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INDUSTRIAL CONTROL HANDBOOK - 2.3 AIR-POWER ACTUATORS AND SOLENOID-ACTUATED VALVES

2.3.1 Actuators

The actuators described so far have all been electrically driven. There is also a wide range of pneumatically driven actuators.

The actuator that is most frequently used in automation is pneumatically driven. This is the common air cylinder.

Air cylinders, shown in Figure 2-5, are extended and retracted by compressed air. They are specified by stroke length and cylinder inner diameter. The diameter limits the force available. Spring return and spring extend cylinders are available, but double-acting cylinders are more frequently used even though they require two air lines instead of one. The powered retraction feature means more dependable operation. Air cylinders can be purchased with magnets in the piston heads, so that reed switches attached to the outside of the cylinder can detect piston extension or retraction.

The piston rod of an air cylinder is often single-ended, so the extension force is greater than the retracting force due to the difference in the air-bearing surfaces on the piston head. If a cylinder has a double-ended piston rod, extension and retraction forces are the same. Both ends of the piston rod may be used to move a load or a sensor can be mounted on one end to detect piston rod position. Non-rotating piston rods are available, although delivery and seal life may not be as good.

Figure 2-3 Push and pull solenoids and force/displacement characteristicsFigure 2-3 Push and pull solenoids and force/displacement characteristics


Figure 2-4 Electromagnetic torque motorFigure 2-4 Electromagnetic torque motor

Figure 2-5 Air cylinders: (a) spring retracted and double-acting; (b) double-acting with one reed switch; (c) double-ended, with non-rotating piston rod; (d) C-section rodless; (e) magnetic rodless.Figure 2-5 Air cylinders: (a) spring retracted and double-acting; (b) double-acting with one reed switch; (c) double-ended, with non-rotating piston rod; (d) C-section rodless; (e) magnetic rodless. (Photographs by permission, Humphrey Products Company, Kalamazoo, Michigan.)

Cylinders can be purchased with an externally threaded cylinder head, or with a (usually rear) pivot mount, so that the cylinder can be rigidly fixed or allowed to pivot.

Air is compressible, which can lead to sluggishness and unpredictability problems. Pistons may stick when they should move, so air pressure builds up until they jump. Lubrication in a compressed air delivery system can help solve the problem, but lubricant delivery is never perfect. The problem becomes worse with large seal-bearing surfaces, so a good compromise is to select cylinders with the minimum bore, and then drive them with full available air pressure. One easily-corrected reason why cylinders may jump is that their air supply lines may be too small or are constricted, so air pressure builds up too slowly in the cylinder. If jumping is a problem or a safety hazard, the best solution is to use a flow control valve on the outlet port of the cylinder to limit the speed of the cylinder and provide lots of air pressure at the inlet port.

 

2.3.2 Valves

When an air-powered actuator is used in an electronically controlled system, the controllers output signals (usually DC) have to control air flow. Conversion of DC signals to pneumatic pressure or flow requires the use of a solenoid-controlled valve.

There are two types of simple air valves. One type, shown in Figure 2-7, is the poppet valve, which is an inexpensive valve for controlling air flow in a single direction through a single line. Such a valve would be adequate for controlling a venturi vacuum generator.

The other type, the spool valve shown in Figure 2-7 is usually purchased with three or with four ports for connecting air lines. One port is for pressurized air, another is for exhaust air and the remainders are for connecting actuators. The spools may be spring-return or detent.

Most two- and three-way valves have spring returns. The spring returns the spool to its normal position when the solenoid is not powered. Two-way valves open and close the air path between two ports. Three-way valves connect one air port to either of two other ports. A three-way valve could switch an actuator between supply air and an exhaust port. The actuator could be a spring-return cylinder. Spring-return three-way spool valves can be used as pressure-release safety valves: the valve must be powered to connect the air supply to the system, and if power fails it exhausts system pressure.

Figure 2-7 Air control valves: direct-acting ("poppet").Figure 2-7 Air control valves: direct-acting ("poppet"). (Photographs and diagrams by permission, Humphrey Products Company, Kalamazoo, Michigan.)


Figure 2-8 Air control valves: spring-return two- and three-way.Figure 2-8 Air control valves: spring-return two- and three-way. (Photographs and diagrams by permission, Humphrey Products Company, Kalamazoo, Michigan.)

 

2.3.3 Air Supply

Air-powered actuators require a source of compressed air or vacuum. Many manufacturing plants have compressors and a ready supply of compressed air. Experienced users of compressed air are aware that the air must be filtered to remove dirt that can jam the actuator or valve, regulated to not exceed the design pressure, and lubricated with a fine oil mist to reduce wear in the actuators. Complete filter-regulator-lubricator (FRL) units (see Figure 2-10) are available.


Figure 2-8 Air control valves: spring-return and double solenoid ("detent") four-way.Figure 2-8 Air control valves: spring-return and double solenoid ("detent") four-way. (Photographs and diagrams by permission, Humphrey Products Company, Kalamazoo/ Michigan.)


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