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A solenoid actuator can be defined as an electromagnetic device that converts an electrical signal into a magnetic field. Solenoids are available in a variety of formats; the two more common types are the linear solenoid and the rotary solenoid.
In a Linear Solenoid (LS), electrical energy is converted into a mechanical pushing or pulling force or motion. Inside the LS, an electrical coil is wound around a cylindrical tube with a ferromagnetic actuator, called plunger. When electrical current flows through the coil, a magnetic field is instantaneously generated. The direction of this magnetic field is determined by the direction of the current flow within the wire. Thus, the coil becomes an electromagnet with its own north and south poles: in such a configuration, the coil behaves exactly as a permanent type magnet. The strength of the magnetic field can be increased or decreased by either controlling the amount of current flowing through the coil or by changing the number of turns (loops) of the coil windings.
When an electrical current is passed through the coil windings, the plunger, which is is free to move (to slide) in and out of the coil body, is attracted by the magnetic flux. Accordingly, the plunger translates towards the center of the coil body. Such translation results in the mechanical pushing or pulling force provided by the LS. The force and speed of the plunger movements is determined by the strength of the magnetic flux generated within the coil (which depends, as mentioned above, on the amount of electrical current). When the supply current is turned off (de-energized) the electromagnetic field collapses and the plunger is allowed to go back to its original rest position. This is usually achieved passively, by means of a return spring (a small compression spring attached to one end of the plunger itself). Both push- and pull-LS types are generally constructed the same with the difference being in the location of the return spring and design of the plunger. The back and forth movement of the plunger is known as the LS’s stroke, in other words the maximum distance the plunger can travel in either in or out direction. LSs can be used to electrically open doors and latches, open or close valves, move and operate robotic limbs and mechanisms, and even actuate electrical switches just by energizing its coil. LSs can also be designed for proportional motion control, were the plunger position is proportional to the input power.
Analogously, a Rotary Solenoid (RS) provides the rotational movement of the plunger in either clockwise, anti-clockwise or in both directions. The coil is wound around a steel frame with a magnetic disk connected to an output shaft. When the coil is energized, the electromagnetic field generates multiple north and south poles. These poles repel the adjacent permanent magnetic poles of the disk, causing the latter to rotate at an angle determined by the mechanical construction of the rotary solenoid. Therefore, the shaft rotation can be controlled by either energizing or de-energizing the RS, or by altering the position of the permanent magnet rotor.
Commonly available RSs have strokes of 25, 35, 45, 60 and 90 degrees, as well as multiple movements to and from a certain angle. RSs can be used to replace small DC motors or stepper motors were the angular movement is very small, with the angle of rotation being the angle moved from a desired start position to a specific end position. RSs are used in vending or gaming machines, valve control, camera shutter with special high speed.
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