2F MEMS Proportional Pneumatic Valve Georgia Institute of Technology Milwaukee School of Engineering North Carolina A&T State University Purdue University University of Illinois, Urbana-Champaign University of Minnesota Vanderbilt University Presenter Alex Hargus, Research Assistant Research Assistants Nebiyu Fikru & Erik Hemstad Principle Investigator Dr. Thomas R. Chase University of Minnesota September 30, 2015
Project Summary Project Goal : Create an efficient miniature proportional valve for controlling air flow in pneumatic systems CCEFP Goal : The valve will contribute to the center s goal of creating compact and efficient systems General appeal because it is general-purpose lowpower, compact valve Particularly appealing for human assist devices like Ankle-Foot Orthosis (AFO) In the AFO, the valve is used to control the actuator for torque assistance Key specification Power consumption (to hold the valve in the fully open state): 5 mw Total volume envelope: ~4 cm 3 TB6 Ankle-Foot Orthosis Currently used solenoid valve in CCEFP s AFO 2
Overview Valve concept Meso-scale valve MEMS valve Orifice plate Actuator array Assembly Conclusion Solid model of a MEMS Unimorph Actuator plate Actual MEMS Unimorph Actuator array 3
Valve Concept Arrays of micro-actuators with corresponding micro-orifices are operated in parallel to provide macro-flow (main parts are orifice plate and actuator array) Actuator Array Orifice Plate Electrode Electrical Contact Actuators are cantilever beams (among other geometries) that are normally flat against the orifice When actuated, the cantilevers lift off the orifice to allow air to flow through Benefits of MEMS valve could also be realized for hydraulic valves, particularly pilot valves 4
Why Piezoelectric Actuation? Use piezobender actuator: Lead-zirconate-titanate (PZT) chosen as piezoelectric material for its large piezoelectric coefficient Unimorph piezobender: One piezoelectric layer deposited on a passive layer Bimorph piezobender: Two piezoelectric layers work against each other to achieve more force/deflection than unimorph for same voltage piezobender piezoelectric cantilever beam actuator Advantages: Low power consumption to hold at fixed deflection Low heat generation Low cost Silent operation 5
Why MEMS Valve? Use micro-electro-mechanical system (MEMS) fabrication techniques MEMS-scale orifice diameters ~50 μm Reduces force due to pressure over orifice to manageable levels to use micro-actuators Small orifices produce low flow Use arrays of orifice/actuator pairs in parallel to achieve macro-flow Actuator Array Advantages: Compact Electrode Fast response time Low actuation voltage Low cost with batch fabrication Light weight Orifice Plate Electrical Contact MEMS Batch Fabrication 6
Why a MEMS Valve?... Advantages for flow control mainly due to power savings and compact size Manufacturer Approx. Dimensions (mm) Max Pressure (bar) Flow Rate* (slpm) Power (W) Response Time (msec) Commercial Valve 1 40x30x22 6.9 23 2.3 20 Commercial Valve 2 71x41x25 10 125 3.6 2.4 Commercial Valve 3 122x51x60 9.3 280 6 35 Commercial Valve 4 45x16x17 4.1 110 2 25 Commercial Valve 5 45x16x17 3.4 35 3 25 Commercial Valve 6** 45x16x17 6.9 550 1 10 Commercial Valve 7*** 43x12x11 3.5 50 0.05 <10 CCEFP: Project 2F Ф18x15 7 40 0.005 2 *Flow rate estimated for a pressure drop of 6 to 5 bar ** Macro-scale piezostack actuator *** Macro-scale piezobender actuator (limited availability in US) 7
Meso-scale Valve (Concept Demonstration Valve) Meso-scale valve employs same concept as MEMS valve but uses traditional machining (~ 20 times larger than MEMS device) Piezo-Systems PZT bimorph piezobender: 35x12.7x2mm Purposes: Validate valve concept Characterize piezoelectric actuator and flow through orifices under operating conditions Utilize capacitive displacement sensor for beam position measurement A similar piezoelectric unimorph valve was brought to market in early 2012 (not a MEMS valve) Capacitive sensor Micrometer head Calibration of capacitive displacement sensor New commercial piezo proportional valve http://www.med-eng.de/uploads/tx_medengbranchen/prodpic-201202021058491-820.jpg 8
Meso-scale Valve Valve successfully tested using in-house built, ISO compatible valve test stand Results: For no pressure, actuator displacement is fairly proportional to input voltage When valve is under pressure, flow is not very linear Use digital control strategy by organizing MEMS actuator array into multiple groups that are switched fully on or off. There is 8% leakage at maximum pressure (further improved by back driving actuator) Power consumption of less than 1mW (to keep valve in the fully open state) Test Stand 9
MEMS Valve Main components of the MEMS valve are the orifice plate (with array of through holes) and the actuator array (with array of actuators for individual through holes) MEMS Valve ~ + Orifice plate Actuator array Current State 1 cm 1 cm Completed Completed 10
MEMS Valve : Orifice Plate Initial problem of through hole etching has been resolved Proprietary masking process developed to produce successful orifice plates with aspect ratio of approximately 20 Proprietary etching process developed to overcome orifice backside blow out and roughness. 380 µm 500 µm SEM image of orifice plate cross-section Bottom side of the orifice Top side of the orifice Size comparison of silicon orifice plate with a dime 11
MEMS Valve : Orifice Plate Several orifice plates with different orifice diameters have been fabricated and tested for flow and pressure One with 80 µm is able to withstand the target specification pressure of 7 bar (100 psi) 29 µm orifice plate withstood pressure over 4 bar (60 psi) The penalty in larger orifice diameters is that, actuator force must also increase Array of small orifices and an equivalent single orifice demonstrated to have similar flow capacity 29 µm and 86 µm orifice plates 1 cm Plot of flow versus pressure differential for various orifice plates. KEY: number of orifices x orifice inlet diameter (μm) / outlet diameter (μm) *The 9x210/210 and 1x630/630 orifices were machined, the rest were etched 12
MEMS valve : Unimorph Actuator Array Deflection (µm) Unimorph actuator array (learning prototype actuator for final bimorph) was successfully fabricated Beam size (1.5mmX0.3mmX12µm) Consists of 1 µm active PZT and 10.5 µm passive elastic material: Si and SiO 2 Max voltage (40V) and max deflection (84µm) Approximate linear displacement validated Approximate ultimate strain (~0.1%, as expected) Unimorph Actuator Array Plate 1 cm 100 80 Deflection Vs Applied Voltage Sample 1 Sample 2 Unimorph actuator Probe applying voltage 60 40 20 0 0 10 20 30 40 Voltage (V) MEMS Unimorph Actuator Array 13
MEMS valve : Unimorph Actuator Array Manufacturing steps for a unimorph (not to scale) Top-down approach Etching precisely through the Si substrate during release is challenging 14
MEMS valve : Bimorph Actuator Array Deflection (µm) Array of bimorph actuators were successfully fabricated and tested Beam size:2.5mmx0.4mmx3µm Two PZT layers each having a thickness of 1µm Max voltage (13.5V) and max deflection (~535µm) Fairly linear actuator for good range of the applied voltage Bimorph Actuator Array Plate 1 cm 500 400 300 200 100 Deflection Vs Applied Voltage Data Points Fitted Curve (Y=42.6X 31.4) Top electrode Top PZT Bottom PZT Middle electrode 0-100 0 2 4 6 8 10 12 Voltage (V) SEM image showing the different layers of the new bimorph actuator during fabrication 15
MEMS valve: Recent Bimorph Actuator Arrays 1) 27 1000x250x2.5 μm cantilever 2) 27 1000x250x2.5 μm fixed-fixed 3) 130 400x100x2.5 μm fixed-fixed P: Pre-wired W: Wire bond 5 different actuator array styles available For each actuator array style, there is a 29 μm and an 80 μm orifice plate option 1 cm 16
MEMS valve : First MEMS Valve Prototype Successful fabrication and assembly of next generation of bimorph actuator arrays and orifice plates Alignment and bond in clean room using photoresist film and adhesive (Dymax) tacked at 4 corners tight tolerances: +/- 20 µm x/y, +/-.5Ɵ (used Karl Suss FC150 Chip Bonder) Aligned the through-holes in orifice plate to contact points on actuator array 1 cm 1 cm 1 cm First MEMS Valve Prototype Photoresist Film First MEMS Valve Prototype Actuator Array Side First MEMS Valve Prototype Orifice Plate Side 17
MEMS valve : MEMS Valve Prototype Test Stand Lead wires connected using conductive epoxy Redesigned custom ISO-compatible test stand to house MEMS valve MEMS valve prototype is on test stand ready for testing 1 cm MEMS valve on test stand 1 cm 1 cm Lead wires connected to electrical contacts with conductive epoxy Exploded view of MEMS valve and test stand 18
MEMS Proportional Valves Goal: Create ultra-efficient miniature proportional pneumatic valves Alignment with CCEFP Strategy: Compactness, Efficiency Test Bed: Ankle-Foot Orthosis (TB6) Original contribution: First macro-flow MEMS valve Competition: MEMS micro-valves, piezoelectric meso-scale valves Major Objectives/Deliverables Potentially revolutionize pneumatic valve technology Exploit piezoelectric materials in pneumatic valves Exploit MEMS technology in pneumatics market Manage leakage in MEMS valves Progress MEMS bimorph actuators prototyped MEMS orifice plate fab process refined MEMS valve assembled Test stand leakage reduced Test stand retrofitted to house MEMS valve Next Steps Test first complete MEMS valve (October 2015) Optimize valve design & fab processes to improve pressure & flow specifications (November 2015) Fabricate MEMS valve for TB6: Ankle- Foot Orthosis (March 2016) 19
Conclusion Concept demonstrated on a meso-scale prototype valve MEMS orifice plate, unimorph and bimorph actuator arrays successfully fabricated and assembled MEMS valve is on the test stand ready for testing MEMS is a potentially revolutionary technology for pneumatic valves 20
Acknowledgements Professor Susan Trolier-McKinstry s group at the Material Research Institute of Pennsylvania State University Industry champions Enfield Technologies Parker Hannifin Corporation Bimba 21