Haptics and Virtual Reality

1. Haptics and Virtual Reality Lecture 3: • Sensors & Actuators M. Zareinejad

2. Outline # Sensors Sensortypes Sensorexamples – – #Actuators Actuatortypes Actuatorexamples – –

3. TheHapticSystem Human

4. Passive & Active joint

5. SensorApplications Eyetracking Headtracking Bodytracking Handtracking     Mostimportantfortypicalhapticinterfaces –

6. Sensortypes Magnetic Optical Acoustic Inertial Mechanical      Mostimportantfortypicalhapticinterfaces –

7. MechanicalTrackers Ground-basedlinkagesmostcommonlyused PositionSensors   digital:opticalencoders analog:Hall-effect(magnetic) – –

8. OpticalEncoders Emitter Howdotheywork? Detector  Afocusedbeamoflightaimedatamatched photodetectorisinterruptedperiodicallybyacoded patternonadisk Producesanumberofpulsesperrevolution(Lotsof pulses=highcost) – – Quantizationproblemsatlowspeeds Absolutevs.Incremental  

9. OpticalEncoders Absolutevs. Incremental  Resolution?

10. OpticalEncoders Phase-quadratureencoder 2channels,90°outofphase   allowssensingofdirectionofrotation –

11. Encoder States & Decoding

12. Hall-EffectSensors Howdotheywork?  – Asmalltransversevoltageisgeneratedacrossa current-carryingconductorinthepresenceofa magneticfield (Discoverymadein 1879,butnotuseful untiltheadventof semiconductor technology.)

13. Hall-Effect Sensors Vh=Hallvoltage Rh=Hall coefficient RhIB t I=Current B=Magnetic fluxdensity t=Elementthickness Vh= Amountofvoltageoutputrelatedtothe strengthofmagneticfieldpassingthrough. Linearoversmallrangeofmotion   Needtobecalibrated – Affectedbytemperature,othermagnetic objectsintheenvironments 

14. Hall-Effect Sensors Vh=Hallvoltage Rh=Hall coefficient I=Current B=Magnetic fluxdensity t=Elementthickness RhIB t Vh= • The voltage varies sinusoidally with rotation angle Resolution?

15. Potentiometers

16. Potentiometers Resolution?

17. Acoustic Tracker Speaker Microphone

18. Acoustic Tracker

19. Magnetic Tracker

20. Magnetic Tracker

21. Optical Tracker • Inside-Looking-Out • Outside-Looking-In

22. Optical Tracker

23. Data gloves

24. Data gloves

25. MeasuringVelocity Differentiateposition  advantage:usesamesensoraspositionsensor disadvantage:getnoisesignal – – Alternative  forencoders,measuretimebetweenticks –

26. Digitaldifferentiation P1-P2 t Manydifferentmethods SimpleExample:   V= Average20readings=P1 Averagenext20readings=P2 wheretisthetheperiodoftheservoloop – – – Differentiation Increases noise 

27. Time-between-ticks Time per ticks rather than ticks per time  useaspecialchipthatmeasurestime betweenticks  Especially good to do at slow speeds – Farespoorlyathighvelocities – p t v=

28. ActuatorTypes Electricmotors  • DC(direct current) • Brushed • PM(permanent magnet) Pneumatic Actuators 

29. PMDCbrushedmotors Howdothey work?  Rotatingarmature withcoilwindings iscausedtorotate relativetoa permanentmagnet currentistransmittedthroughbrushesto armature,andisconstantlyswitchedsothatthe armaturemagneticfieldremainsfixed. – –

30. DCmotorcomponents

31. DCmotorcomponents

32. DCmotorterms Cogging  Tendencyfortorqueoutputtorippleasthebrushes transferpower – Friction/damping  Causedbybearingsandeddycurrents – Stalltorque  Maxtorquedeliveredbymotorwhenoperated continuouslywithoutcooling –

33. MotorEquations Torqueconstant,KT T=KT I Dynamicequation   dI dt V=L +RI+E

34. PneumaticActuators Howdotheywork?  Compressedairpressureisusedtotransferenergy fromthepowersourcetohapticinterface. – Manydifferenttypes Concernsarefrictionandbandwidth  

35. SomeTerms AD/DA  analogtodigital digitaltoanalog – – Interruptroutine ServoLoop Servorate    Usuallyneedstobe>500Hz –

36. D/AandA/D Convertsbetween voltagesandcounts Computerstores informationdigitally, andcommunicates withtheoutside worldusing+/-5V signals 101010101    LSB MSB

37. D/AandA/D Convertsvoltagestocountsandviceversa A12-bitcard:   212decimalnumbers(4096) – 2994 Decimal(base10): Binary(base2): 1 0 1 1 1 0 1 1 0 0 1 0 Hexadecimal(base16): 1 0 1 1 1 0 1 1 0 0 1 0 B B 2 = BB2