•A solenoid is simply a specially designed electromagnet. A solenoidusually consists of a coil and a movable iron core called the armature. Here'show it works. When current flows through a wire, a magnetic field is set uparound the wire. If we make a coil of many turns of wire, this magnetic fieldbecomes many times stronger, flowing around the coil and through its center ina doughnut shape. When the coil of the solenoid is energized with current, thecore moves to increase the flux linkage by closing the air gap between thecores. The movable core is usually spring-loaded to allow the core to retractwhen the current is switched off. The force generated is approximatelyproportional to the square of the current and inversely proportional to thesquare of the length of the air gap.

•Solenoids are inexpensive, and their use is primarily limited toon-off                     applications such as latching, locking, and triggering. They arefrequently used in home appliances (e.g. washing machine valves), officeequipment (e.g. copy machines), automobiles (e.g. door latches and the startersolenoid), pinball machines (e.g., plungers and bumpers), and factoryautomation.

•       Applications

•    An electromechanical relay is a solenoid used tomake or break mechanical contact between electrical leads. A small voltageinput to the solenoid controls a potentially large current through the relaycontacts. Applications include power switches and electromechanical controlelements. A relay performs a function similar to a power transistor but has thecapability to switch extremely large currents if necessary. However,transistors have a much shorter switching time than relays.

•       As illustrated in figure 2, a voice coil consistsof a coil that moves in a magnetic field produced by a permanent magnet andintensified by an iron core. The force on the coil is directly proportional tothe current in the coil. The coil is usually attached to a movable load such asthe diaphragm of an audio speaker, the spool of a hydraulic proportional valve,or the read-write head of a computer disk drive. The linear response andbidirectional capability make voice coils more attractive than solenouds forcontrol applications.

•       (Material taken from Introduction to Mechatronics and MeasurementSystems, Histand & Alciatore McGraw-Hill 1999)

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