Attendance

LEGO project demos!

Updates on Parallels/Bootcamp/VMware Fusion? Count

Finish up Minimum Constraint Design

February 5th: National Teach-in on Global Warming webcast

Motors

• How do we get motion from current?

Current flowing through a wire induces a magnetic field. Motors exploit this fact by mounting coils of wire to an axis and making it turn.
DIY motor

• Driving parameters for choosing a motor:
  1. Do you need speed or torque, or both? Maxon motor how-to
     Torque @ max efficiency is the torque the motor works best at.
  2. Do you want position feedback?
  3. What are you trying to move/spin?
  4. Can you estimate the torque needed? DIY torque measurement
     Always select a motor with higher torque than you actually need
  5. How do you plan to mount the motor? (See Chapter 6 - Mounting Motors, in Building Robot Drive Trains)
  6. What kind of space do you have?

Remember Torque = Force x distance (T=Fd)

Motor Types

• DC Motor (brushed, brushless, gearhead)
  • 2 electrical connections
  • High speed, low torque
  • To reverse the direction, reverse the flow of current
  • Torque proportional to current
• DC Brushed
  • Pros
    • Simple, 2 wire drive
    • Inexpensive
    • Common, easy to find
  • Cons
    • The brushes eventually wear out
    • Because the brushes are making/breaking connections, you get sparking and electrical noise
    • The brushes limit the maximum speed of the motor
    • Having the electromagnet in the center of the motor makes it harder to cool
    • The use of brushes puts a limit on how many poles the armature can have
• DC Brushless
  • Pros
    • No brushes to wear out (armature static, permanent magnets rotate)
    • With the electromagnets on the stator, they are very easy to cool
    • Better speed versus torque characteristics
    • No sparking, noiseless operation
    • More precise, high efficiency
    • Long operating life
    • Higher speed ranges
  • Cons
    • Higher initial cost
    • Need to be controlled through software in conjunction with hall effect sensor or other rotary encoder
• DC Gearhead
  • To give DC motors more torque at the expense of speed, gears can be added to the output
  • A gearbox is added on the shaft of a DC motor to achieve different ratios of mechanical advantage
• Servo
  • Servo motor (in general) is a motor that gives feedback about its position (through a potentiometer or other encoder)
  • RC Servo designed mostly for hobby planes and cars
  • Most will have motor, gearbox, feedback device (pot), servo control circuitry, and drive circuit all together
  • 3 wires: power, ground, control
  • The control signal is a pulse where width of the pulse determines the position of the output
  • Cons – normally don’t move 360° (but you can hack it of course - pg 115)
• Stepper motors
  • Pros
    • Combines precise positioning and full range of motion
  • Cons
    • More complex to control than simpler DC motors
    • Slower than DC motors, upper limit to how fast you can pulse them
  • Permanent magnet attached to shaft
  • A series of coils around the body of the motor create magnetic fields when turned on/off
  • Must constantly pulse the motor to keep it moving
  • If just one coil is energized, the motor will stay fixed (referred to as holding/detent torque)
  • Controlled by microcontroller or stepper motor controller
  • Stepper Motor Basics by Solarbotics
• Solenoids
• AC Motors
  • A typical AC motor consists of two parts:
  1. An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and;
  2. An inside rotor attached to the output shaft that is given a torque by the rotating field.
  • Doug's AC motors
  • AC motors
  • Society of Robots: AC motor guide
  • DIY AC motors
  • Cons
    • You have to plug them in! Or create a DC to AC circuit
    • 120V from the wall is enough to SERIOUSLY hurt you
    • AC motors near logic circuits are likely to make them go crazy

Reference:
Micromo motor calcs
Building Robot Drive Trains
Blackboard:

• Physical Computing Chapter 10 Making Movement
• Chapter 3: Generators and Motors in Understanding Electro-Mechanical Engineering (posted as Mechatronics - Motors)

Motor Control

Easiest way: get a motor control module that interfaces with your microcontroller

• Pg 255 of Physical Computing book has examples

DC brushed

• Can use no control at all to run in one direction, or h-bridge if you want to switch
• PWM for speed control

DC brushless

• Need brushless motor controller, or implement one on microcontroller

Servo

• Microcontroller, chips

Stepper

• Stepper motor controller, microcontroller

How to destroy a motor

• Electrical things die because they get too hot, they don't get hot because they die
  • In motors, the wire insulation melts and causes it not to act like wire, but a useless block of copper
• If you run a motor stalled with too much torque applied, it will get hot
• If you are running a system for the 1st time, check the temp of the motor to see if it's getting hot
  • It can be as warm as a lampshade, but not hot like the bulb
• Steppers are different because they have higher resistances. You effectively stall them with each step.

Motor show and tell

It had long since come to my attention that people of accomplishment rarely sat back and let things happen to them. They went out and happened to things.
-Leonardo da Vinci