Use of Cascade Control

Process Control A Practical Approach Myke King 2011 John Wiley Sons Ltd. ISBN 978-0-470-97587-9 

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This important benefit has led to the extensive use of cascade control in chemical processes. 

Let us describe the use of cascade control in various typical processing systems. 

The dynamics of the secondary loop are much faster than those of the primary loop. Consequently, the phase lag of the closed secondary loop will be less than that of the primary loop. This feature leads to the following important result, which constitutes the rationale behind the use of cascade control The crossover frequency for the secondary loop is higher than that for the primary loop. This allows us to use higher gains in the secondary controller in order to regulate more effectively the effect of a disturbance... 

Another variation on the manipulated distillate scheme is to add a cascade slave control loop for the distillate flow rate as shown in Figure 2.1. The temperature control loop then manipulates the setpoint for the slave distillate flow control loop. Similarly, a slave flow control loop can be used for the reflux flow rate and another for the bottoms flow rate. At one point in time, the use of cascade control loops was called advanced control, it was compared to SISO (single input single output) control, because it required the addition of more hardware PID controllers. Modern computer control systems simply require the addition of software code to the program when the flow sensors are present. 

Advanced control stmetores can be easily implemented in Aspen Dynamics. In this section we illustrate two of the more important methods. A simple distillation column is used to illustrate the installation of ratio elements (multipliers) and the use of cascade control. 

Before attempting to determine the process dynamics we must first explore how they might be affected by the presence of other controllers. One such situation is the use of cascade control, where one controller (the primary or master) adjusts the SP of another (the secondary or slave). The technique is applied where the process dynamics are such that the secondary controller can detect and compensate for a disturbance much faster than the primary. Consider the two schemes shown in Figure 2.9. If there is a disturbance to the pressure of the fuel header, for example because of an increase in consumption on another process, the flow controller will respond quickly and maintain the flow close to SP. As a result the disturbance to the temperature will be negligible. Without the flow controller, correction will be left to the temperature controller. But, because of the process dynamics, the temperature will not change as quickly as the flow and nor can it correct as quickly once... 

To understand cascade control better, we will examine a typical feedback control scheme and consider how it may be improved through the use of cascade control. Let us consider the feedback control loop for a heat exchanger shown in Figure 6.2. 

An example of cascade control could be based on the simulation example DEACT and this is shown in Fig. 2.35. The problem involves a loop reactor with a deactivating catalyst, and a control strategy is needed to keep the product concentration Cp constant. This could be done by manipulating the feed rate into the system to control the product concentration at a desired level, Cjet- In this cascade control, the first controller establishes the setpoint for flow rate. The second controller uses a measurement of flow rate to establish the valve position. This control procedure would then counteract the influence of decreasing catalyst activity. 

We can use a block diagram to describe Fig. 10.1. Cascade control adds an inner control loop with secondary controller function Gc2 (Fig. 10.2a). This implementation of cascade control requires two controllers and two measured variables (fuel... 

If very close control is desired, then any disturbance due to steam pressure changes should be minimized. Figure 7-9 shows how this can be done using a cascade control system. In this case, the temperature of the process stream is measured and compared to its desired value, as before. The output of the controller, however, instead of affecting the control valve, regulates the set point of a second controller, the steam-pressure controller. This controller compares the set point determined by the first controller with the pressure downstream of the steam valve. 

The VPC scheme is a different type of cascade control system. The primary control is the position of the valve. The secondary control is the column pressure. The pressure controller is PI and tuned fairly tightly so that it can prevent the sudden drops in pressure. Its setpoint is slowly changed by the VPC to drive the cooling water valve nearly wide open. A slow-acting, integral-only controller should be used in the VPC. 

L12. The system of Prob. 10.4 is modified by using the cascade control system sketched below,... 

Practical considerations in implementing the hierarchical control framework developed above concern the availability of manipulated inputs to address the control objectives in the slow time scale (it is possible that dim(us) < dim(ys)), as well as achieving a tighter coordination between the distributed and supervisory control layers. Both issues are effectively addressed by using a cascaded control configuration, which extends the choice of controlled variables in the slow time scale to include the setpoints y)p of the distributed controllers. 

The use of a non-square controller (e.g., an MPC), such that the number of manipulated inputs is lower than the number of controlled variables, is certainly possible. While this approach eschews the use of cascaded configurations, it is intuitively detrimental to closed-loop performance due to the reduced number of manipulated variables. 

In tuning a cascade control system, the slave controller is tuned first with the master controller in manual. Often only a proportional controller is needed for the slave loop, since offset in that loop can be treated by using proportional plus integral action in the master loop. When the slave controller is transferred to automatic, it can be tuned using the techniques described earlier in this section. Seborg et al. (1988) and Stephanopoulos (1984) provide further analysis of cascade control systems. 

Suppose we want to use a cascade control system in the process considered in Problem 11.12. The secondary or slave loop will control X by manipulating M. The primary or master loop will control xi by changing the setpoint jcf of the secondary controller. 

The process variables do not always fall into such neat categories. For instance, temperature may be manipulated to adjust average molecular weight. In this case temperature is the manipulated variable for an MW control loop, but may at the same time be the controlled variable for a temperature control loop which uses the flow rate of a coolant as a manipulated variable. In this case the value of the manipulated variable for the MW control loop (temperature) is the desired value for the temperature control loop. This, of course, is the notion of cascade control. 

One way of remedying this issue is to exploit the fact that thermal mass of the burner is much smaller than that of the reformer and use a cascade control structure. Figure 21.16 shows a diagram of such a control structure from [58]. Here the inner, faster, control loop controls the burner temperature by varying the air supply to the burner. The outer, slower, control loop controls the reformer temperature by changing the set point for the burner temperature. 

The concept of cascade control has been traditionally very much used for effective reactor temperature control. An example of the implementation of cascade control is shown in Figures 12.36 and 12.37. 

Consider the control of the top-product purity in distillation column T-201 in the DME process. As shown in Example 21.3. the material balance control is achieved by a flow controller on the reflux stream and a level controller on the top product stream From previous operating experience, it has been found that the top-product purity can be measured accurately by monitoring the refractive index (RI) of the liquid product from the top of the column. Stream 10. Using a cascade control system, indicate how the control scheme at the top of T-201 should be modified to include the regulation of the top-product conposition. 

A commonly used form of cascade control involves a valve positioner, which addresses the problem of valve nonidealities. Valves may stick or exhibit dead zones or hysteresis, and so they may not achieve the same percentage stem position required for a given controller output. The valve positioner senses the valve stem position and uses an internal proportional controller in the inner loop of a cascade control system... 

This workshop will show how the response of feedback control loops can be improved through the use of other control methods. These other methods include measuring common disturbances and taking action before they affect the controlled variable (feedforward control) and using a faster responding loop to decrease the response time of a system with a large time constant (cascade control). You will determine what conditions are necessary for feedforward or cascade control to be useful and identify which parameters reduce the effectiveness of these control methods. 

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