Online Measurement Of Lubricant Film Thickness In Slider-on-Disc Contacts And Its Behavior In Non-steady State

As a novel type of tribological evalution system for lubricants,the apparatus in conformal contact format has been developed recently and successfully used to measure lubrication films under steady state.However,dynamic measurments are unavailable using this system due to the limitations of the light source and the measuring principle.Moreover,the online meaursement techniques used in ball-on-disc elastohydrodynamic lubrication(EHL)contacts can not be directly utilized to the flat-on-flat contacts which are far beyond the measuring scope.Even the popular image tracking algorithm is difficult to recognize the specific fringes at the fixed contact height,because the fringes are always similar along the contact width.Based on the status quo,this dissertation is mainly conducted work on developing a technique for online film thickness measurement which will realize fast measurement of slider inclination and automatic film thickness assessment by interference images.The work consists of the following individual projects:(1)Development of online film thickness measurement system.The feasibility of the multi-beam interferometry and the features of the dichromatic interferometry for film thickness measurement in flat-on-flat contacts have been studied.Some of the units were updated such as the speed control mode,real-time speed measurement unit,image acquisition system as well as control software.(2)Automatic method for fast acquisition of contact inclination.Based on the equal space of fringes generated by the slider-on-disc contact and the periodical variations of interference intensity,a method based on Fourier-transform was proposed for measuring slider inclination,in which a frequency interpolation is used to eliminate frequency leakage by combining the Rife-Jain method and overlapping FFT phase difference method.As a result,accurate and automatic calculation of inclination angles was realized.Experiments showed relative errors of less than 2% for the slider inclination measurement.It is verified that this method is robust with fast calculation and strong disturbance resistance.Moreover,the proposed scheme is suitable for obtaining transient inclinations as well.(3)Approach for online measurement of film thickness.Using dichromatic light source(Right and Green),some specific target fringes can be found which correspond to constant film thickness.Based on optical flow calculation and dynamic time warping(DTW)technique,an algorithm(OF-DTW)has been proposed for tracking those target fringes in the 1-D modulated intensity signals,and then the online film thickness measurement can be achieved.With a monochromatic light,there is usually only a very small shift of straight interference fringes between two successive frame images when variation of film thickness occurs.The same algorithm was employed to calculate the displacement of a 1-D intensity curve across the parallel fringes between two adjacent frames,and the film thickness difference between the two frames can be obtained.From the start,the film thickness variation of each frame can be obtained and the online film thickness measurement can be achieved.As a supplement to the above online approach,an offline method was also presented for film thickness assessment.With Freeman chain code and simple moving average filter technique offline measurement data were smoothed.A normalization algorithm based on threshold value and template matching was combined to determine the useful data region of the offline data,and the interference order could be obtained accurately.The offline automatic calculation of film thickness was thus realized.(4)Experimental studies on lubricant film thickness under non-steady conditions in flat-on-flat contacts.Different acceleration and deceleration,variable speeds and variable loads have been applied.The film thickness measurement for startup,shutdown,load change,speed change,etc.,was conducted based on the online measurement technology developed with dischromatic light source.The online method was thus validated.(5)Numerical solutions of film thickness in flat-on-flat contacts.The film thickness under steady and non-steady conditions was predicted by finite different method.The successive over relaxation iteration was employed to obtain pressure solutions.The optimum relaxation factor and pressure iteration convergence criterion were sought,and the iteration time was greatly reduced.The calculation efficiency was significantly improved by binary search scheme.The results also validated the reliability of the non-steady experiments.This online measurement approach provides an efficient and easy-to-use technique which will contributes to scientific studies on new interface effect,tribological properties of new lubricants and other fundamental topics.In the meantime,it will also serve to some applications in industry area.