| In 2016,the safety committee of the state council issued the "Work Plan for Hid-den Danger Management of Mines in Metal and Non-Metal Underground Mines",which clearly pointed out the importance of safety management in underground mining.De-ploying sensors in underground mining through the Internet of Things(IoT)technology can realize the functions of data identification,positioning,and supervision.Therefore,it is of great significance for mining safety monitoring based on IoT.In view of the need for emergency rescue under the mine,in the case of a mine collapse,conventional power supply cannot be used.Therefore,most of the existing technical solutions are based on the IEEE 802.15.4 low-power MAC(Media Access Control)protocol,such as ZigBee,which adopts CSMA/CA(Carrier Sense Multiple Access with Collision Avoid)algorithm to achieve wireless channel access at the MAC layer.However,unlike open pit mining,the special application environment in underground mining makes existing methods still face challenges:Firstly,a large number of sensors need to be deployed in underground mining,especially personnel positioning sensors,which have the characteristics of mobil-ity.In a certain period,it may cause the aggregation of wireless devices in certain areas and increase the probability of wireless data collision,and CSMA/CA cannot guarantee the reliability of data transmission in the case of large data.Secondly,the CSMA/CA algorithm competes for wireless resources through a random back off.When there are many devices,it may cause a large communication delay.Finally,the IoT in underground mining needs to be deployed according to the direction of the tunnels,so it needs to be designed for special topology in underground mining.Based on the above background,to solve the problems of transmission reliability and communication delay,this dissertation studies the key technology of the MAC protocol based on slot scheduling according to the characteristics of the TDMA(Time Division Multiple Access)method,and design related algorithms based on the different tunnel topological structures in underground mining.At the same time,because clock synchro-nization is a necessary condition for slot scheduling algorithm,this dissertation studies the method of clock synchronization in underground mining.Because the existing synchro-nization methods focus on the study of synchronization accuracy and energy consumption between two devices,and the synchronization methods rely on the sending and receiving of synchronization messages,under special conditions in underground,wireless interfer-ence makes these methods unable to meet the requirements of robustness.Therefore,this dissertation studies a synchronization scheme that meets the needs of slot scheduling in underground mining.The main contributions of this dissertation are as follows:(1)To analyze the basic requirements of the wireless transmission of the IoT in underground,a general multi channel slot scheduling problem is proposed based on the underground construction environment and the deployment of the IoT.Theoretical analy-sis of the computational complexity of this problem proves that it is an NP-Hard problem,and then a general multi-channel slot scheduling algorithm based on greedy thinking is proposed.At the same time,according to the actual situation of a large number of sen-sor data in underground mining,based on the time slot scheduling problem,a multi data stream selection problem is proposed,and corresponding solutions are proposed.Finally,the necessary theoretical analysis is performed.(2)To solve the multi channel time slot scheduling problem of different tunnel topologies in underground mining,the related algorithms are designed and analyzed for two different topologies.Firstly,a hybrid chained topological IoT structure is proposed,and the optimality of the multi channel slot scheduling problem under this topology is an-alyzed.Secondly,for a special application of a hybrid tree topology for the IoT in under-ground mining,a more complex multi channel time slot scheduling problem is proposed.Theoretical analysis of computational complexity is performed,and under the special hy-brid tree topology,the greedy algorithm proposed can achieve the optimal scheduling results on the premise that the problem can be scheduled.Finally,the packet loss rate and average delay of the greedy algorithm under the two topologies are compared and analyzed.The experimental results show that the proposed greedy algorithm has a lower packet loss rate and average delay.(3)To solve the multi base station access problem of underground sensor wireless data,a special multi base station and multi channel time slot scheduling problem based on delay constraints is proposed.The calculation complexity analysis proves that the prob-lem is NP-Hard.This dissertation proposes a heuristic algorithm based on deadline first to solve this problem,which includes two parts:pre-allocation and dynamic adjustment.Firstly,the pre-allocation algorithm with the smallest deadline can guarantee data with smaller deadlines to be prioritized and reduce the delay.Secondly,through dynamic ad-justment algorithm,more data packets can be allowed to access the channel.Finally,a comparative experimental analysis is performed.The experimental results show that the proposed algorithm has a smaller packet loss rate and a lower average delay.(4)To solve the clock offset in underground mining,a clock synchronization scheme is studied and designed,including different clock synchronization mechanisms of the base station and the sensor.Using the idea of dynamic superframe,the superframe is effec-tively divided,and the clock offset is estimated in the last time slots of each superframe,and the size of the next superframe is dynamically adjusted according to the estimation result.Finally,the proposed algorithm is compared and analyzed from the aspects of syn-chronization accuracy,energy consumption and robustness.Experimental results show that the proposed synchronization algorithm has a better performance.This dissertation studies the key technics of the wireless MAC protocol for the IoT in underground mining.The related algorithms are designed to meet the real-time and reliable transmission of wireless data.It has certain theoretical and engineering reference value. |
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