Signal multiplexing on Inductive oil debris sensor

Activities

We invented and tested the signal multiplexing on a multi-channel oil debris sensor (Figure 2) for high throughput detection of metallic wear debris.

2.1 Device Description

Fig. 1. Design concept of the multiplexed four-channel oil debris sensor for metallic wear debris detection in lubricants.

We applied signal multiplexing on a 4-channel Coulter counting oil debris sensor (as illustrated in Fig. 1) to shown the design concept. The sensor consists of four parallel fluidic channel-planar coil assemblies (sensing elements). Each sensing element consists of a meso-scale fluidic channel (1mm in inner diameter) made of glass and a two-layer planar coil wrapped around the fluidic channel. Each sensing coil is electrically connected in parallel with an external capacitor Cpi (i=1, 2, 3, 4) to form a parallel LC resonant circuit that has a unique resonant frequency. The fabrication process of fluidic channel-planar coil assembly is as following: each fluid channel has a two-layer planar coil wound around a glass tube with a 1 mm inner diameter and a 1.2 mm outer diameter. To build a fluidic channel-planar coil assembly, we first drilled a 1.3 mm through hole on a glass slide. Next this glass slide was clamped to another glass slide, with a 175µm gap between the two slides; the gap was created by applying a 175µm spacing, made of seven layers of cellophane tape, at the upmost left and right edges of the two glass slides. Then the glass tube was inserted into the through hole; a small amount of epoxy was applied in the gap between the glass tube and the hole’s edge, and was dried in air to fix the glass tube. Next a 2-layer coil (each layer is a 10-turn planar coil) was built layer by layer by carefully winding AWG 40 copper wire (80µm in diameter) around the glass tube in the 175µm spacing. Once the assembled final device (see Figure 2) was assembled, a syringe pump was used to load the oil sample into the multichannel sensor via a common inlet pipe. A syringe pump, a common inlet pipe and a common outlet pipe were used to pump in and out the oil sample. Multiplexed parallel sensing of oil debris via the multiple sensing channels significantly improves the throughput.

2.2 Performance Testing and Demonstration

Fig. 2. Measurement setup and equivalent electrical circuit for the four-channel multiplexed debris sensor.

Figure 2 illustrates measurement setup and equivalent circuit of the four channel debris sensor. For multiplexed oil debris detection, the four planar coils were electrically connected in serial. Each planar coil is modeled as an inductance Lsi in series with a resistance Rsi (i=1,2,3,4). Cpi (i=1,2,3,4) is the external capacitor, which was connected to each planar coil in parallel to form a parallel LC circuit. The four sensing elements were serially connected to a sinusoidal excitation source, V0, with an internal resistor R0. Parallel LC resonance mechanism was applied to the multichannel oil debris detection. First, specific Cpi for each sensing channel was selected such that each sensing coil has a unique resonant frequency. Second, a combined excitation signal (V0) that consists of four sine waves whose frequencies are close to the resonant frequencies of the four sensing channels was applied, and only one combined response Vout was measured. Because signals from each sensing channel exhibits a peak amplitude at its resonant frequency, the signals for each individual channel can be recovered from the combined response by taking the spectrum components at each resonant frequency with an improved signal-to-noise ratio. Inductance change for each channel can therefore be calculated from individual signals.

An Agilent 33220A Function Generator sent a combined excitation consisting of sinusoidal waves with four frequencies (1.55 MHz, 1.90 MHz, 2.10 MHz and 2.55 MHz) determined above, with 10V peak-to-peak amplitude, to the four channel debris sensor. The output voltage, Vout, was recorded by the 14-bit Digitizer at a 100MHz sampling rate. Next Vout was divided into many segments of data; each segment consists of 0.1 ms data. FFT was conducted for each 0.1ms segment of Vout data. Cubic spline interpolation was used to reduce the peak detection error. Because the signal primarily consists of four sinusoidal waves, the peak magnitude of each Vout(fi) in frequency domain is equal to the Vout(fi) peak in time domain. The peak magnitude of Vout(fi) was obtained by using a peak detection code written in Matlab®. When a debris particle passes through a sensing channel, it causes a magnitude change in Vout(fi) of the channel. By using FFT the change in voltage magnitude of each sensing channel can be obtained. Inductance change of each channel was thus calculated.

Findings

2.1 Findings about the signal multiplexing on Inductive oil debris sensor: Demonstration of a Multiplexed Multichannel Oil Debris Sensor

Fig. 3. Calculated relative inductance change caused by iron particles ranging from 50µm to 75µm and 125µm copper particles, (a) Channel 1, (b) Channel 2, (c) Channel 3, (d) Channel 4

Copper and iron particles suspended in SAE 5W-30 motor oil were used to test the multiplexed sensor. The oil sample mixed with metallic particles was pumped to pass the center of the planar coil by a Kent Scientific YA-12 syringe pump with a controlled flow rate, and was collected with an oil tank. For all experiments the flow rate of oil sample was set to be 21ml/min. 1 mg iron particles with diameters ranging from 50µm to 75µm and 1mg copper particles with 125µm in diameter mixed with 10ml SAE-5W30 lubrication oil were tested. Figure 1 shows the measured relative inductance change caused by iron particles and copper particles for all four channels. Both positive and negative pulses were generated; positive pulses were induced by iron particles, while negative pulses were induced by copper particles. This test demonstrates using the signal multiplexing method, inductance changes caused by debris particles from individual signals were successfully recovered with only one set of electronics and one measurement. We also successfully tested an 8-channel device with higher throughput.

Furthermore, we found that using the parallel LC resonance based signal multiplexing method, the signal-to-noise ratio has been significantly improved because the voltage output was amplified approximately 3 times by the resonance peak near the resonance frequency, making the measurement method possible to detect smaller metallic debris. Figure 4 shows the sensitivity was improved significantly in comparison to the sensitivity without using signal multiplexing. The signal-to-noise-ratio can be further improved by optimizing the inductance and resistance of the sensing coil, and other circuit parameters.

Fig. 4. Comparisons of relative voltage output using LC resonance and non-resonance method. Iron particles ranging from 32µm to 96µm were used for the testing.

The above testing results have demonstrated the multiplexed multichannel oil debris sensor has significantly increased the detection throughout using only one set of detection electronics and one measurement, with improved sensitivity.