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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.
l ( ' Berk ,Jc~. \ kclnrn ·al l-:n!!.inc Ting
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.
P~l'Ql""'~~o n-.se ... fntf~ rec:c,-!tffTClf'IIN:IIC!strtat1:~ ro.,...,,.tt(DQI . ~L, .. a•rr~ . olldtt.Np'nar
'11 l"Y ,-., F: I T
rw•tMbl.M....n:r , ..ltaiJlhttlJ ... ~ ~d\ltN -.ao.tp.h""""'thsterm:r•~·.,..,·· ~wth ~ ,a.i ..... ,titq'091t-.~d
,;.,
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
\l aster· . Rcpt1rt. SJ>()~ \ I c1t the,, CarnL'\
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
L C Bcrkck:,. \ kclrnnicc.11 l:n12.i11ccr111g \laster"..., Ri...'f h)rl. , l>(L' \latthc,, ( ;1rnc\
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
:\h IL'1 ·" Rciw rt. SPO~ \J atth c, , C,1rnc,
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
LC 8crkc k :. \kc h~rn1Gil l ·n!2Jllccnng
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
Bl
\ 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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