Hardware Reference

In-Depth Information

Figure 3.69: Plan view of a microactuator design.

nating voltages on each pair of the metal pads can drive the movable structure

in both directions.

In the straight beam suspension design, the Y direction stiffness k
y
of a

suspension beam is expressed as

k
y
=
Eh
3
w

l
3

,

(3.142)

where E is the Young's modulus, h is the height of the spring, w is the spring

width and l is the spring length along the Z direction. On the other hand, the

X direction (operational direction) stiffness k
x
is expressed as

k
x
=
Ehw
3

l
3

.

(3.143)

The stiffness ratio is

µ

¶

2

k
y

k
x
=

h

w

.

(3.144)

Theratiobetweenthestiffness in the Y direction and the stiffness in the X

direction must be maximized to make the structure very stiff in the Y direction

(normal to XZ plane). If the spring is designed with beam width of 2 µmin

the X direction and 60 µmintheY direction, then the aspect ratio (h/w)of

the spring is 30, and the beam is very stiff in the Y direction. The aspect ratio

is somehow limited by the fabrication process.

The electrostatic driving force F is expressed as

F =
hnV
2

g

,

(3.145)

where is the permittivity of air, h is the height of the structure, n is the

number of the electrode pairs in the comb drives, V is the driving voltage, and

g is the gap width of the comb-drive fingers.