Control element, final

The signal from the controller is used to activate a final control element. In the chemical industry the control element is almost always a valve which controls a flowrate. Changes in flowrate can be used indirectly to change any of the other variables listed in Table 7.5. 

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G = current-to-pressure transducer(I/P) H = actuator and I = final control element. Some devices combine A, B, and C, and (--) represents the line... 

Once the desired control action has been transformed to an analogue signal, it is transmitted to the final control element over the transmission lines. However, the final control element s actuator may require a different type of signal and thus another transducer may be necessary. Many control valve actuators utilise a pressure signal so a current-to-pressure (I/P) transducer is used to provide a pressure signal to the actuator. 

Control Valves and Other Final Control Elements. Good control at any hierarchial level requites good performance by the final control elements in the next lower level. At the higher control levels, the final control element may be a control appHcation at the next lower control level. However, the control command must ultimately affect the process through the final control elements at the regulatory control level, eg, control valves. 

George W, Gassmari/ B S M E / Senior Re.search Specialist, Final Control Sy.stems, Fisher Controls International, Inc., Marshalltown, lA. (Controllers, Final Control Elements, and Regulators)... 

Selective and Override Control When there are more controlled variables than manipulated variables, a common solution to this problem is to use a selector to choose the appropriate process variable from among a number of available measurements. Selec tors can be based on either multiple measurement points, multiple final control elements, or multiple controllers, as discussed below. Selectors are used to improve the control system performance as well as to protect equipment from unsafe operating conditions. 

Other types of selective systems employ multiple final control elements or multiple controllers. In some applications, several manipulated variables are used to control a single process variable (also called split-range control). Typical examples include the adjustment of both inflow and outflow from a chemic reactor in order to control reactor pressure or the use of both acid and base to control pH in waste-water treatment. In this approach, the selector chooses from several controller outputs which final control element should be adjusted (Marlin, Process Control, McGraw-Hill, New York, 1995). 

Figure 8-41 includes two conventional feedback controllers G i controls Cl by manipulating Mi, and G o controls C9 by manipidating Mo. The output sign s from the feedback controllers serve as input signals to the two decouplers D o and D91. The block diagram is in a simplified form because the load variables and transfer functions for the final control elements and sensors have been omitted. 

External control of the process is achieved by devices that are specially designed, selected and configured for the intended process-control application. The text below covers three very common function classifications of process-control devices controllers, final control elements, and regufators. 

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. 

Regulators, though not controllers or final control elements, perform the combined function of these two devices (controller and final control element) along with the measurement function commonly associated with the process variable transmitter. The uniqueness, control performance, and widespread usage of the regulator make it deseivang of a functional grouping of its own. 

The resulting motion of the beam is detected by the pneumatic nozzle amphfier, which, by proper sizing of the nozzle and fixed orifice diameters, causes the pressure internal to the nozzle to rise and fall with vertical beam motion. The internal nozzle pressure is routed to the pneumatic relay. The relay, which is constructed like the booster relay described in the Valve Control Devices subsection, has a direct hnear input-to-output pressure characteristic. The output of the relay is the controller s output and is piped away to the final control element. 

Positioner Application Positioners are widelv used on pneumatic valve actuators, VIore often than not, thev provide improved process-loop control because thev reduce valve-related nonlinearitv, Dvnarnicallv, positioners maintain their abilitv to improve control-valve performance for sinusoidal input frequencies up to about one half of the positioner bandwidth. At input frequencies greater than this, the attenuation in the positioner amplifier netvv ork gets large, and valve nonlinearitv begins to affect final control-element performance more significantlv. Because of this, the most successful use of the positioner occurs when the positioner-response bandwidth is greater than twice that of the most dominant time lag in the process loop. 

In a process loop with a pneumatic controller and a large process time constant. Here the process time constant is dominant, and the positioner will improve the linearitv of the final control element, Some common processes with large time constants that benefit from positioner application are liquid level, temperature, large volume gas pressure, and mixing,... 

A regulator is a compact device that maintains the process variable at a specific value in spite of disturbances in load flow. It combines the functions of the measurement sensor, controher, and final control element into one self-contained device. Regulators are available to control pressure, differential pressure, temperature, flow, hquid level, and other basic process variables. They are used to control the differential across a filter press, heat exchanger, or orifice plate. Regulators are used for monitoring pressure variables for redundancy, flow check, and liquid surge relief. 

An interlock is a protec tive response initiated on the detection of a process hazard. The interlock system consists of the measurement devices, logic solvers, and final control elements that recognize the hazard and initiate an appropriate response. Most interlocks consist of one or more logic conditions that detect out-of-hmit process conditions and respond by driving the final control elements to the safe states. For example, one must specify that a valve fails open or fails closed. 

These tests must encompass the complete interlock system, from the measurement devices through the final control elements. Merely simulating inputs and checking the outputs is not sufficient. The tests must duplicate the process conditions and operating environments as closely as possible. The measurement devices and final control elements are exposed to process and ambient conditions and thus are usually the most hkely to fail. Valves that remain in the same position for extended periods of time may stick in that position and not operate when needed. The easiest component to test is the logic however, this is the least hkely to fail. 

Proportional-action governor is a governor with inherent regulation and a continuous hnear relation between the input (speed change) and the output of the final control element, the governing valve. 

Final Control Element-A device that directly changes the value of the variable used to control a process condition. 

Final control element. Any device that changes the value of a manipulated variable, such as a damper. 

Proportional band In a proportional controller, the control point range through which the controlled variable must pass in order to move the final control element through its full operating range. 

Proportional control A control algorithm in which the final control element moves to a position proportional to the deviation of the value of the controlled variable from the set point. 

To be able to control implies that there is some means of manipulating a variable. The element that makes the change in the variable is called the final control element. This is a valve, switch, or other item that is activated by the controller in order to maintain the measured variable at some desired value or within some set limits. For instance, the level in Figure 7-1 is controlled by opening and closing a valve that changes the flow rate out of the tank. The valve is the final control element. 

When a control system is being used, the designer must indicate the transmission lines that will connect the measuring element to the final control element. The notation for these transmission lines is given in Table 7-3. The final control element has the same code letters and numbers as the controller that regulates it, plus an additional letter to indicate whether it is a valve, switch, or other device. 

Figure 17 Combined Controller and Final Control Element Action. 25...

A measurement element An error detection element A final control element... 

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