Robot sensors A robot can be defined as an intelligent link between - - PowerPoint PPT Presentation

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Robot sensors

• A robot can be defined as an intelligent link

between perception and action

Environment Robot Perception Action

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For a mobile robot

Raw data Environment Model Local Map "Position" Global Map Actuator Commands Sensing Acting Information Extraction Path Execution Cognition Path Planning Knowledge, Data Base Mission Commands Path Real World Environment Localization Map Building

Motion Control Perception

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

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Sensors of the SCI TOS G5 robot

Wheel encoders Laser range finder Sonar sensors Bumper Camera Microphones Emergency stop button

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Robot sensors

Robart II, H.R. Everett

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

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Sensor classification

• 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.

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Sensor classification

• Passive sensors
• energy coming for the environment
• Active sensors
• emit energy and measure the reaction

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Sensor classification

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Sensor classification

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Sensor performance

• Dynamic range
• ratio between lower and upper limits, usually in decibels
• Range
• upper limit
• Resolution
• minimum difference between two values

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Sensor performance

• Linearity
• variation of output signal as function of the input signal
• Bandwidth or Frequency
• the speed with which a sensor can provide a stream of

readings

• Sensitivity
• ratio of output change to input change

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Sensor performance

• Cross-sensitivity
• sensitivity to environmental parameters that are orthogonal

to the target parameters

• Error / Accuracy
• difference between the sensor’s output and the true value

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Sensor performance

• Systematic error
• deterministic errors
• caused by factors that can (in theory) be modeled and

predicted

• Random error
• non-deterministic
• cannot be modelled but can be described probabilistically
• Precision
• reproducibility of sensor results

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Wheel encoders

Measure the position or speed of the wheels

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

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Measuring travelled distances

Dead-reckoning (deduced reckoning) – determining the present location of a vessel from some previous position through known course and velocity information over a given lenth of time Odometry – the course and path length information is directly derived from on board

• dometer (e.g. optical wheel encoders)

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Heading sensors

• Used to determine the robots orientation and

inclination.

• Together with the velocity estimates produce dead-

reckoning information

• Compasses
• I nclinometer

Gyro

• Mechanical gyros
• Optical gyros

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Beacons

• Beacons are signaling

guiding devices with a precisely known position

• Natural beacons

(landmarks)

• Artificial beacons (radio

transmitters, reflectors)

• Active and passive

beacons

• Require the environment

to be prepared and changed for the robot

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

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Global Positioning System

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

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Range sensors

• Range information is the key element for localisation and

environment modelling

• Ultrasonic sensors- measure speed of sound
• Laser range sensors – measure speed of electomagnetic waves

d = c t Where d = distance traveled (usually round-trip) c = speed of wave propagation t = time of flight.

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Ultrasonic sensors

transmit a packet of sound waves distance d of the echoing object can be calculated based on the propagation speed of sound c and the time of flight t.

2 ct d =

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

www.biorobotics.ttu.ee

Ultrasonic sensors

• 30°
• 60°

0° 30° 60°

Amplitude [dB]

measurement cone

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

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Signal reflection and absorbtion

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

www.biorobotics.ttu.ee

Laser rangefinders

• Transmits and receives laser light
• Measures the phase shift of the reflected light

Phase Measurement Target D L Transmitter Transmitted Beam Reflected Beam P

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

www.biorobotics.ttu.ee

Laser rangefinders

π θλ 2 2 ' + = + = L D L D

π θλ 2 = D

Transmitted Beam Reflected Beam

θ λ Phase [m] Amplitude [V]

• 1. R. Siegwart, I. Nourbakhsh, "Introduction to Autonomous Mobile Robots", The MIT Press,2004

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Perception with a sonar

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Perceptions with a laser

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Mapping with a laser rangefiner

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Mapping with a laser rangefiner

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Home assignment: odometry

• Drive around in the simulator with the

SCITOS robot and compare the real position of the robot to the position estimated by odometry readings

• Observe how do the errors accumulate,

what are the maneuvers that more increase the odometry error?

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Home assignment: laser and sensor readings

• Drive around in the simulator with the

SCITOS robot and compare laser and sonar readings.

• Explain how the sensor readings are

different