Continuing my look at final control elements -- those devices that obey the controller output and provide the muscle to modulate the process heat -- this month I'll discuss devices that use position control.
Control Valves. The flow of gas, air, oil, etc., depends on rotation for butterfly valves, ball valves and dampers, and linear travel for plug-and-seat valves. You can readily convert torque to linear thrust and vice versa.
The flow/position relationship often is crude in the face internal valve geometry, linkage design, friction, backlash, pressure and mixing method. To preserve control system gain -- and, therefore, stability -- try to ensure that flow (translating to delivery of heat) is reasonably proportional to the control signal. Bear in mind that some processes are very tolerant of low cost, crudely responding devices and still remain stable.
Valve Actuators. A spring-opposed pneumatic diaphragm actuator has high thrust, equal to the diaphragm area times the air pressure. Pressure (typically 3 to 15 psi, although it could be more) often comes from a current/pressure (I/P) converter whose 4 to 20 mA input comes from the temperature controller. For precise positioning, you can add a pneumatic or electropneumatic positioner.
Analyze the hazards of air pressure failure and loss of controller power. Process safety may demand air-to-close or air-to-open the valve, or current-to-close or current-to-open. Also, consider whether the controller should have upscale (break takes heat off) or downscale (break puts heat on) sensor burnout.
Electric Actuators. The constant speed, single-phase, reversing, induction motor actuator is common and economical. It has two windings connected together at a common terminal. One winding is energized for forward; the other winding is energized for reverse rotation. The non-common ends are joined by a capacitor so that the winding not energized receives a phase-shifted current when its partner is energized. This determines the direction of rotation. The stroke (rotation) is usually limited to 160o, adjustable down to 60o. The motor is geared down to a stroke travel time of typically 15 to 60 sec with torque about 150 lb-in (17N-m). Adjustable switches on the motor limit the travel at each end of the stroke.
In position mode (bounded control), a feedback potentiometer working with the controller's output signal makes a servo control loop within the overall temperature control loop. The motor shaft is positioned proportional to the control signal. Note that if the potentiometer fails or has a faulty wiper or wiring, control will fail. The feedback signal also can drive a meter or digital display showing shaft position, from which you infer valve position.
With velocity mode (boundless control), you can get good temperature control without the potentiometer and servo loop. The controller simply detects deviation from setpoint and pulses the motor in a time-proportioning mode to move the valve to minimize deviation. This makes the average velocity of travel proportional to temperature deviation, thus providing integral action in the control loop. You can still make good use of a potentiometer to show valve position to the operator and to let the controller put top and bottom limits on position. These settings are adjustable at the controller.
Manual Control. With position mode, the operator or an external signal can set the valve to any desired position. If this is a requirement at some stage of your process, you should go with position control. With velocity mode, the operator would have to press open and close buttons until he sees the valve come to the desired position. For added functionality, controllers now are available that work in position mode and switch to velocity mode if the potentiometer or wiring fails.
Motor Linkages. A butterfly valve would have a crank arm linked to one on the motor shaft. You can vary linkage geometry and adjust cam profiles to reduce nonlinearities of the angle/flow relationship and define the zero and span of stroke. Some zero/span adjustment can be done very handily at the controller.
Blower Speed Control. You can waste energy in an induction motor, blowing air and throttling it with a valve or damper. A variable speed AC drive can do the same job without valve.