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

2F – MEMS Proportional Pneumatic Valve

Presenter – Alex Hargus, Research Assistant

Research Assistants– Nebiyu Fikru & Erik Hemstad

Principle Investigator – Dr. Thomas R. Chase

University of Minnesota

September 30, 2015

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

2

Project Summary

TB6 Ankle-Foot Orthosis

Currently used solenoid valve in

CCEFP’s AFO

• General appeal because it is general-purpose low-power, compact valve

• Particularly appealing for human assistdevices like Ankle-Foot Orthosis (AFO)

• In the AFO, the valve is used to controlthe actuator for torque assistance

• Key specification

• Power consumption (to hold the valve in the fully open state): 5 mW

• Total volume envelope: ~4 cm3

• Valve concept

• Meso-scale valve

• MEMS valve

• Orifice plate

• Actuator array

• Assembly

• Conclusion

3

Overview

Solid model of a MEMS Unimorph Actuator plate

Actual MEMS Unimorph Actuator array

• Arrays of micro-actuators with corresponding micro-orifices are operated in parallel to provide macro-flow (main parts are orifice plate and actuator array)

• 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

Valve Concept

Electrode

Actuator Array

Electrical Contact

Orifice Plate

• 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

5

Why Piezoelectric Actuation?

Advantages:

• Low power consumption to hold at fixed deflection

• Low heat generation

• Low cost

• Silent operation

“piezobender” piezoelectric cantilever beam actuator

• 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

6

Why MEMS Valve?

Electrode

Actuator Array

Electrical Contact

Orifice Plate

Advantages:

• Compact

• Fast response time

• Low actuation voltage

• Low cost with batch fabrication

• Light weight

MEMS Batch Fabrication

• Advantages for flow control mainly due to power savings and compact size

7

Why a MEMS Valve?...

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 25Commercial 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)

• 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)

8

Meso-scale Valve (Concept Demonstration Valve)

Capacitivesensor

Micrometer head

Calibration of capacitive displacement sensor

New commercial piezo proportional valvehttp://www.med-eng.de/uploads/tx_medengbranchen/prodpic-201202021058491-

820.jpg

9

Meso-scale Valve …

Test Stand

• 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)

• 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)

10

MEMS Valve

Orifice plate

+MEMS Valve

~

Actuator array

Completed Completed

Current State 1 cm 1 cm

• Initial problem of through hole etchinghas been resolved

• Proprietary masking process developedto produce successful orifice plates with aspect ratio of approximately 20

• Proprietary etching process developed to overcome orifice backside blow out and roughness.

11

MEMS Valve : Orifice Plate

Size comparison of silicon orifice plate with a dime

Top side of the orifice

Bottom side of

the orifice

500 µm

380 µm

SEM image of orifice plate cross-section

12

• Several orifice plates withdifferent orifice diameters have beenfabricated and tested for flow and pressure

• One with 80 µm is able towithstand the target specificationpressure of 7 bar (100 psi)

• 29 µm orifice plate withstood pressure over 4 bar (60 psi)

• The penalty in larger orifice diametersis that, actuator force must alsoincrease

• Array of small orifices and an equivalent single orifice demonstrated to have similar flow capacity

MEMS Valve : Orifice Plate…

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

13

• Unimorph actuator array (learning prototypeactuator for final bimorph) was successfully fabricated

• Beam size (1.5mmX0.3mmX12µm)

• Consists of 1 µm active PZT and 10.5 µmpassive elastic material: Si and SiO2

• Max voltage (40V) and max deflection (84µm)

• Approximate linear displacement validated

• Approximate ultimate strain (~0.1%, as expected)

MEMS valve : Unimorph Actuator Array

Unimorph Actuator Array Plate

Probe applying voltage

Unimorph actuator

MEMS Unimorph Actuator Array0 10 20 30 40

0

20

40

60

80

100

Sample 1

Sample 2

Deflection Vs Applied Voltage

Def

lect

ion

(µ

m)

Voltage (V)

1 cm

14

• Manufacturing steps for a unimorph (not to scale)

• Top-down approach

MEMS valve : Unimorph Actuator Array

• Etching precisely through the Si substrate during “release” is challenging

15

• Array of ‘bimorph’ actuators were successfullyfabricated 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 theapplied voltage

MEMS valve : Bimorph Actuator Array

Bimorph Actuator Array Plate

0 2 4 6 8 10 12-100

0

100

200

300

400

500

Data Points

Fitted Curve (Y=42.6X – 31.4)

Deflection Vs Applied Voltage

Def

lect

ion

(µ

m)

Voltage (V)

Top electrode

Bottom PZT

Top PZT

Mid

dle electro

de

SEM image showing the different layers of the new bimorph actuator

during fabrication

1 cm

16

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

MEMS valve: Recent Bimorph Actuator Arrays

1 cm

17

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

First MEMS Valve PrototypeActuator Array Side

First MEMS Valve PrototypeOrifice Plate Side

First MEMS Valve PrototypePhotoresist Film

1 cm 1 cm 1 cm

18

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

1 cm

1 cm

Lead wires connected to electrical contacts with conductive epoxy

Exploded view of MEMS valve and test stand

MEMS valve on test stand

19

MEMS Proportional Valves Major Objectives/Deliverables

Next StepsProgress

• 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

• MEMS bimorph actuators prototyped

• MEMS orifice plate fab process refined

• MEMS valve assembled

• Test stand leakage reduced

• Test stand retrofitted to house MEMS valve

• 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)

• Potentially revolutionize pneumatic valve technology

• Exploit piezoelectric materials in pneumatic valves

• Exploit MEMS technology in pneumatics market

• Manage leakage in MEMS valves

20

Conclusion

• Concept demonstrated on a meso-scale prototype valve

• MEMS orifice plate, unimorph and bimorphactuator arrays successfully fabricated and assembled

• MEMS valve is on the test stand ready for testing

• MEMS is a potentially revolutionary

technology for pneumatic valves

21

Acknowledgements

• Professor Susan Trolier-McKinstry’s group at the Material Research Institute of Pennsylvania State University

• Industry champions

• Enfield Technologies

• Parker Hannifin Corporation

• Bimba