# 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 system consists of in-hole instruments and orifice instruments as shown in Figure 8, and the corresponding schematic diagram is shown in Figure 9. The working principle is as follows. The measuring nipple, the charging battery cartridge and the generator control nipple are connected to form the instrument in the hole, and it works according to the preset working mode and the start and stop status of the mud pump. The measuring nipple sends the detected parameters to the launch control nipple when data measurement and transmission are required. The launch control nipple sends the measured data through the upper drill string of the insulated nipple and the lower drill string of the insulated nipple in a wireless electromagnetic wave manner according to the preset coding rules, transmitting the data to the orifice through the upper drill string and the coal-bearing formation. The receiving electrode installed in the coal-bearing formation at the hole mouth and on the drilling equipment collects the uploaded electromagnetic wave signal and transmits it to the signal acquisition board in the explosion-proof computer through the wired mode. The signal is demodulated by the acquisition board according to the preset coding rules, and the correct in-hole process parameter data are obtained, then displayed on the screen through the data processing software in the explosion-proof computer [58]. Figure 6: Overall structure composition diagram of mine mud pulse wireless measurement while drilling system. Figure 8: Overall structure design diagram of mine electromagnetic wave wireless measurement while drilling system. Figure 7: Working principle of mine mud pulse wireless MWD system. Figure 9: Wireless MWD system of electromagnetic wave for mining. Automatic vertical drilling technology has been developed on the basis of measurement while drilling and formation evaluation while drilling, real-time drilling data acquisition--processing--application technology, closed-loop rotary steering drilling technology and automatic drilling rig development [59]. This technology is an active deviation prevention technology aiming at vertical drilling. It uses the downhole closed-loop control and ground monitoring technology to measure the hole deviation in real time during the bit drilling process, and makes the bit return to the vertical direction by controlling the action of the deviation correction mechanism on the drilling tool in real time. At present, the existing automatic vertical drilling systems in the world have applied the rotary steering drilling technology, such as the power V system of Schlumberger Company, which integrates the power drive technology; while the VertiTrak system of Baker Hughes integrates the AutoTrak technology, and even has the same key working principle and anti-deviation mechanism [60; 61; 62]. During the drilling process, the drilling track extends along the direction of hole inclination. It is difficult to avoid well deviation due to the influence of many subjective and objective factors. In the actual drilling process, the drilling trend and the extension direction of the drilling track depend not only on the mechanical characteristics of the interaction between the bit and the formation, but also on the structure of the bit and the anisotropy of the formation [63; 64; 65; 66]. At present, the sliding directional drilling technology and the composite directional drilling technology used in the underground directional drilling construction of coal mines are both \"geometric guided\" drilling methods, that is, the trajectory control is based on the deviation of the spatial geometric parameters of the actual drilling trajectory and the design trajectory, which solves the problem of drilling trajectory control in the thicker and stable target strata [67]. However, the drilling track easily deviates from the target stratum when the thickness of the target stratum is small and the fluctuation is large, and it is impossible to ensure the extension along the target stratum.