Master control loop

For the same vacuum level, a crystallizing slurry will have a higher temperature than predicted for the pure solvent because the vapor pressure of the solvent is reduced by the presence of the solute (boiling point elevation). For adiabatic crystallization with the contents temperature as the input to the master control loop, the same temperature profile appropriate for crystallization by jacket cooling would apply here. However, the capability of the vacuum source and the line pressure drop should be considered in conjunction with the boiling point elevation to ensure that the desired final temperature can be met. If this is not satisfied, the desired yield may be achieved by removing some of the distillate, provided the saturation of an impurity is not reached. For most... 

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). 

Most of the autosamplers have a piston metering S3rrmge t)rpe pump to suck the preestablished sample volume into a line and then transfer it to the relatively large loop ( 100 ml) in a standard six-port valve. The simplest autosamplers utilize the special vials with pressuarization caps. A special plunger with a needle, push the cap down to the vial and displace the sample through the needle into the valve loop. Most of the autosamplers are microprocessor controlled and can serve as a master controller for the whole instrument... 

In order to achieve an accurate control of the internal reactor temperature, a cascade controller can be used. In this type of controller, temperature control is managed by two controllers arranged in cascade, that is, in two nested loops (Figure 9.14). The external loop, called the master, controls the temperature of the reaction mixture by delivering a set value to the slave, the inner loop, which controls the temperature of the heat carrier (Tc). 

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. 

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. 

Frequently process plants contain recycle streams and control loops, and the solution for the stream properties requires iterative calculations. Thus efficient numerical methods for convergence must be used. In addition, appropriate physical properties and thermodynamic data have to be retrieved fi om a data base. Finally, a master program must exist that links all the building blocks, physical property data, thermodynamic calculations, subroutines, and numerical subroutines, and that also supervises the information flow. You will find that optimization and economic anafy-sis are really the ultimate goal in the use of flowsheet codes. 

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... 

Using this value of in the slave loop, find the maximum closedloop-stable value of the master controller gain K/. Compare this with the ultimate gain found without cascade control in Problem 8.3. Also compare ultimate periods. 

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. 

For continuous systems, the flow rates of ingredients are controlled. Achievement of the desired composition requires ratio controllers in which all flow rates are tied to a single key component for feedforward control. In addition, flow rate setpoints may be remotely set in a cascade fashion from a master composition loop. 

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 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 the case of material balance control, the correcting actions of the quality controller on the product flow have no effect on the distillation column, i.e. the top product quality, until the level controller adjusts the reflux ratio. The controller should therefore be carefully tuned, such that the dynamics of the level control loop are reduced to a minimum. If material balance control is applied and the reflux drum is large, the power of control can be increased by keeping the ratio R/D constant with a flow ratio controller, which is adjusted by the level controller as a master controller. In that case the reflux is still the adjustable variable. 

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. 

The preceding technique works well for conventional single-loop controls and for secondary or slave loops in a cascade system. But for primary or master controllers, we do something different since the valve-loading signal is no longer meaningful for reset feedback. 

To eliminate reset windup, we break the master controller internal feedback as before, but now we use the secondary measurement for feedback as shown by Figure 9.7. If, for example, we have temperature cascaded to flow, we feed the output from the flow transmitter back into the master controller reset circuit. This means that during normal control the lags in the secondary control loop appear in the reset feedback circuit of the primary controller. If, as usual, Ak slave loop is much faster than the master loop, this technique will not r . >preciably increase the master controller reset time. 

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