Stepper Motors - eecs.umich.edu

Stepper Motors

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

November 14th, 20181

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

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Parts of a Stepper Motor

Stator - Stays Static

Rotor - Rotates the motor shaft

3https://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

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Advantages of Stepper Motors

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● Has high holding torque (maintains its position)

● Moves in discrete amounts

● Inexpensive

● Brushless (can last longer than brushed motors)

Disadvantages of Stepper Motors

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● 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

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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

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

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

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

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

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

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

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

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

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The electromagnets also have stator teeth. Powering the electromagnets cause the tips of the teeth to align.

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Slightly Offset

Aligned


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

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Aligned

Slightly Offset


Great video demonstration of stepper motors!

https://youtu.be/eyqwLiowZiU

Credit: Wikipedia for Stepper Motors 19

https://youtu.be/eyqwLiowZiU

(Electro)magnets. How do they work?

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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

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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

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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)

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● Depends on conventional current flow around solenoid core

Magnetic Field Direction

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● Depends on conventional current flow around solenoid core

Magnetic Field Direction

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Changing Direction on Demand

● Switch direction of current using an H-bridge

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H-Bridge

Changing Direction on Demand

● Switch direction of current using an H-bridge

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H-Bridge(Reverse Current)

Bipolar Control

● Bipolar because each coil can alternate its polarity

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

www.pololu.com

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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

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But H-Bridges are hard...

● 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?

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gearbest.com

But H-Bridges are hard...

● 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?

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Use more solenoids!

adafruit.com

Stacking Solenoids

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Stacking Solenoids

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Stacking Solenoids

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Unipolar Control

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● 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!

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

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Stepper Motorsin Industry

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Industrial

https://www.pcworld.com/article/2069020/from-android-to-automations-rubins-robots-are-googles-next-moonshot.html40

Medical

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

Cameras

42http://www.pngmart.com/image/14730

Interfacing with Stepper Motors

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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

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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)

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

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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° (!)

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Full step:

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


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Half step:

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


Microstepping:

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Motor:

Specs of the 5014-020 - NEMA 14

51https://www.applied-motion.com/products/stepper-motors/5014-020

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

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

Questions?

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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

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Stepper Motors - eecs.umich.edu

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