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

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

Parts of a Stepper Motor Stator - Stays Static Rotor - Rotates the motor shaft https://phidgets.files.wordpress.com/2014/06/stepper_back_web.jpg

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

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

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

How Stepper Motors Work

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

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

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

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

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

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

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

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

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

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

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

How a Stepper Motor Works Great video demonstration of stepper motors! https://youtu.be/eyqwLiowZiU Credit: Wikipedia for Stepper Motors

(Electro)magnets. How do they work?

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! 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!

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

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

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

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

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

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

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

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

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

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?

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!

Stacking Solenoids

Stacking Solenoids I S N

Stacking Solenoids N S I

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

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

Stepper Motors in Industry

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

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

Cameras http://www.pngmart.com/image/14730

Interfacing with Stepper Motors

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

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

Interfacing (Option 2) Do it ourselves! Must drive signals at correct times Remember waving and half-stepping? Easy to use GPIO pins (unipolar) https://upload.wikimedia.org/wikipedia/commons/8/85/Drive.png

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° (!)

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

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

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

Controller: Specs of the STR2 - DC Powered Advanced Microstep Drive https://www.applied-motion.com/products/stepper-drives/str2

Questions?

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