As a fixed node, the minor who is

doing work may far from it, in order to facilitate the miners observed environmental data around the fixed nodes, it uses LED digital display。 At the same time, the large current LED lights and buzzer are designed in the circuit; it makes the fixed node with the function of sound-light alarm。

Considering that it may occur the emergency of without electricity, fixed node also built-in a lithium-ion battery。 Under normal conditions, lithium-ion battery is in charging status, when external cable disconnect, fixed node is automatic switched to battery power, which can ensure the mobile node can deliver the information through fixed nodes in underground。

Without regarding to fixed nodes’ portability, we have a customized shell

that has excellent explosion properties, and the internal space is enough to hold down the 2。4 GHz antenna。 To ensure safety, all cables and the location of sensors are placed with particular glue sealed, so that it has a good seal。

IV。POSITIONING FUNCTION One of the important functions of

the wireless sensor networks is localization, especially in the underground tunnel, it relates to the safe of the miner's life。 Currently most widely used orientation method is GPS satellite positioning, it is a high precision, all-weather and global multifunctional system with the function of radio navigation, positioning and timing。 But the GPS positioning method is not suitable for the underground work environment of coal mine, once you enter the underground, it cannot receive satellite signal, thus unable to achieve targeting [6]。 We need to consider how to use wireless network to realize positioning function, means using wireless signal between the communications of devices for positioning。 The existing distance measuring technology between the wireless-devices basically is the following kinds of methods: TOA, TDOA, AOA and RSSI。

About the TOA method, the distance between the two devices is determined by the product of the speed of light and transmission time [7]。

Although the precision of this method is accurate, but it require a precise time synchronization, so it demand hardware is higher。

TDOA technology need ultrasonic signal，which is setting on a node with receive and transmit function。 When measure the distance, it can sent ultrasonic wave and wireless signals

together。 By measuring the difference between two signals arrival time, we can calculate the distance between two devices [8]。 Using this method can also obtain accurate result, but the method

need to increase ultrasonic sending and receiving device on the node circuit, it will increase cost。

AOA technology needs to install multiple antennas through the nodes so it can obtain adjacent nodes’ signals on deferent directions [9]。 With this it can determine the location information from number of adjacent nodes and calculate its own position。 This method not only need to add additional hardware, but also it's still very vulnerable to external disturbance, therefore it's not suitable for utilize。

RSSI ranging is a cheap and easy technology。 By using this method, we don't need to add additional hardware design。 We also do not need very precise time requirements。 This technique is about with measuring the wireless signals strength in the propagation of the loss, to measure the distance between two nodes。 Because of this method requires hardware equipment is less, algorithm is simple, so it has been using in many wireless communication field。 Comprehensive all conditions, positioning on the use of RSSI ranging technique。

A。Hardware Location Engine The CC2431 wireless

microcontroller chip produced by TI Company has a hardware location engine。 From the software's point of view, CC2431’s hardware location engine has a very simple API interface, as long as writing the necessary parameters and waiting for calculation, it can read the location results [10]。