Stepper Motors – An Overview

1
Stepper Motors An Overview

• Aliasgar Kutiyanawala
• Utah State University

2
Whats a Stepper Motor?

• A stepper motor is a motor that, as the name
  suggests, moves in steps. Stepping motors are
  known in German as Schrittmotoren, in French as
  moteurs pas à pas, and in Spanish as motor paso
  paso.

3
Compare With a Servo

• Stepper motors are similar to servo motors as we
  can perform position control with both.
• However, Servo motors require some form of analog
  feedback whereas stepper motors are often open
  loop.
• Stepping motors can be used in simple open-loop
  control systems these are generally adequate for
  systems that operate at low accelerations with
  static loads, but closed loop control may be
  essential for high accelerations.

4
Compare With a Servo (Contd.)

• For high accelerations with variable loads, all
  rotor information is lost, and we require closed
  loop for accurate control
• Servo motors are not subject to this problem

5
Types of Stepper Motors

• Permanent Magnet
• Employ permanent magnet
• Low speed, relatively high torque
• Variable Reluctance
• Does not have permanent magnet
• Low torque

6
Types of Stepper Motors

• Hybrid
• multi-toothed stator poles and a permanent magnet
  rotor
• High static and dynamic torque

7
Variable Reluctance Motors

• The variable reluctance motor in the
  illustration has four "stator pole sets" (A, B,
  C,), set 15 degrees apart. Current applied to
  pole A through the motor winding causes a
  magnetic attraction that aligns the rotor (tooth)
  to pole A. Energizing stator pole B causes the
  rotor to rotate 15 degrees in alignment with pole
  B. This process will continue with pole C and
  back to A in a clockwise direction. Reversing the
  procedure (C to A) would result in a
  counterclockwise rotation.

8
Permanent Magnet Motors

• Unlike the other stepping motors, the PM motor
  rotor has no teeth and is designed to be
  magnetized at a right angle to it's axis. The
  above illustration shows a simple, 90 degree PM
  motor with four phases (A-D). Applying current to
  each phase in sequence will cause the rotor to
  rotate by adjusting to the changing magnetic
  fields. Although it operates at fairly low speed
  the PM motor has a relatively high torque
  characteristic.

9
Hybrid Motors

• They are constructed with multi-toothed stator
  poles and a permanent magnet rotor. Standard
  hybrid motors have 200 rotor teeth and rotate at
  1.80 step angles. Other hybrid motors are
  available in 0.9ºand 3.6º step angle
  configurations. Because they exhibit high static
  and dynamic torque and run at very high step
  rates, hybrid motors are used in a wide variety
  of industrial applications.

10
Types of Windings

• Unipolar motors
• Bipolar motors
• Bifilar motors
• Multiphase motors

11
Unipolar Motors

• Unipolar stepping motors, both Permanent magnet
  and hybrid stepping motors with 5 or 6 wires are
  usually wired as shown in the figure, with a
  center tap on each of two windings. In use, the
  center taps of the windings are typically wired
  to the positive supply, and the two ends of each
  winding are alternately grounded to reverse the
  direction of the field provided by that winding.
  As shown in the figure, the current flowing from
  the center tap of winding 1 to terminal a causes
  the top stator pole to be a north pole while the
  bottom stator pole is a south pole.

12
Unipolar Motors

• This attracts the rotor into the position shown.
  If the power to winding 1 is removed and winding
  2 is energized, the rotor will turn 30 degrees,
  or one step.
• To rotate the motor continuously, we just apply
  power to the two windings in sequence. Assuming
  positive logic, where a 1 means turning on the
  current through a motor winding, the following
  two control sequences will spin the motor
  illustrated in the figure.

13
Bipolar Motors

• The bipolar motor is similar inc construction to
  the unipolar motor except that it does not have
  any center taps.The advantage gained in the
  simplicity of construction of the motor is lost
  in the control circuitry as that becomes more
  complex. A H-bridge is required for the control
  of the motor.

14
Bifilar Motors

• Bifilar windings on a stepping motor are applied
  to the same rotor and stator geometry as a
  bipolar motor, but instead of winding each coil
  in the stator with a single wire, two wires are
  wound in parallel with each other.

15
Multiphase Motors

• This permanent magnet stepping motor is wired
  with all windings of the motor in a cyclic
  series, with one tap between each pair of
  windings in the cycle. The most common designs in
  this category use 3-phase and 5-phase wiring.

16
What Is Micro Stepping?

• Micro stepping is a way of moving the stator flux
  of a stepper more smoothly than in full- or
  half-step drive modes. This results in less
  vibration, and makes noiseless stepping possible
  down to 0 Hz.
• It also makes smaller step angles and better
  positioning possible.
• In many applications microstepping can increase
  system performance, and lower system complexity
  and cost,
• compared to full- and half-step driving
  techniques. Microstepping can be used to solve
  noise and resonance problems, and to increase
  step accuracy and resolution.

17
Controlling A Stepper Motor

• H Bridges
• UC3717A from TI and other similar devices from
  vendors

18
Acknowledgements

• Jones on Stepper Motors
• http//www.cs.uiowa.edu/jones/step/
• Advanced Micro Systems
• http//www.ams2000.com/stepping101.html