ECE5320 - Mechatronics Assignment 1: Literature Survey on Sensors and Actuators Topic: DC Motors (Actuators)

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ECE5320 - MechatronicsAssignment 1 Literature
Survey on Sensors and Actuators Topic DC
Motors (Actuators)

• Prepared by
• SIDDHARTH P. RAO
• Dept. of Electrical and Computer Engineering
• Utah State University
• Tel [phone](Home) Email
  siddharth_at_cc.usu.edu
• Tel [phone](Work) Email
  siddharthr_at_ext.usu.edu

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Overview

• Actuators are basically the muscle behind a
  mechatronics system that accepts a control
  command (mostly in the form of an electrical
  signal) and produces a change in the physical
  system by generating force, motion, heat, flow,
  etc.
• Under the category of electromechanical
  actuators, a motor is the most common one. It
  converts electrical energy to mechanical motion.
• Motors are the principal means of converting
  electrical energy into mechanical energy in
  industry.
• DC motors are those which operate on a DC voltage
  and varying the same can easily control their
  speed.

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Types of DC Motors

• Permanent Magnet type DC Motors Contains no
  field coils.
• ? Further classified under this type as
  follows
• 1). Conventional Permanent Magnet Motor.
  ? (High efficiency, high peak power, and
  fast response)
• 2). Moving Coil Permanent Magnet Motor.
• ? (Higher efficiency and lower inductance
  than conventional DC motor.)
• 3.) Torque Motor.
• ? (Designed to run for a long periods in a
  stalled or a low rpm condition.)

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Types of DC Motors

• Wound Field type DC Motors
• ? Further classified under this type as
  follows
• 1). Series Wound DC Motor.
• ? (High
  starting torque, high acceleration torque, high
  speed with light load)
• 2). Shunt Wound DC Motor.
• ? (Constant-speed application)
• 3.) Compound Wound DC Motor.
• ? (Low starting
  torque, good speed regulation Instability at
  heavy loads)

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Types of DC Motors

• Electronic Commutation Type DC Motors (brushless
  motors)
• ? Fast response.
• ? High efficiency, often exceeding 75
• ? Long life, high reliability, no maintenance
  needed.
• ? Low radio frequency interference and noise
  production.

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Basic Working Principle

• For any electric motor, its operation is based on
  the principle of simple electromagnetism.
• Now, a current-carrying conductor generates a
  magnetic field.

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Basic Working Principle

• When this is then placed in an external magnetic
  field, it will experience a force proportional to
  the current in the conductor, and to the strength
  of the external magnetic field.
• The internal configuration of a DC motor is
  designed to harness the magnetic interaction
  between a current-carrying conductor and an
  external magnetic field to generate rotational
  motion.

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Basic Working Principle

• The above figure shows a simple 2-pole DC
  electric motor (here red represents a magnet or
  winding with a "North" polarization, while green
  represents a magnet or winding with a "South"
  polarization).
• Every DC motor has six basic parts namely axle,
  rotor (i.e., armature), stator, commutator, field
  magnet(s) and brushes.

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Basic Working Principle

• In most common DC motors, the external magnetic
  field is produced by high strength
  permanent-magnets.
• The stator is a stationary part. This includes
  the motor casing as well as two or more permanent
  magnet pole pieces.

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Basic Working Principle

• The rotor (together with the axle and attached
  commutator) rotate with respect to the stator.
• The rotor consists of windings (generally on a
  core), the windings being electrically connected
  to the commutator.

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Basic Working Principle

• The geometry of the brushes, commutator contacts,
  and rotor windings are such that when power is
  applied, the polarities of the energized winding
  and the stator magnet(s) are misaligned, and the
  rotor will rotate until it is almost aligned with
  the stator's field magnets.
• As the rotor reaches alignment, the brushes move
  to the next commutator contacts, and energize the
  next winding.
• In real life, though, DC motors will always have
  more than two poles (three is a very common
  number). In particular, this avoids "dead spots"
  in the commutator.
• With the above example of two-pole motor, if the
  rotor is exactly at the middle of its rotation
  (perfectly aligned with the field magnets), it
  will get "stuck" there.

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Basic Working Principle

• The above figure shows a three-pole design of a
  DC motor.
• We notice that one pole is fully energized at a
  time (but two others are "partially" energized).

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Basic Working Principle

• As each brush transitions from one commutator
  contact to the next, one coil's field will
  rapidly collapse, as the next coil's field will
  rapidly charge up (this occurs within few
  microseconds).

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Basic Working Principle
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DC Motor Equivalent Circuit
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DC Motor Equivalent Circuit

• The operation equations are as follows
• ? Armature voltage equation
• ? The induced voltage motor speed Vs angular
  frequency
• Where,

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DC Motor Equivalent Circuit

• The operation equations are as follows
• ? The combination of the equations result in
• ? The current is calculated from this equation.
  The output
• power is given by
• And the torque is given by

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Sample Configuration in Application

• There are a number of high volume applications
  for DC motors that require precision control of
  the motors speed.
• Phase locked loop techniques are well suited to
  provide this control by phase locking the motor
  to a stable and accurate reference frequency.
• Thus, the small signal characteristics, and
  several large signal effects, of these loops have
  to be considered.
• PHASE LOCKING GIVES PRECISION SPEED CONTROL.

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Sample Configuration in Application

• The precise control of motor speed is a critical
  function in todays disc drives.
• Other data storage equipment, including 9 track
  tape drives, precision recording equipment, and
  optical disc systems also require motor speed
  control.

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Sample Configuration in Application

• One of the best methods for achieving speed
  control of a motor is to employ a phase locked
  loop.
• With a phase locked loop, a motors speed is
  controlled by forcing it to track a reference
  frequency.

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Sample Configuration in Application

• The reference input to the phase locked loop can
  be derived from a precision crystal controlled
  source, or any frequency source with the required
  stability and accuracy.
• In above figure, a precision crystal oscillators
  frequency is digitally divided down to provide a
  fixed reference frequency.

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Sample Configuration in Application

• The motor speed is sensed by either a separate
  speed winding or, particularly in the case of the
  DC brushless motor, a Hall effect device.
• The two signals, motor speed and reference
  frequency, are inputs to a phase detector.

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Sample Configuration in Application

• The detector output is a voltage signal that is a
  function of the phase error between the two
  inputs.
• The transfer function of the phase detector, Kf,
  is expressed in volts/radian.
• A 1/s multiplier accounts for the conversion of
  frequency to phase, since phase is the time
  integral of frequency.
• Following the phase detector is the loop filter,
  which contains the required gain and filtering to
  set the loops overall bandwidth and meet the
  necessary stability criteria.

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Sample Configuration in Application

• The output of the loop filter is the control
  input to the motor drive.
• Depending on the type of drive used, voltage or
  current, the driver will have respectively, a
  Vout/Vin transfer characteristic, or an Iout/Vin
  transconductance.
• At first glance, it seems that the motor has
  simply replaced the VCO (voltage controlled
  oscillator), in the classic phase locked loop.
• It is, in fact, a little more complicated. The
  mechanical and electrical time constants of the
  motor come into play, making the transfer
  function of the motor more than just a
  voltage-in, frequency-out block.

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Major Specifications of DC Motors

• Basically, specifications of DC motors depend
  mainly on the type of motor and the application
  where it is to be used.
• Among the major performance specifications are
  following
• Shaft Speed No-load rotational speed of output
  shaft at rated terminal voltage.
• Terminal Voltage Usually derived my the designer
  according to the application requirements.
• Continuous Current Maximum rated current that
  can be supplied to the motor windings without
  overheating.

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Major Specifications of DC Motors

• Continuous Torque Output torque capability of
  the motor under constant running conditions.
• Continuous Output Power Mechanical power
  provided my the motor output.
• Apart from the above listed specifications, the
  designer/user may have to consider the type of
  motor (PM, shunt wound, series wound, etc) and
  the type of commutation (brush / brushless).
• Also, the type of gearing has to be considered
  depending on the type of application. Some types
  of gearing choices are Spur, Planetary,
  Harmonic, Worm, Bevel, etc.
• Finally, the type of Shaft is also to be
  decided. Some of the types are In-line,
  offset/parallel, right angle, hollow, etc.

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Application of DC Motors

• DC Motors are used is a very large number of
  applications and its use varies depending on the
  type and the environment of the same.
• Under the category of Appliances, Brushed DC
  motors are used in Coffee Grinders, Bread Makers,
  Can Openers, Ice Makers, Ice Cream Makers,
  Juicers, Vacuum Cleaners, Steam Cleaners,
  Polishers, Waxers, Blenders, Mixers, Food
  Processors, Electric Toothbrushes, Hair Clippers,
  Electric Razors.
• Under the same category of Appliances,
  Brushless DC motors are used in Air Conditioner
  Compressors and Refrigerators.

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Application of DC Motors

• Under the category of Automotives, Brushed DC
  motors are used in Electric Windows, Electric
  Doors, Electric Hatches, Electric Sun Roofs,
  Electric Seats, Electric Retraceable Antenna,
  Windshield Wipers, Electric Starters, HVAC Fan
  and Electric Vehicles.
• Under the same category of Automotives,
  Brushless DC motors are used in Electric Fuel
  Pump, Electric Oil Pump, Electric Radiator Fan,
  Electronic Power Steering (EPS), Anti-Lock Brake
  System (ABS), Electronic Throttle Control, HVAC
  Fan, Electric Vehicles.
• Under the category of Consumer applications,
  Brushed DC motors are used in Moving Toys, Toy
  Robots, Radio Controlled Vehicles, Exercise
  Equipment, Computer PC Fans, Cordless Hand Tools.

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Application of DC Motors

• Under the same category of Consumer
  applications, Brushless DC motors are used in
  Radio Controlled Vehicles, Sewing Machines,
  Computer Hard Drives, Computer Tape Drives,
  Cordless Hand Tools, Camera Motor Drives,
  Camcorders, CD/DVD Equipment, Tape Decks and
  VCRs.
• Under the same category of Consumer
  applications, 2-phase Brushless DC motors are
  used in Computer PC fans.
• Finally, under the category of Industrial
  applications, Brushless DC motors are used in
  Large Movement Control/Hoist, Crane, Elevator and
  in Precision Movement Control Printers, Robots,
  Milling Medical Equipment Servo Control.

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Advantages of DC Motors

• Wide speed range, 10001
• Servo performance.
• Positioning capabilities to within 1/4 of an
  encoder count.
• Precise speed and torque control.
• 0.05 per cent speed regulation.
• Less than 3 torque linearity of the full speed
  range.
• 100 torque at 0 rpm.

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Advantages of DC Motors

• Moderate PWM losses.
• Sensorless operation.
• Low cost (only in case of brushless dc
  trapezoidal motors).

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Limitations of DC Motors

• Brushless DC Servo motors have High PWM losses.
• High radial forces on motor.
• Limited over-speed range.
• Higher torque ripple.
• In case of Brushed DC motors, due to wear and
  tear, the life of he motor is less and thus, more
  maintenance is required.
• In some cases, the noise is more audible.
• Not all DC motors are highly efficient.

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Selection, Cost Buying Info

• Selection of DC motors completely depends on the
  type of applications in which it is being used.
  It can be a brushed DC motor, a brushless DC
  motor, etc.
• Based on the type of application, DC motors range
  from as low as 5 to as high as over 4000 per
  unit.
• There are many online stores from where DC motors
  can be purchased depending on the type of
  application. Some of the good e-stores are as
  follows
• http//www.adcmotors.com/products.htm
• http//www.pennmotion.com/quick_index.html
• http//www.cartermotor.com/
• http//www.hansen-motor.com/dcindex.html

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References

• Motor Control Design Center
• Motors and Drives _at_ Electricity-today
• DC motors principle of operation _at_ solarbotics
• Notes on motors _at_ gizmology
• Design Notes on Precision Phase Locked Speed
  Control for DC Motors
• Lecture notes from Arizona State University
• The Mechatronics Handbook from EngnetBase.

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Thank You!