Solenoids can be found in a wide range of equipment. In residential equipment, solenoids can be found in doorbells, washing machines, and kitchen appliances. Solenoids are commonly used in commercial and industrial control circuit applications such as hydraulics/ pneumatics, refrigeration, combustion, general-purpose controls, and pneumatic robotics.
Solenoid-operated valves typically control hydraulic and pneumatic equipment. A solenoid is used to move the valve spool that controls the flow of fluid (air or oil) in a directional control valve.
A directional control valve is a valve that is used to direct the flow of fluid throughout a fluid power system. Directional control valves are identified by the number of positions, number of ways, and type of actuator.
A manual directional control valve is placed in different positions to start, stop, or change the direction of fluid flow. See Figure 1. A position is the number of locations within the valve in which the spool can be placed to direct fluid through the valve. A directional control valve normally has two or three positions.
Figure 1. Positions are the number of locations within the valve in which the spool can be placed to direct fluid through the valve.
A way is a flow path through a valve. Most directional control valves are either two-way or three-way valves. The number of ways required depends on the application.
Two-way directional control valves have two main ports that allow or stop the flow of fluid. Two- way valves are used as shutoff, check, and quick-exhaust valves. See Figure 2.
Figure 2. A way is a flow path through a valve.
A manual directional control valve uses a handle to change the valve spool position. An electrical control valve uses an actuator to change the position of a valve spool. In an electrical control valve, the solenoid acts as the actuator. See Figure 3.
Figure 3. In an electrical control valve, the solenoid acts as the actuator.
Direct-acting, two-way valves are commonly used in a refrigeration system. Two-way (shutoff) valves have one inlet and one outlet pipe connection. These units may be constructed as normally open (NO), where the valve is open when de-energized and closed when energized; or they may be constructed as normally closed (NC), where the valve is closed when de-energized and open when energized. See Figure 4.
Figure 4. In a refrigeration system, direct-acting two-way valves may be constructed as normally open (NO), where the valve is open when de-energized and closed when energized; or they may be constructed as normally closed (NC), where the valve is closed when de-energized and open when energized.
A number of different solenoids may be used in a typical refrigeration system. The liquid line solenoid valves could be operated by two-wire or three-wire thermostats. The hot gas solenoid valve remains closed until the defrost cycle and then feeds the evaporator with hot gas for the defrost operation. See Figure 5.
Figure 5. Refrigeration systems may use different solenoids, such as liquid line solenoids and hot gas solenoids.
Solenoids may also be used in an oil-fired single- burner system. See Figure 6. The solenoids are crucial in the startup and normal operating functions of the system.
Figure 6. Different solenoids are used for the safe operation of an oil-fired single-burner system.
In addition to commercial and industrial use, solenoids are used for general-purpose applications. Typical general- purpose applications include products such as printing calculators, cameras, and airplanes. See Figure 7.
Figure 7. Solenoids are used for general-purpose applications, such as those in airplanes
Industrial robots are used in all kinds of applications from welding, painting, sorting, and assembling extremely small to extremely large parts. They can replicate human movement with the added advantage of being able to lift objects of almost any size or weight repeatedly in almost any type of environment.
Industrial robots use fluid power (hydraulic and/ or pneumatic) cylinders (linear motion), actuators (rotary motion), and grippers to provide the required power and movement. The cylinders, actuators, and grippers are controlled by solenoid-operated valves. See Figure 8.
In the robotic assembly example, cylinder 1 advances to move the arm out when the system starts. Rotary actuator 1 closes to grasp the part in the part feeder.
Cylinder 2 advances to move the part up and out of the part feeder. Cylinder 1 retracts to move the part away from the part feeder. Rotary actuator 2 rotates counter- clockwise to turn the part over. Cylinder 2 retracts to move the part over the sub-assembly. Rotary actuator 1 opens to release the part that drops into the sub-assembly. Rotary actuator 2 rotates clockwise to return the arm to the start position.
Figure 8. Pneumatic robots can be used to replicate human movement with the added advantage of being able to lift objects of almost any size and weight repeatedly.