STEPPER MOTORS

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STEPPER MOTORS

• BY
• GROUP-II
• SURENDRA KUMAR BATCHU
• DILEEP THAMMAIAHGARI
• SAISAMEER TADIMETI VENKATA
• AMARDEEP REDDY BADDAM
• SRIDHAR SONGOJU

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TOPICS UNDER DISCUSSION

• Introduction
• Types of stepper motors
• Summarize
• Next steps

3

Contd

• Applications
• Conclusion

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Introduction

• A stepper or stepping motor converts electronic
  pulses into proportionate mechanical movement.
• Like many conventional electric motors stepper
  motors also consist of magnets and coils Whereas
  conventional motors spin continuously, a stepper
  motor moves around one small step at a time
  (Hence the name)
• These provide precise positioning and speed
  control without the use of feedback sensors.

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Picture of stepper motor
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Operational Principle
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Inside the Stepper motor

• The simplest way to think of a stepper motor is a
  bar magnet and four coils.
• When current flows though coil "A" the magnet is
  attracted and moves one step to the right. Coil A
  is then turned off and coil "B" turned on.
• Magnet is cylindrical in stepper motor

            
                                                       
A B C D
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Types of stepper motors

• Variable Reluctance motors
• UniPolar motors
• Bipolar motors
• Bifilar motors
• Multiphase motors

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Variable Reluctance Motor

• If the motor has three windings, typically
  connected as above with one terminal common to
  all windings, it is most likely a variable
  reluctance stepping motor

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Contd..

• .Assuming positive logic, where a 1 means turning
  on the current through a motor winding, the
  following control sequence will spin the motor in
  continuous manner
• Winding 1 1001001001001001001001001
• Winding 2 0100100100100100100100100
• Winding 3 0010010010010010010010010

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Unipolar stepper motor

• In Unipolar stepping motors, 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.

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Bipolar stepper motor

• Bipolar permanent magnet and hybrid motors are
  constructed with exactly the same mechanism as is
  used on unipolar motors, but the two windings are
  wired more simply, with no center taps. Looks
  simple but the operation is complex..

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Bifilar Motors

• Bifilar windings on a stepping motor are applied
  to the same rotor and stator geometry as a
  bipolar motor but the windings are different
• In practice, motors with bifilar windings are
  always powered as either unipolar or bipolar
  motors.

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Bifilar motors as unipolar

• To use a bifilar motor as a unipolar motor, the
  two wires of each winding are connected in series
  and the point of connection is used as a
  center-tap

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Bifilar motor as Bipolar

• To use a bifilar motor as a bipolar motor, the
  two wires of each winding are connected either in
  parallel or in series. The above is connected in
  parallel. This allows low voltage high-current
  operation. For reverse we need to connect them in
  series

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Multi Phase motors

• A less common class of permanent magnet or hybrid
  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, or with only
  one end of each motor winding exposed while the
  other ends of each winding are tied together to
  an inaccessible internal connection

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Single Stack Stepper Motor

• There are 3 phase of winding in stator (p
  3)
• Total number of stator poles, ns12
• Total number teeth on rotor nr 8
• Geometrically orthogonal set of four teeth on the
  rotor align themselves perfectly with the
  energized stator poles.
• There is a misalignment of 15 between rotor
  teeth and stator poles
• Stator pitch ? s 360/ns
• Rotor pitch ? r 360/nr

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Single Stack Stepper Motor

• There are 3 phases and total number of 12 stator
  poles
• i.e. ns 12
• There are 8 stack teeth i.e. nr 8.
• When Stator is energized a geometrically
  orthogonal set of four teeth on the rotor
  align themselves perfectly with the four stator
  poles
• Stator pitch ?s 360/ns
• Rotor pitch ?r 360/nr
• Pitch angleIt is the angle between adjacent
  teeth.

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Single Stack Stepper Motor

• Step angle is the smallest misalignment between
  stator teeth and rotor teeth at any stable
  equilibrium position.
• Step angle ? ? ? r ?s .
• Full-stepping sequence for CCW rotation is
  1-2-3-1 for CW it is 1-3-2-1.
• The half-stepping sequence for CCW rotation is
• 1-12-2-23-3-31-1.
• Each switching of phases corresponds to a
  rotation of ?? and
• there are three phases ,the angle of
  rotation for a complete switching cycle of p
  switches is p. ??.

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• Toothed pole construction for stepper motor

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Toothed-pole construction

• Step angle can be reduced by having toothed
  poles in stator.
• Stator teeth equally spaced but not identical to
  rotor teeth.
• In this case ns number of teeth rather than
  number of poles in the stator.
• Toothed stator construction can provide very
  small angles about .72.

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Micro stepping

• Half-stepping It occurs when phase switching
  alternates between one-phase-on and two-phase-on
  states.
• Full-stepping It occurs when either one-phase-on
  switching or two-phase-on switching is used
  exclusively at ever step.
• Micro stepping It is achieved by properly
  changing the phase currents in steps in addition
  to switching the phase ON and OFF.
• Principle Current in one pole is decreased while
  the other current is kept unchanged ,the
  resulting magnetic field will move closer to the
  pole with large current. Rotor stack moves
  depending on the resultant magnetic field .

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Three-stack stepper motor
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Multiple-stack stepper motors

• These are two types
• (i) Equal pitch construction (?r
  ?s)
• (ii) Unequal pitch construction (?r
  gt ?s or ?r lt ?s)
• Equal Pitch
• 1) The teeth in the three stator segments
  are perfectly aligned but the teeth in the three
  rotor stacks are misaligned consecutively by a
  one-third-pitch angle.
• 2) The teeth in the three rotor stacks are
  perfectly aligned ,but the teeth in the three
  stator segments are misaligned consecutively by
  a one-third-pitch angle.

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OPEN-LOOP CONTROL OF STEPPER MOTOR

• In normal operating mode, the stepwise rotation
  of the motor is synchronized with the command
  pulse train.
• This justifies the term Digital synchronous
  motor, which is sometimes denoted as Stepper
  motor.
• Because of the stepwise synchronous operation,
  position error in the stepper motor is generally
  non cumulative so open-loop control is
  adequate.

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OPEN-LOOP CONTROL OF STEPPER MOTOR
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OPEN-LOOP CONTROL OF STEPPER MOTOR

• The pulse generator is typically a
  variable-frequency oscillator.
• For bidirectional motion, it will generate two
  pulse trains
• Position-pulse train
• Direction-pulse train
• The position-pulses identify the exact times at
  which at which angular steps should be initiated.
• The direction-pulses identify the instants at
  which the direction of rotation should be
  reversed.

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OPEN-LOOP CONTROL OF STEPPER MOTOR

• The translator module has logic circuitry to
  interpret a pulse train and translate it into the
  corresponding switching sequence for stator field
  winding.
• The device that has all these capabilities,
  including the standard translation functions, is
  termed a preset indexer.
• The required angle of rotation, stepping rate and
  direction are set either manually or by computer
  commands

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OPEN-LOOP CONTROL OF STEPPER MOTOR

• The Control signal which is in the order of 10mA
  need to be amplified by Switching amplifiers for
  phase excitation.
• The complete unit that consists of the translator
  amplifiers, and the power supply is termed a
  motor-drive system.
• The load may be connected to the motor shaft
  directly or through some coupling devices.

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STEPPER MOTOR RESPONSE

• Step time tr is the time taken by the motor
  shaft to rotate one step angle.
• The settling time ts is the time taken by the
  oscillations or ringing to cease.

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STEPPER MOTOR RESPONSE

• In many applications we need fast response and
  reasonably continuous motor speeds .
• These can be achieved by decreasing the motor
  settling time through increased dissipation.
• So, the fast operation of the Stepper motor can
  be achieved by operation motor at steady state in
  synchronism at a constant pulse called Slew Rate.

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STEPPER MOTOR RESPONSE

• Slew Rate (Rs) is given by
• Rs 1/?t
• ?t is the time between the successive pulses
  under slewing condition

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STEPPER MOTOR RESPONSE

• To attain slewing condition, the stepper motor
  has to be accelerated from low speed by applying
  sequence of pulses with a continuously increasing
  pulse rate R(t).
• In Upramping, the rotor angle trails the pulse
  command, and during Down ramping the rotor angle
  leads the pulse command.

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STEPPER MOTOR RESONANCES

• Stepper motors can often exhibit a phenomena
  called Resonance at certain step rates.
• This occurs when input step pulse rate coincides
  with the natural oscillation frequency of the
  rotor
• In this there will be sudden loss or drop in
  torque at certain speeds, which can result in
  loss of synchronism.

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characteristics of a stepper motor

• Stepper motor manufacturers benefit greatly from
  the ability to measure and analyze the
  characteristics of a stepper motor
• The measurements were obtained by connecting an
  encoder to the shaft of a stepper motor, and
  monitoring the encoder and stepper-motor indexer
  signals using the stepper motor dynamic analyzer

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The Stepper Motor Dynamics Analyzer
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• The Stepper Motor Dynamics Analyzer provides a
  means to quickly
• Analyze and compare stepper motors and drivers
• Measure shaft dynamics and winding currents and
  voltages
• Evaluate motor performance

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Static Step Accuracy

• The angular position accuracy of a stepper motor
  varies from one step to the next
• This inaccuracy is influenced by the construction
  of the motor, the load it is driving, and the
  driver attached to motor.

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Contd..
Static step error vs. The angle of
the shaft
Step response
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Micro-step Accuracy

• Micro-stepping accuracy is determined by the
  construction of the motor and the accuracy of the
  driver
• Micro-stepping is often used to position the
  shaft of a stepper motor between the full step
  positions

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Contd.
Shaft position error vs. Ideal shaft
position
Error introduced by driver current error
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Velocity resonance

• An unloaded stepper motor was ramped in velocity
  up to five Revolutions Per Second.
• At low velocities, resonance caused the velocity
  of the motor to fluctuate severely. At higher
  velocities the velocity became smoother

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Dynamic Position Accuracy

• The shaft of a stepper motor will lag behind the
  ideal step position in the presence of a torque
  load

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Motor Drive Current

• The performance of a stepper motor is strongly
  dependent upon the driver
• As the velocity of a motor increases, the
  driver's ability to deliver current is impeded by
  the back EMF of the motor, the non-zero
  inductance of the motor windings

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Feed back control of stepper motor

• Open-loop control is adequate for many
  applications of stepper motor, particularly at
  low speeds and in steady state operation.
• The main disadvantage of open loop control is
  that the actual response of the motor is not
  measured because of missing pulses
• The main reason for pulse missing is
• Under variable speed conditions, if the
  successive pulses are received at high frequency,
  the phase translator might not respond to a
  particular pulse

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Contd.

• Feedback control is used to compensate for motion
  errors and pulse missing.
• The noted improvement in the feedback control
  scheme is that the actual response of the stepper
  motor is measured and compared with the desired
  response.
• If an error is detected pulse train of the drive
  is modified appropriately to reduce error .

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Block diagram of closed loop stepper motor
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Contd

• Incremental encoder is employed as the motion
  transducer.
• This device provides two pulses that are in the
  pulse quadrature , giving both the magnitude and
  direction of rotation.
• The encoder pitch angle should be made equal to
  the step angle of the motor for ease of
  comparison and error correction.

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STEPPING MOTOR MODELS

• Torque source model
• Mechanical model
• Improved model

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TORQUE EQUATIONS

• Under steady state operation of a stepper motor
  at low speeds, the motor torque equation is given
  by
• T-Tmax Sin(nr ?)
• T- Tmax Sin(2?/(p.??).
• where Tmax maximum torque during a step
• ?? step angle
• nrnumber of rotor teeth
• pnumber of phases.

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TORQUE EQUATIONS

• The torque equation for the mechanical model is
  given by
• T-TL-Tb(?,?.) J?..
• where TL resisting torque on the motor
  by the driven load
• Tb(?,?.) dissipative resisting torque
  on the motor.
• J rotor inertia.

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TORQUE EQUATION

• In the improved model the torque equation is
• 1. The torque equation for the PM motors is
• T-Km iP Sin(nr?)
• where iP is the phase current and
• Km is the torque constant for
• PM motor.
• 2. The torque equation for the VR motors is
• T-Kr iP2 Sin(nr?)
• where Kr is the torque constant for the
  VR motor.

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STEPPER MOTOR SELECTION

• List the main requirements for the particular
  application
• Compute the operating torque and stepping rate
  requirements for the particular application.
• Using the torque versus stepping rate curves for
  a group of commercially available stepper motors,
  select a suitable motor
• If a stepper motor that meets the requirements is
  not available, modify the basic design.
• Select a drive system that is compatible with
  motor and that meets the operational requirements
  in step1

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ADVANTAGES AND DISADVANTAGES

• ADVANTAGES
• Position error is noncummulative. A high
  accuracy of motion is possible, even under open
  loop control
• Large savings in sensor and controller costs are
  possible when the open-loop mode is used.
• Because of the incremental nature of command and
  motion, stepper motors are easily adoptable to
  digital control applications.
• DISADVANTAGES
• Resonances can occur if not properly controlled.
• Not easy to operate at extremely high speeds.

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APPLICATIONS OF STEPPER MOTORS

• Stepper motors are incremental actuators, so they
  are ideally suited for digital control
  applications
• Early application of stepper motors was limited
  to low-speed, low-torque drives.
• A stepper motor is particularly suitable in
  printing applications(including graphic printers,
  plotters, and electronic typewriters).
• Stepper motors are found as joint actuators and
  end effector actuators of robotic manipulators.

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Usage in computer peripherals

Application Use
Floppy Disc position magnetic pickup
Printer carriage drive
Printer rotate character wheel
Printer paper feed
Printer ribbon wind/rewind
Printer position matrix print head
Tape Reader index tape
Plotter X-Y-Z positioning
Plotter paper feed
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Conclusion

• Steppers can be used open-loop without the need
  for expensive encoders
• There are no brushes which eliminates the need
  for maintenance.
• With high precision we can get .72 degrees step
• The advantages of stepper motors outweighs its
  disadvantages.