The Traffic Speed Deflectometer (TSD) is being developed at the Transport Research Laboratory on behalf of the English Highways Agency as a tool to enable network structural surveys to be carried out at traffic speeds of up to 80km/h, avoiding traffic disruption and expensive traffic management. The sensing equipment consists of four velocity measuring lasers mounted on a stiff beam, which is housed in the trailer of an articulated lorry. Three of these lasers (P100, P300 and P750) are used to calculate the slope of the deflection bowl in three positions (100mm, 300mm and 750mm in front of the rear axle) relative to the fourth (Pref) laser, which is in a position where the deflection slope is close to zero. Moreover, the lasers are inclined from the vertical by small angles (around 2 degrees) that have to be known with great accuracy as they have a significant effect on the final outputs, the deflection bowl slopes. Determining the calibration angles (geometric calibration) is a key point in the development of this technology and has been researched in various ways. The most effective of these has been the installation of a high-specification accelerometer into the surface of a relatively strong pavement that enables a comparative measurement of the pavement’s deflection slope under TSD loading. The measurement also provides a simultaneous means of absolute reference calibration of the system where the accelerometers are also installed in a range of pavement constructions. This paper describes the research performed to date, the encouraging results obtained and the steps necessary to take this technology forward.
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