Performance of an optical stand-off control system for laser materials processing

Optics and Lasers in Engineering 21 (1994) 165-180 © 1994 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0143-8166/94/$7.00 ELSEVIER

Performance o f an Optical Stand-Off Control System for Laser Materials Processing

Hussein A. Abdullah, Chris R. Chatwin & Ming-Yaw Huang Laser and Optical Systems Engineering Group, Department of Mechanical Engineering, University of Glasgow, Scotland, UK, G12 8QQ (Received 4 January 1994; accepted 31 January 1994)

A B S T R A CT This paper presents an experimental investigation into a high-bandwidth optical range sensor for laser materials processing stand-off control. Radiation from a low-power laser beam is focused onto a workpiece surface and light reflected from the surface is collected through a main lens and directed into an imaging lens which focuses the signal to two positions after being split by a beam splitter. The irradiances of the two beams are detected by photodiodes placed behind pinhole apertures positioned fore and aft of the two focal positions. A differential amplifier is used to generate an output signal that determines the magnitude and direction of any workpiece displacement. The system facilitates a measuring range of +6 ram. A set of of experiments are performed and results are analysed for different setup configurations. The approximate range of instrument linearity is ±1 mm for the 75-mm focal length main lens and +2 mm for the 120-mm lens; in this linear range the optimal accuracy resolution is 1 tzm. The system's effectiveness in controlling the stand-off distance of a laser cutting machine, and hence cut quality, is assessed.

1 INTRODUCTION

A n optical sensor is, by definition, a n o n - c o n t a c t device which uses visible or invisible light as a m e d i u m for i n f o r m a t i o n transfer. Asso165

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ciated optical and opto-electronic c o m p o n e n t s are used to m o d u l a t e and m e a s u r e r e t u r n e d data. Generally, the principle of operation is based on the focusing of a light b e a m onto an object and the detection of the r e t u r n e d light by means of a photodetector. Optical sensors are used in sensing the location, presence or absence, distance and orientation of an object surface. Simple (binary) optical proximity sensors are designed to detect the reflection of diffuse light from a surface physically close to the sensor, or the interruption of a light b e a m b e t w e e n source and sensor. M o r e complex devices, known as range sensors are fixed on the end of a manipulator to keep it at a specific distance from a surface whilst performing machining or inspection tasks; these are t e r m e d stand-off sensors. O t h e r devices directly inspect the machined surface properties of components. Stand-off sensors are used to m e a s u r e the distance b e t w e e n a workpiece and a processing tool. T h e y can be classified, according to the measuring technique, into the contact type, such as mechanical, or non-contact, based on capacitive, inductive, pneumatic or optical principles. The disadvantage of the mechanical type is the n e e d for physical contact with the workpiece, which m a y cause d a m a g e to surfaces and are thus only for use on hard solids. Capacitive or inductive sensors are susceptible to errors due to the presence of edges, holes or changes in material thickness. P n e u m a t i c devices cannot cope with the presence of holes or slits. A wide variety of optical sensors are available, making use of different m e t h o d s of propagation and collection techniques, optical m e a s u r e m e n t schemes and p h o t o d e t e c t o r layouts.' " Optical sensors can be classified according to their reflected b e a m path as off-axis or on-axis. If the reflected b e a m in the sensor m o d u l e returns along a different path to that of the initial probe beam, then it is in the off-axis class. Conversely if the reflected b e a m returns along the same path as the inspection b e a m the sensor m a y be classed as onaxis. The optical sensor detailed herein is an on-axis system exploiting a laser b e a m and two p h o t o d i o d e detectors. The system provides a measuring range of + 6 mm. A set of experiments were p e r f o r m e d and the results analysed. The approximate range of instrument linearity is +1 m m for the 75-mm focal length mains lens and + 2 m m for the 120-mm lens; in this linear range the o p t i m u m accuracy resolution is 1/zm. D u e to its high bandwidth and accuracy it has proven to be ideal for fine control of laser b e a m stand-off distance.

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2 R E Q U I R E M E N T F O R L A S E R P R O C E S S I N G STAND-OFF CONTROL If the stand-off distance can be accurately measured, the processing beam can be controlled to follow a workpiece profile accurately. In the laser cutting process, the cut quality is mainly assessed by the kerf side wall surface roughness which is characterised by the surface striation frequency. Figure 1 illustrates the gas jet nozzle with coaxial laser beam, a Ferranti MFKP 1.2kW CO2 laser. The cutting nozzle is mounted on a flying optics beam delivery system controlled by a Heidenhein CNC controller. Figure 2 shows the relationship between the kerf surface roughness and the nozzle stand-off distance. Figure 3 illustrates the relationship between the surface striation frequency and the nozzle stand-off distance. Kerf surface roughness increases when the nozzle stand-off distance is increased or decreased. It is therefore important to control the stand-off distance during the laser cutting process. Process stability is essential for a consistent cut quality; kerfs (1) and (3) in Fig. 4 illustrate low cut quality. The figure shows the kerf surfaces of mild steel cut by a laser with different stand-off distances. In order to control the quality of the cutting process the optical stand-off controller has been developed which achieves accurate height control between the cutting nozzle and the workpiece surface. A system diagram that incorporates the optical sensor is illustrated in Fig. l(a). Utilising the stand-off controller a consistently high quality cut was achieved (see Fig. 4 kerf (2)). 3 WORKING PRINCIPLE The working principle is shown in Fig. 5. A laser beam which passes through a beam splitter is focused onto the workpiece surface by the main lens to form a new beam waist. 9"1° The light is reflected from the workpiece surface, collected through the main lens and redirected into the image lens to focus after being split by beam splitter 2. The intensities of the two beams are then detected by two photodiodes placed behind pinholes of equal size. Pinhole 1 is positioned at a distance l behind the beam focal plane and pinhole 2 is placed at the same distance 1 in front of the split beam focal plane. The effect of a small longitudinal displacement of the workpiece in the direction Z2 parallel to the beam axis will substantially alter the magnitude of the irradiance arriving at the planes of the pinholes. As a result the power detected by one photodiode will increase, whilst that at the second will

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decrease. A differential amplifier is then used to condition the output signal and hence determine the magnitude and direction of displacement Z2. It can be seen in Fig. 6 that the relationship between the distance and the output voltage is non-linear, except for a portion between the positive and negative peaks. This linear range is determined by the size and position of lenses, pinholes and photodiodes. This experimental investigation aims to optimise component position, size and output

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kerr (1) kerf (2) kerr (3) Fig. 4. Variation of the cut quality for different stand-off distances. signal, as they control the response of the device in terms of its accuracy and linearity. F u r t h e r m o r e , it identifies the p a r a m e t e r s that are critical to final system design.

4 OPERATIONAL

CHARACTERISTICS

The experimental apparatus can be divided into two sub-systems: (i)

A n optical system which transmits an image of the reflected laser b e a m to form a spot in front of the photodiodes. This spot

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size is related to the spot size of the focused beam on the workpiece surface and hence the stand-off distance. A transducing and signal conditioning system, which transduces the laser power transmitted through the pinholes and converts it into an electrical signal. This signal is then conditioned and amplified to determine the workpiece displacement (Fig. 6).

The experimental optical system, Fig. 5, used an unexpanded h e l i u m - n e o n laser beam (wavelength A = 0.633 p~m, output power of 4 m W ) to provide a bright, highly directional light source. Two main lenses with focal lengths of 75 and 120 m m were used together with a 300-mm focal length image lens. Three pinhole diameters were used (100, 200, 300 p~m) to investigate the effect of pinhole size on the output signal. The distance between the photodiodes and the pinholes was made as small as possible, to reduce truncation and perturbation effects. The output signals from the two photodiodes were fed into a differential amplifier where they were amplified and normalised (see Fig. 6). The output from this was fed to the Y axis of the X - Y recorder. The axial displacement of the workpiece was recorded by the output of a linear voltage displacement transducer and fed to the X axis of the X - Y recorder.

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5 EXPERIMENTAL PROCEDURE Two series of experiments were conducted using focal lengths of 75 and 120mm; for brevity, detailed experimental results are only presented for the 75-ram focal length system. Each series of experiments attempted to determine the effect of changing pinhole size, pinhole offset distance, angle of the workpiece and distance between the main and image lenses. Only the experimental results for the 75-mm main lens are included in the figures; the results for both 75-mm and 120-ram main lenses are discussed in Section 6. Whilst the system successfully operates over the full + 6 m m range the control finesse over the linear response region is significantly better than over the non-linear region. For this reason the design was optimised to maximise the linear range, which is defined by the distance between the positive and negative peaks of the output response (Fig. 6).

6 RESULTS A N D DISCUSSION The results were recorded in graphical form for all experiments using two different focal length main lenses. The horizontal axis of each graph recorded the deviation of the workpiece surface from the beam waist in millimetres, whilst the vertical axis recorded the normalised output voltage from the sensor. Each graph is annotated with a list of main experimental parameters. From Fig. 7 it can be seen that the pinhole offset distance I affects the characteristics of the output signal. It can be concluded that reducing the offset distance of the pinholes reduces the peak-to-peak distance of the output signal and hence gives higher sensitivity. The sensitivity is defined as the horizontal peak-to-peak distance. The magnitude of the normalised output signal is adjustable via the gain of the differential amplifier. From the results it can be seen that for a given pinhole size, when the pinhole offset distance is reduced, the linearity and the sensitivity of the normalised output increases and the peak output voltage decreases. This was the case for both configurations. This occurs because the pinhole offset distance (+l) controls the overlap of the voltage signals V~ and V2. This can be seen in Fig. 8. The differential amplifier produces a summation of these two voltages as a normalised output voltage. At 100% overlap the normalised output voltage is zero (Fig. 8(a)). The output increases as the percentage ratio

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of the overlap decreases, until it reaches a maximum output at 0% overlap (Fig. 8(c)). Any further increase in the distance (+l) forms a section with no overlap between the signals and a consequent loss in linearity. This loss in linearity can be observed from the results for both the 75- and 120-mm main lenses for all pinhole offsets other than +5 mm (curve No. 1, Fig. 7). After extensive experimentation it was established that the optimum distance for the pinhole offset in the configurations studied was +5 mm for pinhole 1 and - 5 mm for pinhole 2. Thus these values were held constant throughout the rest of the experiments. The results in Fig. 9 show that the pinhole size h a s an important effect on output signal. As the pinhole diameter is decreased the sensitivity of the sensor increases; this however, leads to a weaker and noisy signal due to speckle. The optimum size for a particular design is arrived at by compromising between sensitivity, output signal truncation and noise effects. A pinhole of 100/zm diameter (curve No. 3) gives the smallest distance between peaks leading to the most sensitive output, but suffers from large noise effects due to the interaction of speckle and truncation effects. A 300-/zm diameter (curve No. 1) gives a larger distance between the peaks (reduced sensitivity) with less noise; due to its large size speckle noise is integrated out. A pinhole

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size of 200/xm gives results that are intermediate to those obtained with 100- and 300-p~m pinholes. This gives a good compromise between sensitivity and noise immunity, and thus subsequent experiments used a 200-p,m diameter pinhole. Figure 10 shows the effect of changing the distance between the laser and main lens on sensor output. This shows that as the laser is moved away from the main lens the sensitivity and the magnitude of the output increases. This is because the beam has greater divergence and the spot focused onto the workpiece is larger for any particular workpiece. As this larger spot is imaged through the optical system onto the pinholes, it is similar in effect to having smaller pinholes. It is not a particularly

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useful method of increasing sensitivity, as by increasing the vertical measurement resolution the lateral resolution is decreased. The laserto-main-lens distance was selected to be 0.8 m. Curve No. 1 on Fig. 11 was plotted for an angle of incidence on the workpiece of zero degrees: this provided a reference signal. Curve No. 2 shows the output signal after the workpiece was rotated clockwise through 5 and 10 degrees from the horizontal feedrate vector, whilst curve No. 3 was produced by a counter-clockwise rotation of 10 and 15 degrees from the horizontal feedrate vector. The shifting position of the output curves occurred because the rotational axis of the workpiece was not coincident with the focused spot on the workpiece surface. From these results it can be seen that sensor output is largely independent of the angle of the incident beam. The effects of variation in the distance between the main and image lenses are recorded in Figs 12 and 13. Figure 12 illustrates the effect of increasing the distance between the main and image lenses. It can be seen that by increasing this distance the sensitivity is increased. Figure 13 shows the effect of reducing the distance between the main and image lenses, by moving the image lens toward the main lens. This results in decreased sensitivity and magnitude of the normalised output voltage.

Optical stand-off control for laser materials processing

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Fig. 14. Effect of main lens focal length on sensitivity. A more sensitive output is recorded when the distance b e t w e e n the main and image lenses is increased, because the b e a m diameter in front of the image lens changes more rapidly. To show the effect of the focal length of the main lens upon the output an experiment was conducted, the results of which are presented in Fig. 14. It can be seen that curve No. 2 records a more sensitive output than curve No. 1. This indicates that a reduction in the focal length of the main lens produces a more sensitive signal but with a lower output voltage. This occurs due to the increased magnification of the system (any small m o v e m e n t of the surface from the focused position will cause a large change in b e a m spot diameter in front of the pinholcs). In curve Nos 3 and 4 the same main lenses are utilised but without any image lens. This shows a significant drop in output voltage, signifying that an image lens is required to recollect the reflected light beam. The sensitivity of this device d e p e n d s on the size and depth of the focus, the optical system magnification of the reflected light and the sensitivity of the diflerential amplification circuit. The current system facilitates a measuring range of ± 6 r a m . The instrument gives an extremely useful linear response region which, for the o p t i m u m conligurations, has a range of ±1 mm for the 75-ram focal length main

Optical stand-off control for laser materials processing

179

lens and +2 m m for the 120-mm focal length main lens; in this linear range the accuracy resolution is 1/xm. The lateral or spatial resolution for the system is d e t e r m i n e d by the illuminated spot size of the laser b e a m on the workpiece surface. W h e n the workpiece is at the focal point of the main lens the lateral resolution is 22.5/zm for the 75-mm main lens and 37.4/xm for the 120-mm main lens setup. A miniaturised version of the stand-off system has been constructed utilising a laser diode. No further details of this device are reported as aspects of it are the subject of patent applications.

7 CONCLUSIONS A n on-axis non-contact optical sensor capable of accurately sensing and measuring the workpiece surface position in real time has been presented. For materials processing, most workpiece surfaces are matt and hence generate a diffuse reflection which destroys the Gaussian intensity distribution of the laser b e a m upon reflection. This explains why the system still works well when the workpiece is set at an angle. The system gives an extremely useful linear response region which has an accuracy resolution of 1/zm; it achieved a lateral resolution of 22.5 p~m. The control of stand-off distance is important to achieve a good quality cut with laser machining systems.

REFERENCES 1. Porsander, T. & Sthen, T., An adaptive torch positioner system. In Robotic Welding, ed. J. D. Lane. IFS Ltd, 1987, Chap. 2, pp. 157-65. 2. BjOrkelund, M., A true steam-tracker for arc welding. In Robotic Welding, ed. J. D. Lane. IFS Ltd, 1987, Chap. 2, pp. 167-77. 3. Shoham, M., Fainman, Y. & Lenz, E., An optical sensor for real-time positioning, tracking and teaching of industrial robots. I E E E Trans. on Industrial Electronics, IE-31 (2) (May 1984) 159. 4. Kanade, T. & Sommer, T. M., An optical proximity sensor for measuring surface position and orientation for robot manipulation, Proc. S P I E Int. Soc. Opt. Eng., 449 (2) (1984) 667. 5. Kino, G. S., Code, T. R., Hobbs, P. C. D. & Xiao, G. Q., Optical sensors for range and depth measurement, Proc. 5th Int. Congr. on Applications o f Lasers and Electro-optics, I C A L E O 86, Virginia, 10-13 November 1986, p. 93. 6. Sibayama, K., Kubo, M. & Itani, K., Sensory characteristics of the 'Melcut 3DCM'--Laser welding, machining and materials processing, Proc. Int.

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7.

8. 9. 10. 11.

Hussein A. Abdullah et al. Conf. on Applications of Lasers and Electro-Optics, ICALEO, San Francisco, California, USA, 11-14 November 1985, p. 113. Anderson, R. P., Carr, M. F. & Grieve, R. J., Feasibility study into use of laser scanning measuring device and robot as flexibile inspection station. Advance in Manufacturing Technology (Proc. 1st Natl Conf. on Production Research). Kogan Page Ltd, September 1985, p. 270. Simon, J., New noncontact devices for measuring small microdisplacements. Applied Optics, 9 (1970) 2337. Fainman, Y., Lenz, E. & Shamir, J., Optical profilometer: a new method for high sensitivity and wide dynamic range. Applied Optics, 21 (1982) 3200. Dobosz, M., Optical profilometer: a practical approximate method of analysis. Applied Optics, 22 (1983) 3983. Cohen, D. K., Gee, W. H., Ludeke, M. & Lewkowicz, J., Automatic focus control: the astigmatic lens approach. Applied Optics, 23 (1984) 565.