Process control bumpless transfer

The advantage of this form of the PID controller is that it will not act as abruptly to setpoint changes as Equation (15.8). In fact, from Equation (15.8) it can be seen that only the integral action will move the process toward a new setpoint. This reduction in aggressive setpoint tracking has an effect that is similar to bumpless transfer, which is discussed later. 

Figure 15.67 shows the process behavior with and without bumpless transfer. Without bumpless transfer, if the controller is turned on when the controlled variable is far removed from setpoint, the controller takes immediate action and drives the process to setpoint in an underdamped fashion. In certain cases, the controlled variable can be far enough away from setpoint, and the process can be sufficiently nonlinear that the control loop becomes unstable. Even if the control loop does not become unstable, the abrapt action of the feedback controller can significantly upset other control loops in the process. As a result, operators find that the behavior of a controller without bumpless transfer is generally unacceptable, particularly for key loops such as composition and temperature control loops. 

In certain situations the plant operator may wish to override the automatic mode and adjust the controller output manually. In this case there is no feedback loop. This manual mode of operation is very useful during a plant start-up, shut-down, or emergency situation. Testing of a process to obtain a mathematical model is also sometimes carried out in the manual mode. Commercial controllers have a manual/automatic switch for transferring from the automatic mode to the manual mode or vice versa. Bumpless transfers that do not upset the process can be achieved with commercial controllers. 

Early controller designs required balancing of the controller output prior to switching to or from automatic and manual modes. This procedure minimized inadvertent disturbance to the process caused by potentially large differences between the automatic and manual output levels. Later designs featured bumpless or procedureless automatic-to-manual transfer. 

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See also in sourсe #XX -- [ , ]

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