Electropermanent magnet
The direction of magnetization in the latter piece can be switched by a pulse of current in a wire winding about the former.
applications needing a controllable magnetic field required electromagnets, which consume large amounts of power when operating.
Programmable magnets are also being researched as a means of creating self-building structures.
In order to explain the principle of the EPM, the configuration on the following picture is presented.
Two permanent magnets are assembled with two U-shape (horseshoe) iron bars.
During this explanation we use one magnet made of NdFeB and the other made of AlNiCo because both materials had the same remanence (around 1.3T) but AlNiCo has a lower intrinsic coercivity of 50kA/m while NdFeB has an intrinsic coercivity of 1120kA/m.
[5] An additional segment of hiperco is shown to close the circuit and obtains better calculated results.
This will lead to obtain an expression of the EPM's force (exerted over the additional segment of hiperco) as a function of the separation distance.
For the calculation of the values of the components in the circuit we will assume that all the areas in the flow have the same dimensions.
For the AlNiCo magnet we can calculate the magnetomotive force (MMF), the reluctance
A simplified version of the magnetic circuit was presented on the right in the picture above: An equivalent reluctance (
) can be calculated to replace the magnets: For the equivalent MMF, there are going to be two different values.
Since we are going to calculate the force for the two areas corresponding to the gaps then the equation to calculate the force as a function of the gap distance looks like: The first and most important step is to design the solenoid that will create the magnetic field to reverse the magnetization of the AlNiCo.
It is recommended to design the field 3 times higher than the intrinsic coercivity to fully magnetize a material.
Picture below depicts the coil design parameters: The next step to complete the design is to calculate the B field in the middle point of the coil using the equations for thick solenoids[6] (knowing:
To solve this problem it is necessary to fix D1, the solenoid length to L and the current to the maximum value permitted for each wire.
This simplify the optimization problem leading to calculate Bz by changing D2.
A different value for each AWG wire gauge will be generated and the voltage and power to obtain the desired Bz at a maximum current must be calculated.
We will use the full version of the equation for thick solenoids[6] and making z variate between -L/2 and L/2: Using the formula for force mentioned earlier it is possible to plot the force as a function of the gap distance of the external hiperco bar.
This simulation was made using finite element approach by the software COMSOL Multiphysics®.
Project Ara was an open hardware initiative by Google to create a modular phone where all the components are interchangeable and can be replaced while the device is on.
The project was originally announced as using EPMs as the method used to fasten the phone's modules to its endoskeleton.
Bob O’Donnell of TECHnalysis Research said, “This was a science experiment that failed, and they are moving on.”[12] Gripping systems for drones have been developed using electropermanent magnets.
Zubax Robotics, an R&D company, developed the FluxGrip EPM as the next generation payload attachment module.
[14] Related Videos: Using a dice of six sides and in each side include an EPM is the concept behind this Pebbles robots that are capable of interpret a simple shape and reproduce it by selecting which blocks must be attached to the other.
To rapidly switch between modes, the Logitech team engineered a little circuit that energizes a coil which energizes an EPM, which sits still within the internal cavity of the wheel, causing it to pull on the little teeth as they pass, giving the user the illusion of a mechanical detent.
