This chapter presents the sonar sensor model that was developed in this research. Figure 6.1 shows that the model is used to interpret the raw sonar returns from ARNE before the information is passed on to the the other modules on the workstation.

Section 6.1 outlines the operation of the Polaroid ultrasonic rangefinder used by ARNE. Section 6.2 then describes initial experiments to measure the range to a smooth wall in the test environment. The experiments highlight two key features of the sonar sensor: its wide beam and its uneven signal strength. Section 6.3 proposes a sonar model to mitigate the effect of these features by grouping neighbouring range readings. Section 6.4 then describes experiments to verify that the model will be applicable when measuring the range to the variety of objects that ARNE will encounter in the test environment. Section 6.5 then summarises the model.

6.1 The Polaroid Ultrasonic Sensor

Time-of-flight sonar is used in this thesis; distance information is derived from the time taken for a pulse of sound to travel to an object and be reflected back to the sensor.

Figure 6.2 is a simplified diagram of the rangefinder. Voltage pulses are sent to the transducer, which emits 16 cycles of square wave sound at about 50 kHz. As the sound begins, a timer is started. For a short period after transmission, the transducer is disabled (to give enough time for the vibration to die away) and it is then used to listen for an echo.