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2015 Ninth International Conference on Sensing Technology |
==title== |
White Light Triangulation Sensor for Flexible Inspection System |
==authors== |
Abraham Mario Tapilouw*, Yi-Wei Chang, Hsiao-Wei Liu, Hau-Wei Wang, and Hung-Ming Tai |
Center for Measurement Standards, |
Industrial Technology Research Institute, |
Hsinchu City, Taiwan |
*email: mario@itri.org.tw |
==Abstract== |
A triangulation sensor containing two light sources with different wavelength and individual slit has been developed. Differential technique is applied to improve spatial resolution as performing height reconstruction of a specimen. The system can measure up to 7 mm with accuracy up to 1.144 um. Robotic arm with multip... |
==Keywords== |
triangulation; white light; flexible inspection system; sensor; robotic arms |
==INTRODUCTION== |
Laser based optical triangulation system has been widely applied in industry for measuring distance and also surface profile [1, 2]. One of the drawbacks of laser triangulation is it produces speckle and affects measurement uncertainty [3]. Triangular method using low-coherence white light and a slit would reduce speck... |
projected line profile and intensity of the light determines the resolution and accuracy of the measurement. Producing a narrow line source with high intensity requires a strong light |
source. However, efficiency of the light source is very low because most of the light is blocked by the narrow slit. |
In this paper, a white light triangulation sensor is presented. |
The study aims to develop a system with multiple color LED |
light sources achieving higher depth resolution without |
sacrificing light intensity. A differential technique is applied to |
calculate the height of the sample. Measurement range of the |
sample is 7 mm and the accuracy of the measurement is up to |
1.144 um. |
==SYSTEM DESCRIPTION== |
Fig. 1 shows the optical system design of the developed system. The system consists of light source module and color imaging device for capturing images. |
Lateral shift in the image plane is determined by calculating the local peak of the line profile. Therefore the resolution of the peak detection is determined by the width of the line profile; the narrower and sharper the line profile the higher the resolution of peak detection is. In the developed system, a narrower a... |
overlapping. The receiving angle of the camera also has to be adjusted to the enable capturing image of the wholemeasurement range without obtaining defocused images due to limitation of lens’ Depth of Field. The differential technique applied for calculating the peak of the line profile is shown in Fig. 2. A captured ... |
the original red line profile. |
==CALIBRATION PROCESS== |
Eq. 1 shows the relation between image coordinate ( ∆x') and actual height ( ∆z ) in triangulation measurement. Therefore calibration is first performed to generate calibration curve between pixel position and actual sample height. Calibrated gauge block samples are employed as reference. After the calibration curve is... |
In Figure 3, the distance between peaks represents the height difference between two gauge blocks. The smallest height difference that can be made by the gauge blocks is 10 um.Therefore, three height differences are tested, 10 um for the range of 1.0-1.1 mm, 100 um for the range of 1.1-2 mm, and 1 mm for the range of 2... |
After the system has been calibrated, it is put to repeatability test. Each gauge block with height ranging from 1.1 – 1.9 mm is measured five times to estimate the repeatability of the system. Table 1 shows the static repeatability measurement results. Repeatability of the measurement ranges from 0.3 – 1.2 um. |
Static and dynamic repeatability of the sensor is also tested by measuring a flat mirror with accuracy of 1/10λ mounted on a linear stage. The linear stage is equipped with a linear encoder, which is used as position indication. Stage is moved at a total distance of 7 mm and the image is acquired at every 50 um. For ea... |
Repeatability of the static measurement is 2.06 um (1 standard deviation) and. Fig. 5 shows the measured relative displacement at each sampling point. It is shown that the measurement fluctuates more at the end of the measurement range. The standard deviation is 3.41 um and the range of the data is 14.18 um. Accuracy o... |
==EXPERIMENT RESULTS== |
Aspect ratio is one of important parameters of measurement system. It is tested by measuring gap built by two gauge blocks. Figure 6 shows the sample used in the measurement. Thicknesses of the gauge blocks are 5 mm and the gap between two gauge blocks is 1 mm. Aspect ratio of the sample is 5:1. |
Figure 7(a) shows the image captured by the camera and Figure 7(b) shows the peak detection result. |
Figure 8 shows the measured profile of the sample. The red region shows the profile for calculating the depth. The measured depth of the gap is 4.980 mm. This result shows the capability of measuring sample with aspect ratio of 5:1. |
To validate the performance of the system in measuring real workpiece, this study uses the developed system is to measure the depths of the piston oil ring grooves as shown in Fig. 9. There are several ring grooves to be measured for the inspection. Measuring piston oil ring grooves by a side-view camera had been found... |
In the test, we focus on one piston groove at a time to get an image. Figure 10 (a) shows the portion of the piston to be measured and Figure 10 (b) shows the captured image in section 1-3.Measured profile is shown in Figure 11. The portion for calculating depth is marked by red dots. |
The measurement is performed 10 times and the results are compared to measurement with Coordinate Measuring Machine (CMM) as reference instrument for measurement mechanical components as shown in Figure 9. Table 2 summarizes the comparison of the measurement results with the reference instrument. |
The developed system is mounted on robot arm as its motion platform. Table 2 shows the measurement when position of the developed sensor is not changed during repetitive measurement. Another measurement is performed by moving the robot arm into other position and return to the measurement position to test the repeatabi... |
After the system is calibrated, it is used to measure a sample that is hold by other robot arm. Figure 12 shows the setup of the developed system mounted on the robotic arm (left hand side) and another robotic arm holding the piston to be measured (right hand side). Using this approach, the system can be applied for me... |
==CONCLUSIONS== |
A white light triangulation sensor employing multiple light sources has been developed. The system is capable of generating narrow line profile without sacrificing light intensity. The accuracy of the measurement is up to 1.144 um for the whole measurement range. Repeatability of the measurement can be between 0.2~1.2 ... |
==REFERENCES== |
[1] Costa, M.F.M. Surface inspection by an optical triangulation method. Opt. Eng. 1996, 35, 2743–2747. |
[2] Zeng, L.J.; Yuan, F.; Song, D.Q.; Zhang, R. A two-beam laser triangulation for measuring the position of a moving object. Opt. Lasers Eng. 1999, 31, 445–453. |
[3] R. Dorsch, G. Häusler, and J. Herrmann, "Laser triangulation: fundamental uncertainty in distance measurement," Appl. Opt. 1994, 33, 1306-1314 |
2015 Ninth International Conference on Sensing Technology |
==title== |
White Light Triangulation Sensor for Flexible Inspection System |
==authors== |
Abraham Mario Tapilouw*, Yi-Wei Chang, Hsiao-Wei Liu, Hau-Wei Wang, and Hung-Ming Tai |
Center for Measurement Standards, |
Industrial Technology Research Institute, |
Hsinchu City, Taiwan |
*email: mario@itri.org.tw |
==Abstract== |
A triangulation sensor containing two light sources with different wavelength and individual slit has been developed. Differential technique is applied to improve spatial resolution as performing height reconstruction of a specimen. The system can measure up to 7 mm with accuracy up to 1.144 um. Robotic arm with multip... |
==Keywords== |
triangulation; white light; flexible inspection system; sensor; robotic arms |
==INTRODUCTION== |
Laser based optical triangulation system has been widely applied in industry for measuring distance and also surface profile [1, 2]. One of the drawbacks of laser triangulation is it produces speckle and affects measurement uncertainty [3]. Triangular method using low-coherence white light and a slit would reduce speck... |
projected line profile and intensity of the light determines the resolution and accuracy of the measurement. Producing a narrow line source with high intensity requires a strong light |
source. However, efficiency of the light source is very low because most of the light is blocked by the narrow slit. |
In this paper, a white light triangulation sensor is presented. |
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