The self-balancing M-A control station shown in Figure 34 has several controls and indicators that are basically the same as those shown on the M-A control station in Figure 32. However, there are some which differ. In addition, the controller does not require a balancing procedure prior to shifting from one mode (manual or automatic) to another.
Self-balancing describes a control station in which the non-operating mode output signal follows (tracks) the operating mode output signal. When the control station is in the automatic mode, the manual output signal will follow the automatic output signal. Once the controller is transferred to the manual mode, the output signal will remain at its previous value until one of the manual push buttons (discussed below) is depressed. Then, the output will vary. When the controller is in manual mode, the automatic output signal will track the manual output signal. Once the controller is transferred from the manual to automatic mode, the automatic output signal will initially remain at the manual mode value. If a deviation did exist in the manual mode, the automatic output signal would change slowly and return the controlled variable to setpoint. The deviation indicator, setpoint indicator, setpoint adjustment thumbwheel, and output meter of the controller in Figure 34 function essentially the same as those in Figure 32. The remaining controls and indicators are quite different. Therefore, each will be discussed.
Manual push buttons. These buttons are located below each end of the output meter and are used in the manual mode of operation. Buttons are labeled to indicate their effect on the final control element. The labels are “open-close” for valves and “slow-fast” for variable speed devices. The left push button decreases the output signal. The right push button increases the output signal. Either button can be depressed at two different positions, half-in and full-in. At the half-in position, the output signal changes slowly. At the full-in position, the output signal changes about ten times faster.
Mode indicating lights. Located directly below the manual push buttons, these lights indicate the operating mode of the controller. When in manual mode, the left light, labeled “M”, will be lit; when in the automatic mode, the right light, labeled “A”, will be lit.
Mode selection buttons. Located directly under each mode indicating light, each button will select its respective mode of control. If the button below the “M” mode light is depressed, the controller will be in the manual mode of operation; if the button below the “A” mode light is depressed, the controller will be in the automatic mode of operation.As previously discussed, a particular plant will probably have controllers different from the two described here. Although most information provided can be generally applied, it is extremely important that the operator know the specific plant’s controllers and their applications.
Final control elements are devices that complete the control loop. They link the output of the controlling elements with their processes. Some final control elements are designed for specific applications. For example, neutron-absorbing control rods of a reactor are specifically designed to regulate neutron-power level. However, the majority of final control elements are general application devices such as valves, dampers, pumps, and electric heaters. Valves and dampers have similar functions. Valves regulate flow rate of a liquid while dampers regulate flow of air and gases. Pumps, like valves, can be used to control flow of a fluid. Heaters are used to control temperature.
These devices can be arranged to provide a type of “on-off” control to maintain a variable between maximum and minimum values. This is accomplished by opening and shutting valves or dampers or energizing and de-energizing pumps or heaters. On the other hand, these devices can be modulated over a given operating band to provide a proportional control. This is accomplished by positioning valves or dampers, varying the speed of a pump, or regulating the current through electric heater. There are many options to a process control. Of the final control elements discussed, the most widely used in power plants are valves. Valves can be easily adapted to control liquid level in a tank, temperature of a heat exchanger, or flow rate.