Comb-drives capacitive actuators, often used linear actuators that utilize electrostatic forces that act between two electrically conductive combs. Comb drive actuators typically operate at the micro- or nanometer scale and are generally manufactured by bulk micromachining

Bulk micromachining

Bulk micromachining is a process used to produce micromachinery or microelectromechanical systems .Unlike surface micromachining, which uses a succession of thin film deposition and selective etching, bulk micromachining defines structures by selectively etching inside a substrate...

 or surface micromachining

Surface micromachining

Unlike Bulk micromachining, where a silicon substrate is selectively etched to produce structures, surface micromachining builds microstructures by deposition and etching of different structural layers on top of the substrate....

 a silicon wafer substrate.

The attractive electrostatic forces are created when a voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 is applied between the static and moving combs causing them to be drawn together. The force developed by the actuator is proportional to the change in capacitance between the two combs, increasing with driving voltage, the number of comb teeth, and the gap between the teeth. The combs are arranged so that they never touch (because then there would be no voltage difference). Typically the teeth are arranged so that they can slide past one another until each tooth occupies the slot in the opposite comb.

Restoring springs

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

, lever

Lever

In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...

s, and crankshaft

Crankshaft

The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...

s can be added if the motor's linear operation is to be converted to rotation or other motions.



= applied electric potential,
= relative permitivity of dielectric,
= permitivity of free space (8.85 pF/m),

= number of pairs of electrodes,
= thickness in the out of plane direction of the electrodes,
= gap between electrodes

Structure of Comb-drives

• rows of interlocking teeth
• half fixed
• half part of movable assembly
• electrically isolated
• electrostatic attraction/ repulsion
– CMOS drive voltage
• many teeth � increased force
– typically 10μm long

Scaling Issues

Comb drives cannot scale to large gap distances (equivalently actuation distance), since development of effective forces at large gaps distances would require high voltages--therefore limited by electrical breakdown

Electrical breakdown

The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....

. More importantly, limitations imposed by gap distance limits the actuation distance.