Page 140 - An Introduction to Microelectromechanical Systems Engineering

Actuators and Actuated Microsystems 119

well as reduce the area of silicon required for the output pads (which are
large), on-chip n-channel metal-oxide-semiconductor (NMOS) driver circuitry was
implemented on the 104-resistor chip, reducing the number of pads to 36. While
the NMOS transistors require additional chip area and additional processing steps,
the space savings is great enough that the overall chip cost is reduced [40].

Micromachined Valves

A new generation of miniature valves with electronic control would be desirable
among both manufacturers and users of valves. For example, recent trends in home
appliances indicate a shift towards total electronic control [41]. Electronically pro-
grammable gas stoves, currently under development, require low-cost, electroni-
cally controlled gas valves. Moreover, miniature valves are useful for the control of
fluid flow functions in portable biochemical analysis systems [42] and automotive
braking systems [43]. Some potential applications for silicon micromachined valves
include:

• Electronic flow regulation of refrigerant for increased energy savings;
• Electronically programmable gas cooking stoves;
• Electronically programmable pressure regulators for gas cylinders;
• Accurate mass flow controllers for high-purity gas delivery systems;
• Accurate drug delivery systems;
• Control of fluid flow in portable biochemical analysis systems;
• Portable gas chromatography systems;
• Proportional control for electrohydraulic braking (EHB) systems.

The field of micromachined valves remains nascent. In order for silicon
micromachined valves to gain a substantial foothold in the market, they must effec-
tively compete with the relatively mature traditional valve technologies. These cover
a broad range of actuation methods, media handling, pressures, flow rates, and
price. It is unlikely that micromachined valves will displace traditional valves;
rather, they will complement them in special applications where size and electronic
control are beneficial.
The following sections describe three micromachined valves. The devices are
from Redwood Microsystems, Inc., of Menlo Park, California; TiNi Alloy
Company of San Leandro, California; and Alumina Micro, LLC of Bellingham,
Washington. They illustrate the efforts of three small companies in commercializ-
ing microvalves. The first two valves operate on the principle of blocking a vertical
fluid port with a silicon plug suspended from a spring that is sufficiently compliant
to allow vertical displacement during actuation. Accordingly, the inlet pressure
2
limit is relatively low, typically, less than 150 psig (~ 1 MPa). The third valve uses
an elegant pressure balancing scheme to reduce the pressure forces against an
actuated silicon element thus increasing the inlet pressure to nearly 1,000 psig
(~ 7 MPa).

2. The psig is a unit of differential (gauge) pressure equal to one psi (or 6.9 kPa).