cs685-perception-3

cs685-perception-3 - Autonomous Mobile Robots Chapter 4...

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10/29/09 1 Autonomous Mobile Robots, Chapter 4 © R. Siegwart, I. Nourbakhsh Range Sensing strategies Active range sensors Ultrasound Laser range sensor Autonomous Mobile Robots, Chapter 4 © R. Siegwart, I. Nourbakhsh Range Sensors (time of flight) (1) Large range distance measurement -> 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 = c . t Where d = distance traveled (usually round-trip) c = speed of wave propagation t = time of flight. 4.1.6
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10/29/09 2 Autonomous Mobile Robots, Chapter 4 © R. Siegwart, I. Nourbakhsh 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, one million times faster. 3 meters is 10 ms ultrasonic system only 10 ns for a laser range sensor time of flight t with electromagnetic signals is not an easy task laser range sensors expensive and delicate The quality of time of flight range sensors manly depends on: Uncertainties about the exact time of arrival of the reflected signal Inaccuracies in the time of fight measure (laser range sensors) Opening angle of transmitted beam (ultrasonic range sensors) Interaction with the target (surface, specular reflections) Variation of propagation speed Speed of mobile robot and target (if not at stand still) 4.1.6 Autonomous Mobile Robots, Chapter 4 © R. Siegwart, I. Nourbakhsh Ultrasonic Sensor (time of flight, sound) (1) 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. The speed of sound c (340 m/s) in air is given by where : ration of specific heats R: gas constant T: temperature in degree Kelvin 4.1.6
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10/29/09 3 Autonomous Mobile Robots, Chapter 4 © R. Siegwart, I. Nourbakhsh Ultrasonic Sensor (time of flight, sound) (2) Transmitted sound Analog echo signal Threshold Digital echo signal Integrated time Output signal integrator Time of flight (sensor output) threshold Wave packet Signals of an ultrasonic sensor 4.1.6 • Send a wave packet wait until in comes back Autonomous Mobile Robots, Chapter 4 © R. Siegwart, I. Nourbakhsh Ultrasonic Sensor (time of flight, sound) (3) typically a frequency: 40 - 180 kHz generation of sound wave: piezo transducer transmitter and receiver separated or not separated sound beam propagates in a cone like manner opening angles around 20 to 40 degrees regions of constant depth segments of an arc (sphere for 3D) Effective range 12cm, 5m Accuracy between 98-99% Typical intensity distribution of a ultrasonic sensor 4.1.6
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10/29/09 4 Autonomous Mobile Robots, Chapter 4 © R. Siegwart, I. Nourbakhsh Ultrasonic Sensor (time of flight, sound) (4)
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