Stepper Motors - eecs.umich.edu
Transcript of Stepper Motors - eecs.umich.edu
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Stepper Motors
By Brian Tomiuk, Jack Good, Matthew Edwards, Isaac Snellgrove
November 14th, 20181
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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
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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)
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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
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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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How a Stepper Motor Works
Powering a pair of the electromagnets causes the middle bar to align with the electromagnets
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How a Stepper Motor Works
Changing which electromagnets are powered and unpowered causes the plate to realign, turning the motor
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How a Stepper Motor Works
Changing which electromagnets are powered and unpowered causes the plate to realign, turning the motor
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How a Stepper Motor Works
This can be repeated to cause the motor to turn in any direction
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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
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How a Stepper Motor Works
Increasing the number of bars on the rotor can increase the granularity of the movement.
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How a Stepper Motor Works
Increasing the number of bars on the rotor can increase the granularity of the movement.
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How a Stepper Motor Works
Continuing with this concept results in the rotor having dozens of rotor teeth
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How a Stepper Motor Works
The electromagnets also have stator teeth. Powering the electromagnets cause the tips of the teeth to align.
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Slightly Offset
Aligned
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How a Stepper Motor Works
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
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How a Stepper Motor Works
Great video demonstration of stepper motors!
https://youtu.be/eyqwLiowZiU
Credit: Wikipedia for Stepper Motors 19
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(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!
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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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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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● Current flowing through a solenoid coil induces a magnetic field● Right Hand rule points to North (conventional current flow)
Start with a Solenoid
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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
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Changing Direction on Demand
● Switch direction of current using an H-bridge
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H-Bridge(Reverse Current)
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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
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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 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
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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!
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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
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Medical
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https://www.youtube.com/watch?v=4WqaadMqTRA&feature=youtu.be
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Cameras
42http://www.pngmart.com/image/14730
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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
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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
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
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https://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
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Controller:Specs of the STR2 - DC Powered Advanced Microstep Drive
52https://www.applied-motion.com/products/stepper-drives/str2
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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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