1. Motor Tutorial Kevin M. Lynch Laboratory for Intelligent Mechanical Systems Northwestern University Evanston, IL USA

2. Types of Motors DC (brushed) Stepper RC Servo Solenoid

3. Types of Motors DC (brushed) Stepper RC Servo Solenoid Workhorse, high power Simple to use, two wires Torque proportional to current, steady state constant-load speed proportional to voltage Requires gearing Requires feedback

4. Types of Motors DC (brushed) Stepper RC Servo Solenoid Useful for low-torque applications with no surprises No feedback required One step per pulse More involved driving circuit

5. Types of Motors DC (brushed) Stepper RC Servo Solenoid High torque, useful for positioning applications Feedback and gearing built in Position commanded by persistent pulse train Limited motion (less than 1 revolution)

6. Types of Motors DC (brushed) Stepper RC Servo Solenoid For on-off applications Simple to use Short stroke Powered in only one direction; requires external spring for return

7. DC Motors Lorentz Force Law: F = I x B F = force on wire I = current B= magnetic field Right hand rule: index finger along I, middle finger along B, thumb along F N S

8. Inside a DC Motor

9. DC Motors V = IR + L (dI/dt) + k e   = k t I Vvoltage I current R resistance Linductance  speed  torque k e electrical constant k t torque constant 2 - permanent magnet 3 - housing (magnetic return) 4 - shaft 5 - winding   speed-torque curves for two voltages

10. DC Motor Specs

11. Driving a DC Motor Switches and relays Transistors Linear push-pull stage with op amp Ideally: H-bridge and PWM

12. H-bridge and PWM PWM: Rapidly switch between S1-S4 closed and S2-S3 closed Averages to effective voltage across motor between -V and +V depending on time spent in S1-S4 and S2-S3 states Switch control signals are simply digital signals Use an H-bridge chip or build out of transistors

13. L293 H-bridge chip

14. Practical Issues flyback diodes for “inductive kick” heat sinks for transistors capacitors to smooth voltage spikes other noise issues, isolation

15. Gears Gear ratio G  out =  in / G  out =  G  in (  = efficiency) Many types spur, planetary, worm, lead/ball screw, bevel, harmonic...

16. Encoder Feedback Another option: potentiometer US Digital

17. Feedback Control Proportional (Integral-Derivative) Control Multiply position/velocity error by a gain to get control signal (and perhaps add integral and derivative of that error multiplied by other gains) Usually implemented on computer Can be implemented with op amps

18. Stepper Motor Bipolar: 4 wires Unipolar: 5 or 6 wires +V alternately ground one end of coil or other 1423 AB A B R R L L L R R R L

19. Animation of Unipolar Taken fro m http://www.cs.uiowa.edu/~jones/step/

20. Driving a Stepper Use logic on/off signals at 2, 7, 10, 15.

21. RC Servo Motor 3 wires: power, ground, control Control signal sets the position. High pulse every ~20 ms determines set angle; pulse width between ~0.5 ms and ~2 ms, indicating the two ends of angle range Internal gearing, potentiometer, and feedback control.

22. Solenoid Plunger attracted or repelled by current through a coil. May be driven by a relay or transistor.

23. Questions?