Solenoids For Security And Transportation

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I thought that this week, we should talk about solenoids and solenoid valves as they have wide use within industry, commerce, transportation and general security.  This was prompted by an unpleasant memory that involved them some years ago.

I remember once finding myself and a colleague  trapped inside an airport fire station storeroom at night once when the power failed.  That is not a pleasant experience I can tell you as we couldn’t get out of the door we came in.  Luckily we had a glim keyring torch between us and found the building’s MCB distribution board and restored the power.  I was very puzzled why the door wouldn’t open as it should.  On subsequent investigation, I noticed that the personnel access door had a solenoid shoot bolt that failed safe closed when the power went off.  Good for perimeter security but no good for people trying to get out.

So what is a solenoid?  A solenoid is a form of transducer that converts electromagnetic energy into linear mechanical movement.  More particularly, the vast majority of solenoids use a spring-loaded steel (or other magnetic material) plunger that is attracted into a toroidal coil when energised.  When the coil is de-energised, the plunger returns to its rest position.

With such a simple action, it is important that the design closely matches the duty.  For example, the holding force to keep the plunger in only whilst the coil is energised, should sufficiently exceed the return force from the spring to ensure rapid responses for the full cycle but not to cause other problems such as bouncing.   Also, by suddenly cutting off the power to the coil, this induces a sudden voltage surge which will delay the return drop out time.  In the same way,  This surge does assist the other half of the cycle on energisation to improve the pull-in time.

Often, a high value resistor, varister or diode  is incorporated in series with the  coil to dampen this surge.  This however has the effect of increasing power consumption and/or drop out time.

Solenoids can be either AC or DC energised and they work in the same way although the application design will vary considerably.  DC Solenoids are normally used for high frequency operations such as are found used as actuators for 2 or 3 way pneumatic and hydraulic valves on trucks and manufacturing production lines.  AC solenoids are normally found in the security industry where low frequency and low cost but high actuation forces are required.

Another factor is temperature.  DC solenoids’ power requirements are not influenced by the position of the plunger (or armature as it is sometimes called) whereas the AC solenoids are.   AC solenoids are thus limited by the frequency of operation to avoid overheating them, especially enclosed to IP65 to IP68 ones for wet or difficult environments.

Al this tells us that the design of a solenoid is a balance to optimise cycle times whilst reducing the energy used to keep the solenoid energised for typically, long periods.  For this reason the range of solenoids and solenoid valves available is large such as those by industry leader Penny and Giles.   These electromechanical solenoids are therefore used for an eclectic mix of applications such as paintball machines, dot-matrix printers, fuel injectors, automotive air systems and many manufacturing processes.

Latching solenoids are an interesting variation on the simple spring-loaded push-pull solenoid and these use  permanent magnets in conjunction with the solenoid coil to maintain the position of the armature with no current applied.  The solenoid coil is therefore used to electromagnetically move the armature only with a burst of current and it will otherwise remain in either set energised position even if there is a power failure.

The latching solenoid is thus used for automatic door closers, latching mechanisms and other locks, security devises and battery powered equipment.

Another type of of electro-mechanical solenoid is the rotary voice coil in which the permanent magnet stator surrounds a coil, in which when energised,  moves through an arc.  One of the most prolific of applications is for PC disc drives.  It can move bi-directionally.

The principles behind a solenoid action are very simple, but the applications are varied.  There are other non-electrical solenoids that are about using hydraulic and pneumatic power.  These and the fascinating world of fluidics will be the subject of a future post, so keep visiting.

Au revoir.