An Introduction to mems (Micro-electromechanical Systems)

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Figure 31.  Comb-drive electrostatic actuator concept [2,33]. 

Electrostatic rotary motors are another good example of the success of MEMS sacrificial 

capacitive plates that can be driven in correct phase to cause the rotor to turn.  Harmonic or 

‘wobble’ motors rely on the principle of a rotor turning in a slightly larger stator ring, such 

that it ‘wobbles’ around the central axis as it rotates (Figure 32).  Reduction of sliding friction 

and increased electrostatic forces can be achieved with these motors. 

Figure 32.  A MEMS electrostatic ‘wobble’ motor [30]. 

As previously described, the piezoelectric effect can be used in both sensors and actuators.  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.  It should be noted, however, that it is dependent on the geometry of 

the device components.  The main disadvantages of piezoelectric actuation include high 

complexity of fabrication, as well as small actuation displacements.  Larger displacements can 

be achieved using multiple piezoelectric layers known as piezoelectric bimorphs.  Most 

MEMS piezoelectric actuation is used where small strains are required (for example, the tip of 

a scanning tunnelling microscope) [4]. 

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