On-Line Metrology with Conoscopic Holography: Beyond Triangulation

Abstract

On-line non-contact surface inspection with high precision is still an open problem. Laser triangulation techniques are the most common solution for this kind of systems, but there exist fundamental limitations to their applicability when high precisions, long standoffs or large apertures are needed, and when there are difficult operating conditions. Other methods are, in general, not applicable in hostile environments or inadequate for on-line measurement. In this paper we review the latest research in Conoscopic Holography, an interferometric technique that has been applied successfully in this kind of applications, ranging from submicrometric roughness measurements, to long standoff sensors for surface defect detection in steel at high temperatures.

Fringe pattern characterization by OPD analysis in a lateral shearing interferometric profilometer

“…4,10,17,18 Fig. 3 shows an example of the results of such an instrument, in this case with a standoff of 135 mm and a small angle of triangulation (over 15 • ) to provide a wider depth of field.…”

“…These two wavefronts are mutually coherent, so the density of the fringes in the obtained interference pattern contains the information of the radius of this spherical wavefront. The use of interferometry allows for precisions that are beyond those obtained by laser triangulation techniques 10 and which depend mostly on the working distance of the sensor. By proper selection of the setup basic parameters, such as the shear introduced by the birefringent crystal and the working distance, these systems can be easily scaled to cover a wide range of applications.…”

Robust Visual Touch-Up Calibration Method in Robot Laser Spot Welding Application

“…Visual touch-up method can be used for purposes of robot new tool calibration, robot absolute accuracy calibration and also for calibration of several robots carried vision systems as in our case. The noncontact method can use several sensors for calibration procedures: from laser distance sensors based on triangulation [22] and conoscopic holography [23], inductive or capacitive sensors and especially video cameras as Watanabe et al is presenting. In our case we have chosen the video camera approach, because both on robot attached systems are vision based and also the planar robot movement simplifies the calibration procedures approach (Fig.…”

The automated surface inspection system for hot slab

“…The most commonly used technique for 3D measurement used for inspecting slab is laser triangular [I]. And recently conoscopic holography method is adopted to inspect hot slab [2,3]. Each method has its own merits and demerits such as long standoff, amount of precision and operating conditions.…”