lecture 4 sensor 2
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Intensity Based Infrared
Easy to implement (few components) Works very well in controlled environments
Sensitive to ambient light
time
voltage
time
voltage
Increase in ambient light
raises DC bias
Break-Beam sensor
Reflective Sensor
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IR Reflective Sensors Reflective Sensor:
Emitter IR LED + detector photodiode/phototransistor
Phototransistor: the more light reaching the phototransistor, themore current passes through it
A beam of light is reflected off a surface and into a detector Light usually in infrared spectrum, IR light is invisible
Applications: Object detection,
Line following, Wall tracking
Optical encoder (Break-Beam sensor)
Drawbacks: Susceptible to ambient lighting
Provide sheath to insulate the device from outside lighting
Susceptible to reflectivity of objects Susceptible to the distance between sensor and the object
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Modulated Infrared Modulation and Demodulation
Flashing a light source at a particular frequency
Demodulator is tuned to the specific frequency of light flashes.
(32kHz~45kHz) Flashes of light can be detected even if they are very week
Less susceptible to ambient lighting and reflectivity of objects
Used in most IR remote control units, proximity sensors
Negative true logic:
Detect = 0v
No detect = 5v
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IR Distance Sensors Basic principle of operation:
IR emitter + focusing lens + position-sensitive detector
Location of the spot on the detector corresponds to
the distance to the target surface, Optics to covert
horizontal distance to vertical distance
Modulated IR light
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IR Distance Sensors - Example Sharp GP2D02 IR Ranger
Distance range: 10cm (4") ~ 80cm (30").
Moderately reliable for distance measurement Immune to ambient light
Impervious to color and reflectivity of object
Applications: distance measurement, wallfollowing,
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Basic Navigation Techniques Relative Positioning (called Dead-reckoning)
Information required: incremental (internal) Velocity
heading With this technique the position can be updated withrespect to a starting point
Problems: unbounded accumulation error
Absolute Positioning Information Required: absolute (external)
Absolute references (wall, corner, landmark)
Methods Magnetic Compasses (absolute heading, earths magnetic field)
Active Beacons
Global Positioning Systems (GPS)
Landmark Navigation (absolute references: wall, corner, artificiallandmark)
Map-based positioning
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Sensors Used in Navigation
Dead Reckoning
Odometry (monitoring thewheel revolution to compute theoffset from a known startingposition)
Encoders,
Potentiometer,
Tachometer,
Inertial Sensors (measurethe second derivative of position)
Gyroscopes,
Accelerometer,
External Sensors Compass
Ultrasonic
Laser range sensors
Radar
Global PositioningSystem (GPS)
Vision
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Dead ReckoningCause of unbounded accumulation error:
Systematic Errors:
a) Unequal wheel diametersb) Average of both wheel diameters
differs from nominal diameter
c) Misalignment of wheels
d) Limited encoder resolution,sampling rate,
Nonsystematic Errors:a) Travel over uneven floors
b) Travel over unexpected objects onthe floor
c) Wheel-slippage due to : slippery
floors; over-acceleration, fast turning
(skidding), non-point wheel contactwith the floor
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Incremental Optical Encoders
- direction
- resolution
grating
light emitter
light sensor
decodecircuitry
A
B A leads B
Incremental Encoder:
It generates pulses proportional to the rotation speed of the shaft. Direction can also be indicated with a two phase encoder:
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Other Odometry Sensors
Potentiometer
= varying resistance
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Inertial Sensors
Gyroscopes
Heading sensors, that keep the orientation to a fixed frame
absolute measure for the heading of a mobile system.
Two categories, the mechanical and the optical gyroscopes
Mechanical Gyroscopes
Standard gyro
Rated gyro
Optical Gyroscopes Rated gyro
Accelerometers Measure accelerations with respect to an inertial frame
Common applications: Tilt sensor in static applications, Vibration Analysis, Full INS Systems
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Mechanical Gyroscopes
Concept: inertial properties of a fast spinning rotor
gyroscopic precession
Angular momentum associated with a spinning wheel keeps the axis of thegyroscope inertially stable.
Reactive torque t (tracking stability) is proportional to the spinning speed w,the precession speed W and the wheels inertia I.
No torque can be transmitted from the outer pivot to the wheel axis
spinning axis will therefore be space-stable
Quality: 0.1 in 6 hours
If the spinning axis is aligned with thenorth-south meridian, the earths rotationhas no effect on the gyros horizontal axis
If it points east-west, the horizontal axisreads the earth rotation
WI=
4.1.4
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Applications of Gyroscopes
Gyroscopes can be very perplexing objectsbecause they move in peculiar ways and evenseem to defy gravity. A bicycle
an advanced navigation system on the space shuttle
a typical airplane uses about a dozen gyroscopes ineverything from its compass to its autopilot.
the Russian Mir space station used 11 gyroscopes to
keep its orientation to the sun the Hubble Space Telescope has a batch of
navigational gyros as well
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Accelerometer
Main elements of an accelerometer:
1. Mass 2. Suspension mechanism 3. Sensing element
High quality accelerometers include a servo loop to improve thelinearity of the sensor.
kxdt
dxctd
xdmF ++=2
2
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Range Finder
Time of Flight
The measured pulses typically come from ultrasonic, RF
and optical energy sources. D = v * t
D = round-trip distance
v = speed of wave propagation t = elapsed time
Sound = 0.3 meters/msec
RF/light = 0.3 meters / ns (Very difficult to measure short
distances 1-100 meters)
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Ultrasonic Sensors Basic principle of operation:
Emit a quick burst of ultrasound (50kHz), (human hearing:
20Hz to 20kHz)
Measure the elapsed time until the receiver indicates that anecho is detected.
Determine how far away the nearest object is from the sensor
D = v * t
D = round-trip distance
v = speed of propagation(340 m/s)
t = elapsed time
Bat, dolphin,
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Polaroid Ultrasonic Sensors
Ultrasonic
transducer
Electronic boardTransducer Ringing:
transmitter + receiver @ 50 KHz Residual vibrations or ringing may
be interpreted as the echo signal
Blanking signal to block any return
signals for the first 2.38ms aftertransmission
http://www.acroname.com/robotics/info/articles/sonar/sonar.html
It was developed for an automatic
camera focusing system
Range: 6 inches to 35 feet
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Operation with Polaroid Ultrasonic The Electronic board supplied has the following I/0
INIT : trigger the sensor, ( 16 pulses are transmitted ) BLANKING : goes high to avoid detection of own signal
ECHO : echo was detected. BINH : goes high to end the blanking (reduce blanking time < 2.38ms)
BLNK : to be generated if multiple echo is required
t
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Ultrasonic Sensors Applications:
Distance Measurement
Mapping: Rotating proximity scans (maps theproximity of objects surrounding the robot)
Scanning at an angle of 15 apart can achieve best results
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Noise Issues
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Laser Ranger Finder
Range 2-500 meters
Resolution : 10 mm
Field of view : 100 - 180 degrees
Angular resolution : 0.25 degrees
Scan time : 13 - 40 msec.
These lasers are more immune to Dust and Fog
http://www.sick.de/de/products/categories/safety/
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Ground-Based Beacons Elegant way to solve the localization problem in mobile robotics
Beacons are signaling guiding devices with a precisely known position
Beacon base navigation is used since the humans started to travel
Natural beacons (landmarks) like stars, mountains or the sun Artificial beacons like lighthouses
The recently introduced Global Positioning System (GPS) revolutionized
modern navigation technology
Already one of the key sensors for outdoor mobile robotics For indoor robots GPS is not applicable,
Major drawback with the use of beacons in indoor:
Beacons require changes in the environment
-> costly. Limit flexibility and adaptability to changing
environments.
4 1 5
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Global Positioning System (GPS) (1)
Developed for military use
Recently it became accessible forcommercial applications
24 satellites (including three spares)orbiting the earth every 12 hours at aheight of 20.190 km.
Location of any GPS receiver isdetermined through a time of flight
measurement By combining information regarding the
arrival time and instantaneous locationof four satellites, the receiver can inferits own location
4.1.5
Space Segment
Technical challenges:
Time synchronization between the individual satellites and the GPS receiver
Real time update of the exact location of the satellites
Precise measurement of the time of flight
Interferences with other signals
4 1 5
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Global Positioning System (GPS) (2)
4.1.5
4 1 5
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Global Positioning System (GPS) (3) Time synchronization:
atomic clocks on each satellite
monitoring them from different ground stations.
Ultra-precision time synchronization is extremely important
electromagnetic radiation propagates at light speed, Roughly 0.3 m per nanosecond.
position accuracy proportional to precision of time measurement.
Real time update of the exact location of the satellites:
monitoring the satellites from a number of widely distributed ground stations
master station analyses all the measurements and transmits the actual position toeach of the satellites
Exact measurement of the time of flight
the receiver correlates a pseudocode with the same code coming from thesatellite
The delay time for best correlation represents the time of flight.
quartz clock on the GPS receivers are not very precise
the range measurement with four satellite
allows to identify the three values (x, y, z) for the position and the clock correction
T Recent commercial GPS receiver devices allows position accuracies down to a couple
meters.
4.1.5
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Noise Issues
Real sensors are noisy
Origins: natural phenomena + less-than-ideal
engineering Consequences: limited accuracy and precision
of measurements
Filtering:
software: averaging, signal processing algorithm
hardware tricky: capacitor
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Papers to Read
J. Borenstein, H. R. Everett, L. Feng, and D. Wehe,
Mobile Robot Positioing Sensors and Techniques,
Invited paper for the Journal of Robotic Systems,
Special Issue on Mobile Robots, Vol. 14, No.4, pp.
231-249. (You can download this paper from course website onSensing and perception)
This paper defines seven categories for positioning systems: 1.
odometry, 2. inertial navigation, 3. magnetic compasses, 4. active
beacons, 5. global positioning systems, 6. landmark navigation, and 7.
model matching. The characteristics of each category are discussed
and examples of existing technologies are given for each category.
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Papers to ReadJ. Borenstein and Liqiang Feng, Measurement of
Correction of Systematic Odometry Errors in Mobile
Robots, IEEE Trans. on Robotics and Automation,
Vol.12, No.6, December 1996.
(You can download this paper from course website on
sensing and perception.) G. Campion, G. Bastin, and B. DAndrea-Novel, Structural
Properties and Classification of Kinematic and Dynamic Models of
Wheeled Mobile Robots, IEEE Trans. on Robotics and Automation,
Vol. 12, No.1, February 1996.
(You can download this paper form course website on kinematics.)
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