Most modern control systems utilize more than one controllers which receive same inputs, contain same processing logics and generate same outputs.
Out of these, output of only one controller is considered as input for other controllers or process and is known generally as primary controller. The second controller keeps running silently in the background.
In case of bumpless transfer, when the primary controller fails, the secondary controller already has same inputs and same outputs as primary controller, and the only thing required for the control system is to choose the secondary controller as the main source of control which is accomplished by considering the fail state of primary controller as the switchover signal.
