Control configurations types

Shinskey [Chem. Eng. Prog., Vol. 72(5), 1976, p. 73, and Chem. Eng. Prog., Vol. 74(5), 1978, p. 43] proposed the use of a type of control configuration that he called valve position control. This strategy provides a very simple and effective method for achieving optimizing control. The basic idea is illustrated by several important applications. 

In practice, other types of decouplers and decoupling control configurations have been employed. For example, in partial decoupling, only a single decoupler is employed (i.e., either T12 or T21 in Fig. 8-40... 

The central element in any control loop is the process to be controlled. Therefore, the control objectives must be defined (e.g., maintain a desired outlet temperature and/or composition, maintain the level in a tank at a certain height, etc.). Once the control objective is specified, variables are measured in order to monitor the operational performance of the process (sensing element). Next, the input variables. that are to be manipulated are determined. Finally, after the control objectives, the possible measurements, and the available manipulated variables have been identified, the control configuration is defined. The control configuration is the information structure used to connect the available measurements to the available manipulated variables. The two general types of control configurations are feedback control and feedforward control. Details on feedback control are discussed below in this problem. Feedforward control is discussed in the next problem, CTR.2. 

Let us close this section by defining three general types of control configurations. 

What happens, though, if the disturbances cannot be measured None of the control configurations studied so far can be used to control an unmeasured process output in the presence of unmeasured disturbances. This is the type of control problems where inferential control is the only solution. Let us now examine the structure of an inferential control system. 

Later, we will study other types of control configurations, but for the time being we can define a control configuration (or control structure) as follows ... 

Although feedback control is the type encountered most commonly in chemical processes, it is not the only one. There exist situations where feedback control action is insufficient to produce the desired response of a given process. In such cases other control configurations are used, such as feedforward, ratio, multivariable, cascade, override, split range, and adaptive control. 

Shinskey [Chem. Eng. Prog. 72 5) 13, 1976 Chem.Eng. Prog. 74(5) 43,1978] proposed the use of a type of control configuration that he called valve position control. 

The first unmanned surveying robot, called GEOROBOT, was developed in 1982 (Kahmen et al 1984, Kahmen, Steudel 1989). It was used for slope monitoring in an open pit. Hundreds of targets had to be monitored to study the kinematic behaviour of the slopes. By now different types of surveying robots were developed by several companies. The closed loop control configuration of a surveying robot is shown in fig. 6.1. 

One type of decoupling control system for a 2 X 2 process and a 1-1/2-2 control configuration is shown in Fig. 18.9. Note that four controllers are used two conventional feedback controllers, and Gc2, plus two decouplers, T12 and T2i. The input signal to each decoupler is the output signal from a feedback controller. In Fig. 18.9, the transfer functions for the trans-... 

The various stages of this process depend critically on the type of gas, its pressure, and the configuration of the electrodes. (Their distance apart and their shapes control the size and shape of the applied electric field.) By controlling the various parameters, the discharge can be made to operate as a corona, a plasma, or an arc at atmospheric pressure. All three discharges can be used as ion sources in mass spectrometry. 

The process controller is the master of the process-control system. It accepts a set point and other inputs and generates an output or outputs that it computes from a rule or set of rules that are part of its internal configuration. The controller output seiwes as an input to another controller or, more often, as an input to a final control element. The final control element is the device that affects the flow in the piping system of the process. The final control element seiwes as an interface between the process controller and the process. Control valves and adjustable speed pumps are the principal types discussed. 

---