TABLE OF CONTENTS Directional control valves are primarily used to control the direction of flow between the components of a pneumatic circuit. They have a small number of stable states, typically one, two or three that result from their binary input signals. Due to their internal resistance they also throttle the air flow, an effect that is usually not welcome.
Proportional valves have an unlimited number of stable states that are proportional to the analogue input signal, e.g. a voltage between - 10 V and + 10 V or a current between 4 mA and 20 mA. There are different types of proportional valves:
proportional directional control valves: the position of the valve spool is proportional to the electric input signal,
proportional pressure control valves: the output pressure is proportional to the electric input signal,
proportional flow control valves: the mass flow rate is proportional to the electric input signal.
Proportional flow control valves are typically used for paint spray systems, filling systems, fluid dispense or tank flow control, but not for drives; they are not further described. Proportional pressure control valves are used for pneumatic braking, air bearings or blow moulding. They are also applied to generate a proportional force for spot welding, web or paper tensioning or clamping. They are a special kind of pressure regulators and described in Chap. 14.3. Pressure control is commercially most advanced within electro-pneumatic control systems.
In the past, pneumatic drives were used to move a load from one fixed mechanical...
