Chapter 3 Handling of microproducts within the microfactory

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3.1 Introduction

As seen in Chapter 2, the automatic handling of microcomponents is a key task within the assembly microfactory. The main handling operations are:

• Sorting & Sieving • Grasping & Releasing • Feeding & Positioning • Inserting

• Orienting

These different tasks are carried out by means of different handling devices such as microfeeders to transport, locate and orient microparts, microgrippers to grasp and release components, microrobots to support and move microgrippers and micropositioners to carry microparts. Other devices as sorters and sieves are required to organize in preset locations randomly positioned microparts.

It is important to highlight that in this classification are considered as feeders the systems able to directly move microparts, while devices as carriers that are used to carry microcomponents are considered as micropositioners.

In this Chapter, some of these handling devices are analyzed by showing their state of the art. In § 3.2.1 are considered microparts feeders and positioners with particular focus on electrostatic feeders. In § 3.2.2 are treated grasping and releasing devices, in § 3.2.3 microrobots and manipulators and, finally, in § 3.2.4 are considered sorters and sieves.

3.2 State of the art in the handling of microparts

3.2.1 Strategies for transporting, positioning and orienting

microparts

In the microfactory concept, automatic microfeeders are needed to transport parts to the assembly stations and to move them within the assembly workspace. These devices have to locate microparts in the correct position along the required orientation.

Since in the microdomain surface forces equal or overcome gravity (please refer to § 2.1.1), traditional systems are not suitable and so innovative handling strategies have been developed. Most of them take advantage from the very little mass of the handled components: forces usually irrelevant in the macrodomain become adequate to handle parts of a few milligrams.

These novel handling strategies can be distinguished in two macro categories: contact and no contact handling as shown in Table 1.

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Microhandling devices for the assembly of Hybrid Microproducts

Contact handling (H.C)

Principle Actuation Description

Pushing tool Piezo [1][2][3] Use a pushing tool to move and _{rotate microobjects}

Friction Suction + Piezo [4] Use the friction caused by _{suction force to move objects}

Thermal [6][7][8]

Use thermal expansion of polyimide joints to achieve motion

Electrostatic [9][10] Use electrostatic force for the _{activation of cilia}

Magnetic [11] Use magnetic force for the _{activation of cilia} Cilia

Coordinated motion of array of cilia to move microobjects [5][6]

Piezo [10][12] Use piezoelectric to actuate cilia

No Contact handling(H.N.C)

Principle Actuation Description

Air flow Air [13][14] Use air flow to move _microobjects

AC fields [15][16][17]

[18][19]

Use AC electric field to move microobjects

Electric _{DC fields}_[20][21][22]

[23][24][25][26]

Use DC electric field to move microobjects

Standing waves: Small parts are held in the pressure node of standing waves between an ultrasonic source and a reflector Ultrasonic

waves Near field (or squeeze film): The reflector of the standing wave levitation is the levitated part itself. It allows the transporting of heavy objects [27]

Ultrasonic

[28][29]

Use of high intensity ultrasounds (f>20 KHz) to apply pressure forces on the microobjects and to lift them. The transportation of parts can be obtained by the motion of sonodrome or varying its electronic parameters [29]

Laser beam Laser _[30]

Use Focused Laser beam to move microobjects. Thanks to the beam reflection and refraction, an axial force pushes a component forwards in the direction of the beam

Vibration Vibrations _[31][32][33] Use surface vibrations to move _microobjects

Table 1: Contact and no contact transporting strategies.

Contact strategies

The innovative contact strategies make use of different approaches to move and orient microobjects:

• pushing tool

• active friction force field

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The pushing tool method exploits one or more tools with dimensions smaller [1] or comparable [2] to those of the object to be moved and oriented: this tool can be a piezoresistive AFM cantilever [1], a stainless tip sheet piezo actuated [2], a microslider actuated by piezoelectric vibration [3].

The active friction force field approach has been employed by Sin and Others [4] by using an array of pneumatic nozzles and piezoelectric actuators. The pneumatic nozzles make the object adhere to a plane by suction, then the plane, by means of piezo actuators, moves. Since the friction caused by suction force overtakes the one against the motion, the object moves in the same direction of the platform.

In the coordinated motion of array of microactuators, the key idea is to organize the simple motion of microactuators (cilia) to carry and position an object by means of the friction generated among cilia and objects. These cilia (so called in analogy to their biological counterparts in the human respiratory tracks) can be actuated with different principles such as thermal [6][7][8], electrostatic [9][10], magnetic [11] and piezoelectric one [12].

No Contact strategies

Concerning the no contact strategies, they employ various actuation principles: • electric fields

• magnetic fields • air flow

• ultrasonic waves • focused laser beam • surface vibrations • Bernoulli levitation

With regard to air flow manipulation, the lift and motion of microparts is obtained by the coordination of several air flows [13][14]. The key idea is to realize an array of microactuators having two or more nozzles electrostatic driven, that can be opened or closed permitting the air to flow. Konishi and Fujita [13] levitated and carried an object by controlling air flows from many actuators. Each actuator has two on-off electrostatic driven nozzles. Turitto and Others [14] proposed a feeder based on contactless pneumatic distributed manipulation: an array of microactuators having four nozzles allows the part to be suspended and moved by air flows. The motion and the orientation of parts are obtained by the coordination of the different nozzles, each of them actuated by electrostatic force.

The electrostatic manipulation, deeply considered in the following paragraph, exploits the principle that bodies in an electric field are subjected to forces arising either from electrophoresis (direct attraction of a charge in an electric field) or dielectrophoresis (polarization of the matter and its subsequent tendency to move into regions of different strength field) or from a mixture of both [20]. Both AC [15][16][17][18][19] and DC [20] [22][21][23][24][25][26] fields are used in literature to move microobjects.

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In the magnetic force handling, microobjects are levitated and/or moved by means of magnetic fields.

Ultrasonic waves handling makes use of high intensity ultrasound (f>20 KHz) to apply

pressure forces on microobjects and to lift them. The transportation of parts can be obtained by the motion of sonodrome or varying its electronic parameters [29]. A further classification can be done between Standing waves and Near field (or squeeze film) [28]. In “standing waves” approach, small parts are held in the pressure node of standing waves between an ultrasonic source and a reflector. In the “near field” technique, the reflector of the standing wave levitation is the levitated part itself. Thus, it allows the transport of heavy objects [27].

In the focused laser beam manipulation [30], thanks to the beam reflection and refraction, an axial force pushes a component forwards in the direction of the beam.

Similar to traditional vibration feeders, it is possible to feed parts by means of surface

vibration. Vorstenbosch and Others [31] made use of a vibration tooling plate to separate

and orient parts with dimensions between 0.5 and 5 mm.

The above mentioned non contact strategies show good performance in terms of reducing sticking effects and avoiding the damage of the small and fragile manipulated parts. The main advantages of no contact handling strategies are:

• surface forces are reduced or completely neglected;

• fragile and sensitive parts can be manipulated without risk of damage and contamination (these features are very important in case of delicate parts such as biomedical and optical devices);

• there are no problems in manipulating soft parts.

The electrostatic transporting and positioning of microparts

Electrostatic handling has some advantages in comparison with other no contact strategies:

• there is no residual effects on manipulated objects;

• both metallic and dielectric components in a wide dimension range (from a few µm to some mm) can be handled;

• electrostatic fields are quite simple to be generated and controlled in comparison to other methods (e.g. air flows).

In feeding, AC and DC electrostatic fields have been used to manipulate and position microparts. An important issue mentioned in literature with regard to the electrostatic feeding, is the need to eliminate or reduce adhesion forces [20]. In some AC electric feeders microparts are lifted by means of electric force and then adhesion forces are completely removed [15]. In DC feeders, authors reduce them by using air flow [20] or vibration [21][22][24][25].

With regard to AC electric fields, Moesner and Higuchy [15] lifted and conveyed metallic and plastic spheres with a diameter up to 400 µm with a non uniform electric traveling

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field: particles become charged through the application of three or six phase AC voltage to parallel electrodes and move perpendicular to them. A similar approach have been used in [16][17] by using AC traveling field. Gan Mor and Law [16] transported charged particulates with a diameter of 15 µm by means of a three phase AC electric field. This electric field is imposed by a series of parallel electrodes. Deasai and Others [17] realized a MEMS feeder that consists of 3 phase electrode arrays covered by insulators. This system, supplied with 200 Volt, is capable to transport particles with a diameter of 5-10 µm in air. Arai and Others [18] have developed a bio aligner able to control the position and the orientation of little objects (they made tests with glass spheres with a diameter of 100 µm) by means of dielectrophoresis forces arising among electrodes supplied with AC voltage. Fuhr and Others [19] induced the linear motion of dielectric spheres and living cells with a diameter of 5-100 µm by using traveling electric field. An AC voltage with frequency of a few MHz is applied to a linear pattern of electrodes: the resulting electrostatic forces levitate the particles and move them laterally in the direction of the traveling field.

Concerning DC electric field, Gengenbach and Boole [20] have developed an electrostatic feeder able to convey polymer parts of 10x5x1mm: a traveling electrostatic field is achieved by switching high voltage from one electrode pair to another one. Since their conveyor lifts objects thanks to the air flow instead of electrostatic forces, thus it permits to handle big parts. Böhringer and Others [21] positioned and aligned surface-mount diodes and capacitors with rectangular dimensions between 0.75 mm and 2 mm and short pieces of gold wire of 0.25 mm of diameter by using a parallel plate capacitor with apertures on the upper electrodes: the resulting fringing fields induce polarization in neutral parts, so that they are attracted to the apertures, and get trapped there. Fantoni and Santochi [22] used the concept of a moving capacitor between a platform and a series of parallel wires supplied one at a time: microspheres, microcylinders and other microparts with dimensions less than 1 mm move below the connected wire where the electric field is higher. This electrostatic feeder is able to orient and transport microcomponents along a straight line. Porta and Santochi [24] realized an electrostatic system able to center microcomponents. The device makes use of an electrostatic field with a radial gradient increasing towards the center to generate electrostatic forces acting on microparts and also utilizes a vibrating platform to reduce adhesion forces. The good choice of the electrostatic field and of the vibration amplitude and frequency allows the correct centering of microcomponents. Biganzoli e Fantoni [25] developed a device able to manipulate on a plane microbjects with dimensions in the range of 0.5-1 mm. A d.d.p. is applied between a V-shaped wire and an electrode platform: this electric configuration creates a maximum of the electric field under the tip of V. So, micro components are attracted toward this zone and then tend to align with the wire. By moving the wire parallel to the platform, components can be handled. Kikuya and Others [26] coated optical fibers with a thin metal film and aligned them within a “V” groove with electrodes on the two slopes of the groove by applying DC voltage between the fiber and both the electrodes. Controlling the voltage of the two electrodes, electrostatic forces make the fiber exactly aligned inside the groove.

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Table 2 summarizes the DC electric field handling strategies and the features of the manipulated objects.

metallic and dielectric microspheres with diameter of 0.8-1.2mm, microcylinders with diameter 0.5-1.5mm An electrostatic field with a radial gradient

increasing towards the center is used to generate electrostatic forces acting on microparts: parts move to the center where the electric field is higher. Adhesion forces are reduced with vibration.

Porta & Santochi

microspheres,

microcylinders and other microparts with dimensions less than 1 mm

microspheres,

microcylinders and other microparts with dimension less than 1 mm

rectangular surface-mount diodes and capacitors (0.75-2 mm) and short pieces of gold wire with diameter of 0.25 mm

polymer parts of 10x5x1mm

Handled parts

A traveling electrostatic DC field has been obtained by applying a DC high voltage to a series of parallel wires one at a time: parts move below the connected wire, where the electric field is higher. Adhesion forces are reduced with vibration.

Fantoni & Santochi

A d.d.p. between a V-shaped wires and an electrode platform creates a maximum of the electric field in the region under the V: micro components are attracted towards this region and tend to align with the wire. By moving the wire parallel to the platform, components can be handled.

Biganzoli & Fantoni

Using a parallel plate capacitor with apertures on the upper electrodes, the resulting fringing fields induce polarization in neutral parts, so that they are attracted to the apertures, and get trapped there. Adhesion forces are reduced with vibration.

Böringer, Cohn, Goldberg, Howe, Pisano

A traveling electrostatic DC field is achieved by switching high voltage from one electrode pair to another one. Adhesion forces are reduced by an air flow cushion.

Gengenbach & Boole

Scheme of the device Strategies

Author

Electric field

Nozzle plate

Table 2: DC electric field handling strategies.

3.2.2 Strategies for grasping and releasing microparts

In the microfactory, grippers are required to carry out assembly tasks, to handle components within assembly stations, to load pallets moving throughout the assembly workspace. The grippers have to pick microparts up and release them with the correct orientation in the preset place with high accuracy. Furthermore, they have to do that without damaging or contaminating the parts they handle.

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Many strategies have been developed for grasping microparts as shown in [34]. Some microgrippers configurations are downscaled from macro [35][36], while others take advantage, thanks to the very little dimension and mass of handled microparts, of surface forces or other effects for grasping components.

Principle Description

Friction

Objects are grasped by the friction between the fingers and the part. The opening/closing of the fingers is obtained by electrostatic [37][38], piezoelectric [39][40][41][42[43], shape memory alloy [44][45][46][48], magnetic [48] or pneumatic actuators [49].

Suction Objects are grasped by means of air suction [50][51][52].

Van der Waals Objects are picked up by van der Waals forces [53].

Cryogenic Objects are picked up by adhesion forces got freezing a little

volume of liquid [54].

Sound pressure Parts are levitated by ultrasonic radiation pressure [55][56].

Magnetic Parts are grasped by means of magnetic force.

Electrostatic Parts are grasped by means of electrostatic force [57][58]

[59][60][61].

Surface tension Use capillary forces to grasp microcomponents [62][63][64].

Optical pressure

A radial gradient force traps the component in the center of a beam when the component refractive index is higher than that of the surrounding medium [30][65].

Bernoulli

Due to the high pressure supply, air radially flows between the gripper and the component. The high velocity induces a dynamic pressure decrease (Bernoulli effect) leading an upwards attracting force on the component.

Table 3: Grasping strategies (adapted from [34]).

Conversely to the macroworld, in the microdomain the main problem is the releasing phase. Actually the gravity, usually used in traditional assembly to release components, is often less important than adhesion forces. Hence, components tend to remain stuck to the gripper, creating the need for the development of innovative releasing strategies. Table 4 shows the solution adopted in literature to allow the releasing of microparts.

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Principle Description

Sharp edge The object is released by putting it in contact with a

sharp edge [50][66].

Air pressure The object is released by an air pressure flow [50][67] 1 Additional tool

The use of an additional tool which has a contact surface with the object smaller than the gripper permits to detach the object from the gripper [50][68].

Adhesion on substrate

Adhesive force between the part and the substrate overcomes those between the part and the gripper allowing the object to be released [57].

Acceleration An acceleration given to the gripper support permits the

object to be detached thanks to inertial forces [62][69].

Micro heater

The microgripper is provided with a micro heater that permits to reduce the moisture (and consequently capillary forces) between the object and the gripper [70].

Electrostatic force

Parts are released with the inversion of electrodes polarity or shorting the gripper electrodes [57][58][60][61][70] [71].

Roughness change

The roughness change, obtained by vibration, reduces adhesion forces permitting the part to be release by gravity.

Gluing on substrate

Parts are released by gluing them on the deposal place [2][67] .

Varying contact area

The decreasing of the contact area between the object and the gripper allows the parts to be detached [50][53][60] [59][64][68][69][72].

Electrowetting

The surface modification of the liquid drop by mean of an electrostatic field permits to reduce the contact surface.

Scraping The object is transferred from a gripper to another one

by scraping [69].

Different adhesion

The object is transferred from an adhesion gripper to another one with higher adhesion force [69].

Table 4: Releasing strategies.

For parts grasped by means of surface forces, general consideration could be done on microgrippers’ shape, material and roughness to limit the contact area and, consequently, to make simpler the releasing of microparts. In order to limit the effect of surface forces, it is important to pay attention to various aspects:

• fingers’ shape: spherical shape rather than a flat one;

• fingers’ material: hard materials that do not modify their shape during contact rather than rubber or plastic;

• fingers’ roughness: rough fingers rather than smooth ones.

In order to make reliable grippers able to pick and place microparts, often a grasping approach is coupled with one or more releasing strategies. Table 5 (inspired by [73])

Sharp edge

air

Air

pressure Additionaltool

add.

g

Gravity Adhesionon substrate

Micro heater Electrostatic force control Roughness change a Acceleration Gluing on substrate Electrowetting E Varying contact area

scraping _adhesionDifferent T1 T2 T1 T2 edge air Air

pressure Additionaltool

add.

g

Micro heater Electrostatic force control

Electrowetting

E

scraping _adhesionDifferent T1

T2 T1

T2

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shows how grasping methods have been related to releasing strategy in grippers developed in literature. Grasping Principles Friction/ Jaw M agnetic Ice Van der Waals Electrostatic Adhesive f o rces (general) Adhesive ( liquid) Suction

Pressure by an acoustic sour

ce Pr essur e gener ated by a laser beam

Placing on adhesive substrate [57] Gluing the part in the right

place [2][67]

Greater adhesion [69] By positive mechanical

engagement [67] [74] Small puff of gas (blow

away) [67] [50] Mechanical release with

needle [67] Destruction of the gripper

mechanism

[54] [75]

Vibration of the gripper [76] [62] Dynamical effect [69] [62] Component against an edge [66] [50]

Auxiliary tool [50] [68] [50] Decrease the contact surface

(Rolling, varying area etc. ) [72]

[53] [60][59] [68][69] [64] [50] Electrostatic repulsion [70] [57][58] [60][61] Scraping off [69] Pressure generation by

thermal variation into microholes

[77] Roughness variation [70] Thermal control of surface tensions due to air humidity [70]

Relea

sing

Princip

les

Contactless handling [55][56] [30][65]

Table 5: Releasing strategies related to grasping strategies (adapted from [73]).

Since objects that differ in terms of material, shape, dimension etc. require different kinds of grippers, another issue considered in literature is the possibility to have many grippers available at the same time. The approaches used to fulfil this topic are:

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• Gripper changing system

• Revolving turret with different kinds of grippers

In the first approach, different kinds of fingers can be mounted on the same support and used with the same actuators. Clevy and Others [43] made different mechanical end-effectors with two fingers that are fixed and released on the tips of the microgrippers actuators by means of thermal glue. Each gripper is characterized by a finger shape, a gap between fingers and an opening/closing capability. By changing the end-effectors, a wide range of microobjects can be manipulated. Driesen and Others [78] equipped robots with a tool changer in order to increase their functionality and flexibility. The end effectors are adhesion grippers positioned on the tip of a bender piezo actuated. Also in this case, the actuator of the various grippers remains the same.

In the revolving turret approach, various end-effectors are available on the same support and can be selected by the rotation of the turret. A revolver tool changer with six tool positions has been developed within the Microassembly system developed at Karlsruhe Research center [79]. This revolver permits to select the most suitable gripper-tool on the basis of the microassembly task to be carried out.

3.2.3 Strategies for microrobot & micromanipulator

Other significant elements belonging to the microfactory are microrobots and micropositioners. As mentioned in § 3.1 microrobots are systems that support grippers or other end-effectors, while micropositioners are devices (as carriers) that carry and move microparts throughout the workspace. These two kinds of devices are treated at the same time because they usually have similar design and they make use of the same principles for the motion and/or for the actuation.

Like in traditional assembly, the features of microrobots and micropositioners in terms of DOF, versatility, and dexterity depend on the microassembly task. However (as mentioned in § 2.1.2) the precision accuracy required in microassembly is generally lower than 10µm and it can reach values lower than 1 µm. So, microrobots to be used in microassembly tasks have to get these precision capabilities [80].

As shown in Table 6 (adapted from [46] and [80]), a rough classification of precision robots suitable to be used in microassembly can be done by taking into account their structure and design approach. On the basis of their structure [80] the types of microrobot are serial, parallel, hybrid, while considering the design approach [46] the micromanipulators can be realized with a top-down approach or a bottom-up one. In the top down approach, the performance of conventional robots are increased by modifying their existing structures, whereas in the bottom-up conception new robotic systems are specially developed for microassembly tasks [81]. Some microrobots shown in Table 6 are commercially available (c), while others are developed by research institutes (r).

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Design approach

Top-down Bottom-Up

Cartesian

• Sysmelec Autoplace 411 [82] (c) • SPI Precision robots [83] (c) • Feinmess MP series [84] (c)

• MW3R/L Somapatch [89] (c)

• Robot of TOMI project [90] (r)

Serial Scara • Yamaha YK 120-150X [85] (c) • Bosch Turboscara SR6 [86] (c) • Mitsubishi Melfa RH-X [87] (c) • Epson E2C [88] (c) Hy brid

• Parallel hybrid robot [80] (r)

• Kleindeik MM3A [91] (c) • Nanomanip Klocke [92] (c) • Exfo PCS 4100 [93] (c) • NewPort: MRSI [94] (c) Planar • IWF Micabo –f [46] • IWF Micabo f2 [104][105](r) • Mitsubishi RP-series [95] (c) Structure Ty pe Parallel

Volumetric • IWF triglide _•_{IWF Micabo –h}[96] _[97](r) _(r)

Table 6: Classification of precision assembly robots (adapted from [46] and [80]).

In the top-down approach standard robots (e.g. little SCARA) are equipped with extra redundant axes of high resolution, additional sensors, special interface between the robot and the end-effector able to improve the manipulator functionality and precision [98]. This additional equipment give standard robots very good precision performance without losing the wide workspace they own [99]. The main drawbacks are that standard roots are very expensive (many thousands Euros) [80] and they have a high energy and space consumption in relation to microcomponents to be handled and assembled. So that their size make them not suitable to be inserted in little clean rooms where often the assembly takes place.

In the bottom-up approach, the improvements in terms of precision accuracy are usually obtained by reducing the size of manipulators and simplifying their designs. The scope is to improve the system’s immunity to environmental perturbations such as vibrations and thermal drifts and to reduce the overall costs (both production and running costs). Various micromanipulators have been specially designed for microassembly tasks and are commercially available as reported in [100]. Other microrobots exist as prototypes developed in research centers: some of them reach submicron resolution [101][102] while others are much smaller [80] or easier to be manoeuvred than those available on the market and assure a precision accuracy better than 5 µm [103]. In particular, at the Technical University of Braunschweig (IWF), various little parallel microrobots with high performance in term of accuracy have been realized. The “Micabo -f-“[46], especially developed for microassembly tasks, is a 4 DOF robot with hybrid-parallel structure that

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allows a very good repeatability (less than 3 µm) and, thanks to serial drives, a high range of rotation. The “Micabo -f2-“ [104][105] makes use of a 3D vision sensor and improves the performance of the parent robot achieving a repeatability of 0.6 µm.

With regard to the microrobot and micropositioner prototypes developed in literature, three main actuation principles are exploited for their motion: piezoelectric, electrostatic and magnetic ones (Table 7). Table 7 also reports, for robots equipped with control systems, the sensors used for feedback information (when more than one control devices are used, the reference is repeated).

Devices used for control (sensor)

Laser Interf erom eter/ Laser displacement sensor Capacitive senso rs Image processing / vision control AFM Encoder

Eddy currents sensors

Linear sensor Friction [106] [106] Push [99] [107] [111] Inchworm-walking drive [108][109][110] [108] Piezo Stick slip [113] [39] Electrostatic [119] Electromagnetic [48] [126] [123][130] [125][128] [125] Linear motors [104][105] [46][90][105]

Voice coil motor [131]

Motion Princ

iple

Magneti

c

Ferromagnetic fluids [132]

Table 7: Strategies used in micropositioners and microrobots.

Micromanipulators-Microrobots equipped with piezoelectric actuators

For many micromanipulators or robots, piezoelectric actuators are used following four main approaches:

• Push

• Inchworm (or walking drive) • Stick slip

• Friction

In the push strategy [99][107][111],the motion is simply obtained by pushing a slider or another object by means of a piezoelectric actuator. Since in this case the travel of the

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system is limited by the very little motion range of the piezo, often this approach is used as fine positioning only. Other actuators with rough resolution assure the system long travel.

In the “inchworm” or “walking drive” strategy a set of electrostatic [112] or piezoelectric [108][109][110] actuators clamp a slider first and then the motion is obtained with the alternative extension of one or more piezo actuators. This strategy permits to overcome the limit of piezoelectric actuators (the limited travel) without losing their strength (the high resolution). Hence, microrobots and micropositioners with long travel and high accuracy can be obtained.

The stick slip approach has been used in many microrobots and micromanipulators. The microrobots prototype developed in the European projects Robosem [113], Micron [39], Miniman I-II-III-IV, Proham, Robotman and Spider 1-2 [114][115][116] exploit piezoceramic or piezoelectric bimorph actuators as locomotion principle. Piezotubes are actuators exploited for the stick slip motion of the micropositioning of the platform in [117]. The main advantage of the stick slip method is that there is not limit in the length of the travel.

Another method exploits the friction between a platform and various spheres [106], each one actuated by means of two piezos, to obtain the motion of the platform.

Micromanipulators-Microrobots equipped with electrostatic actuators

Also electrostatic actuators can be used to move micromanipulators. In [119] a glass rotor, once polarized using a electrostatic field, is synchronized with a moving electrostatic field: sub micrometric and sub arc second displacements can be achieved. By the coupling of two electrostatic actuators, a comb drive resonator and a parallel-plate capacitor, it is possible to obtain a large displacement and high velocities in the direction of the required motion and stop the paddle in the desired position [119]. Often adhesion forces on the moving platform are reduced by means of various methods such as air flow [120], electrostatic force [121][122] etc.

Micromanipulators-Microrobots equipped with electromagnetic actuators

Magnetic actuators are also widely used. Many positioners have a platform equipped with permanent magnets that can be moved by means of electromagnetic forces generated by an array of wires [123] or coils [47][124][126][128][129]. Some of these micromanipulators utilize (electro) magnetic forces both to levitate and to move themselves [47][123][125][126][130], others only to move mobile magnetic platforms that are previously levitated by squeeze film [126]. In any case, the motion is obtained without contact and this fact assures the absence of friction with good positioning performance of the manipulator within the workspace. Microrobots in pipe for biomedical application [127] exploit a moving fin driven by permanent magnet: the movement can be controlled by the frequency adjustment of an alternative magnetic field.

Particular electromagnetic actuators for microrobots are linear motors that reach high performance in terms of speed and precision as the one shown in [90] (precision less than 1um, acceleration/deceleration 4g) and developed within the TOMI project (please

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refer to Chapter 2). These linear motors are frictionless, but they work at a temperature of 80°C: in order to avoid a thermal expansion in the robot structure, an efficient cooling system is required. Linear drives are also used in micabo robots [46][104][105].

An ultraprecision positioning table with nanometer accuracy and high speed has been obtained by using a hybrid linear motor in noncontact condition. This system exploits a DC motor for rough positioning and a voice coil motor for the fine one [131].

High precision positioning systems with one axis, multi axis and parallel kinematics movements are possible exploiting ferrofluids. Actually, amplifying the magnetic force acting on the surface of a paramagnetic actor which levitates in a ferrofluids, the system can be used for the high precision positioning of tables [132].

3.2.4 Strategies for microsorters and microsieves

Devices able to correctly sort and position many random parts and to select components, according to their shapes, are often the first equipment required in assembly systems. Actually, the first task in a microfactory is to make available to the following assembly operations microparts positioned and oriented according to the assembly features. All the self-assembly methods (please refer to § 2.3.2) can be considered as sorter systems. In general, these devices exploit force fields able to locate many components at the same time in stable predefined positions.

In Table 8 some of the most significant sorting strategies in literature are shown. These devices exploit different driving forces and minimization of energy to correctly sort and position microparts: Böhringer and Others [21] made use of fringing electrostatic fields, Fantoni, Porta and Santochi [133] exploited electrostatic potential minima, Yeh and Smith [134] used gravity in a liquid flow, Xiong and Others [135] combine hydrophobic-hydrophilic surface patterning and capillary forces in a liquid medium, Jacobs and Others employed capillary forces and minimization of the free energy [136].

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Surface mount LEDs (0.8 x1.6mm) and other components in the millimeter range . Many parts contained in a liquid are

positioned at the same time exploiting the hydrophobic-hydrophilic surface patterning and capillary forces of an adhesive liquid between binding sites located both in the objects and in the target areas.

Xiong, Hanein, Fang, Wang, Wang, Schwartz, Böhringer

GaAs/GaAlAs LEDs with a chip size of 280x280x200 mm3

Metallic microspheres with a diameter in the range of 0.9-1.2 mm

rectangular surface-mount diodes and capacitors (0.75-2 mm) and short pieces of gold wire with a diameter of 0.25 mm trapezoidal GaAs Led with basis dimensions 0.02x0.02 mm and 0.01x0.01mm

Handled parts

The sorting of components is achieved by a combination of a vibrating platform, to reduce friction and adhesion, and an electrostatic potential with several minima. Thanks to a suitable design of the electrostatic sorting device, only one component is attracted and trapped in each electrostatic potential minimum.

Fantoni, Porta, Santochi

The substrates present patterned, solder-coated areas that act both as receptors for the components of the device during its assembly and as electrical connections during its operation. The components are suspended in water and gently agitated. Minimization of the free energy of the solder-water interface provide the driving force for the assembly.

Jacobs, Tao, Schwartz, Gracias, Whitesides

Four microparts are sorted and positioned using a parallel plate capacitor with four apertures on the upper electrodes. The resulting fringing fields induce polarization in neutral parts, so that they are attracted toward the apertures, and get trapped there. Adhesion forces are reduced with vibration. Böhringer, Cohn, Goldberg, Howe, Pisano

In a liquid environment microparts are fed by a liquid flow towards a tilted table where a series of pockets with the same shape of the parts are engraved. Gravity makes parts fall down from the top and some of them remain trapped in the hollows.

Yeh, Smith

Scheme of the device Strategies

Author

1

2

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