4.2. APPLICATIONOFTHEAHPTODECISIONPROBLEMSOFTHEROBOTDEVELOPMENTS
49
aspectisthesystemreactiontimetocriticalevents,whichisbestsatisedby real-time
systems(A2andA4). For r thecriterionSafeness,adeterminablereactiontimeis more
importantthanredundancy, becausemobilesystems s especiallyrequirefast andreliable
reactionstocriticalsituations(e.g.,collisions).
Features(F): Theavailablecomputingpowerofasystemarchitecturetoexecutenewsoftware
algorithmshastoconsidered.ThiscriterionisequallysatisedbysystemarchitecturesA1
andA2,becausebotharchitecturesprovideasimilarcomputingpower. Thearchitecture
A3isevaluatedasessentiallymoreimportantthanA1andA2,becausetheofintegration
ofseveralPCs. ThesystemarchitectureA4isweighedtobeconsiderablymoreimportant
thanA1andA2,becauseoftheintegrationofmultiplecontrolmodulesrunningembedded
software.
Costs(C): Thecostsforasystemarchitecturedependontheintegratedcomponents.There-
fore,A3obtainstheworstrating,becausePCsarethemostexpensivecomponentsofa
systemarchitecture.TheredundancyofarchitectureA4alsoproducesadditionalcostsfor
electricalcomponents,powersupplies,andcasings. Themostcosteectivearchitectures
areA1andA2.
ThepairwisecomparisonresultsarepresentedinAppendixA.1.Thederivedweightsaresum-
marizedinTable4.3.
Table4.3: Comparisonresultsforsystemarchitectures.
Characteristics
A
O
U
R
S
F
C
A1
Central
12.5%
36.4%
25.0%
5.4%
5.7%
8.3%
43.4%
A2
Co-Controller
12.5%
36.4%
25.0%
14.6%
26.3%
8.3%
43.4%
A3
MultiplePCs
12.5%
6.6%
25.0%
23.7%
12.2%
41.7%
4.0%
A4
Modular
62.5%
20.7%
25.0%
56.3%
55.8%
41.7%
9.2%
BatteryTechnology
Thebatterytechnologyofamobileplatformiscriticalfortheavailabilityofthesystem,andis
animportantfactorfortheproductionandservicecosts. Relevantparametersforthedecision
aboutabatterytechnologyaretheenergydensity,themaximumchargingenergy,costs,andthe
lifetime.Theenergydensitydenestheamountofenergythatcanbestoredinagivenvolume.
Ahigher energy density allows for longer operation n times s withoutrecharging. . Additionally,
themaximumchargingpowerin uencestheoperationtimeoftherobot,becauseitdenesthe
idlestatetime ofarobotduringrecharging. . Theparameterscosts s andlifetimein uencethe
overallcostsofarobotsystem. Inthecasethatthelifetimeofthebatteryisshorterthanthe
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50
CHAPTER4. THEANALYTICHIERARCHYPROCESSFORDECISION-MAKING
lifetimeoftherobotsystem,productionandservicecostsgeneratedbythebatterysystemhave
toconsideredintheevaluation.
Currently,fourbatterytechnologiesarestate-of-the-artandapplicabletomobileservicerobots:
lead-acid batteries, Nickel-MetalHydride (Ni-MH) ) batteries, , andlithiumbatteries s based d on
LiCoO
2
andLiFePO
4
. ExamplesofrobotsystemsbasedonLead-Acidbatteriesaretherobot
platforms B21r r of f the e company RWI I or the Pioneer r robots s of f the company MobileRobots.
Ni-MHbatteries were, , for r instance, , integrated into the service robot t Nanisha,developedby
theUniversidadPopularAutonomadelEstadodePueblainMexico[Vargasetal.,2009].The
numberofrobotsystemsusinglithiumbatteriesisincreasing,becausethistechnologyprovides
promisingtechnicalparameters. ExamplesaretheservicerobotsREEM-H2(Section2.1.5),the
Care-O-Bot3 (Section2.2.2),orthemobileplatformRollinJustinbytheGermanAerospace
Center[Fuchsetal.,2009].Otherenergystoragetechnologies(e.g,capacitorsorfuel-cells)could
alsobeconsideredfortheevaluationprocess,whichgoesbeyondthefocusofthisthesis.
Theevaluationprocessiscarriedoutonspecicexampletypesofthefourbatterytechnologies.
Eveniftheevaluationprocessdoesnotdependonthebatteryconguration,thespecicbattery
parameters arepresentedon n cellcongurations, applicable to both robot t applications. . It t is
assumedthatthebatteryfortheshoppingrobotandthehome-carerobothasanominalvoltage
ofabout24V.Fortheintegrationofthebatteryinsidetherobot’schassis,avolumeofabout
12dm
3
shouldbeprovided.
BatteryTechnologyB1: Lead-AcidbatteryLC-X1242AP
Thisbatterytypeconsistsofsixseries-connectedsub-cells(eachwithavoltageof2.0V)resulting
inanominalcellvoltageof12.0V.Thenominalcapacityis42.0Ah[Panasonic,2011]. Forthe
evaluationprocess,twoofthesecellsconnectedinseriesareconsidered.
BatteryTechnologyB2: Ni-MHbattery y D9000MAH-FT-1Z
Theselectedbattery cellhasanominalvoltageof1.2Vandacapacityof9.0Ah[Emmerich,
2011]. Acongurationof20cellsinseries s andthreecellsinparallelcouldbeintegratedinto
thegivenspace.Theresultingbattery,assembledfrom60cells,hasanominalvoltageof24.0V
andacapacityof27.0Ah.
BatteryTechnologyB3: LiCoO
2
batteryLP9675135
Thiscelltypebelongstothegroupoflithium-polymercells. Ithasanominalvoltageof3.7V
andacapacityof10.0Ah[DynamisBatterien,2009]. Theevaluatedbatterypackiscomposed
byamatrixofsevencellsinseriesandeightcellsinparallel. Thetheoreticalvolumeofsucha
batteryis6.3dm
3
. Inpractice,suchcellsrequireadditionalspaceforsafetyprecautions. The
56cellsprovideanominalvoltageof25.9Vandacapacityof80.0Ah.
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4.2. APPLICATIONOFTHEAHPTODECISIONPROBLEMSOFTHEROBOTDEVELOPMENTS
51
BatteryTechnologyB4: LiFePO
4
batteryGBS-LFMP60AH
This battery typeis basedon n singlecells s with h anominalvoltage of 3.2Vanda capacityof
60.0Ah[LitePowerSolutions,2011]. Therefore,thecompositionofeightcellsinseries s allows
foranominalvoltageof25.6Vandacapacityof60.0Ah.
SummaryofTechnicalParameters
Relevant technicalparameters s of the four r battery types are e summarized inTable 4.4. . This
overviewincludesnominalvoltages,nominalcapacities,weights,andvolumesofsinglecellsand
theassembledbatteriesaswellasthecellcongurations. The80%-Lifetimeparametersdene
thenumber ofcyclesandtheamount ofenergy that canbe suppliedby a batteryuntilthe
capacitydropsunder80%ofthenominalcapacity. Atthispoint,thebatteryisintendedtobe
exchangedbyanewone. Thecellandbattery y prizes areapproximatedvaluesvalidin2011.
Thelastfourparametersarecalculatedfromgivensystemparameters.Forfurtherinformation
ofbatterytechnologiesreferto[GeneralElectronicsBattery,2008]or[Soderstrom,2008].
Table4.4: Technicalparametersofbatterytechnologies.
Alternative
B1
B2
B3
B4
Chemie
Lead-Acid
Ni-MH
LiCoO
2
LiFePO
4
CellParameter
Manufacturer
Panasonic Emmerich
Dynamis
LitePower
Basis-CellType
LC-X1242AP
D9000MAH
LP9675135
LFMP60AH
NominalVoltage[V]
12.0
1.2
3.7
3.2
NominalCapacity [Ah]
42.0
9.0
10.0
60.0
Weight[kg]
16.00
0.17
0.22
2.00
Volume [dm
3
]
5.7
0.2
0.1
1.5
Prize[e]
80
10
20
75
Conguration
(Series xParallel)
2x1
20x3
7x8
8x1
BatteryParameter
NominalVoltage[V]
24.0
24.0
25.9
25.6
NominalCapacity [Ah]
42.0
27.0
80.0
60.0
NominalCapacity [Wh]
1,008
648
2,072
1,536
Weight[kg]
32.0
10.2
12.3
16.0
Volume [dm
3
]
11.4
12.0
6.3
11.8
80%-Lifetime[Cycles]
300
500
500
1,200
80%-Lifetime[kWh]
302
324
1,036
1,843
Prize[e]
160
600
1,120
600
EnergyDensity [Wh/kg]
31.5
63.5
168.2
96.0
EnergyDensity [Wh/dm
3
]
88.6
54.0
326.9
129.7
Costs-Capacity-Ratio[e/kWh]
158.7
925.9
540.5
390.6
Costs-Lifetime-Ratio[e/kWh]
0.53
1.85
1.08
0.33
52
CHAPTER4. THEANALYTICHIERARCHYPROCESSFORDECISION-MAKING
Evaluationof BatteryTechnologies
Thecomparisonofthebatterytechnologiesiscarriedoutunderfourcriteria:Adaptability,Oper-
ationTime,Flexibility,andCosts. ItisnotexpectedthatthecriteriaUsabilityandRobustness
arein uencedby the typeof battery. . Safeness s isalsonotconsidered,becauseitis s expected
thatthe cellintegrationandallrequiredbattery monitoringandmanagement functionalities
arerealizedappropriately.
Adaptability (A): : Theadaptabilityofarobotsystemtonovelapplicationsisin uencedby
the batterycapacity. . Considering g the available spacefor abattery inside arobot, the
energy density (Wh/dm
3
)canbe used. . BasedonEquation4.8,theweights s are14.8%
(B1),9.0%(B2),54.6%(B3),and21.6%(B4).
OperationTime(O): SimilartoAdaptability,theoperationtimeofarobotdependsonthe
batterycapacityintegratedinagivenvolume.Themaximumchargingpowerofabattery
system,limitingthespeedofthechargingprocess,isnotconsidered. Thisisreasonable,
because inmodern battery systems (with high charging values), the charging speedis
usuallylimitedbythecapabilitiesofthepowersupplyunits.Therefore,thesameweights
ascalculatedforthecriterionAdaptabilitycanbeused.
Features(F): Thiscriterionfollows s the sameargumentsas the Adaptability y andproduces,
therefore,thesameresults.
Costs(C): Fortheevaluationofthebatterycosts,thecosts-lifetime-ratioisapplicable. This
valuerepresentsthebatterycostsunderconsiderationoftheamountofenergythatcanbe
suppliedbyabatteryoveritslifetime.Thisratioisespeciallyimportantforapplications,in
whichthelifetimeoftherobotishigherthanthelifetimeofthebattery.Theweightsforthe
describedbatterytechnologiesare29.4%(B1),8.4%(B2),14.4%(B3),and47.8%(B4).
TheresultsofthisevaluationprocessaresummarizedinTable4.5.
Table4.5: Comparisonresultsforbatterytechnologies.
Technology
A
O
U
R
S
F
C
B1
Lead-Acid
14.8%
14.8%
25.0%
25.0%
25.0%
14.8%
29.4%
B2
Ni-MH
9.0%
9.0%
25.0%
25.0%
25.0%
9.0%
8.4%
B3
LiCoO
2
54.6%
54.6%
25.0%
25.0%
25.0%
54.6%
14.4%
B4
LiFePO
4
21.6%
21.6%
25.0%
25.0%
25.0%
21.6%
47.8%
4.2. APPLICATIONOFTHEAHPTODECISIONPROBLEMSOFTHEROBOTDEVELOPMENTS
53
ChargingSystem
Theintendedrobot platforms forshoppingandhome-careapplications shouldbechargedin
twoways:autonomouslyandmanually. Thegeneraloperationmodeshouldbetheautonomous
rechargeapproach.Inthismode,therobotdrivestothelocationofthechargingstationassoon
asthebatteryisempty.Thedockingprocesstothechargingstationiscarriedoutautonomously
withoutuserinterference.Afterthechargingprocessisnished(orincaseofanexternalevent,
e.g.,a user request),the robot docksofromthechargingstation andcontinues its normal
operations.Tocompensateforpositioninginaccuraciesduringthedockingprocessoftherobot,
metalplateswithanadequatesizecanbeusedforcontactingtherobottothechargingstation.
Themanualchargingmodecanbeusedifnochargingstationisavailableorinapplications,in
whichautonomouschargingisnotrequired(e.g.,tradefairs). Toenablethemanualcharging
process,theuserhastoplug-inthechargingconnectortotherobotandtounplugittonish
thechargingprocess.
For the autonomous charging mode, , thetransfer r of energy fromthe charging station n to the
robotcanbe arrangedbasedonextra-lowvoltage,linevoltage,orelectromagneticinduction
(Figure4.6). Thetransferofextra-lowvoltage(C1,C2)provideshighsafenessforusers. . The
disadvantageofthisprincipleisthehigherelectricalcurrentthathastobetransferred,caused
bythelower voltage(assuminganequalperformanceofallchargingtechnologies). . Charging
principles based d onextra-lowvoltage areused,e.g.,by y  oor-cleaningrobots. . Inthe e eldof
mobilerobotresearch,severaldevelopment groups apply this concept, , like[Silverman et t al.,
2003]or[Kimetal.,2005].
Achargingsystembasedonlinevoltage(C3,C4) requiressignicantly lesselectricalcurrent
totransfer the same amount ofenergy. . Inthis s case, , it t must be assured d thatpersons s never
getintouchwithcontactsprovidinglinevoltage. Theintegrationofcertiedpowerplugsfor
thetransfer oflinevoltagewouldsolvethisproblem. . Unfortunately,suchplugs s usuallyhave
high demands s onthepositioning accuracy of arobot. . Possiblesolutions s are the integration
ofadditionalsensorstobetterdetectthepositionofthechargingstationortheintegrationof
mechanicalguidancesystemstoforcetherobotintothecorrectposition. Anotherpossibility
would be the usage of arobot t manipulatorto execute thedockingprocess [Meeussen n et t al.,
2010].
Thethirdapproach uses s acontactlesstransferofenergy based d onelectromagneticinduction
(C5,C6).Suchasystemallowsforhigherinaccuraciesofthedockingprocess(arangeofsome
centimeters) andis safe forusers. . Regrettably, , inductive chargingtechnologies s createhigher
systemcostscomparedtocontactbasedchargingprinciples.Anexamplefromtheeldofrobot
systemsinpresentedin[RyanandCoup,2006].
For the manual l charging mode, the integration of an extra-low voltage chargingtechnology
54
CHAPTER4. THEANALYTICHIERARCHYPROCESSFORDECISION-MAKING
Robot
(C1)
Charger
Line Voltage
Charging Station
AC     DC
Battery
AC     DC
Robot
(C2)
Charger
Line Voltage
Charging Station
AC     DC
Battery
AC     DC
Robot
(C3)
Charger
Line Voltage
Charging Station
Battery
AC     DC
AC     DC
Robot
(C4)
Charger
Line Voltage
Charging Station
Battery
AC     DC
AC     DC
Robot
(C5)
Charger
Line Voltage
Charging Station
AC     DC
Battery
Trans
Robot
(C6)
Charger
Line Voltage
Charging Station
Battery
AC     DC
Trans
Trans
Trans
Figure 4.6: : Chargingsystemsfor r therobotplatforms. . Every y system consistsof amanual
mode(upper part)andanautonomousmodebasedonachargingstation(lowerpart). . The
manualmodeprovidestwoversions:anextra-lowvoltagechargingprocessincombinationwith
anexternalAC/DC-Converter(C1,C3,C5),andalinevoltagechargingprocessincombination
withanintegratedAC/DC-Converter(C2,C4,C6). Theautonomousmodesarerealizedby
extra-lowvoltage(C1,C2),linevoltage(C3,C4),oraninductivechargingprinciple(C5,C6).
or aline voltage chargingtechnologyis possible. . Thedierence e isthelocationofthe power
converter(AC/DC-Converter) either outsidetherobot(C1,C3,C5)orinsidetherobot (C2,
C4,C6). Thedecision n for r thebestsolution n depends s ontherequirements of the application.
Alinevoltagechargingprincipleallowsforahigher usability,becauseoperatorsdonotneed
externalpowerconverterstochargetherobot.Therealizationofanextra-lowvoltagecharging
systemreducesthecostsoftherobotsystem,whichcanbebenecialforservicerobotsusing
theautonomouschargingmodebydefault.
4.2. APPLICATIONOFTHEAHPTODECISIONPROBLEMSOFTHEROBOTDEVELOPMENTS
55
EvaluationProcessof Charging Systems
Consideringthethreeautonomous andthetwomanualchargingprinciples,six combinations
arepossible(Figure4.6).Thefollowingstatementsandratingscanbederived:
Adaptability (A): : Thechargingsystemhasnoimpactonthiscriterion,becauseallpresented
solutionsareequallyapplicabletonewapplications.
OperationTime(O): Servicerobotsareusuallychargedintheautonomouschargingmode,
whichisexclusivelyconsideredfortheweightingunder thiscriterion. . Fromexperiences
itcanbeassumedthattheprovidedchargingpower,whichin uencestheoperationtime,
is equalfor extra-lowvoltagesolutions (C1,C2)andinductivesolutions(C5,C6). . The
higher amount of transferable powerofline voltage based d chargingprinciples s (C3, , C4)
essentiallyincreasethechargingspeed.
Usability(U): Theautonomouschargingmodedoesnotrequireanyuserinterference. Still,
itshouldbeconsideredthattheusageofaninternalpowerconverter(C2,C4,C6)forthe
manualchargingmodeisweaklymoreimportantfortheusabilitythananexternalpower
converter(C1,C3,C5).
Robustness(R): Chargingtechnologiesusingelectricalcontactsareassumedtohavesimilar
robustness(C1,C2,C3,C4).Themainreasonsformalfunctionsofcontactbasedcharging
technologies arecorrosion,impurity,anddeformation. . These e sources ofdefects donot
applytocontactlesschargingsystems(C5,C6).Therefore,theyareweightedasstrongly
morerobustthancontactbasedsolutions.
Safeness(S): Everychargingtechnologymustpreventthecontactofpersonswithdangerous
voltagelevels,whichisaddressedinthemanualmodebytheapplicationofstandardpower
plugs. Fortheautonomouschargingtechnologies s itshouldbeconsideredthatsolutions
withextra-low-voltage(C1,C2)areweaklymorepreferablethansolutionsbasedonline
voltage(C3,C4).Inductivechargingsystemsprovidethehighestsafeness. Therefore,C5
andC6areevaluatedas weakly moreimportant thanC1andC2,andessentially more
importantthanC3andC4.
Features(F): Thesatisfactionofthiscriterionisnotin uencedbytheappliedchargingsys-
tem.
Costs(C): It t should d be assumed that t the e costs of available e power r converters (AC/DC-
Converters)areequaltothecosts ofinductivechargers. . Nevertheless,theusage e ofthe
autonomouschargingtechnologiesC3andC4isevaluatedasweaklymoreimportantthan
othersolutions (C1,C2,C5,C6). . Thereasons s arethelowercosts for thechargingsta-
tion,benecialforapplications,inwhicharobotshouldbechargedatdierentlocations
(applicabletobigstoresorthehomeenvironment).
56
CHAPTER4. THEANALYTICHIERARCHYPROCESSFORDECISION-MAKING
Thecomparison results for r the described d evaluationprocess s are presented d inAppendixA.2.
ThecalculatedweightsaresummarizedinTable4.6.
Table 4.6: : Comparisonresultsforchargingtechnologiesbasedonextra-lowvoltage(ELV),
linevoltage(LV),orinductivetransmission(IND).
Manual
Autonomous
A
O
U
R
S
F
C
C1
ELV
ELV
16.7%
7.1%
8.3%
7.1%
13.0%
16.7%
10.0%
C2
LV
ELV
16.7%
7.1%
25.0%
7.1%
13.0%
16.7%
10.0%
C3
ELV
LV
16.7%
35.7%
8.3%
7.1%
5.3%
16.7%
30.0%
C4
LV
LV
16.7%
35.7%
25.0%
7.1%
5.3%
16.7%
30.0%
C5
ELV
IND
16.7%
7.1%
8.3%
35.7%
31.7%
16.7%
10.0%
C6
LV
IND
16.7%
7.1%
25.0%
35.7%
31.7%
16.7%
10.0%
DriveSystem
Avarietyofdrivesystemsareappliedintheeldofmobilerobots.Technicalrealizationsdepend
onindividualrequirementsofthedrivingbehavior(e.g.,speed,maneuverability). Mostofthe
systemscanbeclassiedinthreegroups: systemswithoverdetermineddierentialkinematics,
systemswithdierentialkinematicsandcastorwheels,andsystemswithomni-directionalkine-
matics(overviewby[Staab,2009]). Drivesystemswithoverdetermineddierentialkinematics
weredevelopedforoutdoorapplications,inwhichtheredundancyofdrivingwheelsareadvan-
tageousforthemovabilityonroughterrains.Systemswithdierentialkinematicsareoftenused
inindoorapplications,wherearobustandcosteectivesolutionisneeded. Omni-directional
kinematicsarealsousedinindoorapplications.Theadvantageofanomni-directionalmovement
iscombinedwithaveryhighrealizationeort. Theworkofthisthesisfocuses s ondierential
platformswithcastorwheels,becausethistechnicalapproachismostapplicablefortheintended
robotapplications.
Tocharacterizeadierentialdrivesystem,threeparametersareimportant. First,themaneu-
verability of the platform, , whichdepends s on n therequiredarea of the robot t duringrotation
A
ROT
comparedtotheareaoftheplatformfootprintA
PL
. Ifbothvaluesareequal(realized
by acircular robot basewitharotationpoint inthe center of the platform),the robot can
rotatewithouttheriskofacollision. IftheratioR
PL/ROT
=A
ROT
/A
PL
increases,theturning
curve increases andthemaneuverability decreases. . The e secondparameter is the stabilityof
theplatform. ThisdependsonthestabilityareacreatedbyallwheelsA
ST
. Ahighervalueof
A
ST
meanshigherstability. Theratioofthestabilityareatotheareaoftheplatformfootprint
R
ST/PL
=A
ST
/A
PL
describesthesaturationoftheavailableplatformspacefortherealization
ofthestabilityarea.Thisratioshouldbemaximal.Thecenterofgravityoftheplatformshould
beinthecenterofgravityofthestabilityarea. Thethirdparameterdescribes s themaximum
4.2. APPLICATIONOFTHEAHPTODECISIONPROBLEMSOFTHEROBOTDEVELOPMENTS
57
Wheel
 = 2,916 cm²   A
= 7,571 cm²   A  = 1,015 cm²
PL
ROT
ST
R
= 0.39   R
= 0.35
PL/ROT
ST/PL
 = 2,290 cm²   A  = 2,290 cm²   A  = 0 cm²
PL
ROT
ST
R
= 1.00   R
= 0.0
PL/ROT
ST/PL
 = 3,456 cm²   A  = 5,507 cm²   A  = 1,400 cm²
PL
ROT
ST
R
= 0.63   R
= 0.41
PL/ROT
ST/PL
(D5)
(D6)
(D1)
1 2
1
2
(D3)
Wheel
Castor
 = 2,290 cm²   A
= 2,290 cm²   A  = 300 cm²
PL
ROT
ST
R
= 1.00   R
= 0.13
PL/ROT
ST/PL
(D2)
1
2
 = 2,290 cm²   A
= 2,290 cm²   A  = 600 cm²
PL
ROT
ST
R
= 1.00   R
= 0.26
PL/ROT
ST/PL
1/2
 = 2,630 cm²   A
= 5,281 cm²   A  = 580 cm²
PL
ROT
ST
R
= 0.50   R
= 0.22
PL/ROT
ST/PL
(D4)
1
2
1/2
Figure4.7: Drivesystemswithdierentialkinematics. . Theexamplesassumearobotwidth
of540mm.ThecharacterizingparametersaretheareaoftheplatformA
PL
,therequiredarea
oftheplatformduringrotationA
ROT
,thestabilityareaA
ST
(dashedlines) describedbythe
positionsofthewheels,aswellastheratioR
PL/ROT
oftheplatformareatotherotationarea
andtheratioR
ST/PL
ofthestabilityareatotheplatformarea.Therotationpointofaplatform
ismarkedby1,theoptimalpointforthecenterofgravityismarkedby2.
58
CHAPTER4. THEANALYTICHIERARCHYPROCESSFORDECISION-MAKING
stepheightthatcanbecrossedbytheplatform. Thisvaluedependse.g.onthewheels’diam-
eters,thewheels’softness,thepowerofthemotors,ortheweightdistributionoftheplatform
components.
Fortheevaluationprocess,sixdierentialplatformscanbeconsideredthat areapplicableto
theintendedservicerobotapplications(Figure4.7). ThedrivesystemD1consistsofjusttwo
drivenwheels. Ithasaverygoodmaneuverabilityandcanpasshighbarriers,becauseitdoes
notincludecastor wheelsthatimpedethecrossingofbarriers. . Unfortunately,suchplatforms
areunstableandmustbebalanced. Thisworsensthestabilityoftherobotandcreateshigher
costsformotorcontrolandsensorsystems.AnexampleistheSegwayrobotplatform[Nguyen
etal.,2004].
Thestabilityis improvedinthedrivesystemD2,whichcontainsadditionally acastor wheel
at the e back k side of the e robot. . Still, , this platform m can fall over r to o the front. . A A (limited)
compensationmightbeasmartplacementofsystemcomponentstobringthecenterofgravity
tothecenterofthestabilityarea(markedby thenumber2inFigure4.7). . Abettersolution
ofthisdrawbackisrealizedinthedrivesystemD3,whichconsistsoftwocastorwheels. This
platformcanstillrotatewithoutexceedingthegivenfootprintoftheplatform.Theplacementof
heavycomponents(i.e.,thebattery)ismore exiblethaninsystemsD1andD2.Adisadvantage
ofthisplatformistherequirementofaspringsystem.Thisisnecessaryforplatformswithmore
thanthreewheels to always provideaconstant surfacepressureof the driving wheels. . Such
aspringsystemincreases thecosts for the drive systemanddecreases the robustness of the
platform.AnexampleusingthedrivesystemD3istherobotKompai [RobosoftSA,2011].
ThedrivesystemD4combinesthelow-costrealizationofathree-wheeledplatformwithahigh
stability. Two o driven wheels s areplacedoutside e the center ofthe platform’s footprint. . The
disadvantage is that theturningcurveappearstobebigger thanthe footprint of the robot.
Thishastobeconsideredduringthemovementoftheplatformtoavoidcollisions.Anexamples
forthisdrivesystemis theservicerobot Charles,buildonaPeopleBot t platform[Kuoetal.,
2008].
Thedrive systemD5 includes two drivenwheels at the front sideandtwo castor wheels at
theback. Suchplatformsareusuallyrealizedbasedonrectangularfootprintstominimizethe
requiredarea. TheplatformD5allowsforagoodstability,buthassomedisadvantagesinthe
maneuverability,because of thesignicantly enlarged area for r rotation A
ROT
. An n example,
applyingthisdriveconcept,istherehabilitationrobotFRIENDII [Volosyaketal.,2005].
Thedrive systemD6consists of two drivenwheels at the center line andfour castor wheels
placedatthecorners oftherectangularfootprint. . Themaneuverability y andstability of this
platform is s better comparedto D5, , but t the complexity andcosts s for the e suspension of the
wheelsarehigher. AnexampleimplementationistheCare-O-Bot t I I byFraunhofer r IPA[Graf
etal.,2004].
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