of instantaneous valve changes makes me suspect that you do not have a modulating valve because modulating valves always have a time to full open and a time to full close and the fastest I have ever seen a modulating valve move from any supplier would preclude the description of "chattering"

So assuming that you have a modulating valve then in a PID loop if you set P to a value and leave I and D equal to zero then you would get the exact result you are describing. (when you get to to your set point the valve suddenly only opens and closes). This would be because your PID is actually putting out only a minimum and maximum signal out to your analog output. To stop this effect you must put some "I" into the equation.

With "i" put in (leaving "D" at zero) you will be allowed to not position correct so instantly and there will be some time lag to changes.

For example set your P to 50 and your I to 20 You should have a more gradual change when approaching and leaving your setpoint.

If you do indeed have an on/off only valve then the tactic needs to change, but I am not going to go through the time to write all that down not knowing what you actually have.

Give some more information on your system.

You have a sensor that ranges fromn 0 - 1000mBar

What is your valve control input?

What is your valve main power voltage?

What is the time of travel for full open and full closed (spring return motors will have different times for opening and closing)?

What is it you are actually controlling Boiler, Compressor, Injector, ....Etc

You gave the MI for the pressure sensor what is the MI for your analog output to your valve?

and anything else you think we might need to know to help


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Hi emil

Thanks for your reply.

I have been through the pid help but I'm still struggling with how to connect the pid to an output and also how to use my linearised transmitter signal as an input

Normally, to tie a PID output to a single physical digital output you will send it through a PWM block. You will want the cycle time to be long if you want to slow down the switching.

Your proces value should go directly into the PID block.

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