T 7 .49 2008 C376 ENGi

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!\. Multi--Axial Tribo-System: Developing a Rolling, Sliding, and Rotation Tribological Testing Machine for Assessment of

Total Joint Replacements

By

Matthew Eli Carney

B.S., California Polytechnic State University , San Luis Obispo 2004

A report submitted in partial satisfaction of the Requiremenb for the degree of

Ma ~rer of Science, Plan II

m

Mechanical Engineering

at the

Univer ity of California at Berkeley

Committee in Charge:

Sprin g Seme ster 2008

UC BERKELEY ENG NEERING UBRA

ENGi

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ABSTRACT

e mcnt of the failure modes of total joint replacement bearing surfaces requir s robu ' t and configurable te ting equipment . Physiological motions of total knee

r placem nt include rolling, Ii ding and rotation motion that arc not fully characterized

b tandard tribological te ting machin . In order to more fully understand the effect of

motion an advanced multi-a , ial trib logical te ting machine wa developed by the UC

Berkeley Medical P lymer and Biomaterial Group in conjunction with Smith & ephew, Inc. In ugu t 2007, two tudent , Matthew Carney and Eli Patten began

de igning and con tructing a high} modular and configurable tribotester based on a

computer numericall ontrollcd (C C) milling machine platfom1.

The basic etup included multipl te t tations , each providing loading through a

pneumaticall a tuatcd tern capable of achieving up to 3200 ewtons of nom1al loading fore applied to a modular ample holder. Additional axes ofloading and motion

could also be attached to thi platform allm, ing complex wear path involving rolling and rotation a well a two-dimen ional sliding. A 6-axis load cell provided real-time

collection of loading and friction force . Integrating the motor controls and force tran ducer with Lab IE\\' allovved for a imple graphical user interface through which

motion could be controlled and data could be acquired .

\ ith two talion for a 2D pin-on-disk test fully operational, the ba ic

etup wa hown to funcbon a cl :igned. Ba sic friction mea urement data could be tracked and ba ic motor control u ing Lab VIEW had also been demonstrated. A conceptual design for advanced rolling and rotation motion was also completed by Eli

Patten and wa read y to move to the detailed de . ign stage. Upon completion of the additional motion axe a full integrati on with Lab VIEW would be po ible and a final

product completed.

Both report . that by Camey and Patten have ignificant necessary o erlap and

further infonnati n can be found in the report by Eli Patten "A Multi-Axial Tribo­System: Rolling, Sliding, and Rotation of IlMWPE in Total Joint Replacement ."

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CONTENTS

ABSTMC1 ' ................................................................................................................................................. II

1 INTRODUCTION ................................................................................................................................ 1

I . I MOTi ATI01 ........................ ................... ... .......... ... ... .. . .... ... ..... . .. . .... ... ....... .. ...... . ..... ... .......... ....... 1

1.2 PROJECT B AC KGRO D D HI STORY ....... ..... ..... .. .. ...... ... .... .. ........ ... . .. ......... ...... .. .. ..................... 2

2 GENERAL DESIGN OBJECTIVES OF THE TRJBOLOGICAL TESTING MACJllNE .......... 3

2.1 B ASIC D ESIG PHIL OSOPHY ........ ...... .................... ......... .. .. ...... ... .. .. .. . .............................. ... .. . ... ... .4

2.2 M L T IPH E PPROACH ........................... .... ...... .. ........ .. .......... .............................. .. ... ................. 6

3 PHASE I DESIGN SUJ\11\1 ,..\RY ......................................................................................................... 7

3.1 DES IG"\ o, ERVIE\v ........... ...... .................................. ............ ... .. ....................................... .. ......... .

3.2 CHEDL,LE ....................................... ......... .... ... .......... ................... .. ................... .......................... 12

3.3 BLDGET ........ . ..... ... .................................................... ........... ...... ....... ............ ... ... ... .... ................. 13

4 PI-1,\SE II Sl';\11\1 ,\RY .................... ........ ...................... .............................. ....... ........... .................... 13

-l. 1 Moo LAR DES IG'-. Ov ERV IEV. ... .................. ................................ ............................................... 13

-L2 P HASE II D ESJG'-. 0 El A ILS ............... ...... ...... ......... ................ ERROR! BOOK L\RK 'OT DEFl 1 ED.

-U MOT IO"\ CONTROL A D D ATA COLLECT IO ................................................ ........ ....................... 14

-l.4 LABVJEW Tr \ SKS ................................................ ............................... ................... .......... ... . ... .... 14

-l .5 LOAD CELL D ATA ACQL, ISIT IO ............ ............... ............. ............................ ...................... ... .. .. 17

4 .6 CO \I P TER TO I OTOR CO TRO LLER I TERFAC I G .. ............ ....... .. .. ..................................... 18

-U I IT IAL TEST ! G .................................... . ................................ ........................ ............................ 19

-l.8 PH ASE 11 SCI I EDU LE .................................... .......... .. ... ......... ............. . ... ....... .... ... . .................. .. .... 21

5 CONCLCS ION S ............... ..... ..... ................................. ............................................. ...... ................... 22

1\PPE'.\IDIX A: REFERENCE S ........................................................................... ......................................... I

APPENDIX B: TACHOMETER TROL'BLESJIOOTING ................... .................................... .... ............ 1

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ACKNOWLEDGE 1\JENT

l would like to thank Eli Patten for his xtreme dedication to the project as well a

thank to Dr. Li a Pruitt and the member of th Medical Polymer Group for their

guidance and ncouragement and Smith & Neph w for their graciou support. We would

al ' O like to thank our undergraduate a si tants for help with machining and design work:

Dmitriy Shindich , Mike Wat on, PeITy John on, Chris Chaplin, and Romy Fain. The

tribo-te ter would be an expen ive and w rthless pile of crap metal without the vast

machining expertise and patience of Gordon Long and Mick Franssen , as well as El

Benn ett and v endy Penning. Special thanks also to Scott McCormick , Tom Clark ,

Georg Anwar Sharon Hurd , and all other staff who assi ted u .

I 'v ould al o Jik to thank arah Houghland for her undying support.

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1 INTRODUCTION

1.1 M oti vation

In our rnor acti , aging population total joint arthrop la ·tie (TJA) arc becom ing

incr a ingl prevalent - mor than 500 000 TJAs are perfonned every year in the US

(Kurtz, 2005) in order to relie e joint pain and re tore function lost due to o tcoaiihriti

and other di ea e and injurie , and th number i tead ily rising. Unfortunately, about

0 000 (- 15%) of the will require risky revision totaling more than two bi Ilion dollars

per ear (Krniz, 2005). Modem TIA are typically comprised of a CoCr metal counter­

bearing and an Ultra-High Molecular Weight Polyethy lene (UHMWPE) bearing urface

(although ceramic-on- HMWPE and metal-on-metal al o exi t). A common mode of

failur involve HMWPE wear pariicle generation. These wear particles instigate an

inflammatory response leading to o teolysi s and implant loo ening. Und r tanding the

fundamental science behind thi \ ear particle generation i crucial to improving

HMWPE performance , decreasing the number of rcvi ion procedures , and allowing

patients to recei\'e their quality -of-life restoring joint replacements at younger age .

Re earchers first tried to investigate the tribo logica l is ue invol ed in TJA

u ing single directional 'pin-on -disk' wear te ters with continuou or reciprocating path

However , wear rate re ults from the se tests does not agree well with in i o ob rvation

and it was di covered that wear of HMWPE is very sensitive to multi-directional

sliding and cross- hear (Wang, 1997: Bragdon, 2001 ). Many research group are no\

using two-directional wear testing machine s to ana lyze arious a pect of HMWPE

wear and finding wear rates that are at least on the same order of magnitude oh ,, hat i ,

seen in-vivo (Bragdon. 2001; Van Cittcr , 2004; aikko; 2005). Howe er, the e wear

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te ter only utilize up to two dimension of motion, be it 2D wear tracks, rolling/ Jiding

in one direction, or one-directional translation combined with rotation. Joint kinematics

in th hip and knee can be quite complex and involve multi-directional liding combined

with rolling (motion about an axis parallel to contact urface) and rotation (axi

perp ndi ular to urface). In order to analyze pecific implant design , many pro thesi

compam utilize hip and knee joint imulators that can recreate typical gait motion and

loading. Joint simulators howe er, are not well suited for comprehensive and

fundamental re earch about specific wear mechanisms and they are not always successful

at predicting linical outcomes. To accompli h this, a robust and adaptable multi-axial

wear te ter i needed. A y tern capab le of sim ulating a loading sy tern a complex a the

pine but remain en itive enough to look at fundamental tribological issue s uch a

surface modification in hip and knee replacement would be a valuable re earch tool and

make significant contributions to the total joint replacement field.

1.2 Project Background and History

De ·igning and building a tribosystem that is robust enough to apply a lar ge range

of load for ery high cycles and flexible enough to te st many type of amples with any

sort of wear motion po e many challenge . An excellent way to simp lify the ystem and

tart with a relatively inexpen ive robust frame already capable of automated 2D motion

would be to u eat\ o-axi CN ertical-knee rni11ing machine a the ba e. It could be

easily retrofitted by replacing the milling head with a frame that can be u ed to attach

specimen holders and loading rnechani m .

To de ign, build, and te st such a robu t and adaptable C C machine-ba ed multi­

axial wear te ter , Dr. Lisa Pruitt and her lab (Medical Pol ymer and Biomaterial Group)

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entered into a ba ic research contract agreement with Smith & Nephew, Inc., beginning

the fir t of June, 2006. The research agreement, "Asse sment of Rolling, Sliding, and

Rotation Ratio on the Wear Mechanism ofUHMWPE in Total Knee Replacements,"

called for $66, 97 in funding from Smith & Nephew, which includes $36,000 for

personnel expen es and $19,000 for quipment acqui ition and development. Smith &

ephe \ a al o to provide material and supplies for testing, including UHMWPE

CoCr and Bovine Serum.

Although the project wa originally intended to be led by Jevan Furrnanski , delays

in funds transmi ion and milling machine acquisition meant he was only able to

purcha e some basic equipment before he had to move on to another project. The idea

wa then taken up by Matt Carney and Eli Patten in August , 2007. The project was

developed through extremely clo , e collaboration between Patten and Carney. There i

therefore some overlap between the reports and further infonnation including hi torical

background and det ail of the advanced rolling/rotation axes configurations can be found

in the MS 2008 project report by Eli Patten, "A Multi-Axial Tribo-System: Rolling ,

Sliding , and Rotation of UHMWPE in Total Joint Replacements."

2 GENERAL DESIGN OBJECTIVES OF THE TRIBOLOGICAL TESTING MACHINE

Controlled experiments inve stigating the long-tern1 friction and wear

characteristics of materials require a robust machine with the capability of te ting a

variety of loading conditions. The Medical Polymers and Biomaterials Group wa

interested in creating a tool who e main purpo e would be the xarnination of failure

modes of implantable load-bearing devices, such as hip and knee , while retaining the

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capacity and modular adaptability to perfonn a wide range of experimental procedures

that may be necessitated in future research and development perfonned by the lab.

Hence the o erall goal of this project was to not only develop a product capable of

perfonning tandardized biornechanical tests but to build a tool capable of simulating

phy iological motions and controlled adaptations of these motions for analysis of very

specific failure modes such as the ro1ling/sliding wear mechanisms and continuing to be

adaptable and modular so as to be capable of cont1ibuting to the research endeavors of

the university. n initial survey of the state of the field revealed that for this project to

establish itself it would require addressing issues of: strength , modularity, loading

mechanism, ample ize sample orienta tion and ease of use.

2.1 Basic Design Philosophy

Preliminary design of the tribological system required identification of specific

design requirement . There already existed a number of commercial machine on the

mar ket and one of which had been used to perfonn research by some members of the lab.

It wa therefore , important to learn from the commercial products and either improve

upon their capabilities or develop different approaches so as to contribute to the scientific

community.

Strength and stiffness were major concerns a in any mechanical testing machine

it is important to be sure the samples are being tested rather than the machine it elf.

Particularly when perfonning friction testing chatter induced by unwanted flexing of the

sample holder can drastically effect the loading measurements. The in-hou se pin-on-disk

tribology machine was unfortunately rather su ceptible to stiction/chatter beha ior when

loaded in standardized UHMWPE testing configuration . It was therefore detern1ined that

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the de ign of the new machine shou ld be significant ly stronger and sti ffcr than the

expected interfacial forces induced in any of the biomechanica l testing condition

expected to be ncountered.

Tightly coupled to the strength of the machine is the loading capacity. A

mentioned abo e, if the load were too high they could induce unwanted vibrations in the

te ting apparatu . The trength of the machine was therefore dependent on the type of

loads expect d. Phy iological imulations and controlled trajectorie of those conditions

would be required by this machine. Future enhancements to the machine to include gait

motions were al o of interest to the re earchers and so the ability to produce teady-state

and potentially cyclic load at the range of forces expected in the human body were

considered as de ign requirement .

In order for the testing conditions de ribed abo e to materialize into respectable

result s the t sting r giment mu t be framed within its tatistical ignificance and so the

ability to ha ve multiple station t ting imultaneously was paramount to the design.

From a manufactwing point of view, fewer setup operations required by the technician

operating the equipment would re ult in increased reliability ofresults due to a decrea e

in entry point s for error to manage their way into testing conditions. Con equently the

ability to quickly se tup and remove multiple sample for internal mea urement during

high-cycle testing wa equally valuable.

Further , increasing the o erall capabil itie and ca c of use of the y tern invol ed

the consideration of modularity and manufacturing requirements of the de ice. It was

determined that off-the- helf items would be used whenever po ible to al I w more time

to be pent on the overall design. Interchangeability between comp nent and 111 dular

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attachm nt also led th de ign crite,ia, lea ing open the option for future enhancement

Controlling the y tern would also nece sarily be modular and readily configurable for

highly specialized testing capabi 1 ities.

2.2 Multiphase Approach

The high-le el design c1iteria e tablished in the previous ection led the two

re earchers to break the project into multiple pha es of manufacturing and testing. While

a great deal of design can be done through 30 imulation software such a SolidWorks,

there i no replacement for actual prototyping and t ting of physical machinery. With

this in mind the project was broken into two phases. The initial pha e was considered the

proof-of-concept phase . Design, machining and a sembly of the foundation of the device

was to be con tructed along with the main loading mechanisms , sample holders, serum

contaim11ent , and the capability of two-di men ional pin-on-disk motion profiles. The

econd phase of design included designing additional axe of motion for controlled

ro lling, sliding and rotating wear motions and developing a Lab VIEW interface for

producing and mea uring these loads.

The development of phase one was a significant hurdle and it was completed

through collaboration of the two researchers. Phase two wa then plit into two eparate

tasks , the first focusing specifically on the type of rol I ing sliding motions that have been

establi hed in the literature and designing equipment that would provide further tool

the scientific community, while the second focused on communicating between the

motion controllers and sensors. The establ i hment of the e mu! tip le pha e evidenced the

di er ity of engineering principles utilized in the de elopment of thi project and the true

necessity of identifying these two complimentary engineering tasks.

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3 PHASE I DESIGN SUMMARY

The most critical design feature in any testing device is the decoupling of the testing

machine from the testing sample; an extremely strong and stiff machine is required to test

mechanical properties . Similarly , the next most critical component is the ability to

generate powerful and reliable motions. A cheap and readily available method of

satisfying those tw requirements became the foundation of the project, a tool

specifically designed to perform those two characteristics, a computer numerically

controlled (CNC) milling machine, whose main purpose in life was to be sturdy while

performing complex two -axis motions. Figure one shows the 3D CAD model that was

developed for the initial phase. The next sections describe the specific components of the

machine, followed by overviews of the schedule and budget.

Figure 1: Three dimensional model of the first phase of the Tribotester construction and a

photograph of the assembled device . In the model, the worktables and ram of the mill are visible,

however the remaining portions of the CNC machine, including the pneumatic lines, are not shown, for simplicity . The upper table remains stationary and the lower table has X and Y axis CNC control. Very few mid-construction design changes were needed .

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3.1 Design Overview

Th design of the first phase required ignificant forethought as it provided the

primary foundation for the future components to be built upon. The majority of the

component of the first phase of construction can be een in the image above. Striking

chara teri tic are the ·h er magnitude of the con truction where steel worktables offer

modular mounting opportunitie and multiple pneumatic cylind ers allow large finely

tuned con tant-force loads.

AC C milling machine was chosen as the basic framework for thi tribology

machine becau e of it inherent mass , strength and stiffnes . Additionally, purcha ing a

u ed mill uch a thi wa an efficient u e of funds as the price of the used machinery

included gearing, motor and dri er amplifier , part that would, if independently

purcha ed, co t e era] thousand dollar more and would ha e taken mu h more time to

design and build. The linear motion stages of the CNC mill were designed with

automatically lubricating ball-screw and ways that had already be n optimized for heavy

load and frequent operation. The horizontally mounted worktable, whose length was

defined as the x-axis and whose short direction referred to as the y-axis pro ided a sturdy

platform that offered the ability to use off-the-shelf !-slot fixturing componentry. The

attached motors were capable of mo ing the stages at rate in excess of 60mm /s - more

than enough to co er the a erage joint sliding velocity of 30mm / . As mentioned abo e

becau e thi device was to be used in a researc h laboratory environment, modularity and

the capacity to modify the test setup was a key criteria of the de ign. The horizontal ba e

atisfied thi requirement and following in suit another worktable wa cavenged from a

u ed machinery dealer mounting flange s and hardware (which proved non-trivial t

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manufacture) were designed and machined and finally the second worktable was

mounted to the ram in a vertical orientation to allow for stationary, modular fixturing

abo e th mo ing platf 1111.

The loading mechani m (the entire assembly mounted on the vertical worktable

hown in figure one) wa designed to be apable of producing the loads that may be

expected in the gait of a human and irnilar1y to allow the flexibility for future dynamic

loading. Linear guide rails mounted to the large u-channel allow for vertical motions with

maximum transverse rigidity and allow for ea y access to sample components drning

experiment and potential for rigidly upported dynamic loading conditions. Pres urized

air was cho en as the mean for delivering constant vertical loads across the multiple test

tation . The capacity for four pneumatic cylinders, two of two inch diameter and two of

three-quaiier inch diameter were built into each test station thereby providing nearly 3200

Newton of force (720 lbf) , or 40 MPa of stre son a flat 10 mm diameter pin-on-disk

expenm nt. Precision regulators and gauge a11ow a resolution of twenty Newton of

force in the maximum load condition, or 0.7% of full scale. ot all experiment reqmre

suc h a magnitude of applied force and so the system is also capable of u ing the smaller

cylinders to fine-tune the load. Similarly, it is also possible to arrange the pneumatic line

to provide positive and negative ve rtical forces on the individual loading cylinders

(similar to a method u ed for calibrating pressure transducer ), that provides the capacity

to load just enough force to compensate for the weight of the loadin g mechanism , for u e

wi th minimal contact force experiments. Further , the same bolt pattern was used on all of

the supporting rib of the cylinders and L-brackets o that the cylinders could be

rearranged in any balanced configuration as may be nece sary for specific experiments.

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The loading mechanism was designed with a versatile fixturing end so that it may

accept a simple pin sample holder or instead the additional rotational hardware to

generate the rolling/sliding behavior which was to be designed in the second phase of the

project. For this first phase the machine would be used for pin-on-disk testing and so

simple round cylindrical samples would be tested. An off-the-shelf item, the SC coll et

and a quick release collet holder, standard work holding devices for lathes and milling

machines were fi und to be the ideal candidates for holding the sample pins. These items

seen in figure two were chosen for their satisfaction of one of the main design criteria:

the collet holder allowed quick part removal and setup for simplified in-process

examination . The SC collet was available in a range of standard sizes including square

shapes that provided a sturdy and standard method of tracking sample orientation. The

association of wear and shear orientation with microstructure was of particular interest to

the Medical Polymer and Biomaterials Lab and this capability was considered part of the

design criteria . Finally , the collet holders would be ideal for future knee and hip testing

where solid fixturing of standard tapered shafts would be necessary.

Figure 2: Pin and disc sample holder assembly. The pin is held by an orientation specific ½ inch

square 5C collet (the lever of the quick release collet fixture can be seen in the foreground) . The

bottom disc sample holder has a basin for lubricant as well as an outer splash shield and

securing bolts .

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The di c sample holder wa , designed to be ab le to receive a range of samp le sizes,

material and hygienically retain a basin of lubticant. The outer po lycarbonate cylinder

was included a a afety shield to pre ent lubricant from leaking or splashing onto the

v ork table. Lubricant uch as bovine serum can be retained within the well fom1ed by the

intennediate lamping plate and a of yet evaporation rates have been low and were

cornpen at d for b a controlled deionized water drip. The slotted disc clamps the sample

di c by mean of an inte1mediate adju table plate. The slots allow for quick removal of

the clamp without the necessity to fully remove the clamping screws. Adjustability i

pro ided by machining a new intermediate di c for the corre ponding sample size with

diameter up to ix inche being acceptable. While this tep initially appear

cumber ome , once a et of intennediate piece are made through relatively simple lathe

machining operation s the sample holder can be reu ed indefinitely. The part were made

of 6061 aluminum for easy manufacturing and prototyping and as of yet conosion had

not been an i sue alth ough, it could be possible that galvanic conosion would require

anodi zing the aluminum or u ing stainles steel 316L or titanium in the future.

The whole of the c component were de igned and a single station wa

manufa ctur ed by th e end of the fall 2007 sernc ter. The re ults of this endea or can be

seen in figure three . For the second pha e of the project a parallel effort was establish d

to both de ign the multi -ax ial tes ting and comput r control int rfaces while a team of

undergraduate student s were recruited to manufacture two more identical te ting tation . .

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Figure 3: Phase one completion of a single testing station capable of two axis cross shear pin­on-disk testing .

3.2 Schedule

Although the scope of the project had been laid out in the contract and had been in

the works for sometime , it was not actively pursued until the beginning of the Fall 2007

semester. Initial estimates outlined a completed design, parts procurement , machining

and final a sembly of the first phase by December , the end of the semester.

The full design of the modular platfonn and biaxial pin-on-disk phase one was

completed by mid September . Specification and procurement of components was

completed within another two weeks and machining began on the eighteenth of

September. Machining time was unfortunately deeply underestimated and the mounting

flange for the vertical worktable proved far more cumbersome than originally estimated .

A custom 50 degree dovetail had to be manufactured by a local tooling company so that

the mating surface of the flange mount could properly match the British worktable that

had been scavenged. By the end of the semester all of the components had been

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manufactured and assembly was continued through to the beginning of the Spring

ernester. Final Phase l as embly, including the pneumatic piping was completed by mid

February and the tribote ter was finally operational (pin loading , 2D motion control,

force data collection) by February 22nd, 2008.

3.3 Budget

The final tally of purchase for con truction of the first phase of the project

reached a total of 9,430. The raw m aterials , fittings , pneumatics regulators, and

fastening devices were only 30% of the cost, while the multi-axial load cell (around

$5,000) requiring the remaining majority of the bill. Total hours spent on the project were

difficult to e timate with the late nights , but total time in the machine shop for both

re earchers and the occaisio nal help of an undergraduate student totalled roughly 627

man hour from the fourteen weeks of machining.

4 PHASE II SUMMARY

4.1 Modular Design Overview

Adding a degree of motion has thre e basic aspects: generating motion ,

transmitting the loads and motions, and mounting the test sample s. Keeping each motion

aspect independent from the other wou ld ma ximize the adaptability of sys tem. Thi way

the u er may accommodate many potential tests th at might be performed . Further

infonnation regarding the clinical applications of additional axes of motion a well as

their implementation can be found in the report by Patten (2008).

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4.2 Motion Control and Data Collection

The goal of the econd pha e of the proj ct was to incorporate additiona l axes of

motion along\ ith a user friendly interfac for operating the equipment as well as

acquiring data from the force sensors. Eli Patten designed the additional motion axes and

Matthew Carne d eloped the motion control. The first pha e version of the tribotester

made u e f the original Anilam CNC controller software, where a standard G-code

oftwar \ a u . ed to program the motors. Thi control software was not capable of

controlling th additional axes of rolling/sliding nor were there motor driver amplifiers

available for the addition of the rolling and sliding axes . In order to simplify the u age of

the sy ' tern it wa determined that a central oftware should command all of the motor

actuator '. ational In truments Lab V1EW was determined to be capable of pro iding a

configurable graphical u er interface that al o offered the ability to interface with motion

control hardware and force sensor .

4.3 LabVIEW Ta ks

Programming for the required protocol was laid out and divided into several

concrete ta k that could be developed either individually or among a group of

<level per working in eries or parallel.

4.3.1 Load Cell Data Acqui ition: Lab VIEW provided a graphical user interface and

programming environment that c uld be readily configured to gather data and dri e

actuator \ hilc retaining a rapid learning curve for technicians to successfully operate

experiments. The software was programmed through a graphical interfac here

functions were represented a blocks and data wa pa ed along wires that wer

connected between the se blocks. The logic\ as c sc ntially the programming language

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and in fact C code could be inserted as custom functions. There was also a front panel

that provided the interface between the computer code and the user. The application

de elopment environment provided commonly used controls and indicators that would

allow the user to adjust settings with knobs and switches and also view and save data in

arrays of data or waveforms .

The basic design criteria of the Lab VIEW code included the ability to record

forces from the load cell, generate the trajectory of the motors and start and the stop the

system remotely . A code, although still in its infancy was primarily based upon the

Lab VIEW code provided by ATI Industrial Automation. The user interface of the initial

program can be seen in the figure four. The primary attributes of the code include the

ability to change the calibration setting of the load cell, watch real-time force data and

program loading trajectories by applying equations of curvature.

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rw•tMbl.M....n:r , ..ltaiJlhttlJ ... ~ ~d\ltN -.ao.tp.h""""'thsterm:r•~·.,..,·· ~wth ~ ,a.i ..... ,titq'091t-.~d

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Th!icdb at:W"n,m;.1s:,w*dledbr~!h~C19JOJ1,wl¥,Je~ frdl:il!I ~edhir <a~~ ~,r Mt f-,.R'nJ'Y~a~UibatmlMt'ltt

Figure 4: Example of the LabVIEW interface . Access to settings and raw data are readily noticeable . The main graph shows real-time forces from the load cell. The upper right image shows how equations can be used to generate loading trajectories.

15

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4.3.2 Wear Path Definition: The u er interface to input advanced wear path planning

input method would also need to be developed. The wear path of a single particle, for

in tance on an acetabular cup, would traver e the curvature of the mostly spherical

femoral head tracin g a path mapped to a spherical coordinate system. Intuitively one can

imagine th tip of a pen dragging along the urface of the sphere as perfon11ed by Saikko

(2002). Then hold the pen steady and instead rotate the sphere. These rotations of the

phere would b plit into independent coordinate sy terns referenced to each of the

appropriate a e and finally transformed to instructions to the controller which would

then plan appropriate independent motor movements. The pherical coordinates of the

ample could then be mapped to two cylindrica l coordinate ystem , one for each

independent rolling and sliding axis. The height term of the cylindrical coordinate could

then be applied towards the linear motion of the x and y stages and finally geared back

through the ball-screw gearing to the actua l rotation of the motors. It would then be

possible to generate a mathematical model that tran sfon11s the desired ingle point wear

motions as presented in the literature and hown by Eli Patten to the rotational motion of

the independent axes of motion.

4.3.3 Motor Control: A second task would be programming the motion control package

included with Lab VIEW t apply these transfon11ations to actual motor motions with

proper scaling of dimen sions. Additionally, two dimensional trace patterns shown in the

literature could be mapped onto the curved surface and trajectorie could be planned

through the tran sfon11ations generated by the first two tasks .

4.3.4 Graphic Wear Paths: Writing code to accept comp uter-aided-cl sign files as w 1l

as other arbitrary vecto r grap hic s that could de sc ribe loading trace to be mapped to the

16

surfac of the , ample. Similarly providing a mean to graphically display the real-time

load path tracking as applied by the first two tasks , thereby producing a complete

package capable of generating any user spec ified physical loading parameters and the

capabi lit y of real-time loading measurement .

4 . ..., .5 Further

de crib d fu1iher task would include remote notification of experiment statu and further

proce ing of load cell data. Continuous operation of the testing machine should not

r quire con tant upervision, ye t, there may be periodic eve nt s of interest wher e changes

in loading mea urements could indicate changing and interesting behavior or the

nece it for maintenance of the equipmen t. A piece of co de could easily b written to

take advantage of the remote operation feature of Lab VIEW that would se nd an email or

text me age to the technician at a certain trigge1ing event such a when a critical numb er

of cycles had pas ·eel or perhaps if there had become ignificant changes in sensor

readings.

4.4 Load Cell Data Acquisition

The primary strengt h of Lab VTEW is its data acqui sition implicity and a 1x ax1

load cell \,Va purchased to exp loit these capahi liti es and to allow signific ant

experimentation development. For initial experime nt al capacity a ingle multi-axial loa d

cell with high (200 ewton nonnal load ) and low ( 100 ) ca librati on data wa ob tain ed

to be in talled on one of the sta tions. The load cell use six strain gage arra ngeme nt s and

has internal Wheatstone bridge circuit: o that ix channe l of data co uld be read through

twel e of the sixteen ana log differential input on th e SCB-6 breakout box co nnect ed to

th e 16258 M-Series data acqui ition card mounted within the co mput er . This left f ur

17

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remaining analog channel that could be used for additional differential voltage ensors

as\ 11 as twent -four digital 1/0 pins. Future tasks would include the construction of an

additional force transducer to make u e of the remaining data acquisition channels and

po ibl I2C or SPI ba ed temperature sensors or other digital devices uch as vision

tern .

4.5 Computer - to - Motor Controller Interfacing

;;:·:;::?\ \ -C-ell) ' '-..__...,/

1 :aplu.cal User ( Ir~e r face : I LablJiel.J DAQ and ~ Trajectory planrnng

\.

(, IIT62:(/ '..______.,.~ ( --- ---. \, ~~~iJ t!:~~~~er ] +-+ t !1ot~~

7i~~,,-J-r ..... -... Card j \

l __ / ' /

SCB68 __... Bi:e8l-:out

/ Box

/

/~---. / x-aX1_:;\ \ motor }

\ ...__ _ _/

/~0 ( y-axis \ moto r

'---

Figure 5: The graphical user interface , Nl6250 and Nl7358 all reside within a single computer stat ion . The Nl7358 controller card handles encoder feedback and PIO position control . The Nl7604 sends power to the motors . The Nl6250 Data Acquisition Card collects input from the load cell.

Th e comm uni ca tion betwee n the motor s and user int erfac e i don throu gh a high-

perfo rm ance contro ller card and dri er amplifier . The s hematic illu tratin g the

hard wa re and commun ica tion co nnec tion can be see n in figur e fi e. Th e T 73 58 eight

ax1 hi gh-p erfonn ance contr oller card was int erfa ced to the N I 7604 four axi stepper

motor drive r. The I7358 is cap able of receiv ing high re oluti on optic al enc oder

qu adra tur e po iti on feedback for dri v ing eight motor axes . imult aneo u ·] . Th e Lab I W

pro gram runnin g on th e comput er tation instruc t · th c ntroller with the de ired

18

L'C Bcrh.ck:. \kchan icc.11 I ng1nccrim~

trajectory. Th controller then end an analog ± 1 OV signal to the driver amplifiers to

drive th motor with a given speed, force i curTent limited. A rotary encoder on the

motor haft pul e signal.. on two channels that the controller uses to track position (and

in some case velocity) 1 and the enor from the commanded positions trajectory. A

proportional integral differential control loop within the controller then commands an

updated signal to the driver .

The I7604 t pper motor driver was only capable of driving four stepper motors

and so only four axes were accessible in thi configuration. Future upgrades would

include the purcha e of th UMI7774 four axi motion interface, basically a breakout­

box , to communicate w ith servo motor driver amplifiers. The NI7604 can also be routed

a purely an interface or breakout-box for ervo motors until stepper motors are acquired.

4.6 Initial Testing

Upon completion of the fir st pha e initial te ts were perfonned and a break-in

period of continuou operation wa initiated where a critical upgrade in reliability was

realized. Te ting conditions included spec ifications of a standard accelerated wear

program of 130 load and a 35mm /s travel rate in an 8 mm diameter circular path.

Unfortunately, within the first week of te ting one of the brushed DC motors began

emitting an amplified noi e. The machine was stopped and troubleshooting began -

break-in period was a rather fitting description of the initial testing. The problem was

detennined to be a failed analog tachometer on the x-axis motor. Thi caused the dri ver to

1 The machine came with Brushed DC Motors. the::.e gene rally ha\e an ana log tachomet er relaying ,elocity

feedback to the dri er board for it own internal PID trnck ing control. \~ hile enc der data still goes to the contro ller. A bru ·hie ·s D motor driver doe not require an ana lng tachome ter though hall effec t sensor.

are used to determine co il timin g. The BLOC controller will rece ive the encoder signa ls and ca lculate

position and veloc ity from the quadrature signal.

19

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lo e it ability to track velocity and hence become unable to close the feedback loop

resulting in un stable conditions. lt was dctennined that the high cycle , short path

recipr eating nature of th e experiment would not be reliable on brushed DC servo motors

because of electrical arcing aero s the brushes in repeated positions would shorten the life

of the y tern. Reliable operation would require brushless de servo motors; a

configuration ithout contacting electrical elements and greater efficiency.

ting the motor and driver led to the conclusion of a failed tachometer and a

significant in e tment of time in resolving the issue (see Appendix B for more details).

An off-the-shelf item was found specifically designed for this task of generating

analog tachometer signal for ser o motion driver applications - the ETACH2 from US

Digit al (a high performance electronic bipolar tachometer). The ET ACH2 use s a digital

signal proce ing microcontroller , the 16-bit MC68HC912B32 to count pulses at 2.5Mhz,

or 0.4µs timing inter als. A 12-bit ana log output wa capable of generating a± 1 OV

ignal, that although lower range than the original 9V /1 OOOrpm tachometer , would still

be within a range of magnitude that could be compen ated for by adjusting gains on the

driver board. Installation of the device and retuning the driver s al1owed the motor s to be

put back online so that interfacing between compo nent s could be developed as the new

brushless DC motors were to be procured.

Splicing wires into the original Ani lam CNC controller allowed the Nati nal

Instruments 7358 and Lab View to directly communicate and control the motor s,

amplifi er and safety features whi le ·ti}] retaining the capacity to operate the Anilam

contro11er. Figure five show s a simplified orga niza tional structure of data pathway s

between the ys tem s. The NI 7604, while designed fi r driving stepper motor s, can also

20

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used be u ed a a breakout box to rece ive the digital ignal from encoder and limit s

swi tche and analog output pin s to drive the Anilam amplifier and digital inhibit pins for

each of the axe . The Nl7604 then communicates with the NI 7358 which performs the

actua l controllin g functions. The motor control is actually implemented through the Nl

Motion control oth are package that provide a mean for sup er isory control through

the main Lab IEW user interface panel. The Lab VIEW user int erface then allows the

u er to p cif loading trajectorie and to monitor the mious ystem ensor , producing

the final packaged product.

4.7 Phase ll Schedule

The ba ic function of the project have been de igned and implemented or are in

the proce s of being implemented over the next two months. Rema inin g work includ e

purcha e. machining and a sembly of rolling tsliding motion axes, as we ll as complex

motion programming through the Lah VIEW interface. Abou t four thou and dollars

remain in the budget for u eon equipment and thi money wi ll be used to cover the cost

of the additional rolling /sliding axes hardware as we ll as the upgrade to brushless DC

motors and drivers. Machining the additional axes is planned to begin by the first of July

and is expected to require no more than one month to be comp leted. The interface for the

motion planning software will be de eloped in parallel and wi11 be ready by the end of

July. The detailed calibration of the oftware will require a work ing multi-axial setup and

so the final details will require another month to be comp leted. Project completion is

expected to be satisfied by the beginning of September , one year from the initiation of the

project and atisfying the initi al one year estimate as specified in the contract .

21

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5 CONCLUS IONS

B u ing a CNC milling machine for a base and a two-phase approach, a robust

and ersatil wear te ting machine was d igned and built and showed great potential for

uniqu and ignificant total joint replacement res arch. The basic setup for each te t

tation in ol ed a pneumatic loading system that could achieve up to 725 lbs of normal

loading force and a modular platform to which a test specimen could be attached.

Additional axes ofloading and motion could also be attached to this platfonn, which

a1lo\ for very complex wear paths involving rolling and rotation a well as two-

dimen ional liding. 6-axi load cell allows real-time collection ofloading and friction

force . Connecting the motor control with Lab VIEW allowed for an easy, graphical u er

interface through which motion could be controlled and data could be collected.

With two station s for a 2D pin-on-disk test fully operational, the basic etup has

been hown to function a de. igned. Basic friction mea urement data can be tracked and

basic motor control using Lab VIEW has also been demonstrated. A conceptual design for

advanced rolling and rotation motion is al o complete and ready to mo e to the detailed

design stage. Although the existing C C milling machine frame has proven to be heavy

duty and adaptable , it is recommended to replace the bru hed D servo motors with

brushless DC ervo motor s in ord er to last for a greater number of loading cycles.

22

la tcr ·-., Rc11Prt. SPU/ - l· Ii Patten

APPENDIX A: REFERENCES

[l] S Kurtz F Mowat. et al: Prevalcnc of Primary and Re ision Total Hip and Knee

A1ihropla t in the United State From 1990 Through 2002. Bone Joint Surg Am. 2005·

7:14 7-l-l-97.

[2] . Wang, D . Sun, et al: 01ientation softening in the deformation and wear of

ultra-high molecular v eight polyethylene. Wear. 203-204 (l 997) 230-241.

[3] C Bragd n, DO' onnor, et al.: New Pin-on-Disk Wear Testing Method for

Simulating W ar of Pol ethylene on Cobalt-Chrome Alloy in Total Hip Arthropla ty.

The Journal of Artlzroplasty: ol. I 6 o. 5 2001

[4] Van Citter, Kennedy, ct al: Multi-Station Rolling-Sliding Tribotester for Knee

Bearing Mat rial . Transactions of the ASME. Vol. 126, April 2004

[5] Saikko: hip wear ~imulator \ ith I 00 test station . Proc. !Mech£ Vol. 219 Part

H: J. Engineering in Medicine. _005

[6] M.P. Kaclaba, H.K. Ramakri hnan, M.E. \ ootten, Measurement oflower

extremity kinematic during level walking, J . Orthop. Re , . 8 ( 1990) 383- 392.

[7] Hamilton, M. .. ucec, M. ., Fregly, B. J., Banks , S. A., and Sawyer, W. G.

2005, "Quantif ing Multidirectional liding Motions in Total Knee Replacement ," J.

Tribol., 127, pp. 2 0- 286.

[8] Wang. ., et. al. Lubrication and wear of ultra-high molecular weight

polyethylene in total joint replacements. Tribology International 1998. Volume: 31

I sue: 1-3 , Page : J 7-33.

[9] PauL J.P.; pproache to D e ign - Force Actions Transmitted by Joints in Human

Body. Proceeding of th Ro al Society of London Serie . B-Biological Sciences , 1976.

olume: 192, I ue: 1107, Page : 163-172.

[10] Saikko. .: hlro o , T.: Caloniu ·. 0 .; three-axi knee\ ear imulator with ball-

on-flat contact.\ ear, 2001. o lurn c: 249. Issue: "-4 Page s: 310-315.

[11] Schwenke. T. Experimental Detern1ination of Cro , s- hear Dependency in

Polyeth y lene Wear. 200 . ORS Conference Po ter.

[12] Ge vac 1i, M. R.; LaBergc. M.: Gord n. J . M.: Dc sJardin , J. D .· Th quantification

of physi logicall y rele ant cross- hear wear phenomena on orthopedic bearing material

using the MAX- hear wear te ting _ y tern . Journal of Tribology-Transaction of the

ASME, 2005. olume: 127; I ue: 4~ Pag e : 740-749.

[13] http: /amti.biz 1

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[ 14] Kan g, L. ; Galvin A. L. ; Brown , T. D. · Jin , Z. M. ; Fisher , J. · Quantificabon of the effec t of cross-shear on the wear of conventional and highly cross-linked UHMWPE. Journal ofBiomechanics, 2008. Volume: 41; lssue: 2; Pages: 340-346.

[15] Saikko, V. · Ca]onius , O.; Slid Track Analysis of the relative motion between femora l head and ace tabular cup in walking and in hip simulators. Journal of Biomechanics , 2002. Volume: 35; Pages: 455-464.

[ 16] Patten, Eli; A Multi-Axial Tribo-System: Rolling , Sliding, and Rotation of UHMWPE in Total Joint Replacements . MS Report , Department of Mechanical Engineering, UC Berkeley. Sp1ing 2008.

2

\la . rcr·" RcpPrl. ' P08 - 1: li Patten

APPENDIX B: Tachometer Troubleshooting

Para!] l solutions were con idered to resolve the broken tachometer with the

minimal budget re ource a ailablc. Initially a microcontroller was considered b ut

research led to\ ard the po sibiJity of a discrete electronic component solution, both

method relied upon counting pul e signals from the digital optical encoder and

con erting th frcquenc of pulses to an analog signal. The first step required identifying

the proper signal and power wire coming from the wiring harness and splicing into these

by oldering additional wire·. The component approach in olved the u e of a frequency

to voltage conve1ier LM2907 chip that ba ically u cd a charge pump circuit to count the

pulse from one f the encoder channel and generate an analog voltage. A high-pass

filter v ith a 1 hz cutoff frequency wa included at the input of the circuit to remove the

voltage offset from the encoder digital logic commands. The charge pump included two

capacitor that would alternatively charge and then dump their voltages through a

follower op-amp. The op-amp acted a a buffer to decouple the circuit from the

impedance of the output connections. The response of this circuit was very fast, with a

1 Ons settling time. However, when attached to the driver board it was reahzed that the

driver internal control circuitry required a bipolar analog tachometer signal, while thi

frequency to vo ltage component was on ly capable of tracking a unipolar velocity

magnitude, without po ·iti ve or negati e direction component , leaving the system only

marginally stable.

The microcontroller, an 8-bit PIC l 8F4520, was also capable of counting pulses

and determining direction and ve locity through software and had the option of outputting

the results through a digital to analog converter to genera te the neccs ary simulated

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\ 1ntth~\\ Carney

tachometer ignals. Nurnerou ' techniques were trialed to calculate directional velocity as

well a multiple method ' f digital to analog conver ion. The encoder send a two

channel quadrature ( or 90° out of phase) signal, triggered by a egmented disc and

photore istor . By tracking the change from previous to next signals on both channels it

wa pos ibl to determine position , direction and by tracking a time stamp, velocity was

also readily calculable . Ri . ing and falling digital edges triggered interrupts within the

microcontroller and the e were used to track position and calculate timer overflows. One

highly accurate method wa u ed but it turned out the 1000 count /revolution encoder

would quickly outpace the maximum frequency tracking capacity of the microcontroller.

The PIC op rated at 20 MHz, but with four clock cyc les per instruction and roughly

eighty in tructions for the implest program the minimum time requirement for a single

iteration of the code required 16~ts. Wlii le ri ing and falling triggers turned the 1 OOOct

signal into a 4000ct signal that would trigger an interrupt every 6.25µs if the motor spun

at the full 2400rpm peed. Although it was assumed the motors could be operated well

below their maximum peed thi turned out to be unacceptable when the driver card

initiated its sequence by quickly jogging the motor and immediately outpacing the

tracking capacity of the PIC. Additional approaches were made including methods of

t1iggering on fewer edge and capturing internal timers for rapid calculations.

Simultaneously two method of digital to analog conversion were explored.

The first and initially most promising technique to output an analog voltage was

through a R-2R resistor ladder. single byte of data could be attached to th e resi tor

network through eight pins. Each of the eight bit of data wou ld be directly connected to

the network changing the overa ll vo ltage output by summing the mo t to lea t significant

bit generated voltage . This method required high accuracy resistors and could have its

2

Ma, tcr·~ Rcpo11. SPO 1 - Matth ew Carney

bit resolution increased by addressing multiple 8-bit channels. Unfortunately, when using

an 8-bit microcontroller only 8-bits could be addressed at a given instant and so

increasing resolution also increased instructions and minimum capturing frequency.

Similarly , a 16-bit integrated circuit DAC was acquired to try to increase the resolution of

the output. The AD5422 made use of a high speed Serial Peripheral Interface (SPI) to

communicate with the PIC by sending clock pulses that cycled through a 24-bit control

and data addr es ing protocol and a high-precision internal R-2R ladder. Again, the high

speed clocking and latching of the 24-bit commands proved too cumbersome for the

limited bandwidth 8-bit microcontroller and unreliable operation resulted.

3

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