## 2 Operating Principle and Structure
### Sensor Structure  
Since the sensor mentioned in this paper is installed on the wrist devices, it will be in contact with the human body frequently. In addition, the motion signals of the wrist are relatively weak and easily disturbed. Based on the above, the specific structure of the sensor was designed as Figure 2. It consists of seven layers: a protective layer, a PDVF film layer, an adhesive layer, a backing layer, a substrate layer, an electrode layer, and a signal preprocessing circuit board layer.  
A protective layer is needed on the surface of PVDF to avoid wear or corrosion, which is a piece of transparent thin film made of high-molecular polymer PET (polyethylene glycol terephthalate). The PVDF film mentioned in this paper (area: 6 \(\times\) 20 mm; thickness: 50 \(\upmu\)m) was provided by Jinzhou Kexin Electronic Material Co., Ltd. (Jinzhou, China). The substrate (area: 11 mm \(\times\) 8 mm;  
Figure 2: Structure diagram of wrist PVDF sensor.  
thickness: 0.4 mm) is plastic plate. There are two square through holes (area: 6 mm \(\times\) 0.8 mm) on the substrate, which are used to pull the PVDF from the front to the back of the substrate.  
The backing is a pad behind the PVDF film, which directly affects the performance of the sensors. The backing was designed as a half-cylinder (diameter: 5 mm; high: 6 mm), which could increase the deformation amplitude of the PVDF film effectively and produce more charges for a higher sensitivity and better directivity of the sensor. The output response of the sensor was based on silicone rubber and ABS (acrylonitrile-butadiene-styrene) backing, which were easily prepared; then, they were tested and compared. The silicone rubber backing was prepared through a mold, and the ABS backing was prepared through a three-dimensional (3D) printer. The silicone rubber was chosen as backing because of its better performance. The film and backing need to be anchored with adhesives to reduce the relative displacement between them. The curing agent and PSA (pressure-sensitive adhesive) can be used as adhesive. The sensor's output responses based on them were tested and compared. The PSA was selected as the adhesive because of its better performance. The test details of adhesive and backing are described in Chapter 3.  
The two printed electrodes (each area: 2 mm \(\times\) 2 mm; material: Cu) on the back of the signal preprocess circuit board were designed for signals transduction. The electrodes were connected with the film through silver glue to improve the electrical conductivity. The output signal of the sensor needs to be preprocessed in order to achieve a high signal-to-noise ratio because of the complex electromagnetic environment when it is used in wearable devices such as smart watches. A differential charge amplifier, which is developed using one amplifier and a second-order Butterworth low-pass filter, has been integrated onto the circuit board of a signal preprocess (area: 11 mm \(\times\) 8 mm; thickness: 0.4 mm). The main function of the charge amplifier is to amplify weak charge signals and match the impedance of the sensor, which transforms the medium or high-impedance input to a low-impedance output [20]. The main function of the filter is to filter out the high-frequency noise. The signal preprocess circuit board and the sensor were directly combined in the structure design for three advantages, which include greatly reducing the size, ease for clamping the PVDF piezoelectric film, and convenience for modular use of the sensor.