Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 5M

A) Mechanical Actuators

Electrostatic actuation: The fundamental actuation principle behind electrostatic actuators is the attraction of two oppositely charged plates. Their use is extensive in MEMS devices, since it is relatively simple to fabricate closely spaced gaps with conductive plates on opposite sides. For a parallel plate capacitor, the energy (W) stored at a given voltage (V) is equal to: W = ½ CV2 where: C=capacitance between the plates And the force between the plates is: F=∂W/∂x=1/2 ∂C/∂x V^2

Comb-drive-type actuators make use of a large number of fine interdigitated fingers that are actuated by applying a voltage between them

ii) Piezoelectric actuation : In piezoelectric actuation, the electrically induced displacement (or strain) is proportional to the applied potential difference. Despite small displacements, relatively high forces (in the region of tens of MPa) can be achieved using lower voltages than those required for comparable electrostatic actuation.

B) Radiation (Optical) Actuators : The two most common forms of optical actuation include light emitting diodes and light modulators such as liquid crystal displays and reflective micromechanical light modulators (technology used in Texas Instrument’s DMD projection system).

C) Thermal Actuators : Thermal actuation in MEMS is usually as a direct result of incorporating tiny heaters, or resistors. These resistors can be controlled to locally heat specific areas or layers as in the case of a bilayer actuator. As already detailed, basic thermal actuation utilizes the difference in thermal coefficients for expansion of two bonded materials and is referred to as thermal bimorph actuation.

Shape memory alloy actuation: Shape memory alloys (SMAs) exhibit considerable changes in their length (contraction) when heated. These include titanium/nickel alloys, of which some, once mechanically deformed, would return to their original undeformed state when heated.

D) Magnetic Actuators : Magnetic actuation is based on the fact that a current-carrying conductor generates a magnetic field. If this conductor is a wire (or coil) and interacts with another external magnetic field (e.g. from a similar conductor or coil) a mechanical force is produced.

Magnetostrictive Actuators : These rely on the magnetostrictive effect, which is the change of shape or size of a ferromagnetic material induced by a magnetic field, for example, the contraction of a nickel rod under a longitudinal magnetic field.

E) Chemical Actuators : These electrochemical transducers are based on the simple electrochemical electrode concept in which current is transduced from the circuit domain into the chemical domain through oxidation or reduction of chemical species at the electrode surface. These structures are amongst the simplest (they can be as simple as a region of bare metal in solution) and play a major role in biological interfacing (e.g. neurophysiological probes).