Condition based monitoring has become essential in maintaining and extending the health of high-speed rotating and reciprocating machinery. One effective approach to detect signs of potential failure of a rotating or reciprocating machine is to examine the wear debris in its lubricating oil. Magnetic inductive sensors have gained certain success due to the capability of differentiation of ferrous and non-ferrous wear debris, which is important for condition monitoring of rotating and reciprocating machinery. However, they are limited to debris larger than 100 µm in size because of the low sensitivity of 3-D solenoids. To overcome this problem, the design concept and preliminary testing results of an inductive coulter Counter that uses a planar coil as sensing element are presented.
We first demonstrate the feasibility of the inductive Coulter counting principle to detect metal particles in lubrication oil using a microfluidic device. The device detects the passage of ferrous and nonferrous metal particles by monitoring the inductance change of an embedded planar coil. The device was tested using iron and copper particles ranging in size from 50 µm to 125 µm. The testing results have demonstrated that the device is capable of detecting and distinguishing ferrous and nonferrous metal debris particles in lubrication oil.
Next, to overcome the limitations of the microfluidic device, a meso-scale oil debris sensor was studied. The sensor used a two-layer planar coil with a
meso-scale fluidic pipe crossing the planar coil's center as the sensing element. The testing results indicated that the throughput is significantly higher than that of the microfluidic channel without sacrificing the sensitivity. However, the throughput still needs to be improved to meet the online monitoring requirements. To further improve the throughput, we designed and tested an inductive Coulter counter with seven detection channels to detect and count metal particles in lubrication oil. The testing results indicated that using multichannel sensor we are able to improve the detection throughput of the sensor seven times in contrast to a single channel sensor. Negligible crosstalk among channels was observed.
Finally, a proof-of-principle resonant frequency division multiplexed four-channel inductive sensor for detecting microscale metallic debris in lubrication oil using only one set of detection electronics is presented. Using the resonance frequency division multiplexing method, debris particles in four parallel sensing channels were successfully detected and counted simultaneously. Testing results indicate the sensor is capable of differentiating and detecting ferrous and non-ferrous metallic debris. Crosstalk among four sensing channels is negligible.