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Vladimirov, et al. Karimov, R. Dalbokova, M. Kavvas, et al. Charou, et al.

Integrated Technologies for Environmental Monitoring and Information Production | SpringerLink

Mikhailov, A. Kuzin, et al. Vorontzov, et al. Satallite Observation of Aral Sea; S. Stanichny, et al. IX: Transfer of Data into Information. Vyazilov, et al. Based on these considerations, an integrated environment monitoring system for underground coal mines is constructed, which can comprehensively monitor the coal mine environment. Figure 1 shows the architecture of the integrated environment monitoring system. In this architecture, the WSN is composed of three types of nodes: sink nodes, routing nodes and sensor nodes. The routing node and the sensor node connect to a sink node to join a WSN.

Environmental information about the underground coal mine are collected by the sensor nodes, and are sent to the GMDC through the sink nodes. According to the landform of the coal mine and the environmental monitoring requirements, different WSNs may consist of different kinds of sensors to collect different environment information. After receiving the information, the GMDC can know the environmental conditions of the underground coal mine in real time, such as the level of gas and CO, and can rapidly respond to urgent cases.

In this case, the monitoring of the environment of the underground coal mine and early warning of dangers could be achieved effectively. In addition, the connection between the GMDC and Internet make it possible for the remote manager to monitor and manage the overall safety conditions of local coal mines. What is a good network topology? Based on network connectivity and coverage, by adjusting the transmitting power of nodes and selecting some nodes as backbone to deal with data processing and transmission based on certain principles, one can optimize the network topology and extend the lifetime of the whole network.

Due to the harsh environment and complex conditions of wireless communication in underground coal mines, higher requests are put forward on the stability and reliability of the WSN. According to the zonal structure of the coal mine tunnel [ 7 — 9 ], a kind of mesh WSN for underground coal mine is designed based on the ZigBee technology, as shown in Figure 2.

In addition, the coal mine is where personnel can come to, so WSN nodes are deployed manually, which is helps optimize and manage the network. As shown in Figure 2 , the WSN for an underground coal mine consists of two layers: the upper backbone network and the lower perception network.

The backbone network consists of a sink node and routing nodes; they can communicate with each other directly within their coverage, and mainly gather and forward data. According to the zonal structure of the coal mine tunnels, the routing nodes are deployed along the tunnel in two symmetrical lines. In this way, every routing node can communicate with multiple routing nodes directly.

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If a routing node is broken, communication can still go on by using another routing node for relaying, thus the robustness of wireless communication in WSN is ensured. The perception network consists of sensor nodes in charge of collecting environment information. They are deployed around the routing nodes and with the consideration of the environmental monitoring needs.

Because the sensor node is a Reduced Function Device RFD , it is designed only to communicate with the nearest routing node as its parent node. Besides, the transmission power of a sensor node is adjusted appropriately by using an adaptive power control mechanism. By adjusting the transmission power of sensor nodes, not only is the power consumption of the sensor nodes reduced, but also the interference among neighboring nodes is reduced.

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As is known, sensor nodes are powered by battery, so their working time is limited by power. In order to prolong the lifetime of sensor nodes, and at the same time ensure the environment of underground coal mine be monitored efficiently and constantly, two working modes are used in WSN, called periodic inspection mode and interrupt service mode. Based on the deployment of the WSN nodes showed in Figure 2 , the scheduling of the periodic inspection mode [ 11 ] is used, as described in Figure 3.

In each cycle, the sink node starts to broadcast a parameter collection command to the routing node. After receiving the command, the routing node first forwards it, and then determines whether it has children sensor nodes itself. If it has none, the routing node will send an active status report to the sink node after a random time period; otherwise, it will broadcast the parameter collection command to its children sensor nodes, carrying the network address of the children nodes that it has messages to transmit to, since the sensor node can only communicate with its parent node in an indirect manner.

In other words, the sensor node needs to poll its parent node to determine whether it has any messages pending. When sensor node receives the command, it will first determine whether the parent node has data for it. If yes, it requests the data. After this, it collects environmental parameters according to the command, and sends them in its time slot, then goes to sleep to save power. The routing node gathers the parameters received from its children nodes, and then sends them to the sink node in a multi-hop relay way, and finally to the GMDC via the gateway.

At this point, a cycle comes to an end. The natural conditions of underground coal mines are so rough that there may be a sudden incident, so it is very important for the safe production in underground coal mines to accurately monitor and provide timely alarms for sudden incidents [ 7 ]. Therefore, as the supplement of the periodic inspection mode, the interrupt service mode is designed for the WSN to warn of any abnormal situation.

When the sensor node is asleep, the processor and the communication module will be shut down to save power, thus it can neither send nor receive messages, but the sensors will stay active and keep collecting environmental parameters. Once there is an overrun in a certain parameter, the sensor node will be woken up by an interrupt generated by the voltage comparator of the processor. Then, the sensor node marks the parameter as an emergency, and sends it to the GMDC immediately.

When GMDC receives the emergency data, it will alarm the corresponding areas about the abnormal situation so as to predict the danger in time. As for the WSN, there are some technologies which are very important for its performance in underground coal mines. These technologies are called key supporting technologies, and mainly include the routing mechanism, collision avoidance, data aggregation, unified parameter gathering, network synchronization and interconnection with the CMS [ 12 , 13 ].

To construct an integrated monitoring system for underground coal mines, all these key supporting technologies have been discussed. As stated in Section 2. For the perception network, the sensor node can only communicate with its parent node, without routing functions. Therefore, the routing mechanism mentioned here is only applied to the backbone network. Because the WSN nodes are limited in electric power and computing capability, in the first place the routing protocol cannot be complex.

Secondly, the underground wireless communication condition is so complex that the routing protocol should be robust enough to guarantee the WSN will work stably and reliably. Thirdly, in the WSN, the sensor node mainly transmits environmental parameters, whose destination is the sink node [ 14 — 16 ]. Based on the above considerations, a kind of routing mechanism with minimum hops, multiple paths and based on link quality indication LQI is designed. In this routing mechanism, every routing node establishes three routes, i. The destination is the sink node, so the destination address item is omitted.

In order to reduce the power that consumed for transmission, the hops information is carried along in the parameter collection command. The value of hops is zero when the command is first broadcast by the sink node, and adds 1 each time it passes a routing node. After receiving the command, the routing node extracts the hops and LQI information, then the link cost will be obtained based on the definition stated in the ZigBee protocol [ 6 ].

The routing node updates the routing table according to the following rules:. If yes, add it into the routing table as the parent route; otherwise, go to 2. If yes, add it as the minimum route; otherwise, compare it with the minimum route. If the link cost is less than that of the minimum route, replace it and replace the previous minimum route with the backup route; otherwise, go to 3. If the backup route is empty, or the link cost is less than that of the backup route, add or replace it as the backup route; otherwise, discard directly. At this point, the routing node finishes updating the routing table in a cycle.

When the routing node forwards data, it first chooses the minimum route. If it fails to send, it chooses the parent route then. If it fails to send again, it finally chooses the backup route. In order to reduce the channel interference among WSNs, orthogonal channels are assigned to the WSNs within the interference range. That means the channel of any WSN in the integrated monitoring system is orthogonal to the channels of neighboring WSNs within the interference range. If they transmit simultaneously, collisions will happen, and the interference will lead to the failure of data transmission [ 17 ].

Collisions happen on both the layers of a WSN: collisions among the routing nodes in the backbone network, and collisions among the sensor nodes with the same parent routing node in the perception network. After the avoidance time, the node starts to listen to the channel. If the channel is idle, it sends immediately; on the contrary, if the channel is busy, it waits and then tries to access to channel again.

In the WSN, a routing node normally has more than one child sensor node. When these sensor nodes send data to the routing node simultaneously, collisions will happen. For this reason, the time is divided into slots and allocated to each sensor node according to the order in which it joins the routing node. The order is computed through Equation 1 :. By using this timeslot allocation mechanism, collisions among sensor nodes under the same parent routing node are effectively avoided. It also helps the routing node in gathering environmental information and managing the children sensor nodes.

In the integrated monitoring system for underground coal mines, a sensor node is equipped with multiple sensors so as to collect different types of environmental parameters. To improve the transmission efficiency, the collected environment data needs to be aggregated before reporting it to the parent routing node. Therefore, a data aggregation mechanism is employed on the sensor nodes. The frame format of the aggregated data is illustrated in Table 2.

Here, the RFD field shall be set to 0X0F, indicating that this frame is environmental information that the sensor node sends; Limit field shall be set to 0X01 if a parameter overruns, otherwise, be 0; Type field indicates the type of parameters; Para field carries the specific value of parameters. When it receives environment information from its children sensor nodes, the routing node caches it firstly. After it receives the data from all the children nodes, it aggregates it and then sends it to the sink node.

The frame format of the packaged data is illustrated in Table 3.

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Here, the ROUTER field shall be set to 0X0E, indicating that this frame is environmental information that the routing node is sendsing; Num field indicates the number of sensor nodes in this frame; Addr field means the network address of each sensor node; fields of Type and Para have the same meaning with the corresponding items in Table 2. Because the overrun parameter is sent directly to the sink node by the sensor node, Limit field is not included in the frame.

Environmental Monitoring Systems

In the environmental information delivery process, data aggregation is adopted by the sensor node and the routing node, which decreases the amount of data, reduces data collisions, saves power, and extends the lifetime of the whole network. Meanwhile, for the overrun parameters, the sensor node directly sends the to the sink node without aggregation, realizing urgent transmission of the emergency data. As mentioned above, there are many kinds of sensors in the WSN in charge of monitoring the environment comprehensively. How to gather and recognize the different sensing data efficiently?

A kind of unified parameter gathering mechanism is presented. For example, we need to collect the following eight kinds of parameters: gas, CO, coal dust, smoke, temperature, humidity, wind speed and pressure, 1-byte Type field in the frame as shown in Table 3 is used for expressing the type of parameters. Table 4 shows the relationship between Type-value and the corresponding parameter type, where 0x00 means that he sensor nodes are allowed to go to sleep. As shown in Table 4 , each bit of the Type Byte represents a kind of environmental parameter. If a kind of parameter is collected, the corresponding bit is set 1, otherwise, set 0.

After receiving a parameter collection command, the sensor node starts to collect parameters according to the Type value in the command. If the requirement of the sensor types is more than the types of sensors with which that the sensor node is equipped, then the Type value should be reset according to the actual sensor types. For instance, a sensor node is equipped with two sensors: a gas sensor and a CO sensor. When it receives a command of Type value 0x07, which means collecting three gas parameters, CO and coal dust, then the Type value should be set 0x In this way, various environment parameters are gathered uniformly although different sensor nodes may be equipped with different sensors.

The mesh WSN with multi-parameter monitoring for underground coal mines mainly works in the periodic inspection mode, but the accuracy of the crystal oscillators of the WSN nodes is generally low, which means their clocks are not accurate, so it is hard to keep WSN nodes working synchronously. In this case, a relative synchronization mechanism is proposed.

In this mechanism, the sink node, which is the coordinator of the WSN, is selected as the basic benchmark node of the WSN. The other nodes select their own parent node as their benchmark node. The designed network synchronization means relative synchronization on working and sleeping, rather than absolute synchronization on a physical clock [ 3 ]. As the coordinator, the sink node is selected as the benchmark node of the whole WSN, and other WSN nodes select their own parent nodes as the benchmark node.

As designed, the sink node periodically broadcasts the parameter collection command. The command frame is also the synchronous frame in the proposed synchronous mechanism. When a WSN node receives a synchronous frame, it will check whether the synchronous frame comes from its parent. If not, it discards it directly; otherwise, it compares the sequence number of the frame with that of the last one.

If they are identical, it discards the frame directly; otherwise, it records the sequence number, and regards that moment as the beginning of a new cycle. In addition, in order to reduce the impact of clock skew, the sensor node listens to the channel both at the beginning and the end of each cycle until it receives a synchronous frame, rather than only at the beginning. In this way, the WSN nodes are capable of working synchronously without accurate clocks, ensuring the reliable delivery of environmental information.

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