Magnetic lock

Frbiz Site


History


An electromagnetic lock, also known as magnetic lock, was patented on May 2, 1989, by Arthur, Richard and David Geringer of Security Door Controls, an access control hardware manufacturing firm. The device outlined in their designs was the same in principle as the modern magnetic lock consisting of an electromagnet and armature plate. The patent did not make any reference to the manufacturing methods of the electromagnet and detailed several variations on the design including one that used a spring-loaded armature plate to bring the armature plate closer to the electromagnet. The patent expired on May 2, 2009.


This device was a 'shear' magnetic lock, as opposed to the original (and now ubiquitous) 'direct pull' electromagnetic locks and was an improvement on a 1984 patent cited in the same document.
embossed aluminum


Operation
corrugated metal sheets


The magnetic lock relies upon some of the basic concepts of electromagnetism. Essentially it consists of an electromagnet attracting a conductor with a force sufficiently large enough to prevent the door from being opened. In more detailed examination, the device makes use of the fact that a current through one or more loops of wire (known as a solenoid) produces a magnetic field. This works in free space, but if the solenoid is wrapped around a ferromagnetic core such as soft iron the effect of the field is greatly amplified. This is because the internal magnetic domains of the material align with each other to greatly enhance the magnetic flux density.


Equations


Using the Biot-Savart law, it can be shown that the magnetic flux density B induced by a solenoid of effective length l with a current I through N loops is given by the equation:


The force F between the electromagnet and the armature plate with surface area S exposed to the electromagnet is given by the equation:


In both equations, 0 represents the permeability of free space and r represents the relative permeability of the core.


Although the actual performance of a magnetic lock may differ substantially due to various losses (such as flux leakage between the electromagnet and the conductor), the equations give a good insight into what is necessary to produce a strong magnetic lock. For example the force of the lock is proportional to the square of the relative permeability of the magnetic core. Given the relative permeability of a material can vary from around 250 for cobalt to around 5000 for soft iron and 7000 for silicon-iron the choice of magnetic core can therefore have an important impact upon the strength of a magnetic lock. Also relevant is the choice of current, number of loops and effective length of the electromagnet.


Technical comparison


Magnetic locks possess a number of advantages over conventional locks and electric strikes. For example, their durability and quick operation can make them valuable in a high-traffic office environment where electronic authentication is necessary. Nevertheless in a number of applications economic or security reasons can lead to conventional locks or electric strikes being a better choice.


Advantages


Normally easy to install


Magnetic locks are generally easier to install than other locks given there are no interconnecting parts.


Quick operation


Magnetic locks unlock instantly when the power is cut allowing for quick operation in comparison to other locks.


Competitive with conventional locks


Despite common misperceptions, today's magnetic locks are competitive with conventional locks. For example, the ASSA Heavy Duty High Security Modular Lockcase is designed to withstand an end load of 5,000 N. In comparison, the Securitron 1200 Pound Magnetic Lock is designed to withstand a load of 5,300 N and there are higher power models available. Magnetic locks may also suffer less damage from multiple blows.


Disadvantages


Requires continuous power when locked


To remain locked the magnetic lock requires a constant power source. At around 3 watts, the power drain of the lock is typically far less than that of a conventional lightbulb (around 60 watts), but it may cause security concerns as the device will become unlocked if the power source is disrupted. In comparison, electric strikes can be designed to remain locked should the power source be disrupted. Nevertheless, this behaviour can actually be preferable in terms of fire safety.


Cost


The Securitron 1200 Pound Magnetic Lock costs around US$340 for the lock alone. It must then be connected to an electronic access-control system (for example a keycard reader). In comparison, conventional locks can be purchased for under US$40. Annual operation of the magnetic lock will also cost around US$3 in terms of electric power consumption.


Installation


The magnetic lock should always be installed on the inside (secure side) of the door. Installation is as simple as installing on the header of the door frame for out-swinging doors or using a Z-bracket for in-swinging doors. It is important to make sure the armature plate and the electromagnet align as closely as possible to ensure efficient operation. Magnetic locks are almost always part of a complete electronic security system. Such a system may simply consist of an attached keycard reader or may be more complex involving connection to a central computer that monitors the building's security. Whatever the choice of locking system, fire safety is an important consideration.


References


^  Geringer A. Geringer R. Geringer D. Electromagnetic Door Lock Device, U.S. Patent 4,826,223, May 2, 1989.


^  Sadiku, M. Elements of Electromagnetics (3rd edition), Oxford University Press, 2001 (ISBN 0-19-513477-X).


^  Performance Specifications, http://www.assa.co.uk/resources/Services/docs/PSG%20Word.rtf, ASSA Limited (Last updated 7th, May 2003)


^  Securitron Magnetic Locks, http://www.nokey.com/secmagloc.html, The Keyless Lock Store (Accessed 12th, September 2005)


^  Purchase Rates, http://www.westernpower.com.au/home/products_services/renewable_energy_buyback/purchase_rates.html, Western Power (Accessed 12th, September 2005) (assumes 3 W power consumption)


^  The Complete Book of Locks and Locksmithing (4th edition), Bill Phillips, McGraw-Hill Inc. 1995.


Categories: Locks