Cascade controller

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Fig. 12. Cascade control signal flow diagram, where SPP = primary control variable setpoint PVP = primary control variable measurement ...

Schemes to control the outlet temperature of a process furnace by adjusting the fuel gas flow are shown in Figure 13. In the scheme without cascade control (Fig. 13a), if a disturbance has occurred in the fuel gas supply pressure, a disturbance occurs in the fuel gas flow rate, hence, in the energy transferred to the process fluid and eventually to the process fluid furnace outlet temperature. At that point, the outlet temperature controller senses the deviation from setpoint and adjusts the valve in the fuel gas line. In the meantime, other disturbances may have occurred in the fuel gas pressure, etc. In the cascade control strategy (Fig. 13b), when the fuel gas pressure is disturbed, it causes the fuel gas flow rate to be disturbed. The secondary controller, ie, the fuel gas flow controller, immediately senses the deviation and adjusts the valve in the fuel gas line to maintain the set fuel gas rate. If the fuel gas flow controller is well tuned, the furnace outlet temperature experiences only a small disturbance owing to a fuel gas supply pressure disturbance.

Fig. 13. Cascade control schemes, where TC = temperature controller FC = fuel gas flow controller and LC = liquid level controller, (a) Simple circuit having no cascade control (b) the same circuit employing cascade control and (c) and (d) Hquid level control circuits with and without cascade control,...

Both control schemes react in a similar manner to disturbances in process fluid feed rate, feed temperature, feed composition, fuel gas heating value, etc. In fact, if the secondary controller is not properly tuned, the cascade control strategy can actually worsen control performance. Therefore, the key to an effective cascade control strategy is the proper selection of the secondary controlled variable considering the source and impact of particular disturbances and the associated process dynamics. 

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. 

Many misconceptions exist about cascade control loops and their purpose. For example, many engineers specify a level-flow cascade for every level control situation. However, if the level controller is tightly tuned, the out-flow bounces around as does the level, regardless of whether the level controller output goes direcdy to a valve or to the setpoint of a flow controller. The secondary controller does not, in itself, smooth the outflow. In fact, the flow controller may actually cause control difficulties because it adds another time constant to the primary control loop, makes the proper functioning of the primary control loop dependent on two process variables rather than one, and requites two properly tuned controllers rather than one to function properly. However, as pointed out previously, the flow controller compensates for the effect of the upstream and downstream pressure variations and, in that respect, improves the performance of the primary control loop. Therefore, such a level-flow cascade may often be justified, but not for the smoothing of out-flow. 

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. 

While the single-loop PID controller is satisfactoiy in many process apphcations, it does not perform well for processes with slow dynamics, time delays, frequent disturbances, or multivariable interactions. We discuss several advanced control methods hereafter that can be implemented via computer control, namely feedforward control, cascade control, time-delay compensation, selective and override control, adaptive control, fuzzy logic control, and statistical process control. 

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... 

Cascade controllers used where the supply and extract air temperatures or return water from heater or cooler batteries requires complete control... 

Tabakoff B, Nelson E, Yoshimura M et al (2001) Phosphorylation cascades control the actions of ethanol on cell cAMP signalling. J Biomed Sci 8 44—51... 

Fig. 5.4-23 shows a sketch drawing of a BSC (Brogli et al., 1981). The stirred-tank reactor made of glass (a metal version is also available) is surrounded by a jacket through which a heat-transfer fluid flows at a very high rate the jacket is not insulated. The temperature of the circulation loop is regulated by a cascaded controller so that the heat evolution in the reactor is equilibrated by heat transfer through the reactor wall. The temperature in the loop is adjusted by injection of thermostatted hot or cold fluid. 

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. 

Figure 2.35. Cascade control to maintain product concentration by manipulating the reactant concentration in the feed.

With this arrangement, the output of one controller is used to adjust the set point of another. Cascade control can give smoother control in situations where direct control of the variable would lead to unstable operation. The slave controller can be used to compensate for any short-term variations in, say, a service stream flow, which would upset the controlled variable the primary (master) controller controlling long-term variations. Typical examples are shown in Figure 5.22c (see p. 235) and 5.23 (see p. 235). 

Figure 5.23. A typical stirred tank reactor control scheme, temperature cascade control, and reagent ...

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. 

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

A very common design found in process engineering is cascade control. This is a strategy that allows us to handle load changes more effectively with respect to the manipulated variable. 

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 10.1. Cascade control of the temperature of a furnace, which is taken to be the same as that of the outlet process stream. The temperature controller does not actuate the regulating valve directly it sends its signal to a secondary flow rate control loop which in turn ensures that the desired fuel gas...

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... 

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.

So far, we know that the secondary loop helps to reduce disturbance in the manipulated variable. If we design the control loop properly, we should also accomplish a faster response in the actuating element the regulating valve. To go one step further, cascade control can even help to make the entire system more stable. These points may not be intuitive. We ll use a simple example to illustrate these features. 

With cascade control, we know from part (d) that the system is always stable. Nevertheless, we can write the closed-loop characteristic equation... 

A Routh-Hurwitz analysis can confirm that. The key point is that with cascade control, the system becomes more stable and allows us to use a larger proportional gain in the primary controller. The main reason is the much faster response (smaller time constant) of the actuator in the inner loop.2... 

To counter probable disturbances, we can take an even more proactive approach than cascade control, and use feedforward control. The idea is that if we can make measurements of disturbance changes, we can use this information and our knowledge of the process model to make proper adjustments in the manipulated variable before the disturbance has a chance to affect the controlled variable. 

In Section 10.1, the fuel gas flow rate is the manipulated variable (M) and cascade control is used to handle its fluctuations. Now, we consider also changes in the cold process stream flow rate as another disturbance (L). Let s presume further that we have derived diligently from heat and mass balances the corresponding transfer functions, GL and Gp, and we have the process model... 

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