Laser-optic triangulation

The principle of optical triangulation has been known since the time of the early Greeks, and indeed optical triangulation has been used for hundreds of years in applications such as surveying, camera auto-focus and even smart-bombs. With the advent of low-cost, compact electro-optic components such as lateral-effect photodetectors, diode lasers and micro-optics, laser-based triangulation sensors can now be employed for applications that were, heretofore, considered uninspectable. 

Optical Laser Displacement Measuring laser based triangulation principle Micro-Epsilon optoNCDT 2200... 

R. Dorsch, G. Hausler, and J. Herrmaim, Laser Triangulation Fundamental Uncertainty in Distance Measurement, Applied Optics, Vol. 33, No. 7 (1994). 

A typical active triangulation method is depicted in fig. 4.2. Two motor driven computer controlled theodolites (surveying robots) perform the measurements. The first instrument projects profiles of laser spots on the object, the laser being coaxial with the optical axis of the telescope. Thus structured light is used for target generation and serves as an active oriented sensor. The second instrument, a video theodolite, is activated to track the spots where they intersect the surface. 

Mechanical, optical, and electrical systems are mainly used to detect burrs. The stylus method, a mechanical system, can measure burr height, but optical methods can also be applied for this purpose. Among the most important optical systems are microscopes, autofocus methods, laser triangulation, and camera systems. For automatic burr detection in a production process, an electrical measuring method, for example, an inductive sensor system, can be applied. 

A wide range of techniques can be used to capture the surface roughness of a component using noncontact methods. Some of the more common instruments used to captore topographic data include confocal microscopy, laser triangulation, focus detection, and optical interferometry. A relatively recent branch of microscopy known as scanning probe microscopy (SPM) yields over 20 other instruments which are defined based on what probe-surface interaction they are monitoring. The SPM family is described in more detail elsewhere in this book and will be briefly mentioned in this article. 

Figure 2 shows results from an optical profilometer (ProScan 2000 made by Scantron Industrial Products, Taunton, UK) for an abraded steel surface. In this instrument, laser triangulation enables fine displacements of the sample in the X, Y and Z directions to be recorded. Figure 2(a) shows a plan representation of the area scanned. Fig. 2(b) a three-dimensional representation. Surface profiles along the two lines indicated in Fig. 2(a) are given in Figs. 2(c) and (d). 

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