An optical technique using laser for non-contact measurement of surface roughness and form accuracy of ground surfaces is presented.
When a ground surface is illuminated by a beam of laser light, the pattern of radiation along the reflection angle varies with its surface texture. It is found that, among various statistical properties of the surface texture, the slope distribution has the most significant influence on the radiation and it maintains an unique Gauss distribution relationship with the surface roughness. The principle idea of the optical measurement system is therefore monitor the radiation, and then calibrate it in process against surface roughness by means of necessary digital data processing.
On the other hand, measuring the form accuracy of a ground surface is accomplished by using a triangular method, which is based on observing the movement of an image of a spot of light projected onto the surface. The image moves along with form error when the surface is scanned in the direction in which the form accuracy is to be measured. The image is focused, through a series of lenses for magnification, on a photodetector array of line configuration. The spatial location of the image within the array can be found by monitoring the signal levels of the photodetector elements, and then the relative movement of image and consequently the form accuracy of the surface can be obtained through appropriate calibration procedures.
The optical measurement system developed in this work for basic experiment was designed to meet the following specifications:
a) The measurement must be suitably fast for real-time adaptive control of the grinding process, i.e., one sampling time should be less than 20 msec.
b) The measuring range of surface roughness should be 0.1 to 1.0 μm CLA with a resolution of 0.1 μm minimum.
c) The measuring range of form accuracy should be 4.0 mm with a resolution of 0.001 mm minimum.
The measurement system is composed of four main parts: a laser emitting unit, an optic system consists of lenses, a charge-coupled device(CCD) image sensor, and a digital image processor. A 0.8 mm-diameter laser beam is obtained from 4 mW-power HeNe red laser head, and this beam is transmitted through an optical fiber cable and reduced to 0.4mm by lenses and then projected onto the target surface. The CCD image sensor with a line array of 4096 photodetector elements is used with a series of optical lenses to collect the reflected light with a spatial resolution of 1 um. Each photodetector element can produce an analog signal level individually up to 5 Vdc in proportion to light intensity. The image processor based on 16 bit CPU (Intel 8088) receives the optical information from the image sensor through appropriate interface, and then calculate and display the surface roughness and the form accuracy.