Attendance

Mouse trap car races!
- Lessons learned

Force, Friction, Torque

Force, mass, wine, and elevators
Measuring Force

• Mechanical - scales, spring scales
• Electrical - scales, FSRs, Flexiforce pressure sensors, luggage/fish scales

Torque and embarrassing doors
SOHCAHTOA and soup cans, Torque = Force * Distance
Measuring Torque

• Torque wrenches, watches generally $100+, so it's easier to measure force and use T=F*d

Power, Work, Energy

Cool thing of the week: inDOOR energy harvester

Work - we talked about this with simple machines: Work = Force * distance, also Work = Torque * rotation (in radians)
Energy is the capacity to do work. Types:

• Potential energy: of a mass = mass*gravity*height, of a spring = 1/2 * k * x^2, of a torsional spring = 1/2 * k * angle^2
• Kinetic energy: 1/2 * mass * velocity^2

Power is the rate the rate that work is performed or that energy is used

• Mechanical power: Power = Torque * rotational velocity (rotational) OR Power = Force * velocity (translational)
• Electrical power: Power = Current * Voltage

All things that move need some source of energy. Energy can't be created or destroyed - it just changes form (1st law of thermodynamics). Transduction is the conversion of one form of energy to another, and anything that does this is called a transducer.

Powering your projects

• Benchtop Supply
• Portable Options
  • Batteries - what different sizes mean
• Plug-In Options
  • Computer power supplies
  • Power converters/AC adaptors/DC power supplies
• Alternative Energy Sources
  • Solar - direct and charge controllers. See SADbot pics, check here for ITP's resources and check out Jeff Feddersen's Sustainable Energy class
  • Wind, water, and other fluids
  • Food - anything acidic (tomatoes, lemons...)
  • Humans! - hand cranked flashlights, wind up toys
• Springs and elastic energy storage

Motors

• How do we get motion from current?

Current flowing through a wire induces a magnetic field around the wire. Motors exploit this fact by mounting coils of wire to an axis next to a magnet that makes it turn.
Project 6-1: DIY motor

• Driving parameters for choosing a motor:
  1. Do you need speed or torque, or both?
     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? Always select a motor with higher torque than you actually need.
  5. How do you plan to mount the motor?
  6. What kind of space do you have?

Motor Types

• DC Motor (brushed, 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 can get sparking and electrical noise
    • The brushes limit the maximum speed of the motor
• 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 is a motor that gives feedback about its position (through a potentiometer or other encoder)
  • RC Servo designed originally for hobby models
  • Most will have motor, gearbox, feedback device (pot), control and drive circuitry 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). You can also find an Instructable here.
• 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 can be push or pull, continuous or intermittent
• 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
  • Society of Robots: AC motor guide
  • DIY AC motors
  • Cons
    • You have to plug them in! Or create a DC to AC circuit
    • 120V and up to 15A from the wall is enough to SERIOUSLY hurt you
    • AC motors near logic circuits are likely to make them go crazy

Motor Control

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

• Pg 255 of Physical Computing book has examples

DC motor

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

Servo

• Arduino - pulse or use servo library

Stepper

• Stepper motor controller module (like Sparkfun's EasyDriver - part #ROB-09402), Arduino with Stepper Library

Motor example

Example: How to get the RPM you want out of a motor

• Choose motor based on speed
• Peter's midterm project: gearing up to increase torque (which decreases speed)
• Buy or create a speed controller

• DC toy motor and Gearhead motor
• Buy gears, pulleys, etc. that have hubs with set screws or clamps
• A flanged shaft collar can help you mount hub-less components
• Also hubs, adaptors and shafts

mini-Midterm projects: in-progress discussion