Mouse trap car races!
- Lessons learned
Force, Friction, Torque
Force, mass, wine, and elevators
- Mechanical - scales, spring scales
- Electrical - scales, FSRs, Flexiforce pressure sensors, luggage/fish scales
Torque and embarrassing doors
SOHCAHTOA and soup cans, Torque = Force * Distance
- 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
- 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:
- Do you need speed or torque, or both?
Torque @ max efficiency is the torque the motor works best at.
- Do you want position feedback?
- What are you trying to move/spin?
- Can you estimate the torque needed? Always select a motor with higher torque than you actually need.
- How do you plan to mount the motor?
- What kind of space do you have?
- 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
- Simple, 2 wire drive
- Common, easy to find
- 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 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
- Combines precise positioning and full range of motion
- 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:
- An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and;
- An inside rotor attached to the output shaft that is given a torque by the rotating field.
Easiest way: get a motor control module that interfaces with your microcontroller
- Pg 255 of Physical Computing book has examples
- Can use no control at all to run in one direction, or an h-bridge if you want to switch
- PWM for speed control
Example: How to get the RPM you want out of a motor
mini-Midterm projects: in-progress discussion