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electropermanent_magnet [2017/01/30 14:40]
stan_zurek -review
electropermanent_magnet [2020/07/08 23:00] (current)
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 +====== Electropermanent magnet ======
 +|< 100% >|
 +| //[[user/Stan Zurek]], Electropermanent magnet, [[|]], {accessed @YEAR@-@MONTH@-@DAY@}// |
 +**Electropermanent magnet** (also **electro-permanent magnet**) - a type of [[electromagnetic device]] which uses a combination of [[magnetically soft materials|magnetically soft]] (e.g. [[iron]]), switchable [[magnetically semi-hard materials|semi-hard]] (e.g. [[Alnico]]) and [[magnetically hard materials|hard]] (e.g. [[Nd-Fe-B]]).
 +<box 100% left #f0f0f0>
 +[[FEM]] simulation of electropermanent magnet operation:
 +  * left (drawing): red - switchable magnet, blue - permanent magnet, orange - magnetising coils, light grey - [[yoke|yokes]], dark grey - part being lifted
 +  * centre (FEM): magnet configured for lifting (light blue = low flux density, purple = high flux density)
 +  * right (FEM): magnet inactive
 +===== Operating principle =====
 +Mechanically switchable magnet technology was invented in 1960's[(Hizook)][(Knaian>[[|Ara Nerses Knaian, Electropermanent Magnetic Connectors and Actuators: Devices and Their Application in Programmable Matter, PhD thesis, Massachusetts Institute of Technology, 2010]])], but electro-permanent magnets were patented much later.[(Pignataro>[[|Dominic F. Pignataro, Electrically switchable magnet system, Patent US6002317, 2001]])][(Lifter_patent)]
 +In some aspects the operation in similar to such devices as [[magnetic chuck|magnetic chucks]]. If the [[magnetic flux]] is guided in a closed internal magnetic circuit then there is no [[magnetic force]] produced outside of the device. The difference is that the switching action is achieved not mechanically, but electromagnetically.
 +Changing the path of magnetic flux and forcing it outside of the electropermanent causes any [[magnetic material]] in the vicinity to be included in the resulting [[magnetic circuit]] because the magnetic flux to close through the surrounding volume. This leads to magnetisation of the material and hence attraction, so that holding or lifting is possible.
 +[[Electric current]] is used to change the level of [[magnetisation]] of the semi-hard part. Once the magnetisation is changed the state is stable and no electric energy is required to sustain such state.[(Hizook>[[|Travis Deyle, Electropermanent Magnets: Programmable Magnets with Zero Static Power Consumption Enable Smallest Modular Robots Yet,, {accessed 2012-12-30}]])][(Motor_patent>[[|Ara Knaian, Neil Gershenfeld, Maxim Lobovsky, Electropermanent magnet-based motors, Patent US 20100289359, 2010]])]
 +If the two magnets (permanent and switchable) can have similar value of [[remanence]] (e.g. around 1.25T) so if their volumes are equal the can cancel each other completely (no external force). 
 +As an example, the [[coercivity]] of the permanent magnet (e.g. grade N40 NFeB magnet) can be 1000 kA/m and for the switchable magnet (e.g. sintered Alnico 5) can be 48 kA/m.[(Knaian)] The coil must be capable of generating an impulse of [[magnetic field strength]] significantly exceeding the coercivity of the switchable magnet, but significantly below the coercivity of the permanent magnet. Such impulse would ensure that the polarity of the switchable magnet is reversed without affecting the magnetisation of permanent magnet. 
 +For instance, for the values given above, for a magnet 3.2 mm long, the suitable impulse could be 5.3 A into a coil of 80 turns, which would result with //[[H]]// exceeding 130 kA/m. This value is more than 275% of the coercivity of the switchable magnet, which is sufficient to ensure the reversal of polarity. On the other hand, it is only 13% of the coercivity of the permanent magnet, so that its magnetisation is not affected.
 +===== Practical applications =====
 +Because relatively low average [[power]] is required for the operation of the device (only pulse switching) such actuators can be used as miniature actuators in small-scale robotics (a few millimetres in size).[([[|David L. Chandler, The robotic equivalent of a Swiss army knife. Reconfigurable robot a step toward something that can become almost anything., MIT News Office, {accessed 2012-12-30}]])][(Motor_patent)]
 +On the other hand, the fact that no electrical energy is required during passive operation the devices has some inherent safety advantages.[(Lifter_patent>[[|Gregory E. Elias, Lifter with electropermanent magnets provided with a safety device, Patent US6104270, 2000]])]
 +Lifting very heavy load (up to 40 tons) without danger of dropping the load when the supply of electricity is cut off (unlike ordinary [[lifting electromagnet|lifting electromagnets]]) is possible.[(Lifton>[[|Catalogue 2011, Lifton Magnets, {accessed 2015-02-04}]])]
 +In large devices during normal operation the electrical energy used is less than 5-10% as compared to conventional electromagnets.[(Lifton)][([[|W. Cassing, T. Pohl, F. Steger, Using electro-permanent magnets to lift loads in modern logistic networks, ThyssenKrupp techforum, 1/2008, p. 80 {accessed 2013-11-26}]])]
 +===== See also =====
 +  *[[Electromagnet]]
 +  *[[Electromagnetic actuator]]
 +===== References =====
 +{{tag>Electropermanent_magnets Counter}}

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