Stepper Motors

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Stepper Motors

• ME-191
• Computer Applications in Mechanical Engineering
  II
• Dr. William Farrow

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Stepper Motor

• An output transducer that converts a digital
  signal to discrete rotational motion.

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Types of Stepper Motors

• Permanent Magnet
• Permanent Magnet Rotor (the part that rotates
• Smaller size, Faster possible speeds
• Lower max torques
• Variable Reluctance
• Iron or Steel Rotor
• Larger size, Slower possible speeds
• Larger max torques

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Permanent Magnet

• Permanent magnet rotors magnetic field is driven
  around by attraction to and repulsion from
  electrically switched stator field coils (or
  poles).

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Variable Reluctance

• Steel rotor seeks position of minimum reluctance
  under influence of the electrically switched
  stator field coils (or poles).

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Performance

• Torque
• PM limited to 500 oz-in
• VR limited to 2000 oz-in
• Speed
• PM smaller rotor size ? higher speeds
• VR larger rotor size ? smaller speeds

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Performance

• Input speed
• Measured in steps/second
• Some PMs can reach 30,000 steps/sec
• Resolution
• Measured in steps/revolution
• Commonly 12, 24,144, 180, 200, and 400
• Or from 30/step down to 0.9/step
• Half-stepping can further enhance resolution

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Important Parameters to Maintain Open Loop Control
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In Addition

• VR holding torque is zero when power is off
• PM holding torque is generated by PM field and
  steel stators (detent torque)

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Typical Stepper Motor Characterics
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Lab 3 XY Table

• Apply the concept of stepper motors to drive a XY
  positioning table

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XY Table Layout
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Stepper Motor Driver

• Digital logic used to command stepper motor
• This lab applies the UCN5804 IC to control the
  stepper motor via digital outputs from the
  LabJack
• MOSFET power switching transistors used to
  amplify power to the stepper motor pole coils

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Stepping Signals

• X-Axis step
• D1 - direction
• Low (0) or High (gt0) opposite directions
• D0 - move one step
• High (gt0) followed by a Low (0)
• Note requires two digital output calls
• D8 D9 - limit switches for travel
• Inputs go high when switch is closed

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Lab Operation

• Connect hardware and test using LJLogger
• Write m-file to move X-Axis 100 steps

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Lab Operation cont.

• Locate middle of Axis
• Move x-axis until limit switch A is set
• Will require While-loop and edigitalin() to
  monitor switch A
• Reverse direction and count of steps to B
• Will require While-loop
• Need to increment value in a variable steps
• Reverse direction and move ½ the steps
• Require a For-loop to go ½ of steps