## 2 Materials and Methods  
### Electronic Tongue System  
Electronic tongue systems can be divided into several types according to data measurement technology, such as potentiometry [30], voltammetry [31] and so on. In addition, spectrophotometric [32] is another very popular means of measurement. Each type of electronic tongue can utilize different types of working electrodes, such as bare electrodes, modified electrodes and biosensors [33]. Typically, bare electrodes are gold, silver, and palladium; the modified electrodes are carbon paste processed by double phthalocyanine compounds or conductive polymers; biosensors are carbon biocomposite or graphene electrode containing enzyme and different metal catalysts. Although different types of electronic tongue systems collect sensors response in a different way, their features extraction and pattern recognition methods are similar. In this paper, to verify the validity of features extraction and classification methods, the widely used voltammetry electronic tongue system is adopted to collect sensors response.  
As the structure shown in Figure 3, we designed a voltammetry electronic tongue hardware system composed of controller, multi-frequency pulse voltage occurrence circuit, constant potential circuit, three-electrode module, micro-current detection circuit and low-pass filter. Three-electrode module consists of working electrodes (WE), reference electrodes (RE) and counter electrodes (CE), which is the core part of the electronic tongue hardware system. Different electrode materials have different response characteristics, the details of the electrode sensor array used in the proposed model are described in Table 1. Figure 4 shows the physical diagram of the electronic tongue hardware system we designed.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Electrode** & **Material** & **Role** \\ \hline
1 & WE & gold, silver, palladium & Produce a chemical reaction on the electrode surface. \\
2 & CE & platinum & Make measurement results more stable and reliable. \\
3 & RE & silver/silver chloride & Provides a reference for the working electrode potential. \\ \hline \hline \end{tabular}
\end{table}
Table 1: The details of the electrode sensor array.  
Figure 2: The overview processing of convolutional neural network (CNN) based auto features extraction strategy.  
The micro-control unit of the electronic tongue hardware system is the STM32 controller, whose functions include generating scan signals, micro current detection, noise filtering, signal sampling and signal storage. Specifically, the controller drives the external circuit through the DA conversion to generate different voltage waveforms in the first and analog voltage waveforms are equivalent added to RE through the constant potential circuit under the feedback of CE. Then, driven by potential of RE, the Faraday current is generated between WE and CE, the micro current detecting circuit connected to the WE converts the Faraday current into a voltage signal. Next, the filter circuit is designed to eliminate clutter in the converted signal. Finally, the amplified voltage signal is converted to a digital voltage signal by AD module conversion and the response signals from different WE are stored. Two images in Figure 5 represent the typical sensor data of different kind of tea and electrodes, respectively. Figure 5a shows five kinds of tea sampling from silver WE and Figure 5b displays the response of Tieguanyin tea from three different WE (Gold, Silver, Palladium). It should be noted that for the same concentration of tea from the same brew, the three different WE work sequentially. In detail, the gold electrode is applied to collect samples first, and the silver electrode is replaced to get samples subsequently, and the palladium electrode is replaced for data sampling finally. All the sampling signals are stored in the SD card.  
Figure 4: The designed voltammetry electronic tongue hardware system.  
Figure 3: The structure of designed voltammetry electronic tongue hardware system.