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A COMBINED SYSTEM FOR ABSOLUTE 3-D GAUGING IN AUTOMATED MANUFACTURING PROCESSES 
L. Biancardi, U. Minoni, E. Gelmini, G. Sansoni 
Dipartimento di Elettronica per l'Automazione, Laboratory of Optoelectronics 
Università degli Studi di Brescia, via Branze 38, 25123 Brescia, Italy 
Tel. +39-30-3715442, Fax. +39-30-380014 
e-mail: Sansoni@bsing.ing.unibs.it 
KEY WORDS: 3-D profilometry, absolute interferometry, image preprocessing, system calibration 
ABSTRACT 
We report on the development, characterization, and testing of a combined electrooptical system suitable for the 
absolute 3-D gauging of surfaces. The system combines an absolute point-by-point distance measurement based onto 
a dual-wavelength interferometer, operated in the scanning mode, to a whole field profilometer based on structured- 
light projection using a liquid crystal display. The dual-wavelength interferometer uses two Nd:YAG laser sources 
locked in frequency, for increased measuring range and locking stability. The profilometer has been implemented with 
image preprocessing utilities in order to be effectively used in harsh environments. The two units have been arranged 
in a master-slave configuration, with the interferometer operating on a scanning basis over a discrete set of points and 
providing absolute coordinates to the whole field profilometer. 
1. INTRODUCTION 
In flexible manufacturing there is a growing need for non-contact, accurate instrumentation to monitor the distance of a 
target in an absolute way, as well as to gauge the target profile and to provide 3-D reconstruction of its shape. Tracking 
a robot arm during the operation for error compensation, measuring transient deformations in thermal processing of 
the workpiece, performing visual inspection and quality control of a workpiece, evaluating the 3-D shape of an object in 
order to transfer its shape to a CAD system are typical areas of application for this instrumentation. 
3-D profilometry can be developed using projection of fringes, and successive deconvolution of the deformed pattern. 
Adaptiveness of the projection light is one of the key factors leading to the success of profilometry techniques (Sansoni 
et al., 1993). 
The availability of an absolute measuring technique is essential to sample the surface of interest at a convenient 
number of points with a sufficient accuracy, while performing profilometric measurements on the whole surface. In this 
context, absolute interferometry can provide a useful way of solving ambiguity problems related to structured light 
illumination. Absolute interferometry, that makes use of multiple wavelength operation and of heterodyne detection 
(Dándliker, 1988, Sódnik, 1991), is a rapidly emerging technique for remote measurement of the target distance with 
an extended Non Ambiguity Range (NAR). This technique can be used on both co-operative targets (retroreflectors, 
"scotch-lite" ribbon) as well as on non co-operative targets (diffusive surfaces). Several laser sources can be used to 
build up an absolute interferometer. 
The activity carried out by our laboratory in cooperation with the Institute of Technical Optics (University of Stuttgart) 
and CISE (Italy) led to the development, the characterization and the integration of (i) a 3-D macro profilometer based 
on the projection of light patterns by means of a Liquid Crystal Projector (LCP), to provide full adaptiveness of the 
measurement to different target shapes, and (ii) a 3-D point-by-point profilometer based on an absolute dual 
wavelength interferometer, able to provide absolute measurements on convenient points of the target, to support the 
operation of the 3-D macro profilometer (Docchio et al., 1994). The specifications of the combined system are: (i) a 
field of view of 1m by 1m, (ii) a depth range up to 0.15 m, (iii) a height resolution of 0.1 mm, and (iv) a data acquisition 
rate of up to 10 measurements/sec. Following this development, research has been carried out in our laboratory with 
the aim of adding several features to the macro profilometer setup to render it more suitable for 3-D gauging in harsh 
target conditions. Moreover, the use of tunable laser sources has been tested to obtain a greater operating flexibility of 
the system. In this contribution, details of the activities carried out in the above directions will be presented. 
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IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995