Control cascading

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

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. 

However, over the years a number of slightly more complex structures have been developed that can, in some cases, significantly improve the performance of a control system. These structures include ratio control, cascade control, override control, etc. We will devote much of this chapter to these subjects. 

Propose a control system based on feed-forward - feedback control, cascade control and inferential control to achieve these control objectives. 

The resulting hierarchical control structure is represented schematically in Figure 5.2. Note that, while controller design proceeds in a bottom-up manner, starting from the fastest time scale, during the operation of the process there will exist a tight top-down interconnection via control cascades between the supervisory and regulatory layers. 

It is well known that there are closed loop locations which can not be reached by constant proportional control using less than full state feedback. The common approach in the case where proportional output feedback cannot yield a satisfactory design is to add an observer to the system. A similar but somewhat different approach is to use a dynamic controller. As an example, consider the control of a second order SISO plant by an ideal PID controller cascaded with a first order filter, which is... 

The carbolithiation of unactivated alkenes has also proven very successful for the synthesis of complex polycyclic systems. This has typically been achieved by reaction sequences utilizing an intramolecular carbolithiation process to generate a variety of carbocycles185 and heterocycles186. To achieve the intermolecular carbolithiation reaction required to initiate a controlled cascade reaction sequence for the generation of indole ring scaffold, Kessler and coworkers44 have expanded the synthetic utility of the styrene... 

Greg Shinskey (1988), over the course of a long and productive career at Foxboro, has proposed a number of advanced control" structures that permit improvements in dynamic performance. These schemes are not only effective, but they are simple to implement in basic control instrumentation. Liberal use should be made of ratio control, cascade control, override control, and valve-position (optimizing) control. These strategies are covered in most basic process control textbooks. 

Figure 7.2c illustrates how a variable-speed drive (a steam turbine) can be used to control throughput. The turbine is driven by high-pressure steam (600 psia) and discharges into a low-pressure steam header (25 psia). A flow controVspeed control cascade structure is used. The output signal from the flow controller adjusts the setpoint of the turbine speed controller, which manipulates the flow of high-pressure... 

A pilot-scale distillation column located at the University of Sydney, Australia is used as the case study [60]. The 12-tray distillation column separates a 36% mixture of ethanol and water. The following process variables are monitored temperatures at trays 12, 10, 8, 6, 4, and the reflux stream, bottom and top levels (condenser), and the flow rates of bottoms, feed, steam, distillate and reflux streams. The column is operated at atmospheric pressure using feedback control. Three variables are controlled during the operation top product temperature, condenser level, and bottom level. Temperature at tray 8 is considered as the inferential variable for top product composition. To maintain a desired product composition, PI controllers cascaded on flow were used to manipulate the reflux, top product and bottom product streams. 

Multiple loop control (cascade, selective, split range)... 

Single-loop controllers are usually the only backup required. Sometimes a cascade backup in which the computer adjusts the secondary loop through the direct digital controller mode of the primary is desirable. The backup modes include manual, local secondary-set, and local cascade-set. To assure complete backup synchronization, the primary controller tracks the secondary variable through a feedback connection whenever the cascade mode is deactivated. The result is procedureless, nonshift switching of on and off computer control. Cascade backup is required only on loops that can break up an entire system. 

Column pressure is controlled by adjusting the speed of the column compressor through a steam flow control-speed control-pressure control cascade system. Reflux is flow controlled. Reflux drum level sets distillate flow. Base level sets bottoms flow. 

The djmamic response of the Btu control may create instability. A step increase in inlet temperature will directly and linearly increase heat duty, but changes in flow and outlet temperature have a slower and more interactive effect as described earlier. Because of this problem, it has been recommended (362) to have the Btu controller cascade onto a flow controller rather than control the valve directly. 

The Fig. 19.86 system can be modified to become an improved version of the Fig. 19.8a system by cascading the composition controls onto the product ratio controls (68). If needed, an internal vapor flow control can be used to control the side-draw rate, and the bottom level controller cascaded onto the steam ratio control. 

Feedforward Control Cascade Control Set points are given by the sensed conditions upstream of the process. Chemical process control... 

Little, R. G. 2002. Controlling Cascading Failure Understanding the Vulnerabilities of Interconnected Infrastmc-tmes. Journal of Urban Technology 9(1) 109 -123. 

As indicated by the reaction steps (9), (10), and (11), oxidants, namely O ", H O, and OH are intermediate products of the enzymatically controlled cascade. Their reactivity and presnmable site of action can be assessed by physico-chemical parameters, such as standard reduction potential ( ) and half-life (tj j) of the given species (cf Table 4). 

MBC-Tool an environment facilitating the testing of conventional and advanced model based control strategies within Matlab. It includes feed-forward control, cascade control, delay compensation, IMC control as well as decoupling control techniques. It can be used for both single-input single output and multi input multiple output processes and allows the computation of different robustness measures to compare alternative designs. 

The same correction would be necessary if we change the MV of the controller, for example changing a primary controller cascaded to a flow controller so that it instead directly manipulates the control valve. This is often a quick fix if the secondary flow transmitter has a problem. We will show in Chapter 6 that adding a ratio-based feedforward can also require recalculation of Kc again because the effective range of the MV may be changed. 

It could be configured as a virtual analyser controller cascaded to the temperature controller. The approach needs some care since the input to the inferential is effectively the output of the temperature controller. It is only because we use the actual reboUer duty rather than its SP that introduces a lag that makes the cascade stable. 

The options for applying this correlation are equivalent to those for the bottoms composition controller. With care, it could be set up as composition controller cascaded to the temperature controller. Or a correction term could be incorporated into the temperature measurement. 

The most efficient operation of the compressor is at maximum capacity, therefore the compressor is on flow control (cascaded to speed of rotation) however, there is a protection against high pressures. 

If cascade control is involved, as, for example, liquid level control cascaded to flow control, the secondarv station not only has manual-automatic switching, but also another function— remote-local. In the remote position, the secondary controller set point comes from the output of the primary controller. In the local position, valve position may be set manually or the controller set point may be set by the operator ( local-auto ). Although cascade functions are sometimes combined into one station (or CRT faceplate ) for space and money-saving reasons, we recommend dual stations. Most single-station designs with which we are familiar are ver inflexible and complicated they do not permit ready implementation of feedforward, overrides, and so forth. 

If a smooth flow to the next step in the process is needed, a reflux drum, with averaging level control of distillate, should be employed. If the column top product is a vapor, takeoff should be by averaging pressure control. As an alternative, vapor may be taken off on flow control cascaded from top composition control while column pressure is controlled by heat input. 

Pumped-back reflux, proportional level control cascaded to reflux flow control, th 3—5 minutes, th is the level control time constant (see Section 3.10). 

Gravity-flow reflux, proportional level control cascaded to reflux flow control, Th 3—5 minutes. 

For maximum flow smoothing, we recommend a PI level controller cascaded to flow control. The PI level controllo- should have, as indicated by Figure 4.12, auto overrides to keep the liquid level between the level transmitter taps. Quantitative design is discussed in Chapter 16. 

A true cascade system (see Figure 12.3) has at least two controllers a primary or master one and a sectMidary or slave one, whose set point is normally set by the primary controller. An example would be column-top temperature control cascaded to reflux flow control. C5ur preferred scheme uses the secondary variable for external reset feedback to the primary ccxitroller. This gieady minimizes the need for manual—automatic switching—none is needed in the primary control station. 

Base Level Control Cascaded to Steam Flow Control... 

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