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Determining Surface Roughness by Laser Light Backscatter

Abstract

The University of Hartford has been involved with the measurement of surface roughness using non-contact methods. The type of surface finish under study is created by a grinding operation in industrial applications. This surface must meet a specified pre-determined finish. Surface roughness is a measure of localized small-scale height variations or imperfections on the surface of a material. The measurement parameter of surface roughness is described by the term Ra, which is the average roughness expressed in micro-inches. One method used for non-contact measurement of a ground surface is to measure the back-scatter of a laser light source. The amount of back-scatter is directly proportional to the Ra value.

One drawback to the current design is that it is not portable. A computer handles data acquisition and provides a user interface. The inspection probe is attached to the computer via a limited length of wire. In an industrial setting, it would be more efficient and flexible to bring the inspection probe to the surface in question rather than to bring the surface to the inspection probe. This design will eliminate the need for the wire tether and the computer altogether. The functionality for both data acquisition and user interface will be replaced by an embedded system. This embedded system will consist of a microcontroller, a multi-channel analog to digital converter, an LCD display, and a keypad input device.

Introduction

As essential part of quality control in many industrial manufacturing operations is the measurement of the surface roughness in a metal. Surface roughness is a measure of localized small-scale height variations or imperfections on the surface of a material. These variations are caused during the manufacturing process. These variations consist of indentations, scratches, gouges, and any imperfections in the metal.

In recent years the need for more precisely cut machines has increased. The measurement parameter of surface roughness is described as Ra, which is the average roughness expressed in micro-inches. The standard method is to physically measure the surface. The device for measuring roughness is called a contact profilometer. This is considered to be the baseline method. A profilometer consists of a flat inspection table and a small stylus attached to an adjustable holder. The stylus is then dragged across the grain of the work piece. As the stylus moves across the work piece, it moves up and down generating an analog signal. By evaluating this analog signal and by comparing it to signals created by set standards, an average roughness can be determined. The problem with this method is that the stylus may scratch the surface when the surface is being measured. Also the surface may not be readily accessible. A non contact method would solve the issues related to destructive testing and accessibility while still providing a valid measurement.

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Moslehpour, S., & Cloutier, G., & Brown, M. (2007, June), Designing A Portable Surface Roughness Inspection Probe Paper presented at 2007 Annual Conference & Exposition, Honolulu, Hawaii. 10.18260/1-2--2219