Normally, the router of ZigBee network has no sensor equipment; it is only responsible for data forwarding。 But considering the practical application, we believe that add sensor devices on the router will be better on monitoring underground coal mine environment。 So

in our design, the router also has an environment monitoring function which is usually designed in end device。

Fixed node will sent received data from mobile node to the gateway, then the gateway transmits data to monitor computer through RS232 or optical fiber。

The PC monitor software in the computer will process all data and display them in a visualization window。 The PC software also calculates each mobile node’s real-time location through the specific localization algorithm, according to the received signal strength (RSSI) obtained from mobile nodes。

III。NODE DESIGN

Since the ZigBee wireless network platform sold on present market was designed for the general environment, for special underground so they are not suitable for the environment。 Therefore, we need to customize the system for underground environment whit a special hardware circuit。 Node photo are shown in Fig。 1

Then wireless microcontroller CC2530 chip is the core processor of the node device, it can constitute a ZigBee network with very few peripheral circuits。 The CC2530 is an IEEE

802。15。4 compliant true System-on-Chip, supporting the proprietary 802。15。4 market as well as the ZigBee, ZigBee PRO, and ZigBee RF4CE standards。

Unlike other wireless chip, CC2530 built-in 8051 monolithic integrated circuits kernel, therefore we no longer need to use a single MCU to control the circuit, and this save us a lot of cost [5]。

A。Mobile Node

The mobile node is the end device of a ZigBee network that can be carried by miner; it should be a portable and low power consumption node。 So the mobile node we designed is only as

small as a mobile phone, and it is by built-in lithium ion battery power supply。 In power loss, the core processor CC2530 is a low power consumption chip, when it is in the sleep mode, it

only need to use less then 1uA work current。 In order to reduce power consumption as much as possible on the display, a 100*32 pixel matrix with no backlighting LCD screen was used。 The battery’s capacity of the mobile node is 1500mAh, so it is enough to meet the miner’s long hour works in the underground。 The battery charge management chip is TP4057, the maximum charge current can up to 500Ma。

Figure 1。 Node photo。

The mobile node circuit includes the gas concentration sensor MJ4。0 and temperature sensor PT-1000。 As far as we know, many wireless sensor platforms use the digital type sensor。

The communication between the digital sensor and the MCU need strict timing requirements。 But considering the actual application, the wireless MCU usually has a real-time operating system in general, if we use the microcomputer to simulate the strict timing, it will affect the real-time of whole operating system。 These two sensors output analog signals not digital signals。 Only input this signal into a differential amplifier, can we get an appropriate signal that can be converted to a digital signal by an ADC mode within the CC2530 chip。 In order

to facilitate the carrying, external

antenna was not used in our mobile node, instead of using a 2。4GHz patch antenna。 And we customize a shell like a cell phone size; it is enough to put all PCBs, sensors and battery in it。 Taking into account the small shell of the explosive performance is not very good, the design of PCBs and the selection of component are all carried out the safety assessment。

B。Fixed Node

Fixed node is installed in the wall of the underground tunnel。 Because it is big than the mobile node, it is not appropriate to carry around。 The circuit of the fixed node is almost same with the mobile node, it also use a CC2530 chip as core processor。 Because of underground tunnels generally deploy with power cable, fixed nodes can use cable power-supply modes。 At the same time, because we use wireless signal transmission, the deployment of new fixed nodes become very convenient, which also resolves the problem of the signal lines deployment。