External reset feedback

Now, the internal state of the controller, whether it be electronics or a computer algorithm for integration, will have an upper limit. External reset feedback, on the other hand, makes use of measurements of the manipulated variable. You may find such implementation details in more applied control books. 

EXTERNAL RESET FEEDBACK TO PREVENT RESET WINDUP... 

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 this chapter, we illustrate how external reset feedback can be implemented in Aspen Dynamics and demonstrate the improved dynamic performance using two process examples. 

Windup problems can be significantly reduced by applying external reset feedback as suggested by Shinskey over 45 years ago and extensively applied by Buckley to a variety of process applications, particularly distillation columns. 

Figure 18.2 External reset feedback circuit for PI controller.

Two examples are presented later in this chapter that demonstrate the windup problem when external reset feedback is not used and the improvement in control when it is used. 

First, we will show how external reset feedback is implemented in the widely used commercial dynamic simulator Aspen Dynamics. Developing effective control structures for processes often require the use of override controllers to handle operating up against constraints. Unfortunately, Aspen Dynamics does not have a module for an external reset feedback controller. The following section shows how one can be implemented using the available control element blocks and points out some of the problems in getting the simulation to initialize and run. 

Figure 18.3 External reset feedback in Aspen Dynamics.

We start with a simple process example to illustrate the procedure and demonstrate the improvement in performance provided by the use of external reset feedback. Only the normal controller has integral action in this example. The override controller is a proportional-only controller, so it does not need reset-windup protection. 

Override Control Structure Without External Reset Feedback... 

Figure 18.6 shows the Aspen Dynamics flowsheet with a standard PI flow controller (no external reset feedback) and a high-level override controller. Liquid level is normally controlled by the liquid valve. The normal level is at 2 ft, so the setpoint of the normal level controller is 2 ft. The level transmitter span is 4 ft. A simple P-only high-level override controller is set up to come into action when the level gets too high. The setpoint of the... 

The external reset feedback control structure discussed in Figure 18.3 is inserted in the Aspen Dynamics flowsheet for the flash tank process as shown in Figure 18.8. The feed flow controller has its output signal OPpc sent to a low selector. The other input to the low selector is the output signal OPqrc of the high-level override controller. The override controller is proportional-only, so it does not need anti-reset windup protection. 

Figure 18.9 gives the AllVariables views of the individual blocks in the feed flow controller with external reset feedback. The setpoint of the flow controller SPpc is set by a fixed-variable stream. The low selector inputs are the output signals from the two controllers (OP FC and OP ORC). The level transmitter for the override controller is simulated by using a Multiply block in which the liquid level (with units of ft ) is... 

Figure 18.10 Comparison with and without external reset feedback Ti = 5.

Figure 18.16 Override controller without external reset feedback.

Figure 18.17 30% feed disturbances no external reset feedback. 

Both controllers can windup since they both have integral action. Therefore, two external reset feedback controllers were configured in Aspen Dynamics. Figure 18.18 gives the elements of the control stmcture. The process variables and the setpoints of both controllers use Scale blocks to convert the values from process units (°C or bar) to percent-of-scale units. At the other end of the loops, Multiply blocks are used to convert percent-of-scale units to reboiler duty units (MMkcal/h but are shown as GJ/h in some of the blocks). 

Override control with external reset feedback +30% feed 6000... 

Figures 18.21 and 18.22 provide direct comparisons between the override control structures with and without external reset feedback. The bump in temperature when the...

These dynamic results clearly demonstrate the improvement in control that the use of external reset feedback provides in override systems. 

The implementation of external reset feedback in Aspen Dynamics has been presented. The available Aspen Dynamics control blocks can be configured to simulate external reset feedback. Getting the simulation to run requires that the lag elements used for integral action must first be initialized with fixed signals and then connected to the rest of the blocks. 

However the situation becomes more complex with cascaded controllers. The situation can arise where the secondary is controlling at SP but with its output at minimum or maximum. It is important therefore that the primary makes no changes to the secondary s SP which will cause it to saturate. DCS controllers have external anti-reset windup protection, sometimes described just as external reset feedback, to prevent this. 

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