A MEMS magnetic actuator is a device that uses the microelectromechanical systems (MEMS) to convert an electric current into a mechanical output by employing the well-known Lorentz Force Equation or the theory of Magnetism.
Overview of MEMS Micro-Electro-Mechanical System (MEMS) technology is a process technology in which mechanical and electro-mechanical devices or structures are constructed using special micro-fabrication techniques. These techniques include: bulk micro-machining, surface micro-machining, LIGA, wafer bonding, etc.
A device is considered to be a MEMS device if it satisfies the following:
If its feature size is between 0.1 μm and hundreds of micrometers. (below this range, it becomes a nano device and above the range, it is considered a mesosystem) If it has some electrical functionality in its operation. This could include the generation of voltage by electromagnetic induction, by changing the gap between 2 electrodes or by a piezoelectric material. If the device has some mechanical functionality such as the deformation of a beam or diaphragm due to stress or strain. If it has a system-like functionality. The device must be integrable to other circuitries to form a system. This would be the interfacing circuitry and packaging for the device to become useful. For the analysis of every MEMS device, the Lumped assumption is made: that if the size of the device is far less than the characteristic length scale of the phenomenon (wave or diffusion), then there would be no spatial variations across the entire device. Modelling becomes easy under this assumption.
Operations in MEMS The three major operations in MEMS are:
Sensing: measuring a mechanical input by converting it to an electrical signal, e.g. a MEMS accelerometer or a pressure sensor (could also measure electrical signals as in the case of current sensors) Actuation: using an electrical signal to cause the displacement (or rotation) of a mechanical structure, e.g. a synthetic jet actuator. Power generation: generates power from a mechanical input, e.g. MEMS energy harvesters These three operations require some form of transduction schemes, the most popular ones being: piezoelectric, electrostatic, piezoresistive, electrodynamic, magnetic and magnetostrictive. The MEMS magnetic actuators use the last three schemes for their operation.
Magnetic actuation The principle of magnetic actuation is based on the Lorentz Force Equation.
F → m a g = q v → × B {\displaystyle {\vec {F}}_{mag}=q{\vec {v}}\times B}
When a current-carrying conductor is placed in a static magnetic field, the field produced around the conductor interacts with the static field to produce a force. This force can be used to cause the displacement of a mechanical structure.
Governing equations and parameters A typical MEMS actuator is shown on the right. For a single turn of circular coil, the equations that govern its operation are:
The H-field from a circular conductor:
H ( z ) = I r 2 2 ( r 2 + z 2 ) 3 / 2 {\displaystyle H(z)={\frac {Ir^{2}}{2(r^{2}+z^{2})^{3/2}}}}
The force produced by the interaction of the flux densities:
F z = B I A m a g ∫ z z + h m a g d H z d z d z {\displaystyle F_{z}=B_{I}A_{mag}\int _{z}^{z+h_{mag}}{\frac {dHz}{dz}}dz}
The deflection of a mechanical structure for actuation depends on certain parameters of the device. For actuation, there has to be an applied force and a restoring force. The applied force is the force represented by the equation above, while the restoring force is fixed by the spring constant of the moving structure. The applied force depends on both the field from the coils and the magnet. The remanence value of the magnet, its volume and position from the coils all contribute to its effect on the applied Force. Whereas the number of turns of coil, its size (radius) and the amount of current passing through it determines its effect on the Applied Force. The spring constant depends on the Young's Modulus of the moving structure, and its length, width and thickness.
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