# Monitoring of Liquid Viscosity for Viscous Dampers through a Wireless Impedance Measurement System
## 2 Monitoring Method of Damper Fluid Viscosity
### Design of Wireless Impedance Measurement System  
The overall scheme of the wireless impedance measurement system used in this paper is shown in Figure 2. The system consists of the following four modules: power supply, impedance measure, main controller, and communications. The power supply module can provide +3.3 V working voltage for the system. The main control module STM32F103C8T6 provides control and information processing for AD5933, NB, and other modules. The communications module is responsible for sending measurements to the host computer or cloud server.  
The impedance measurement module uses the AD5933 as the core, and the internal functional block diagram of the chip is shown in Figure 3. The AD5933 contains four main parts: the transmitter stage, the receiver stage, the digital signal processor (DSP), and the data transmitter. The transmitter stage generates a sinusoidal excitation signal of up to 100 kHz with a resolution of 0.1 Hz. The receive stage conditions the impedance response signal and then inputs the conditioned signal into a 12 bit, 1 MSPS analog-to-digital converter (ADC). The built-in DSP module performs discrete Fourier transform (DFT) processing on the digital data output from the ADC. The data transfer records the real and imaginary registers after DFT processing at each frequency point through the IIC interface, and finally the impedance of the measured object can be calculated according to Equation (2).  
Figure 1: One dimensional model of electromechanical impedance.  
\[|Z|=\frac{1}{\text{k}\times\sqrt{\text{Re}^{2}+\text{Im}^{2}}} \tag{2}\]  
where \(|Z|\) denotes the impedance modulus, k denotes the gain coefficient, and Re and Im denote the real and imaginary parts of the impedance, respectively.  
NB260 module is selected as the wireless transmission module. NB260 module is a Narrowband IoT (NB-IoT) series module with high performance, small volume, and low power consumption. It supports NB-IoT all over the world. The minimum power consumption is only 0.2 mW and the size is 33 \(\times\) 25 \(\times\) 5 mm. NB-IoT is an international Internet of Things standard. It has the characteristics of flexible deployment, wide coverage, and low cost, supporting the access of a large number of devices, and has the advantage of direct access to the web server. NB260 module and MCU control module interact with each other through serial port. The NB260 module is connected to the master module through a serial port, and the initialization settings and data interaction are performed through AT commands. The NB260 module has three operating states, including a connection mode, an idle mode and an energy saving mode. When the data transmission is completed, the master control module commands the NB260 module to enter energy-saving mode to reduce system power consumption and improve the battery life. The functional block diagram of NB260 is shown in Figure 4. The physical diagram of the wireless impedance measurement system is shown in Figure 5.  
Figure 3: Block diagram of impedance measurement function.  
Figure 2: Wireless impedance measurement system model.