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Process control systems serve two different purposes Generally the first of these purposes is to effect a change in a certain output variable commonly encountered in the startup of a process The second of these is to regulate or hold an output variable constant despite any changes in an input variable which usually cannot be easily managed 1 In order to discuss process control in detail it is necessary to define several terms A control loop is comprised of several variables Among these are manipulated variables MV process variables PV and disturbance variables DV Manipulated variables refer to variables that are easily controlled such as stream flowrates Process variables are those that are desired to be set at a certain datum level commonly a desired temperature Disturbance variables create deviations in the process variable from the desired datum level or set point SP The controller receives process variable information and in turn attempts to maintain the process variable at the set point In particular there are two types of control systems feedforward and feedback control Feedback control operates by feeding back process variable data to the controller In feedback process control the controller receives process variable data and makes appropriate changes in the manipulated variable via the final control element FCE The uniqueness of feedback control is that it utilizes prevailing process variable information in order to determine what measures need to be taken to return the process variable to the set point 2 Figure I illustrates a typical feedback control loop The modus operandi of a feedfoward controller is that the controller receives a direct signal of the disturbance variable Here the disturbance variable is measured directly rather than the output variable that is desired to be regulated The disturbance variable is measured before the process is perturbed and the controller endeavors to neutralize the disturbances effects usually by some prescribed process model The controller does not
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