A Dichromatic Interference Intensity Modulation Approach To Measurement Of Lubricating Film Thickness

In this study, an efficient approach on the basis of dichromatic interferometry has been developed for the measurement of lubricating film thickness. Modulated interference signals are obtianed by linear substraction of the intensity signals of two different colors from dichromatic interference fringes, and the interfrence order can be obtained readily from some characteristic points of the modulated signal, and therefore, film thickness measurement under non-steady state conditions can then be achieved. The main work carried out includes:1. The old lubricating film measurement system was updated, and the interference image quality has been improved significantly. A new 3CCD camera was incorporated into the image capture unit, and the control system for the servo-motors was updated. The software of the apparatus has been re-designed and built into one, including the image capture, friction force measurement and motor-control section.2. A red-green dichromatic interference intensity modulation approach(DIIM) has been developed with a large measurement range of 4 μm. Based on multi-beam interferometry, a colorimetric model was established for the multi-layer measurement system, which would do benefit to reveal the formation mechanism of interference colour and intensity. Similar to the heterodyne interferometry, a relative large equivalent wavelength and a modulated signal, obtained by intensity subtraction between the red and the green components, were used to infer the fringe order of characteristic locations. Thereafter the film thickness of other location can be determined by red or green monochromatic intensity. The measurement principle was analyzed with both the two-beam and the multi-beam interference, and the influences of wavelengths and the FWHM of the light source were discussed. The differences in the pattern of the fringe colour and the modulated interference intensity were taken good use in the first three half-cycles of the beat wave, and thus to extend the measurement range of dichromatic interferometry. Using red and green lasers with wavelength of 653 and 532 nm, respectively, the measurement range can be as high as 4 μm with resolution of 0.8 nm. Since there is no need to count the fringe order, the DIIM approach can be used to evaluate film thickness efficiently.3. Dynamic responses of the hydrodynamic lubrication film under transient condition were studied. There is always some time lag or film thickness hysteresis in the h-u(film thickness vs. speed) curve. However, film changing rate varies in phase with the experimental speed. During start-up/shut-down, the film thickness approaches the final steady value asymptotically showing a stretched sigmoid shape. During the start-up process, the transient film thickness is always smaller than the corresponding steady film thickness, while it is reverse during the shut-down process. With cyclic triangular wave speeds, the deceleration process presents larger film thickness than the acceleration one. In fact, the squeeze effect dominates the time lag and amplitude reduction of the transient h-u curve, which is influenced by the frequency and amplitude of the input speed, the load and also the oil viscosity. The experiment data measured by DIIM agrees well with the solution of Reynolds equation, which could validate the classical hydrodynamic lubrication theory directly and quantitatively.