Process Control Functions
Pneumatic controllers and pneumatically driven control valves can generate and transmit very high forces to modulate large valves through very minimal and inexpensive tubing between the controller and the valve. Since there is no appreciable air flow between the sensor transmitter and the controller at steady state, the system is very accurate because pressure drop effects are nil. Process controllers compare a process variable such as a temperature from a temperature transmitter to a desired set point signal and move a proportional control valve such as a steam valve until the temperature equals the set point. In pneumatic control, a proportional 3 to 15 psi (pound per square inch) pneumatic signal communicates zero to 100 percent of full range. For a full temperature range of zero to 300 degrees F, 3 psi would correspond to zero degrees, 15 psi would indicate 300 degrees, and 9 psi, or halfway between 3 and 15 psi, would indicate 150 degrees F.
Proportional vs. On-Off Control
Continuous proportional control, as opposed to on-off control such as used in home thermostats, throttles a control valve proportionately somewhere between fully on and fully off instead of only on or off. This is the same type of control action used in automotive cruise control where the accelerator is continuously modulated to maintain a desired speed. Proportional control allows sustained precise control right at a desired set point instead of the hunting or oscillation around a set point that is characteristic of on-off systems.
Versatile Control Functions
Pneumatic controllers use opposing bellows or diaphragms along with adjustable timing functions to achieve needed mathematical control algorithms. The most common controller type is proportional plus integral plus derivative, commonly termed PID control. Proportional action provides an immediate proportional output response to a process upset, integral ramps to gradually re-establish equilibrium, and derivative adds anticipatory capability. These actions working together and outputting through the pneumatic pilot to the control valve enable the process variable to be closely controlled at set point when the system is properly tuned.
Tighter process control enables manufacturers to consistently make better products and achieve better business results. Practically, pneumatic controllers do not require electro-pneumatic conversion transducers between the controller and valve, simplifying the overall control system. Since simple pneumatic controllers are reliable and particularly cost-effective, they are also extensively used in heating ventilating and air-conditioning systems in large buildings and complexes.