Autonomous Mobile Robots, Chapter 4 4

Perception
 Sensors
 Uncertainty

 Features

Localization "Position" Cognition

Global Map

Environment Model Path

Local Map

Perception Real World Motion Control

Environment

© R. Siegwart, I. Nourbakhsh
 4
4a
3 Sensors for Mobile Robots

 Why should a robotics engineer know about sensors?

 They are the key components for perceiving the environment
 Understanding the physical principles enables appropriate use

 Understanding the physical principle behind sensors enables us:

 To properly select the sensors for a given application
 To properly model the sensor system, e.g. resolution, bandwidth,
uncertainties


 Classification of Sensors
 What:
 Proprioceptive sensors
• measure values internally to the system (robot),
• e.g. motor speed, wheel load, heading of the robot, battery status
 Exteroceptive sensors
• information from the robots environment
• distances to objects, intensity of the ambient light, unique features.
 How:
 Passive sensors
• Measure energy coming from the environment
 Active sensors
• emit their proper energy and measure the reaction
• better performance, but some influence on environment
 Characterizing Sensor Performance (2)
 Basic sensor response ratings (cont.)

 Range
• upper limit - lower limit

 Resolution
• minimum difference between two values
• usually: lower limit of dynamic range = resolution
• for digital sensors it is usually the A/D resolution.
• e.g. 5V / 255 (8 bit)

 Linearity
• variation of output signal as function of the input signal
• linearity is less important when signal is treated with a computer
x  f ( x)   x    y  f (  x    y)    f ( x)    f ( y)
y  f ( y)
 Characterizing Sensor Performance (3)
 Basic sensor response ratings (cont.)

 Bandwidth or Frequency
• the speed with which a sensor can provide a stream of readings
• usually there is an upper limit depending on the sensor and the sampling rate
• lower limit is also possible, e.g. acceleration sensor
• one has also to consider phase (delay) of the signal
 In Situ Sensor Performance (1)
Characteristics that are especially relevant for real world environments

 Sensitivity
 ratio of output change to input change
dy
dx
 however, in real world environment, the sensor has very often high sensitivity to
other environmental changes, e.g. illumination

 Cross-sensitivity (and cross-talk)

 sensitivity to other environmental parameters (e.g. temperature, magnetic field)
 influence of other active sensors

 Error / Accuracy
 difference between the sensor’s output and the true value
error
m = measured value
v = true value
 In Situ Sensor Performance (2)
Characteristics that are especially relevant for real world environments

 Systematic error -> deterministic errors

 caused by factors that can (in theory) be modeled -> prediction
 e.g. calibration of a laser sensor or of the distortion caused by the optics of a
camera

 Random error -> non-deterministic

 no prediction possible with given sensors
 however, they can be described probabilistically

 Precision
 reproducibility of sensor results:
 Encoders
4a
 Definition:
32
 electro-mechanical device that converts linear or angular position of a
shaft to an analog or digital signal, making it a linear/anglular transducer
 Wheel / Motor Encoders
 Use cases
 measure position or speed of the wheels or steering
 integrate wheel movements to get an estimate of the position -> odometry
 optical encoders are proprioceptive sensors
 typical resolutions: 64 - 2048 increments per revolution.
 for high resolution: interpolation

 Working principle of optical encoders

 regular: counts the number of transitions but cannot tell the direction of motion
 quadrature: uses two sensors in quadrature-phase shift. The ordering of which wave
produces a rising edge first tells the direction of motion. Additionally, resolution is 4 times
bigger
 a single slot in the outer track generates a reference pulse per revolution
 Heading Sensors
 Definition:
 Heading sensors are sensors that determine the robot’s orientation and
inclination.

 Heading sensors can be proprioceptive (gyroscope, accelerometer) or

exteroceptive (compass, inclinometer).
 Allows, together with an appropriate velocity information, to integrate the
movement to a position estimate.
 This procedure is called deduced reckoning (ship navigation)
 Gyroscope
 Definition:
 Heading sensors that preserve their orientation in relation to a fixed reference
frame
 They provide an absolute measure for the heading of a mobile system.
 Two categories, the mechanical and the optical gyroscopes
 Mechanical Gyroscopes
• Standard gyro (angle)
• Rate gyro (speed)
 Optical Gyroscopes
• Rate gyro (speed)
 Mechanical Gyroscopes
 Concept:
 Inertial properties of a fast spinning rotor
 Angular momentum associated with a spinning wheel keeps the axis of the gyroscope
inertially stable.
 No torque can be transmitted from the outer pivot to the wheel axis
 spinning axis will therefore be space-stable
 however friction in the axes bearings will introduce torque and so drift ->precession
 Quality: 0.1° in 6 hours (a high quality mech. gyro costs up to 100,000 $)
 Rate gyros
 Same basic arrangement shown as regular mechanical gyros

 But: gimbals are restrained by torsional springs

 enables to measure angular speeds instead of the orientation.
 Optical Gyroscopes
 Optical gyroscopes
 angular speed (heading) sensors
using two monochromic light (or laser) beams
from the same source.
 One is traveling in a fiber clockwise,
the other counterclockwise around a cylinder
 Laser beam traveling in direction opposite to
the rotation
 slightly shorter path
 phase shift of the two beams is proportional to
the angular velocity  of the cylinder
 In order to measure the phase shift, coil
consists of as much as 5Km optical fiber
 New solid-state optical gyroscopes based on
the same principle are build using
microfabrication technology.

3-axis
© R. Siegwart & D. Scaramuzza, ETHoptical
Zurichgyro
- ASL
 Mechanical Accelerometer
 Accelerometers measure all external
forces acting upon them, including
gravity
 accelerometer acts like a spring–
mass–damper system

Where m is the proof mass, c the

damping coefficient, k the spring
constant
 at steady-state:
4a
42
 Mechanical Accelerometer
 On the Earth's surface, the
accelerometer always indicates 1g
along the vertical axis
 To obtain the inertial acceleration (due
to motion alone), the gravity must be
subtracted. Conversely, the device's
output will be zero during free fall
 Bandwidth up to 50 KHz
 An accelerometer measures
acceleration only along a single axis.
By mounting three accelerometers
orthogonally to one another, a three-
axis accelerometer can be obtained
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 Inertial Measurement Unit (IMU)
 Definition
 An inertial measurement unit (IMU) is a device that uses measurement systems such
as gyroscopes and accelerometers to estimate the relative position (x, y, z), orientation
(roll, pitch, yaw), velocity, and acceleration of a moving vehicle.

 In order to estimate motion, the gravity vector must be subtracted. Furthermore,

initial velocity has to be known.

 IMUs are extremely sensitive to measurement errors in gyroscopes and

accelerometers: drift in the gyroscope unavoidably undermines the estimation of
the vehicle orientation relative to gravity, which results in incorrect cancellation of
the gravity vector. Additionally observe that, because the accelerometer data is
integrated twice to obtain the position, any residual gravity vector results in a
quadratic error in position.

 After long period of operation, all IMUs drift. To cancel it, some external reference
like GPS or cameras has to be used.
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4a
50 Ground-Based Active and Passive 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.
 Global Positioning System (GPS) (1)
 Facts
 Recently it became accessible for commercial applications (1995)
 24+ satellites orbiting the earth every 12 hours at a
height of 20.190 km.
 4 satellites are located in each of 6 orbits with
60 degrees orientation between each other.

 Working Principle
 Location of any GPS receiver is determined through a time of
flight measurement (satellites send orbital location (ephemeris)
plus time; the receiver computes its location through
trilateration and time correction)

 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
Global Positioning System (GPS) (2)
 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
 Light travels 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 to each of the
satellites
 Exact measurement of the time of flight
 the receiver correlates a pseudocode with the same code coming from the satellite
 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.
 Differential Global Positioning System (dGPS) (4)
 DGPS requires that a GPS receiver, known as the base station, be set up on a precisely
known location. The base station receiver calculates its position based on satellite signals
and compares this location to the known location. The difference is applied to the GPS data
recorded by the roving GPS receiver
 position accuracies in sub-meter to cm range
 Range sensors
 Sonar

 Laser range finder

 Time of Flight Camera

 Structured light
4a
56
 Range Sensors (time of flight) (1)
 Large range distance measurement  thus called range sensors
 Range information:
 key element for localization and environment modeling
 Ultrasonic sensors as well as laser range sensors make use of
propagation speed of sound or electromagnetic waves respectively.
 The traveled distance of a sound or electromagnetic wave is given by

 d = distance traveled (usually round-trip)

 c = speed of wave propagation
 t = time of flight.
 Range Sensors (time of flight) (2)
 It is important to point out
 Propagation speed v of sound: 0.3 m/ms
 Propagation speed v of of electromagnetic signals: 0.3 m/ns,
 Electromagnetic signals travel one million times faster.
 3 meters
• Equivalent to 10 ms for an ultrasonic system
• Equivalent to only 10 ns for a laser range sensor
• Measuring time of flight with electromagnetic signals is not an easy task
• laser range sensors expensive and delicate

 The quality of time of flight range sensors mainly depends on:

 Inaccuracies in the time of fight measurement (laser range sensors)
 Opening angle of transmitted beam (especially ultrasonic range sensors)
 Interaction with the target (surface, specular reflections)
 Variation of propagation speed (sound)
 Speed of mobile robot and target (if not at stand still)
 Ultrasonic Sensor (time of flight, sound)
 transmit a packet of (ultrasonic) pressure waves
 distance d of the echoing object can be calculated based on the
propagation speed of sound c and the time of flight t.

c t
d
2
 The speed of sound c (340 m/s) in air is given by

Where c    R T
: adiabatic index ( isentropic expansion factor) - ratio of specific heats of a gas
R: gas constant
T: temperature in degree Kelvin
 Factsheet: Ultrasonic Range Sensor

emitter
1. Operational Principle
An ultrasonic pulse is generated by a piezo-
electric emitter, reflected by an object in its path,
receiver and sensed by a piezo-electric receiver. Based
on the speed of sound in air and the elapsed time
from emission to reception, the distance between
v  t the sensor and the object is easily calculated.
d
2

2. Main Characteristics
• Precision influenced by angle to object (as
illustrated on the next slide)
• Useful in ranges from several cm to several
meters
• Typically relatively inexpensive

3. Applications
• Distance measurement (also for transparent
surfaces)
<http://www.robot-electronics.co.uk/
• Collision detection
shop/Ultrasonic_Rangers1999.htm>
 Ultrasonic Sensor (time of flight, sound)
 typical frequency: 40kHz - 180 kHz
 Lower frequencies correspond to longer maximal sensor range
 generation of sound wave via piezo transducer
 transmitter and receiver can be separated or not separated
 Range between 12 cm up to 5 m
 Resolution of ~ 2 cm
 Accuracy 98%  relative error 2%
 sound beam propagates in a cone (approx.)
 opening angles around 20 to 40 degrees 0° measurement cone
 regions of constant depth -30° 30°
 segments of an arc (sphere for 3D)

-60° 60°

Amplitude [dB]

Typical intensity distribution of a ultrasonic sensor

 Ultrasonic Sensor (time of flight, sound)
 Other problems for ultrasonic sensors
 soft surfaces that absorb most of the
sound energy
 surfaces that are far from being
perpendicular to the direction of
the sound  specular reflections

a) 360° scan b) results from different geometric primitives

 
4a
64
Ultrasonic Sensor (time of flight, sound)
 Bandwidth
 measuring the distance to an object that is 3 m away will take such a
sensor 20 ms, limiting its operating speed to 50 Hz. But if the robot has a
ring of 20 ultrasonic sensors, each firing sequentially and measuring to
minimize interference between the sensors, then the ring’s cycle time becomes
0.4 seconds => frequency of each one sensor = 2.5 Hz.
 This update rate can have a measurable impact on the maximum speed
possible while still sensing and avoiding obstacles safely.
 Laser Range Sensor (time of flight, electromagnetic) (1)
 Laser range finder are also known as Lidar (LIght Detection And Ranging)

SICK

Alaska-IBEO

Hokuyo
 Laser Range Sensor (time of flight, electromagnetic) (1)

D
Transmitter
P

L Target

Phase Transmitted Beam

Measurement Reflected Beam

 Transmitted and received beams coaxial

 Transmitter illuminates a target with a collimated laser beam
 Receiver detects the time needed for round-trip
 A mechanical mechanism with a mirror sweeps
 2D or 3D measurement
 Laser Range Sensor (time of flight, electromagnetic) (2)

 Operating Principles:
 Pulsed laser (today the standard)
• measurement of elapsed time directly
• resolving picoseconds
 Phase shift measurement to produce range estimation
• technically easier than the above method
 Laser Range Sensor (time of flight, electromagnetic) (3)
 Phase-Shift Measurement
D
Transmitter
P

L Target

Phase Transmitted Beam

Measurement Reflected Beam

 c
D  L  2D  L   
2 f
Where:
c: is the speed of light; f the modulating frequency; D’ the distance covered by the
emitted light is.
 for f = 5 MHz (as in the A.T&T. sensor),  = 60 meters
 Laser Range Sensor (time of flight, electromagnetic) (4)

 Distance D, between the beam splitter and the target


D 
4
 where
 : phase difference between transmitted and reflected beam
 Theoretically ambiguous range estimates
 since for example if  = 60 meters, a target at a range of 5 meters = target at 35
meters
Amplitude [V]

lambda

q Phase Transmitted Beam

Reflected Beam
 Laser Range Sensor (time of flight, electromagnetic) (5)
 Uncertainty of the range (phase/time estimate) is inversely proportional to
the square of the received signal amplitude.
 Hence dark, distant objects will not produce such good range estimated as
closer brighter objects …
 Laser Range Sensor (time of flight, electromagnetic)

 Typical range image of a 2D laser range sensor with a rotating mirror. The length of
the lines through the measurement points indicate the uncertainties.
 3D Range Sensor (4): Time Of Flight (TOF) camera
 A Time-of-Flight camera (TOF camera, figure ) works similarly to a lidar with the advantage that the whole
3D scene is captured at the same time and that there are no moving parts. This device uses an
infrared lighting source to determine the distance for each pixel of a Photonic Mixer Device (PMD) sensor.

Swiss Ranger 3000

(produced by MESA)
 Triangulation Ranging

 Use of geometrical properties of the image to establish a distance measurement

 If a well defined light pattern (e.g. point, line) is projected onto the environment.
 reflected light is then captured by a photo-sensitive line or matrix (camera) sensor device
 simple triangulation allows to establish a distance.
 If size of a captured object is precisely known
 triangulation without light projecting
 Structured Light (vision, 2D or 3D): Structured Light

b
a b
u

 Eliminate the correspondence problem by projecting structured light on the scene.

 Slits of light or emit collimated light (possibly laser) by means of a rotating mirror.
 Light perceived by camera
 Range to an illuminated point can then be determined from simple geometry.