Multiple PID Control

A

Thread Starter

Alok Khatlawala

We have an application wherein two control valves are set up side by side so as to control two parameters in the process.
Flow Control and Pressure Control.

We used two PID controllers to control each parameter independently but saw disastorous results.
What happens is in case pressure increases, the flow gets disturbed. The PCV tries to control the pressure, but in this process, because the flow was disturbed against setpoint, the FCV tries to control the flow also.
This keeps oscillating in an infinite loop.

Same applicable for the reverse combination also (flow variations effect on pressure)

Any ideas how these parameters can be controlled. Can anyone suggest standalone controllers which can take care of controlling both the parameters?

Thanks and regards


Alok
 
What you are looking for is know as a two-loop cascade control. The first loop looks to maintain the primary variable within range but using its control output as the setpoint for the second variable loop.

 
C

ControlNovice

Do you have two separate control valves on the same vessel/pipeline with two separate controllers?

Assuming the pressure affects the flow more...You can try CASCADE control, with the Flow controller's ouput going to the Pressure controller's input. Then the Pressure controller's output goes to the (ONLY ONE) valve. The Flow is set and on error (SP-PV), will send a signal to the pressure controller, which tries to obtain this input. Any change in pressure on the vessel/pipeline will affect the output of the pressure controller also.

The pressure tuning has to be faster (integral time) than the flow.

If the pressure and flow are to maintain a certain ratio, you can try a RATIO controller, but this may be much more difficult if the two affect each other.

ControlNovice

 
S

S.Idayachandran

What is the application? Since it is involving pressure control surely the control will be for a vessel. Hence you can control the pressure by using a control valve in the inlet of the vessel and the flow by using a control valve in the outlet of the vessel with independent controllers.

Iday
 
Pehaps one solution is to try a de-couple by changing loop speed. When loops interact together, we need to select response speeds that differ. If one loop is about 3 to 5 times faster than other, the cycling caused by interactions between them will decrease. To do this, you could tune one loop as fast as possible and de-tune the other, decreasing the proportional gain, as slow as possible until reach about 3 to 5 times slower than the first. You can use the settling time to measure their speeds.
 
U
you cannot control with this configuration two parameters if they are in the same direction of the flow, the first valve can control the in press and the second can control the flow.
 
Firstly, I would suggest installing the PCV and FCV in the same line, and not side by side.

Secondly, Yokogawa has a YS170 controller, which can easily be set up to control both pressure and flow loops. (Dual PID, dual display, 5 Analog inputs, 3 Analog outputs, 6 Digital I/O etc.)

It may be possible to use the flow signal as a feedforward signal to the pressure controller, or visa versa. This will add "predictive" control to the secondary controller and avoid the oscillation.
 
Y

Y.K.JARIWALA

Would you give your application including downstream requirement, then probably we can give you better solution to your problem.

But the way you have described the issue would never solve your problem.

Jari
[email protected]
 
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