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Physics 124: Lecture 3 Motors: Servo; DC; Stepper Messing with PWM (and 2-way serial) The Motor Shield From T. Murphy’s PHYS124 lectures

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Page 1: Physics 124: Lecture 3physics124.barreiro.ucsd.edu/wp-content/uploads/...Valid Divider Options PWM pins Register scaler values Divisor Approx.frequencies (Hz) 5, 6 TCCR0B 1, 2, 3,4,

Physics124:Lecture3

Motors:Servo;DC;StepperMessingwithPWM(and2-wayserial)

TheMotorShieldFromT.Murphy’sPHYS124lectures

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ThreeTypes(forus)

• Servomotor– PWMsetsposition,usedforR/Cplanes,cars,etc.– 180° rangelimit,typically– 5Vsupply

• Steppermotor– Forpreciseangularcontrolorspeedcontrol– Canrotateindefinitely– Lotsofholdingtorque

• DCmotor– simplesttechnology;giveuponprecisecontrol– goodwhenyoujustneedsomethingtoSPIN!

2Phys124:Lecture3

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WhenanyoldPWMwon’tdo

• ThefunctionanalogWrite() givesyoueasycontroloverthedutycycleofPWMoutput– butnocontrolatalloverfrequency

• ConsidertheHitec servomotorswe’llbeusing:

• Wantsa50Hzpulserate,andadutycyclefrom4.5%to10.5%(11/255to27/255)todrivefullrange

3Phys124:Lecture3

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WhatfrequencyisArduino PWM?

• Dependsonwhichoutputisused• Pins5and6:default~977Hz– 16MHzclockratedividedby214 =16384

• Pins3,9,10,11:default488Hz– 16MHz/215

• Neitherisatalllikethe50Hzweneedfortheservomotor

Phys124:Lecture3 4

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Whatchoicedowehave?• Wecanchangetheclockdivideronanyofthreecounters

internaltotheATMega328– timer/counter0,1,and2,considerregistermap:

– noteinparticularthelowest3bitsinTCCR2B– Timer/CounterControlRegisters,WaveformGenerationMode,

CompareMatchOutput– settingtheseaccordingtothefollowingrubricscalesspeed

Phys124:Lecture3 5

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ValidDividerOptionsPWMpins

Register scaler values Divisor Approx. frequencies(Hz)

5, 6 TCCR0B 1,2,3, 4,5 1,8,64,256,1024 62500,7812, 977,244,61.0

9,10 TCCR1B 1,2,3,4,5 1,8,64,256,1024 31250,3906,488,122,30.5

3,11 TCCR2B 1,2,3,4,5,6,7 1,8,64,256,1024… 31250,3906,977,488,244,122,30.5

Phys124:Lecture3 6

• Defaultsinred• Limited choices,wecan’tpreciselyhit50Hztarget• Closestistousetimer0withdivideroption5(61Hz)• 0.9to2.1mspulsescorrespondto14/255to33/255• only20possiblestepsbythisscheme

Canyoufindoutwhyabovescalervaluesinbinarydon’tmatchtheCStableabove?

http://playground.arduino.cc/Main/TimerPWMCheatsheet

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HowtosetdividerandchangePWMfreq.

• It’sactuallynotthathard– candoinsetuporinmainloop

• BrokenDown:– modifyingTCCR0Bassociatedwithpins5&6– & isbitwiseAND operator– 0b11111000isbinarymask,saying“keepfirstfiveas-is”– whilezeroingfinalthreebits(because0ANDanythingis0)– | isbitwiseOR operator,effectivelycombiningtwopieces– 0x05ishexfor5,whichwillselect61.0HzonTimer0– ifTCCR0Bstartedasvwxyzabc,itendsupasvwxyz101

Phys124:Lecture3 7

TCCR0B = TCCR0B & 0b11111000 | 0x05;

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• Ifyouwanttoknowmore,understandbetter:Lesson12ofthisclassSecretsofArduinoPWM:https://www.arduino.cc/en/Tutorial/SecretsOfArduinoPWMTimerPWMcheatsheet:http://playground.arduino.cc/Main/TimerPWMCheatsheetUnderstandingTCCRnA/B:http://www.righto.com/2009/07/secrets-of-arduino-pwm.html

Phys124:Lecture3 8

FastPWM:

Phase-correctPWM:

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CodetointeractivelyexplorePWMfrequencies

• Willuseserialcommunicationsinbothdirections

– continuedonnextslide…

Phys124:Lecture3 9

const int LED = 5; // or any PWM pin (3,5,6,9,10,11)char ch; // holds character for serial command

void setup(){

pinMode(LED,OUTPUT); // need to config for outputSerial.begin(9600);

}

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…continued

Phys124:Lecture3 10

void loop(){

analogWrite(LED,128); // 50% makes freq. meas. easierif (Serial.available()){ // check if incoming (to chip)ch = Serial.read(); // read single character, or Serial.parseInt()

if (ch >=‘0’ && ch <=‘7’){// valid rangeif (LED == 3 || LED == 11){ // will use timer2TCCR2B = TCCR2B & 0b11111000 | int(ch - ‘0’);Serial.print(“Switching pin ”);Serial.print(LED);Serial.print(“ to setting “);Serial.println(ch);

}}if (ch >=‘0’ && ch <=‘5’){// valid for other timers

if (LED == 5 || LED == 6){ // will use timer0TCCR0B = TCCR0B & 0b11111000 | int(ch – ‘0’);Serial.print(same stuff as before…);

}if (LED == 9 || LED == 10){ // uses timer1TCCR1B etc.

} } } } // would indent more cleanly if space

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Usingtheinteractiveprogram

• Useserialmonitor(Tools:SerialMonitor)– makesurebaudrateinlowerrightissameasinsetup()– cansendcharacterstoo– inthiscase,typesingledigitandreturn(orpresssend)– getbackmessagelike:

• Switching pin 11 to setting 6

– andshouldseefrequencychangeaccordingly

Phys124:Lecture3 11

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RiggingaServotosort-ofwork

• Originalmotivationwasgettinga50Hzservotowork

– continuednextslide…

Phys124:Lecture3 12

const int SERVO = 5;char ch; // for interactive serial controlint level = 23; // 23 is 1.5 ms; 14 is 0.9; 33 is 2.1

void setup(){

pinMode(SERVO, OUTPUT); // set servo pin for outputSerial.begin(9600);TCCR0B = TCCR0B & 0b11111000 | 0x05; // for 61 HzanalogWrite(SERVO, level); // start centered

}

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Continuation:mainloop

• Beinglazyandonlyacceptingsingle-charactercommands,limitedtotenvalues,mappingonto20– themap() functionisusefulhere– thech - ‘0’ does“ASCIIsubtraction”

Phys124:Lecture3 13

void loop(){

if (Serial.available()){ // check if incoming serial datach = Serial.read(); // read single characterif (ch >=‘0’ && ch <=‘9’){// use 10 step range for demo

level = map(ch-’0’,0,9,14,33); // map 0-9 onto 14-33analogWrite(SERVO, level); // send to servoSerial.print(“Setting servo level to: “);Serial.println(level);

}}delay(20); // interactive program, so slow

}

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Abetter(andeasier!)way• Thepreviousapproachwasapoorfit– poormatchtofrequency,andnotmuchresolution

• Arduino hasalibraryspecificallyforthis:Servo.h• VariouslibrariescomewiththeArduino distribution– in/Applications/Arduino.app/Contents/Resources/Java/librariesonmyMac

– Handlesstepperandservomotors,LCDs,memorystorageineitherEEPROM(on-board)orSDcard;severalcommoncommunicationprotocols(ethernet—forusewithshield,SPI,2-wire,andemulatedserial)

– canlookatcodeasmuchasyouwant

Phys124:Lecture3 14

EEPROM/ Firmata/ SD/ Servo/ Stepper/Ethernet/ LiquidCrystal/ SPI/ SoftwareSerial/ Wire/

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ExampleusingServolibrary

• Watchhoweasy:onedegreeresolution

Phys124:Lecture3 15

// servo_test . . . . slew servo back and forth thru 180 deg#include <Servo.h>

Servo hitec; // instantiate a servoint deg; // where is servo (in degrees)

void setup(){hitec.attach(9,620,2280); // servo physically hooked to pin 9// 620, 2280 are min, max pulse duration in microseconds// default is 544, 2400; here tuned to give 0 deg and 180 deg

}void loop(){for(deg = 0; deg <= 180; deg++){ // visit full range

hitec.write(deg); // send servo to degdelay(20);

}for(deg = 180; deg >= 0; deg--){ // return trip

hitec.write(deg); // send servo to degdelay(20);

}}

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AvailableServoMethods• attach(pin)

– Attachesaservomotortoani/opin.• attach(pin,min,max)

– Attachestoapinsettingminandmaxvaluesinmicroseconds;defaultminis544,maxis2400

• write(deg)– Setstheservoangleindegrees.(invalidanglethatisvalidaspulsein

microsecondsistreatedasmicroseconds)• writeMicroseconds(us)

– Setstheservopulsewidthinmicroseconds(givesveryhighresolution)• read()

– Getsthe lastwrittenservopulsewidthasananglebetween0and180.• readMicroseconds()

– Getsthe lastwrittenservopulsewidthinmicroseconds• attached()

– Returnstrueifthereisaservoattached.• detach()

– Stopsanattachedservofrompulsingits i/opin.

Phys124:Lecture3 16

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Libraries:Documentation

• Learnhowtousestandardlibrariesat:– http://arduino.cc/en/Reference/Libraries

• Butalsoanumberofcontributedlibraries• Upside:workanddeepunderstandingalreadydone• Downside:willyoulearnanythingbypickinguppre-madesophisticatedpieces?

Phys124:Lecture3 17

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DCMotor

• Coiltoproducemagneticfield,onrotatingshaft• Permanentmagnetorfixedelectromagnet• Commutator toswitchpolarityofrotatingmagnetasitrevolves– the“carrot”isalwaysoutfront(andwillalsogetpushfrombehindifswitchoveristimedright)

Phys124:Lecture3 18

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DCTorque-speedCurve• Seehttp://lancet.mit.edu/motors/motors3.html

• Stallsatτs;noloadatωn

• Outputmechanicalpowerisτω– areaofrectangletouchingcurve– maxpoweristhenPmax =¼τsωn

Phys124:Lecture3 19

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ElectricalExpectations

• Windinghasresistance,R,typicallyinthe10Ω range

• Ifprovidedaconstantvoltage,V– windingeatspowerPw =V2/R– motordeliversPm =τω– currentrequiredisItot =(Pw +Pm)/V

• Atmaxpoweroutput(Pm =¼τsωn)– turnsoutwindinglossiscomparable,for~50%efficiency

Phys124:Lecture3 20

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Example2.4Vmotor

• Randomonlinespecfor2.4Vmotor(bewareflippedaxes)– noteatpowermax0.0008Nm;0.7A;8000RPM(837rad/s)

• totalconsumption2.4×0.7=1.68W• outputmechanicalpower0.0008×837=0.67W;efficiency40%• atconstantV=2.4,totalpowerconsumptionrisesà 3Wtowardstall• 1.25AatstallimplieswindingR =V/I =1.9Ω

Phys124:Lecture3 21

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Anotherrandomexample

• Noteprovisionofstalltorqueandno-loadspeed– suggestsmaxoutputpowerof

¼×2π(5500)/60×0.0183=2.6W– abouthalfthisatmaxefficiencypoint

2π(4840)/60×0.00248=1.25W– atmaxefficiency,0.17×12=2.04W,suggesting61%eff.– impliedcoilresistance12/1.06≈11Ω (judgedatstall)

• Lesson:forDCmotors,electricalcurrentdependsonloadingcondition– currentismaximumwhenmotorstrainingagainststall

Phys124:Lecture3 22

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ServoInternals

• AServomotorisjustaseriouslygear-reducedDCmotorwithafeedbackmechanism(e.g,potentiometer)toshutitoffwhenitissatisfiedwithitsposition– anddrivemotorfasterorslowerdependingonhowfarofftarget

Phys124:Lecture3 23

DCmotor

potentiometer

gearreduction

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CleverSteppers

• Steppermotorsworkinbabysteps• Insimplestversion,therearetwoDCwindings– typicallyarrangedinnumerousloopsaroundcasing– dependingondirectionofcurrentflow,fieldisreversible

• Rotorhaspermanentmagnetsperiodicallyarranged– butadifferingnumberfromtheexternalcoils

Phys124:Lecture3 24

teethonrotor

8“dentures”aroundoutside

NEMAstandard(NationalElectricalManufacturersAssociation),NEMA17:faceplate1.7in

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ACarefullyChoreographedSequence

• Fourdifferentcombinationscanbepresentedtothetwocoils(A&B;eachbi-directional)– eachcombinationattractstherotortoa(usu.slightly)differentposition/phase

– steppingthroughthesecombinationsinsequencewalkstherotorbythehandtothenextstep

Phys124:Lecture3 25

Inpractice,rotorhasmanypolesaround (inteeth,often), soeachstepismuchfiner.

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ToothedAnimation• Noteteethare

notphasedwith“dentures”allthewayaround– eachis90° from

neighbor• Thissequenceis

typicalofcenter-tapsteppers– canactivateone

sideofcoilatatime

• Noteusuallyhavemorethanfour“dentures”aroundoutside

Phys124:Lecture3 26

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SteppingSchemes

• Cangoinfullsteps,halfsteps,orevenmicrostep– fullstepiswhereonecoilisonandhasfullattentionofrotor– iftwoadjacentcoilsareon,they“split”positionofrotor– sohalf-steppingallowsfinercontrol,buthighercurrentdraw

• everyotherstepdoublesnominalcurrent

– insteadofcoilsbeingallonoralloff,canapplydifferingcurrents(orPWM)toeach;calledmicrostepping• socanselectacontinuousrangeofpositionsbetweenfullsteps

• Obviously,controllingasteppermotorismorecomplicatedthanourotheroptions– mustmanagestatesofcoils,andstepthroughsequencesensibly

Phys124:Lecture3 27

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TheStepperLibrary

• PartoftheArduino StandardLibraryset• Availablecommands:– Stepper(steps,pin1,pin2)– Stepper(steps,pin1,pin2,pin3,pin4)– setSpeed(rpm)– step(steps)

• ButArduino cannotdrivestepperdirectly– can’thandlecurrent– needtransistorstocontrolcurrentflow– arrangementcalledH-bridgeideallysuited

Phys124:Lecture3 28

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Examplestrippedcode

Phys124:Lecture3 29

#include <Stepper.h>#define STEPS 100 // change for your stepper

Stepper stepper(STEPS, 8, 9, 10, 11);

int previous = 0;

void setup(){stepper.setSpeed(30); // 30 RPM

}

void loop(){int val = analogRead(0); // get the sensor value

// move a number of steps equal to the change in the// sensor readingstepper.step(val - previous);

// remember the previous value of the sensorprevious = val;

}

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AUnipolar StepperMotor:CenterTap

• Aunipolar stepperhasacentertapforeachcoil– halfofcoilcanbeactivatedatatime– candrivewithtwoArduino pins(leftarrangement)– orfourpins(right)– bothuseULN2003orULN2004DarlingtonArray

Phys124:Lecture3 30

Centertapnotshownoncoil!

Centertap->

Drivingacoil:

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Phys124:Lecture3 31

ADarlingtonarrayprovidesbufferstohandlecurrent

EachchannelisapairoftransistorsinaDarlingtonconfiguration

• wheninputgoeshigh, theoutputwillbepulleddownnearground.(detailsinnextslides)

• whichthenpresentsmotorwithvoltagedropacrosscoil(COMMONisatthesupplyvoltage)

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Parenthesis:Transistorasaswitch

Phys124:Lecture3 32

Operateineithercut-off(OFF)orsaturation(ON)regions

Example:Bipolarjunction transistor(BJT)

http://www.electronics-tutorials.ws/transistor/tran_4.html

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Phys124:Lecture3 33

Parenthesis:Transistorasaswitch

OFF

ON

http://www.electronics-tutorials.ws/transistor/tran_4.html

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Phys124:Lecture3 34

NPNTransistorasaswitch

PNPTransistorasaswitch

Sinkscurrent

Sourcescurrent

http://www.electronics-tutorials.ws/transistor/tran_4.html

𝐼" = 𝛽𝐼&

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BacktotheDarlingtontransistor

Phys124:Lecture3 35

EachDarlington:

Exampleuses:

Whatisthatdiodenexttothemotor?

Typeequationhere.

𝐼"~𝛽6𝐼&

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Studentquestion:Flyback/freewheeldiode?

Phys124:Lecture3 36https://en.m.wikipedia.org/wiki/Flyback_diode

Eliminatesthesuddenvoltagespikeseenacrossaninductive loadwhenitssupplycurrentissuddenly reduced/interrupted.

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Unipolar hookup;controlwithfourpins

• Yellowmotorleadsarecentertap,connectedtoexternalpowersupply𝑉8 (jackhangingoffbottom)

Phys124:Lecture3 37

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ABipolarStepperMotor:NoCenterTap

• Inthiscase,thecoilmustseeonesideatgroundwhiletheotherisatthesupplyvoltage

• Atleftis2-pincontrol;rightis4-pincontrol– H-bridgeisL293Dorequiv.– transistorsjustmakeforlogicinversion(1inopp.2in,etc.)

Phys124:Lecture3 38

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H-bridgeInternals• AnH-bridgeisso-calledbecauseofthearrangementof

transistorswithamotorcoilspanningacross– twotransistors(diagonallyopposite)willconductatatime,witha

motorcoilinbetween,e.g.,onebridgepercoil

Phys124:Lecture3 39Noticetheflyback diodes.

24waysofmakinganH-bridge:http://www.talkingelectronics.com/projects/H-Bridge/H-Bridge-1.htmlhttp://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridges-the-basics/

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L293D:quadrupolehalf-Hdrivers

Phys124:Lecture3 40

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BipolarHookup;controlwithfourpins

• Inputsupplyshownasjackhangingoffbottom

Phys124:Lecture3 41

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Steppermotorwiring

Phys124:Lecture3 42

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Full-stepandhalf-stepbipolar drives

Phys124:Lecture3 43

Fullstep

Halfstep

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Internalwiringexamples…todriveyou&yourmotorcrazy

Phys124:Lecture3 44

Moreathttp://www.allaboutcircuits.com/textbook/alternating-current/chpt-13/stepper-motors/

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TheMotorShield

• Wehavekitshieldsthatcandrivea“motorparty”– 2servosplus2steppers,or– 2servosplus4DCmotors,or– 2servosplus2DCmotorsplus1stepper

• Allowsexternalpowersupply:motorscantakealotofjuice

Phys124:Lecture3 45

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TheMotorShield’sAssociatedLibrary

• Seeinstructionsat– http://learn.adafruit.com/adafruit-motor-shield– Installlibrarylinkedfromabovesite– followinstructionsfoundattopofabovepage– mayneedtomakedirectorycalledlibraries intheplacewhereyourArduino sketchesarestored• specifiedinArduino preferences

– andstoreinittheunpackedlibrariesasthedirectoryAFMotor

• Onceinstalled,justincludeinyoursketch:– #include <AFMotor.h>

• Openincludedexamplestogetgoingquickly

Phys124:Lecture3 46

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ExampleCode

• StepperCommandsinAFMotor– #include <AFMotor.h>

• grablibrary

– AF_Stepper my_stepper(#S/R,port);• my_stepper isarbitrarynameyouwanttocallmotor• argumentsarestepsperrevolution,whichshieldport(1or2)

– my_stepper.setSpeed(30);• setRPMofmotorforlargemoves(here30RPM)

– my_stepper.step(NSTEPS,DIRECTION,STEP_TYPE);• takeNSTEPSsteps,eitherFORWARD orBACKWARD• candoSINGLE,DOUBLE,INTERLEAVE,MICROSTEP

– my_stepper.release();• turnoffcoilsforfreemotion

Phys124:Lecture3 47

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StepTypes• SINGLE

– oneleadatatimeenergized,insequence3,2,4,1• ascounteddownwardonleftport(port1)onmotorshield

– normalstepsize• DOUBLE

– twoleadsatatimeareenergized:1/3,3/2,2/4,4/1• splitspositionofprevioussteps;tugofwar

– normalstepsize,buttwicethecurrent,power,torque• INTERLEAVE

– combinesbothabove:1/3,3,3/2,2,2/4,4,4/1,1– stepsarehalf-size,alternatingbetweensinglecurrentand

doublecurrent(so50%morepowerthanSINGLE)• MICROSTEP

– usesPWMtosmoothlyrampfromofftoenergized– inprinciplecanbeusedtogoanywherebetweenhardsteps

Phys124:Lecture3 48

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DCMotorswithmotorshield/AFMotor

• DCmotorsarehandledwiththefollowingcommands– #include <AFMotor.h>

• grablibrary

– AF_DCMotor mymotor(port);• portis1,2,3,or4accordingtoM1,M2,M3,M4onshield

– mymotor.setSpeed(200);• justaPWMvalue(0−255)tomoderatevoltagesenttomotor• notRPM,notload-independent,etc.— crudecontrol

– mymotor.run(DIRECTION);• FORWARD,BACKWARD,orRELEASE• depends,ofcourse,onhookupdirection

Phys124:Lecture3 49

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ServosontheShield• TwoServohookupsareprovidedontheshield• Reallyjustpower,ground,andsignalcontrol

– signalcontrolisArduino pins9and10– useServo.h standardlibrary– pin9à Servo2onshield;pin10à Servo1onshield

Phys124:Lecture3 50

Powerjumper

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Thisismykindof(stepper)party:12!

Phys124:Lecture3 51

BeagleBone GreenBNCtotriggermotion

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RJ45&ethcablesgotosteppers

Phys124:Lecture3 52

BeagleBone Green

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Announcements

• RememberTAofficehours:– TBD

• Turninprev.week’slabbystartofnextlabperiod,at2PM(daydep.onMon/Tuesection)– candropinslotonTAroominbackofMHA3544anytime

Phys124:Lecture3 53


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