Tham khảo tài liệu 'an introduction to mems engineering - nadim maluf and kirt williams part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 120 MEM Structures and Systems in Industrial and Automotive Applications Micromachined Valve from Redwood Microsystems Early development of this valve took place in the mid 1980s at Stanford University 44 . Redwood Microsystems was founded shortly thereafter with the objective of commercializing the valve. The actuation mechanism of either normally open or normally closed valves3 depends on the electrical heating of a control liquid sealed inside a cavity. When the temperature of the liquid rises its pressure increases thus exerting a force on a thin diaphragm wall and flexing it outward. In a normally open valve the diaphragm itself occludes a fluid port by its flexing action hence blocking flow see Figure . Upon removal of electrical power the control liquid entrapped in the sealed cavity cools down and the diaphragm returns to its flat position consequently allowing flow through the port. The flexing membrane is in intimate contact with the fluid flow which increases heat loss by conduction and severely restricts the operation of the valve. A more recent demonstration from Redwood Microsystems shows a thermal isolation scheme using a glass plate between the heated control liquid and the flexible membrane. Small perforations in the isolation glass permit the transmission of pressure to actuate the diaphragm. The normally closed valve uses mechanical levering activated by a liquid-filled thermo pneumatic actuator to open an outlet orifice. The outward flexing action of the diaphragm under the effect of internal pressure develops a torque about a silicon fulcrum. Consequently the upper portion of the valve containing the actuation element lifts the valve plug above the valve seat permitting flow through the orifice. The pressure that develops inside the sealed cavity results from the heating of the control liquid which must meet several criteria in order to yield efficient actuation. In particular the control liquid must be inert and noncorrosive. It must be .
