Introduction to Motors

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Introduction to Motors

• Understanding the CEENBots muscles

Presented by Alisa N. Gilmore, P.E. Senior
Lecturer, UNL Computer and Electronics
Engineering Dept. NSF ITEST SPIRIT Workshop
Summer 2008 The Peter Kiewit Institute Omaha, NE
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Robot Defined

• A robot is an autonomous system which exists in
  the physical world, can sense its environment,
  and can act on it to achieve some goals. The
  Robotics Primer by Maja Mataric
• Autonomous programmable
• Exists in physical world has a body
• Can sense its world has sensors
• Can act on it possesses effectors actuators,
  i.e. legs, arms (wheels) muscles (motors)

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Overview

• Motors in context of robotics, different types of
  robots have different types of motors
• Overview of motor types / characteristics
• All motors convert electric energy to mechanical
  motion
• Motor characteristics AC or DC power source,
  torque, speed performance
• Industrial robotics AC servo motor
• Mobile robotics Hobby robots dc motor, dc
  servo motor, and stepper motors
• Principle of operation of a DC motor
• Inside a DC motor
• Principle of operation of stepper motors
• Performance advantages of stepper motor over DC
  motor and DC servo motor
• CEENBot stepper motor operation/control

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Motor Basics

• Either an AC or DC electrical energy source
  serves as the input to the motor.
• The result is mechanical motion of the output
  shaft, most often a rotation about the shaft,
  provided the load carried by the shaft does not
  exceed the maximum load the motor is designed to
  carry.

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Choosing a Motor

• There are numerous ways to design a motor, thus
  there are many different types of motors.
• The type of motor chosen for an application
  depends on the characteristics needed in that
  application.
• These include
• How fast you want the object to move,
• The weight, size of the object to be moved,
• The cost and size of the motor,
• The accuracy of position or speed control needed.

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Motor Parameters

• The level of performance a motor can provide is
  described by its parameters. These include
• Rated Speed
• Speed measured in shaft revolutions per minute
  (RPM)
• Torque
• Rotational force produced around a given point,
  due to a force applied at a radius from that
  point, measured in lb-ft (or, oz-in).
• HorsePower Speed x Torque / 5252.11...
• A measure of work expended 1 HP 33,000
  foot-pounds per minute.
• Torque-Speed performance of a motor

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Types of Motors

• The different types of motors possess different
  operating characteristics.
• A brief overview of some operation
  characteristics of
• AC motors
• DC motors
• DC servo motors
• Stepper motors

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AC Motor Characteristics

• When power is applied, AC motors turn in one
  direction at a fixed speed.
• Both reversable and non-reversable models
  available
• Usually high voltage (110V AC and up)
• Inexpensive and commonly available
• Optimized to run at a fixed, usually high speed.
• If the applied load is greater than the capacity
  of the motor, the motor will stall and possibly
  burn out.

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DC Motor Characteristics

• When power is applied, DC motors turn in one
  direction at a fixed speed.
• They are optimized to run at a fixed, usually
  high speed.
• Most common found in toys, hobby planes,
  inexpensive robots, blender, toothbrush,
  screwdriver, etc.
• Speed can be varied if a (pulse width modulation)
  PWM controller is added.
• Almost all can be reversed.
• Inexpensive and commonly available.
• Not suitable for positioning unless some kind of
  position feedback is added.
• If the applied load is greater than the capacity
  of the motor, the motor will stall and possibly
  burn out.

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DC Servo Motors

• Applications that require Servo motors involve
  control of acceleration, velocity, and/or
  position to very close tolerances. These motors
  allow for fast starts, stops and reversals, and
  very accurate control.
• DC servo motors consist of a DC motor combined
  with feedback for either position or speed.
• A servo system is closed loop with a motor,
  feedback signal, desired input signal, and a
  controller which constantly adjusts the position
  or speed in reaction to the feedback.
• Servo motor controllers are complex.

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Stepper Motors

• A stepper motor will not automatically turn when
  power is applied.
• It requires a separate controller circuit to
  cause the motor to move.
• Controllers for stepper motors are easier to
  implement than closed loop servo systems.
• Precise positioning is possible by keeping count
  of steps, no feedback is required. It is open
  loop.
• They are inexpensive and commonly available,
  especially in salvaged computer equipment.
• Note If the applied load is greater than the
  capacity of the motor, the motor may not step,
  thereby making precise positioning no longer
  possible.

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DC Electric Motors

• DC Electric Motors use Direct Current (DC)
  sources of electricity
• Batteries
• DC Power supply
• Principle of How Motors Work
• Electrical current flowing in a loop of wire will
  produce a magnetic field across the loop.
  
• When this loop is surrounded by the field of
  another magnet, the loop will turn, producing a
  force (called torque) that results in mechanical
  motion.

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Motor Basics

• Motors are powered by electricity, but rely on
  principles of magnetism to produce mechanical
  motion.
• Inside a motor we find
• Permanent magnets,
• Electro-magnets,
• Or a combination of the two.

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Magnets

• A magnet is an object that possesses a magnetic
  field, characterized by a North and South pole
  pair.
• A permanent magnet (such as this bar magnet)
  stays magnetized for a long time.
• An electromagnet is a magnet that is created when
  electricity flows through a coil of wire. It
  requires a power source (such as a battery) to
  set up a magnetic field.

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Current in a coil creates a Magnet
Current Flowing through a coil or wire LEFT
Current Enters A North Pole on
Top RIGHT Current Enters B (Reversed) North
Pole on Bottom
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A Simple Electromagnet

• A Nail with a Coil of Wire
• Q How do we set up a magnet?
• A The battery feeds current through the coil of
  wire. Current in the coil of wire produces a
  magnetic field (as long as the battery is
  connected).

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A Simple Electromagnet

• A Nail with a Coil of Wire
• Q - How do we reverse the poles of this
  electromagnet?
• A By reversing the polarity of the battery!

S
N

-
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The Electromagnet in a Stationary Magnetic Field

• If we surround the electromagnet with a
  stationary magnetic field, the poles of the
  electromagnet will attempt to line up with the
  poles of the stationary magnet.
• The rotating motion is transmitted to the shaft,
  providing useful mechanical work. This is how DC
  motors work!

OPPOSITE POLES ATTRACT!
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DC Motor Operation Principles

• Once the poles align, the nail (and shaft) stops
  rotating.
• How do we make the rotation continue?
• By switching the poles of the electromagnet. When
  they line up again, switch the poles the other
  way, and so on.
• This way, the shaft will rotate in one direction
  continuously!

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Brushed DC Motor Components

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How the Commutator Works

• As the rotor turns, the commutator terminals also
  turn and continuously reverse polarity of the
  current it gets from the stationary brushes
  attached to the battery.

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Inside a Toy Motor(Similar to TekBot Motor)
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Inside the Motor, cont.
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Advantages of Stepper Motor

• The DC motors on the TekBot offer limited speed
  control and low torque.
• The CEENBot uses a stepper motor for each wheel.
• The stepper motors on the CEENBot enables
  accurate wheel positioning with high holding
  torque and allows for open-loop speed control
  (wheel position feedback is option).

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Stepper Motor Operation

• A stepper motor consists of
• A permanent magnet rotating shaft (or rotor)
• Electromagnets on the stator the stationary
  portion that surrounds the motor
• The stepper motor moves as the permanent rotor
  magnet attempts to line up with the poles of the
  electromagnets on the stator.
• The electromagnets are digitally switched to
  change their pole orientation, which when done in
  a sequence produces continuous rotation of the
  rotor.
• http//www.interq.or.jp/japan/se-inoue/e_step1.ht
  m

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• The smallest step of angular rotation a stepper
  motor can make is called its resolution.
• Unlike the example, which had 90 degrees per step
  resolution, real motors employ a series of
  mini-poles on the stator and rotor to increase
  resolution.

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• The same sequence of 4 stepping phases is used to
  control this scenario. There is no increase in
  control complexity.
• http//www.interq.or.jp/japan/se-inoue/e_step1.htm
  
• CEENBot stepper motors have a resolution of 1.8
  degrees per step.
• Q How many steps are needed to make 1 complete
  wheel revolution?

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• Because the rotor is fixed by magnetism in the
  stationary condition, the stationary torque is
  large. It allows one to make a precise stop at
  some angle and hold it there.
• The CEENBot can better hold its position on a
  ramp.
• Speed control is achieved by digitally cycling
  through the phases at a desired speed of
  rotation.
• A microcontroller is used to reverse the current
  after each step, which changes the poles of the
  corresponding electromagnets.

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Unipolar Bipolar Steppers

• The stepper motor example is similar to the
  CEENBot motor, except that it is unipolar.
• It has 6 wires to connect, verses the 4 wires of
  the bipolar stepper motors you will install on
  the CEENBot.
• The difference is the bipolar provides greater
  torque since an entire coil is energized instead
  of a half coil for each state of the
  electromagnet.
• The unipolar is simplier to control since the two
  coils that make up the stepper are centertapped,
  a wire is connected midway on each coil and is
  tied to power. To reverse power, simply
  alternate the grounding of one of the two
  terminals connected to a coil. This reverses
  current flow, and thus reverses the poles of the
  electromagnet. However, only one half of each
  coil is energized at a time.
• Bipolar motors require a slightly more involved
  controller that must reverse the current flow
  through the coils by alternating the polarity of
  the terminals.
• This is done simply with the aid of a
  microcontroller.

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References

• The Difference Between Stepper Motors, Servos,
  and RC Servos by Roger Arrick http//www.arrickro
  botics.com/motors.html
• Making Things General Information on Motors
  http//www.makingthings.com/teleo/products/documen
  tation/app_notes/motors_general.htm
• How Stepper Motors Work by Images Scientific
  Instruments http//www.imagesco.com/articles/picst
  epper/02.html
• CEENBot Stepper Motor PM DC Motor Testing Unit
  Operations Manual by Ben Barenz, CEEN Student
• Hansen Corp. Servo motors http//www.hansen-moto
  r.com/servo-motors.htm
• Animated operation of a Unipolar stepper motor
  http//www.interq.or.jp/japan/se-inoue/e_step1.htm
  
• Basic Motor Theory by Reliance Electric
  http//www.reliance.com/mtr/mtrthrmn.htm