Page 145 - An Introduction to Microelectromechanical Systems Engineering

124 MEM Structures and Systems in Industrial and Automotive Applications

Sliding Plate Microvalve

Alumina Micro, LLC, of Bellingham, Washington, is developing micromachined
valves under license based on technology developed at GE NovaSensor of Fremont,
California. These valves are intended for use in such automotive applications as
braking and air conditioning, which require the ability to control either liquids or
gases at high pressures—as high as 2,000 psi (14 MPa)—over a wide temperature
range (typically from –40°C to +125°C).
In micromachined valves that use a vertically movable diaphragm or plug over an
orifice, such as the two examples discussed previously, the diaphragm or plug sus-
tains a pressure difference across it. This pressure difference, when multiplied by the
area, results in a force that must be overcome for the diaphragm to move. For high
pressures and flow rates, the force becomes relatively large for a micromachined
device. By contrast, the valve under development by Alumina Micro belongs to a
family of valves known as sliding plate valves, in which a plate, or slider, moves
horizontally across the vertical flow from an orifice. With appropriate design, the
forces due to pressure can be balanced to minimize the force that must be supplied to
the slider.
As shown in Figure 4.36, the valve is comprised of three layers of silicon
[46, 47]. The inlet and outlets ports are formed in the top and bottom layers of
silicon, respectively. For the normally open valve shown, fluid flows past the top
controlling orifice formed between the slider and the top wafer, through the thick-
ness of the second layer of silicon, and down out of the outlet port formed in the bot-
tom wafer. Fluid flow also passes through the slot in the slider, under the slider,
through the lower controlling orifice, and out of the outlet port. To reduce or turn
off the flow, an actuator moves the slider to the right in the figure, reducing the area
of the two controlling orifices. The pressure inside the slot is equal to the inlet pres-
sure p . Therefore, the horizontal pressure forces acting on the internal surfaces of
in
the slot are equal and opposite and balance each other. Similarly, the horizontal
pressure forces acting on the external surfaces of the slot balance each other because
the pressure outside the slot is equal to the outlet pressure p . The pressure forces
out
are also balanced vertically, as the pressures on the top and bottom surfaces of the
slider are equal to the inlet pressure [47]. In practice, small pressure imbalances due
to flow are present, so some force is still required to move the slider, limiting opera-
tion to a few MPa (hundreds of psi).
The actuator is formed entirely in the middle silicon layer. There is a small
(approximately 0.5 to 1 µm) gap above and below all moving parts to allow motion.
The thermal actuator consists of a number of mechanically flexible “ribs” sus-
pended in the middle and anchored at their edges to the surrounding silicon frame.
Current flow through these electrically resistive ribs heats them, resulting in their
expansion. The centers of the ribs push the movable pushrod to the left in the draw-
ing [5], applying a torque about the fixed hinge and moving the slider tip in the
opposite direction. When the current flow ceases, the ribs passively cool down by
conduction of heat, both out the ends of the ribs and through the fluid. The mechani-
cal restoring force of the hinges and ribs returns the slider to its initial position.
Depending on the geometry of the actuator ribs, the actuation response time can
vary from a few to hundreds of milliseconds. The depth of the recesses above and