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Sensors
Material taken from Robotics with the
Boe-Bot
Where Are We Going?
Sumo-Bot competitions
Devices that Contain Sensors
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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.
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.
Not So Simple to Connect
Theory of Operation
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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.
Limited Detection Range
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,
"Devantech SRF04 Demo", CR,
"--------------------", CR,
"Raw........... ", CR,
"Centimeters... ", CR,
"Inches........ "
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
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
Light Sensors
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Light sensors are
also used in a variety
of applications:
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Automatic street
lights
Camera flash and
exposure controls
Security alarms
Introducing the Photoresistor
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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.
Why?
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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.
Basic Circuit
As the photoresistor’s 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
A Better Idea: Measuring Using
Time
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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.
Introducing the Capacitor
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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.
Polled RC Time
In the Polled RC Time
circuit the following
occurs:
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Button is pressed charging
the capacitor.
The button is released, the
BASIC Stamp begins
timing and the capacitor
begins to discharge.
5V
Polled RC Time (continued)
o
The BASIC Stamp continues timing until
input P7 changes to a low (drops below
1.4V).
o
Time is displayed in tenths of seconds.
V => 1.4V
Logic 1
V < 1.4V
Logic 0
Polled RC Time (continued)
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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.
Reading RC-Time with BASIC
Stamp
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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.
Build & Test
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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?
Using A Sample Plot
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This image shows a plot of the light level.
Note how the value increases from left to
right then drops again suddenly. Why?
Object Detection Using IR
The IR Detector
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The IR detector is only looking for infrared
that’s flashing on and off 38,500 times
per second.
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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.
Schematics
Detecting IR
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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 detector’s output in a
variable.
FREQOUT 8, 1, 38500
irDetectLeft = IN9
The IR detector’s 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 detector’s output only stays low for a fraction
of a millisecond after the FREQOUT command is done
sending the harmonic, so it’s essential to store the IR
detector’s output in a variable immediately after
sending the FREQOUT command.
Simple Display Program
IR Detection Range
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Less series resistance will make an
LED glow more brightly.
Brighter IR LEDs can make it
possible to detect objects that are
further away.
OBJECT DETECTION AND
AVOIDANCE
OBJECT DETECTION AND
AVOIDANCE