## 3 Shearer Positioning Technology
### The Measurement Method of Longwall Retreat and Creep Displacements  
In Section 3.1, the utilization of auxiliary sensors, ZUPT, DZUPT, and kinematics model reduced INS drift error and improved the positioning accuracy. However, due to the lack of periodic GPS calibration, the INS longtime error is still relatively large. After several cutting cycles, the shearer positioning accuracy cannot meet the requirement of the longwall mining automation. According to the longwall mining method, the shearer reciprocates between two roadways along the longwall face. When the shearer reaches the end of the longwall face, the longwall retreat and creep displacements become significant parameters to measure, which can be utilized to back-correct the shearer trajectory after the completion of each cutting cycle. As shown in Figure 11, according to the longwall mining method, the retreat displacement is actually the advancing distance of the mining equipment along the advancing direction every time. Due to the different levels of inclination of the longwall face, there will be a height difference between the two ends of the longwall face. When the mining equipment is installed on the inclined longwall face, the force acting on the equipment is not balanced, which makes the equipment slide along the longwall-face direction and produces the displacement of sliding up and down. In the LASC, [PERSON] et al. [20] installed the scanning laser on the roadway conveyor structure linked to the AFC to scan the roadway profile. Accordingly, the first generation of a scanning device with SICK LMS200 as the core sensor and the second generation of an explosion-proof 3D laser scanning device called ExScan have been developed. The current scan was matched to the global map with the scan-matching algorithm to acquire the retreat and creep displacements, as shown in Figure 12. However, the dust and moisture in an underground mine would greatly increase the attenuation and distortion of the laser beam, which affect the laser measurement accuracy. Then, [PERSON] et al. [29; 30; 31] used  
Figure 11: The diagram of the longwall retreat and creep displacements. Figure 10: The diagram of (**a**) the underground field test in Shanxi and (**b**) the shearer trajectory acquired using the SPAS [28]. the FMR 250 radar from Endress-Hauser company in Germany to measure the retreat and creep displacements, and a field test in Australia's Beltana mine was carried out to verify the effectiveness of the proposed method, as shown in Figure 13. The radar is sensitive to distance information, and the creep displacement was obtained through measuring the distance between the radar and roadway wall. The number of bolt-plates on the roadway surface represented the position information, thus enabling the measurement of retreat displacement through the recognition of these bolt-plates. However, in the majority of mines, the bolt-plates lack position information, thereby limiting the applicability of this method. In CUMT, the authors of this paper provided a novel measurement solution with the ultra-wideband (UWB) radar imaging method [32]. UWB radar is characterized by a radar with a fractional bandwidth exceeding 25%. Through transmitting electromagnetic wave signals with a strong ability to penetrate a coal-dust environment, UWB radar can obtain target or scene images all day and in all weather, which has the technical advantage of application in the complex environment at the end of a longwall face. As shown in Figure 14a, the radar was mounted on a slide-way, and the slide-way was placed on the roadway conveyor structure connected to the head of the AFC. The radar was moved along the slide-way to image the bolt-plates. Two imaging results were obtained before and after the movement of the longwall face. Then, the feature-based image registration algorithm was employed to identify the correct-match pairs, and subsequently, the similarity transformation model was calculated to determine the retreat and creep displacements.