Valve Groups and Air Connections
The figure below shows directional solenoid valves bundled into a single manifold, with the intake and exhaust ports of the individual valves interconnected. Dual-coil and single-coil solenoid valves in this assembly are referred to as a valve group. When connecting the pneumatic lines, only one supply tube is required to feed the intake port.
Coil Wiring and Control
The solenoid coil has two wires: a red wire connected to the DC 24V positive of the power supply and a blue wire connected to the DC negative. The red indicator LED inside the solenoid is polarized; if the wires are reversed, the indicator will not light, although the valve can still operate. In practical control circuits, solenoid valves can be driven by various relays or by PLC output terminals.
Single-Acting vs Double-Acting Solenoid Valves
Solenoid valves are categorized as single-acting and double-acting. A single-acting solenoid valve is used to control movement in one direction of a cylinder and enables extension and retraction. A double-acting solenoid valve can control both inlet and outlet of a cylinder, also enabling extension and retraction. On sorting production lines, pneumatic grippers typically use double-acting valves, while valves used for sorting on conveyor belts are often single-acting.
Key Differences
The main difference is that a single-acting valve’s initial position can be arbitrary, allowing it to switch between two positions as needed. A double-acting valve has a fixed initial position and controls movement in a single defined direction.
Operating Principle
Using a pneumatic gripper as an example, the operation of a solenoid valve can be described as follows. For a single-acting solenoid valve: when the coil is energized, its internal normally-open contact closes and the 24V supply energizes the air valve. The valve changes from closed to open, allowing high-pressure air to flow through the tubing into the pneumatic gripper, causing it to clamp. When the solenoid is de-energized, the internal valve closes under spring force and the gripper opens. Thus, a single-acting solenoid valve uses coil energization and de-energization to open and close the pneumatic line, and it has two pneumatic lines: an inlet and an outlet, as shown below.
Double-Acting Control and Practical Use
When a gripper is controlled by double-acting solenoid valves, one valve is used for clamping and another for releasing. This arrangement requires two sets of four pneumatic tubes to provide inlet and outlet paths. When controlling double-acting valves, it is important to avoid energizing both valves simultaneously.
Applications
Solenoid valves are widely used in hydraulic and pneumatic control systems. For example, automated assembly lines and hydraulic control circuits use large numbers of solenoid valves to control the flow of gases and liquids. When gas is used as the transmission medium, a compressor supplies the high-pressure gas, such as with an air compressor.
