Microgripper

Sensors and Actuators A 133 (2007) 218224

A microgripper using piezoelectpung

School , 50 Nbruary2006

Abstract

Design, f ipperstresses wer del wand deforma e criwas manufa nt amachieved. Th latiothe microgri tion aof the micro 2006 Else

Keywords: G

1. Introduction

Gripping and manipulating micro-objects is required for awide rangemicro-partsin electronnipulator sshapes steators shouldto avoid anless than 1

Over thoped for celectronicsogy coveriengineeringconsiderabmicro-partsfor a substa

CorresponE-mail ad

A microgripper is one of the key elements in micro-roboticsand micro-assembly technologies for handling and manipulat-ing micro-objects without damage. An essential component of

0924-4247/$doi:10.1016/jof important applications such as the assembly ofto obtain miniature systems or component assembly

ics packages [1]. An effective mechanical microma-hould possess the ability to grasp objects of differentdily with high positioning accuracy. The manipula-be able to accurately control grasping forces in order

y damage to the small-size delicate objects, which aremm in diameter.e years, micro-scale technologies have been devel-onsumer products and specialized applications in, information technology, optics, medicine and biol-ng areas such as diagnostics, drug delivery, tissue

and minimally invasive surgery [25]. Althoughle developments have been achieved in fabrication of, the assembly of these micro-systems still accountsntial portion of the final cost.

ding author. Tel.: +65 6790 5588; fax: +65 6791 1859.dress: [email protected] (Z.W. Zhong).

all microgrippers is the actuator, which provides the requiredapplied force to make the device operate as a gripper. Vari-ous prototypes of microgrippers of different actuation methodshave been developed, including electrothermal actuators [69],electrostatic actuators [1015], piezoelectric actuators [16,17],electromagnetic actuators [18] and shape memory alloy actua-tors [1921].

Microgripping is different from conventional gripping.Micro-parts with major dimensions less than 100m are oftenfragile and can be easily damaged during gripping, and thusspecial grasping techniques are required. A miniature grippermay be of interest to achieve safe transport of small objectssuch as electronic devices, and to have a new end-effectortool to grasp cells for bio-application as well as for endoscopemanipulations. The specifications to realize such a gripper arequasi-static motion to have high accuracy in micro-positioning,a large-stroke to grasp the maximum types of object, and theuse of special actuation method like piezoelectric actuation[22].

Piezoelectric actuators are widely used in various applica-tions [2333] as well as in microgrippers, because of their

see front matter 2006 Elsevier B.V. All rights reserved..sna.2006.03.014micro-object maniS.K. Nah, Z.W. Zho

of Mechanical and Aerospace Engineering, Nanyang Technological UniversityReceived 10 November 2005; received in revised form 7 Fe

Available online 27 April

abrication and tests of a monolithic compliant-flexure-based microgre considered through the finite element analysis. The simulation motion. The maximum stress in the microgripper is much smaller than thctured using micro-wire electrode discharge machining. A displacemee use of piezoelectric actuation allowed fine positioning. Micromanipupper with piezoelectric actuation in handling micro-objects. The simulagripper.vier B.V. All rights reserved.

ripper; Actuator; Manipulation; Piezoelectric actuationric actuation forlationanyang Avenue, Singapore 639798, Republic of Singapore2006; accepted 14 March 2006

were performed. The geometry design and the materialas used to study in detail profiles of von Mises stresses

tical stress values for fatigue. The microgripper prototypeplification of 3.0 and a maximum stroke of 170m were

n tests were conducted to confirm potential applications ofnd experimental results have proven the good performance

S.K. Nah, Z.W. Zhong / Sensors and Actuators A 133 (2007) 218224 219

advantages [34]: large generated force, stable displacement, highresponse speed and ease of use.

In this study, design, fabrication and tests of a mono-lithic compliant-flexure-based microgripper were performed.The geometry design and the material stresses were consideredthrough the finite element analysis. The simulation model wasused to study in detail profiles of von Mises stresses, deforma-tion and contact forces. A microgripper prototype was fabricatedusing micro-wire electrode discharge machining. Micromanipu-lation tests were carried out to confirm the potential applicationsof the microgripper with piezoelectric actuation in handlingmicro-objects.

2. Design and fabrication of the microgripper

The micromanipulation solution is a monolithic compliantmechanism (shown in Fig. 1). The dimensions of the micro-gripper were designed in such a way that micro-manufacturingtechnologies could be used, leading to low cost and an easy adap-tation to diwas also lasions rangedesign enathus to be a

The comby utilizingjoints and gsurfaces prcontaminanThe designstrain of th

The micflexible andthe elasticnism is usein order toamplificatitric translaforce and smicrogripp

Fig. 2. FEA model of an half of the mechanism.

ctively controlled microgrippers are well suited for grasp-egular objects in both static and dynamic environments.

microgripper was manufactured from a single piece ofstee

). Anmateg ma

Stress concentration occurs at the notch hinge points (the unit of thethe figure is kPa).fferent micromanipulation needs. The microgripperrge enough to manipulate components with dimen-from a few microns to hundreds of microns. The

bles the microgripper tips to move in parallel andlways mutually aligned.pliant mechanism moves solely by deformation andits flexural hinges instead of conventional bearings,ears. The absence of conventional joints and bearing

oduces a clean device that is free of lubricants or otherts, and therefore it can be used in clean environments.uses flexures and the motion created is a result of thee elastic linkages [35].rogripper compliant mechanism was designed to beits motion transfer function was made possible by

deformation of the flexible elements. The mecha-d to amplify the initially small actuating displacementachieve larger output displacements. The complianton mechanism combined with a solid-state piezoelec-tor can yield a microgripper with good bandwidth,troke control. For general micromanipulation, thiser can be actively controlled to open and close their

Fig. 1. The microgripper designed and fabricated.

tips. Aing irr

Thespring(EDMTheseneerin

Fig. 3.stress inl using a micro-wire electrical discharge machineother material used to fabricate it was aluminium.rials were selected because they are common engi-terials and are easy to be machined by EDM.

220 S.K. Nah, Z.W. Zhong / Sensors and Actuators A 133 (2007) 218224

Fig. 4. Components of the gripping device.

The micro-wire EDM technology can generate the micro-gripper widimensiona0.1m andmethod is smicrogrippthick.

3. Finite e

Finite ethe complimodel the mratio, stressper mechamicrogrippmicrogrippstress concin Fig. 3. Tsmaller thamaterial.

4. Prototy

The fabdevice. It isactuator anready for udevice are

g. 5.

Schemand the microgripper.

vious work [3638] indicates that piezoelectric ceram-upled with an appropriate transmission, can provide the

actuation performance. Typical leadzirconatetitanatepiezoelectric actuators can perform step movements withution on the order of a nanometer. These actuators offeroop stable operation with the power and bandwidth nec-for the specified motion. However, the limitation of thelectric material is that only a typical strain of 0.1% isable, thus limiting its actuation stroke.s research work attempts to overcome this limitation of thectuator by the amplification mechanism. The advantagemethod is that the grasp of an object can be perfectly

lled because the actuation of the gripper can be controlledth good manufacturing precision. With achievablel tolerances of 5m, a surface roughness of aboutthe capability to use 20m wires, the wire cuttinguitable for micro and high precision machining. Theer prototype is 36 mm long, 30 mm wide and 3 mm

lement analysis

lement analysis (FEA) was performed to investigateant mechanism. 2D and 3D elements were used toicrogripper mechanism and predict the amplificationconcentration and displacements of the microgrip-

nism. Fig. 2 shows the finite element model of theer. The material properties used for the FEA of theer are that of spring steel or aluminium. The bulkentration occurs at the notch hinge points as shownhe maximum stress found is 2.1 MPa, which is muchn the yield stress and the elastic modulus of the

pe of the gripping device

ricated microgripper itself is still not a functionalnecessary to attach it to a fixture, connect it with an

d position it on the operating platform. Only then it istilization. The components required for the grippingshown in Fig. 4.

Fi

Fig. 6.actuator

Preics, codesired(PZT)a resolopen-lessarypiezoeachiev

ThiPZT aof thiscontroFig. 7. Inspection of the notch hinge and linkThe prototype of the gripping device with a PZT translator.

atic diagram of the experimental setup for testing of the PZTage features.


the microgripper.

and stoppedits tips in thtransport o

The propiezoelectr

Fig. 10. The tulations and tFig. 8. Various displacement modes ofFig. 9. A vertical Teflon wire gripped by (a) the flat ends or (b) the curved surfaces of the microgripper.

at any time. Thus, it allows the microgripper to closee desired position. This is useful to achieve a secure

r immobilize an object without damaging it.totype of the gripping device is shown in Fig. 5. Aic actuator (Model LVPZT P-842 from Physik Instru-

ip displacements of the microgripper obtained from the FEA sim-he experiments.

Fig. 11. Displacements of the PZT actuator and the gripper tip vs. the appliedvoltage to the actuator.


on th

mente) wasgrasping mactuator fuage of 010wear and teform billioare operateused for thestatic and dhighly relinon-magneThe maximIt is electro

5. Perform

Fig. 6 sdevices forThe experithe amplifimicroscopeplacementsactuator weattached. Tthe displactasks were

Under thposition. Emicrogrippper to prev

Beforeconductedof the protoobserved a

Duringthe microgusing the Pthe gripperobtained usrange of th200800

Using ptrolled opegripper isforward tow

mechT actool,zes b

ng acaceded s

. 10ed frondis werela

dispobtais re

ic ins oflatioe tipcom

crogtedtic so tipThited.exam

r delly wgripds oatchrippgrasectioFig. 12. Grasping action of the microgripper

combined with the microgripper to achieve preciseotion due to its very fine positioning feature. The PZTnctions with a typical open-loop at an operating volt-0 V. The lifetime of the PZT actuator is not limited byar. Tests have shown that the PZT actuators can per-

ns of cycles without any loss of performance, if theyd under suitable conditions [39]. The PZT translatortests is a high-resolution linear actuator suitable for

ynamic applications. The translator is equipped withable multilayer PZT ceramic stacks protected by atic stainless steel case with internal spring preload.um displacement provided by the translator is 60m.nically controlled by an amplifier unit.

ance testing

hows the experimental setup including measuringtesting of the PZT actuator and the microgripper.

mental setup consisted of the micro-gripping device,er unit used to drive the PZT translator and an optical

linked to a monitor for the measurement of the dis-on the microgripper. The displacements of the PZTre measured using a microscope with a CCD camerahe images of the displacements were captured andements were measured. Additionally, some grippingperformed using this setup.e normal condition, the microgripper is in its closed

xternal actuation is used for opening the tips of theer and offsetting the gripping force of the microgrip-ent damage to delicate objects.testing, a visual inspection of the microgripper wasusing the optical microscope to verify the feature sizestype. The flexural notch hinges and the linkages weres shown in Fig. 7.the functional testing of the prototype, actuation of

pliantthe PZas thewith sigraspiwire plor curv

Figobtainideal culation

Thetionalresultswhichceram

motionThe reand th

Bythe micalculaan elasthe twments.fabrica

Angrippetypicamicro-hundreture wmicrogtips tothe dirripper by hand was performed, as well as actuationZT translator, to check kinematical correctness of

. The displacement modes of the microgripper tipsing actuation by hand are shown in Fig. 8. The sizee part that can be grasped using this microgripper ism.

iezoelectric actuation, it is possible to perform con-ning and closing of the microgripper tips. The micro-actuated by the translating motion. A movementards the microgripper tip is transformed by the com-

gear and thof the geardamaging t

6. Conclu

Design,flexure-basprototypecharge mace miniature gear.

anism kinematics into the tip opening. Retraction oftuator closes the microgripper. With the microgripperit is possible to grasp objects of different materialsetween 100 and 500m and to release them. Sometions were made on a 500m Teflon wire. The Teflonvertically could be gripped by the flat ends (Fig. 9(a))

urfaces (Fig. 9(b)) of the microgripper.compares the tip displacements of the microgripperom the FEA simulations and the experiments. Undertions, the displacements obtained from the FEA sim-re larger than those obtained from the experiments.tionships between applied voltages and the opera-

lacements of the PZT actuator were identified. Theined in Fig. 11 show the expected hysteresis effect,lated to the material properties of the piezoelectric

side the actuator. The strokes of the open and closethe microgripper tips were observed and measured.nships between the applied voltage to the actuatordisplacement are also shown in Fig. 11.parison of the results in Fig. 11, it was evaluated thatripper mechanism has a mean amplification of 3.0by the average value of three sets of test readings andtroke range of up to 170m. It was also observed thats of the microgripper had slightly different displace-s could be due to the imperfection of the prototype

ple of manipulation was performed using the micro-vice. The assembly of miniaturized gear systems,ith diameters below 2 mm, requires the use of specialping tools. The miniature gears are typically of somef microns in size. Tasks of pick and place of minia-gears were carried out. Fig. 12 shows images of theer tips approaching the gear part and closing of thep the gear. In order to visualize the grasping process,n of the mechanical contact between the miniature

e microgripper was perpendicular to the rotation axis. The grasping motion was controlled so as to avoidhe tiny teeth of the miniature gear.

sion

fabrication and tests of a monolithic compliant-ed microgripper were performed. The microgripperwas manufactured using micro-wire electrode dis-hining. A displacement amplification of 3.0 and a


maximum stroke of 170m were achieved. The use of piezo-electric actuation allowed fine positioning. Micromanipulationtests weremicrogrippobjects. Sigood perfobe scaled to

Acknowled

The authter of Nanyresearch scAerospacethe first aut

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Biographies

S.K. Nah received his BE in mechanical and production engineering fromNanyang Technological University (NTU) Singapore in 2003. He pursued MEstudies in NTU from 2003 to 2005. Presently, he is working as a mechanicalengineer in Defence Science Organisation (DSO) laboratories, Singapore. Hisresearch interests include micro-manipulation as well as thermal and mechanicaldesign and analysis for electronic packaging.

Z.W. Zhong worked at the Institute of Physical and Chemical Research, Japanafter he received his Doctor of Engineering in precision engineering in 1989.He has also worked at the Gintic Institute of Manufacturing Technology, Singa-pore, and is currently at the Nanyang Technological University, Singapore. Hisresearch and development areas are electronics packaging, precision engineer-ing, mechatronics and design.

A microgripper using piezoelectric actuation for micro-object manipulationIntroductionDesign and fabrication of the microgripperFinite element analysisPrototype of the gripping devicePerformance testingConclusionAcknowledgementsReferences

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Microgripper