HI There
I am using control logix pid as a cascade loop. I am trying to control the temperature of the drum with the flow control valve. So outside loop (final controling element) loop is control the temperature of by adjusting the flow of the liquid with the help of opening and closing the valve. I am not using a true cascade function inside the pid block instead i am using two standard pids and the first pid output is being feed as a setpoint to the second pid loop. The problem is even though the second loop setpoint becomes zero and the error (PV-SP) is either zero or some negative numbers (sometime there is some flow through orfice plate), but my output to the valve is still about 20 to 23% Hence the valve is open almost 20 %. This output does not seem to go away, unless i delete the integral term from the second loop (which is controling the valve). After setting the integral portion zero, then the output goes to 0% and the valve is fully closed, lets say i put the some integral again, the valve again become open about 20%. This phenomena can only go away completely when i put my gain and integral term equal to 0. Anybody have any clue what is going on? Thanks in advance
I am using control logix pid as a cascade loop. I am trying to control the temperature of the drum with the flow control valve. So outside loop (final controling element) loop is control the temperature of by adjusting the flow of the liquid with the help of opening and closing the valve. I am not using a true cascade function inside the pid block instead i am using two standard pids and the first pid output is being feed as a setpoint to the second pid loop. The problem is even though the second loop setpoint becomes zero and the error (PV-SP) is either zero or some negative numbers (sometime there is some flow through orfice plate), but my output to the valve is still about 20 to 23% Hence the valve is open almost 20 %. This output does not seem to go away, unless i delete the integral term from the second loop (which is controling the valve). After setting the integral portion zero, then the output goes to 0% and the valve is fully closed, lets say i put the some integral again, the valve again become open about 20%. This phenomena can only go away completely when i put my gain and integral term equal to 0. Anybody have any clue what is going on? Thanks in advance