LEVEL CONTROL BY PID

[Sali]

How can I connect PID Output to a valve. In ladder logic program is there any logic do I want to add between valve and PID? PV=SP What will be the O/P of PID? Input of PID is from level transmitter.

 

[Ken Moore]

Is the control valve on the inlet or outlet of the tank?

 

[MaxK]

PV=SP What will be the O/P of PID? \

I-term value ))
Could you be more specific please? Feel free to decompouse your question. Step after step.

 

[Sali]

I want to do this program in Siemens s7 Tia portal . Tank level is detected by a level transmitter. Scaled value from from level transmitter is given to PID Compact and I want to control a valve from PID output ( Valve should open when level is low and close when level is high) . Valve is inlet of the tank. There is a set point. Tank level should be in set point level. Is PID Out put is 0-100% . How PID Controls a ON/OFF Valve from its varying 0-100 analog value? What will be the PID Output when level of the tank = setpoint ( ie input of the PID = set point to the PID) ?

 

[Robb B]

If you want continuous level control (tank level stays at setpoint with varying outflow), you'll probably be best served with a proportional valve at the inlet, that way it can vary the inlet flow to match the outlet. If you just want tank fill control (shutoff valve when it reaches setpoint), you can probably just use the level transmitter and digital output to turn the valve on/off when you reach the setpoint. Desired accuracy of fill level will determine how refined your fill control has to be.

 

[robertmee]

If it's just an on/off valve, do as Robb suggested, and just compare the tank level to a deadband, and open valve until upper limit of deadband is reached, then close. Reopen when lower deadband is reached. You could get fancy and turn the PID output to a Timer Preset (similar to SCR Heater controls), but why bother, and likely no more accurate than opening and closing to a deadband.

 

[MaxK]

If you need accurate level control with on/off valve then feel free to read about pulse width modulation

But most likely Robb B and robertmee absolutely right.

If current_level > max_level then valve_open_comand = 0
If current_level < min_level then valve_open_comand = 1

in any other case
current_level (current scan) = current_level (previous scan)

Aren't you asking us to do your homework?

Who forces you to use a PID controller?

What will be the PID Output when level of the tank = setpoint ( ie input of the PID = set point to the PID) ?

As I written, steady-state (PV=SP) PID-out = I-term out, it is obvious.

But I suppose your question is formulated as follows:
How is the output value of the PID controller related to the valve position (eg at steady-state)?

Answer:
PID (steady-state) out is a number that, taking into account the “conversion factor,” corresponds to the inflow value that exactly compensates for the outflow.

I would suggest that you think carefully before asking the next question (I suppose I can guess what the question will be)

 

[Bit_Bucket_07]

I have seen (I didn't write it) a program in which a PID was used to control a tank level via an on/off valve. The CV was limited to a span of 1, so it could only be on or off.

I don't know how well it worked. I never got to see it in operation. I just found it to be an interesting concept. I'd have done as others have already suggested and used simple setpoint and deadband logic, had I written it.