Ž .Sensors and Actuators 78 1999 190–197www.elsevier.nlrlocatersna

Out-of-plane magnetic actuators with electroplated permalloy for fluiddynamics control

Chang Liu a,), Thomas Tsao b, Gwo-Bin Lee c, Jeremy T.S. Leu d, Yong W. Yi a,Yu-Chong Tai b, Chih-Ming Ho d

a ( )Micro Actuators, Sensors, and Systems MASS Group, UniÕersity of Illinois at Urbana-Champaign, Urbana, IL 61801 USAb Electrical Engineering, California Institute of Technology, Pasadena, CA 9125 USA

c Engineering Science, National Cheng Kung UniÕersity, Tainan, Taiwan, Chinad Mechanical, Aerospace and Nuclear Engineering, UniÕersity of California at Los Angeles, Los Angeles, CA 90024 USA

Received 23 June 1998; accepted 1 June 1999

Abstract

We have developed millimeter-scaled magnetic actuators capable of achieving large out-of-plane displacement and large forces usingŽ .surface micromachining techniques in conjunction with electroplating of Permalloy Ni Fe . Each actuator consists of a Permalloy80 20

piece attached to flexural cantilever beams, which are 400 mm long and 100 mm wide. Experiments show that under a 6=104 Armexternal magnetic field, an actuator with the volume of the magnetic piece being 1 mm=1 mm=5 mm can reach a 658 angulardisplacement and exert a 87-mN force in the direction perpendicular to the substrate. We also discuss one potential application ofcontrolling macroscopic fluidic mechanical systems using micromachined actuators. Results on using developed actuators to achieverolling motion in a macroscaled delta-wing airfoil are presented. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Magnetic actuation; Permalloy; Fluid control

1. Introduction

Technologies for developing micromachined actuatorshave been advanced significantly in the past two decades.

ŽHowever, to achieve large output force on the order of. Žtens of micronewtons and long actuation range 100 mm

. Ž .and above in microelectromechanical systems MEMSstill poses many challenges. The electrostatic actuation,which is widely used in MEMS actuators, cannot alwayssatisfy the desired range of force and displacement simul-taneously under the constraints of practical microsystems.Other actuation methods, such as the ones based onbimetallic thermal actuation or shape memory alloy materi-als, are capable of producing the required displacementand force; however, their actuation capabilities are limitedby the required temperature.

) Corresponding author. 313 Microelectronics Laboratory, 208 NorthWright Street, Urbana, IL 61801, USA. Tel.: q1-217-333-4051; fax:q1-217-244-6375; E-mail: changliu@uiuc.edu

Microfabricated actuators are being increasingly usedfor fluid-control purposes. For these applications, a largeforce is required in order to allow efficient interaction withthe fluid and to generate desired control effects. Magneticactuation is potentially capable of realizing both largeforce and large displacement in an energy-efficient mannerw x w x1–3 . Wagner et al. 4 manually attached miniature per-manent-magnet pieces on microfabricated suspensions andutilized integrated in-plane coils on the same chip to

w xgenerate an external magnetic field. Liu et al. 5 devel-oped an integrated coil-type magnetic actuator capable ofachieving out-of-plane rotational displacement on the orderof several hundred micrometers and magnetic forces withmagnitude in the tens of micronewtons range. However,coil-type actuators typically require large biasing currentŽ .typically 50 mA which, when coupled with a largenumber of turns in the coil, can translate into rathersignificant ohmic heating. Judy et al. demonstrated in-planemotion of a suspended polycrystalline silicon structure

w xwith an electroplated magnetic piece 6 . The actuator wasdriven by an external magnetic field and large deflection

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 99 00238-1

( )C. Liu et al.rSensors and Actuators 78 1999 190–197 191

Ž .angle over 1808 has been demonstrated. In addition,w x w xMiller et al. 7 and Judy and Muller 8 have both

developed individually addressable magnetic actuators.Moreover, complex electromagnetic subsystems, including

w x w xa planar electromagnet 2 and magnetic micromotors 3 ,have also been realized in the past.

2. Fluid dynamical applications

Micromachined actuators have traditionally been usedto control objects with similar or smaller dimensions ormass. Examples of these applications include microfluidic

w xdevices, microassembly 9 and mirrors for optical beamsteering. As a novel objective of our current project, weplan to demonstrate that a collection of micromachinedactuators can control macroscale objects, provided that a

w xproper controlling mechanism exists 10 . The macroobjectis a delta-wing airfoil, which is one of the fundamental

w xconfigurations for generating lift forces 11,12 . Thoughthe sizes of microactuators are much smaller comparedwith that of the delta wing, there exists a known fluidmechanism that allows microscale actuation to have an

Ž .amplified, macroscopic effects Fig. 1 .When laminar air flow impinges on the two leading

Žedges of the wing at a certain angle-of-attack Fig. 1a and.b , two counter-rotating leading-edge vortices separate from

Žthe laminar flow and propagate over the wing’s top Fig..1c . These high-momentum, low-pressure vortices con-

tribute two vortex lifting forces on the two sides of thewing; the sum of these two forces constitute approximately

w x40% of the total lifting forces at high angles of attack 13 .The strength and position of these two vortices are deter-mined by the condition of the boundary layer, which isroughly 1–2 mm thick at a wind-tunnel flow speed of lessthan 20 mrs for a wing model with a chord of 30 cm.

The position and strength of the vortex is controlled bythe location of the vortex separation, which can be influ-enced by airfoil geometry within the boundary layer itself.

Fig. 1. Schematic diagram of the rolling moment generation.

Fig. 2. Schematic of a Permalloy magnetic actuator with two cantilever-Ž . Ž .beam supports. a Top view and b cross-sectional view.

Small perturbation to the boundary layer can drasticallychange the characteristics of the separated vortices and thebalance of lifting forces. We propose a new leading-edgecontrol mechanism by employing arrays of millimeter-scaled, micromachined actuators. Specifically, two linear

Žarrays of surface micromachined out-of-plane actuators so.called microflaps can be placed along two leading edgesŽ .of the delta wing Fig. 1d . Flap arrays at rest remain at the

bottom of the boundary layer, having no effect on the flowand vortices; when one array is deflected out of plane,however, it interacts with the boundary layer and artifi-cially modify the separation point of the correspondingleading-edge vortex. The two vortex structures becomeunbalanced, and a macroscale rolling moment can becreated. This unique mechanism can provide a new modeof maneuvering by eliminating the use of tail stabilizers, toincrease energy efficiency and to achieve fast maneuveringof aircrafts.

In this study, micromachined actuators are required toŽdeflect 1 to 2 mm out-of-plane comparable with the

.thickness of boundary layers , and withstand large aerody-namic loading on the order of several hundreds of mi-cronewtons. Microactuators can potentially sustain highfluid loading because they reside in boundary layers, inwhich the flow velocity and momentum is lowered com-pared with the free stream. These offer much highermechanical response speed compared with macroscopicmechanical elements.

3. Actuator design

Fig. 2 illustrates the schematic diagram of a microma-chined magnetic actuator, which consists of a suspended

( )C. Liu et al.rSensors and Actuators 78 1999 190–197192

magnetic piece. Its length, width, and thickness are desig-nated as L, W, and T , respectively. The magnetic piece issupported by two cantilever beams, with their length,width, and thickness being l, w, and t, respectively. Thebending displacement of the actuator due to its own gravi-tational weight is negligibly small.

The analysis of displacement as a function of the ap-plied magnetic field is presented in the following. Sincethe regime of large angular displacement is the focus ofour design, the Permalloy is modeled as a permanentmagnet with the magnitude of its magnitization vectorbeing equal to the saturation magnetization M . It shoulds

be noted, however, that under low magnetic field andangular displacement, the magnetization is not a constantbut changes according to the applied magnetic field

w xstrength. Discussion of this topic can be found in Ref. 8 .Under the current assumption, we assume that two

concentrated magnetic forces act at the upper and lowerŽ .edges of the Permalloy piece Fig. 3b . The magnitude of

these two forces are:

F sM WTH1 s 1

F sM WTH , 1Ž .2 s 2

with H and H being the magnetic field strengths at the1 2Žtop and bottom edges of the plate H )H in the current2 1

.configuration . The magnitudes of H and H are experi-1 2

mentally found and are linearly dependent on the distanceŽ .to the substrate Section 5.1 .

The structure plate, along with the Permalloy piece, hasa thickness of tqT. Nominal values for t and T are 1 and5 mm, respectively. Its moment of inertia, I, is propor-

Ž .3tional to tqT and is much greater compared with thatof the cantilever beam, which has a thickness of t. Thecombined structure plate and the Permalloy piece can thusbe considered as a rigid body. Based on this assumption,an equivalent force system is created by translating F to1

Fig. 3. Schematic illustration of the mechanism of magnetic actuatorŽ .biased using an external electromagnet. a Rest position of the actuator

Ž .when the external magnetic field, H , equals 0; b out-of-plane actua-extŽ .tion when H /0, is provided by an external electromagnet; c aext

Ž .simplified force system containing M and F acting at the free-endsmag

of the cantilever beams.

coincide with F ; this results in a torque termed M and2 magŽ .a point force F Fig. 3c , both acting on the bottom edge

of the structural plate. The magnitude of M and F canmag

be expressed as:

M sF L cos u ,mag 1

FsF yF sM WT H yH . 2Ž . Ž .2 1 s 2 1

Because the displacement of the actuator is relativelylarge, conventional linear treatment is not valid in thiscase. A closed-form analytical solution cannot be easily

w x Žfound 14,15 . A finite element simulation software AN-.SYS is therefore used. However, the displacement must

be calculated recursively and manually as the magnitude ofthe forces are related to the vertical displacement, which interm depends on the magnitude of forces. The calculationprocess is inefficient. We are therefore motivated to find asimplified analytical solution.

Using the ANSYS finite element simulation, we havefound that it is a good approximation to treat the displace-ments due to the moment and the force independently andadd these results to calculate the actuator bending underthe combined force and momentum. This assumption isvalid when the force is relatively small. It significantlysimplifies the analysis.

Beam displacement under M is solved first. Themag

cantilever beam assumes the shape of an arc, with theradius of curvature being rsEIrM . The angular dis-mag

placement at the free-end of the cantilever beam is:

lu s . 3Ž .torque r

The y coordinate at the corresponding location is there-fore:

ly sr 1ycos . 4Ž .torque ž /r

Ž .The maximum angular displacement u and verticalforceŽ .displacement y due to force F can be simply foundforce

w xusing established linear models 15 . The maximum verti-Ž . Ž .cal deflection y and angular displacement u at themax

end of the rigid structural plate are:

y sy yy qL sin u yu . 5Ž .Ž .max torque force torque force

usu yu . 6Ž .torque force

We have designed the area of the Permalloy piece to be1=1 mm2. The supporting cantilever beams are 400 mmlong and 100 mm wide, with the nominal thickness being1 mm.

4. Fabrication process

The fabrication process for the magnetic actuator issummarized in Fig. 4 and described in detail below. In the

( )C. Liu et al.rSensors and Actuators 78 1999 190–197 193

Ž .first step Fig. 4a , a 3-mm-thick phosphosilicate glassŽ .PSG sacrificial thin film is first deposited on top of thesilicon substrate at 450 C8 as a sacrificial material. It ispatterned to form individual mesas on top of which actua-tors will be located. These mesas isolate individual actua-tors and limit the total amount of lateral undercut duringthe sacrificial-layer etching processing. This important fea-ture provides robust process control and results in highstructural yield and high area device of devices even whenover-etching is encountered.

After removal of the photoresist layer, the wafer isannealed in nitrogen at 10008C for 1 h. It is then coveredby a layer of LPCVD polycrystalline silicon, which issubsequently coated with a 0.5-mm-thick PSG layer thatserves as a complimentary doping source. During a 1-h,9508C stress-relief anneal in nitrogen ambient, the polysili-con is doped symmetrically from both sides. This measurereduces the intrinsic-stress gradient across the thickness ofthe polysilicon and minimizes residue beam bending.

Fig. 4. Major fabrication steps of a Permalloy magnetic actuator.

Ž .Fig. 5. Calibrated B–H loop in both easy and hard axes of thePermalloy thin film.

˚ ˚ŽA conductive seedlayer 200-A-thick Cr and 1800-A-. Ž .thick Cu is thermally evaporated Fig. 4b . During the

Ž .electroplating process Fig. 4c , the wafer is affixed to thecathode with a pure Ni piece as the anode. An external

Ž .magnet 450 Oe is applied with the field lines beingparallel to the wafer substrate. This bias establishes direc-

w xtions of the easy axis within the Permalloy piece 16,17 .The easy axis is parallel to the length of the cantileversupport beams. Electroplating takes place at a rate of 5mmrh under a bias-current density of 8 to 12 mArcm2.Experimental B–H hysteresis curves of the Permalloy

Ž w x.material 80% Nickel and 20% Iron 18 along both theeasy axis and the hard axis are shown in Fig. 5. ThePermalloy material used in the current study show only aminor difference in magnetic properties, e.g., saturationmagnetization, between the easy and hard axes. The rela-

Ž .tive permeability m is 4500. Following the electroplat-r

ing process the photoresist is removed and the exposedseedlayer material is then etched away by using Cu etchantŽ .100:5:5 wt. water:acetic acid:hydrogen peroxide followed

Ž w xby a Cr-mask etchant either a commercial etchant 19 orŽ .diluted HCl 10 water:1 HCL .

Actuators are subsequently released by 49% HF within20 min. The Permalloy material sustains HF etching with-out any structural or chemical damage. The polysiliconmaterial is etched slightly, reducing the thickness of thecantilever beams. Since the structure plates have large

Žsurface areas and the supporting beams are soft springconstant;100 mNr1 mms0.1 Nrm for cantilever

.beams , they can be easily pulled down by surface tensionto the substrate and form permanent bonds if conventional

Ž . 1drying techniques e.g., spin drying are used. To ensurehigh yield, the structural plate is levitated away from thesubstrate surface via magnetic interactions. This methodeffectively prevents the actuators from coming into contactwith the substrate, therefore guaranteeing that 100% yield

1 Cr mask etchant, Transene, USA.

( )C. Liu et al.rSensors and Actuators 78 1999 190–197194

Ž . Ž .Fig. 6. SEM micrograph of a top view and b perspective view of aType-1 actuator.

is routinely achieved. Shown in Fig. 6 are top and perspec-tive views of fabricated actuators.

5. Results and discussions

5.1. Actuator calibration in still air

We used an electromagnet to provide an uniform exter-nal magnetic field. The variation of H with respect to thedistance, d, to the substrate plane, is calibrated experimen-

Ž .tally using a Gauss meter Edmund Scientific . In a regionnear the surface of the core, H decreases linearly withincreasing spacing d, according to the following expres-sion:

HsH 1ya d , 7Ž . Ž .0

where H is the strength of the magnetic field at the0Žsurface of the substrate, a is a scaling constant as0.0267

.as determined by calibration .

Ž .Fig. 7. Sequential video images of a Type-1 magnetic actuator a beforeŽ . 4 Ž .applying the magnetic field; b when H s3.34=10 Arm; c at theext

maximum displacement.

Fig. 7 contains a sequence of video images of anactuator at the resting and the activated positions. Fig. 8shows the measured magnitude of angular displacementstogether with the results obtained by analysis. The magne-tization of the material is 1.5 T as determined by the

Ž .measurement Fig. 5 . Due to small but finite etching ofpolysilicon by the HF solutions, the thickness of the

˚cantilever beams is 9700 A instead of 1 mm.We have demonstrated that the beams can be bent by

more than 1808 without failure. In pure bending mode, thesupport cantilever beams may be fractured when the maxi-

Ž .mum normal strain near the surface at its fixed endexceeds the fracture strain of the beam material. Forpolysilicon, fracture strain equals 9.3=10y3 according to

w xan early report by Tai and Muller 20 .

5.2. Fluid dynamic control

Ž .A scaled aluminum delta-wing airfoil Fig. 9 has aspan of 40.35 cm and a sweep angle of 56.58. It is mountedon a six-component transducer that records moments andforces in three axes. The magnetic biasing is provided by arotational magnetic bar embedded within the leading edgesof the delta wing and powered by a miniature steppingmotor. The location of the actuators is indicated by theangle b which is zero at the lower surface and 1808 at theupper surface of the wing.

It was found that the region close to flow separation isw xvery sensitive to small perturbation 21 . The innovation of

this study is applying microactuators to manipulate the thinboundary layer near the leading edges of a delta wing, andconsequently, alter the flow separation location. We used a

w xmicromachined shear-stress sensor 22 to measure theseparation point along the leading edges of the delta wing.

Initially, time-averaged measurement of the rolling mo-ment M was taken with the flaps kept on for at least 4roll

min. The resulting M at different wind-tunnel flowroll

speed is normalized with respect to the vortex lift momentM . Since only a single vortex is controlled, the resultingvl

M at different wind-tunnel flow speed is normalizedroll

with respect to the moment produced by a single vortex,

Fig. 8. Angular displacement of the structure plate with respect to thebiasing magnetic field intensity H.

( )C. Liu et al.rSensors and Actuators 78 1999 190–197 195

Fig. 9. Photograph of a delta-wing model mounted in a windtunnel, together with a close-up view of a leading edge of the wing, where actuator arrays areinstalled.

M . Here, M is the product of the vortex-lift force onvl vl

one of the leading edges multiplied by the distance fromthe centerline to the centroid of the half wing. The rollingmoment is measured as a function of b for various angles

Ž .of attack a as20, 25, 30, and 358 . The free-streamvelocity is also varied from 10 to 35 mrs. Approximately

Ž15% torque was achieved at an angle of attack of 258 Fig..10 . At as258, a negative torque can be as large as

y12% at bs808.

The interaction between microactuators and vortices canw xbe seen from flow visualization experiments 23 . Fig. 11a

shows the vortex structure with an array of actuatorslocated at bs508 on the right-hand side of the wing. Alsoshown on the left-hand side in Fig. 11a is a picture of asingle vortex without any actuator for comparison. It isclearly noticeable that vortex structure has been distortedby the presence of the microactuators. Since the location ofthe microactuators is ahead of the separation point, the

Fig. 10. Normalized rolling moment vs. locations of actuators at AOAs258, with 1 mm-sized actuators.

( )C. Liu et al.rSensors and Actuators 78 1999 190–197196

Ž . Ž .Fig. 11. Flow visualization of vortical structure after actuation at a bs508 and at b bs808. Microactuators are located on the right-hand side of theairfoil.

flow could separate earlier due to the higher pressuregradient caused by the presence of the microactuator. Itwas observed that the core of the vortex has been movedoutboard. A positive rolling moment could be generated bythese unbalanced vortices.

At b)608, the rolling moment becomes negative in allcases. Fig. 11b is the flow visualization picture of thevortex with a linear array of actuators located at bs808.It was found that the core of the vortex had shifted inboardand the direction of the shear layer shedding from theleading edge had been suppressed. The possible mecha-nism for the phenomena is as follows: the flap actuatormoves away from the surface and reduces the local curva-ture locally such that the adverse pressure gradient isreduced. As a result, the separation is delayed and thevortex moves inboard. A negative rolling moment could begenerated by these unbalanced vortices accordingly. Thenegative rolling moment can be employed in conjunctionwith the positive-working rolling moment generated on anopposite leading edge. We have observed the sum ofindividual incremental torque equals the value obtained bythe simultaneous actuation along both leading edges; itindicates that modifications of the vortices on both sides ofa wing by the actuators seem to be independent.

6. Conclusions

Surface micromachined out-of-plane Permalloy actuatorarrays have been developed for controlling the rollingmoment of a tailless delta wing. The actuator consists of amillimeter sized electroplated Permalloy plate with sup-porting polysilicon beams and is driven by an external

Ž .magnetic field. Large angular deflections over 608 andŽ .vertical deflections on the order of 1–2 mm have been

demonstrated. The magnetic forces and flow loading in-volved in the flap operation is on the order of hundreds ofmicronewtons. Linear arrays of such flaps are positionednear the leading edges of a delta wing; wind-tunnel testsconfirm that a rolling moment on the wing can be gener-ated by the flap actuation. These magnetic actuators canhave a number of applications, including fluid mechanicsapplications that are illustrated in this paper. Others in-

clude optical light modulators and microrobotics assemblysystems.

Acknowledgements

This work is supported by the Defense Advanced Re-search Program Agency. The authors thank the many helpand discussions from colleagues including Drs. Steve Tung,Weilong Tang, Raanan Miller, Fukang Jiang and Jin-BiaoHuang, who were members of the URIrDARPA team.

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