I have an application that requires control of the pressure in a pumped line but with flow rate limiting. The pump is inverter-controlled and the PLC is an AB compactlogix L33ER. I have achieved this before in a Siemens plc but with limited stability during flow limiting.
I suspect that I am reinventing the wheel, but my method is to set up two PID blocks; one to control the pressure to a setpoint and the second (configured PV-SP, with its own setpoint) to monitor the flow rate. The output of the flow-sensing PID function is then subtracted from the output of the pressure PID function and the result is the speed value to the inverter.
The pressure loop output is sent to the inverter unaffected by the subtraction until the flow rate approaches the flow limit setpoint, at which point the flow PID output starts to rise, reducing the output value to the inverter via the subtract function.
I'd prefer to pre-scale and send the F PID o/p directly to the P PID output limit tag but it doesn't appear to be accessible and in any case, I'm sure there's a more orthodox method already built-in, so I'd be happy to be enlightened.
Thanks.
I suspect that I am reinventing the wheel, but my method is to set up two PID blocks; one to control the pressure to a setpoint and the second (configured PV-SP, with its own setpoint) to monitor the flow rate. The output of the flow-sensing PID function is then subtracted from the output of the pressure PID function and the result is the speed value to the inverter.
The pressure loop output is sent to the inverter unaffected by the subtraction until the flow rate approaches the flow limit setpoint, at which point the flow PID output starts to rise, reducing the output value to the inverter via the subtract function.
I'd prefer to pre-scale and send the F PID o/p directly to the P PID output limit tag but it doesn't appear to be accessible and in any case, I'm sure there's a more orthodox method already built-in, so I'd be happy to be enlightened.
Thanks.