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Stepper Motors 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 S 9 How a Stepper Motor Works Changing which electromagnets are powered and unpowered S causes the plate to realign, turning the motor S 10 How a Stepper Motor Works Changing which electromagnets are powered and unpowered S causes the plate to realign, turning the motor S 11 How a Stepper Motor Works This can be repeated to cause the motor to turn in any S direction S 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. S 14 How a Stepper Motor Works S Increasing the number of bars on the rotor can increase the granularity of the movement. S 15 How a Stepper Motor Works S Continuing with this concept results in the rotor having dozens of rotor teeth S 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) I 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 S N 25 Magnetic Field Direction ● Depends on conventional current flow around solenoid core I S N N S I 26 Magnetic Field Direction ● Depends on conventional current flow around solenoid core I S N N S I 27 Changing Direction on Demand ● Switch direction of current using an H-bridge S H-Bridge N 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 I S N 35 Stacking Solenoids N 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 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: 50 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 54.
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