Robot Actuation: Motors

1. N S - - + + v v Robot Actuation: Motors Stepper motors DC motors Servo motors Physics “review” Things seek lowest energy states. Nature is lazy. • iron core vs. magnet N S • magnetic fields tend to line up Electric fields and magnetic fields are the same thing. Author: CIS Torque is a good scrabble word.

2. Stepper Motors stator S rotor N electromagnets

3. S N Stepper Motors stator S rotor N “variable reluctance” stepper motor electromagnets How does rotor angle affect the torque?

4. S N Stepper Motors stator S rotor N “variable reluctance” stepper motor electromagnets torque angle

5. S N Stepper Motors stator S rotor N “variable reluctance” stepper motor electromagnets torque angle

6. S N Stepper Motors stator S N S rotor N “variable reluctance” stepper motor on to the next teeth… electromagnets

7. S N Stepper Motors stator S N S rotor N “variable reluctance” stepper motor on to the next teeth… electromagnets printers computer drives machining • Direct control of rotor position (no sensing needed) • May oscillate around a desired orientation • Low resolution can we increase our resolution?

8. Increasing Resolution S N S N Half-stepping energizing more than one pair of stator teeth

9. Increasing Resolution torque S N S angle N Half-stepping energizing more than one pair of stator teeth

10. Increasing Resolution torque S N S angle N Half-stepping energizing more than one pair of stator teeth More teeth

11. Increasing Resolution torque S N S angle N Half-stepping energizing more than one pair of stator teeth More teeth on the rotor and/or stator Question 2 this week…

12. Motoring along... • direct control of position • very precise positioning http://www.ohmslaw.com/robot.htm • What if maximum power is supplied to the motor’s circuit accidently ? • Underdamping leads to oscillation at low speeds • At high speeds, torque is lower than the primary alternative… Beckman 105 ?

13. DC motors -- exposed !

14. N S DC motor basics permanent magnets N rotor S stator brushes + V commutator on shaft -

15. N S N S DC motor basics permanent magnets N rotor S N S stator brushes + + V V commutator on shaft - -

16. S N N S N S DC motor basics permanent magnets N rotor S N S N S stator brushes + + + V V V commutator on shaft - - -

17. N rotor S N stator S Who pulls more weight? electro-magnets stator S rotor N Stepper motor DC motor

18. N rotor S N stator S Who pulls more weight? electro-magnets stator S rotor N Stepper motor DC motor • Position control • High holding torque • Durability (no brushes) • Energy used is prop. to speed • Higher torque at faster speeds • More popular, so they’re cheaper • Smoother at low speeds

19. Open-loop control An “open-loop” strategy desired speed w V w Motor and world Controller solving for V “the plant”

20. Bang-bang control General idea works for any controllable system... desired speed w V w Motor and world Controller solving for V actual speed desired position q V(t) q Motor and world Controller solving for V(t) actual position

21. Returning to one’s sensors But the real world interferes... desired speed wd V wa Motor and world Controller solving for V desired speed wd  actual speed wa t R Vr = + k w We don’t know the actual load on the motor. k

22. Closed-loop control Compute the error and change in relation to it. Error signal e wd - wa wa V desired wd compute V using the error e - The world actual speed wa how do we get the actual speed?

23. Proprioceptive Sensing • Resolver • = measures absolute shaft orientation • Potentiometer • = measures orientation by varying resistance, it has a range of motion < 360º Power/Contact

24. Servomotors potentiometer Direct position control in response to the width of a regularly sent pulse. A potentiometer is used to determine the motor shaft angle. modified to run continuously

25. Optical Encoders • Detecting motor shaft orientation potential problems?

26. Gray Code # Binary 0 1 2 3 4 5 6 7 8 9 0 1 10 11 100 101 110 111 1000 1001 000 001 011 010 110 111 101 100

27. Gray Code # Binary 0 1 2 3 4 5 6 7 8 9 0 1 10 11 100 101 110 111 1000 1001 000 001 011 010 110 111 101 100 1100 1101 with FPS applications !

28. Gray Code # Binary 0 1 2 3 4 5 6 7 8 9 0 1 10 11 100 101 110 111 1000 1001 000 001 011 010 110 111 101 100 1100 1101 among others... wires?

29. Absolute Optical Encoders • Complexity of distinguishing many different states -- high resolution is expensive! something simpler ?

30. Relative Encoders • Track position changes light sensor decode circuitry light emitter grating

31. Relative Encoders • Relative position - calibration ? - direction ? light sensor - resolution ? decode circuitry light emitter grating

32. Relative Encoders • Relative position - calibration ? - direction ? light sensor - resolution ? decode circuitry light emitter grating

33. Relative Encoders • Relative position - calibration ? - direction ? light sensor - resolution ? decode circuitry light emitter grating A A A lags B B B

34. Relative Encoders • Relative position - calibration ? - direction ? light sensor - resolution ? decode circuitry light emitter grating A B A leads B quadrature encoding 100 lines -> ?

35. Relative Encoders mask/diffuser • Relative position light sensor A decode circuitry light emitter grating B A diffuser tends to smooth these signals Ideal Real With motors and sensors, all that’s left is...

36. Control

37. Closed-loop control Compute the error and change in relation to it. Error signal e wd - wa wa V desiredwd compute V using the error e - The world actual speedwa Feedback

38. Initial Feedback “First” feedback controller

39. Other Systems Biological feedback systems Chemical feedback systems intelligent hydrogels

40. Additional Feedback Chemical feedback systems for insulin delivery ph dependant at low pH values, the carboxylic acid groups of PMAA tend to be protonated, and hydrogen bonds form between them and the ether oxygens on the PEG chains. These interpolyer complexes lead to increased hydrophobicity, which causes the gel to collapse. At high pH values, carboxylic groups become ionized, the complexes are disrupted, and the gel expands because of increased electrostatic repulsion between the anionic chains. Why I’m not a chemist:

41. Robotic use of EAPs

42. Short Assignment #3 Remember that these may be done either individually or in your lab groups. Choose 1 of these four papers on design/locomotion: Reading: • Designing a Miniature Wearable Visual Robot • An Innovative Locomotion Principle for Minirobots Moving in the Gastrointestinal Tract • Get Back in Shape! A reconfigurable microrobot using Shape Memory Alloy • Walk on the Wild Side: The reconfigurable PolyBot robotic system A second page and picture(s) for Lab Project #1. work in a citation for the paper you read! problem 1 Putting the step into stepper motors… problem 2 Implementing one-dimensional PD control (Nomad) problem 3 Implementing two-dimensional PD control (Nomad) Extra Credit

43. Wednesday Controling motion by controlling motors: PID Coming soon! The ancient art of motor arranging...

44. Spherical Stepper Motor complete motor rotor stator applications

45. Returning to one’s sensors But the real world interferes... desired speed wd V wa Motor and world Controller solving for V desired speed wd  actual speed wa t R Vr = + k w We don’t know the actual load on the motor. k

46. How robotics got started...

47. Proportional control better, but may not reach the setpoint

48. PI control but I thought PI was constant... better, but will overshoot

49. PID control Derivative feedback helps damp the system other damping techniques?

50. And Beyond Why limit ourselves to motors? Nitinol -- demo stiquito robot ? Electroactive Polymers EAP demo Wiper for Nanorover dalmation