Surface Quality Measurement System Using Laser Technology for Lathe Machines

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1 Surface Quality Measurement System Using Laser Technology for Lathe Machines Sheragin Tavassoli, Suresh Gobee and Vickneswari A/P Durairajah Asia Pacific University of Technology and Innovation Technology Park Malaysia, Bukit Jalil, Kuala Lumpur, Malaysia Abstract A simple, yet efficient system for the surface roughness measurement of metal round workpieces using laser scattering method is developed. The system is designed to be mounted on lathe machines and it features non-contact and real time measurement. It also allows the user to take measurements online (while machining) and offline (after machining). The system uses LabVIEW for its user interface and it is able to record the measurement data. Keywords Surface roughness, round work-piece, lathe machine, laser, non-contact measurement, scattering method, interferometry method, microscope method, diffraction method, optical sensors, differential signal, GUI, LabVIEW and Data Acquisition Cards. 1 INTRODUCTION In the machine tool industry, the finished quality of objects being machined is important. For instance; the round surface of a piston inside a cylinder has to be smooth and flat in order for it to move up and down while causing minimum friction with the cylinder s body. Therefore it is necessary to monitor the surface roughness of such objects during production. The roughness of a surface can be defined as the displacement of the surface along a length or an area. Mathematically, roughness is expressed as the arithmetical mean roughness as shown in figure below and expressed by the following formula: Figure 1. Arithmetical Mean Roughness [1] (1) Where R a is the arithmetical mean roughness, l is the sampling length and f(x) is the function of the height between peaks and valleys in the range of the sampling length l, respectively [1], [2]. Generally, roughness measurement systems are categorized into two methods; Contact method (A.K.A Mechanical Profiler) and Non-contact. Contact methods use a stylus in contact with the object/work-piece for taking measurement. However in the Non-contact method, as the name indicates, the roughness of work-piece is measured without making any contact with it. There are several approaches to roughness measurement which are described under Surface Profilers. 2 PREVIOUS WORKS ON SURFACE PROFILERS There have been many approaches on surface profilers in the past. A brief explanation of previous works done on surface profilers is provided. ISBN: SDIWC 25

2 2.1 Surface Profiler - Contact Method Contact method (also known as mechanical profiler) uses a mechanical stylus in contact with the work-piece for taking measurements. The stylus is normally positioned perpendicular to the work-piece. Thus, it detects the changes in height or steps that exist on the surface. The reading is done sequentially and is taken at low speed; the data is processed in a computer. The resolution of this method depends on the size and shape of the tip of the stylus; the smaller and narrower the tip of the stylus, the bigger the resolution of the system would be and more changes in average roughness (R a ) can be detected as shown in Figure 2. Therefore, as the roughness measurement is taken while in contact with the work-piece, and due to the above drawback, the measurement cannot be taken online or during the machining process [2]. Figure 2. Mechanical Profiler [3] In the above figure, data is calculated by the arithmetical roughness formula given by Eq. (1). Waviness can be simply expressed as outside the shape of the work-piece, the changes in waviness are more sensible and wider than the changes in roughness. In other words the waviness has a longer spatial wavelength than roughness [2]. For instance, by assuming the roughness as sinusoidal waveforms, by decreasing the frequency, the space between these waveforms increases and the changes in the waveform are obvious. In addition, by increasing the frequency, the space between waveforms decreases and it looks like a flat line. However, changes in the amplitude of signals are visible. This amplitude represents the waviness of the surface profile [4]. In collection of data by stylus method, many approaches have been taken for transducing the movement of the stylus into analog or digital signals. The LVDT concept is one the more popular methods used to detect the up and down movement of the stylus. The stylus is the core of the LVDT and results in an analog output voltage. Another method that has been developed for this is the use of laser interferometry to detect the movement of the stylus. Basically it is a combination of an optical transducer and a mechanical stylus [5]. 2.2 Surface Profiler - Noncontact Method Another approach to surface displacement measurement is the use of optical transducers. Due to its significant advantages over contact method, it has attracted a lot of attention in recent years. The concept of this method is normally the detection of reflected illuminated light or laser beams from the surface of the specimen. A major advantage of non-contact methods over contact methods is that they provide online (in-process) measurement. Non-contact measurement methods are: Microscope methods, Interferometry methods, Diffraction methods and methods based on Scattering Modeling [6]. The figure below shows the difference between the spacing and the height of the roughness for different non-contact methods: ISBN: SDIWC 26

3 and fragile. Therefore such tools are used more in laboratories than in the production industry [9] Interferometry Methods Figure 3. Height and spacing parameters and ranges of vertical-lateral resolution for different methods of roughness measurement [6] Microscope Methods In this method, optical microscopes are used to obtain images of surfaces, they are mostly used for measuring the roughness of super smooth surfaces. Scanning Probe Microscope (SPM), Scanning Electron Microscope (SEM), Scanning Tunneling Microscope (STM) and Atomic Force Microscopy (AFM) are techniques of microscope methods. In recent developments, the resolution of such techniques for lateral and vertical are in the range of nanometers and micrometers respectively. In Figure 3, STM, AFM and SEM regions show the exact relation of such methods resolution in height and spacing of roughness [6]&[7]. Therefore, beside the advantages that this method provides; such as high accuracy, topography and non-destructive measurement, these methods can only be used for very small specimens that are able to fit in the microscope specimen cell and the measurement distance (between the measurement tool and specimen) is quite short which only lets a small area of the specimen be available for scanning [6]&[8]. Apart from that, optical microscopes are costly, sensitive This method uses an interferometer for roughness measurement. In a recent study by Han[10]. Interferometry is described as a study of interference between wave fronts of light beams exiting the same source and also an interferometer is described as an optical device that divides a beam of light exiting a single source (like a laser) into two beams and then recombines them. In this method, coherent light waves have a certain frequency and are split into two by a splitter. One beam is taken as a reference and the other is used for surface roughness measurement and then the waves will be recombined. Normally two photo detectors and CCD cameras are used to detect the reflected waves from the surface and the reference waves. Both waves are passed through several stages (beam splitters, mirrors, spatial filters, and phase detectors) and the phase shift between beams is detected, data is gathered and transferred to a computer for processing. A typical configuration of this technique is shown in Figure 4. Vertical Scanning Interferometry (VSI), White Light Interferometry and Phases-stepping Interferometry are techniques developed in this method. In a recent study on comparison between Vertical Scanning Interferometry and AFM, the result shows that VSI provides a longer distance in measuring the surface roughness which results a larger and faster scanning of the area of the specimen [8]. It also provides high speed scanning. Furthermore, in another recent study on Phase Stepping Interferometry indicates that this technique provides a highly sensitive, accurate and rapid surface measurement [6]. Moreover, white light interferometry is used in commercial surface roughness ISBN: SDIWC 27

4 measurement instruments for providing subnanometer, high speed and repeatable 3D profile measurements [6]. Despite the good factors that interferometry techniques provide in context of accuracy of high lateral and vertical resolution in the range of micrometers (different for different techniques, however a general interferometry resolution is shown in Figure 3) 3D topography and high speed measurement mostly have very complex structures (as shown in Figure 4) and are sensitive to vibration (vibration influences the phase shift and thus the result). Moreover, instruments used in such techniques are too expensive and their operation is not simple and convenient for production factories. Thus such instruments are only used for further study and inspection of surface roughness in laboratories [11]. of the material can be extracted. This method can be used to measure the surface roughness of different materials such as: Metals, non-metals, hard ceramics and soft plastics [6]. Moreover, many approaches have been developed to observe and correlate the changes in the diffraction pattern. Mostly video cameras are used to record the patterns of diffraction. Then, by the help of image processing and filtering, suitable images can be extracted to analyze the surface roughness of specimen tested. In Xu s research [12], he uses CCD cameras to record and capture images of the reflection of laser sub-waves from a machined metal work-piece, and uses image processing and fuzzy logic [13]. The set-up of his project is shown in as shown in figure 5. Figure 5. Sketch map of set up and configuration[9]. Figure 4. Configuration of phase stepping interferometer [6] Also the interferometry and microscopy methods are mostly used for milling and other machine tools rather than lathes due to their configuration, set up limitations and short measurement distance Diffraction Methods This method is based on the concept of diffraction phenomenon. When a light or laser beam is illuminated on a rough surface, the light or beam reflects and is diffracted. Therefore by observing this diffracted beam and its patterns, the roughness The advantage of this method is that, it can be used to measure surface roughness of different types of materials while having a good accuracy, however the resolution of this method is quite small and is within 1-200nm [6]. In addition this method requires complex and intensive techniques to study and extract the data of the surface roughness of an object Scattering Methods A laser beam has coherent sub-waves, when these sub-waves strike on a rough material, they will be scattered away. Therefore, depending on the existing elements on the surface (roughness), both the amplitude and phase of the scattered subwaves varies Also interference of the sub-waves with surface elements can occur if roughness at some spots is quite high. By observing the ISBN: SDIWC 28

5 reflection of beams on a surface this interference of sub-waves and surface elements results in dark or less bright points compared to other reflected sub-waves from the surface [6] & [13] In other words, if the surface is rough, the intensity of reflected speckles on the screen is less and results in more dark points. On the other hand, as the surface is less rough and the surface is as a mirror, the intensity of the speckles on the screen are high and more bright points appears on the screen. Therefore, this method is suitable in measuring machined metal surfaces which have smoother surfaces, small rough elements on the surface can be easily detected [2] & [13] Figure 6. Laser scattering from a rough surface and intensity distribution [6] Moreover, the figure above shows a sketch map of the concept of this method. The figure shows that when the angle of the reflected beam is close to the angle of illumination, the beam has a higher intensity value than a beam which is reflected with an angle bigger or smaller than illumination angle. Since this method provides qualitative data, there have been many investigations on correlating this qualitative data with quantitative data of surface roughness for instance the arithmetical mean roughness. Therefore, there have been many approaches, for analyzing the relation of electromagnetic theories and scattering parameters. Despite the intensive mathematical calculation, many theory models have been developed and modified such as: Small Perturbation Method (SPM), Kirchoff Approximation (KA), Beckmann, recently developed methods are: Modified Beckman- Kirchoff (K-A), Small Slope Perturbation (SSP), Integral Equation Method (IEM), and Local Curvature Approximation (LCA) and so on. Each of such models provides quantitative data on the intensity of the scattering method and they are sufficient for measuring the surface roughness [6]. In Ragheb and Hancock s research [14], they have used modified B-K theory with visible light and a digital camera to measure the roughness; they were able to measure the surface roughness of a die-electric and metallic material. The digital camera was used to capture images of the specimen being illuminated by visible light, the images were then processed and the roughness of the specimen was determined using pixel brightness measurement. The scattering method provides a much longer range of measurement compared to other methods; it provides high accuracy and rapid measurement using simpler measuring instruments. Due to such characteristics, surface profilers of this type are mostly used on measuring the surface roughness of round work-pieces machined by lathes. 3 METHODOLOGY Based on the principle of scattering method, a U shaped design is developed to detect and measure the intensity of the reflected laser beam from the work-piece. The structure of this design is shown in the figure below. α Laser Diode Work-piece Figure 7. Design of U-shaped detector and work-piece. α is the incident angle and β is the reflected angle of the laser beam. β Detectors ISBN: SDIWC 29

6 A laser diode is positioned in an angle of 45 o to strike a laser beam on the work-piece. A differential photodiode (shown in blue color Model No: BPX48) and two phototransistors (shown in green color Model No: BPX81) are used to detect the angle of reflection of the laser beam as shown in figure above. A differential photodiode is nothing but a double photodiode with extremely high homogeneousness and high photosensitivity separated with a distance of 0.09mm. Its outputs are measured differentially which produces positive and negative voltage whenever the reflected angle varies across each photodiode. The two phototransistors are positioned at each side of the differential photodiode to detect farther angles of reflection. The outputs of the phototransistors are measured differentially as well. Differential measurement is used to prevent the effects of surrounding lights and noises on the detectors. Furthermore, the figure below illustrates the variation of the laser beam reflection on the detectors. The distance d can be calculated through triangulation. As the incident/reflected point on/from the work-piece has to be in the center of the length (l) (between the two sides of the U- shaped design) and based on an Isosceles triangle; the distance d has to be half of l as shown in the figure below: Figure 10. Description of the triangle and its relation to d (the height of triangle) Furthermore, this U-shaped design (header) is attached to a power screw mechanism powered by a stepper motor that allows it to move forward and backward with an accuracy of 0.1mm through micro-stepping. Figure 11 below shows the design of the body of the abovementioned mechanism. Figure 8. Variation of angle of laser beam reflection on the detectors Moreover, it is important that this U- shaped design should have a specific distance from the work-piece which is fixed throughout the measurement. Figure 9 below shows the importance of this matter. Figure 11. Design of the body of the system designed in Autodesk Inventor Figure 9. From left to right: Correct distance between work-piece and header "d. Greater distance and its effect. Lesser distance and its effect. ISBN: SDIWC 30

7 4 SYSTEM OPERATION AND PROGRAM The operation sequence of the system can be described by the following flowchart. (DAQ) takes measurements at a sampling rate of 1 khz (1000 points per second). By use of a statistics block, the mean of the data is taken for every 100ms to reduce the small fluctuations in data and stabilize them. A mathematical relation between voltage signals and roughness is then developed based on the analysis of the results in comparison with the given actual roughness of the specimen. Figure 13 below shows the specimen used for measurement. The specimen has 8 parts machined at the different cutting speeds and feed rates Figure 13. Reference work-piece that has 8 different roughnesses on each section Figure 12. System s flow chart. A program is developed in LabVIEW to cover the whole operation of the system and a Data Acquisition Card USB-6008 from National Instruments (NI) is used to process the inputs and outputs of the system. The developed program allows the user to have a user interface system to key in the nominal diameter of the work-piece and observe the position of the header as well as monitoring and recording the variation of voltages caused by the variation of the angle of reflection across the optical sensors. The outputs of optical sensors are then processed through several calculations to convert the electrical signals to roughness. Measurements taken of the specimen are shown in the Figure 14-a with the respective section number (shown in green color) a) ROUGHNESS CALCULATIONS Data on the measuring of the surface roughness of a reference work-piece with known roughness is collected and recorded. The Data Acquisition card Figure 14. a. Recorded measurement of surface roughness at a sampling rate of 1k Hz. b. Arithmetical Mean of the measurement graph b) ISBN: SDIWC 31

8 Roughness As per the formula in equation (1); the roughness of a material is calculated as the arithmetical mean of a function. It is difficult to get the exact function of a roughness mathematically. Therefore Taylor s approximation is used to estimate the behavior of such a function. For round work-pieces, the roughness function is dependent on two parameters: the feed rate and the cutting speed. In the finishing stages of machining, these two parameters are kept constant for better finishing quality. Based on the result, it was found out that the function is quite linear. Therefore an arithmetical mean is applied on the function of measured voltage to get the roughness of each section of the work-piece. Figure 15 below shows the relation of calculated arithmetical mean of voltage with actual roughness. Note that the negative sign is due to the taken differential measurement. The above formula is then applied on the arithmetical mean voltage signal in LabVIEW and the output is displayed numerically on the front panel of the system to allow the user to monitor the roughness. 6 DISCUSSIONS AND ANALYSIS 6.1 Inaccuracy of system Several measurements have taken place to ensure the operation, repeatability and accuracy of the system. Therefore, based on collected data, the inaccuracy of the system (percentage error) is calculated as follows: Table1. Percentage error of roughness measurement system Arith. Mean Voltage Figure 15. Relation in between actual roughness and arithmetical mean of voltage signal. The figure above shows the linear relation between the arithmetical voltage and roughness. Therefore a mathematical relation between roughness and measured voltage can be calculated as follows: Where m is the slope of the linear function. Measured Roughness Act. Roughness Percentage Error (%) The table above shows that the system has an inaccuracy of 3.96% at most. The inaccuracy of the system could be reduced by the use of more optical sensors placed at the reflection part to cover a bigger area as the system has certain limitations. (2) ISBN: SDIWC 32

9 6.2 Limitation of System One of the main limitations of the system is that it is not able to measure higher changes of roughness as only two sets of optical sensors are used for measurement. Therefore, to measure the roughness of rougher materials, more arrays of detectors need to be used in order to detect higher variations in the reflection of the laser beam from the surface. Another limitation of this system is that it can only take online measurements of the machining process when no coolant is used. This is because the coolant would interfere with the measurement. However the measurements for machining that requires coolant can be taken offline. 7 CONCLUSION The surface quality measurement system using laser technology for lathe machines provides the advantage of sufficient measurement systems for the machine industry by the use of a simple and cheaper mechanism compared to existing measurement systems. The system can be easily mounted on the carriage of a lathe machine that moves along the length of a work-piece and can easily take the online and offline measurement with an accuracy of 96.4%. 8 REFERENCES [5] Krar, S., Gill, A. & Smid, P., Technology of Machine Tool. 6 ed. Kuala Lumpur: McGraw Hill [6] Xu, X. & Hu, H., Development of Non-contact Surface Roughness Measurement in Last Decades. IEEE, 8(9), pp [7] Zhang, J. Z., Optical Properties and Spectroscopy of Nanomaterials. Santa Cruz: World Scientific. [8] Koyuncu, I., Barnt, J., Luttge, A. & Wiesner, M. R., A comparison of vertical scanning interferometry (VSI) and atomic force microscopy (AFM) for characterizing membrane surface topography. ELSEVIER-memsci, 278(2006), pp [9] Ignat, M., Zarnescu, G., Peter, I. & Borsos, A., The applications of the interference microscope on the electrical machines field. IEEE, 1(8), pp [10] Han, S., Interferometer Alignment. Tucson, The University of Arizona. [11] Nilanthi, W. & Kim, M. K., Quantitative Phase Imaging Using Multi-Wavelength Optical Phase Unwrapping. In: N. Costa & A. Cartaxo, eds. Advance In Laser and Electro Optics. s.l.:intech. [12] Xu, X., Non-contact Surface Roughness Measurement Based on Laser Technology and Neural Network. IEEE, 5(9), pp [13] Dong, Z. G., Deng, Y. j. & Li, Y. Z., Surface Roughness Measurment Based on Image Comparison. IEEE, 6(10), pp [14] Ragheb, H. & Hancock, E. R., The modified Beckmann Kirchhoff scattering theory for rough surface analysis. Elsevier, 1(10), pp [1] JIS, Surface Roughness - Technical Data, Tokyo: Japanese Industrial Standard Committee. [2] Jia, H., Study of Non-contact On-site Surface Roughness, Hamilton: McMaster University. [3] Vorburger, T. V. & Raja, J., Surface Finish Metrology Tutorial, Galthersburg: National Institute of Standards and Technology. [4] ASME, Surface Texture (Surface Roughness Waviness and Lay) B46.1. New York: The American Society of Mechanical Engineers. ISBN: SDIWC 33