Comparison of Special Methods with Commercial Products for Optical-based Measurement of Dimensions

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1 XXIX. ASR '2004 Seminar, Instruments and Control, Ostrava, April 30, Comparison of Special Methods with Commercial Products for Optical-based Measurement of Dimensions MATELA, Lukáš 1 & KMOCH, Petr 2 1 Ing, Department of Measurement, Faculty of Mechatronics and Interdisciplinary Engineering Studies, TU Liberec, Hálkova 6, Liberec, , lukas.matela@vslib.cz 2 petr.kmoch@vslib.cz Abstract: This contribution compare developed and implemented special methods for opticalbased precision measurement of dimensions with two nowadays commercial products. Implementing of special methods was the goal of diploma thesis solved at the Department of Measurement at FMIES in Liberec. Two different commercial software products are used for building machine vision applications and we use them for education purposes. The request for precision of measurement was very strict and exceeded possibilities of both commercial products. So it was necessary to find and implement special algorithms which allows us to obtain requested results. The subpixel measurement is possible to ensure by several different methods, some of them are described in this contribution. Final measurement is realised by using algorithm that combines polynomial regression of line profile and Hough transform. The two commercial software products are Vison Builder for Automated Inspection from National Instruments and Simatic Spectation from Siemens company. Keywords: subpixel, videometry, measurement, jewellery stones, Hough transform 1 Introduction Formation of this paper is evoked by comparison of diploma thesis methods with results of available commercial software products. Company Preciosa presented demand for precision measurement of edged jewellery stones with 10 µm resolution. So the student Petr Kmoch at Department of Measurement (Technical University of Liberec) has been solving the diploma thesis called Videometric way of measurement of the geometric characteristics of jewelery stones where he implemented several subpixel measuring methods. Present stage allows us to use commercial software products to built machine vision applications. Sometimes it is better choice to built machine vision or optical-based measurement application using commercial products, especially in the case of simple problems. But if the demand for precision is strict or if the machine vision application is complicated, it is often necessary to implement special algorithms. The contribution tries to find point of necessity to develop customer application. 2 Commercial software products We tested two commercial products developed by companies National Instruments and Siemens. Both software products have comparable possibilities but NI Vision Builder for Automated Inspection seems to be more user friendly. Graphic user interface of National Instrument software is shown in the figure nr. 1. The basic differences between commercial software are possible to see in the table nr. 1.

2 XXIX. ASR '2004 Seminar, Instruments and Control, Ostrava, April 30, Table 1 basic differences between two commercial products NI Vision Builder for AI Siemens Spectations - licence costs + free licence + possible to use with various hardware - fixed to Siemens hardware + user friendly - little more complicated control It is not possible to ensure constantly position and orientation during image capturing. So in the both software product the first step is pattern (jewellery stone) finding and matching in the image. After finding pattern the procedure is similar in both programs: finding new coordinate system and than transforming all next measuring steps into new coordinates. Figure 1 Graphics User Interface of NI Vision Builder for AI, jewellery stone, normally transparent, is dark in backlight illumination 3 Special algorithms To achieve the subpixel precision it is possible to use two basic concepts. First of them use thresholded picture and its edge image. Pixels which corresponds with edges of picture (called edgels) are processed by some statistical method. In our case is possible to consider Hough transform. The second concept is to process edge detection on the grey-level images and to find subpixel location at the line profile perpendicular to edge line. In the diploma thesis the best results are achieved by combination of both approaches. 3.1 Hough transform After obtaining black and white (binary) thresholded image, we could apply Hough transform. Its principle could bring us subpixel precision even when the original image is not in high quality.

3 XXIX. ASR '2004 Seminar, Instruments and Control, Ostrava, April 30, Hough transform allows us to obtain parameters of objects, which could be described by geometric parameters, from the edge image. In our case we are looking for abscissas lying on the straight lines. Every bisector can be described by formula: y = kx + q and after substitution q = D sinυ k = tanα = cotυ we obtain formula in polar system of coordinates: D = x. cosυ + y. sinυ Transform into polar coordinate system is necessary in the case that in image occur abscissas, which are perpendicular to x-axe. Then the coefficient k is not Figure 2 original image with edgels Figure 3 Hough accumulator 3.2 Subpixel precision There are several ways how to achieve primary subpixel dimension information. First of them makes interpolation by polynomial function (polynomial regression) and than after derivation we are looking for inflection point which is determinated with subpixel precision. The other way is interpolation by spline functions. After obtaining such curve it is possible to determine edge by comparing area under and above curve. Last approach that was used in the diploma thesis is interlaying of the two biggest edgels in gray-scale image by parabolic function. The top point give us edge also in subpixel precision. Figure 6 Interpolation of line profile by spline functions Figure 5 Subpixel edge is given by equal areas

4 XXIX. ASR '2004 Seminar, Instruments and Control, Ostrava, April 30, Measure conditions To accomplish repeatability of measurement we have to ensure the standard conditions of measurement. In this case it is very important to endeavour to acquire as quality images as possible. The measuring assembly should consist from specific parts: lightning, optical system, camera, grabber and evaluation software. In the case of using analog area-scan camera, we have to ensure that the pixels are square-shaped. In the opposite it is necessary to correct it by coefficients. In the machine vision applications there are often used analog area-scan cameras, where the number of CCD photo-elements in the horizontal line is much lower than should be. Such CCD camera should interpolate the output signal by low pass filtering. Obtained signal is not often usable in videometry systems than. The concrete devices that were used for images capturing are telecentric lens Computar 55 mm, progressive-scan analog camera Sony XC-8500 and frame grabber NI The problem of measuring is inflection of light ray on the stone edge. This problem is possible to eliminate thanks to calibration. The deviation is systematic and for the same type of measured objects is the same value. So this additive error is independent on the dimensions of objects in the case the object dimensions are bigger than error value. This problem is shown at the Fig. nr. 5. Figure 5 Measuring assembly. Errors caused by ray inflection must not exceed object dimension Figure 6 The principle of telecentric lens. Parallel beam of rays entering into lens is projected as parallel beam onto a CCD sensor

5 XXIX. ASR '2004 Seminar, Instruments and Control, Ostrava, April 30, Telecentric lens and CCD sensor, which were used for image capturing, were adjusted in following magnification: 2x2 mm large object covered approximately area 200x200 pixels. So the length of one pixel (and also height) correspond to 10 µm. 5 Results All geometric measurement of jewellery stones is passed by the same procedure, which consists of two basic steps. Finding edges of stone in image with subpixel precision and then computing corner of rectangle (practically general quadrangle). The algorithm that gave us the best results is polynomial regression followed by Hough transform. This results are presented in table nr. 2 with label special algorithm. The interpolating of line-profile brought worse results and spline interlaynig was not correctly implemented yet. Results of comparing are apparent from the table nr. 2. Achieved maximal variance is in the case of special algorithm 0,1. The standard deviation is than σ = 0,32 3, 2µm For Normal (Gausian) distribution we could expect the 99,7 % of measurement is in the range x 3σ ; x + 3σ x 9,6; x 9, 6 µ m Table 2 - Achieved variance in pixels for each stone edge stone 1 stone 2 Special algorithm 0.03; 0.03; 0.05; ; 0.10; 0.03; 0.02 NI software 0.16; 0.15; 0.95; ; 0.10; 0.09; 0.05 Siemens software 0.18; 0.31; 0.39; ; 0.13; 0.04; Conclusions From attained results we can come to a conclusion, that for common videometric measurement it is sufficient to use available commercial software products. Configuring of measurement project takes short time. Algorithms for line detection are implemented with sufficient precision for most of applications. The other way to enhance precision is to use CCD sensors with higher resolutions. This could increase precision without related costs for programming labour. In the case of demand for really high precision measurement which is not possible to achieve by increasing of CCD sensor resolution it is necessary to implement special algorithms. We checked up the algorithm of polynomial regression followed by Hough transform for increasing precision give us approximately three-times better results than two tested algorithms used in commercial software products. References FISCHER, J. Optoelektronické senzory a videometrie. 1. vyd. Praha: Vydavatelství ČVUT, s. ISBN X. HLAVÁČ, V. & SEDLÁČEK, M. Zpracování signálů a obrazů. 1. vyd. Praha: Vydavatelství ČVUT, 2002, 220 s. ISBN