RobotArmChallenge
Description
Inthis activity studentsareintroducedtofluidpower, articulated robot arms andend effectors.Studentswill work with a partner todesignand construct an arm thatallowsthemtomove anumberofsmall objects from one location toanother.
This activity helpsstudentsby:
•Introducingthemtofluidpower(hydraulicsandpneumatics)
•Introducingthemtothe concept ofmoment(torque)
•Developingdesignandmanufacturingskills
•Demandingteamworkand cooperation toaccomplish a task
LessonOutcomes
Studentswill be ableto:
•Describe theoperationof a fluidpowersystem,including:
–Howpressureandpistonarea affect force
–Therelationshipbetweenpistonarea,strokelengthand force
–Whypistonsare good atpushingbut poor atpulling
•Identify liveloadsanddeadloadsandtheirimpact on arm performance
•Apply techniquestoreducetheimpactofdeadload on arm performance
•Describe theroleof force anddistanceincalculatingmoment(torque)
•Usethetoolsandequipmentprovidedtocreate a hydraulicrobot arm
•Usetherobot arm theycreatetoaccomplish a predefinedtask
Assumptions
Studentswillhavebasicunderstanding or backgroundknowledgeinthefollowingareas:
•Useofsimplehandtools:
–Wirestrippers
–Hot glue
–Solderingirons
–Sharpknives
•Sketchinganddrawing
FoundationalLearning
•Fluidpowertransmission
–Pressure / force / arearelationship
–Pushingvs.pulling(youcan’t“pull”)
•Howpushing force anddistance affect moment(torque)
–Effect ofpushingatananglewhencalculatingmoment
KeyTerminology
Area: a measurementof surface.
Cylinder: the outercontainer of a fluid poweractuator.
Degreesof freedom: the numberofaxesupon which a devicemaymove or rotate.Forexamplethehuman arm hassevendegreesoffreedom:threeattheshoulderjoint,oneattheelbow,andthreeatthewrist.
Endeffector:the toolatthe endof a robot arm thataccomplishesthe desiredtask.Often usedtograspobjects,itmay be anytypeoftool,including a spraygun or welder.
Moment:a twist, or torque,created bya forceactingat a distance from a pivot point.
Piston: the slidingcomponent inside acylinder.
Pressure: a force distributedevenlyovera surface. Force: a push or pullupononeobjectexertedby a second object.
EstimatedTime
Totaltime8–12hours:
•2–3hoursof lesson time
•5–7hoursofbuildandtestingtime
•1–2hoursofactivity/competitiontime
RecommendedNumberofStudents
Twostudentsper arm to a maximum of20 students,based on BCTechnologyEducators' BestPracticeGuide
Iftimeandresourcespermit,havingeachstudentbuildtheirownrobot arm allowsthemtotakethe arm home attheendoftheactivity.
Facilities
A multipurposetechstudiesshop or labwith access to:
•Drawing or sketchingresources
•Drills (ideallydrillpresses)
•Fine-toothedsaws or sturdyknivesfor cutting woodensticks
•Ideallyone or morebandsawsfor cutting plywood pieces
•Hotgluearea
•Waterand“wet work space”withtowels or papertowelfor drying parts
Tools
•Drill press (orsuitablehanddrillarrangement)
•Whitneypunch(ifavailable)
•Wirestrippers
•Screwdrivers
•Scissors
•Hotglueguns
Materials
•Syringes:available from medical supplystores or online.The “Luer-Lok” tip on therightinFigure 1 holdstubingmuchbetter.10ccsyringesareabouttherightsizeforthisproject.
•Tubing:¼" clear vinyltubing from Home Depotworkswell.Testthetubingwith a syringefirsttoensure a good fit.
•Woodstrips
•Plywood forrobotbase platform
•Assorted blocksofwood
•Dowels
•Screws,nuts,bolts
•Stiffwire or thin rod (1⁄16"and⅛"weldingrods work nicely)
•Cardboardandthumbtacksfor“Cardboard Aided Design”
•¼"–1"gameelementsfortherobotchallengeofmovingsmallobjects.You can use arangeofsizes,givingmorepointsforthe little ones.Nutsandbolts work well. Otherpossibilities include:
–Marbles andballbearings
–Emptyaluminum cans
–M&M candies (The team can shareasmany M&Ms asthey can placeinto a cupin60seconds.)
•Youmaywanttouse a plasticcup,about10 cm high,asthe“goal”wheretherobotwillplacethegamepieces.
Resources
Figure1—Syringes
Thisis a common STEM activity performed inslightlydifferentwaysinmanydifferentschoolsaroundtheworld.Therearemanyexcellentresourcesavailablebysearchingfor“syringerobotarm,”“hydraulicrobotarm”andsimilarcombinations. Some currentresources(asof2016)include:
TuftsUniversity“TeachEngineering”EducationalOutreachisanexcellent,alternative activityguideincluding a shortvideo:
The “Syringe Hydraulic Arm” atIdeas-Inspire.comisanotherexcellent write-up withphotos,videosandinstructionsforbuildingarms:
A well-documentedbuildofan articulated 3 DOF syringe arm withgripper,includingplans,isavailableat:
Thissitepresentsinstructionsforassembling a commerciallypreparedkit.Thefinedetailoftheseinstructionsmay serve as a guideinthedesignofyourstudents’arms:
TheInstructablessitehas a nice arm built from cardboardandducttape:
Commerciallyproducededucationalrobot armkitsareavailableforpurchase.However,theycan be manufactured in-house for a fraction ofthe cost:
Therearealso 3D printablerobot arms onThingiverse:
Demonstration
If a samplesyringe arm isavailable,useittodemonstratethechallenge. Otherwise youmaywishtouseoneofthemanyvideosavailableon-line.Linksareprovidedabove.
Procedure
Followingisanoutlineoftheproceduresforthislesson.Detailedprocedureguidelinesareprovided belowinthe DetailedProcedureGuidelinessection on page9.Itemsmarkedwith
an asteriskhave supporting materials includedintheLessonSupport Materials section ofthisactivityguide.
Day1: / Lesson: / IntroduceActivity.*Discuss types ofrobotarms.*Discuss types ofend effectors.
Activities: / Putstudentsintoteams.
Begindrawing / modellingexercisetodetermine arm typeanddimensions.*
Day2: / Lesson: / Fluidpower*
Activities: / Fillandbleedsyringes.
Comparethe force on differentsizesyringes.
Comparethe difference betweenpushing on a syringeandpulling on it.Wheredothebubbles come from whenpulling?
Finishdesignofrobot arm if timeallows.
Day3: / Lesson: / Momentsandcounterbalances*
Activities: / Finishdesignofrobotarm.Begin construction ofrobotarm.
Day4: / Lesson: / Reviewfluidpowerandmoment concepts
Activities: / Continue arm construction.
Day5: / Lesson: / Quiz on fluidpowerandmomentconcepts*
Activity: / Continue arm construction.
Day6: / Lesson: / Cableandtubingmanagement
Activity: / Continue arm construction andtest.
Day7: / Lesson: / Reviewchallenge
Activity: / Arm practice andrefinement
Day8: / Activity: / RobotArmChallengeDay!
LessonSupportMaterials
Thefollowing lesson support materialsareprovidedinthis activity:
•RobotArmChallengepage 7
•FluidPowerWorksheetpage18
•MomentsandCounterbalancesWorksheetpage25
•RobotArmQuizpage28
TheRobotArmChallenge
Name:Date:Block:
Robot arms areusedeverywhere!Fromanexcavatorto a chocolate factory totheCanadarm2on theInternationalSpaceStation,therobot arm isoneofthemost practical applicationsforrobotics. Manyrobot arms operateusingmotorsforcontrol,but some ofthemost powerful onesusehydraulics.Inthischallengeyouwill work with a partner todesignandbuild a hydraulicrobot arm andhaveitcomplete a taskas efficiently aspossible.
TheChallenge:
Thegame object foryourchallengewillbe:
Thegame objects will be placed on a 20 cm × 20 cm playingfield.
Theonlythingthatmayenterthespaceabovetheplaying field or touchthegame objects shallbe yourrobotarm.
Thebaseofyourrobot arm must be atleast 5 cm from theplaying field atalltimes.
Your arm willhaveto pick upthegame objects andplacethemin a containerapproximately10cm tall.
Yourtaskwill be togetasmanygame objects intothecontaineraspossiblein: seconds.
YourSupplies:
Youwill be given a collection ofbuildingmaterialsbyyourteacher.Yourrobotmust be built fromthematerialssupplied.
Youwill be given:plasticsyringesandtubingtojointhem.Thesewill form yourhydraulic actuators.
YourProcess:
Youwillneedtostartbydesigningyourrobotarm.Thesix common types ofrobot arm areshownbelow.
Forthischallengethe articulated robot arm isthemost common solution.Yourteacherwillguideyouthroughanexerciseindesigningyourarm.
Once youhave a designthatworks,sketchitat ½ scale usingthebackofthishandout.
RectangularcoordinaterobotCylindricalcoordinaterobot
Sphericalcoordinaterobot
Articulatedarmrobot
GantryrobotSCARArobot
Figure2—Six mostcommontypes ofrobotarm
DetailedProcedureGuidelines
Day1Lesson:ArmGeometry:exerciseandtipsondesigninganarticulatedarmRobot arms are often described intermsof“degreesoffreedom”(DOF).A degreeoffreedomisprovidedby a jointthat can eithertranslate(slide) or rotate.
IntheCNCworldthesearesometimesdescribedas “axes” (plural ofaxis,notthetoolsforlumberjacks). Astandardthree-axisCNC router or3Dprinterisa“3DOFCartesian” system.
Youcanseea5DOFCNCmachineinaction here:
Most ofthesix common robot arm designshaveatleastone DOF from a translationalelement.Unlessyouhaveprovidedthestudentswith a waytobuild a linearslide,theywillhave aproblem.Theymight be abletosolve it—it’s uptothemtofigureitout.
Most ofthe arms builttosolvethischallengearethe articulated type. Usingeightsyringesallowsforthree DOF plusoneend effector (“gripper”).
Typicallytheyarereferredtoasthe:
•base(rotation)
•shoulderjoint(rotation)
•elbowjoint(rotation)
Tosave on syringesyoumaywanttoallowstudentsto turn thebasebyhand.
Designingan articulated robot arm isfairlyeasy;it’s connecting thehydraulicsthatisthechallenge. 3D parametricmodellingprogramssuchasAutoDeskInventormakethiseasier.
Tohelpinthedesign,haveyourstudentsuseCAD:Cardboard Aided Design(credittoRandySchultz atBCITfortheterm).Students can mark outthebaseline,playing field and “lifting line”at1:1 scale on a piece ofcardboard.
They can cut a representationoftheirbase,upper arm andlower arm from cardboard.They canusethumbtacks,pushpins or evenbradsto serve asjoints.
The process isdetailedinFigures3–14 on thefollowingpages.
Figure3—ThestartingpointFigure4—Pintheparts
Pin down base and armjoints.Armshouldreachfarthestpointon playing field.
Figure5—Test nearest reachpoint
Armshouldalsoreachnearestpointon playing field.
Figure6—Heightclearance
Armshouldclearthelifting line.
Figure 7—Test-fitshouldersyringe
Test-fitshoulderjointsyringe at fullextension.
Markmountingpointsforthe syringe.
Figure 8—Test-fitretractedsyringe
Test-fitshoulderjointsyringe at fullretraction. ItMUST fit thesamemountingpoints as at fullextension.
Figure 9—Test-fitelbowsyringe
Test-fitelbowjointsyringe at fullextension.Markmountingpoints. Note that thissyringemay notintersect
withtheshoulder syringe. It maybeeasier to usetwosyringes in this exercise.
Figure10—Test-fit retractedelbow
Test-fitelbowjointsyringe at fullretraction. Note thatit cannotretractthearm to the near side of theplaying field.Thismountingpointwill not work.
Figure11—Adjustsyringestofit
Try a new mountingpointfortheelbowjointsyringe. Markthe new mountingpoints.
Figure12—Pay attentiontotorque
The new syringelocation might work, but note that it is almostperfectly in line withthe lower arm.Thismeans that it will notbedelivering a lot of torque to theelbowjoint. It might work,but there might be a betterdesign.
Figure13—Try curvedarms
Students might want to try using bent orcurvedarmsin order to improve mountingpoints and torquetransfer. They may find someinspiration in thebackhoeorexcavator,although they are
designedfordouble-actingcylinders.
Figure14—Curved armsmay offer betterperformanceAprocess of iterationshouldlead students to develop aworkabledesign.Once they have finalized theirdesign,have
them sketch a ½ scalemodelontheback of theirhandout.
Theteachermaywish to havethe students demonstratetheirdesign using theircardboard model..
EndEffectorsor“Grippers”
Thedesignoftheend effector will be based on thechosengameobjectandshould be designedatthesametimeasthearm; a similar CAD process may be used.Elastics or springs can bevery useful. As youuse a syringetopushthe effector open,theelasticswill hold ittightwhen
thesyringeisretracted.
See theDay 2 notes on pulling on a hydrauliccylinder.You could pull,ofcourse,justnot verywell.
Linksto some examples can be foundinthe“Resources” section ofthisactivity.
Day2Lesson:FluidPower
Thesenotesareintendedto support thefluidpowerworksheet.Itishelpfultohave a fewsyringesandtubingfilledinadvanceofthe lesson to serve asdemonstrations. A smallamountoffoodcolouringinthewaterwillmakeiteasiertoseethefluid.Placingthesyringes on anoverheadprojectorwillallowallstudentstoseethesyringesmoveastheinstructorpushesandpullsthepistons.Havingbucketsfilledwithwater(perhaps a couple ofbucketswithdifferentcoloursofwater)willhelpstudentsfillandbleedtheirsystems.
Fluidpowersystemsincludebothpneumaticandhydraulicsystems.Inboth,poweristransmitted from one location toanotherbymoving a pressurizedfluid.Gases,likeair,areconsideredfluidsbecausetheyflow.Liquids,likewater,arealsofluids.
Hydraulicsystems
Inhydraulicsystemsthefluidis a liquid.Weusewaterforourhydraulic arm (that’swherethehydro comes from).
Industrialequipmentusesoil, which helpsprevent corrosion inthesystem.
Hydraulicfluidsareincompressible.Theydon’tchangetheirvolumewhenyoupush on them.Whenusing a hydraulicfluid,itis important togetalloftheairoutofthesystem.Eventinyairbubbleswill compress underpressure.Thismakesthesystem “spongy” or “springy.”Removingthebubbles from thesystemis called bleeding thesystem.Haveyoueverheardofsomeonebleedingtheirbrakes?
Pneumaticsystems
Inpneumaticsystemsthefluidisusuallyair(that’swherethe“pneum” comes from … likepneumonia).
Gasesarecompressible.Compressedgasactslike a spring:itstores a lotofenergy!This canbe usefulwhenusedin a paintballgunbutdangerouswhenusedwith “home-made” storagetanksliketheSchedule 40 ABSpipeinpotatocannons.When a pneumaticvesselfails,ittendstoexplodeviolently.
Steampower
Questionforstudents: Wouldsteampower be considered hydraulic orpneumatic?
Answer:Ituseswatertomakesteam,butsteamis a gas so itispneumatic.Inreality,however,steam often getsitsown category becauseofthehightemperaturesinvolvedandthefactthatthesteam often condensesintoliquid.
Steampoweris a veryimportantpartof industrialoperations inBC. Itsuses include:
•heatingbuildings
•runningindustrialplantssuch a pulpmillsandrefineries
•creating electricity throughturbinesdrivenbysteamgeneratedbyburningnaturalgas orevengarbage!
People who run andoperatesteampowerplantsare called powerengineers or steamengineers.Therearemany good jobsforpeople who can work safelywithsteam.You can learnto be a powerengineeratBCIT or otherschools.
Hydraulicsvs.Pneumatics:QuickReference
Hydraulic / PneumaticTypeofFluid / Water, oil or other liquid / Air or other gas
Compressible? / No. Does not change volume whenunderpressure. / Yes. It compresses when underpressure.
Movement / Very precise. Good for movingactuators a specific distance. / Veryflexible.Good for gripping objectsofunknownsize.
Explosion Hazard / Verylow. If a cylinder ruptures, a smallleak will instantly relieve all of thepressure in the line.(Justdon’tget hitby the stream … it will be a veryhigh-pressure jet!) / Veryhigh. If a cylinder ruptures, thestored energy in the compressed
gas will be released as an explosion.Higher pressures bring higher danger!
Spill Hazard / Leakscan create a big mess,particularly when using oil. / No clean uprequired.
Speed / Liquids are more viscous than air.Theymove more slowly. / Pneumatic actuators canmoveveryquickly, as air flowseasily.
Force / Can be used safelyat high pressure,so can transmit high forces safely. / High pressures create an explosionhazard. The working pressure is lowerthan for hydraulicsystems.
General Uses / Larger,heavier,slowermovingequipment that transmits a lot of force. / Smaller,quickermoving machines thatmay pick up objects ofunusualsizes orshapes
Pressure,AreaandForce:HowtoMoveaPistonwithFluidPower
Our syringes work like a hydraulic or pneumaticcylinder.Thecylinderisthe“outside”partthatstaysstillandcontainsthefluid.Thepistonisthe“inside”partthatmoves.Thefluidinthesystemisunder pressure—the samepressureateverypointinthesystem.
Pressureismeasuredintermsof force dividedbyarea.Inthemetricsystemweusethepascal,or onenewtonof force persquaremetre. Since thepascalis a very smallamountofpressure,wetendtousethekilopascal(kPa). One kPais 1000 newtonsof force persquaremetre.Thisisroughlythe force exertedby100kgofmassatthe surface ofthe Earth. Standardairpressureisabout100kPa.Standardairpressureis101.3kPaatsealevel.
Intheimperialsystemweusepoundspersquareinch(psi).Standardairpressureisabout15psi.
Imagine a 1"square on thepalmofyourhand.Theairpressurepushing on thatsquareis15 pounds.
•Howmuchairpressureispushing on your whole palm?
•Whydoesn’tyourhandmoveunderthepressure?
•Whydon’tyoufeel it?
•Whatwouldhappen if theairpressurewasn’tthere?
Within a closed hydraulicsystemitisassumedthatthepressureisthesame everywhere inthesystem.
Force can be calculatedbymultiplyingthepressurebythearea: F = P × A
WorkedExampleofPressureCalculation
Ifyouhave15psiacting on 3 squareinches,youhave:F = 15psi × 3 squareinches = 45 poundsof force
Ifyouhave100kPaacting on a 10-cmdiametercircle:
1.Convert 1 cm to0.01m.
2.Calculatetheradiusofthecircle:0.1 m diameter / 2 = 0.05 m radius
3.Calculatetheareaofthecircle:3.14 x (0.05 m radius)2 = 0.008 m2
4.Convertthepressuretopascals:100kPa = 100,000Pa
5.Nowcalculatetheforce:100,000Pa x 0.008 m2 = 800 N
Toestimatetheequivalent“weight,”dividethenewtonvalueby10:
800 N / 10 = approximately80kgof force will be acting on thecircle.
Themetricexamplemayseemmorecomplex,butthemetricsystemisusedbyscientistsandengineers. Metric calculations become mucheasierwithpractice.
PushingaPiston:MechanicalAdvantage
Typically a pistonwillhavetwo forces acting on it:
1.Theload:appliedbysomething or someone“outside”thehydraulicsystem.
2.Thepressurizedfluid:acting on thepistonbythefluid“inside”thesystem.
Whenthetwo forces areequalthepistonwon’tmove.Whenthetwo forces areunequal,thepistonwillmove.
Thepistonwillmove “out” if thepressurizedfluidpresseswithmore force thantheload.Itwillmove“in” if theloadpresseswithmore force thanthefluid.
Whenyouhavetwopistonswithdifferentareas,the force acting on eachpistonwill be different(Figure15).Thelargerpiston,withmore surface area,willpushwith a higher force butitwillmove a shorter distancethanthesmallpiston.
Thesmallerpiston,with less surface area,willpushwith a lowerforce.Itwillmove a greaterdistancethanthelargepiston.Thisissimilartothe principles ofgearing, or leverage:
Smaller force × longerdistance = bigger force × shorter distance
Two single acting cylinders joined by a hose.Which will push with the most force?
Which will travel the longer distance?
Figure 15—Twosingle-acting cylindersof different diameters joinedby abase
Consider a car’sbrakingsystem(orhydraulicdiscbrakes on a mountainbike).Thebrakepadneedstomove a very smalldistancebutpush very hard.Thebrakepedal can move a longdistancebutneedsto be easytopush.Thepedalgets a smallcylinderandthebrakegets alargecylinder.
Since largercylinderswillexertmore force atthesamefluidpressure,youneedtodesignyourarm to work withthecylindersthatyouhave.Ifthehosekeepspoppingoffthecylinder,thenperhapsyourfluidpressureistoohigh.
Do youneed a biggercylinder? Do youneedtoreducetheload on thecylinder? See thenotesunderDay3:MomentsandCounterbalances,below.
Pushingvs.PullingaPiston
Questionforstudents: When you pull on one piston, what pushes on the other piston?
Answer: Theotherpiston is“pushed”byambient airpressure,which isusuallymuchlower thanthepressureinsidethesystem.
Youcan’tpull on a hydraulicsystem.Well,you CAN … butnotwithasmuch force asyou canpush.Whathappens if youpull harder? Bubbles form insidethesystem.Demonstratebyplacingsyringes on anoverheadprojectorandpulling.Wheredothebubbles come from?Dissolvedgasesinthewater.
Consider a bottle ofsodapop. Do youseebubblesinitwhenitisclosed?Whatpressureisitatwhenitisclosed?Whathappenswhenyouopen it? Whatpressureisitat after opening?Thereis a LOTof carbon dioxidegasdissolvedinsoda.
Studentsmayhaveheardofscubadiversbeing affected by“thebends.”Thishappenswhenoxygenand carbon dioxidebuildupintheirbloodathighpressurewhentheyareunderwater.Ifthey come uptothe surface toofast,theirbloodbubbles,justlikethesoda pop whenitisopened.Thebubbles block theflowofbloodand can kill.
Ifyoupullhardenoughyouwillgetwatervapour. As waterpressureisreduced,theboilingtemperaturedrops.Thisisusedinvacuumdehydrationoffoods.Butindustrialhydrauliccylinders can pushANDpull.Howdotheydo it?
Our syringes are asingle-actingpiston: fluid contacts only one end of the piston. The pistons aredesignedtopushinonedirectiononly.
Most industrialcylindersaredouble-acting: fluid contacts bothendsofthe cylinder(Figure16).Thepiston can be pushedinbothdirections. Do youthinkitwillpushharderinonedirectionthanthe other? Take a look attheareaofthepiston on eachside.The shaft on onesideslightlyreducesthearea.Thereisn’tasmucharea, so thereisn’tasmuchforce!
Figure16—Double-actingcylinder
FluidPowerWorksheet
Name:Date:Block:
Theseare some ofthewordsandideasthatengineersusewhenworkingwithfluidpower.Theyarealsousedby mechanics andequipmentoperatorswhencontrollingandrepairingheavyequipment.Canyouthinkofanyother careers where people needtoknowaboutfluidpower?
Terms:
Hydraulicsuse asuch asor
tomakethingsmove.
Pneumaticsuse asuch astomakethingsmove.
A liquidisanfluid.Thatmeansthatitdoesn’t “shrink” whenyoupush on it.
A gas is afluid. That means that it acts kind of likea spring whenyoupush on it.
Pressureis a measureofdividedby.
Air pressureacts on usallthetime. One “atmosphere”ofairpressureisthepressurewefeelon Earthatsealevel.Inmetricmeasure,oneatmosphereismeasuredas
kPa.Inimperialmeasureitismeasuredaspsi.
Our robotwillusesyringestomakethingsmove.Inthephotobelow,labelthepartofthesyringethatisthe“piston.” Label thepartthatisthe“cylinder.” Label thepistonringsthatkeepthefluid
inthesyringe.
Thefluid can push on thepistontomakeitmove.Howhardthepistonpushesis called the
Itiscalculatedbymultiplyingthe ofthefluidbythe ofthepiston.
Whenfilling a hydraulicsystemwithfluid,itis important togetalltheoutofthesystem.Thisis called thesystem.
Calculations
Whenengineers, mechanics andequipmentoperatorsusefluidpower,theyneedtoknowhowmuch force theirsystem can apply.Theseare some ofthecalculationsthattheyuse.Fillinthespaces.
10 ccsyringe / 5ccsyringeDiameterofpiston / 14.5 mm / 12 mm
Diameterofpistonin metres(dividemm by1000)
Radiusofpiston(dividediameterby2)
Area ofpiston(3.14 × radiussquared)
Forceonpistonat 100,000 Pa
(multiply area by100,000)
Which pistonexertsthelarger force at100kPa?
Whydoesthelargerpistonexertthelargerforce?
10 ccsyringe
5ccsyringe
Inthephotoabove, a 10ccsyringeisconnectedto a 5 ccsyringe so thatthefluid can flow fromonetotheother.Ifyoupush on thepistoninthe10ccsyringe,the 5 ccsyringe’spistonwill
moveoutward.Itwillmovewithforce thanispushing on the10
ccsyringepiston,butwillmove adistance.
Ifyoupush on thepistoninthe 5 ccsyringe,the10ccsyringe’spistonwillmoveoutwards.Itwillmovewith force thanispushing on the 5 ccsyringepistonbut
willmove adistance.
Fluidpowerworksbestwhenpushing.Whenyou“pull” on onesyringeyouarerelying onambientpressure topushtheotherpistoninward.Ifyoupulltoohard,then will form inthehydraulicfluid.
Day3Lesson:MomentsandCounterbalances
Thesenotesaretocomplementthe “Moments andCounterbalancesWorksheet.”Having a long(1–2m)lever arm with holes drilledatregularintervalsand a selection ofweightstohang fromthe arm willhelpillustratethe concepts described here.
A moment,alsoknownastorque,is a twisting force thatistheresultof a force pushingat adistance from a pivotpoint. A 100 g massattheendof a 10-cmlongpivotwillcreate a momentof10 cm × 100 g = 1000 g × cm (Figure17).
10 cm
100 g
Figure17—Amoment
Technically,themetricunitfortorqueisthenewtonmetre. Since inEarth’sgravitational field 100g ofmasscreates 1 N offorce,and10 cm = 0.1m,ourcalculationwould be betterstatedas:
1 N × 0.1 m = 0.1 Nm
Theimperialunitformomentisthefoot-pound,althoughyou’llalsofind inch-pounds used onsmallertorquewrenches. One newtonmeteris0.738foot-pounds or 8.85 inch-pounds.
Regardlessofwhetheryouareusingmetric,imperial,inch-pounds, g x cm, Nm or foot-pounds,thebasic concept isthesame: a moment, or torque,issimply force x distance.
Since the force ofgravityisalwayspullingdown,momentschangeas a robot arm lifts an object(Figure18).
When a 10-cm arm isrotatedto 45 degrees,theloadisnowonly 7 cm from thepivotpoint.Themomentaboutthispivotpointisnow100 g × 7 cm = 70 g × cm, or moreaccurately,
1 N × 0.07 m = 0.07Nm.
Figure18—Load isclosertopivotpoint
Whatwouldthemomentbewhenthearmisperfectlyvertical?
Youmaywishtohave a studentdemonstratebyholding a weightinonehandwiththeir armextendedhorizontally.Havethemslowlyrotatetheir arm to vertical whilekeepingtheir armstraight.Ask,“Whendidtheloadfeelheaviest?”
Tocounteract a clockwise moment,weneedtoapply a counter-clockwisemoment.InFigure19,thesyringeisapplyinganupward force at a distanceof 5 cm from thepivot, so itmustcounteract a clockwise momentof100 g × 10 cm = 1000 g × cm.
Thesyringemustcreate a counter-clockwisemomentof 1000 g × cm.
Figure19—Pistoncounteractsmoment
Since thepistonis 5 cm from thepivot,we can calculatetheupwardforce:1000 g × cm / 5 cm = 200g.
Hadwecalculateditallinnewtonmetres,wewouldhavearrivedat 2 N ofupwardforce.Thismakessense:theloadistwiceasfar from thepivotasthesyringe, so thesyringemustexerttwiceasmuchforce.
Sometimes a hydraulicactuator(syringe)can’tprovideenough force toliftthearm.Adding acounterbalance can help.InFigure20,the100 g counterbalanceat a distanceof 5 cm from thepivotprovides100 g × 5 cm = 500 g × cm ofcounter-clockwisemoment.Thismeansthatthesyringeonlyneedstocreate a 500 g × cm counter-clockwisemoment.Thesyringeonlyneedstopushwith a force of100 g (1N).
Figure20—Counterbalancesupportingload
Sometimesadding a counterbalance can makethe arm tooheavy or tooslow.Springs or elasticbands can be usedtoprovide a counteracting force (Figure21).
Figure21—Elastics orspringsreplacingcounterweight
It can be more difficult tocalculatethe effect of a spring or elasticband.Theamountoftensionchangesasthebandisstretched or relaxed. One alternativeistouse a constant force spring,which is a coil ofthinmetalstripthat can be usedatthepivotpointtocounteracttheload.
Therearetwomain types ofloadthat a robot arm experiences:
1.The liveload:theweightofwhatever isbeing lifted
2.The deadload:theweightofthearm, grippers andactuators
Normallycounterbalancesareusedtocounteractthedeadloadofthearm.Theliveloadis leftfortheactuators(pistons or motors)tomanage.
Anadditionalliveloadtoconsideristhemomentumofthearm. A heavy arm doesnotwanttostart(orstop)movingquickly.Fast-moving arms haveto be either very light or very powerful.
When a heavyliveloadmust be heldinpositionforanextendedtime,itmay be helpfultoalsoapply a brake or lockingmechanismtothepivotpoint.Thisreducesthestrain on themotors orcylindersholdingthe arm inplace.
Your arm designwilllikely be a bitmorecomplicatedtocalculate.Thesyringeswillprobably beatanangletothearm.This can be calculatedbyhand or modelledbycomputer.
ThesyringeinFigure 22, forexample,hastopushharderthan a vertical syringebecauseitis
atanangle. So longasyouunderstandthebasicprinciplesofmoment,however,youshould beabletomakeyour arm work.
Figure22—More complexpistonarrangement
MomentsandCounterbalancesWorksheet
Name:Date:Block:
Yourrobot arm willrotateaboutjoints, or pivotpoints.Theamountof force acting on the armandthedistanceofthat force from thepivotpointwill be important inmakingsurethatyour armcan liftitsload.Learningaboutmomentsandcounterbalanceswillhelpyoudesignandbuild abetterrobotarm.
Ais a twisting force,alsoknownas.Itis
theresultof apushingat a distance from apoint.
Inmetricmeasurethestandardunitfor a momentisthe
orNm.Inimperialmeasure people usethe ______
orft.lb.
Theequationtocalculate a momentis×.Inthisdiagramtheliveloadis100 g andactsat a distanceof10 cm from thepivot.
100 g ofmassinEarth’sgravitationgives a force ofN.A distanceof10 cm expressedinmetresis m.
Themomentis:
N ×m =Nm.
Thesyringemustcounteractthemomentoftheliveload.Thesyringeisat a distanceof
m, so we can calculatethe force on thesyringeasNm /
=N.
This force isroughlythesameasthatexertedby a massofg.Thismakessensebecausethesyringepushes ashard,butat
thedistanceastheload.
Themoment can changeasthe arm rotates.Inthisdiagramthe arm isstill10 cm long,butnowtheloadis closer tothepivotpoint.Themomentisnow:
N ×m =Nm.
Tohelpbalancean arm andmakeiteasierto lift, wesometimesadd a weight on theoppositesideofthepivotpoint from themainload.We call thisa .
Inthis arm itcreates a counter-clockwisemomentofN ×m =
Nm.
Thismeansthesyringeonlyhastocreate a momentofNm tobalancethearm.
One problemwithcounterbalances(also called counterweights)isthattheymakean arm
.
Robot arm designers can help support theloadbyaddingor
tohelppullthe arm up.
Therearetwomain types ofload on thearm.Theloadisthe
weightofthe object the arm is lifting, whiletheloadistheweightofthe arm itself.Theweightofthe object mightchange,buttheweightofthe arm usually
remainsconstant.Forthisreason,robotdesignerswillusuallyusethecounterbalanceto supportthemomentcausedbythedeadload,andletthepiston or motor support theweightoftheliveload.
RobotArmQuiz
Name:Date:Block: ______
Score:/16
1.Matching—Place theletterthatbestrepresentstheterminthecolumnindicated.(0.5 mark each–6marks)
PlaceLetterHere / TermHydraulicpower / A
Force/Area / B
About 100kPa in metric,
about15 psi inimperial / C
A metric
unitof force,about equalto a mass of100 g
Pneumaticpower
Pressure
Airpressureat sealevel / D
10cm
100g / E
Fluidpowerusing a liquidsuch as oilorwater as thefluid / F
The metricunitoftorque,the newtonmetre
Bleeding
Moment
Newton / G
A weight or spring added to theback ofyour robot armto assistwith lifting the load / H
Fluidpowerusing a gas such asairasthe fluid / I
The imperialunitoftorque,the footpound
Nm
Ft.lb.
Live load / J
The weightof the structure; inthis case, the weightof the arm / K
Removing airbubbles froma hydraulicsystem / L
The weightof the objectyou aremoving
Dead load
Counterbalance