AN EVALUATION OF LIBELLA AS A PROFILOMETER

The purpose of this study was to investigate how an instrumentation system called libella worked as a profilometer and how well it can measure and classify road roughness. This analysis is based completely on simulations. A simple mathematical model of the libella has been developed and programmed in FORTRAN. Two manually measured road profiles and three theoretically generated profiles corresponding to very good, good and bad roads have been used as test roads. As a measure of road roughness we have used the so called iri-value (international roughness index) and the rms-value (root mean square). The different roughness measures computed on the real road profile and the libella signal are correlated against each other and the standard deviation is computed. The different measures show high correlation if the profiles are high pass filtered with a cutoff wavelength of 15 m before the rms-computations. This correlation indicates that the libella manages to distinguish between roads where the roughness measures differ by a factor of two. If the difference in roughness measure is only about 20-40% the correlation will be much lower. The libella overestimates in general, but not always, the iri-value depending on the amplification of the road profile wavelengths around 5 m. The results indicate that this overestimation can become up to 50% of the real iri-value.