J
jeremy
4 minutes
Bumpless Transfer Between Manual and Auto
When you switch a controller from manual to auto, the PID takes over the output. If its terms do not add up to the output you had in manual, the output jumps at that moment. On the trend you see a step in the output right at the switch, then the PV kicks away from where it sat. That bump can trip a limit or upset the process. Bumpless transfer stops it. At the moment of the switch, the controller sets its integral term so that P plus I plus D equals the current manual output. The output then starts from where you left it and changes only as the error does. Many controllers also let the setpoint track the PV while in manual, so the error is zero when you switch. Check your controller's manual to see which it does. The same trick matters when you change a gain on a running loop. In some forms of PID a new gain changes the output at once, so good code resets the integral to keep the output steady. If your code does not do this, change gains while in manual, or make small changes. Other things can look like a bump. A setpoint step with derivative on error gives a spike, and a wrong action sign sends the output to a limit. After this Concept you can switch a loop between manual and auto, or change its gains, without kicking the process, and tell a transfer bump from other jumps.