CONTROL METHODS
GENERAL
An automatic control system is classified by the type of energy transmission and the type of control signal (analog or digital) it uses to perform its functions.
The most common forms of energy for automatic control systems are electricity and compressed air. Systems may comprise one or both forms of energy.
Systems that use electrical energy are electromechanical, electronic, or microprocessor controlled. Pneumatic control systems use varying air pressure from the sensor as input to a controller, which in turn produces a pneumatic output signal to a final control element. Pneumatic, electromechanical, and electronic systems perform limited, predetermined control functions and sequences. Microprocessor-based controllers use digital control for a wide variety of control sequences.
Self-powered systems are a comparatively minor but still important type of control. These systems use the power of the measured variable to induce the necessary corrective action. For example, temperature changes at a sensor cause pressure or volume changes that are applied directly to the diaphragm or bellows in the valve or damper actuator.
Many complete control systems use a combination of the above categories. An example of a combined system is the control system for an air handler that includes electric on/off control of the fan and pneumatic control for the heating and cooling coils.
Various control methods are described in the following sections of this manual:
— Pneumatic Control Fundamentals.
— Electric Control Fundamentals.
— Electronic Control Fundamentals.
— Microprocessor-Based/DDC Fundamental.
See CHARACTERISTICS AND ATTRIBUTES OF CONTROL METHODS.