# Methane Gas Density Monitoring and Predicting Based on RFID Sensor Tag and CNN Algorithm
## 1 Introduction  
Because of the existence of methane and other toxic gases, underground mines have proved to be dangerous [1; 2; 3]. For example, 33.8% of accidents in underground mines are caused by methane gas density exceeding the normal range [4]. Generally speaking, these underground mine accidents are difficult to eliminate because human beings always work underground. In all accidents, methane exposure is the most common and the most serious [5; 6]. The minimum explosion percentage of methane gas is 5%. However, when exposed to low methane gas density for a long time, the human body will still be severely damaged [7; 8; 9]. The methane gas density is traditionally measured by human beings [1; 3]; however, this method is not only time-consuming and laborious, but dangerous to these people in underground mines. A search-and-rescue robot system was introduced for the remote sensing of the underground coal mine environment [10]. It can achieve environment sensing and rescue automatically. However, it is still not a real-time monitoring system. In the past few decades, the cable monitoring system (CMS) has been widely employed in underground mines for real-time environment monitoring [2]. Still, due tothe complex situation of underground mines, the CMS requires high installation and maintenance costs. Therefore, the environmental monitoring of underground mines urgently needs a flexible and economic method. In recent years, due to their advantages of fast deployment and low cost, wireless sensor network (WSN) technologies have been wildly applied in industry [11; 12; 13]. In particular, ZigBee technology has become the mainstream technology for underground environment monitoring [14; 15; 16]. Nevertheless, the WSN network does not care about the location of the sensor node; so, when an abnormal situation happens, the abnormal node cannot be identified quickly. Hence, the research on integrating WSN with radio frequency identification (RFID) arouses great interest [17; 18; 19]. Each RFID tag has a unique identification (ID) number. The integration of these two technologies is beneficial for the WSN network to concentrate on the data transmission. When the information of a specific node is considered, this node can be quickly located by using the RFID function. Furthermore, because the RFID sensor employs a backscattering communication scheme, it can work in passive mode for long-term application. Hence, RFID sensor technology is a beneficial supplement to the current WSN technology. While there are always several research projects focusing on the combination of RFID and WSN technology [20; 21; 22], this paper studied the feasibility of monitoring methane gas density in an underground mine environment by using a wireless sensor based on RFID technology for the first time. RFID sensors can be classified into two categories, the chipless group [17; 23] and chip-based group. Chipless RFID sensors adopt a sensor's signal to directly modulate the backscattering performances of the antenna. They have the advantage of being ultra-low cost; however, they are not suitable for composing the sensor network due to the lack of digital blocks. Chip-based RFID sensors can also be divided into two groups, integrated circuit (IC)-based RFID sensors [18; 19] and printed circuit board (PCB)-based RFID sensors [24; 25]. IC-based RFID sensors integrate the sensors with an RFID tag chip, resulting in small circuit area and high stability. However, these sensors require a long design period. Moreover, only several kinds of sensor can be fabricated by the complementary metal oxide semiconductor (CMOS) process. PCB-based RFID sensors incorporate an RFID chip, sensors, and digital blocks on the same board. These sensors show the advantages of short design period, high extension capability, and low cost. Besides real-time monitoring, gas density prediction is also important for mine safety.