A hybrid-type micro-gripper with an integrated force sensor

J. Park, W. Moon

Abstract A new hybrid-type micro-gripper that uses anintegrated force sensor to control the gripping force wasdeveloped for handling micro-objects. The micro-gripperis composed of a piezoelectric multilayer bender for ac-tuating the gripper fingers, silicon fingertips fabricated byuse of silicon-based micromachining, and supplementarysupports. The micro-gripper is referred to as a hybrid-typemicro-gripper because it is composed of two main com-ponents: micro-fingertips fabricated using micromachin-ing technology to integrate a very sensitive force sensor formeasuring the gripping force, and piezoelectric gripperfinger actuators that are capable of large gripping forcesand moving strokes. A systematic design approach wasapplied to the design of each of components of the de-veloped gripper, which made it possible to establish thefunctional requirements and design parameters of themicro-gripper. The micro-gripper was installed on amanual manipulator to assess its performance in taskssuch as moving micro-objects from one position to a de-sired position. The gripping force signal was found to havea sensitivity of 667 lN/V and several micro-objects weresuccessfully moved (grasped and released) with the de-veloped gripper. It was found during the testing experi-ments that the frictional forces between the working planeand the micro-object could be utilized to facilitate therelease of micro-objects from the micro-gripper.

1IntroductionSince the first tools were proposed for handling micro-objects, considerable progress has been made in themanipulation of micro-objects for assembling micro-partsinto micro-systems or for manipulating bio-objects such

as cells [1, 2]. Micro-objects are harder to manipulate thanmacro-objects because they are invisible to the naked eyeand they have different behavior. This difference in be-havior is due to so-called ‘scale effects’, which are believedto become important when the object size is less than1 mm [1]. These scale effects arise because the effects ofphysical quantities such as surface tension and electro-static force, which can be ignored in the world of macro-sized objects, dominate in the world of micro-objects. Inother words, physical parameters that can be ignored whenhandling macro-objects may be the most significant oneswhen manipulating micrometer-sized objects. For thisreason, the techniques adopted for the manipulation ofmicro-objects should be based on ‘micro-physics’, thephysics of the behavior and interactions of micro-objects.

Systems for manipulating micro-objects must containthree essential components: end-effectors that influencethe micro-objects directly, a micro-positioning system thatmoves the end effectors to the desired position and con-trols their orientation, and an assistant vision system thatenables the human operator to observe the positions andorientations of the invisible micro-objects. The operatorcan control the system in two ways. One way is for theoperator to ascertain the relative positions of the gripperfingertips and the micro-object by watching the imagefrom the vision system and then to operate the positioningactuators directly by adjusting levers on the control panel.The other way is to use a haptic device which sendscommand signals to the finger actuators and receivessensing signals from sensors integrated into the gripperfingers or the actuators. Either way, the accuracy of posi-tioning should be less than 1 lm. Regardless of the op-erating strategy employed, the end effectors of system (i.e.the gripper fingers and fingertips) are the most importantcomponents because they are directly involved in the pe-culiar micro-physics that causes micro-objects to behavein ways that seem extraordinary when compared with thebehavior of macro-objects.

Since the early 1990s, various types of grippers havebeen devised as components of systems that assemblemicro-parts or manipulate bio-cells [3–6]. Many notablemicro-gripper designs have been developed and fabricatedby a range of manufacturing methods to demonstrate thefeasibility of handling micro-objects with gripping tools[6–9]. Kim et al. developed a micro-gripper actuated by a‘micro comb-drive’ fabricated by silicon-based microma-chining [4, 5]. George et al. invented a micro-gripperbased on bimetal-type actuators; this micro-gripperobtains its motive force from the mismatch in thermal

Microsystem Technologies 9 (2003) 511–519 � Springer-Verlag 2003

DOI 10.1007/s00542-002-0267-6

Received: 30 May 2002/Accepted: 24 July 2002

J. Park, W. Moon (&)Pohang University of Science and Technology(Postech) San 31 Hyojadong, Namgu, Pohang,Kyungbuk 790-784, KoreaE-mail: wkmoon@postech.ac.kr

We would like to thank Dr. W. K. Chung and Dr. S. S. Lee fortheir helpful discussions. We also express our appreciation to Mr.S. J. Kwon and S. C. Ko for their help in the experiments. Thiswork is supported partly by the project ‘‘Development of CoreTechnologies for Fabrication of Micro Tele-Manipulators’’ fromMinistry of Science and Technology, partly by the project ‘‘BrainKorea 21’’ from Ministry of Education, and partly by Posco.

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deformations of two adhered aluminum and silicon plates[7]. Lee et al. utilized a shape memory alloy for the actu-ating material [9], and Arai et al. developed a piezoelectricgripper equipped with force sensors [10]. In 1996, Tanik-awa et al. introduced a chopstick-style micro-gripper as acomponent integrated into a manipulation system [11].From the late 1990s, researchers have been actively de-veloping micro-manipulation systems [10–14].

In spite of the research in this area, most of the micro-grippers developed to date cannot perform such funda-mental functions as gripping and stable releasing. Thislack of success can be attributed to the fact that mostgrippers have been designed in the same fashion as grip-pers for macro-objects, despite the large differences be-tween the behaviors of micro- and macro-objects. Giventhat the ultimate objective of a system for the manipula-tion of micro-objects is to place micro-objects at a desiredposition in a given orientation, micro-grippers should bedeveloped through a design process that from the earlystages takes into account the peculiar behavior of micro-objects.

One important constraint on the design of micro-grip-pers for manipulating micro bio-objects or cells is thatthe gripping force should be properly controlled during thegripping and releasing of objects because an excessivegripping force may fatally damage micro-bio-objects. In thispaper, a new gripper is developed that uses piezoelectricmulti-layer bender and stack actuators and micro-ma-chined cantilever-type fingertips with an integrated forcesensor. In this design process used to devise this system, thebehavior of micro-objects was taken into consideration inthe early conceptual design stages in order to achieve asmany of the functions required of a micro-gripper as pos-sible. This approach entailed devising methods for mini-mizing nominal problems such as sticking during thehandling of micro-objects and then integrating thesemethods into the gripper. Through the design processes thefunctional requirements and the corresponding design pa-rameters were established using a systematic design ap-proach. As a consequence of these efforts, a hybrid-typemicro-gripper was developed that was equipped with pi-ezoelectric multiplayer benders as finger actuators and fin-ger tips containing force-sensors fabricated by micro-machining. We refer to the system as a ‘hybrid-type micro-gripper’ because the finger actuators and finger tips aredesigned and fabricated by different technologies.

2Functional requirements and design parameters

2.1The required functions of micro-manipulation systemsMicro-manipulation systems for precisely manipulatingmicro-objects have the following essential functions:

FR I. Ability to observe the micro-object of interest.FR II. Ability to place a micro-object in a desired position.FR III. Ability to change the physical condition of a micro-

object to a desired condition.FR IV. Ability to posture a micro-object in a desired ori-

entation.

Vision systems based on techniques such as electron mi-croscopy or scanning probe microscopy can be adoptedfor FR I, end effectors can be used for FR II and FR III, anda precise positioner can be used for FR IV. Of thesecomponents for fulfilling the functional requirements, theend-effectors are the most important because they interactdirectly with the micro-objects. The micro-gripper is onetype of end-effectors, which grasps a micro-object bycontact and releases it for manipulating micro-objects forassembly or operations.

2.2Selection of a proper gripping methodDiverse gripping mechanisms have been proposed for thehandling of micro-objects. They can be classified intothree categories: (1) mechanical grippers that operate bydirect contact between the micro-objects and fingertips,(2) adhesive grippers that use adhesive forces between thefingertips and micro-objects [15], and (3) absorptivegrippers that use an air or acoustic pressure gradient tomanipulate micro-objects [14, 16, 17]. Among the grippersthat use the adhesive method, those that use liquids to gripthe micro-objects have problems when releasing the mi-cro-objects; these problems are caused by surface tensionand contamination of the micro-object by the liquids [18].For this reason, this approach has been seldom used inspite of the fact that it has no trouble in gripping objects.In addition, absorptive grippers that use air pressure orelectrostatic force to absorb objects have little flexibility incontrol. Given the drawbacks of the adhesive and ab-sorptive grippers, we chose to concentrate on developinga mechanical gripper that fulfils all of the functionalrequirements.

2.3Design parameters corresponding to FR I and FR IIThe fundamental functions of mechanical grippers aregripping and releasing micro-objects. In this section weestablish systematic design processes for the developmentof a micro-gripper by considering the functional require-ments for micro-gripper operation and the correspondingdesign parameters. It is expected that the design approachestablished here can be applied in future to the design ofother types of micro-grippers with improved functions.

The two essential functions of a mechanical micro-gripper – gripping and releasing – have the followingspecific functional requirements:

FR 1.1 (or 2.1) Ability to correctly grasp (or release) amicro-object at a desired position.

FR 1.2 (or 2.2) Ability to grasp (or release) a micro-objectwithout damaging the object.

FR 1.3 (or 2.3) Ability to grasp (or release) a micro-objectstably and repeatedly.

To achieve these functional requirements, a mechanicalmicro-gripper must be able to precisely position the fin-gertip of the gripper and it must be able to approach themicro-object from any direction so as to grasp (or release)the object at the desired position and in the desired ori-entation. The extent to which a micro-gripper fulfils the

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functional requirements FR 1.1, 1.2, 2.1, and 2.2 is corre-lated with the selection of an appropriate actuatingmethod, the number of actuators, the arrangement of theactuators, the number of degrees of freedom of the actu-ators for the fingertips, and the precision and speed ofpositioning of the actuators.

In contrast to FR 1.1, 2.1, 1.2, and 2.2, the design pa-rameters of FR 1.3 (gripping a micro-object stably andrepeatedly) differ from those of FR 2.3 (releasing a micro-object stably and repeatedly) because it is much moredifficult to release a micro-object than to grasp it. For thisreason, it is necessary to independently analyze the rela-tions between functional requirements and design pa-rameters for FR 1.3 and FR 2.3. Comparison of theseindependent analyses will then reveal the differences andsimilarities in the design parameters that result from thedifferent functional requirements. One design parameterthat should be important for both FR 1.3 and FR 2.3 is theselection of fingertips. The type, number and size of thefingertips will influence the ability of the gripper to stablyand repeatedly grasp or release a micro-object. In addi-tion, the ability to sense the gripping force is potentially avery important function when manipulating or operatingmicro bio-objects because excessive gripping force candamage the object being handled. The ability to control themagnitude of the gripping force by means of a force sensorshould additionally contribute to reducing the adhesioncaused by van der Waals forces.

In addition to the design parameters related to theselection of fingertips, supplementary methods can beincorporated into the fingertip to assist in the stable re-leasing of micro objects in repeated trials. It is well knownthat surface forces such as electrostatic forces, van derWaals forces, and surface tension have a greater influenceon the handling of micro-objects of less than 100 lm insize than body forces such as gravitational forces [1].Hence, adhesion of micro-objects to the fingertips viasurface forces can be overcome by removing the forcescausing the adhesion, by applying repulsive forces betweenthe fingertips and micro-object, or by controlling thephysical characteristics of the fingertips. In order to solvethe problems associated with adhesion by adding supple-mentary devices with new functions, the methods forimplementing these devices, as well as the approach thatwill be taken to reduce inherent adhesive forces betweenfingertips and micro-objects, should be considered in theearly stages of design.

The adhesive forces between micro-objects and finger-tips can be reduced if the origin and mechanism of theforces are understood. For example, studies on van derWaals forces have revealed that this adhesive force can beconsiderably reduced by coating the surface appropriatelyor by roughening the surface [10, 19]. In the case ofelectrostatic adhesive forces, the charges on the targetobject should be the main parameter that determines themagnitude of the adhesive force; hence, if these chargescan be removed as much as possible by a method such asgrounding, the electrostatic adhesive force should be re-duced considerably. The forces due to surface tension,another adhesive force that causes problems when han-dling micro-objects, can be almost completely eliminated

by drying the object surface. This can be accomplished bydecreasing the humidity of the atmosphere or by applyingheat to the surface of the fingertips. In addition to themethods outlined above for removing the causes of ad-hesive forces, other techniques can be used to apply ad-ditional forces between the fingertips and the micro-objectthat cancel out the adhesive forces. For example, electro-static or electromagnetic forces can be applied to the fin-gertips and micro-objects in order to cancel out theadhesive force. Another possible approach is to use theinteraction forces between the working table and micro-objects to assist in the manipulation. For example, appli-cation of attractive forces between the working table andthe micro-object facilitates the release of the object fromthe fingertips.

3Design processesThe peculiar behavior of micro-objects in comparison tothe more familiar macro-objects means that a systematicapproach to the design of micro-grippers has the greatestchance of success. The design of micro-grippers formoving micro-objects is complicated by the fact that themain cause of the adhesion of micro-objects to the fin-gertip is not clearly known. Given this problem, one ap-proach to micro-gripper design that has good chances ofsuccess is to eliminate all possible adhesive forces. Toachieve this, we need to consider all the possible responsesof micro-objects to external actions that can be predictedon the basis of physical principles as well as their knownspecial behaviors. Then, we must decide how the predictedproblems can be solved. If methods can be found to solveexpected problems, the design of the gripper should in-corporate these methods. The use of a systematic approachin such design processes is known to be very helpful [20].The following sections describe how the systematicapproach is adopted in the design of a micro-gripper.

3.1Design based on the functional requirements

3.1.1Selection of actuatorsThe actuators control the shifting of the fingertips of thegripper. Hence, the selection of actuators is crucial to thedesign of a gripper that satisfies the basic functional re-quirements FR 1.1, 2.1, 1.2, and 2.2. Various kinds of ac-tuators can be used for this purpose, for examplepiezoelectric stacks, piezoelectric unimorphs, electrostaticcomb-drives, and combined shape memory alloy struc-tures. In this study, two piezoelectric multi-layer actuatorsare adopted: one piezoelectric multi-layer bender and onepiezoelectric stack. The piezoelectric multi-layer bender isan example of an actuator that converts longitudinal de-formation into bending deformation when a voltage isapplied. It can provide larger displacements to the fin-gertips than other piezoelectric stack actuators because itis a type of bender, and it can apply a larger force thanactuators such as the unimorph or bimorph because it iscomposed of multiple thin layers of piezoelectric ceramics.

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Since the piezoelectric multi-layer bender can offer a rel-atively large displacement and gripping force to the fin-gertips, it was chosen as the finger actuator that applies thedisplacements and forces necessary for gripping and re-leasing. The advantageous characteristics of the piezo-electric multi-layer bender allow the maximum separationbetween the fingertips to be more than 200 lm. In addi-tion to the piezoelectric multi-layer bender, the widelyused piezoelectric stack was chosen for adjusting the endsof the two fingertips in order to minimize the mismatchbetween the two fingertip ends when they are broughttogether to grasp an object. This mismatch is usually in-troduced during fabrication. Since the piezoelectric stackcan exert a displacement of up to 5 lm with an appliedvoltage of 60 V, it can compensate for the mismatch of thefingertips [21]. In comparison to previously developedgrippers that use a special transformer to amplify thedisplacement, the gripper developed here using a piezo-electric stack and multi layer bender is capable of muchgreater displacements and gripping forces without a spe-cial transformer.

3.1.2Finger and fingertipsAnother design parameter to consider in connection withgripping is the finger configuration, as mentioned inSect. 2. The close relationship between the finger config-uration and functional requirements FR 1.3 and 2.3 has ledto the use of various kinds of fingers in previous studies.For example, fingertips have been constructed from siliconwafer, shape memory alloy or cylindrical glass needle [4, 8,11]. Construction of the entire fingers from a silicon waferby micro-machining has the merit that the resultinggripper can be fabricated in a micro-size and an arbitrarytwo-dimensional shape. However, this approach has thedisadvantages that the micromachining fabrication tech-nology constrains the size and shape of the resulting fin-gertips, which limits its working space [4], and that themicro-machined gripper is easy damaged in the assemblyprocess because it is so small and composed of brittlesilicon [5]. However, the fragility problem can be over-come by minimizing damage to the gripper tip duringassembly by means of a protector or special packaging inconjunction with precise control of the positions andforces.

In the present study, we develop a hybrid-type micro-gripper composed of fingertips constructed by silicon-based micro-machining and piezoelectric actuators con-structed by a conventional manufacturing process. Theadvantage of this approach is that it gives a gripper that iscapable of large displacement strokes of the fingertips, dueto the use of a piezoelectric multilayer bender as the fingeractuator, as well as delicate control of the gripping forcedue to the use of a micro-cantilever that contains a pi-ezoresistive force sensor fabricated by micro-machining.

3.1.3Design of the fingertipsIn order to manipulate micro-bio objects without dam-aging them (i.e., to satisfy FR 1.2 and 2.2), the grippingforce must be properly controlled [2]. One method to

achieve force control is to attach a force sensor to themicro-fingertip. If the signals from the force sensor inte-grated into the finger are delivered to a haptic system, thehuman operator can feel the gripping force through his orher hand. Using this system while observing the situationvia the vision system, the operator can easily control thegripping force with the hand or finger in real time. Thisadaptive operation considerably reduces the possibility ofdamaging the micro-objects. When micro-objects otherthan delicate bio-objects are manipulated, force controlmay still be useful because it can help to minimize van derWaals forces by minimizing the contact area between themicro-objects and fingertips [1].

To be suitable for use in a micro-gripper, a force sensormust have very high sensitivity and a relatively broadsensing range. In particular, the force sensor should be ex-tremely sensitive because it must detect the force in a micro-gripper whose gripping force should be controllable to anaccuracy of less than 1 lN in order to handle such delicatemicro-objects as bio-cells without damaging them. How-ever, it is difficult to achieve such a high sensitivity with aforce sensor that uses structures, strain gauges, laser inter-ferometers, and piezoelectric sensors of the conventionalsize [1, 22]. Hence, we adopted a micro-cantilever with in-tegrated piezoresistive strain sensors as a fingertip. Thissystem can detect very small forces by sensing the bendingstrain in the micro-cantilever induced by the contact forcebetween the fingertips and the micro-object in a similarmanner to the micro-cantilever probe of an Atomic ForceMicroscope (AFM). Since the micro-cantilever fingertip alsoacts as a force sensor for the micro-gripper, the two func-tions of the micro-cantilever place opposing demands on themechanical characteristics of the cantilever structure. To actas an extremely sensitive force sensor, the micro-cantilevershould be as flexible as possible, but its stiffness must besufficiently large to sustain the gripping forces imposed on itwhen it is used as a fingertip. Hence, the micro-cantilever isdesigned to be as stiff as possible on condition that itssensitivity is high enough to detect the smallest force de-sired. An important additional consideration in the designof the micro-cantilever is finding a way to reduce the ad-hesive forces that hinder the release of objects. It is knownthat a rough surface causes a smaller van der Waals forcethan a smooth surface [10, 19]. Hence, we constructed mi-cro-bosses on the surface that contacts directly with themicro-objects to reduce the adhesive forces between thefingertips and the micro-objects.

3.1.4Design process for vibration isolationVibrations of the fingertips during the processes of grip-ping and releasing an object make it impossible to carryout the task. To block structural vibrations, the grippersupporter should be designed to have a large stiffnessvalue and a high fundamental natural frequency [23].Since flexible joints in a structure lower its fundamentalfrequency, bolts were used, where possible, to join thecomponents of the gripper assembly. In this study, thenatural frequency of the fingertip and gripper structureswas checked by calculations using simplified modelscomposed of mechanical beams. The structure of the

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gripper assembly was designed to have a fundamentalnatural frequency greater than 100 Hz because the exci-tations due to movement of the positioning actuator arepredicted to be less than 100 Hz. After the micro-gripperassembly had been designed in detail, the fundamentalnatural frequency was confirmed using the commercialfinite element analysis package ABAQUS. The calculatedfrequency was about 560 Hz, which is sufficiently high tosatisfy the requirement.

3.2A hybrid-type silicon-fingertip gripperThe structure of the gripper designed through the pro-cesses described above is illustrated in Fig. 1. The gripperassembly is composed of a gripper supporter, a piezo-electric multilayer bender, a piezoelectric stack actuator,and a structure connecting the supporter to the actuators.The two silicon cantilever fingertips are installed at theends of the two actuators after first attaching each fingertipto a Printed Circuit Board (PCB) plate as shown in Fig. 1.The detailed geometric modeling and design as well as theprocess for assembling the parts were executed by use of a

commercial CAD package, SolidWorks from SolidWorksInc. The maximum distance between the fingertips wasdesigned to be about 200 lm because the objects to behandled have diameters up to 100 lm.

4FabricationMicro-fingertips equipped with an integrated force sensorwere fabricated using bulk micro-machining technology;Fig. 2 shows schematics of the fingertips. The sensitivity ofthe fabricated integrated-force-sensor is about 667 lN/V, ascan be seen from the relation between applied force andoutput voltage shown in Fig. 3. Since the sensor usespiezoresistivity to detect the bending strain of the micro-cantilever fingertip, its sensitivity changes with temperatureand the relationship between the gripping force and theoutput voltage is not very linear. However, these problemscan be solved by optimizing the fabrication conditions.

The most difficult of the assembling processes is theconnecting of the micro-machined silicon fingertip to thefingertip actuator, which may be a piezoelectric multi-layer bender or a piezoelectric multi-layer stack. Thefabricated silicon fingertips are too small and weak to bedirectly attached to the piezoelectric actuators. Hence, theyare first attached to the larger PCB plates, as shown Fig. 4,and then the PCBs are glued to the actuators. Micro XYZaxis stages, micro-rotational stages, and a holding jig wereused when the PCBs were combined with the actuators toensure that the fingertip ends were aligned as preciselyas possible. Throughout the assembling processes a12-times magnifying CCD camera was used to monitor thealignment. The final micro-gripper assembly is shown inFig. 5.

5ExperimentsThe objectives of the experimental testing were as follows:

1. To assess whether the actuator could move the finger-tips properly.Fig. 1. Composition of the hybrid microgripper

Fig. 2. The micro fingertips equippedwith an integrated force sensor

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2. To assess the performance of the fabricated hybrid-typemicro-gripper through tests of its ability to grasp, place,and release micro-objects at a desired position.

3. To look for any special interacting behavior betweenthe micro-objects and fingertips. Information on theinteracting behavior may be helpful for improving theperformance of micro-grippers in the future.

5.1Experimental setupThe entire experimental setup is shown in Fig. 6. Toconstruct a manually-operated gripping system for micro-objects, the fabricated micro-gripper was combined withtwo XYZ axis micro-stages, a rotational stage, a tiltingstage, three right-angle brackets, and a power supply. Thepositioning system in this manually-operated grippingsystem enabled movement of the micro-gripper over fivedegrees of freedom: the three translation dimensions, thetilting angle of the micro-gripper assembly, and the

torsional angle of the gripper assembly. The ability toadjust the torsional angle allowed the edges of the twofingertips to be maintained parallel to the ground, which isnecessary for stable grasping of micro-objects becausethere is a few micrometers mismatch between the fingertipends. The entire experimental apparatus was installed on avibration isolation table to minimize external vibrationaleffects.

Manipulation of the invisible micro-objects on theworking plate can only be done with the assistance of aproper vision system for observing the micro-objects. Thevision system should provide images from differentviewpoints to enable the human operator to easily com-prehend the relative positions and orientations of themicro-objects in three-dimensions. In the experiments, a12-times magnifying CCD camera was used for the sideview, a 10-times magnifying CCD camera for the other sideview, and a microscope for the top view. A backgroundlighting system was needed to clearly observe the micro-objects and the micro-gripper fingertips with the CCDcameras. The entire experimental procedure was recordedwith a digital camcorder.

The micro-gripper was tested for its ability to manip-ulate steel beads of size 50–100 lm. Steel beads werechosen because they represent the simplest objects withwhich to develop and test the force-sensor-integrated mi-cro-gripper. Ultimately, the micro-gripper should be test-ed on real micro bio-objects since the purpose ofintegrating a force sensor into the system is to preciselycontrol the gripping force when handling delicate micro-objects such as micro bio-cells. However, using live bio-objects involves many additional difficulties arising from

Fig. 3. Output voltage to applied force in the piezo-resistivestrain gauge

Fig. 4. Assembly procedures of the silicon gripper

Fig. 5. Setup for assembly and the manufactured silicon gripper

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the fact that the target undergoes random movements.Hence, in the present study we concentrate on the per-formance of the system when manipulating steel beads.

6Results and discussionIn the experiments, three micro-objects of diameter about80 lm were moved from one position to another. Theoperating procedure was as follows. First, the optical stagewas actuated to translate the fingertips to the locationwhere the target micro-object lay at rest. The fingertipswere then slowly rotated to approach the object by rotatingthe tilting stage, after which they were separated suffi-ciently to accommodate the object and translated slightlyso that the object was between the two fingertips. Theobject was then grasped by closing the fingertips. Subse-quently, the object was lifted by rotating the tilting stageupward and shifted to the desired position by adjusting theXYZ axis stage. When the fingertips were at the desiredlocation, they were slowly tilted down until they touchedthe surface of the work plate by actuating the tilting stage.The object was set down by opening the fingertips suffi-ciently, taking advantage of the friction and attractionforces between the micro-object (micro-bead) and thework plate surface. This procedure was repeated until allthe target objects had been moved.

The signal of the integrated force sensor was monitoredduring the testing of the gripping and releasing functions. Itwas found that the signal from the force sensor is directlyrelated to the voltage applied to the piezoelectric multi-layerbender actuators; hence, the sensor signal is proportional to

Fig. 6. Experimental setup for the grip-ping test of force sensing gripper

Fig. 7. A releasing method using frictional force

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the gripping force. The ultimate goal is to be able to adjustthe gripping force using a haptic device that receives themeasured force signals and sends command signals tocontrol the gripping force. The present results confirm thatthe force-sensor-integrated silicon fingertip developed hereis suitable for use as a component in such an adaptive sys-tem. The incorporation of a haptic device into the controlsystem will be undertaken in future work.

Since the micro-machined fingertip is a long thin platewith a rectangular cross section, its contacting surface withthe micro-objects is planar; hence, grasping was expectedto be easy. In our experiments the micro-objects tended tobounce from the fingertips unless the target object wasaligned in the center of the fingertips before grasping andthe entire operating procedure was conducted slowly andsteadily to grasp and release the micro-object with precisecontrol and without vibrations. The inclusion of micro-bosses on the contacting surface of the fingertips is be-lieved to reduce adhesive van der Waals forces by mini-mizing the contact area. Another interesting fact observedduring the experiments was that the friction and adhesionforces between the work plate surface and the micro-objectcould be used to facilitate the release of the micro-bead. Asshown in Fig. 7, we found that the success percentage oftrials to release a micro-object was considerably improvedwhen the interaction forces between the micro-object andthe work plate surface were exploited when the object wasreleased. Figure 8 shows sequential images taken whileusing the micro-grippers to move three steel beads of di-ameter less than 100 lm so as to align them in a straightline.

7Conclusions and future worksThe main achievements of the present work are as follows:

1. The functional requirements and their correspondingdesign parameters were established for a micro-gripperwhile developing a hybrid-type mechanical micro-gripper as an example.

2. A new hybrid-type micro-gripper was developed thatcombines micro-machined silicon fingertips containingan integrated force sensor with piezoelectric multi-layerbender and stack finger actuators. This micro-gripperwas designed on the basis of systematic design pro-cesses that from the early stages took into consideration

the peculiar behavior of micro-objects. The resultinghybrid-type micro-gripper has several merits:

2.1. The micro-machined fingertips are so flexible thatthey should hardly damage micro-objects by thegripping force.

2.2. The force sensors can be easily integrated into thefingertips provided they are realizable by surfaceand bulk micro-machining technologies.

2.3. The surfaces of the fingertips in contact with themicro-objects can be roughened to reduce thecontact area between the fingertips and micro-object.

2.4. The piezoelectric actuators can exert relatively largeactuating forces and provide large displacements ofthe fingertips.

3. In the experiments, micro-objects were successfullymoved from one position to another using the devel-oped hybrid-type micro-gripper installed on a manualmanipulator.

4. On the basis of observations of the behavior of themicro-objects during experimental testing, a methodwas proposed to use the frictional forces between amicro-object and the working table to facilitate the re-lease of micro-objects from the micro-gripper.

In future work we plan to manipulate micro bio-objectsusing a hybrid-type micro-gripper into which a position-ing actuator controlled by a remote haptic device, and ahaptic device to command the actuators for the gripperand the positioning actuators, are integrated. In addition,we intend to develop an improved micro-gripper designfor perfectly controlling the adhesive forces between thetips and the micro-objects.

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