Primary loop, cascade control

Figures 16.2 and 16.4 clearly show that cascade control will effectively reduce the effects of pressure disturbances entering the secondary loop (i.e., D2 in Fig. 16.4). But what about the effects of disturbances such as Di, which enter the primary loop Cascade control can provide an improvement over conventional feedback control when both controllers are well tuned. The cascade arrangement will reduce the response times of the elements in the secondary loop, which will, in turn, affect the primary loop, but the improvement may be slight.

Cascade control was discussed qualitatively in Section 4.2. It employs two control loops the secondary (or slave ) loop receives its setpoint from the primary (or master ) loop. Cascade control is used to improve load rejection and performance by decreasing closedloop time constants. 

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. 

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). 

Example 10.1 Consider a simple cascade system as shown in Fig. 10.2a with a PI controller in the primary loop, and a proportional controller in the slave loop. For simplicity,... 

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... 

Cascade control was discussed quahtatively in Sec, 8.2. There is a secondary (or "slave ) loop and a primary (or master ) loop. Both load rejection and performance can sometimes be improved by using cascade control. 

It is useful to compare these values with those found for conventional control = 19.8 and oi. = 1.61. We can see that cascade control results in higher controller gain and smaller dosedloop time constant (the reciprocal of the frequency). Figure 11.26 gives a root locus plot for the primary controller with the secondary controller gain set at Two of the loci start at the complex poles s = rj which Come from the dosedloop secondary loop. The other curve... 

The reactor temperature controller (loop 2) is the primary controller, whereas the jacket temperature controller (loop 3) is the secondary controller. The advantage of the cascade control is that the reactor temperature control quickly reacts by the cascade system to disturbances in cooling fluid inlet conditions. The d3mamics of the transfer function G32 is faster than that of G 22-In the CSTR cascade control there are two control loops using two different measurements temperatures T and Tj, but only one manipulated variable Fj. The transfer function of the primary controller is the following ... 

In order for the cascade loop to be effective, the slave should be more responsive (faster) than the master. The slave s time constant should be one quarter to one tenth that of the master loop and the slave s period of oscillation should be one half to one third that of the master loop. The goal is to distribute the time constants between the inner (slave or secondary) and outer (master or primary) loops, while making sure that the largest time constant is not placed within the inner loop. When that occurs, such as in the case when the valve has a positioner (the slave) on a fast flow or liquid pressure controller (the master), stability will be sacrificed because the slave (valve)... 

An addition to the noted advantages is that the set point of the secondary controller can be limited. In addition, by speeding up the overall cascade loop response, the sensitivity of the primary process variable to process upsets is also reduced, whereas the secondary loop can reduce the effect of control valve sticking or actuator nonlinearity. The primary or outer control loop of a cascade system is usually a PI or PID controller. A properly selected secondary will reduce the proportional band of the primary controller. 

Flow as a secondary cannot only overcome the effects of valve hysteresis, but also insures that line pressure variations or badly selected valve characteristics will not affect the primary loop. For these reasons, in composition control systems, flow is usually set in cascade. Cascade flow loops are also useful in feedforward systems. Flow controllers invariably have both proportional and integral modes. If their proportional band exceeds 100%, they must have an integral mode. 

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. 

Since high-purity products are usually produced only in situations where the separation is relatively easy, most of these columns have fairly large temperature gradients. Therefore it is possible to use tem-perature/composition cascade control systems. The secondary temperature controller serves as a fast loop to detect disturbances quickly and hold the temperature profile in the column. The primary composition... 

Cascade control is one solution to this problem (see Fig. 8-35). Here the jacket temperature is measured, and an error signal is sent from this point to the coolant control valve this reduces coolant flow, maintaining the heat transfer rate to the reactor at a constant level and rejecting the disturbance. The cascade control configuration will also adjust the setting of the coolant control valve when an error occurs in reactor temperature. The cascade control scheme shown in Fig. 8-35 contains two controllers. The primary controller is the reactor temperature coolant temperature controller. It measures the reactor temperature, compares it to the set point, and computes an output, which is the set point for the coolant flow rate controller. This secondary controller compares the set point to the coolant temperature measurement and adjusts the valve. The principal advantage of cascade control is that the secondary measurement (jacket temperature) is located closer to a potential disturbance in order to improve the closed-loop response. 

Cascade control significantly reduces the effect of certain types of disturbances by applying two control loops in tandem, i.e., the output of one controller is the setpoint for the other controller. The secondary or slave controller receives its setpoint from the primary or master controller and operates on a much faster cycle time than the primary. As a result, the secondary controller can eliminate certain disturbances before they are able to affect the primary control loop. 

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... 

Cascade control discusses the properties of the commonly used cascade flow loop, as well as the use of temperature measurement in the secondary loop and how to protect the primary controller against windup. 

The three major types of process heater cascade control systems are shown in Figures 10-2, 10-3, and 10-4. Each system has a primary and a secondary loop. The secondeiry loop detects and acts on as many upsets as possible so that the primary loop has the least disturbances. 

The tuning procedure for a cascade control system is to tune the secondary controller first and then tune the primary controller with the secondary controller on automatic. As for the types of controller used, we often use a proportional controller in the secondary loop. Since it has only one tuning parameter, it is easy to tune. There is no need for integral action in the secondary controller because we don t care if there is offset in this loop. If we use a PI primary controller, the offset in the primary loop will be eliminated, which is our control objective. 

EXAMPLE 9.1. Consider the process with a series cascade control system sketched in Fig. 9.le. A typical example is a secondary loop in which the flow rate of condensate from a flooded reboiler is the manipulated variable M, the secondary variable is the flow rate of steam to the reboiler, and the primary variable is the temperature in a distillation column. We assume that the secondary controller Gci and the primary controller Gci are both proportional only. 

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