Metrologica I Limitations of Optica I Probing Techniques for Dimensional Measurements

L. De Chiffre (2), H. N. Hansen Institute of Manufacturing Engineering, Technincal University of Denmark

Received on January 10,1995

Abstract Based on experimental results from a series of investigations, this paper deals with the metrological limitations of optical probing techniques in the field of dimensional metrology. The optical probing systems considered herein comprise an optical coordinate measuring machine, an experimental optical roundness tester and an optical measuring station for dimensions which is under development for integration in a production line. In all these cases, a single measurement micrometer accuracy is required but hard to achieve. The influences from sensor, optical system, illumination, object, background, and mechanical system, are evaluated. Guidelines are suggested which can be used to optimize the accuracy in measuring situations.

Keywords : Dimensional metrology, Optical probe, Accuracy

1. Introduction Optical probing techniques for dimensional metrology have been investigated during the last decade, and have been used for a number of different laboratory and industrial applications /1-5/. Optical techniques feature much higher measuring speeds than tactile measurements, and are therefore potentially indicated for industrial applications integrated into production. However, the accuracy obtainable using optical tech- niques is low, compared to the single measurement micrometer requirement that is often encountered in the production, and which can be achieved with tactile instruments. Based on experimental results from a series of investigations carried out at the authors’ university during the last three years, this paper deals with the metrological limitations of optical probing techniques in the field of dimensional metro- logy. The paper considers three applications of optical probing techniques for dimensional metrology: a CCD-based system used on a coordinate mea- suring machine (CMM), an experimental roundness tester equipped with a line-scan camera, and an on- line measuring station using a CCD-camera for mea- surement of dimensions in a production line.

2. Optical probing systems Many parameters influence the measurement result when measuring dimensions with an optical mea- suring system. In the investigations referred to in this work special attention was paid to the following factors: sensor, optical system, illumination, object, background and mechanical system, as illustrated in Fig. 1. In this paper, the impact of each of these factors on the measuring accuracy will be illustrated using experimental results from investigations of three different applications of optical probing techniques.

The CMM used in the first investigation was a Zeiss OMC 850 based on a CCD sensor with 510*492 pixels and using autofocusing and edge detection /6,8,9/. The camera can be equipped with different objectives, giving different resolutions as described in Table 1. The sensor used in the experimental round- ness tester developed at the authors’ university was a linescan camera with 2048 lines placed in connec- tion with a microscope and giving a resolution of 1.7 pm /7/. The sensor of the optical measuring station for the production line was a CCD camera with 752*582 pixels having the dimensions 20 pm * 20 pm. This measuring station is intended for dimen- sional measurements on a tubular plastic item /lo/.

3. Experimental results Sensor. The influence of the pixel size of the CCD ca- mera on the resolution of the measuring system was investigated using both the CMM and the measuring station. In an investigation on the CMM the diameter of a 4 mm ring was measured using different sizes of the measuring field, and it was verified that the pixel size determines the secondary resolution and thus the accuracy /8/. For the optical measuring station a strong interaction was shown between the sensor accuracy and the mechanical set-up. The position of the sensor relatively to the object influences the result because of the large pixel size, and a theoretical model was developed for this /lo/. Fig. 2 shows the results of theoretical model and experimental inves- tigation. The pixel network was moved in 2 pm steps over one pixel length (20 pm) and the influence on the measurement result of a particular diameter on the plastic item was determined. A total variation of approximately 5 pm was observed both in theory and practice /lo/. This figure represents the lower accu- racy limit for this particular measurement.

Annals of the ClRP Vol. 44/1/1995 501

Optical system. The influence of the optical system was investigated on the CMM. Results of autofocusing on two different surfaces are shown in Fig. 3. Two different objectives were used, and the light intensity was varied. The standard deviation of the single measurements was less than 1 pm /6/. The depen- dence on the light intensity becomes higher for in- creasing focal length. This was explained by the fact that an objective with a larger focal length also has a larger aperture angle and it thus receives more light /6/. Discrepancies up to 40 pm can be recognised on Fig. 3, when two different optics are considered. These investigations emphasize the importance of the optical system as set-up parameter and stress the need for identifying the objective that has been used for a given measurement.

Illumination. The influence of illumination was investi- gated using the CMM. Fig. 3 shows a strong depen- dence of the autofocusing result on the light intensity /6/. Fig. 4 shows the effect of different illuminations and light intensities using edge detection on a white ceramic gauge block. The length of a 1.004 mm gauge block was measured optically, without moving the measuring head on the CMM. The standard deviation of measurements was less than 1 pm /6/. The light sources were checked for non-linearity and no effect was found / 8 / . When increasing the inten- sity, the pixels experience blooming and measure- ment is no longer possible /6/. Deviations from the certified gauge block length up to 20 pm were ex- perienced using different types of illumination and different intensities /6/. Also this set-up parameter was shown to be of great importance for the mea- surement result.

Object. The influence from the object characteristics was investigated using both the CMM and the optical roundness tester. Fig. 3 indicates the differences in autofocusing results when measuring different sur- faces. Fig. 5 shows results obtained with the optical roundness tester, which also indicate that the object surface influences the measurement result. The deviation from the reference roundness error measu- red on a conventional form tester was approximately 5 pm for clean plug gauges, slightly higher for black chromium-plated items, and it increased to 20 pm in the presence of surface contamination /7/.

Background. The influence of a single environmental parameter was investigated using the CMM. A 1 mm thick steel plate was placed on different backgrounds and measured by means of edge detection. Results are shown in Fig. 6. There is a tendency that reflec- ting backgrounds give other results than dark back- grounds. The variation is up to approximately 5 pm / 8 / . Also the influence of the objectives is quite evident here.

Mechanical system. The interaction of the optical system with the mechanical system including the measurement setup was investigated both on the CMM and on the optical measuring station. Fig. 7 shows the deviation from the best result obtained with edge detection as a function of distance out of focus. The deviations can be calculated theoretically, as shown in Fig. 7 /6/. Several different positioning errors can be expected when the measuring station is implemented in production. Both theoretical models and measurements have shown the importance of different parameters on the results. Fig. 8 shows the effect of slight part rotations on a particular diameter measurement /lo/. It should be noticed that even small angles of rotation give large deviations in the measured diameter. Other characteristic deviations for the measuring station lay in the range of 10-20 pm, while 1-2 pm accuracy is required.

4. Discussion The accuracy of optical probing techniques in dimen- sional measurements is limited by a number of factors which must be considered in planning and performing measurements. If all set-up parameters are under control, accuracies in the 1-2 pm range can be reached, while typically 10-20 pm accuracy can be expected in most applications. In the worst cases, deviation as high as 30-40 pm may be experienced.

In order to reach the best measuring results, it is very important to control the additional influence parame- ters of an optical nature. This should be done at least by recording all parameters connected to a measure- ment set-up, e.g. using a data sheet as shown in Table 2.

5. Conclusions Three applications of optical probing techniques for dimensional metrology were investigated experimen- tally, and the metrological limitations evaluated considering the following factors: sensor, optical system, illumination, object, background, and mecha- nical system.

It was concluded that optical probing techniques allow dimensional measurements with accuracies of the order of 1-2 pm, when all set-up parameters are under control, and approximately ten times worse accuracy in most cases. In most of the cases consi- dered, a single micrometer accuracy is required but hard to achieve using optical probing techniques. Guidelines were suggested which can be used to optimize the accuracy in measuring situations.

6. Acknowledgements The authors want to thank Jan L. Andreasen and F. Grannegaard for their help in preparing this paper.

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7. References 1. Takesa, K., Sato, H., Tani, Y., 1984, Measurement

of Diameter Using Charge Coupled Device (CCD), Annals of the CIRP, Vol. 33/1, p.377- 381

2. Poulter, K.F., 1985, Vision Systems for Dimensio- nal Metrology, Annals of the CIRP. Vol. 34/2,

3. Batchelor, B.G., Hillard, D.A., Hodgson, D.C., 1985, Automated Visual InsPection, IFS (Publica- tions) Ltd and North-Holland

4. Tonshoff, H.K., Janocha, H., Seidel, M, 1988, Image Processing in a Production Environment, Annals of the CIRP, Vol. 37/2, p.579-590

5. Huser-Teuchert, D., Trapet, E., Garces, A., Torres- Leza, F., Pfeifer, T., Schrarsich, P., 1994, Perfor- mance test procedures for optical coordinate mesuring probes. BCR report EUR 15314 EN.

6. Larsen, M.G., 1993, Optical Coordinate Metrology, M.Sc.-thesis, Department of Mechanical Proces- sing of Materials, Institute of Manufacturing Engi- neering, Technical University of Denmark, (in Danish)

7. Andreasen, J.L., Glibbety, S.J., 1993, Optical Roundness Measurement, Department of Mechani- cal Processing of Materials, Institute of Manufac- turing Engineering, Technical University of Den- mark

8. Hansen, H.N., 1994, Influence Parameters in Optical Coordinate Metrology, Department of Mechanical Processing of Materials, Institute of Manufacturing Engineering, Technical University of Denmark, (in Danish)

9. Hansen, H.N., De Chiffre, L., 1994, Applications and Limits of Optical Coordinate Metrology, 16th Nordic Conference "Measuring Techniques and Calibration", Association of Notwegian Engineers.

lO.Grsnnegaard, F., 1994, Industrial Application of Computer Vision for Geometrical Measurement, M.Sc.-thesis, Department of Mechanical Proces- sing of Materials, Institute of Manufacturing Engi- neering, Technical University of Denmark, (in Danish)

p.593-598

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Table 1. Data for the objectives used on the CMM /8/.

I - [ I Camera

I / \ ' , 6

2 Optical system

%\

4 Objed ,\\ - 6 Mechanical system

Fig. 7 Factors influencing the accuracy of measure- ments by optical probing techniques.

6 T

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Shift from nominal position [pm]

L-TJeory Experiment! _ _ _ -

Fig. 2 Influence on measured diameter when moving the CCD camera over a 20 pm pixel in 2 pm steps. Deviation from initial diameter /l O/.

Fig. 3 Results from autofocusing in z-direction on CMM. Ring illumination. Objective sizes refer to Table 1 / 6 / .

503

lo T Deviation

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fig. 4 Deviation from certified length using edge detection on white ceramic gauge block on CMM. 25 mm objective. A: co-axial illumination; B: ring illumination /6 / .

0 1 I 0 20 40 60 80

Diameter [mm]

A Lapped, pped, Oiled A Black lapped ___-

Fig. 5 Deviation from reference roundness error, as obtained with the experimental optical roundness tester. Measurements on different plug gauge diame- ters as indicated / 7 / .

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Fig. 6 Results of measurements of thin item on differ- ent backgrounds on CMM. Objectives refer to Table 1 / 8 / .

[ W I

Distance out of focus bm]

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1 Experiment -Theory1

Fig. 7 Effect of defocusing on edge detection on CMM /6/.

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Fig. 8 Effect of rotation of item on measured diame- ter on the optical measuring station. Theoretical and experimental results. Deviation from initial diameter /to/.

Object:

Sketch:

Operator:

Date:

2 autofocusing: - Area - Edge

Ob],lectiVe: 25mm 40mm 63mm

Illumination: - Coaxial -Ring

Intensity:

lllumination/intensity constant during measurement: Yes - No If not: Show actual illumination on sketch above

- Other:

Special remarks: Synthetic edge (14): Object background colour: Background illumination: Size of measuring field: - 7*7 - 17*17

Other commZts: 25*2!5 - 21*7 -9*25

Table 2. Example of sheet to record optical CMM set-up data / 8 / .

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