Pneumatic switch control for pneumatic actuator in air conditioning control systems

Heating, ventilating and air conditioning control systems commonly use pneumatic actuators. A digital logic control has first and second output ports applied to control the actuator through dual channels, including an air supply channel and an air exhaust channel. Various sensed and related input signals are processed by the logic device and generate control signals at the control signal ports. Each channel includes an electrically controlled switch unit having an electrical operator and a pneumatic switch for controlling the flow to and from the pneumatic actuator. The switch may take the form of an electromagnetically actuated flow valve opening and closing a flow path from a supply or exhaust to the pneumatic actuator or, in the case of the supply channel, the form of a small electrically operated pump unit. The actuation of the actuator is controlled either on a time or frequency basis to set the actuator in the proper position. Pneumatic restrictor means in output lines to the actuator provide fluid rate control to and from the actuator.