Feedback controllers cascade control

Propose a control system based on feed-forward - feedback control, cascade control and inferential control to achieve these control objectives. 

One such approach is called cascade control, which is routinely used in most modern computer control systems. Consider a chemical reactor, where reac tor temperature is to be controlled by coolant flow to the jacket of the reac tor (Fig. 8-34). The reac tor temperature can be influenced by changes in disturbance variables such as feed rate or feed temperature a feedback controller could be employed to compensate for such disturbances by adjusting a valve on me coolant flow to the reac tor jacket. However, suppose an increase occurs in the... 

Apply classical controller analysis to cascade control, feedforward control, feedforward-feedback control, ratio control, and the Smith predictor for time delay compensation. 

Example 10.2 Consider the temperature control of a gas furnace used in heating a process stream. The probable disturbances are in the process stream temperature and flow rate, and the fuel gas flow rate. Draw the schematic diagram of the furnace temperature control system, and show how feedforward, feedback and cascade controls can all be implemented together to handle load changes. 

One of the most useful concepts in advanced control is cascade control. A cascade control structure has two feedback controllers with the output of the primary (or master) controller changing the setpoint of the secondary (or slave) controller. The output of the secondary goes to the valve, as shown in Fig. 8.2n. 

Imperfections in feed-forward control can often be overcome by the addition of suitable feedback action. A typical design is shown in Fig. 7.70 where any variations in xd which occur bring the feedback control loop into action. The reflux flow is shown on flow control in cascade with the boiling temperature of the liquid at an appropriate point within the column. The inner (or slave) flow controller maintains... 

Figure 9.5 Cascaded feedback-control scheme for supersaturation control. Reprinted with permission from Liotta (2004).47...

A disadvantage of feedback controllers is that corrective action is not taken until after the controlled variable deviates from the set point. Cascade control can significantly improve the response to disturbances by employing a second measurement point and a second feedback controller. The secondary measurement is located so that it recognises the upset condition sooner than the controlled variable. Note that the disturbance is not necessarily measured. 

Cascade Control. The response of the simple feedback control to changes in the cooling water temperature can be improved by measuring the cooling water temperature and taking control action before its effect has been felt by the reacting mixture as shown... 

Loop 1 is cascaded with loop 2 to improve the response to disturbances in the cooling water temperature Tc w- The cooling water temperature is measured (TT2) and the signal sent to a second feedback controller TC2 which is normally a P-controller (see Figure 14). 

A potential choice of manipulated inputs to address the control objectives in the slow time scale is [ 3 Mrsp]t, i.e., the product flow rate from the column reboiler, and the setpoint for the reactor holdup used in the proportional feedback controller of Equation (3.35). This cascade control configuration is physically meaningful as well intuitively, the regulation of the product purity 23 is associated with the conversion and selectivity achieved by the reactor, which in turn are affected by the reactor residence time. 

The approach assumes that all the constrained variables can be measured or estimated on-line at a sampling period much smaller than the time constant of the controlled plant. Notice that the decision variables u in the RTO problem may very well be set points of feedback controllers acting directly on the plant manipulated variables. In this case, the constraint controller can be viewed as a primary controller in a cascade control configuration that corrects the set points produced at the RTO level. 

A pilot-scale distillation column located at the University of Sydney, Australia is used as the case study [60]. The 12-tray distillation column separates a 36% mixture of ethanol and water. The following process variables are monitored temperatures at trays 12, 10, 8, 6, 4, and the reflux stream, bottom and top levels (condenser), and the flow rates of bottoms, feed, steam, distillate and reflux streams. The column is operated at atmospheric pressure using feedback control. Three variables are controlled during the operation top product temperature, condenser level, and bottom level. Temperature at tray 8 is considered as the inferential variable for top product composition. To maintain a desired product composition, PI controllers cascaded on flow were used to manipulate the reflux, top product and bottom product streams. 

Figure 20.1 Temperature control of jacketed CSTR (a) conventional feedback (b) cascade.

The two controllers of a cascade control system are standard feedback controllers (i.e., P, PI, PID). Generally, a proportional controller is used for the secondary loop, although a PI controller with small integral action is not unusual. Any offset caused by P control in the secondary loop is not important since we are not interested in controlling the output of the secondary process. 

Hardware components computer systems, 552-57 control loops, 28-30, 32, 561-66 digital control loops, 557-61 Hardware instructions, 553 Heat exchanger cascade control, 399 control loops, 364 feedback control, 243 feedforward control, 413 modeling, 69-70... 

Although feedback control is the type encountered most commonly in chemical processes, it is not the only one. There exist situations where feedback control action is insufficient to produce the desired response of a given process. In such cases other control configurations are used, such as feedforward, ratio, multivariable, cascade, override, split range, and adaptive control. 

Figure 20.2 Schematic representation of (a) open-loop process (b) conventional feedback (c) cascade control.

For the new system, a sophisticated computer control algorithm was designed and implemented to quickly reach the target flow rate and then maintain that flow rate within a very tight range. This system includes multiple orifice runs, each consisting of an orifice meter and two actuated valves, one upstream of the orifice meter and one downstream. The algorithm that drives these two valves includes a cascade of feedback controls and a... 

To illustrate the cascade control approach, consider the control of the temperature of a jacketed crystallizer by manipulating the ratio of the flows to the crystallizer jacket from a heating stream and a cooling stream. The crystallizer temperature can be influenced by disturbances in the temperature of the heating and cooling streams and the heat transfer to the surroundings. Obviously, the crystallizer temperature could be measured and used in a feedback control scheme as shown in Figure 9.9 however, the manifestation of the disturbances would be slow, and the corrective action taken by the controller would be delayed. 

Series cascade, (a) Openloop process, (b) Conventional feedback control, (c) Series cascade, (d) Reduced block diagram, (e) Example 9.1. 

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