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Journal of Advanced Manufacturing SystemsVol. 8, No. 2 (2009) 137152c World Scientific Publishing Company

A FLEXIBLE MICRO MANUFACTURING SYSTEM FOR

MICRO PARTS ASSEMBLY VIA MICRO VISUAL SENSING

AND EAP BASED GRASPING

DUGAN UM

Mechanical Engineering, Texas A&M University Corpus Christi

Corpus Christi, Texas, 78412, [email protected]

BAHRAM ASIABANPOUR and JESUS JIMENEZ

Manufacturing Engineering, Texas State University

San Marcos, Texas, 78666, [email protected]@txstate.edu

In this paper, we developed a flexible micro workcell for micro part assembly and bio

materials handling. The primary focus of the research is on the flexible manufacturingsystem that can handle parts in the size of 100 to 500 m for various applications. Flex-ibility in micro assembly, though important, has not been examined in depth due to thecomplexity of micro operations of small parts. Micro gears, micro glass fibers or fragile biomaterials require flexibility in gripping and haptic feedback control for further operation.To that end, we design grippers made out of electro active polymer, controlled by highprecision micro manipulator for a novel micro assembly process, namely flexible microassembly system (FMAS). The areas of research include micro/nano electro-mechanicalsystem (MEMS) material and structure, micro sensor/actuator system, visual feedbackcontrol system, micro-robotic arm motion control and flexible micro-gripper system. Inorder to verify the functional aspect of the FMAS, we made micro mechanical gears fab-ricated via bulk micromachining technology for 3D micro vision capability and handling

precision of micro robotic manipulator.

Keywords: Micro assembly; micro workcell; MEMS; flexible manufacturing; micro vision.

1. Introduction

Demands for micro/nano products and assembly systems have been significantly

raised to meet the ever complex technical needs for modern society. The market for

such microsystems has been climbing at a rate of 20% to 30% per year since 1990;

the worldwide market for microsystems has been estimated to approach 30 billiondollars in 2009.13 Full-blown application of microsystems, however, is hindered by

manufacturing constraints together with a high assembly cost For example a com

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138 D. Um, B. Asiabanpour & J. Jimenez

capability still falls short of industrial implementation in terms of mass production.

Successful development of a micro-manufacturing system will open up wide-ranging

application opportunities in homeland security, transportation, aerospace, biomed-

ical, advanced manufacturing, and many other commercial applications.

Existing micro-manufacturing systems have been developed primarily to assem-

ble a matrix of arranged micro components for various micro-structures. Novel

processes like LIGA (lithography, electroplating, and molding), microcasting, and

micro electrical discharge machining (microEDM) have been developed to fabricate

microcomponents for miniature machines. When considering the manufacturing

aspects of such miniaturized components, however, neither an efficient nor an auto-

mated assembly system has been reported for a full-fledged solution. This research

aims to develop a novel microassembly system that can see and feel whenhandling micro objects by 3D vision and haptic feedback respectively.

Development of microdevices, micro sensors, and microactuators has been esca-

lating due to increasing trends for product miniaturization and advances in science

and technology. Greater flexibility and capability in microassembly systems will

be facilitated by the development of improved microgrippers. Ideal microgrippers

would be able to handle a variety of micro/nano components for biological, medical,

aerospace, manufacturing, chemical, or security applications. Most commercially

available micro assembly devices are open loop systems, i.e. a trained operator is

required to operate the system via high magnification. Without sensitive feedback,handling of fragile biological cells and advanced materials such as carbon nano

tubes or optical fibers proves difficult since an operator has to constantly watch for

minute shape change of the sample or risk damaging it. Although microassembly

of prototypes and low volume microsystems could be done with skillful operators,

having human operators assembling high volume microsystems would not be eco-

nomically justifiable due to high labor and training cost, fatigue, low yield, and

inconsistent product quality. The risk is even higher when having a human oper-

ator working on hazardous chemicals, radioactive components, or contaminated

biological microtissues.While some preexisting micro-manufacturing systems, such as the one from

Zyvex, Inc., provide excellent accuracy, they lack the flexibility to span the assembly

system to the next level of technical domains, including military, homeland secu-

rity, transportation, aerospace, and other commercial applications due to material

constraints and lack of flexibility. The micro gripper system developed by Sandia

National Laboratory demonstrates excellent object identification and manipulation

capability (James). Their micro-manufacturing cell, composed of several compart-

ments, can cope with up to 100

m parts with its multi-camera micro-vision system.However, since the image processing system works only off-line, the application area

is significantly limited Furthermore next generation micro manufacturing systems

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A Flexible Micro Manufacturing System 139

either by manual or autonomous assembly operations. Due to the tedious and time

consuming assembly works by manual operator, innovative autonomous assembly

platforms are in essential demand. The general domain of micro devices assem-

bly (MDA) now thus emerges into the field involving the study of computer-based

methods to accomplish the assembly of micron-sized parts described as automated

micro devices assembly (AMDA). To that end, delicate and dexterous micro assem-

bly grippers are the core technology and have been under in-depth scrutiny so far.

In Bohringers study, thorough reviews have been made as to the role of the

interactive forces at the micro level including sticking effects, adhesion, electro-

static forces, etc. In the course of the wide ranged study of the micro forces, force

measuring devices for micro level have been proposed and tested.

One example of the micro-grippers design is introduced in Carrozzas study,where a LIGA-based micro-gripper is manufactured with semiconductor strain

gauges. The micro-gripper system is composed of a micro-gripper, 3 DOF micro-

positioners, a PC, and Phantom 1.0 haptic device. In the decision of strain-gauge

location, they utilized FEM to find the maximum stress point.

Another important aspect of micro assembly for AMDA development is on the

machine vision and virtual reality technology for micro-parts localization. The pri-

mary usage of machine vision in micro assembly is to facilitate object identification

and to measure the gripping forces during the grasping operation. Virtual reality in

conjunction with the machine vision enables users to propose and estimate assem-bly operation beforehand. Some optimization can be made on robotic motion and

path planning via VR technology as well.

As is addressed in the literature survey, micro assembly system, as an inte-

grated manufacturing cell, requires various technical components, such as intelligent

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micro-grippers, machine vision, and robotic manipulators. To meet these needs,

researches are being conducted to develop a novel micro-manufacturing system that

is autonomous, intelligent, and flexible in structure as shown in Fig. 1. The proposed

micro-manufacturing systems can assemble or disassemble arbitrary components

based on assembly sequence maps to deal with mission critical parts such as micro

sensors, micro actuators or biologically/chemically hazardous embodiments, etc.

2. Research and Results

The complete embodiment of the novel microassembly system is composed of a

micro-robot, micro-grippers, and a microscopic visual feedback system. In order

to make a micro-work cell functional, a 3 degree-of-freedom (DOF) mini robotichand is utilized. To form optimally shaped fingers for this robot hand, the fingers

are fabricated from piezoelectric materials, namely polyvinylidene fluoride (PVdF)

ionic polymer.

A prefabricated prototype of the piezoelectric materials used for the robotic

hand and fingers is shown in Fig. 10. Such electroactive polymers (EAPs), which

can be molded into an intricate shape, can move micro to millimeter distance when

a small voltage is applied to the grippers. Use of EAP for robotic actuators was also

recommended due to the material characteristics (i.e. high power density, low cost,

easy to form into special shapes), and ability to combine sensing and actuation intoa single device.10

Both the vision system and micro-grippers are integrated into an autonomous

assembly system to recognize, grasp, and then manipulate a micro object. The

image processor converts the captured infrared image to 3D geometry, followed by

template matching for object localization. During the template matching process,

the current geometry obtained from an infrared image is compared against the

reference template in a database from which incremental changes will be calculated,

thereby determining the exact position and orientation of the to-be-manipulated

object.

2.1. Micro gear fabrication

In order to provide mechanical components of micro size for assembly testing,

silicon-based micro gears are fabricated. Bulk micro machining, one of the MEMS

technologies, has been utilized for fabrication of the gears in a class-10,000 certified

cleanroom.15 Figure 3 illustrates the overall process of bulk micro machining of

MEMS gears from oxidation to KOH etch. The critical dimension that is soughtafter is 1 micro meter. The wafers used to create the aforementioned micro parts

are from doped P type silicon with a Miller Indices crystal orientation of

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Fig. 2. Photomask of micro gears, latch, piston and assembly bed masks.

Unwanted SiO2 is then removed during the buffered oxide etch process. In the final

step of the process, the parts are released from the wafer using an etch solution of

potassium hydroxide.

In order to fabricate micro gears, the thickness of the SiO2 layer needs to be

at least 20,000 Angstroms thick.17 This was accomplished by baking the wafers at

1,000C for 15 hours while introducing 2 liters per minute or 58 millimeters on the

flow meter scale of wet oxygen gas. In the photolithography process, an image ofthe desired parts is optically transferred onto the SiO2 layer using ultra violet (UV)

light. The photomask for this research project was created using a CAD model and

a photo reduction camera and appears in Fig. 2. Details of the bulk micro machining

process has been excerpted from the previous work described in the paper by Dugan

& Scott as below.

The optical transfer of the image on the photomask to the silicon substrate

is accomplished by adding an even layer of photoresist to the wafer through the

use of a vacuum hold and a high RPM programmable spinner. Once the wafer

is baked, it is placed in the photolithography machine. Proper alignment of thewafer with the photomask is essential in this stage. The wafer is then exposed to

UV light for 2.5 min. Best results are achieved using 2.5 ml of AZ5124E photoresist

and applying it to the wafer at 2,000 RPM spin rate. In the develop process the

unwanted photoresist is removed leaving only the desired parts protected by the

photosensitive polymer. Satisfactory results were achieved by using a 1:1 ratio of

developer and de-ionized water and gently agitating the wafer for 10 seconds. In

order to remove unwanted SiO2 on the wafer, Buffered Oxide Etch (B.O.E.) solution

is used. Finally, micro parts such as gears, latch, piston, etc. . . are removed fromthe wafer using a potassium hydroxide (KOH) solution.

The growth of the oxide layer has been estimated and observed with filimetry

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Fig. 3. Micro gear fabrication process.

The bulk micro machining technology utilizes discriminate etching between

materials; pure silicon etches at a rate of 100 micron per hour while the SiO2

etches at the much slower rate of 800 nm per hour at 90C. A method experimentwas then conducted to attempt the optimization of the part removal technique.

The initial method experiment run had the parts wafer in the vessel face up with

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Growth over Time

19698.3

17740.0

15218.3

12991.7

8369.4

8507

11839

14450

16703

18897

0.0

5000.0

10000.0

15000.0

20000.0

25000.0

3:21 6:11 9:00 11:51 15:00

Time

AVGS

iO2Growth

Results

Calculations

(a)

(b)

Fig. 4. (a) Silicon-Oxide (SiO2) layer thickness growth, (b) KOH Etching rate at 45% solution.

into the bottom of the tray, they can be removed from the solution by lifting the

vessel out of the KOH. The preliminary results using this method, however, were

less than successful. The gears were over etched and formed square angles on all

surfaces. In order to resolve this issue, a gravity driven stop etch process used to

create cantilever beams for atomic force microscopes was adopted as a possible

solution.11

It was determined that an etch stop needs to meet the following three criteria.

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Fig. 5. Etch stop bed.

(2) The temperature of the etchant should be easily controlled with the heating

energy transmitted through the etching stop solution.

(3) The etch stop solution must be chemically inert to both silicon and KOH.

In order to meet the criteria for etch stop, a etch stopper has been designedand fabricated by a Rapid Prototype machine (Fig. 5). As an etch stop solution,

Diiodomethane (CH2I2) was chosen due to the fact that the CH2I2 is a very dense

liquid and naturally separates itself from the KOH into two distinct layers. The

basic idea behind this process is that as the parts release from the silicon wafer

they fall through the KOH layer into the CH2I2 and the etch process immediately

stops preventing the parts from being over etched, thereby preserving the integrity

of the parts. This allowed us to fabricate a gear of micro size as shown in Figs. 6 and

7. The square feature appearing in Fig. 7 has been formed by the crystallographic

arrangement of the silicon wafer and reaction with the KOH. Based on the various

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Fig. 7. Released gear with unwanted square features.

conducted experiments, squaring of the features is unavoidable. The three main

orientations of crystal silicon are ,,; of these only the

orientation was tested.11 The subsequent orientations will need to be tested in

order to determine whether they can be used to improve the final parts. Wafers with

different crystal orientation, or even amorphous orientation, can also be tested. The

anisotropic nature of KOH etch may be used as an advantage in creating preciseparts. If the nature of the etch rate and selectivity can be predetermined the parts

can be designed to begin as an amorphous non-descript shape but become perfectly

defined as the etch process evolves.

2.2. Micro 3D infrared vision

In order to realize a microscopic 3D visual feedback system, a monovision based

3D visualization system is studied and developed using an infrared proximity array

(IPA) to capture image and geometric data of a 3D object. As a 3D visual device,the IPA system includes light sources utilizing infrared light-emitting diodes, an

infrared light filtering lens, and a charge coupled device array. The principle of 3D

visual sensing via monovision IPA is;

Ep (dp)2, (1)

where Ep is the reflected infrared light energy and dp is the distance to an object

from the pixel, p.14 This relationship allows the scanning of the infrared energy of

all the pixels to obtain not only an accurate geometry of the object but also tocapture the position and orientation of the object relative to the sensor.16 As a

result one IPA system is equivalent to 307 200 infrared sensors in a 2 5 2 5 mm2

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The goal of the 3D infrared vision technology is to determine x,y,z coordinates

of the micro gears fabricated by MEMS technology. The vision system determines

the location of the gears on the x-y plane so that the robot will be able to locate

the gear and grip the gear. The micro vision system uses infrared sensor technol-

ogy to accurately measure distance with a 2D array image and, thus, synthesizes

3D geometry of sensed objects in real time. In order to generate a micro parts

image, the vision system is assembled together with a microscope for miniature

parts recognition (Fig. 8). To make the gear recognition simple, a profile averaging

technique is used to find the centroid of a gear as detailed below.

Scan the 3D image profile produced by the infrared camera.

If the profile is higher than the threshold, then add the values ofx and y location

into x-sum and y-sum.Divide x-sum and y-sum by the number of data points to get the centroid of

the part.

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(a) No voltage applied. (b) Voltage applied.

Fig. 9. States of the IPMC-based Electro-Active Polymer before and after voltage is applied.

2.3. EAP gripper

Electro-Active Polymer (EAP) is a material that contracts when voltage is applied

as illustrated in Fig. 9. The response of this material is similar to the contraction of a

human muscle, and therefore is commonly referred to as an artificial muscle. Several

types of artificial muscles are commercially available. The type of artificial muscle

used in this research is based on an ionic polymeric gel treated with platinum,

material also known as ionic polymer metal composite (IPMC). To manufacturean IPMC, a thin strip of polymer undergoes the oxidation process to composite a

metal coating on either side of the polymer surface. To create movement, voltage

is run through the material by applying opposite charges to either side of the strip.

Voltage produces an electric field that causes redistribution of charges in the IPMC,

which then causes the muscle-like movement of the polymer. In general, IPMCs have

shown to be effective for micro-gripping because of their large motion sensing and

actuation and low cost. See Ref. 12 for a detailed description of IPMC.

2.4. Load carrying capacity vs voltage

A specific amount of voltage is required to produce a force sufficient enough to

lift the mass of the gears. The trends found by previous researchers show that

both displacement and force of the IPMC increases proportionally to increases in

voltage. For instance, Deole and Lumin found that applying a voltage of five volts

or less can produce large displacements in micro-object manipulation. The method

of experimentation used by Lumin and Deole characterizes load carrying capacity

as a function of voltage by using a trial and error approach (i.e. lifting several loadsof varying weights and noting the applied voltage). Higher load carrying capacity

enables heavier load lift without failure

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width of micro-gripper (Factor B), and applied voltage (Factor C). As for the length

of micro-grippers, two different lengths (i.e. 5 mm and 15 mm) were chosen to be

proportional to the size of the gears. The two levels of width, 1 mm and 2 mm, were

chosen in the same manner as length. The two limits of applied voltage were chosen

to be 3 volts, as a minimum necessary to cause a significant force displacement, and

6 volts, to avoid damaging the material from overvoltage.

The experimental results were analyzed using the analysis of variance (ANOVA)

(see Fig. 10). The significant factors can be observed graphically in Fig. 11 on a

normal probability plot. Any significant factor lies a considerable distance from the

theoretical normal line. Results show that width, length, voltage, and width/voltage

interaction were significant at = 0.05. The increase in width and the decrease

in length resulted in better performance. Initially, the increase in voltage causedan increase in force but became unpredictable throughout the experiment. This

experimentation suggests that a 5 mm 2 mm EAP-based micro-gripper should be

used. Similarly, 3V should be applied in order to lift the objects used in this research.

2.5. Micro robotic system

Fuzzy logic is a methodology used to make computers assimilate our way of thinking

to solve problems.1 Thus, fuzzy logic is useful to provide reasoning for systems that

may be too complex or for which a dynamic model is not available. The purpose ofusing fuzzy logic in this research is to provide a system capable of fast and accurate

manipulation of MEMS products with a cost effective and simple methodology.

Attaching the gripper to the robotic system was facilitated by developing a stage

attachment for the gripper assembly. The robotic system is positioned outside of

the cameras field of view with the EAP micro-grippers extended into the work

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Fig. 11. Normal probability plot.

Fig. 12. MM-3M-F robotic arm with holder and EAP micro-grippers.

area to be seen by the camera. The integrated system will provide for complete and

autonomous manipulation of any object within the cameras field of view (Fig. 12).

The code for the robot control is to receive the location information of a target

object from the real-time vision system and identify the X, Y and Z coordinates.Once the program reads the coordinates it then incorporates them into the fuzzy

logic controller to help decelerate the approach speed of the robot as it nears its

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robot specific case scenarios. To successfully complete this task, one must first deter-

mine the input and output variables that affect motor speed. For the needs of this

experiment, encoder counts are read to control the speed and position respectively.

The development of the fuzzy logic speed control is to be broken down into

three parts: Fuzzification, Rule Declaration, and Defuzzification.8 Fuzzifi-

cation will be between our input (encoder counts) and output (speed). From this

point a group of sets will be established to create the necessary membership value

needed for our defuzzification as represented by Fig. 13.

Rule declaration is the stage when one will develop a series of IF-Then state-

ments to go along with our membership values created in the fuzzification (Fig. 14).

These rules are produced in a way that, when the input falls within a particular

set, the defuzzification will produce the appropriate output actions. Defuzzificationwill then convert the first two steps into a viable output being used to tell the motor

how fast to move. This is done by determining how much the input falls within a

certain set or sets. Output speed can be found for the formulated fuzzy logic by

determining the centroid of the membership sets (2). Xi represents centroid of the

ith fuzzy set and Yi being weighted membership in ith set. X will be the output

speed which will then be supplied to the motors.

X =

XiYi

Yi. (2)

Fig. 13. Fuzzy logic rule.

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The robot is set to move a predetermined distance and then notify if the position

has been reached. If the robot has not reached the desired location the program will

loop to determine the actual position, and apply the fuzzy logic. The program will

continue looping as the robot moves slower and gets closer to the desired location.

3. Conclusion

In this paper, we proposed a novel design of micro manufacturing work cell com-

posed of micro-grippers, micro vision, and micro robotic manipulator for flexible

assembly of micro parts. In order to overcome the current system limits of non-

flexible micro assembly operation, micro visual feedback system has been proposed

as well as EAP-based flexible micro-grippers. Micro robotic system in conjunction

with the micro-grippers and micro visual feedback system demonstrated the capa-

bility of assembly of micro parts.

For assembly tests, micro scale gears are fabricated via bulk micromachining

technology. Potassium hydroxide is used to carve the silicon substrate controlled

precisely by photo lithography. Fabricated gears are used to verify the micro manu-

facturing cells performance, especially for micro visual feedback system and micro-

gripper system. As for the micro-grippers, the experimental results reveal that

further research should be done pertaining to the lifespan of the IPMC. As discussed

before, an equation to account for longevity would be ideal. Since it is known thatdeterioration is caused by the loss of water molecules, humidity monitoring would

help predict the lifespan of the EAP grippers in a precision assembly process. Micro

infrared proximity array (IPA) technology has been utilized to capture images of

micro parts. Further image processing technique such as object recognition has

been adopted for micro gear localization. Thus, the micro robot react to the gear

location. Fuzzy logic-based distance and speed control loop has been implemented

on a micro-robotic platform for assembly operation.

Overall, a novel system design for flexible micromanufacturing is demonstrated.

Further development of each compartment and precision turning, however, will helpproduce a commercially functioning workcell for the micromanufacturing industry.

References

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5. S. K. Chollet, F. Bourgeois and J. Jacot, Economical justification of flexiblemicroassembly cells, in Proc. on IEEE Symposium on Assembly and Task Planning(2003), pp. 4853.

6. U. Deole and R. Lumia, Measuring the load-carrying capability of IPMC microgripperfingers, in Proc. on IEEE Industrial Electronics (France, 2006), pp. 29332938.

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15. D. Um, Microelectronics Lab Manual, Texas State University San Marcos,Photomask Preparation (2006).

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