Final control element action

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

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. 

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. 

With proportional control, the final control element has a definite position for each value of the measured variable. In other words, the output has a linear relationship with the input. Proportional band is the change in input required to produce a full range of change in the output due to the proportional control action. Or simply, it is the percent change of the input signal required to change the output signal from 0% to 100%. 

Derivative cannot be used alone as a control mode. This is because a steady-state input produces a zero output in a differentiator. If the differentiator were used as a controller, the input signal it would receive is the error signal. As just described, a steady-state error signal corresponds to any number of necessary output signals for the positioning of the final control element. Therefore, derivative action is combined with proportional action in a manner such that the proportional section output serves as the derivative section input. 

You must remember that these response curves are drawn assuming no corrective action is taken by the control system. In actuality, as soon as the output of the controller begins to reposition the final control element, the magnitude of the error should begin to decrease. Eventually, the controller will bring the error to zero and the controlled variable back to the setpoint. 

As indicated previously, the control action of the controller seeks to alter the position of the final control element in such a way as to minimise the error in the least possible time with the minimum disturbance to the system. The control action selected depends largely upon the dynamic behaviour of the other components in the control system. 

A setting knob is provided on the more traditional stand-alone controllers for the adjustment of Kc as well as a pointer to set the desired value. The setting knob for Kc is sometimes graduated in terms of proportional band. This quantity is defined as the error required to move the final control element over its whole range and is expressed as a percentage of the total range of the measured variable (see Example 7.1). In the newer MBC installations, the control action is simulated in the form of software and values of Kc and of the desired value are entered via an appropriate keyboard with the relevant values displayed on a VDU (Visual Display Unit)—see Section 7.19. 

Control action due to the derivative mode occurs only when the error is changing (equation 7.4). The presence of the derivative mode contributes an additional output, KD(de/dt), to the final control element as soon as there is any change in error. When the error ceases to change, derivative action no longer occurs (Fig. 7.8). The effect of this is similar to having a proportional controller with a high gain... 

Figure 15.3 shows a block diagram of a generalized feedback control system for the system shown in Figure 15.2. That is, this example has a controller, a final control element, a process, and a sensor, in that order, along with feedback of the measured value of the controlled variable to the controller. In addition, the example process is affected by disturbances. Note that the sensor reading, y, is compared with the setpoint, and the controller chooses control action based on this difference. The final control element is responsible for implementing changes in the level of the manipulated variable. The process for a control loop is only the part of the system that determines the value of the controlled variable from the inputs. The overall process can be based on a number of processing units.

To apply control to a process, one measures the controlled variable and compares it to the setpoint and, based on this comparison, typically uses the actuator to make adjustments to the flow rate of the manipulated variable. The industrial practice of process control is highly dependent upon the performance of the actuator system (final control element) and the sensor system as well as the controller. If either the final control element or the sensor is not performing satisfactorily, it can drastically affect control performance regardless of controller action. Each of these systems (i.e., the actuator, sensor, and controller) is made up of several separate components therefore, the improper design or application of these components, or an electrical or mechanical failure of one of them, can seriously affect the resulting performance of the entire control loop. The present description of these devices focuses on their control-relevant aspects. Later, troubleshooting approaches and control loop component failure modes are discussed. 

The discrete-time nature of a digital computer implies that when a computer is used to control a process, the control commands are given periodically as impulses at particular time instants and not continuously in time. Such sequence of control impulses cannot maintain a final control element continuously in operation. Thus a valve opens when a control impulse from the computer reaches the valve, but then it closes until the next control impulse arrives at the valve. Such control action is undesirable and the question is How can we construct a continuous signal from its discrete-time values ... 

ACTION - Refers to the action of a controller. It defines what is done to regulate the final control element to effect control. 

MODULATING CONTROL - A mode of automatic control in which the action of the final control element is proportional to the deviation, from set point, of the controlled medium. 

Depending on the degree of potential catastrophe, there usually is more than one safety interlock for a potential catastrophic event. Each of these safety interlocks including the sensor/transmitter, control function, and final control element are usually independent of the other safety interlocks for the same event. For maximum protection, each sensor for the same event should be unique to eliminate the potential of a common failure. The safety interlock must be fail-safe. This means that any loss of interlock power—electricity, air, hydraulics, etc.— loss of signal, must produce the same action as the safety interlock produces when it is activated (tripped). 

A final control element is a device that receives the manipulated variable from the controller as input and takes action that influences the process in the desired manner. In the process industries valves and pumps are the most common final control elements, because of the necessity to adjust a fluid flow rate such as coolant, steam, or the main process stream. 

Figure 5.50 shows the typical components found in a control loop. It can be seen that there are four main components involved process, measuring device, controller, and the final control element. The controller mechanism gets the value of the set point and directs the final control element to carry out the actions. Disturbances enter the system through the process, which affect the process variables. 

In this example, the liquid level inside a tank is controlled using a DPC (dilferential pressure cell). The DPC sends out the measured variable to the comparator, where it is compared with the set point. Based on the error, the control action is transmitted to the final control element (a control valve) in order to either increase or decrease the outflow rate of liquid from the tank, as shown in Figure 5.55. 

Not only because final elements contribute 50% of PFD share, but also final control elements are the key components of any control loop in any system, be it BPCS or SIS. Therefore selection of final elements needs special attention. Final element implements the action determined by the logic system. This final control element of interest to SIS is typically a pneumatically/hydraulicaUy actuated on-off valve operated by solenoid valve(s). But it could be other types also. It is important to keep in mind the applicability SIL assignment to final elements. When needed, assignment criteria may be applied. Using perspective of lEC 61508 and 61511 same assignment criteria could be fixed. 

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