Traffic Monitoring System using Acoustic Sensors Yuping Dong May 14, 2008

Traffic Monitoring System Architecture

Sensor Schematic

Acoustic Sensor

Cross Correlator

Sonar (Sound Navigation and Ranging) Whales and Dolphins use Sonar. Two kinds: active and passive. Mainly used as underwater navigation, can be used as acoustic location in air. Frequencies used vary from infrasonic to ultrasonic.

Active Sonar Uses a sound transmitter and a receiver. Creates and transmits a pulse at constant frequency and listens for echo of the pulse. Lower frequencies can communicate over longer distances. Twice Distance = Time of flight X Speed of sound. Convert difference in frequency between transmitted and received signals into velocity. Use multiple transducers to calculate relative positions of objects. Detectors must be sensitive to pick up the echoes. Receivers have to be enabled after a predetermined time to prevent hearing the transmission of the pulse.

Hitechnic Active Sonar Sensor Emits a short burst of 40kHz sound from a transducer. Reflected sound energy is received by another transducer. Calculates the distance between sensor and object. Performs well when sensor face is parallel to the surface of the nearest object. Time of flight information can be misleading if the object deviates significantly from the ideal place. Detection range is approximately 80 degrees.

Passive Sonar Listens without transmitting. May have large sonic databases. Can not control the amount of transmitted energy. Covertness can be advantageous. Feasibility depends on number of receiving sensors, observation time and SNR.

Passive Sonar for Obstacle Detection Signals received at two or more receiving sensors are used to estimate the position and velocity of a source. Time delay is the travel time of the acoustic wavefront between pairs of receivers. B is the bearing estimate, D is the time delay estimate, C is the speed of sound. System outputs cross correlation of received signals. When estimated time delay equals real time delay, it achieves a maximum. Detecting source location

Possibility of Sonar Sensors being used in Traffic Monitoring System It’s possible to use both active and passive sonar sensors as traffic monitoring sensor according to their principles. But the currently commercial sonar sensors have small detection range around 6.5m. Active sonar, carefully choose transmitting and receiving directions, set the predetermined enable time of receivers, etc. Passive sonar, set specific frequency range. We need more research on sonar sensors to decide whether it would be a good idea to use it or not.