Cascade controls

In control situations with more then one measured variable but only one manipulated variable, it is advantageous to use control loops for each measured variable in a master-slave relationship. In this, the output of the primary controller is usually used as a set point for the slave or secondary loop. 

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. 

Cascade control or the temperature of the outlet stream from a process heater 

from Fig. 7.67b, the primary closed-loop transfer function is  

Usually the dynamics of the secondary loop are sufficiently faster than those of the primary loop for G / (s) to be approximated by its steady-state gain. For the same reason it is possible to tune the cascade system by tuning first the inner loop and then the outer loop. 

Cascade control also removes any control valve issues from the primary controller. If the valve characteristic is nonlinear, the positioner poorly calibrated or subject to minor mechanical problems, all will be dealt with by the secondary controller. This helps considerably when tuning the primary controller. 

Cascade control should not normally be employed if the secondary cannot act more quickly than the primary. Imagine there is a problem with the flow meter in that it does not detect the change in flow for some time. If, during this period, the temperature controller has dealt with the upset then the flow controller will make an unnecessary correction when its measurement does change. This can make the scheme unstable. 

Cascade control is also widely used in the chemical process industries and especially in cases where there may be nonlinear behavior in the dynamics of the con- 

The output signal of the primary (frequently referred to as the master ) reactor temperature control loop serves as the set point of the secondary (frequently referred to as the slave ) reactor feed temperature control loop. 

The secondary controller responds rapidly to any temperature disturbances in the reactor feed line and provides improved reactor temperature control. As discussed in Ref. 7, a similar cascade control scheme can be implemented in the case where the reactor is jacketed and the reactor temperature is controlled by manipulating the cooling medium (typically water) inlet stream. In this case the additional measurement is the temperature of the jacket, which is compared with a set point provided by the master reactor temperature controller. The resulting error signal is the input to the controller for the cooling water makeup. 

It is obvious from the preceding discussion that for a cascade control system to function effectively, the secondary control loop must be selected and tuned so as to have a faster response than the primary controller. According to Ref 7, the secondary controller is normally a P or PI controller with derivative action hardly used. The primary controller is typically a PI or PID controller. 

A cascade control strucmre features a primary controller and a secondary controller. The OP signal of the primary controller is the SP signal of the secondary controller. 

In this chapter we have attempted to present many of the useful features and tools available in Aspen Dynamics. Detailed step-by-step procedures have been given for performing the various tasks required to run a dynamic simulation. These methods will be applied in the dynamic simulations reported in subsequent chapters. 

Design and Control of Distillation Systems for Separating Azeotropes. By William L. Luyben and I-Lung Chien Copyright 2010 John Wiley Sons, Inc. 

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. 

There are two purposes for cascade control (1) to eliminate the effects of some disturbances, and (2) to improve the dynamic performance of the control loop. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

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. 

This system also is a good illustration of the improvement in dynamic performance that cascade control can provide in some systems. As we will show quantitatively in Chap. 11, the closedloop time constant of the reactor temperature will be smaller when the cascade system is used than when reactor temperature sets the cooling water makeup valve directly. Therefore performance has been improved by using cascade control. 

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. 

In cascade control, we therefore have two control loops using two different measurements but sharing a common manipulated variable. The loop that measures the controlled variable (in the example, the reacting mixture temperature) is the dominant, or primary control loop (also referred to as the master loop) and uses a set point supplied by the operator, while the loop that measures the second variable (in the example, the cooling water temperature) is called the secondary (or slave) loop and uses the output from the primary controller as its set point. Cascade control is very common in chemical processes and the major benefit to be gained is that disturbances arising within the secondary loop are corrected by the secondary controller before they can affect the value of the primary controlled output. 

If there are any differences it then adjusts the steam valve. If the downstream pressure changes, a correction in the control valve is made immediately, instead of waiting for a product temperature change. Should the output temperature of the process stream rise, this would cause a set point change of the steam-pressure controller, which would cause a decrease in the steam pressure in the heat exchanger. Cascade control is very useful when the variation in the quality of a utility or other manipulable stream can cause deviations from the desired output. 

When close control is desired, usually the variable that is to be closely controlled is monitored and no changes are made until the measurement differs from what is desired. This is feedback control. It obviously is not an ideal system, since the controller can only react to changes. A better system would be one that anticipates a change and takes corrective action that ensures an unvarying output. This is a feedforward control system. This type of control is very advantageous when the input variables have a wide range of variation. 

Source Friedman, P.G., Moore, J.A. Tor Process Control Select the Key Variables, Chemical Engineering, June 12, 1972, p. 90. 

The neutralizer in the previous example might be controlled differently if the main fluctuation in the load occurs in one or two of the streams. Instead of combining all the streams together before they enter the neutralizer, those streams that vary widely might enter an additional holding tank, where they would be neutralized using traditional feedback control. They would then be added to the main neutralizer, which also has a feedback controller. Which system is best can be determined by running an economic analysis (see Chapters 10 and 11). 

Both of the control schemes for the neutralizer took measurements on the major varying streams before they were diluted in the large blending tank. This is usually desirable because once the streams are mixed the measurable differences are smaller, and the possibility of noise (the equivalent of static in radio signals) affecting the measurement accuracy is greater. 

Let us now consider another possible type of upset a supply side upset. If the steam Fs, is coming from a supply header that is also servicing other users, then there is a possibility that, as the other users needs vary, pressure upsets will occur and cause changes in the steam supply Fs. Suppose that another user demanded steam quantities that caused a pressure drop in the header, thus resulting in a drop in steam flow to the exchanger. The only way this drop in Fs could be measured would be as a drop in the 

In both cases the process variable T2 would dampen out in the period t . However, for the supply upset situation the feedback control may cause the temperature to be in a constant state of flux. For instance, if the valve (when open to 50 per cent) supplies the amount of steam needed, then the outlet temperature will be at the set point. However, then the flow through the valve is a function of the pressure drop across it. Therefore, if 

With cascade control in place, if a set-point change is made in the temperature controller, or if a load upset occurs that changes Fy, then the output of the temperature controller will change the steam flow controller set point. The flow loop operates so much faster than the temperature loop that the temperature controller does not in fact know whether its output is going directly to a valve or as a set point to another controller. In general, the control loop closest to the controlled variable is called the 

Both the primary and secondary loops have their own response period, independent of whether they are in a cascade configuration or not. The response period of the primary loop is ri and that of the secondary loop is T2. In order for cascade control to work effectively, r i 4t2. What this rule of thumb implies is that the primary loop should never know that there is a secondary loop. The secondary loop should be able to respond as quickly as the FCE. If this rule is followed, then there will be httle interaction between the two loops and the control scheme will function properly. 

1 Place the primary controller in manual or the secondary controller to the local set point. This will break the cascade and allow the secondary controller to be tuned. 

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Cascade control Cascade cycles Cascading Cascara sagrada Cascarine CA Search... 

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. 

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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