APPLICATION OF AN OPTICAL
SENSOR FOR MICRON LEVEL
SURFACE PROFILE MEASUREMENTS
OF FLAT AND CYLINDRICAL
SURFACES
SHAIK RIYAZ ALI*
Electronics and Communication Engineering Department, Sri Chaitanya Engineering College, Kommadi , Visakhapatnam, Andhra Pradesh 530016, India.
*riyazali06@gmail.com.
K.S.RAVI KUMAR**
Electronics and Communication Engineering Department, Sri Chaitanya Engineering College, Kommadi , Visakhapatnam, Andhra Pradesh 530016, India.
**srinivas.ravikumar260@gmail.com
Dr.M.SRINIVASA REDDY***
Director, Faculty for Science and Technology, ICFAI Foundation for Higher Education Dontanapalli, Shankarpalli Road, Hyderabad, Andhra Pradesh 501203, India.
***drmsrinivasareddy@yahoo.com
Abstract:
The theory and application of a stray light sensor for micron level surface-profile measurements on fine machined industrial products is discussed here. The stray light sensor has already proven its worth on products like automotive gear shafts, iron bar’s surface peeling process, electronic silicon and germanium wafer roughness measurement etc. where accuracy in measurement and perfection in surface profile is of utmost importance. This study showcases principle behind the sensor operation, results of micron level surface profile measurement made on a flat multi-profiled sheet spread over an iron plate and a cylindrical iron rod.
Keywords: Optical light scattering; Micron level surface profile measurements; Sensor alignment; Flat and cylindrical profile specimens.
1. Introduction
The creation of a competitive product is associated with the growth of amount and accuracy of technical measurements at various stages of planning, production and application. Measurements became a basic means of preventing defects and ensuring high product efficiency [1].There is a great interest in fast, robust and non-destructive measurement of high quality surfaces at the work-place itself.
require smoothness less than 1nm which can be achieved using surface profile measurements. Roughness measurement can be achieved using two different techniques.
Profiling
Area Averaging
In profiling technique surface height is measured using a high resolution probe such as stylus or a focused optical beam. This method gives an accurate and quantitative measurement but is time consuming and may even come in contact with the surface under test. In contrast, Area profiling gives out a quantity depending on the statistical average of the surface roughness. Examples of area averaging include optical scatter, parallel plate capacitance and low electron energy diffraction. Optical scatter techniques are preferred amongst all because they are fast, accurate and non-contact avoiding the potential of surface damage associated with stylus techniques [4].
In contrast to tactile profilometers, which only measure a linear profile of a surface, the optical scattered light method always captures an area of a surface. In view of the above backdrop, an attempt is made to make measurements on different
textured surfaces and record values at micron level accuracy by using an optical sensor. 2. Principle and Working of the Optical Sensor
2.1. Sensor principle
The optical surface measuring sensor uses Angular Resolution of Scattered light (ARS) and can capture Surface roughness as well as the reflection angle of machined surfaces by evaluating the distribution curve. The sensor collects the reflections from the surface under test and gives out a proportional intensity distribution curve on the basis of simple theory of geometrical reflections, also called as Mirror-facet theory [5,6]. The measuring principle and set up of the basic components inside the sensor are shown in Fig.1.
Fig. 1 Sensor Principle
Labels:
a – Surface with spot size e – Scattered Light Distribution b – Measurement object L – LED light source
c – Stray light Δ– Local form Angle d – Diode detector Array ɸ– Stray Light Angle
M – Average stray Angle Iɸ– Stray light Intensity
2.2. Working of the sensor
Internally the optical sensor consists of an LED light source, a measuring lens and a linear photo diode detector array. The measurement spot on the surface to be measured depends on the LED spot size and the focal ratio of the collimator optic and measuring lens. Depending on the application the spot diameter could be 0.9 or 0.3 mm.
The light radiation reflected or scattered by the surface is collected by the measuring lens and then refracted on to the diode array, which measures a line sample of the scattered light beam to obtain an angular distribution of light intensity and is recorded. The pattern of scattered light depends on the surface roughness and texture. As shown in fig.2 (a) and 2(b) unidirectional patterns of marks, caused by grinding and turning processes, typically result in an elliptical scattered light distribution at 90° to the marks direction across and along the surface[7].
Fig. 2 Scattered light sensor setup measuring roughness across the finishing marks (a), along the finishing marks (b) and form deviations(c)
3. Hardware Components and Software Package of the Sensor 3.1. Hardware components
The stability provided by the hardware components to fix the sensor position to give optimal and reliable measurements, is a pre-requisite. With each unique model of sample taken for test the physical hardware could be a rotor (for circular measurements) or a tabular mounting surface (for linear measurements).
3.1.1. Tabular surface: The set up of a tabular mounting surface is shown in Fig.3. It is used for samples of short length and linear measurements only . Flat surfaces and Cylindrical Rods can be placed on it, while the moving base displaces the sample placed over it. The tabular surface linear movement speed and distance can be controlled using the Sensor software.
Fig. 3 Tabular Mounting Surface
3.2. Software package of the sensor
Fig. 4 Parameters window
The Sensor alignment is performed by observing the parameters I and M. Best position of the sensor reads maximum I value and M value should be as close to zero as possible. The sensor is capable of recording more than 1500 measurements per second which can be tabulated on an excel sheet [7].
4. Mathematical Expressions used in Measurements
Data of the sample under test is provided by the user. All measurement values required are then calculated by the sensor software based on this data input by the user. Few basic parameters to be keyed in for tabular mounting surface can be viewed in the control tab [7] and the same isshown in Fig.5.
Fig.5 Measurement drive parameter window
Fahrstrecke (mm): Denotes distance to be travelled by the moving base in mm.
Geschwindigkeit (mm/s): length to be measured in milli meters per second.
Delta X: Distance between successive measurements (Fig.6) is automatically calculated from the input speed and number of measurements.
Fig. 6 Distance between successive measurement spot sizes ‘ΔX’
Sensor Timer (ms): Time interval between successive measurements taken by Sensor. This value should be more than 2milli seconds [7].
Mathematically the above parameters are calculated as
Sensor Clock: Sensor clock frequency in Mega Hertz.
Spot Size (ø): Spot size diameter (0.9 mm or 0.3 mm). 5. Measurement Results
In this study, ambient light falling on the sensor is assumed to be constant throughout for each measurement made.
In all profile plots i.e., the measurements of Slope, Intensity, Scattered Light, the x-axis is always length of measurement in mm.
5.1.Multi-profile flat surface spread over an iron plate
The surface under test here is a flat iron plate of 70mm length with 5-different standard roughness profiles of length 12mm each, equally spaced on it. The order in which the 5 samples are placed is –P0.1, P0.25, P0.6, P1, P1.6 (P- polished) [9, 10].The diode array in the sensor is along the processed groves of the surface under test and the corresponding Lengthwise Roughness) measurements are shown in Fig.7.
Fig. 7 Lengthwise Roughness (Aq) and Skewness Measurements of a flat surface
INFERENCE:
(1) As the roughness profile of the individual samples increases from left to right (P0.1 to P1.6) we find the Scattered light value (Aq) increases accordingly.
(2) There is a groove observed at every 12mm interval from the start of measurement indicating the change in roughness profile between individual samples.
Figure 8 Crosswise Roughness (Aq) and Skewness Measurements
INFERENCE
(1) As shown above the roughness value has increased to 70, as the measurements are taken across the grooves of the sample.
5.2. Cylindrical iron rod
The sample under test is a cylindrical rod with a well-polished surface. The length of the rod is 27.7cm and radius is 1.28 cm. The entire length of the rod cannot be placed for measurement, as it cannot be balanced over the moving platform of 6cm (Fig. 3). Hence only 20 cm excluding both edges of the rod is measured. The plot is shown in Fig. 9.
INFERENCE
(1) The slope is around zero, which means that the measurements made are on the same plane of the cylindrical sample under test.
(2) The Intensity curve varies in a range from 800 to 1000 on y-axis and is also linear. Hence there are no possible irregularities or scratches on the surface.
(3) The profile has a deviation of 1μm, which may be result of possible misplacement of the rod on the moving platform.
6. Conclusions
The measurements on a multi-profile iron plate and a cylindrical iron rod have been made and the results have been found. As expected all profile deviations even lesser than 1μm can be recorded using the sensor. The skewness, intensity and roughness values of surfaces are also found to be consistent. Hence it can be concluded that an optical sensor measurement system as proposed in this paper can be adapted for high speed, accurate and non-contact measurement of sensitive samples.
The alignment of the sensor before starting measurements would take time, hence introduction of a Robotic Arm is proposed for quick alignment of the sample with the sensor. A Robotic Arm can also help to improve the accuracy in movement of samples, repeatability in measurements at a faster pace and finally making the entire process much automated with minimum human intervention.
REFERENCES
[1] N. Sychev-‘Measurement Techniques’-Optical Society of America, Vol-38, No.4, 1995.
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[3] C.R.Coggarave, ‘Wholefield Optical Meteorology’: Surface Profile Measurement’ Scholarly Article-Phase Vision Limited, 2002. [4] T.V.Vorburger ‘Methods for Characterizing Surface Topography’- Tutorials in Optics, Optical Society of America, 2010.
[5] Brodmann, R. Brodmann, H. Bodschwinna, J. Seewig, ’Theory and measurements of a new light scattering sensor’. In- The 12th
International Conference on Metrology and properties of Engineering Surfaces. Rzeszow, Poland, 07/2009.
[6] Boris Brodmann, Rainer Brodmann, Tobias Hercke, ’Function-oriented measurements of fine machined automotive parts by means of a new light scattering sensor’,In-The12th International Colloquium on Surfaces, Chemnitz ,Germany 01/2008.
[7] Stray light sensor Operating Manual English Version 2,1. 08/2008.
[8] J.Seewig,G.Beichert H.BodchWinna,M,Wendal, R. Broadmann ‘Extraction of Shape and Roughness using Scattering Light’Applied Optics, Vol. 49, Issue 30, 2010.
[9] Shaik Riyaz Ali. “Performance Analysis of a Stray light sensor with Robotic Arm for micron level surface profile measurements”, Master Thesis Report submitted at University of Applied Sciences, Karlsruhe, Germany 03/2010.
