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Research Papers: Hydrodynamic Lubrication

Wide Range Measurement of Lubricant Film Thickness Based on Ultrasonic Reflection Coefficient Phase Spectrum

[+] Author and Article Information
Pan Dou

Key Laboratory of Education Ministry for
Modern Design and Rotor-Bearing System,
School of Mechanical Engineering,
Xi'an Jiaotong University,
Xi'an 710049, China
e-mail: doupan001@stu.xjtu.edu.cn

Tonghai Wu

Key Laboratory of Education Ministry for
Modern Design and Rotor-Bearing System,
School of Mechanical Engineering,
Xi'an Jiaotong University,
Xi'an 710049, China
e-mail: wt-h@163.com

Zhaopeng Luo

Key Laboratory of Education Ministry for
Modern Design and Rotor-Bearing System,
School of Mechanical Engineering,
Xi'an Jiaotong University,
Xi'an 710049, China
e-mail: lzp3117301096@stu.xjtu.edu.cn

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received March 27, 2018; final manuscript received September 12, 2018; published online November 1, 2018. Assoc. Editor: Noel Brunetiere.

J. Tribol 141(3), 031702 (Nov 01, 2018) (9 pages) Paper No: TRIB-18-1129; doi: 10.1115/1.4041511 History: Received March 27, 2018; Revised September 12, 2018

The ultrasonic technique is very effective in measuring lubricant film thickness in a noninvasive manner. To estimate the film thickness with reflection signals, two main ultrasonic models are often applied in cases of different film thicknesses; they are the spring model for thin films and the resonant model for thick films. However, when measuring oil film thicknesses distributed in a wide range, there is an inherent blind zone between these two models. This problem is especially prominent in online monitoring because the abrupt variation of film thickness is highly correlated with the occurrence of abnormal conditions. To address this issue, we further proposed a method using the phase spectrum of reflection coefficient which can cover a wide range of film thicknesses. The slight variation of reflection signal in the blind zone can then be identified and bridged the measurement gap between those two traditional models. A calibration rig was used to verify the theoretical analysis and the results indicated that the developed model is capable of providing reliable ultrasonic measurement of lubricant film thicknesses in a wide range.

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References

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Figures

Grahic Jump Location
Fig. 2

Variation of reflection coefficient amplitude with the product of frequency and thickness. The measurement range is divided into three regions by the vertical dashed lines.

Grahic Jump Location
Fig. 1

Illustration of an ultrasonic wave traveling through a three-layered system

Grahic Jump Location
Fig. 3

Variation of reflection coefficient phase with the product of frequency and thickness. The solid line represents the result of complete reflection coefficient phase (Eq. (8)) and the dotted line represents the result of the spring model (Eq. (9)).

Grahic Jump Location
Fig. 4

Comparison of the phase spectrum of reflection coefficient including oil attenuation and no attenuation

Grahic Jump Location
Fig. 11

Oil film thickness calculated by Eq. (8) showing the variation with frequency in the transducer −3 dB bandwidth

Grahic Jump Location
Fig. 9

Reflection coefficient amplitude and phase spectra of reflected signal from oil films of different thicknesses in the resonant model zone: (a) amplitude spectrum and (b) phase spectrum. The FFT amplitude spectra of reference signal are also shown (dotted line), with arbitrary vertical scale, to demonstrate the bandwidth of the transducer.

Grahic Jump Location
Fig. 5

Schematic diagram of experimental setup to calibrate the lubricant film thickness

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Fig. 6

Time-domain plot of reference signal reflected from a steel–air interface

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Fig. 7

The FFT of reference signal reflected from the steel–air interface: (a) amplitude spectrum and (b) phase spectrum

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Fig. 8

Time-domain plot of reflected pulses from oil films of different thicknesses: (a) in resonant zone and (b) in the blind zone and spring model zone

Grahic Jump Location
Fig. 10

Reflection coefficient amplitude and phase spectra of reflected pulse from oil films in blind and spring model zones: (a) amplitude spectrum and (b) phase spectrum. The FFT amplitude spectrum of the reference signal is also shown (dotted line), with arbitrary vertical scale to demonstrate the bandwidth of the transducer.

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Fig. 12

Measured film thickness with the number of measurement for different position of movable steel disk

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Fig. 13

Comparison of five models of film thickness measurement with the actual film thickness; the minima and zero-crossing methods represent the results from the resonant model (Eq. (3)) by measuring the frequency of minima and zero-crossings, the amplitude |RK| and the phase ΦK method represent the result from spring model (Eqs. (7) and (9)), the phase method Φ represents the result from the complete reflection coefficient phase model (Eq. (8))

Grahic Jump Location
Fig. 14

The absolute error of actual measured value to predicted one for five experiments by using the reference signal acquired before the experiment. The theoretical reflection coefficient phase spectrum of center frequency is also shown (dotted line), with arbitrary vertical scale.

Grahic Jump Location
Fig. 15

The absolute error of measured value to actual one for fourth and fifth experiment using the reference signal recorded before and after experiment, respectively. References 1 and 2 represent the reference signal acquired before and after experiment, respectively. The theoretical reflection coefficient phase spectrum of center frequency is also shown (dotted line), with arbitrary vertical scale.

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