## 2 The Concept and Design: The Function Design of the CBTC System
### The Function Design of the CBTC System for the Solutions to the Practical Problems
#### 2.2.1 Safety Problem: The Rear-End Collision Problem  
The ATP (Automatic Train Protection) system is the core system to control the speed of the transport units and the safe distance between the front and rear transport units. The ATP system controls and regulates the minimum interval and over-speed protection in the operation of the transport units to avoid the rear collision of the transport units. Its main components include range measurement equipment, vehicle speed monitoring equipment, vehicle-ground interaction equipment, and emergency braking equipment. It should be noted that in the rail transportation of an intelligent underground mine, the maximum speed of the transport units should not exceed 40 km/h in order to avoid rollover caused by track bump. Therefore, in the application of the ATP system, when the speed of the transport units is detected to exceed 40 km/h, it will be slowed down by the control system.  
At the 2020 IEEE 23 rd International Conference on Intelligent Transportation Systems (ITSC), [PERSON] et al. [34] published research on the safety function of the ATP system, and they proposed a new analysis method for the ATP system, the Dynamic Fault Tree analysis method. The Dynamic Fault Tree is a system of methods for evaluating the reliability and safety of a complex system, and it is also a kind of deductive method, which is based on the fault event and is analyzed step by step according to the top-down sequence through certain logical reasoning steps until the result is reached. The Dynamic Fault Tree analysis method is devoted to modeling the fault logic according to the dynamic fault behavior, and it conforms to the functional design of the signal system. It is worth mentioning that logical analysis based on the Dynamic Fault Tree analysis method often analyzes the emergence of problems at the same time, instead of analyzing it layer by layer, which can greatly improve the efficiency of problem solving.  
The order of problem solving based on the Dynamic Fault Tree analysis method is as follows: (1) Identify the possible fault events. (2) According to the possible fault events, establish the analysis process of them. (3) Through the control system (as in the ATP control system), analyze whether the fault events occur. (4) If the fault events occur, solve them through the control system (as in the ATP control system).  
For the ATP system, the fault events in the underground transportation might caused by over speed, the safety distance, Vehicle Rollover, or an emergency. Therefore, the analysis of the fault events has been established as follows: (1) Is the speed of any transport unit over 40 km/h? (2) Are all transport units within a safe distance? (3) Have there been any rollovers? (4) Has an emergency appeared?  
Through the monitoring and control functions of the ATP system and other auxiliary functions, the rear-end collision problem could be solved with the assistance of the Dynamic Fault Tree analysis method: (1) If the speed of the transport units is detected to exceed 40 km/h, it will be slowed down by the control function of the ATP system. (2) If the distance between the transport units may cause an accident, the distance will be controlled by the control function of the ATP system through controlling the speed of the transport units. (3) If a vehicle rollover accident is detected, the control function of the ATP system will halt the transport units nearby the accident. (4) If the emergency appears, the control function of the ATP system will halt all transport units in the underground mine.  
With the improvements based on the Dynamic Fault Tree analysis method, the control function and working principle of the ATP system in the intelligent underground mine are shown in Figure 4.