# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 2 Fluid Property Sensing  
### Fluid Property Sensor  
A physical diagram of the fluid property sensor is shown in Figure 2. The measurement parameters of the fluid property sensor are shown in Table 1. The fluid property sensor directly and simultaneously measures the viscosity, density, dielectric constant, and temperature of nonconductive fluids. Relying on patented tuning fork technology, the sensor monitors the direct and dynamic relationship between multiple physical properties to determine the quality, condition and contaminant loading of fluids such as engine oil, fuel, transmission and brake fluids, hydraulic and gear oils, refrigerants, and solvents. The capacity for multiparametric analysis improves fluid characterization algorithms. The fluid property sensor provides in-line monitoring of fluids for a wide range of OEM and aftermarket installations, including fluid reservoirs, process lines and pressurized high flow conduits (e.g., engine oil gallery) and for applications that include on- and off-road vehicles, compressors, industrial equipment and turbines. This paper will focus principally on engine oil applications, but most of these analyses can be adapted or replicated for other applications [23; 24].  
Figure 2: Physical view of quartz tuning fork sensor.  
The hardware circuit design of this paper consists of two parts, an STM32F103 microcontroller system control circuit and impedance measurement circuit based on AD5933 impedance measurement chip. The hardware structure of the system is shown in Figure 3. The AD5933 chip is integrated with a DDS frequency generator, I-V amplifier, low-pass filter circuit, A/D conversion circuit, and FFT module, which can realize impedance measurements based on the vector current-voltage method in a high integration environment with certain accuracy. The chip realizes data communication through the I\({}^{2}\)C interface.The hardware structure of the system is designed as follows:  
The peripheral circuit design of AD5933 is shown in Figure 4. The chip is powered by a 5-V power supply. The SCL and SDA pins of the microcontroller are used as I\({}^{2}\)C communication pins and are connected to the 15 and 16 pins of the AD5933 chip through a pull resistance of 10 k\(\Omega\). The AVDD and AGND of the analog circuit are directly connected with the VCC and GND, while the AVDD and DGND of the digital circuit are connected with the VCC and GND by a 10-uH chip inductor, respectively, to realize the separation of the analog and digital circuits. The input and output ends of the quartz tuning fork sensor are, respectively, connected to the VOUT, RFB and VINT pins of the AD5933 chip through the connectors; VOUT is the excitation voltage signal output, RFB is the external feedback resistance, and VINT is the transconductance amplifier input. Because the component to be tested is a quartz crystal oscillator, its impedance is large, so it is not necessary to increase the external amplifier circuit to improve the load capacity, and the output end of the chip is directly connected to the crystal oscillator, the measurement can be realized.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Measurement Parameter** & **Measurement Range** & **Precision** & **Resolution Ratio** \\ \hline
**Viscosity** & 0.5–50 cP & \(\pm\)2\% & 0.015625 cP \\
**Density** & 0.65–1.5 g/cm\({}^{3}\) & \(\pm\)1\% & 0.00003052 g/cm\({}^{3}\) \\
**Dielectric constant** & 1.0–6.0 & \(\pm\)1\% & 0.00012207 \\
**Temperature** & \(-\)40–150 \({}^{\circ}\)C & 0.1 \({}^{\circ}\)C & 0.03125 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table}
Table 1: Measurement parameters of the sensor.  
Figure 3: Design of hardware circuit of the measurement system.