Debris monitoring

Details Debris monitoring Debris monitoring

1997-02-27

1998-09-22

Raevis, Robert

Measuring and testing

Liquid analysis or analysis of the suspension of solids in a...

Lubricant testing

324204, G01R 3312

Patent

active

058116648

DESCRIPTION:

BRIEF SUMMARY
This invention relates to machine wear debris monitoring and, in particular, to transducers for use in monitoring machine wear debris. More specifically, the invention concerns an improved sensor for such transducers.
There is considerable interest in developing accurate and reliable techniques for continuous, "on-line" monitoring of metallic particles in the lubricating oil of machines such as engines and gear boxes. Such metallic particles are produced by mechanical wear in the machines and the presence of metallic wear particles can indicate incipient machine failure. The rate at which wear debris is generated can be used as a measure of the condition of a machine.
Wear debris monitoring transducers are known which utilise an inductive coil having a plurality of turns which are wound around a non-magnetic pipe containing a lubricant stream. As metallic particles in the lubricant pass through the coil they are magnetically coupled to the coil.
Ferrous particles have a high relative permeability and so a ferrous particle passing through the coil will increase the inductance of the coil. Non-ferrous metallic particles will decrease the inductance due to eddy currents which are induced in the particles by the magnetic field of the inductive coil. By resonating the inductive coil with a fixed capacitor in the resonant or tank circuit of an oscillator it is possible to monitor changes in the coil inductance as changes in the frequency of the oscillator since the resonant frequency of the tank circuit is dependent upon the inductance of the coil. An increase in the coil inductance will cause a decrease in the oscillator frequency indicating the presence of ferrous particles. A decrease in inductance will cause an increase in the oscillator frequency, indicating the presence of non-ferrous metallic particles. The magnitude of the frequency change can be used to indicate the size of a metallic particle in the lubricating fluid passing through the coil.
In practice most such transducers are used in systems where the lubricating fluid stream contains air bubbles or water droplets. This is a common phenomenon in lubrication systems and pumped lubrication systems in particular may include significant amounts of foam within the fluid (air content may be in excess of 50%), as well as lubricating fluid which has become contaminated with water. The inductive coil of the transducer has a parasitic capacitance arising from stray capacitance effects between adjacent turns of the coil. Where the coil is placed around a relatively thin-walled pipe this parasitic capacitance will be affected by the dielectric properties of the contents of the pipe. Since the relative permittivities of air, oil and water are approximately unity, two point five, and eighty-one respectively, the parasitic capacitance of the coil may vary considerably as air or water droplets present in the lubricating oil or fluid pass through the coil. As the resonant frequency of the oscillator tank circuit is dependent on capacitance as well as inductance, changes in the capacitance of the coil result in changes in the resonant frequency. An increase or decrease in capacitance will result in a decrease or increase, respectively, in the resonant frequency. Thus the responses of the transducer to air bubbles and water droplets will be similar to those cause by metallic particles. This can lead to unreliable results and undesirable machine "down-time" due to false alarms and may lead in time to the monitor being deliberately inactivated as a result of user frustration with repeated false alarms, with the potentially highly dangerous consequence of the engine or gearbox being left without any means for indicating failure. Moreover, turbulent flow containing trapped air or foam will give a continuous noisy output signal from the transducer which may mask signals from metallic debris.
A probe member for a machine wear debris monitoring transducer is described in U.S. Pat. No. 5,041,856 which attempts to overcome the problems associated with stray capacitance effect

REFERENCES:
patent: 3744044 (1973-07-01), Vosteen
patent: 4439734 (1984-03-01), Weber
patent: 4837511 (1989-06-01), Whittington et al.
patent: 4926120 (1990-05-01), Veronesi et al.
patent: 5315243 (1994-05-01), Kempster et al.
"An On-Line Wear Debris Monitor" Measurement Science and Technology, vol. 3, No. 7, Jul. 1992, Bristol GB, pp. 656-661 (H.W. Whittington, et. al.).

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