The 60 cm long profile is read with an accuracy of 0.05 mm by means of aA.black and white video camera viewing the image of a laser line projected at anangle onto the surface (figure 3).Any gaps in the profile are filled in. Thisprofile, recorded digitally as 12,000 ordinates, is divided into 4 bands with afifthorderBesselFilter.If the average absolute slope of the coarsest component is greater than anB.arbitrary 0.1 the surface is regarded as "rough" and the program goes to C fordrain path length computation. If the surface is "smooth" the program goes toD where a longer drainage path is computed.The number of large asperities in the hypothetical contact patch of the tyre onC.the rough surface, that part of the texture in contact with the tyre and thedrainagepath lengths are computed. Then move toE andF.D.Thedrainage path length on the smooth surface is assumed equal to the radiusof the contact patch.Then moveto E and FE.The average absolute slope of the texture, is computed, of that part of eachscale that is in contact with the tyre. (The average absolute slope of the twosides of an equilateral triangle for example is 3)The average water film thickness and the ratio of a/d is computed.a' is the asperityF.height 'd' of the second finest scale of texture minus the water film thickness.This is done for each of the speeds 18, 48 and 80 km/hr and for sidewaysforce coefficient and the locked wheel braking force coefficient.Then move toI
The damping factor of the rubber is determined for each of the three speeds ofG.sliding and for sideways and for locked wheel friction. The effect oftemperature rise during locked wheel braking is accounted for. Zankin [11]provided this information.H.The coefficient of dry hysteretic friction C, is calculated for each scale oftexture using the damping factor of the rubber and the average absolute slopeofthatscaleoftexturetogiveCo,C,CCI.The coefficient of wet (not flooded) hysteretic friction is equal to the sum ofthe coefficients of dry hysteretic friction each modified by the effect of surfacewater film thusThe Coeff of Wet Friction = Co + 三 ci +Cwhere a/d is the film thickness ratio, a is the height of the coarsestcomponent of "microtexture" not masked by the water film, d is theaverage depth of the coarsest component of "microtexture"and C arethe coefficients of hysteretic friction computed from average absoluteslope and tread damping factor using the mechano-lattice analysis [10]Theexpression is Tanueerananon's hypothesis.2.2TheTextureMeasuringDeviceThe texture measuring device is housed in a light weight case measuring 40 cm by50 cm and 40 cm high. A slot in the base of the case allows the laser to be projectedonto the road surface and viewed by the video camera from inside the case. Thisportable unit is connected by a long cable to a 486 Compac P.C. which controls the
operation. The components of the texture measuring device are the laser source, thecamera and thelaser-camera transport.2.2-1 The TransportThe laser and the camera are fitted to a cross carriage which is sequentially moved intothree alternate positions by fixed slides situated at each end of the main slide.Themain carriage carrying the cross slide is driven by a motor through a lead screw for adistance of 20 cm shown in figures 4 and 5. In this way three parallel profiles each 20cm long can be recorded automatically in the one operation.2.2-2TheLaserSourceThe laser is a 5 mw 670 nm laser diode.Its fine cylindrical beam is changed to a flatknife by passing it through a cylindrical lens. It impinges on a miror which reflects itOnto the road surface in view of the video camera.See figures 4 and 5.2.2-3TheCameraandLensThe black and white video camera views the laser line impinged on the road surfacethrough powerful magnifying lenses. The laser beam and the line of sight of thecamera are mutually at right angles so the line is always in focus. The aperture of thelens is adjusted automatically. The magnitifcation of the lens is such that onecentimetre of the surface is viewed ata time
2.3TheComputerOperation andOutputThe texture measuring device is placed on the road.Upon iniation the computerquickly tracks the edge of the laser line to give a profile of texture 10 mm long asshown in figure 6.60 of these profiles are sequentially shown on the monitor andrecorded whilethecarriageis transportedautomaticallyone centimetre ata time.Oncethe profile data is stored in the P.C. the processing is effected as shown in section2.1.The 60 parts of the profile are accurately connected and missing pieces are flledin.All the ordinates are divided by 2 to give the vertical resolution of the 45°view of theprofile.Figure 7shows thetotal texture and its four components as shown onOutputthe monitor screen.Figure 8 shows the main screen display with coefficient offriction values average and peak texture depths and the menu for other operations.The inappropriate three decimal places will be modified to a more appropriateaccuracy.The contents of other files can also be shown on the screen and or printed.Forexample figure9 is an example of a block file showing the chainage,texture depth,locked wheel and sideways force friction for any number of readings along a road.Figure 10 is an example of a pointfile which gives the six friction readings plus filmthickness ratios anddryhystereticfrictionvalues.CORRELATIONWTTHDIRECTLYMEASUREDFRICTION3.There are a great number of devices which measure pavement friction directly with atesttyre.Thereis seldomcompleteagreementbetween anytwo,measuringfrictionon the same surfaces.For example a Runway Friction Tester [15] and a Pavement