# Review on the Influence of Complex Stratum on the Drilling Trajectory of the Drilling Robot
## 2 Detection of Complex Formation and Measurement while Drilling Technology
### Measurement while Drilling (MWD) Technology  
The MWD system is mainly used for monitoring while drilling during the construction of horizontal directional drilling in the coal mine. It can measure the main parameters such as the drill angle, azimuth angle and tool face angle while drilling. At the same time, it can realize the real-time display of the drilling parameters and drilling trajectory. It is convenient for the drilling personnel to understand the drilling construction situation at any time, and adjust the face angle and process parameters of the screw drilling tools in a timely manner, so that the drilling can be extended as far as possible according to the design trajectory [40; 41]. According to different signal transmission modes, MWD systems can be divided into two categories: wired MWD systems and wireless MWD systems. Among them, the mine wireline MWD system is the most widely used and most developed MWD system in the field of directional drilling in coal mines [42; 43; 44]. The system is composed of in-hole equipment and orifice equipment, in which the in-hole equipment is composed of a measuring probe pipe, a non-magnetic drill pipe and other supporting equipment, while the measuring probe pipe is generally composed of a measuring unit, a signal transmission unit and a power supply unit [45]. The orifice equipment is composed of an explosion-proof computer, keyboard, memory and other supporting equipment, among which the explosion-proof computer is generally composed of a signal receiving unit, a data processing and display unit, a power supply unit, etc. As shown in Figure 4, the in-hole measuring probe pipe is installed in the non-magnetic drill pipe behind the screw drilling tool, the orifice equipment is installed near the rig console, and the central wireline drill pipe is connected from the hole bottom to the orifice in turn and the cable is used to connect with the orifice equipment when the mine wireline MWD system is used. During signal transmission, the central cabled drill pipe is equivalent to a coaxial cable. The drill pipe body and the signal transmission device form a signal transmission circuit. The signal transmission device in the center of the drill pipe is used as the signal line, and the drill pipe body is used as the signal ground to transmit the data in the hole to the orifice in real time. Wired signal transmission is widely used with the advantages of fast transmission speed and two-way signal transmission [46; 47]. The research focus of the mine wireline MWD system is mainly on the power supply mode, signal transmission mode and signal transmission channel of the in-hole probe tube, and the related products are mainly the RS485 or RS232 wired transmission mode. The transmission channel is a specially designed central wireline drill pipe, and the power supply mode is the in-hole battery barrel, such as the DGS drilling guidance system of the Australia VLD Company, and the YHD1-1000 (A) MWD system of Xi'an Research Institute, etc. [48; 49; 50]. However, the use of an in-hole battery cartridge for power supply has some disadvantages, such as affecting the signal transmission and working stability, easy damage to the instrument, measurement lag, and increased use and maintenance costs. In view of the shortcomings of a battery power supply, the mine wire MWD system YHD2-1000 (A), based on explosion-proof computer power supply, was developed through the innovation of the power supply mode of the measuring probe tube and the signal transmission technology [51]. The system is composed of an explosion-proof computer, explosion-proof keyboard, explosion-proof data memory and explosion-proof measuring probe, as shown in Figure 5. The explosion-proof computer at the orifice uses the explosion-proof keyboard for human-computer interaction, and the explosion-proof data memory is used for file import and export, which can receive and process the measurement signals while drilling in real time and can also supply power for the in-hole measuring probe tube.