Stepper Motors

By Brian Tomiuk, Jack Good, Matthew Edwards, Isaac Snellgrove

November 14th, 2018 1 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

2 Parts of a Stepper Motor

Stator - Stays Static

Rotor - Rotates the motor shaft

https://phidgets.files.wordpress.com/2014/06/stepper_back_web.jpg 3 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

4 Advantages of Stepper Motors

● Has high holding torque (maintains its position) ● Moves in discrete amounts ● Inexpensive ● Brushless (can last longer than brushed motors)

5 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

6 How Stepper Motors Work

7 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

8 How a Stepper Motor Works

S Powering a pair of the electromagnets causes the middle bar to align with the electromagnets

9 How a Stepper Motor Works

Changing which electromagnets are powered and unpowered S causes the plate to realign, turning the motor

10 How a Stepper Motor Works

Changing which electromagnets are powered and unpowered S causes the plate to realign, turning the motor

11 How a Stepper Motor Works

This can be repeated to cause the motor to turn in any S direction

12 How a Stepper Motor Works

S Two groups can be powered to cause the plate to land between S the two in a process called half stepping

S S

13 How a Stepper Motor Works

Increasing the number of bars on the rotor can increase the S granularity of the movement.

14 How a Stepper Motor Works

S Increasing the number of bars on the rotor can increase the granularity of the movement.

15 How a Stepper Motor Works

S Continuing with this concept results in the rotor having dozens of rotor teeth

16 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

17 How a Stepper Motor Works

Aligned

Powering a different pair of electromagnets cause a new S group of teeth to align, causing the whole rotor to slightly shift.

S Slightly Offset

18 How a Stepper Motor Works

Great video demonstration of stepper motors!

https://youtu.be/eyqwLiowZiU

Credit: Wikipedia for Stepper Motors 19 (Electro)magnets. How do they work?

20 Stator Magnet Needs

1. Must be turned on/off - Allows motor movement

2. Must be able to change magnet direction - Allows us to both push and pull rotor - Greater torque and speed

21 Stator Magnet Needs

1. Must be turned on/off Electromagnets - Allows motor movement do these!

2. Must be able to change magnet direction - Allows us to both push and pull rotor - Greater torque and speed

22 Start with a Solenoid

● Current flowing through a solenoid coil induces a magnetic field ● Right Hand rule points to North (conventional current flow)

23 Start with a Solenoid

● Current flowing through a solenoid coil induces a magnetic field ● Right Hand rule points to North (conventional current flow)

24 Start with a Solenoid

● Current flowing through a solenoid coil induces a magnetic field ● Right Hand rule points to North (conventional current flow)

25 Magnetic Field Direction

● Depends on conventional current flow around solenoid core

S N

N S I

26 Magnetic Field Direction

● Depends on conventional current flow around solenoid core

S N

N S I

27 Changing Direction on Demand

● Switch direction of current using an H-bridge

S H-Bridge

28 Changing Direction on Demand

● Switch direction of current using an H-bridge

N H-Bridge (Reverse Current) S

29 Bipolar Control

● Bipolar because each coil can alternate its polarity

● Requires current reversal (which typically means an H-Bridge)

www.pololu.com

30 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 www.pololu.com

31 But H-Bridges are hard... 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?

32 But H-Bridges are hard... 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!

33 Stacking Solenoids

34 Stacking Solenoids

35 Stacking Solenoids

S I

36 Unipolar Control

● Unipolar because each coil has one polarity (and can only be switched on or off)

● No H-Bridge! N

● Requires at least 3 wires per solenoid set (2 to control direction, 1 common ground) S ● Smaller coils mean weaker magnetic fields! I

37 Differences in 2-Phase Stepper Motors

Bipolar Unipolar

● Fewer wires (4) ● More wires (5-8) ● Higher torque ● Lower torque ● Current reversal ● No current reversal ● Advanced controller and/or ● Much simpler controller H-Bridge

38 Stepper Motors in Industry

39 Industrial

https://www.pcworld.com/article/2069020/from-android-to-automations-rubins-robots-are-googles-next-moonshot.html 40 Medical

https://www.youtube.com/watch?v=4WqaadMqTRA&feature=youtu.be 41 Cameras

http://www.pngmart.com/image/14730 42 Interfacing with Stepper Motors

43 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

44 Interfacing (Option 1)

● Buy a controller ● Tell the controller when to step and in which direction ● (Good for bipolar)

https://www.hobbyist.co.nz/?q=stepper-motor-controller-A4988 45 Interfacing (Option 2)

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

46 https://upload.wikimedia.org/wikipedia/commons/8/85/Drive.png Step modes:

● Full step ○ 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° (!)

47 Full step:

https://www.rs-online.com/designspark/stepper-motors-and-drives-what-is-full-step-half-step-and-microstepping

48 Half step:

https://www.rs-online.com/designspark/stepper-motors-and-drives-what-is-full-step-half-step-and-microstepping 49 Microstepping:

https://www.rs-online.com/designspark/stepper-motors-and-drives-what-is-full-step-half-step-and-microstepping Motor:

Specs of the 5014-020 - NEMA 14

https://www.applied-motion.com/products/stepper-motors/5014-020 51 Controller: Specs of the STR2 - DC Powered Advanced Microstep Drive

https://www.applied-motion.com/products/stepper-drives/str2 52 Questions?

53 References

https://www.youtube.com/watch?v=eyqwLiowZiU https://learn.adafruit.com/all-about-stepper-motors/what-is-a-stepper-motor https://en.wikipedia.org/wiki/Stepper_motor https://www.youtube.com/watch?v=0qwrnUeSpYQ https://www.linengineering.com/industries/medical/ https://www.elprocus.com/stepper-motor-types-advantages-applications/ https://www.linengineering.com/industries/security-surveillance/ https://photo.stackexchange.com/questions/24109/what-does-stm-mean-on-a-canon-lens http://www.machinetoolhelp.com/Automation/systemdesign/stepper_dcservo.html