Sensors

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Sensors

• Material taken from Robotics with the Boe-Bot

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Where Are We Going?
Sumo-Bot competitions
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Devices that Contain Sensors

• The boebot uses sensors to interact with its
  environment.
• There are a variety of sensors used for a variety
  of purposes Pressure, rotation, temperature,
  smoke, tilt, vibration, light, proximity and so
  on.

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Ultrasonic Proximity Sensor

• Devantech Sonic Range Finder

• The ultrasonic distance sensor provides precise
  distance measurements from about 3 cm to 3
  meters.
• It works by transmitting an ultrasonic burst and
  providing an output pulse that corresponds to the
  time required for the burst echo to return to the
  sensor.
• By measuring the echo pulse width the distance to
  target can easily be calculated.

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Not So Simple to Connect
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Theory of Operation

• The sensor emits a short ultrasonic burst and
  then "listens" for the echo.
• Under control of a host microcontroller (trigger
  input), the sensor emits a short 40 kHz
  (ultrasonic) burst.
• This burst travels through the air at about 1.125
  feet per millisecond, hits an object and then
  bounces back to the sensor.
• The sensor provides an output pulse to the host
  that will terminate when the echo is detected,
  hence the width of this pulse corresponds to the
  distance to the target.

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Limited Detection Range
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Basic Program Initialization

• ' ----- I/O Definitions ------------------------
  --------------------
• Trigger PIN 0
• Echo PIN 1
• ' ----- Constants ------------------------------
  --------------------
• Trig10 CON 5 ' trigger pulse 10 uS
• ToCm CON 30 ' conversion factor to cm
• ' ----- Variables ------------------------------
  --------------------
• samples VAR Nib ' loop counter
• pWidth VAR Word ' pulse width from sensor
• rawDist VAR Word ' filtered measurment
• cm VAR Word ' centimeters
• inches VAR Word
• ' ----- Initialization -------------------------
  --------------------
• Setup
• LOW Trigger
• DEBUG CLS,

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Main Routine

• ' ----- Program Code ---------------------------
  --------------------
• Main
• DO
• GOSUB Get_Sonar ' take sonar
  reading
• DEBUG CRSRXY, 15, 2, DEC rawDist, CLREOL
• cm rawDist / ToCm ' convert to
  centimeters
• DEBUG CRSRXY, 15, 3, DEC cm, CLREOL
• inches cm / 03EF ' x 3.937 (to 0.1
  inches)
• DEBUG CRSRXY, 15, 4,
• DEC inches / 10, ".", DEC1 inches,
• CLREOL
• PAUSE 250 ' delay
  between readings
• LOOP
• END

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Get the Sonar Reading

• ' ----- Subroutines ----------------------------
  --------------------
• Get_Sonar
• rawDist 0 '
  clear measurement
• FOR samples 1 TO 5 ' take
  five samples
• PULSOUT Trigger, Trig10 ' 10 uS
  trigger pulse
• PULSIN Echo, 1, pWidth ' measure
  pulse
• rawDist rawDist (pWidth / 5) ' simple
  digital filter
• PAUSE 10 '
  minimum period between
• NEXT
• RETURN

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Light Sensors

• Light sensors are also used in a variety of
  applications
• Automatic street lights
• Camera flash and exposure controls
• Security alarms

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Introducing the Photoresistor

• While there are a variety of light sensors, a
  very popular one is the photoresistor in that it
  is easy to use and inexpensive.
• As the name implies, it is a resistor that reacts
  to light. The active ingredient Cadmium Sulfide
  (CdS) allows electrons to flow more easily when
  light energy hits it, thus lowering it resistance
  (opposition to current flow).
• The brighter the light the lower the resistance.

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Why?

• A photoresistor is made of a high resistance
  semiconductor.
• The photons of high frequency light are absorbed
  by the semiconductor giving bound electrons
  enough energy to jump into the conduction band.
  The resulting free electrons (and their hole
  partners) conduct electricity thereby lowering
  resistance.

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Basic Circuit
As the photoresistors resistance changes with
light exposure, so does the voltage at Vo. As R
gets larger, Vo gets smaller, and as R gets
smaller, Vo gets larger. Vo is what the BASIC
Stamp I/O pin is detecting when it is functioning
as an input. If this circuit is connected to
IN6, when the voltage at Vo is above 1.4 V, IN6
will store a 1. If Vo falls below 1.4 V, IN6 will
store a 0.
R
V0
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A Better Idea Measuring Using Time

• The photoresistor can also be used with the BASIC
  Stamp in an RC circuit (Resistor-Capacitor
  circuit) to obtain a value in relation to the
  amount of resistance, or, in this case, the
  amount of light hitting the sensor.
• In a RC-network, the capacitor is charged and
  discharged at different rates determined by the
  resistor and the capacitor sizes.

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Introducing the Capacitor

• The capacitor is a device which can store an
  electron charge. Its size is expressed typically
  in microfarads (?F) or millionths of Farads.
• Certain types of capacitors are polarity
  sensitive, that is, they can only be connected in
  one direction.

• Connecting a polarity sensitive capacitor
  backwards can cause the device to explode.
• Wear safety glasses.
• Ensure proper polarity when connecting.

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Polled RC Time

• In the Polled RC Time circuit the following
  occurs
• Button is pressed charging the capacitor.
• The button is released, the BASIC Stamp begins
  timing and the capacitor begins to discharge.

5V
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Polled RC Time (continued)

• The BASIC Stamp continues timing until input P7
  changes to a low (drops below 1.4V).
• Time is displayed in tenths of seconds.

V gt 1.4VLogic 1
V lt 1.4VLogic 0
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Polled RC Time (continued)

• The time to discharge the capacitor is in
  proportion to the size of the resistor and
  capacitor network (RC).
• The larger the capacitance (C), the greater the
  charge it can hold, increasing time.
• The larger the resistance (R), the slower the
  capacitor will discharge, increasing time.

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Reading RC-Time with BASIC Stamp

• The BASIC Stamp has an instruction to perform
  much of the timing operation automatically
• RCTIME Pin, State, Variable
• Where
• Pin is the pin the RC network is connected.
• State is the initial state when timing begins.
• Variable is the memory location to store the
  results. Just like PULSOUT the time is the
  number of 2uS increments.

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Build Test

• The RC Time circuits is configured so that the
  capacitor is charged by the output (P2 in this
  case) and the time to discharge though the
  resistor is measured.
• In this case, as light level changes, discharge
  time will change.

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• What happens to the value of time as the light
  level changes? When is it lowest? Highest?

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Using A Sample Plot

• This image shows a plot of the light level. Note
  how the value increases from left to right then
  drops again suddenly. Why?

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Object Detection Using IR
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The IR Detector

• The IR detector is only looking for infrared
  thats flashing on and off 38,500 times per
  second.
• It has built-in optical filters that allow very
  little light except the 980 nm infrared.
• It also has an electronic filter that only allows
  signals around 38.5 kHz to pass through.
• This prevents IR interference from common sources
  such as sunlight and indoor lighting.

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Schematics
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Detecting IR

• The key to making each IR LED/detector pair work
  is to send 1 ms of 38.5 kHz FREQOUT harmonic, and
  then, immediately store the IR detectors output
  in a variable.
• FREQOUT 8, 1, 38500
• irDetectLeft IN9
• The IR detectors output state when it sees no IR
  signal is high. When the IR detector sees the
  38500 Hz harmonic reflected by an object, its
  output is low.
• The IR detectors output only stays low for a
  fraction of a millisecond after the FREQOUT
  command is done sending the harmonic, so its
  essential to store the IR detectors output in a
  variable immediately after sending the FREQOUT
  command.

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Simple Display Program
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IR Detection Range

• Less series resistance will make an LED glow more
  brightly.
• Brighter IR LEDs can make it possible to detect
  objects that are further away.

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OBJECT DETECTION AND AVOIDANCE
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OBJECT DETECTION AND AVOIDANCE