Slave control loop

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. 

Another variation on the manipulated reflux scheme is to add a cascade slave control loop for the reflux flow rate as shown in Figure 2.1. The temperature control loop then manipulates the setpoint for the slave reflux flow control loop. Similarly a slave flow control loop can be used for the distillate flow rate and another for the bottoms flow rate. 

This is in particnlar the case if the process contains two large time constants, of which one is eontained in the slave loop. This is schematically shown in Fig. 33.7. The slave control loop is shown with an ideal measurement (no measurement dynamics). 

When Ti = 10 minutes and = 9, the slave control loop can be replaced by a first-order transfer function with a time constant of 1 minute. The value of the gain K is usually limited by secondary effects, such as non-linearities and smaller time constants, which also play a role). 

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. 

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). 

Imperfections in feed-forward control can often be overcome by the addition of suitable feedback action. A typical design is shown in Fig. 7.70 where any variations in xd which occur bring the feedback control loop into action. The reflux flow is shown on flow control in cascade with the boiling temperature of the liquid at an appropriate point within the column. The inner (or slave) flow controller maintains... 

In selective and cascade control loops, external feedback is the most-often-applied solution. Here, instead of looking at its own output, which can be blocked, the integral mode of the controller looks at an external feedback signal (such as the opening of the valve), which cannot be blocked. In surge control or reactor heat-up applications, the chosen solution usually is to use the slave measurement as the external reset signal to prevent saturation. 

Cascade loops consist of two or more controllers in series and have only a single, independently adjustable set point, that of the primary (master) controller. The main value of having secondary (slave) controllers is that they act as the first line of defense against disturbances, preventing these upsets from entering and upsetting the primary process, because the cascade slave... 

Providing external reset for the cascade master from the slave measurement is always recommended. This guarantees bumpless transfer when the operator switches the loop from slave control to cascade control (Figure 2.45). The internal logic of the master controller algorithm is such that as long as its output signal (m) does not equal its external reset (ER), the value of m is set to be the sum of the ER and the proportional correction (Kc(e)) only. 

Adding a cascade slave to a fast loop can destabilize the primary if most of the process dynamics (time lags) are within the secondary loop. The most common example of this is using a valve positioner in a flow-control loop. The... 

In reactor temperature control applications, a slave controlling the jacket outlet temperature is recommended so that the dynamics of the jacket is transferred from the primary to the secondary loop. In temperature-on-temperature cascade systems, such as shown in Figure 2.45, the secondary controller should have little or no integral. 

One effective method of keeping the valve gain (GJ perfectly constant is to replace the valve with a linear flow control loop. The limitation of this cascade configuration (in addition to its higher cost) is that if the controlled process is faster than the speed of response of the flow loop, cycling will occur. This is because the slave—in this case, the flow control loop—in any... 

In heat exchanger applications, cascade loops are configured so that the master detects the process temperature and the slave detects a variable, such as steam pressure, that may upset the process temperature. The cascade loop, responds immediately and corrects for the effect of the upset before it can influence the process temperature. The cascade master adjusts the set point of the slave controller to assist in achieving this. Therefore, the slave must be much faster than the master. A rule of thumb is that the time constant of the primary controller should be ten times that of the secondary, or the period of oscillation of the primary should be three times that of the secondary. One of the quickest (and therefore best) cascade slaves is the simple and inexpensive pressure regulator. 

A pressure regulator is an ideal slave controller in a cascade loop. This is possible due to its speed of response. 

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. 

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. 

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. 

The rate of heat removed from the vessel is varied by manipulating the rate of solvent evaporated. The pressure and, thus, the evaporation rate are manipulated by varying the recycle rate of the stream exhausted from the steam ejector. A control loop that links the recycle valve position directly to the operating temperature would not permit compensation for short cycle variations in the steam supply pressure, permitting rapid swings in pressure and surface-solution temperature that could result in spontaneous nucleation. Therefore, a cascade configuration that uses the temperature measurement in a master loop and a pressure measurement in a slave loop is employed. 

The pressure of the vessel is controlled by a bypass valve that recirculates exhausted gas to the suction side of the vacuum source, giving the fast response that is required of the pressure loop to compensate for the varying vapor load to the condenser. Nevertheless, the contents temperature responds more slowly to pressure changes due to the time required to mix the surface with the vessel contents and the capacitance of the vessel. To decrease the response time, the contents temperature can be controlled by cascading the temperature to the pressure loop. The master temperature loop will then adjust the pressure set point at a rate commensurate with the temperature process response while maintaining the solution at the surface within the metastable zone the slave pressure loop will react to the pressure fluctuations during boiling. 

Using a TRC/TRC cascade control loop, assume that the output of the master TRC is Input 1 and the output of the slave TRC is Input 2. Suppose... 

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. 

In cascade control, at least two control loops exist the primary (slow or outer loop) and a secondary (fast or inner loop). The process is separated into two parts one part contains the external disturbances while the other part contains the long time constants. The slave or secondary loop is used to reduce the effects of supply-side disturbances. The master or primary controller senses the desired controlled variable. 

The slave robots are remotely controlled with the input devices of the master station. As previously explained, commercial systems to date do not allow force feedback and a simple position-control loop is used to pilot the system so that the position and orientation... 

The reason for the non-satisfying control performance of batch process units very often is the slave process that can have a more complex dynamics than the master loop has. As the slave process is determined by the mechanical construction, it is straightforward to design a model-based controller based on a nonlinear tendency model of the slave process. It has been shown that the parameters of the model-based slave controller (namely the parameters of the tendency model) can be easily determined by simple process experiments, and the complexity of the controller is comparable to that of a well furnished PID controller. Real-time control results showed that the proposed controller effectively handles the constraints (no windup) and gives superior control performance. 

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. 

In this configuration, the outer portions of IPMC serve as one sector (Sector 1) and are driven by input i(0> while the middle portion serves as a second sector (Sector 2) and is driven by input U2(t). Thus, toe IPMC comprises two controllable sectors. To control the performance of the sectored IPMC, toe feedforward architecture in Fig. 15 can be used. In this diagram, Ci(s) and C2 s) are the controllers and Gi(5) and G2(s) are their respective transfer functions associated with Sector 1 and Sector 2, respectively. Tstructure represents an open loop version of the master-slave control system. The advantage of the open... 

Design material-balance control loops to be at least a fector of 10 slower than related composition control loops. Similarly, in cascade systems, make the secondary or slave loop at least a factor of 10 faster than the master loop. 

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