Image deflection measuring technique

The image deflection measuring technique for surface deflection measurements has recently been tried on pavements. This non-destructive method consists of the projection of one light pattern (line, grid, etc.) on the pavement surface. The surface is captured by a camera from a different viewpoint, and software analyses the pattern. This allows the measurement of surface deflection and its gradient. 

The first experiments have been carried out by the LCPC putting the projector and the camera on a static fatigue device. The first applications showed interesting results, which validate the choice of measuring deflection basin with fringe projection. However, this technique needs to be improved in order to obtain an operational device. An ongoing project aims to mount the system on a heavy truck (Muzet et al. 2009). 

The imaging technique, although promising, is still in the early stage of development. 

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Very similar to scanning tunnelling microscopy (STM). In this technique, however, the attractive Van Der Waals forces between the surface and the probe cause a bending of the probe. This deflection is measurable by a variety of means. Because this technique does not require a current between the probe and the surface, nonconducting surfaces may be imaged. 

Atomic force microscopy (AFM) has become a standard technique to image with high resolution the topography of surfaces. It enables one to see nanoscopic surface features while the electrode is under potential control. This powerful probe microscopy operates by measuring the force between the probe and the samples (56,57). The probe consists of a sharp tip (made of silicon or silicon nitride) attached to a force-sensitive cantilever. The tip scans across the surface (by a piezoelectric scanner), and the cantilever deflects in response to force interactions between the tip and the substrate. Such deflection is monitored by bouncing a laser beam off it onto a photodetector. The measured force is attributed to repulsion generated by the overlap of the electron cloud at the probe tip with the electron cloud of surface atoms. 

The restriction to mica was overcome by a relatively recent technique the atomic force microscope (AFM). sometimes also called the scanning force microscope [69. AFMs are usually used to image solid surfaces. Therefore a sharp tip at the free end of a cantilever spring is scanned over a surface. Tip and cantilever are microfabricated. While scanning, surface features move the tip up and down and thus deflect the cantilever. By measuring the deflection of the cantilever, a topographic image of the surface can be obtained. 

Since the introduction of the STM a number of variations have been devised, such as ATM (atomic force microscope). The basic concept is that piezoelectric actuators move a miniature cantilever arm (with a nm-sized tip) across the sample while a non-contact optical system measures the deflection of the cantilever caused by atomic scale features. The deflection is proportional to the normal force exerted by the sample on the probe tip and images are generated by raster scanning the sample [201]. One application of this technique was to measure the thickness and size distribution of sub-micron clay particles with diameters in the 0.1 to 1 pm size range and thickness from 0,01 to 0.12 pm [202]. 

AFM (atomic force microscopy) was developed about five years after STM (see Atomic force microscopy). It relies upon the measurement of the force of interaction between a sharp tip and a sample. Being based upon the measurement of force rather than current, it is applicable in principle to any material. The technique has great versatility, and as in the case of STM, imaging may be carried out under ambient or fluid conditions. The tip is attached to a flexible cantilever, which is rastered across a sample surface. As the interaction force between the tip and the sample changes, the deflection of the cantilever varies. The cantilever deflection is readily measured (by optical deflection in most commercial systems) and is proportional to the interaction force leading to quantification, provided the spring constant of the lever is known. Either the cantilever or the sample is mounted on a piezoelectric crystal in order to exact fine control over the relative movements of the tip. 

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