Control system cascade

The dynamics of the secondary control loop should be approximately two to four times as fast as the dynamics of the primary control loop in order to achieve stable control. The secondary controller is actually part of the primary controller s process system. Hence, changes in the secondary controller tuning constants change the process system of the primary controller. Therefore, cascade control loops should always be tuned by first tuning the secondary controller and then the primary controller. If the secondary controller tuning is changed for any reason, the primary controller may need to be retuned also. 

Cascade control strategies are among the most popular and usehil process control strategies. Modem control systems have made thek implementation and operation both easier from the standpoint of operations personnel, and cost effective as they are implemented in software rather than hardwiring the connections. 

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... 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

There are many advanced strategies in classical control systems. Only a limited selection of examples is presented in this chapter. We start with cascade control, which is a simple introduction to a multiloop, but essentially SISO, system. We continue with feedforward and ratio control. The idea behind ratio control is simple, and it applies quite well to the furnace problem that we use as an illustration. Finally, we address a multiple-input multiple-output system using a simple blending problem as illustration, and use the problem to look into issues of interaction and decoupling. These techniques build on what we have learned in classical control theories. 

A cascade control system can be designed to handle fuel gas disturbance more effectively (Fig. 10.1). In this case, a secondary loop (also called the slave loop) is used to adjust the regulating valve and thus manipulate the fuel gas flow rate. The temperature controller (the master or primary controller) sends its signal, in terms of the desired flow rate, to the secondary flow control loop—in essence, the signal is the set point of the secondary flow controller (FC). 

Figure 10.2a. Block diagram of a simple cascade control system with reference to the furnace problem.

Figure 10.2b. Reduced block diagram of a cascade control system.

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. 

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. 

EXTRACTCON - Extraction Cascade with Backmixing and Control System... 

However, over the years a number of slightly more complex structures have been developed that can, in some cases, significantly improve the performance of a control system. These structures include ratio control, cascade control, override control, etc. We will devote much of this chapter to these subjects. 

With the cascade control system, the steam flow controller will immediately see the increase in steam flow and will pinch back on the steam valve to return the steam flow rate to its setpoint. Thus the reboiler and the column are only slightly affected by the steam supply-pressure disturbance. 

We will also talk in Chap. 11 about the two types of cascade control series cascade and parallel cascade. The two examples discussed above are both series cascade systems because the manipulated variable affects the secondary controlled variable, which then affects the primary variable. In a parallel cascade system the manipulated variable affects both the primary and the secondary controlled variables directly. Thus the two processes are basically different and result in different dynamic characteristics. We will quantify these ideas later. 

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. 

Eliminate minor disturbances by using cascade control systems where possible. 

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

In the last chapter we used Laplace-domain techniques to study the dynamics and stability of simple closedloop control systems. In this chapter we want to apply these same methods to more complex systems cascade control, feedforward control, openloop unstable processes, and processes with inverse response. Finally we will discuss an alternative way to look at controller design that is called model-based control. 

Example 11.1. Consider the process with a series cascade control system sketched in Fig 11.le. The secondary controller B, and the primary controller Bj are both proportional only. 

Figure 11.3d shows a process where the manipulated variable affects the two controlled variables and in parallel. An important example is in distilla tion column control where reflux flow aSecte both distillate composition and a tray temperature. The process has a parallel structure and this leads to a parallel cascade control system. 

If, however, a cascade control system is used, as sketched in Fig. ll.3b, the closedloop characteristic equation is not that given in Eq, (11.21). To derive it, let us start with the secondary loop. 

MARK V University of Washington Source Test Cascade Impactor Pollution Control Systems Corporation, Renton, Washington. 

M i (input) and Jfr0 (output), and Gyb(s) is the product of all blocks in the whole loop—often termed the open-loop transfer function of the control system. It is possible to apply the same rule successively to simplify certain multiple loop control schemes (e.g. cascade control—Section 7.13). 

Flo. 7.67. Block diagrams of cascade control system shown in Fig. 7.66 (a) original block diagram (b) condensed block diagram... 

The constant G is the gain of the cascade. This is an important parameter for tuning the dynamics of the temperature control system too low a gain results in slow temperature control where the set point may be surpassed, causing a hazardous situation (see Section 7.8.3). On the other hand, too high a gain results in oscillations that may cause loss of control of the reactor temperature. 

FIG. 8-37 Block diagram of the cascade control system. For a chemical reactor would correspond to a feed temperature or composition disturbance, while Gd2 would be a change in the cooling water temperature. (Source Seborg et al, Process Dynamics and Control, 2d ed., Wiley, New York, 2004.)... 

It is imperative that the kinetics of drug release are aligned with the kinetics of the restenotic cascade, as well as the pharmacologic mechanism of the drug being released (7,36), Rate-controlling systems, such as polymers, are ideal for this... 

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