Traffic Speed Deflectometer

Traffic Speed Deflectometer (TSD) is a rolling wheel Deflectometer for network level bearing capacity measurement. TSD uses patented Doppler technology to measure the deflection of roads while travelling at normal traffic speed - up to 80 km/h.

TSD data provides continuous pavement deflection profiles, from which bearing capacity indicies can be derived and pavement fatique / residual life can be estimated. The hign accuracy and resolution makes TSD perfect to pin point location with bearing capacity deviations.

ProfilographLaserProfTraffic Speed DeflectometerRight-of-Way ImagingSurface Imaging

Traffic Speed Deflectometer - Results

Dedicated post processing software has been developed for use with the data produced by the Traffic Speed Deflectometer.

The software produces a file in text format that, among other information, contains:

• Chainage
• Time
• Temperature (air and road)
• Deflection velocities
• Deflection slopes
• Structural Curvature Index 300, SCI 300
• Maximum Deflection under the load

The text file is based on a binary file created by the data acquisition software during measuring. Parameters such as filter lengths and output frequency can be changed by the user. The software also has a plot facility for quick viewing of the results. Extra files containing the raw data or information about data quality can be created.

Below the different results/parameters are discussed:

Chainage

The chainage is determined with an odometer using an optical encoder. The pulses from the encoder are also used to determine the driving speed.

Temperatures

The air temperature is measured using a standard PT100 sensor. The road temperature is measured using an infrared type of thermometer.

Deflection Velocity

To obtain the correct deflection velocities, the data from each of the Doppler lasers needs to be compensated for the sensors own movements. The fact the laser beams are not exactly perpendicular to the road and that the angle of incident varies also makes corrections necessary. This is done using data from an inertial unit consisting of 3 accelerometers and 3 fibre optic gyros. One laser Doppler sensor placed outside the deflection bowl is also important to the compensation of data.

Deflection Slope

The Deflection Slopes are obtained by dividing the Deflection velocities by the driving speed.

Structural Curvature Index 300 and Maximum Deflection

Close to the load we can assume, that the road behaves like an elastic beam on a foundation of linear springs. Based on the Deflection Slopes we can therefore use the Euler-Bernoulli Beam Equation to determine the Structural Curvature Index 300 and Maximum Deflection.

Curve fitting

The existing TSD devices has 4 Doppler lasers, which results in three slopes. Based on a set of three slopes (the figure to the left) we can determine the best curve through the measuring points (the middle figure) and then integrate the found curve to obtain the deflection bowl (the figure to the right). From the deflection curve (to the right) we can simply read the values for the maximum deflection and also make a reading at 300 mm to obtain the Structural Curvature Index 300.

Readings further out are "not that reliable", because the curve fit is based on only three measurement points placed near the load, in 100 mm, 200 mm and 300 mm. If there is an interest of for example the Structural Curvature Index 900, more sensors can be added and a new model will be used to determine the deflection bowl.

Examples of results

The figure below shows SCI300 data from app. 100 m of the same section of road measured at 4 different driving speeds.

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Traffic Speed Deflectometer
Product Information
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Traffic Speed Deflectometer
Latest Article - TRA2008
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The Traffic Speed Deflectometer is patented under the following patent numbers: 819196 in Europe, 694347 in Australia, 6119353 in USA and 3850875/3851342 in Japan.