Lectures 7,8,9

• how do servomotors work

  can be positioned from 0-180 degrees.

  easy three wire PWM, 5V interface.
• what are the different ways to control a servo?

  by pulse: write a pulse of desired length to the motor in microseconds

  by angle: value between 0-180 degrees, followed by a non-digit char
• what are the three main methods in the arduino servo library?

  attach(): attaches the servo variable to a certain pin

  write(): writes a value to the servo, controlling the shaft accordingly

  read(): reads current angle of the servo (last value passed with write())
• how to setup a port for serial communication in Processing


  import processing.serial.*;

  Serial port;

  void setup() {

  
  

  // list all available serial ports

  println(Serial.list());

  
  

  // once you know which port is yours, use that one.

  port = new Serial(this, Serial.list()[0],9600);

  
  

  }
• what are the different factors to consider when sensing movement?

  position: where is the object?

  orientation: how is the object rotated around each axis?

  velocity: how fast is the object? in which way is it moving?

  absolute vs relative position: which one do we need to know?

  identity: do we need to discern between multiple objects?
• what do ranging sensors do? and what is their range?

  they sense distance of a body from the sensor

  they sense movement in front of the sensor

  most sensors have range under 9ft
• what could you do to make the sensing of of movement more accurate, without applying more complex technology?

  apply constraints to the environment
• what is the operating principle of ranging sensors (how do they work)?

  work by sending out a reference signal:

  • light
  • magnetism
  • sound

  then measure the amount of energy that reflects off the target and compare with the energy that went out.
• what are the characteristics of a sensor that detects presence? (motion detector)


  binary output (present or not)

  simple digital sensor

  uses switches, either photoelectric or magnetic
• how does a photoelectric switch in a motion detector (presence detector) work?

  a light beam hits a target sensor.

  when the beam is broken by something between sensor and light source, switch is activated.
• how does a magnetic switch in a motion detector (presence detector) work?

  very thin pair of contacts in a protective housing

  when they are exposed to a magnet, they are drawn together, closing switch.
• how does an IR sensor for position work? and what is its range

  they send out an infrared beam and rea the reflection of the beam off a target.

  they can range anywhere from 1.5 inch to 56 inch
• how does an ultrasonic sensor for position work? and what is its range

  sends out a ping of ultrasonic sound, times how long it takes to bounce back.

  ranges anywhere from 6 inch to 34 feet
• do distance ranging sensors operate uniformly at all distances?

  no, they have zones in which they are most sensitive (cones usually)
• can you use a distance ranging sensor to find location over a large 2D area?

  its not great for large areas.

  need to use several sensors to cover a room. but no two sensors can operate at the same instant, this would cause interference. must switch constantly.
• how does video tracking work?

  dense array of thousands of photocells.

  lense that scales the range of lights we are interested in.

  can adjust lighting conditions.

  ability to separate incoming light by colour.

  works off pixel-by-pixel manipulation by a multimedia computer (rather than frame-by-frame)
• why is video tracking difficult?

  computer vision.

  cameras/computers do not automatically adjust for changing conditions like lighting differences or shadows.

  object recognition.
• what is the challenge with determining identity (discerning one object from another) with a sensor?

  must track more than one object: where each object is, which object is where.

  but many sensors do not support multiple targets, IR and ultrasonic sensors work by bouncing off the closest target.
• what are some identification technologies?

  barcode scanners.

  radio frequency identification (RFID).

  magnetic swipe card readers.
• what is trilateration


  the use of three ranging sensors to determine precise position.

  with one sensor there is an arc that the position could be in.

  with two sensors there are two possible points for the location.

  with three sensors you will know the precise position of the object.
• what are the components of physical identity?

  presence: is it near me?

  address: where is it?

  identification: what is it?
• what techniques to computers use to identify objects?


  optical recognition

  radio frequency identification (RFID)
• what forms can optical recognition take?

  video color tracking

  shape recognition

  barcode
• why is recognizing the shape of a physical object more challenging compared to recognizing its colour? 


  2D geometry must be known from every angle.

  we must be able to compare any view we get from the object.

  the computer simply compares patterns, it does not understand the object as a discrete entity
• what is a barcode and how is it scanned?

  a barcode is a pattern of dark and light lines or cells used to encode an alphanumeric string

  the computer scans the lines or cells as 0s or 1s.

  scanning is done by camera or a photodiode.
• what are the two different types of barcodes

  1D barcode: traditional barcode, scanner only needs to read the image along one axis

  2D barcode: QR code (Quick Response code), 2D matrix so more information density.
• what are the drawbacks of barcodes?

  distortion from analog to digital conversion through the camera causes many errors.

  line of sight.

  barcode has to be centered, otherwise recognition wont work due to distortion at edges.
• how does radio frequency identifiation (RFID) work?

  tags objects in order to identify them.

  tags do not need to be visible to be read.

  RFID reader sends out short-range radio signal, which will be picked up by an RFID tag.

  tag transmits back a short string of data.
• how does a passive RFID system work?

  tags contain an integrated circuit that has a basic radio transceiver and small amount of nonvolatile memory.

  powered by the current that the reader's signal induces in their antennas.

  received energy is just enough to power the tag to transmit its data once.
• how does an active RFID system work?

  tag has its own power supply and radio receiver.

  transmits a signal in response to a received message from reader.

  these have longer range than passive system, but are more expensive.
• what to consider when choosing an RFID reader?

  the environment it is going to be used: is there lots of RF noise? what ranges of frequencies? How long is the range that you need to read.

  not all readers read all tags.
• what do force-sensitive resistors do? 


  convert mechanical force into electrical resistance.

  these are generally small and flat.
• what are flex sensors?

  they look and work like force-sensitive resistors.

  vary resistance based on how much they are bent.

  uses flat plastic strip that can bend up to 180 degrees.
• what do pressure sensors do

  measure the pressure exerted by a gas or fluid.
• how do capacitance sensors work


  reads the small electric charge that is always stored in the human body.
• how can you measure touch using heat?

  you can use a thermistor.

  if someone is touching a thermistor, there will be an increase in heat which can be sensed.