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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 20 — Oct. 4, 2004
  • pp: 4905–4911
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High-speed measurements of steel-plate deformations during laser surface processing

Matija Jezeršek, Valter Gruden, and Janez Možina  »View Author Affiliations


Optics Express, Vol. 12, Issue 20, pp. 4905-4911 (2004)
http://dx.doi.org/10.1364/OPEX.12.004905


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Abstract

In this paper we present a novel approach to monitoring the deformations of a steel plate’s surface during various types of laser processing, e.g., engraving, marking, cutting, bending, and welding. The measuring system is based on a laser triangulation principle, where the laser projector generates multiple lines simultaneously. This enables us to measure the shape of the surface with a high sampling rate (80 Hz with our camera) and high accuracy (±7 μm). The measurements of steel-plate deformations for plates of different thickness and with different illumination patterns are presented graphically and in an animation.

© 2004 Optical Society of America

1. Introduction

Surface-processing techniques that involve the use of lasers—for example, marking, engraving, cutting, cleaning, welding and forming—have become increasingly common as industrial processes over the past few years [1

1. V. Kovalenko and R. Zhuk, “Systemized approach in laser industrial systems design,” J. Mat. Processing Technol. 149 (1–3), 553–556 (2004). [CrossRef]

]. A common feature of all these techniques is the need for them to operate as close as possible to the optimal working point. However, this situation usually involves a compromise between the operation time for the process and the quality of the finished product. For example, increasing the average power of the laser and the traveling speed of the laser beam usually decreases the operating time; however, at a certain point the quality of the processed surface is no longer acceptable as a result of unwanted permanent deformations. These deformations, which are caused by various thermoplastic mechanisms, such as the temperature-gradient mechanism, the buckling mechanism or the upsetting mechanism [2–4

2. K. G. Watkins, S.P. Edwardson, J. Magee, G. Dearden, and P. French, “Laser Forming of Aerospace Alloys,” Proceedings of the 2001 Aerospace Congress SAE Aerospace Manufacturing Technology Conference Seattle (2001).

], must be minimized for processes like engraving, marking, cutting, and cleaning. On the other hand, during laser-forming processes the deformation should generate the required shape with the minimum of deviations [5–8

5. J. Bao and Y. L. Yao, “Analysis and Prediction of Edge Effects in Laser Bending,” ASME Trans. J. Manufacturing Sci. Eng. 123(1), 53–61 (2001). [CrossRef]

].

At present there is a lot of effort being put into investigating the fundamentals and possible applications of laser-forming processes. Indeed, many analytical and numerical models have already been proposed; however, these are mainly restricted to very simple laser-beam patterns and initial plate shapes [3

3. M. Dove, J. Možina, and F. Kosel, “Optimizing the final deformation of a circular plate illuminated by a short laser pulse,” J. Phys. D: Appl. Phys. 32, 644–649 (1999). [CrossRef]

,5

5. J. Bao and Y. L. Yao, “Analysis and Prediction of Edge Effects in Laser Bending,” ASME Trans. J. Manufacturing Sci. Eng. 123(1), 53–61 (2001). [CrossRef]

,9

9. H.S. Hsieh and J. Lin, “Thermal-mechanical analysis on the transient deformation during pulsed laser forming,” International J. Machine Tools and Manufacture 44 (2–3), 191–199 (2004).

]. Experimental measurements of the deformed surface have also been limited to some characteristic values, such as the bending angle or the radii of curvature [2

2. K. G. Watkins, S.P. Edwardson, J. Magee, G. Dearden, and P. French, “Laser Forming of Aerospace Alloys,” Proceedings of the 2001 Aerospace Congress SAE Aerospace Manufacturing Technology Conference Seattle (2001).

,9

9. H.S. Hsieh and J. Lin, “Thermal-mechanical analysis on the transient deformation during pulsed laser forming,” International J. Machine Tools and Manufacture 44 (2–3), 191–199 (2004).

,10

10. Z. Hu, R. Kovacevic, and M. Labudovic, “Experimental and numerical modeling of buckling instability of laser sheet forming,” International Journal of Machine Tools and Manufacture 42(13), 1427–1439 (2002).

]. Furthermore, these measurements are mainly performed after the process is complete, using a linear variable differential transformer (LVDT), a coordinate measuring machine (CMM), or techniques such as laser-point or line-triangulation. So far, only one group has published some preliminary experimental results from a full-field dynamic shape measurement during laser-based plate bending [11

11. M. Reeves, A. J. Moore, D. P. Hand, and J. D. C. Jones, “Dynamic shape measurement system for laser materials processing,” Opt. Eng. 42(10), 2923–2929 (2003). [CrossRef]

].

This paper presents a high-speed technique for a three-dimensional measurement of plate deformation during laser processing. The full-field absolute-distance measurement is performed on a single acquired image, where the illuminating laser-light pattern is composed of multiple light planes. The laser-light source was used because of its several advantages over ordinary light sources (such as halogen or xenon light sources). Firstly, the ambient light can be efficiently filtered out due to the monochromatic nature of the laser light. Secondly, the laser pattern can be held in tight focus over a long range. And thirdly, the heat dissipation of lasers (especially semiconductor diode lasers) is much less than with conventional projectors, which consequently reduces the measuring errors due to any thermal expansion of the apparatus [12

12. B. Curless, New Methods for Surface Reconstruction from Range Images (Stanford - Ph.D. Dissertation1997).

].

2. Experimental set-up

The experimental set-up for the laser surface processing is shown in Fig. 1. The main components are the processing laser with its XY scanning head, the shape-measuring apparatus and the personal computer. The experimental set-up is constructed in such a way that the specimen’s top side is laser processed, while the measurements are made simultaneously on the bottom side of the specimen. The problem of the very intense scattered light from the processing laser, which disables the shape measurement near this point [11

11. M. Reeves, A. J. Moore, D. P. Hand, and J. D. C. Jones, “Dynamic shape measurement system for laser materials processing,” Opt. Eng. 42(10), 2923–2929 (2003). [CrossRef]

], is effectively solved by using such a configuration.

The specimens are 0.1, 0.2 or 0.5 mm thick. The material is Ck 101 [13

13. DIN 17222, “Kaltgewalzte Stahlbänder für Federen,” Deutsche Normen August 1997.

] with a yield stress of 1275 N/mm2, a tensile stress of ~1500 N/mm2 and an elongation of 6%.

Fig. 1. Experimental set-up.

The processing laser is a diode-pumped Nd-YAG laser with a wavelength of 1064 nm. It can operate in either pulsed or continuous modes. The pulse duration varies from 50 to 600 ns and its maximum average output power is 8 W. Both the pulse duration and the maximum power are functions of the frequency and the electrical current that drives the diode. The XY galvo-type scanning head has an f-theta lens with a 160-mm focal length. It has a square scanning region with side lengths of 90 mm. The resolution along each axis is 1.5 μm, the maximum traveling speed of the beam is 90 m/sec, and the minimum traveling speed is 1.8 mm/sec. This laser system is primarily designed for marking or engraving applications, but the laser drilling of small-diameter holes and the cutting of thin plates can also be accommodated.

The shape-measuring apparatus is based on the multiple-line laser triangulation principle, similar to the one used in [14

14. D. Bračun, M. Jezeršek, and J. Možina, “Apparatus for determining size and shape of a foot,” PCT patent nr. WO2004037085 (2004).

]. It consists of a laser projector and a camera. The laser projector (Lasiris SNF-533L, 20 mW, 670 nm) generates a light pattern of 33 equally inclined light planes that are directed toward the measured surface. The middle light plane has a higher intensity than the others. The camera (Basler 301f, 656×494 pix., 8 bit, 80 frames/sec, Fire Wire) records the illuminated surface from a different viewpoint, and consequently, the light pattern is distorted by the shape of the surface. To improve the image contrast an interference filter (10 nm FWHM, center at 670 nm) is placed between the lens and the camera’s CCD sensor.

The measuring apparatus is designed to operate in two modes: high-speed and real-time. In the high-speed mode the image sequence is acquired first, and the processing is done later. The maximum acquisition speed is limited to 80 Hz by the camera. In the real-time measuring mode all the processing is done after each image is acquired. The maximum measuring speed is, therefore, lower. However, it is still fairly high, approximately 5 Hz, when using a 1 GHz Celeron processor.

The measuring range of the apparatus is approximately 40×30 mm in the horizontal plane and approximately 10 mm in the vertical direction. The calibration is made in situ, simply by replacing the specimen with the reference sample: a groove-shaped plate, which is measured at various heights. The transformation parameters are then numerically optimized until the minimum deviation (the sum of the squared errors) between the measured points and the reference surface is found. The major advantage of this procedure is that all the transformation parameters can be determined in a single measuring step, i.e., the camera’s internal parameters (focal length, central point and distortion), the projector’s distortion and the projector’s position relative to the camera (rotation and translation). The accuracy of the calibrated apparatus in vertical direction is ±7 μm, which is calculated as a standard deviation between points of a measured and nominal reference surface.

This relatively high accuracy was achieved by employing some additional actions. In the case of a shiny surface, the specimens were given a removable white coating (HELLING gmbh, Standard-Check, medium nr. 3) on their bottom, i.e., measurable, surface. The influence of the speckle noise was further minimized by setting the aperture of the camera lens to its highest value (f/1.4). The remaining high-frequency noise – mostly due to speckles and surface roughness – was filtered out by applying a spatial average over a line length of approximately 1 mm during the line-detection phase. And finally, the calibration and measuring procedure were always performed after constant temperature conditions were achieved.

3. Results and discussion

Fig. 2. Plate deformation during laser-spot illumination. The beam diameter was 200 μm for the first row, and 1 mm for the second row. The illumination time was 2 sec in both cases. The vertical axes in the graphs are magnified 10 times. [Media 1]

As a result of the rapidly growing number of published numerical simulations involving laser bending processes where the beam-propagation path is a straight line along the entire plate width, a second set of experiments was undertaken to look at plate deformation during such an illumination procedure. The 0.2-mm-thick, 25×40-mm plate was freely supported on its two shorter sides. The laser was operated in continuous mode, the beam diameter was 200 μm, and the beam-propagation speed was 1.8 mm/s. From Fig. 3 and from the movie clip the bending process can be clearly observed; it is also clear that quite complex thermo-mechanical effects are present. The most interesting of these are the phenomena at the start of the illumination. We can see, for example, that the deformation during the first scan has the opposite sign to the deformation during subsequent scans. In addition, the location of the current beam is clearly visible throughout the entire process because the position of the current beam is reflected in the point of maximum deformation. The plate was scanned four times, and the time between two consecutive scans was ~20 sec.

Fig. 3. Plate deformation during laser illumination with a linear beam-propagation path. The image shows the plate deformation during the third scan at the moment when the beam reaches ¾ of the plate width. The vertical axis is magnified 10 times. [Media 2]

Fig. 4. Plate deformation after laser drilling with various frequencies.

Fig. 5. Laser-based flattening of a convex-shaped plate deformation.

The proper shape-measuring apparatus plays an important role in the various stages of the laser-based flattening process: first, it locates the deformed features; second, it helps to select the proper beam-propagation path; and third, it helps us to observe the progress of the flattening—and tell us how well the process is going—in real time.

4. Conclusions

Our high-speed technique for three-dimensional measurements of plate deformation presented above is opening up new possibilities for monitoring and controlling various laser-processing techniques, e.g., laser forming, drilling, cutting, and engraving. The measuring apparatus is designed to operate in high-speed mode with subsequent image processing and a measuring speed of 80 Hz. Alternatively, we can operate in real-time mode, and the speed of the surface-deformation observation is 5 Hz. In both cases the accuracy is ±7 μm. The full-field absolute-distance measurement is performed on a single acquired image. The problem of very intense scattered light from the processing laser, which disables the shape measurement near this point, is effectively solved by using a configuration where the plate is measured from the opposite side relative to the processing laser. When this is not possible, due to, for example, hollow specimens without any convenient opening, there is still a front-side solution. But in this case a more powerful laser projector should be used together with the already included narrow-band interference filter, which is placed in front of the camera and is intended to transmit only the light that has the same wavelength as the projector emits.

Reference and links

1.

V. Kovalenko and R. Zhuk, “Systemized approach in laser industrial systems design,” J. Mat. Processing Technol. 149 (1–3), 553–556 (2004). [CrossRef]

2.

K. G. Watkins, S.P. Edwardson, J. Magee, G. Dearden, and P. French, “Laser Forming of Aerospace Alloys,” Proceedings of the 2001 Aerospace Congress SAE Aerospace Manufacturing Technology Conference Seattle (2001).

3.

M. Dove, J. Možina, and F. Kosel, “Optimizing the final deformation of a circular plate illuminated by a short laser pulse,” J. Phys. D: Appl. Phys. 32, 644–649 (1999). [CrossRef]

4.

L. Zhang and P. Michaleris, “Investigation of Lagrangian and Eulerian finite element methods for modeling the laser forming process,” Finite Elements in Analysis and Design 40(4), 383–405 (2004). [CrossRef]

5.

J. Bao and Y. L. Yao, “Analysis and Prediction of Edge Effects in Laser Bending,” ASME Trans. J. Manufacturing Sci. Eng. 123(1), 53–61 (2001). [CrossRef]

6.

Jitae Kim and S. J. Na, “Feedback control for 2D free curve laser forming,” Optics & Laser Technology In Press, Corrected Proof, Available online 26 April 2004.

7.

G. Thomson and M. Pridham, “A feedback control system for laser forming,” Mechatronics 7(5), 429–441 (1997). [CrossRef]

8.

An. K. Kyrsanidi, Th. B. Kermanidis, and Sp. G. Pantelakis, “Numerical and experimental investigation of the laser forming process,” J. Mat. Processing Technol. 87(1–3), 281–290 (1999). [CrossRef]

9.

H.S. Hsieh and J. Lin, “Thermal-mechanical analysis on the transient deformation during pulsed laser forming,” International J. Machine Tools and Manufacture 44 (2–3), 191–199 (2004).

10.

Z. Hu, R. Kovacevic, and M. Labudovic, “Experimental and numerical modeling of buckling instability of laser sheet forming,” International Journal of Machine Tools and Manufacture 42(13), 1427–1439 (2002).

11.

M. Reeves, A. J. Moore, D. P. Hand, and J. D. C. Jones, “Dynamic shape measurement system for laser materials processing,” Opt. Eng. 42(10), 2923–2929 (2003). [CrossRef]

12.

B. Curless, New Methods for Surface Reconstruction from Range Images (Stanford - Ph.D. Dissertation1997).

13.

DIN 17222, “Kaltgewalzte Stahlbänder für Federen,” Deutsche Normen August 1997.

14.

D. Bračun, M. Jezeršek, and J. Možina, “Apparatus for determining size and shape of a foot,” PCT patent nr. WO2004037085 (2004).

OCIS Codes
(110.6880) Imaging systems : Three-dimensional image acquisition
(140.3390) Lasers and laser optics : Laser materials processing

ToC Category:
Research Papers

History
Original Manuscript: July 16, 2004
Revised Manuscript: September 24, 2004
Published: October 4, 2004

Citation
Matija Jezeršek, Valter Gruden, and Janez Možina, "High-speed measurements of steel-plate deformations during laser surface processing," Opt. Express 12, 4905-4911 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-20-4905


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References

  1. V. Kovalenko, R. Zhuk, �??Systemized approach in laser industrial systems design,�?? J. Mat. Processing Technol. 149 (1-3), 553-556 (2004). [CrossRef]
  2. K. G. Watkins, S.P. Edwardson, J. Magee, G. Dearden, P. French, �??Laser Forming of Aerospace Alloys,�?? Proceedings of the 2001 Aerospace Congress SAE Aerospace Manufacturing Technology Conference Seattle (2001).
  3. M. Dovc, J. Možina, F. Kosel, �??Optimizing the final deformation of a circular plate illuminated by a short laser pulse,�?? J. Phys. D: Appl. Phys. 32, 644-649 (1999). [CrossRef]
  4. L. Zhang, P. Michaleris, �??Investigation of Lagrangian and Eulerian finite element methods for modeling the laser forming process,�?? Finite Elements in Analysis and Design 40(4), 383-405 (2004). [CrossRef]
  5. Bao, J., Yao, Y. L., "Analysis and Prediction of Edge Effects in Laser Bending," ASME Trans. J. Manufacturing Sci. Eng. 123(1), 53-61 (2001). [CrossRef]
  6. Jitae Kim, S. J. Na, �??Feedback control for 2D free curve laser forming,�?? Optics & Laser Technology In Press, Corrected Proof, Available online 26 April 2004.
  7. G. Thomson, M. Pridham, �??A feedback control system for laser forming,�?? Mechatronics 7(5), 429-441 (1997). [CrossRef]
  8. An. K. Kyrsanidi, Th. B. Kermanidis, Sp. G. Pantelakis, �??Numerical and experimental investigation of the laser forming process,�?? J. Mat. Processing Technol. 87(1-3), 281-290 (1999). [CrossRef]
  9. H.S. Hsieh, J. Lin, �??Thermal�??mechanical analysis on the transient deformation during pulsed laser forming,�?? International J. Machine Tools and Manufacture 4 (2-3), 191-199 (2004).
  10. Z. Hu, R. Kovacevic, M. Labudovic, �??Experimental and numerical modeling of buckling instability of laser sheet forming,�?? International Journal of Machine Tools and Manufacture 42(13), 1427-1439 (2002).
  11. M. Reeves, A. J. Moore, D. P. Hand, and J. D. C. Jones, �??Dynamic shape measurement system for laser materials processing,�?? Opt. Eng. 42(10), 2923-2929 (2003). [CrossRef]
  12. B. Curless, New Methods for Surface Reconstruction from Range Images (Stanford - Ph.D. Dissertation 1997).
  13. DIN 17222, �??Kaltgewalzte Stahlbänder für Federen,�?? Deutsche Normen August 1997.
  14. Bracun D., Jezeršek M., Možina J., �??Apparatus for determining size and shape of a foot,�?? PCT patent nr. WO2004037085 (2004).

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