PID Control digital output RS Logix 5000

i have a control valve that is used to heat a tank of water. the steam valve only has digital signal to control it and no position feed back. when i open the valve
it takes about 40 seconds and 20mm travel ( valve is a linear actuator)

whats the best way to control the valve if i use PID hows it best to track the distance traveled?

thanks

Does the valve have two signals, one to power open and one to power closed? If so use the split range time proportional (srtp) function block

Simon08 said:

hows it best to track the distance traveled?

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You don't. You can't. Basically all you are doing is opening the valve a little if the tank is too cold or closing it a little if the tank is too hot. The valve basically integrates the control output so you shouldn't need an integrator in the controller. The control output should be use to determine how long the valve is to be opened or closed.

The trick will be to chose an update interval. Hopefully your tank has a long time constant. For instance make an output of 100% being opening or closing for 10 seconds. So you would run the loop every 10 seconds. An output of 50% would open or close the valve for 5 seconds.
If the gain appears to be too much lengthen the interval to 20 seconds so at max the valve can only open for 10 seconds every 20 seconds.

You should use a derivative gain or time constant and the proportional gain only.

Heed raffles advice.

Siemens has a valve controller that does this. It is called a step controller. I am sure other process companies have a similar controller.

Interesting problem.

One DIY solution might be to divide the 0-100% output of the PID into 20 "ticks" of 5% each. Every time the output crosses another 5% threshold on the way up, pulse the valve open signal for 2 seconds. Same on the way down. If the output sits above 95%, just hold the valve open signal on. If it sits below 5%, hold the valve closed signal on.

To eliminate chatter, you could implement a simple deadband so that once the output crosses a "tick" on the way up, it would have to cross two "ticks" on the way down before a close pulse would occur. And vice-versa.

I have had to use a control scheme very similar to what Peter described. Paddle could be raised or lowered using digital outputs (would have been much easier if it was a linear actuator) to increase or restrict the flow of molten aluminium into a casting wheel.

I took the output of the PID, and then scaled the 0-100% into a duty cycle of a simple PWM control scheme. 50-100% UP time from 0 to max time, 50-0% DOWN time from 0 to max time. Max time was the maximum on time in a cycle. I had a deadband as well

There are many ways to skin this, the easiest is use the instructions already, you'll get to the outcome you want much easier