Driving Principles for Actuation

The driving principles used to drive various types of micromachined MEMS systems are primarily four types, namely,

Microlevel dimension

Fig. 2.1. Microelectromechanical systems

• Electrostatic drive

• Magnetic drive

• Piezoelectric drive

• Electrothermal drive

Each driving principle has specific advantages and disadvantages with respect to deflection range, required force, environmental durability, and most importantly the response time. Furthermore, the required power supply mainly depends on the driving principles involved (Table 2.1).

Table 2.1 typical power requirements for respective driving principles

Voltage

Current

Electrostatic Drive

ten» of volts - hundreds of volts

nA ~ nA

Piezoelectric Drive

tens of volts -- hundreds of volts

nA ~ nA

Electromagnetic Drive

about 1 V

hundreds of mAs

Electrothermal Drive

a few volts ~ tens of volts

niA - tens of mAs

Electrostatic drive is based on electrostatic forces between the microelectrodes. When an external voltage is applied between the electrodes, a potential energy is stored which enables the actuation. The electrostatic forces act perpendicular to the parallel electrode. Electromagnetic actuation is primarily a current controlled process. The driving mechanism again requires currents of the order of several hundreds of milliamps, and voltages in the range of less than one volt. Magnetic drive is an attractive driving principle and very suitable for applications like dust-filled environments and in environments where low driving voltages are acceptable or desired. Piezoelectric driving is based on the material properties of crystals, ceramics, polymers, and liquid crystals. In a piezoelectric material, the internal dielectric displacement is developed via an applied electric field and mechanical stress. Electrothermal devices use electrically generated heat as an energy source for actuation. The electrothermal effects can be divided into three different categories: shape memory alloys, electrothermal bimorphs, and thermo-pneumatic actuators. Many electrostatic MEMS actuators that are investigated include micromotors, comb drive actuators and microvalves. Accelerometers, ink jet printer heads, color projection displays, scanning probe microscopes, pressure, temperature, chemical and vibration sensors, light reflectors, switches, vehicle control, pacemakers and data storage devices are examples of high-end applications of MEMS sensors and actuators.

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