Automating a Motor Controlled Valve Using a PID Controller

Hi

Currently we are asked to automate a valve to exhaust steam from a pressure cooker. The condition is we have to turn the valve one step at a time every 5 minutes, when the pressure inside the cooker reaches about 2 bar. Meaning the process of depressurization will take about 30-45 minutes to fully exhaust the steam from the cooker

A solution that we are thinking is using a PID controller, a pressure transmitter and a motorized valve for the process.

The problem is I'm not quite sure if we can use the PID controller, assuming that the process is more of a time based control. Do you suggest using a PLC instead.

Thanks.

You certainly can control this process, but a PID controller is not the correct tool. PID is designed to control a process (pressure) at a steady state, which is not what you have described. You have described a batch process and a procedure that is designed for manual control. The result will be a product with lots of variation from one batch to the next. I would just not implement the control of the process this way.

Yes, use a pressure transmitter, but I recommend use of a PLC with a program for batch control. You can use a motorized actuator for the control of the exhaust valve. The objective of this batch control logic is to perform the procedure in a highly repeatable way. I also assume that you will need some type of control on the heating source, which you have not described. Your objective is to heat the contents at some rate that will not harm the batch to a maximum pressure. Then turn off the heat source and relieve that pressure at some rate which will finish the cooking cycle until atmospheric pressure is attained. If you program the relief rates in terms of a pressure profile, rather than in terms of exhaust valve positioning, you will achieve a more consistent product from batch to batch. PLCs are a good tool for this type of sequential control. You may want to use a PID function block for pressure control within the PLC batch software, but look for the ability to do setpoint ramping during the exhaust period.

Dick Caro

You could use a PID controller with some extra logic to ramp the setpoint from 2 bar to zero over 30-45 min once the pressure reaches 2 bar. Logic and PID could be in a PLC.

Or you could use discrete logic in a PLC to start a sequence when the pressure reaches 2 bar to open the valve 1 step every 5 min.
Pressure input could be a pressure switch or a transmitter.

good luck