## 7 Summary  
This paper is a systematic description and review on the research status and development of underground intelligent LHD, based on the relevant literature collected by the mainstream literature database in domestic and foreign countries. Through the literature statistics, it is found that the research history for the reviewed vehicle has been over 20 years. Foreign countries kept ahead in either industry or theory, while China started late and developed slowly. Through arranging and reviewing the mainstream technologies from four aspects as the mine pile perception and modeling technology, bucket loading trajectory planning methods, autonomous navigation technology, real-time monitoring and fault diagnosis technology, it can be concluded that even though those technologies for underground intelligent LHD have developed rapidly in recent years, there still needs to be further progress both domestically and overseas; the research directions can be proposed as follows:  
1. For better mile pile perception in the future, how to complement and optimize the information of multi sensors in a multi-level and multi-dimensional manner, improve the data processing speed and establish the three-dimensional model would be the critical scientific issues, as a single sensor perceives poorly for the heaps in underground roadways that are dark, dusty and face field interference.
2. In the research for bucket shovel loading trajectory plan and optimization, the planning method based on reinforcement learning will be one of the mainstream directions under the background of artificial intelligence, big data and cloud computing in the future, while how to complete the shoveling most efficiently with the least energy consumption is the key goal for this method.
3. As for autonomous navigation technology, it is one of the key researched technologies for underground intelligent LHD, both at home and abroad, and it directly determines whether the transport of the ore will succeed or not. Thus, the research on multi-sensing information fusion technology and the positioning accuracy improvement and speeding should be focused on. The combination of the global path planning with the local path planning methods to plan a travel path, which is without collision and has shortest time consumption, will be the mainstream direction in the future.
4. With the introduction of digital twin technology into the intelligent mine construction field, synchronous mapping and real-time interaction between physical equipment and virtual equipment can be achieved. By building digital twin models for the intelligent LHDs in the coming future, remote monitoring, fault diagnosis, control optimization and health prediction for the physical machine are expected to be attained through modeling on the extracted feature from the faults and the corresponding process and analyzing the interference factors.  
The first author [PERSON] advanced the topic, collated the literature resources and wrote original draft. [PERSON] provided financial support on literature searching and provided significant suggestions to the structure of this article. [PERSON] revised and re-edited the manuscript on not only the language but also the concept. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 51875415 and Special Project for the Central-Guided-Local Science and Technology Development in Hubei Province, grant number 2019 ZYYD010.  
Not applicable.  
Acknowledgements to [PERSON] in Geology Institute of China Chemical Geology and Mine Bureau for help in technical support for the Citespace database.  
The authors declare no conflict of interest.