Lecture 4 Sensor 2

8/14/2019 Lecture 4 Sensor 2

1/31

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


2/31

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


3/31

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


4/31

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


5/31

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,


6/316

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


7/317

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


8/318

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


9/319

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:


10/3110

Other Odometry Sensors

Potentiometer

= varying resistance


11/3111

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


12/3112

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


13/31


14/31

14

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


15/31


16/31

16

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


17/31

17

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)


18/31

18

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,


19/31


20/31

20

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


21/31

21

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


22/31

22

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


23/31

23

Noise Issues


24/31

24

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/


25/31

25

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


26/31

26

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


27/31

27

Global Positioning System (GPS) (2)

4.1.5

4 1 5


28/31

28

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


29/31

29

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


30/31

30

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.


31/31

31

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.)

Documents

Lecture 4 Sensor 2