1. Stepper Motors
By Brian Tomiuk, Jack Good, Matthew Edwards, Isaac Snellgrove
November 14th, 2018

2. What is a Stepper Motor?
A motor whose movement is divided into discrete “steps”
“Turn 10 steps clockwise”
Holds its position without additional control
No sensor or feedback loop

3. Parts of a Stepper Motor
Stator - Stays Static
Rotor - Rotates the motor shaft

4. Different Types of Torque
Holding torque - How much load can the motor hold in place when the coils are energized
Detent torque - The torque the motor produces when the windings are not energized, sometimes call residual torque
Detent torque can be beneficial in stopping the motor, even though it reduces the actual running torque, caused by the permanent magnet in some stepper motors

5. Advantages of Stepper Motors
Has high holding torque (maintains its position)
Moves in discrete amounts
Inexpensive
Brushless (can last longer than brushed motors)

6. Disadvantages of Stepper Motors
Uses the same amount of power regardless of load
Lower power efficiency
Torque decreases rapidly as speed increases
No internal feedback
Cannot tell when a step was missed
Must step slowly to ensure accuracy
Low torque to inertia
Cannot accelerate loads very rapidly

7. How Stepper Motors Work

8. How a Stepper Motor Works
Unpowered Electromagnets
Bar with magnetic ends
A basic stepper motor consists of a series of electromagnets surrounding a magnetically charged bar

9. How a Stepper Motor Works
Powering a pair of the electromagnets causes the middle bar to align with the electromagnets S

10. How a Stepper Motor Works
Changing which electromagnets are powered and unpowered causes the plate to realign, turning the motor S S

11. How a Stepper Motor Works
Changing which electromagnets are powered and unpowered causes the plate to realign, turning the motor S S

12. How a Stepper Motor Works
This can be repeated to cause the motor to turn in any direction S S

13. How a Stepper Motor Works
Two groups can be powered to cause the plate to land between the two in a process called half stepping S S S

14. How a Stepper Motor Works
Increasing the number of bars on the rotor can increase the granularity of the movement. S S

15. How a Stepper Motor Works
Increasing the number of bars on the rotor can increase the granularity of the movement. S

16. How a Stepper Motor Works
Continuing with this concept results in the rotor having dozens of rotor teeth S

17. How a Stepper Motor Works
Slightly Offset S
The electromagnets also have stator teeth. Powering the electromagnets cause the tips of the teeth to align.
Aligned S

18. How a Stepper Motor Works
Aligned
Powering a different pair of electromagnets cause a new group of teeth to align, causing the whole rotor to slightly shift. S S
Slightly Offset

19. How a Stepper Motor Works
Great video demonstration of stepper motors!
Credit: Wikipedia for Stepper Motors

20. (Electro)magnets. How do they work?

21. Stator Magnet Needs Must be turned on/off - Allows motor movement
Must be able to change magnet direction - Allows us to both push and pull rotor - Greater torque and speed

22. Electromagnets do these!
Stator Magnet Needs
Must be turned on/off - Allows motor movement
Must be able to change magnet direction - Allows us to both push and pull rotor - Greater torque and speed
Electromagnets do these!

23. Start with a Solenoid
Current flowing through a solenoid coil induces a magnetic field
Right Hand rule points to North (conventional current flow) I

24. Start with a Solenoid
Current flowing through a solenoid coil induces a magnetic field
Right Hand rule points to North (conventional current flow) I

25. Start with a Solenoid S N
Current flowing through a solenoid coil induces a magnetic field
Right Hand rule points to North (conventional current flow) I S N

26. Magnetic Field Direction
Depends on conventional current flow around solenoid core I S N I N S

27. Magnetic Field Direction
Depends on conventional current flow around solenoid core I S N I N S

28. Changing Direction on Demand
Switch direction of current using an H-bridge S
H-Bridge N

29. Changing Direction on Demand
Switch direction of current using an H-bridge N
H-Bridge
(Reverse Current) S

30. Bipolar Control Bipolar because each coil can alternate its polarity
Requires current reversal (which typically means an H-Bridge)

31. Bipolar Control Bipolar because each coil can alternate its polarity
Requires current reversal (which typically means an H-Bridge)
Only two wires for each set of solenoids

32. But H-Bridges are hard... Sometimes an H-Bridge cannot be used
gearbest.com
Sometimes an H-Bridge cannot be used
They can (potentially) be larger than the motor in some cases
They generate a lot of heat
You can’t be bothered
How do you change the magnetic direction of a solenoid without changing the current direction?

33. But H-Bridges are hard... Use more solenoids!
adafruit.com
Sometimes an H-Bridge cannot be used
They can (potentially) be larger than the motor in some cases
They can generate a lot of heat
You can’t be bothered
How do you change the magnetic direction of a solenoid without changing the current direction?
Use more solenoids!

34. Stacking Solenoids

35. Stacking Solenoids I S N

36. Stacking Solenoids N S I

37. Unipolar Control
Unipolar because each coil has one polarity (and can only be switched on or off)
No H-Bridge!
Requires at least 3 wires per solenoid set (2 to control direction, 1 common ground)
Smaller coils mean weaker magnetic fields! S N I

38. Differences in 2-Phase Stepper Motors
Bipolar
Fewer wires (4)
Higher torque
Current reversal
Advanced controller and/or H-Bridge
Unipolar
More wires (5-8)
Lower torque
No current reversal
Much simpler controller

39. Stepper Motors in Industry

40. Industrial

41. Medical

42. Cameras

43. Interfacing with Stepper Motors

44. Ease of interfacing stepper motor:
Rotation is proportional to number of input pulses
Speed is proportional to frequency of input pulses
Quick response to starting, stopping, and reversing
Very precise
3-5% and error does not accumulate from one step to the next

45. Interfacing (Option 1) Buy a controller
Tell the controller when to step and in which direction
(Good for bipolar)

46. Interfacing (Option 2) Do it ourselves!
Must drive signals at correct times
Remember waving and half-stepping?
Easy to use GPIO pins (unipolar)

47. Step modes: Full step Half step Microstepping
200 teeth / 360° = 1.8° per step
Half step
One winding energized, other two alternate
Half the distance per step (0.9°) and smoother operation, but 30% less torque
Microstepping
Newer technology that divides each step up to 256 microsteps, resulting in a step angle of 0.007° (!)

48. Full step:

49. Half step:

50. Microstepping:

51. Motor: Specs of the 5014-020 - NEMA 14

52. Controller: Specs of the STR2 - DC Powered Advanced Microstep Drive

53. Questions?

54. References