Qin Y. Micromanufacturing Engineering and Technology

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coatings. Gels with different levels of adhesive

strength are available depending on the sizes

of the components.

3. Belt magazines: a belt magazine has a band

made with depressions into whi ch the compo-

nent is locked by a covering layer of foil. For

small pieces, plated and embossed belts are

available to enclose the components.

4. Tube magazines: in the case of tube maga-

zines, the components are either stacked or

lined up. The lengths of the tubes, which are

made of plastic, are shaped like the compo-

nents themselves, thus preventing the compo-

nents from becoming distorted.

In general, a dimensionally specific carrier is well

suited for large volume production where changes

of micro-component types are not frequent.

Conventional Transport Equipment

Conventional transport equipment such as stock-

ers, vehicles, conveyors, manipulators, indexers

and lifters are used for transporting materials,

components and tools between the different sta-

tions in a linear pick-and-place plant based on

carriers.

Conveyors usually consist of a looped belt

or chain moving along a predetermined path.

Instead of conveyors, vehicles could be more flex-

ible and easier to adapt to changes in the environ-

ment. Both conveyors and vehicles are used to

transport the carriers and cassettes between the

different stations. Some examples of transport

equipment are: a person-guided vehicle (PGV); an

overhead-hoist transport (OHT); a rail-guided

vehicle (RGV); and an autonomous-guided vehicle

(AGV).

1. An OHT is a conveyor suspended from the

ceiling that loads/unloads carriers or cassettes.

2. An AGV is a trackless mobile robot typically

with a single gripper, a single manipulator and

two buffers. A buffer refers to a place on the

vehicle where one or two carriers can be

placed. Loading/unloading of carriers and

transfer between two carriers can be per-

formed.

FIGURE 18-2 Linear pick-and-place concept based on carriers.

CHAPTER 18 Handling for Micro-Manufacturing 301

3. An RGV is a rail-guided mobile robot of

which there are several types with various

numbers of grippers, manipulators and buf-

fers.

4. Finally, a PGV is a person-guided vehicle.

There are several types of vehicle, as with the

RGV.

Other components such as optical character

recognition (OCR), ionization units, carrier ID

or light curtains are occasionally added, based

on system requirements.

INTRA-MACHINE

MICRO-HANDLING SYSTEMS

A key problem area limiting the emergence of

automated micro-handling technology is the

non-availability of flexible and highly precise

micro-handling machinery [2].

The most significant difference between

meso- and micro-handling is the required posi-

tional accuracy of automatic handling systems.

Micro-handling often re quires sub-micron pre-

cision. This degree of precision is beyond the

calibration range of the normal open-loop

precision-handling devices used in industry.

Closed-loop strategies, such as real-time vision

feedback, are required to compensate for poor

kinematic models and thermal effects. Addition-

ally, in micro-handling the structural vibration

due to link flexibility must be controlled at a

sub-micron level.

Another major difference between handling in

the meso- and micro-domains is the mechanics of

object interactions. People usually think in meso-

scale terms, but when talking about handling

methods at the micro-scale, gravity and inertial

forces cannot be considered the main forces being

applied to the part. In the micro-world, surface

forces dominate due to scaling effects, and the

mechanics of manipulation can be unpredictable.

For example, when a gripper opens, the part may

not drop downwards.

Finally, it is necessary to emphasize that micro-

handling requires greater care in manipulation

and in cleanliness because the micro-parts are very

fragile. Usually it is mandatory to control the

grasping force s and to work under clean-room

conditions.

Since micro-scale positioning becomes a diffi-

cult problem, manual micro-handling is the

most commonly used method. In this case micro-

handling tasks are carried out by operators, who

position and align objects manually in a specific

station. The application of tele-operating micro-

handling, which transforms the human operator’s

hand motion by means of a joystick into the finer

3D motion of the system’s manipulator systems,

is normally tailored to specific complex tasks.

Finally, an automated serial pick-and-place task

is only applied to micro-parts which can be

handled and released using a well-suited gripper.

A micro-handling system is composed of the

following sub-systems:

1. Micro-part feeder sub-systems;

2. Fixture and gripper sub-systems;

3. Sensor a nd control sub-systems.

Significant Forces at the Micro-scale

In micro-handling, surface forces such as friction

or adhesion are immensely more significant than

in meso-handling. Therefore at the micro-scale

(masses <10

6

kg), gravity forces are not signif-

icant compared to surface forces, and releasing

an object becomes a real challenge due to adhe-

sion between the micro-part and the tool (see

Fig. 18-3).

Formally, a force is defined to be short-range if

it decreases with distance quicker than d

n

where

n is the dimensionality of the system (usually 3).

Short-range interactions are commonly dealt with

by imposing a cut-off to the potential V(d), d

beyond which V(d) is set to zero. On the other

hand, long-range forces have a range of infinity.

Inter-molecular and surface-force inte ractions

are classified into several categories. The first cat-

egory includes long-range attractive interactions

that bring particles to surfaces and establish adhe-

sive contact. These forces include van der Waals,

electrostatic, and magnetic forces. The second

category of forces is focused on adhesion, includ-

ing diffusion, condensation, diffusive mixing,

mutual dissolution, liquid and solid bridges

302 CHAPTER 18 Handling for Micro-Manufacturing

between particles and surfaces, and capillary

forces. The third category includ es very short-

range interactions, which can contribute to adhe-

sion only after an adhesive contact area has been

established. These forces include chemical bonds,

intermediate bonds and hydrogen bonds.

Balancing these forces depends on environ-

ment conditions, such as humidity and tempera-

ture, the surface condition, material properties,

geometry, and relative motion. Micro-handling

is quite different from meso-object manipulation.

To manipulate micro-objects, one must consider

micro-physics and pay attention to environmental

conditions.

There are three main adhesive forces that are

important for handling micro-parts: van der

Waals, capillary and electrostatic forces [3]. Van

der Waals forces come from inter-molecular

potentials and are always present. Capillary- or

surface-tension forces depend on the humidity of

the environment and electrostatic forces are due

to the tribo-electrical phenomenon. Much work

has been done by several authors, who have pre-

sented the principles of van der Waals forces, sur-

face-tension forces and electrostatic forces [4–5].

Some authors have described numerical methods

to compute the van der Waals force between a

smooth sphere and a smooth rectangular block

[6] or to compare capillary effects with surface-

tension forces [7]. However, micro-parts have

different shapes with considerable surface rough-

ness and often show large deformation in the con-

tact region. As a consequence, theoretical predic-

tions are rarely applicable in practice.

In order to improve micro-part manipulation,

different strategies have been set up to deal with

these surface forces [6,8]. First, the effects of sur-

face forces, F

adhesion

, can be reduced by choosing

an adapted set of manipulation parameters. This

can, for example, be done by changing the coat-

ing. Surface-tension effect s can be reduced with

hydrophobic coatings, electrostatic forces by

using conductive materials, and van der Waals

forces by increasing the roughness profile.

Micro-part Feeders

A micro-part feeder always uses a certain number

of functions to ensure the perfect treatment of a

micro-part at the workstation. These functions

are to: index; extract (isolate, disentangl e, sepa-

rate); sort; turn over; locat e; guide; move; load/

unload; release/seize; and recognize a micro-com-

ponent. Micro-part feeders, which singularize and

orientate parts prior to the station processes, are a

significant bottleneck in successful autom ation.

The basic kinds of part-feeding, which are

designed to feed and to orient the micro-parts,

may be classified as follows:

1. Mechanical feeders;

2. Manipulator-based part-feeders.

Mechanical Feeders. Commonly used mec han-

ical feeders for micro-handling are vibratory-

bowl feeders and tape-and-reel systems .

The most common approach to automated

feeding is the vibratory-bowl feeder, which con-

sists of a bowl filled with parts guided by a

mechanical track. The bowl and track are made

to vibrate, causing parts to move on the track,

where they encounter a sequence of mechanical

devices such as grooves, gaps, balconies, etc. Most

of these devices are filters that reject parts in all

orientations except for the desired one. Thus a

stream of oriented parts emerges at the end after

successfully running the full track.

Another particular form of mechanical feeder

is known as tape-and-reel for feeding parts of

FIGURE 18-3 Significant forces depending on part size.

CHAPTER 18 Handling for Micro-Manufacturing 303

relatively small sizes, which can be placed on

tapes of standard width. Tape-and-reel is a

method of housing parts in separate cavities in a

long continuous strip. The cavities are covered

with a plastic sheet to facilitate winding the strip

around a reel for component presentation or feed-

ing to automated placement equipment. These

particular mechanical feeders are based on stan-

dard belt or tube magazines.

For large-volume manufacturing, the employ-

ment of a dedicated mechanical part-feeding

apparatus may be justified. However, mechanical

feeders often fail due to jamming, and, most sig-

nificantly, generally require retooling when a

micro-component is changed.

Manipulator-based Part-feeders. Another com-

mon feeding c oncept is the use of a specially de-

signed carrier for each micro-part family to

maintain suffi cient accuracy for a completely

preprogrammed manipulator. In this case, the

manipulator w ill perform a serial pick-and-

place task using a specific g ripper mounted on

the manipulator.

Conventional manipulators do not meet the

repeatability and accuracy requi rements needed

in the micro-world because joint backlash, tem-

perature drift and structural vibration must be

controlled at the sub-micron level.

The state-of -the-art of precision industrial

manipulators is summarized in Table 18-2. This

table shows the most important parameters of the

workspace, the accuracy, the velocity and the load

capacity of different manipulators. The most

common configurations are SCARA (RRP) and

Cartesian (PPP) manipulators, but other config-

urations are also available. There are important

differences in degrees-of-freedom (DOF), accu-

racy, workspace and load. The most accurate

manipulators have reduced speeds. Cartesian

manipulators are usually slower than SCARA-

type robots, but have greater precision.

Micro-handling flexibility would denote that

different products with similar scales and com-

plexity levels are handled at the same station with-

out any hardware modifications, but with soft-

ware changes. Although there are technological

and economic limitations to reach in such a goal,

shorter product life and constant product changes

demand greater flexibility in the han dling process.

The most common princ iple for achieving flexi-

bility is vision-based flexible part-feeding, which

is a concept used in conjunction with a manipu-

lator, whereby the manipulator uses a vision sys-

tem to locate micro-parts that can be randomly

scattered on a conveyor belt. Randomly oriented

parts have to be positively identified and accu-

rately located everywhere within the camera’s

field of view.

The cost and performance of a micro-han dling

system can be significantly improved by carefully

considering issues at multiple scales: precision,

compliance, modeling, gripping, fixturing, toler-

ance, and control. One of the basic challenges in

precision handling is the need for very high accu-

racy ov er a large range of motion. This fact

involves the design of handling tools and pro-

cesses at multiple scales, and their integrat ion

into coherent system architectures. One possible

approach to micro-handling systems is to improve

the performance of standard manipulator sys-

tems. A conventional robot for coarse motion

with low accuracy but long traveling distance is

used and there is a fine-positioning device be-

tween the end effector and the robot with high

accuracy and a very small traveling distance.

Existing solutions have the common feature of

being expensive and bulky. Due to their dimen-

sions, they are sensitive to environmental pertur-

bations such as vibrations or temperature drifts. A

general trend is to reduce manipulator size. The

aim is to improve the system’s immunity to envi-

ronmental perturbations such as vibrations and

thermal drifts. The devel opment of such robots

is now be ing made possible by new technologies,

in particular by zero-backlash micro-gears and

highly dynamic micro-motors with integrated

incremental encoders, which allow manipulator

structures to be miniaturized.

Fixtures and Grippers

Fixtures are used to hold micro-components dur-

ing machining, inspection and assembly pro-

cesses. The use of precision fixtures can mitigate

304 CHAPTER 18 Handling for Micro-Manufacturing

TABLE 18-2

Some State-of-the-Art Industrial Micro-manipulators

Model By Type DOFS Rep. Acc. (um) WS (mm) Load (kg) Speed (mm/s)

E2C Epson Scara 4 8 R250 Z100 5 P&P

0.39 s

RP-1AH Mitsubishi Scara 4 5 150  105  25 1 400

P&P 0.28 s

YK-120X

YK-150X

Yamaha Scara 4 5–10 R110 Z30 0.5 700–2000

P&P < 300

Tusboscara

SR4-Plus

Bosch Scara 4 50 R400 Z200 2 1600

Autoplace400 Sysmelec Cart 4 2.5 150  150  150

85  85  75

MP63-25DC Feinmess Cart 3 5 25  25  25 2 5

MP84 Feinmess Cart 3 3 25  25  25

100  100  100

1.5 250

1940 Kopf Cart 3 1 128  128  128

MM3A Kleindiek RRR

(antr)

3 1 100 cm

z:12 x,y:180



Klocke Nanomanip RRP

(spher)

3 0.001 5  5  19 5

MRSI

Exfo PCS-4100 RPPPR 5 0.4 25  25  25 2

Semprex Univ. pipette

manipulator

PPPRPP 6 75  33  25



75,200

Somapatch MW3R/L CART 3 0.25 5  5  5

MRSI Newport PPPR 4 10 415  415 9000

CHAPTER 18 Handling for Micro-Manufacturing 305

the need for high positional accuracy. V-groove

structures, datum points or other minimum

energy surfaces should be used as much as possi-

ble to guide the micro-parts into their desired

positions and orientations.

Micro-grippers are used to grasp, hold and

release micro-components during the handling

process. Compliance analysis of micro-handling,

including both the gripper and the micro-part, is

critical for reducing the necessary positional accu-

racy of the system, and therefore the cost. The

handling system need not require the same preci-

sion as the tolerance of the locations if proper use

is made of co mpliance, gripping and fixturing.

Grasping Principles. A grasping principle is the

physical principle which produces the necessary

force to get and maintain a part in a position with

respect to the gripper. Some grasping princ iples

are well known in the meso-domain: friction-

based gripping [9]; form-closed gripping [10];

suction-based gripping [11–12]); and magnetic

gripping.

Other principles, based on adhesive forces, are

particularly applicable in the micro-domain [13].

One example is the use of electrostatics, or the

charge difference between the gripper and the part

[14–16]. For small, low weight parts, the capillary

force and the surface tension of a liquid between

the gripper and the part can be sufficient to hold

the part [17–18]. Van der Waals force is ano-

ther grasping principle [19]. Cryogenic gripping

means that a small amount of liquid is frozen

between the gripper and the part, so that the adhe-

sive property of ice produces the required force.

To release the part, the frozen material is broken,

and/or molten and evaporated [20]. Ultrasonic

pressure waves can also be used to lift a pa rt. Since

these forces are small, only extremely light parts

can be handled in this way [21]. A focused light

source, for example a laser source, can produce a

pressure which is sufficient to lift small parts. To

compensate for the mass of the part, the operation

may take place in a liquid [22,23]. The Bernoulli

effect has also been demonstrated to be applicable

for raising small parts; an airflow between gripper

and part causing a force which brings the gripper

and the part close together.

Grasping Requirements. Some grasping princi-

ples can only be applied when the environment in

which the operation takes place meets certain

demands, or, the other way around, the environ-

ment puts constraints on the selection of the

grasping principle. In some cases conditioned

environments are applied. Micro-handling may

for instance take place in clean environments, in

dry environments, or in a vacuum. The substrate

is important because adhesive forces occur

between both the part and the gripper and also

between the part and the substrate. One possible

attractive alt ernative is to manipulate parts while

they are immersed in a fluid, which eliminates

electrostatic and surface-tension effects. Fluidic

transport provides a powerful means for handling

components in many micro-systems and is

increasingly being employed in a number of such

applications.

Some grasping principles put constraints on the

type of material that can be gripped. Where

appropriate, the material type of the top layer, a

coating for example, must be considered. Adding

coatings to parts is a possibility that enables the

application of a grasping principle which could

not have been applied otherwise. Some examples

of the relation between the grasping principle and

the material type are: (1) the hydrophilic proper-

ties of the material are important for the appli-

cation of adhesive gripping; (2) gripping using

magnetism demands ferromagnetic materials.

The interaction between the gripper and the

part takes place via the part’s force-interaction

surface. This surface must be available for force

operation throu ghout the entire pick-and-place

cycle, for example from picking up a part at a

feeding position through to releasing the part in

the final position. Technical executions of grasp-

ing principles demand a certain configuration and

shape of the force-interaction surface. Friction-

based gripping will demand at least two locations

on the side of the part for finger placement. Sev-

eral other principles only demand one accessible

part surface. In developing a grasping solution, it

should be noted that geometric constraints may

impose limitations on the availabi lity of part sur-

faces.

306 CHAPTER 18 Handling for Micro-Manufacturing

In the case of a contact-grasping principle, one

possible situation that may occur during the

release procedure is that the part may stick to

the gripper and not remain static during the

release action. In this case F

adhesion

is greater than

gravity

(see Fig. 18-4). This situation would

require a device that would hold the part in place

during the gripper-release procedure. Such

devices do not have to be complicated to design;

one common method to overcome this problem is

to use sticky paper with an adhesive force slightly

greater than that of the adhesive force acting on

the part and grippe r.

Grippers. Suction grippers, such as vacuum grip-

pers, consist mainly of a thin tube or pipette con-

nected to a vacuum pump. This makes this kind of

gripper cheap and easy to replace. This is impor-

tant since micro-tools are fragil e and have to be

replaced frequently. The disadvantage is that the

vacuum gripper can obviously not be employed in

a vacuum.

Tong grippers grasp a part employing friction

between the part and the gripper, usually imple-

mented using a pair of fingers. Gripping parts

based on friction certainly provide sufficient force

in many cases. Another advantage of this kind of

gripper is the ability to center the part between the

gripping jaws and to align it parallel to the jaws.

This allows more precise handling of micro-parts

by putting them in a defined location after being

gripped. This is an important attribute in a han-

dling system with a high degree of accuracy and

speed. The more reference points that can be

applied to the location of the micro-part, the

fewer sensors that will be needed to determine

the same information. However, with decreasing

part size, the parameters of the gripper-finger

dimensions need to be small also. Here a technical

problem may occur, with this method having to be

eliminated due to technical or cost perspectives.

Adhesive grippers utilize the surface forces dis-

cussed earlier. The easiest force to control is the

electrostatic force, but it is not suitable for the

manipulation of charge-sensitive devices. Sur-

face-tension forces due to air humidity can be

controlled by incorporating a micro-heater in

the gripper. In cold conditions, an object can be

picked up simply by touching it. To release the

object, the heater evaporates the water in the con-

tact. Common manipulation methods with sur-

face forces include vibration of the gripper, sliding

and inclining tools or using dual manipulators.

The disadvantage of contact strategies is that

friction between the tool and the part may gener-

ate micro-dust from the object. Contactless

grippers have several advantages compared to

conventional contact grippers. A homogeneously

distributed force enables the sensitive gripping of

fragile parts. Disadvantages include low flexibility

for different part shapes and the need for addi-

tional equipment, such as ultrasonic sources and

pressure supplies. Contactless grippers include

air-cushion grippers and ultra-sound levitation.

The principle of an air-cushion gripper is based

on a vacuum prestressed air bearing. The gripper

includes several arrays of pressure and vacuum

FIGURE 18-4 Force vectors in a contact-handling

principle.

CHAPTER 18 Handling for Micro-Manufacturing 307

nozzles. These create an air cushion, separating

the gripper and the micro-part, which is levitated

about 10 microns underneath the gripper. The

weight of the part has to be in equilibrium with

the pushing and pulling forces exerted by the air

cushion.

Another contactless gripping method is ultra-

sound levitation, which is based on a squeeze film

or acoustic standing waves. Parts can be picked

from above by the combination of ultrasonic

waves and vacuum. The vacuum forces the parts

to the gripper surface. The air cushion generated

by the high frequency oscillatio n of the ultrasound

sonotrode produces a repulsive force on the part.

Finally, there is balancing of these two forces and

the weight of the handled part. A further phenom-

enon that can be used for handling parts without

contact is levitation based on acoustic standing

waves. The arrangement consists of a reflector

and a vibrating sonotrode, the assembly being

called a resonator. The distance between the two

elements is an integer multiple of half the wave-

length. In the resonator, small parts are levitated

in the pressure nodes of the standing wave.

Sensors and Control Systems

Obtaining accurate sensor information is difficult

at a micro-scale as sensors can be too large to be

placed in a tiny environment. The main sensors

used at a micro-scale are displacement, vision and

force sensors because motion control, visual ser-

voing and force-control strategies are often

required at micro-domains. These sensors have

to be extremely sensitive as the forces and displa-

cements involved are very small. Thus, to enable

micro-manipulation, miniature sensitive sensors

are needed.

Vision Sensors. Vision systems are often used

for the recognition and positioning processes to

locate the individual micro-components. The

camera resolution must be compatible with the

size of the manipulated objects. Optical micro-

scopes and scanning-electron microscopes are

therefore used. Applications in confined spaces

require compact camera systems such as fiber-

scopes or micro-cameras.

In a meso-domain, visually servoing has been

shown to effectively compensate for uncertainty

in the calibration of cameralens systems, manip-

ulators, and workspaces. However, manufactur-

ing engineers usually prefer strongly calibrated

parts-handling systems due to cost and reliability

issues. In micro-domains precise calibration is

highly dependent on precisely modeled kinemat-

ics, which is subject to thermal-growth errors.

Two common techniques for compensating for

thermal errors include the use of expensive cool-

ing systems, or waiting several hours for the ther-

mal equilibrium of the devic e to stabilize. These

types of factors greatly affect the cost and reliabil-

ity of handling systems, therefore real-time visual

feedback can be used effectively and economically

in a micro-domain. However, there are problems

with visual control or image processing in control-

ling these systems. Some of the main problems are

low processing speeds, high costs, programming

difficulties and being error prone due to glare,

reflection and other unwanted contaminants.

Moreover, the manipulation tools may obstruct

the view. Additionally, it is important to empha-

size the problems in setting up image-processing

equipment and the long down-times that are

involved if an error occurs. Even processes that

may seem straightforward such as focusing and

aligning the camera properly become complicated

and extremely difficult to carry out at these micro-

scales.

In micro-h andling, structural vibrations due to

link flexibility must be controlled at the sub-

micron level. Control of machine vibration

becomes very important as designers attempt to

advance the state-of-the-art with faster and lighter

machines. Many researchers have examined dif-

ferent controller configurations in order to con-

trol machines without exciting resonances. Even

with a sophisticated controller it is difficult to

rapidly move flexible machines without deflec-

tions and vibrations. A more achievable goal is

to eliminate residual vibration once the machine

has achieved a desired set-point. Input shaping is a

command-generation technique that reduces

residual vibration when a machine is moved from

one set-point to another. Input shaping works like

308 CHAPTER 18 Handling for Micro-Manufacturing

a notch filter that is designed to eliminate decay-

ing sinusoidal responses.

Micro-force Sensors. A typical micro-force sen-

sor structure is fabricated by micro-machining

technology on a silicon wafer. A diode sensor

has a cold-field emission cathode, which is a sharp

silicon tip, and a movable diaphragm anode.

When a positive potential difference is applied

between the tip and the anode, an electric field is

generated which allows electrons to tunnel from

inside the cathode to the vacuum. The field

strength at the tip and the quantity of electrons

emitted (emission current) are controlled by the

anode potential. When an external force is

applied, the anode deflects and changes the field

and the emission current.

Impedance control is a strategy adequate for

both free- and constrained-motion control. It con-

sists of an imposition of behavior on the system

rather than tracking a reference value. Mechani-

cal impedance is the dynamic relation between a

force acting on a body and its motion:

ZðsÞ¼FðsÞ=vðsÞ¼MðsÞþBðK=sÞ

where Z is the impedance, F the force, v the veloc-

ity, M the mass, B the damping and K the stiffness

of the system.

Opposition of the body to the force consists of

its stiffness, viscous dampi ng and mass. The func-

tion of the impedance control is to impose the

desired values for these three parameters instead

of the real value s.

APPLICATIONS

The serial pick-and-place task has been used

extensively in the micro-manufacturing industry.

There are specific pick-and-place machines used

for placing micro-components. The aim of this

section is to show a few examples of handling

applications which may refer to different kinds

of processes, for example in micro-fabrication,

inspection and a ssembly.

Micro-fabrication

The main intr a-machine micro-handling task in a

micro-fabrication station is the packaging of the

fabricated micro-parts into a standard carrier.

Intra-machine Handling System

in a Micro-forming Application

This example consists of the design of a micro-

handling system, which arranges the micro-parts

produced by a bulk-forming machine into a stan-

dard carrier.

The forming machine is set in a specific loca-

tion and will leave the micro-pieces in an output

buffer. This buffer guides the micro-parts along a

mechanical track, like a vibra tory bowl feeder

system. A vision control strategy for actuating

on the vibration will allow the individual micro-

parts to flow in order to synchronize the buffer

release process with the load process into the car-

rier cells.

The carrier has been defined following the

DIN-32561 standard. This carrier is a small tray

able to store hundreds of small pieces on it (see

Fig. 18-5). The shape of the cells for locating the

pieces depends on the piece shape. An empty car-

rier will be attached to a 2 DOF manipulator,

which will move the carrier under the vibration

sub-system, so that the empty cells in it are exactly

below the buffer output, permitting the pieces to

FIGURE 18-5 Vibration sub-system details.

CHAPTER 18 Handling for Micro-Manufacturing 309

fall into them. The vision system indicates to the

manipulator when a micro-part is into the cell to

proceed to locate the next empty cell.

When the carrier is full, the 2 DOF manipula-

tor will move it to a specific location where the

unload sub-system will take the carrier and put it

into a cassette. The load sub-system will take

another empty carrier and load it on the manipu-

lator.

Inspection

Usually component inspection requires different

points of view. A micro-handling system can be

used to obtain all the views required for a com-

plete inspection. For example, a rotational move-

ment can be used for inspecting cylindrical sur-

faces.

Stent Inspection System. A cardiovascular

stent is a small, expandable, slotted, metal tube

that acts like a mechanical scaffold in an artery.

Cardiovascular stents have hundreds of critical

features with tight tolerances. These stents require

100% inspection over all surfaces.

Stent inspection has been traditionally accom-

plished by manually operated stereo-microscopy.

The typical drawbacks of inspecting cylindrical

surfaces with this method are non-referenced

fields of view and the focal depths of stereo-

microscopes, which increase operator fatigue

and error. Often parts are rotated under the

microscope while the operator is looking from

checkpoint to checkpoint.

Nowadays the quality control of stents is

made by an automatic visual-inspection system

but the handling process is usually manual. In

the sampling plant, the stent is inside a plastic

tube, which protects it throughout the process.

This tube is also useful for identifying one stent

from the others. An operator manually extracts

the stent from the plastic tube and inserts it into

a needle. The operator fixes the needle on a

rotary-gripping system in front of a vision sys-

tem, and then the system rotates the needle while

the vision system s cans the stent surface. Once

the automated inspection has been done, the

operator removes the n eedle and extracts the

stent from it, puts the stent into the tube again

and screws its top on.

The aim of this example is to describe a stent

quality control application without human inter-

vention. In this application all the manipulation is

done by a 6-degrees-of-freedom mani pulator. The

whole handling system is composed of different

sub-systems (carriers, unscrewing/screwing grip-

per, manip ulator, funnel system, rotating disk,

insertion pin).

The inter-machine transport system consists of

a linear pick-and-place architecture based on car-

riers. The tube arrives in a carrier that is able to

transport nine tubes (Fig. 18-6). This carrier

approaches the inspe ction station on a powered

belt conveyor. The manipulation process is

divided into the following parts:

1. Picking: the manipulator picks the plastic tube

from the carrier.

2. Unscrewing: the manipulator carries the tube

to the unscrewing/screwing gripper and re-

moves the screwed cap.

3. Extraction: the manipulator transports the

uncapped tube to the funnel zone and empties

its content (the stent).

4. Gripping: the funnel guides the stent to a ver-

tical pin on the rotating disk, where it is

inserted by gravity and vibration principles.

A piezoelectric system mounted on the pin

holds the stent.

5. Inspection: the pin is rotated to the horizontal

inspection zone. There is a horizontal position

to perform the machine vision quality control.

6. Collection: the pin carrying the analyzed stent

is transported to the collection position, where

the stent is released and put into the tube.

7. Screwing: the tube with the stent is taken to

the unscrewing/screwing gripper where its cap

is located and then screwed on.

8. Placing: the tube is placed in a new carrier.

The first task must be the opening of the tube.

This must be done by a rotating four-fingered

gripper. The fingers of the gripper are developed

in order to accurately grip the top of the transport-

ing tube.

A 6-degrees-of-freedom manipulator is needed,

in order to be able to move the tube around the

310 CHAPTER 18 Handling for Micro-Manufacturing