Basic, but specific question about Proportional Band

Hello,

I am trying to get more accurate control of one of my basic plant processes, which essentially reads an input signal which goes to a controller, which then sends a 4-20 mA signal to the final control element. In this case, a small pump with a max flowrate of 2000mL/min. The control setup I have may be way too simple for what it’s being used for, however, in this case a great deal of accuracy is not required.

An example:
I have a SP of, say, 6 (arbitrary PV).

The way the controller is designed, I need to set the Pb as the value ‘away’ from the SP at which the controller output is maximum. If I’m understanding this correctly, I could set the Pb at 2.0 and that would mean that the controller would send it’s maximum output value at a PV of 4 (since I have the controller setup to INCREASE the PV to a SP of 6.)

This would also mean that if my controller max output was 50%, then when the PV decreases to 4, the controller would send that 50% output to the pump. As long as I’m on the right track so far, I’m seeing this all as fairly straightforward.

My specific questions:

  1. If the PV, for any reason, was to fall below 4 (or outside the Pb), what would happen to the controller output? ie: if the PV fell to 3.2, would the controller still just send the maximum output of 50% to the pump?

  2. As the PV approaches the SP of 6, does the output value decrease gradually from 50% to 0% until the SP is achieved?

As of now, I’m thinking that the Pb on the controller is set too low. The controller/pump always overshoots the SP. It’s not critical, just looking for better, more accurate performance.

I’m a Power Engineer/Process Operator by trade/education, and although I do have some basic knowledge of instrumentation, it is indeed basic and limited.

Any help is greatly appreciated!

Hi,

Its not possible to get accuracy using Proportional-only controller.

Please check below articles, for reference.