micromouse lecture #2 sensors, mini project #2 11/6/2014
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MICROMOUSE LECTURE #2SENSORS, MINI PROJECT #2
11/6/2014
WHAT IS A SENSOR?
• Converts outside parameters to voltage signals
0 2 4 6 8 10 12 14 160
1
2
3
4
5
6
7
Reading
Distance (cm)
Vo
ltag
e (
V)
SENSORS IN A MICROMOUSE
Encoders Gryoscope Infrared
GRYOSCOPE
• Outputs angular velocity
Mouse turns
Gryo outputs angular velocity
Code integrates angular velocity
Determine what mouse should do
HOW TO CHOOSE A GRYO
• Range
• Need at least +/- 1000 degrees/second
• Can be analog or digital
• Only need to measure 1 axis
INFRARED SENSORS
• Infrared LEDs emit light in wavelengths from 870nm to 950nm
• Receivers (phototransistors) sensitive to IR light
Emitter emits light
Light bounces off wall
Receiver measures reflected light
Determine wall presence
HOW TO CHOOSE AN IR RECEIVER/EMITTER
Emission Angle
Power Density
LED EMITTER
• Make sure to choose a narrow emission angle
• Maximize power efficiency
• Maximize signal amplitude
• Power Density
• Measures solid angle
• Often measured in mW/sr (miliwatt/steradians)
LED RECEIVER
• Make sure to match the receiver with emitter wavelength
• Make sure to choose small angles=high directivity
• Reduces interference
• Emitter/receiver pairs generally sold together
Note that max sensitivity at 940nm
Note high directivity
CALIBRATION• Mostly trial and error
• Vary distances and measure voltage with MCU
• IR calibration is non-linear
0 1 2 3 4 5 6 70
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
Distance vs. Voltage
Series1
Distance
Volt
age
THE SENSORS YOU WILL BE USING
• IR Emitter: an IR Light Emitting Diode
• IR Receiver: An NPN phototransistor
• Amplifies current going into the Baseand sends it out the Emitter
• An overly-simplified analogy:think of it as a dimmer switch
IR SENSOR SCHEMATIC
IR SENSOR SCHEMATIC
• Emitter: you can drive the LED directly from the MCU
• Make sure you use a series resistor so you don’t burn out the LED!
• Orientation matters! Anode goes to power, cathode goes towards ground
• Receiver: need to use a pull down resistor at the Emitter
• BJTs work with current, but MCU reads voltage.
• Resister allows current to pass and have a readable voltage
AMBIENT LIGHT
• Ambient light will definitely confuse your mouse
• Might seem like there is a wall when there isn’t
• How to deal with softer ambient light:
• Turn IR emitter off and read from receiver
• Turn IR emitter on and read from receiver
• Subtract the first reading from the second
• Won’t work if light is too bright and saturates receiver
IR SENSORS SAMPLE PROGRAM
JEREMY HAUGENGUEST LECTURE ON SENSORS
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