Fluid Power Talk

MEMS Proportional Control Valves Enable Lightweight Orthoses

Ten research projects out of the Center for Compact and Efficient Fluid Power (CCEFP) were presented at the Fluid Power Innovation and Research Conference (FPIRC) last fall. The projects pertain to five sectors in fluid-power research: human-scale technologies, mobile applications, manufacturing, stationary equipment, and fluids/tribology.

The first project—a MEMS Proportional Fluid Control Valve (PDF)—is meant to replace bulky solenoid valves for motion control in orthoses, wearables, and other human-scale technologies. Like most MEMS, which is an acronym for micro-electro mechanical systems, it contains tiny components on a lightweight chip that perform mechanical tasks as part of a larger system. 

Clean-room technology allows for MEMS with microscopic actuators that open and close orifice arrays for proportional flow control. (Courtesy of Pneumatic Tips)

Designed by a team at the University of Minnesota, the hybrid MEMS valve uses a baseplate with tiny orifices interfacing with another plate containing microscopic piezoelectric actuators. (Actuator arrays are shown on the left.)  The actuators are arranged in parallel to increase the flow capacity of the valve. The MEMS valves are designed to work at pressures that are sufficient to supply airflow to larger pneumatic actuators. The team works with the University of Illinois at Champaign-Urbana to power their pneumatic ankle-foot orthosis. 

Each piezoelectric actuator is a “unimorph” cantilever beam, fabricated by depositing a piezoelectric layer on top of a passive base layer.  The piezoelectric actuators respond to a small voltage applied across two electrodes included on each side of the piezoelectric layer. Depending on the strength of the electrical signal, the cantilever beam will bend to proportionally open and close the orifices. 

The research meets the CCEFP's mission to produce streamlined fluid power actuators and systems.  Because the piezoelectric actuators are low energy, they could be powered throughout the day using only a small battery, improving the overall comfort and weight of orthoses as patients recover from injuries. 

Testing

Initial proof-of-concept testing was done on an ISO test bench using a meso-scale prototype twenty times larger than the MEMS. The team then went on to test MEMS prototypes with different orifice diameters and sealing designs. They found that an orifice diameter of 80 microns could withstand target pressures of up to 100 psi without requiring very high actuator force, which can be hard to provide, since the actuators are very thin. To learn more about leakage ratings and other factors measured during testing, view the 2016 project slides (PDF) and 2015 project slides (PDF). 

The team is still investigating more clean-room technologies and materials before they can implement their MEMS valves in orthoses. The Hybrid MEMS Proportional Fluid Control Valve team continues to receive funding from the C​CEFP and NSF to advance their research. 

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