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Old July 28th, 2021, 11:03 PM   #1
wes_h_13
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PID Output To Electric Actuator With Duty Cycle

I have an electric actuator with a 75% duty cycle. I am sending an analog output from a PID to a controller board on the actuator. My concern is if I just send the analog output direct to the controller board how do I make sure that the valve doesn't cycle to much and go outside its duty cycle. I have some ideas on possibly setting up a deadband on the output. Just looking for any ideas of how to program this or examples of what you have done to accomplish this would be appreciated.
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Old July 29th, 2021, 06:26 AM   #2
Corsair
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If the controller board on the valve has a deadband then it's not a PLC programming problem. Adjust it to something that that you can live with and tune the loop on the PLC without worrying about the duty cycle of the valve.
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Old July 29th, 2021, 07:21 AM   #3
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Valve's own deadband don't make any good if PID loop is tuned poorly and can't control flow or pressure to point where constantly controlling isn't needed.

For electric valves own 3 point control on ladder is sometimes only needed. (deadband, control step and a delay between adjustments.)



It can be done also with PID, but then delay between adjustments needs to be long enought so that valve isn't moved all time.

(Also check from PID that time delay is really delay between adjustments and not only PID output update delay.

If PID is controlling behind all time but output is updated with time intervals then PID outputs "jumps" with big steps if time is changed bigger and valve is moved also with big steps.





Try search, there is plenty of threads of electric valves.



http://www.plctalk.net/qanda/showthr...electric+valve
http://www.plctalk.net/qanda/showthr...1&page=1&pp=15
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Old July 29th, 2021, 11:26 AM   #4
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LOL gotta love the 'net.

yyy.png

Anyway, I am trying to understand OPs process.

Is the duty cycle of the actuator used when moving the valve, i.e. when either opening or closing, presumably to keep the actuator motor from overheating, which it would do if it ran continuously?

So the command to the actuator is one of three discrete values:
  • Be closing - actuator motor runs, with 75% duty cycle, moving valve toward fully closed
  • Be opening - actuator motor runs, with 75% duty cycle, moving valve toward fully open
  • Be holding - actuator motor does not run, valve holds position
So there is no feedback of the actual valve position, e.g. 0% (closed), 100% (open), 50% (half-closed/-open), 42%, etc.

Meanwhile the PID tries to drive a pseudo-analog measured Process/Present Variable feedback value to a Set Point by varying its pseudo-analog output Control Variable value.

And OP's query is how to map that pseudo-analog, continuous PID output CV signal to one the three non-analog, discrete states* of the physical control device, i.e. to the controller board of the electric actuator?

Also, I am not sure I understand this:
Quote:
I have an electric actuator with a 75% duty cycle ... how do I make sure that the valve doesn't cycle to much and go outside its duty cycle.
Isn't the duty cycle a property of the actuator and its controller board? How could the valve (motor) "go outside its duty cycle?" Is that problem that, if the input signal to the controller board is shorter than the cycle time, the motor could start, get part way through its 75%, then stop briefly before another input signal starts it on another 75%, etc.?

* be closing, be opening, be holding
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Old July 29th, 2021, 03:44 PM   #5
Lare
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Quote:
Originally Posted by drbitboy View Post
LOL gotta love the 'net.



Also, I am not sure I understand this:
Isn't the duty cycle a property of the actuator and its controller board? How could the valve (motor) "go outside its duty cycle?" Is that problem that, if the input signal to the controller board is shorter than the cycle time, the motor could start, get part way through its 75%, then stop briefly before another input signal starts it on another 75%, etc.?

* be closing, be opening, be holding



Electric valves which I have used would move all time if controlled and will overhead eventually.
Duty cycling is needed to done on PLC code.


Setting PID to manual for hold time (2s) and back to auto (1s) for new control is one approach and will give 33% cycle for control. I assume that control don't need to be fast as there is electric actuator.
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Old July 29th, 2021, 09:45 PM   #6
drbitboy
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Quote:
Originally Posted by Lare View Post
Electric valves which I have used would move all time if controlled and will overhead eventually.
Duty cycling is needed to done on PLC code.

Setting PID to manual for hold time (2s) and back to auto (1s) for new control is one approach and will give 33% cycle for control. I assume that control don't need to be fast as there is electric actuator.
So OP does not have "an electric actuator with a 75% duty cycle," they have an electric actuator that will run whenever it is given a signal to run, as long as that signal is supplied.

That electric actuator, when the PLC decides it is time to move the valve, needs be run, by the PLC, using a 75% duty cycle in the signal supplied to the actuator.

Is that correct?
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Last edited by drbitboy; July 29th, 2021 at 09:51 PM.
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Old July 30th, 2021, 12:21 PM   #7
Tom Jenkins
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There are two potential problems with the electric actuator and analog positioner.

1) The dead band on the actuator control card can be too tight, causing the valve to hunt continuously even if the analog command remains at a constant value.

2) The control signal can be hunting, causing the valve to hunt as well.

Most electric actuators can be ordered with a "continuous duty" motor that can accommodate hunting.
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