Bumpless transfer

Bumpless transfer is the smooth transfer of a controller from one operating mode to another. The balance (BAL) position provides this smooth transfer when transferring the controller from the automatic to manual mode. In the BAL position, the controller is still in the automatic mode of operation, but the deviation meter now indicates the difference between outputs of manual and automatic modes of control. The manual output is adjusted until the deviation meter shows no deflection. Now, the controller can be transferred smoothly from automatic to manual. 

To ensure a bumpless transfer from manual to automatic, the manual output signal, indicated by the output meter, is adjusted to match the controlled variable value to setpoint. This will be indicated by no deflection of the deviation meter. Once matched, the M-A transfer switch can be switched from manual (MAN) to automatic (AUTO) control. 

The positional algorithm is preferred when the measurement is noisy, because it works with the error and not the rate of error change when calculating its proportional correction. Velocity algorithms have the advantages of providing bumpless transfer and less reset windup (0 or 100%), and are better suited for controlling servomotor-driven devices. Their main limitations include noise sensitivity, likelihood to oscillate, and lack of an internal reference. 

Providing external reset for the cascade master from the slave measurement is always recommended. This guarantees bumpless transfer when the operator switches the loop from slave control to cascade control (Figure 2.45). The internal logic of the master controller algorithm is such that as long as its output signal (m) does not equal its external reset (ER), the value of m is set to be the sum of the ER and the proportional correction (Kc(e)) only. 

Boiler controls have already been described in Section 2.2, so their discussion here will be limited and oriented toward power generation. The level control of the steam drum of an HRSG is very similar to that of fired boilers, except that up to 30% of nominal steam flow, a single-element controller is usually used. Above 30%, the loop is bumplessly transferred to a three-element control (Figure 2.116). 

Apply an input signal to the loop controller equivalent to 50% of the instrument range and adjust the output of the manual pneumatic regulator to 50%. Adjust the loop controller setpoint to 50% and, by switching the auto/manual transfer switch, check for bumpless transfer. Using the manufacturer s instructions, adjust where necessary until a satisfactory bumpless transfer is achieved. 

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. 

For bumpless transfer, there are two types of setpoints the true setpoint, which corresponds to the desired operating point, and the internal setpoint, which is used for bumpless transfer. When a control loop is turned on, the setpoint used by the controller is actually different from the true setpoint when applying bumpless transfer. When the controller is turned on, the internal setpoint... 

FIGURE 15.67 The startup response of a feedback system without bumpless transfer and with bumpless... 

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. 

Reset windup problems can occur in override control structures when controllers have integral action. Large swings (bumps) in control valve position can result as control is transferred from one controller to another. One method for achieving bumpless transfer is the use of the Shinskey/Buckley control structure called external reset feedback. Unfortunately, this type of controller is not available in commercial dynamic simulators. 

In order to increase availability redundant subsystems may be deployed. The redundant controllers may work in l-out-of-Z principle (or dual cross wired) comprise two subsystems of identical design. They could be kept electrically isolated from one another, and are synchronized over fiber-optic cables. In the event of a fault, there should be a bumpless transfer from the active subsystem to the backup subsystem. Though it is possible to keep two subsystems but in the same rack, or spatially it is better to keep them apart so that in case of fire both systems may not be lost. 

Transfer from manual to automatic or automatic to manual cannot cause a sudden change in valve position, i.e., bumpless transfer is... 

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