System structure is shown in Figure 1.
Fig.1 System Structure
Personnel positioning system on the ground is composed of monitoring host computer, backup computer, switch, server, communication interface, arrester and
other peripheral devices. Monitoring center stores and analyzes data by corresponding PC and database system, then can obtain each miner’s position information at any time. Personnel positioning system underground is wireless sensor network, it is composed of substation, wireless receiver, code emitter, mine explosion-proof and intrinsically safe power, etc, which will complete collection and transmission of miner’s position, time and other information. Hardware design for personnel positioning system underground will be described in detail as follows.
B. Substation design
Mine substation is the first level sub-node. Each substation is designed to load six wireless receivers. It sends periodic detection instructions to receivers to determine which wireless receiver’s information is to be received at one time, then receives miner’s identity information from the wireless receiver. Substation processes the received data for storing and displaying. It will wait for command from host computer and sendits data to monitoring center on the ground through field bus. Usually there are hundreds of miners in one coalmine, and mine tunnels are staggered within a few kilometers. So 11 substations are designed, which can load 66 wireless receivers at most.
In this scheme LPC2114 CPU is selected for substation design [6]. The LPC2114 is based on a 16/32-bit ARM7TDMI-S CPU with real-time emulation and embedded trace support, with their compact 64-pin package, low power consumption, various 32-bit timers, 4-channel 10-bit ADC, PWM channels and 46 fast GPIO lines with up to nine external interrupt pins. Multiple serial interfaces including two UARTs (16C550), fast I2C and two SPI. With a wide range of serial communication interfaces, which are very suitable for receive data.
C. Wireless receiver design
When miner with wireless code emitter in mobile pass through a wireless receiver, the code emitter would deliver miner’s information of ID number to the wireless receiver, then it begin to analyze the information. When detection instruction arrives, it willsend data to substation in accordance with the established protocol.
ATmega128 chip of Atmel Corporation is used for the microcontroller unit, and smart RF CC2420 chip of Chipcon Corporation with ZigBee function is used as the wireless transceiver. The
ATmega128 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC structure. CC2420 is a true single-chip 2.4GHz IEEE 802.15.4 compliant RF transceiver designed for lowpower and low –voltage wireless applications. It includes a digital direct sequence spread spectrum providing an effective data rate of 250kbps. It is suitable for both full function device (FFD) and reduced function device (RFD) in wireless sensor network [7]. The configuration interface andtransmit/receive FIFOs of CC2420 are accessed via an 4 wire SPI interface (SI, SO, SCLK, CSn). As mentioned above, the many excellent features make CC2420 and Mega128 particularly suit for our position system. Interface between Mega128 and CC2420 is shown in Figure 2. Wireless receivers communicate with substation through RS485. Baud rate for data transmission is set to 1200bps. The maximum communication distance between substation and wireless receiver is up to 5km. 75LBC184 chip is used in the receiver for voltage transformation between RS485 and RS232 in order to transmit data with UART pin of CPU. When DE pin of 75LBC184 is of high level, transmitting is effective, after that CPU’s UART pin transmit data to wireless receiver. When /RE pin of 75LBC184 is of low level, receiving is effective, then the CPU can receive data from RS485. If the receiver’s address meets the detected address in instruction, the receiver will be configured to transmit state and send back data block tosubstation.
Fig.2 CC2420 to MCU Interface
D. Power Supply design
Explosion proof and intrinsically safe power supply is commonly used in dangerous environment with gas and coal dust. It uses double protection measures for over-voltage and over-current, three 18DC voltage output, 500~1000mA current output. It can supply power for 1 substation and 6 receivers at the same time, with compact size and light weight.
E. Wireless code emitter
Mine code emitter transmits independent identification digital code at intervals (every 3 seconds). The code is used to identify personnel information. Code emitter is composed of high-frequency transmitter (modulator), microcontroller, batteries and antenna, It makes use of ZigBee technology, 2.4GHz transmit frequency, FSK modulation, transmission power less than 0 dBm, supply voltage 2.0~3.4V. It has two types of state: work and sleep. Work current is less than 1mA, and sleep current is less than 10_A, so as to reduce its power consumption. With mini package it can be fixed in miner’s lamp box, helmet, belt, or pocket. If it is used for vehicles, it should be fixed on non-metallic parts, more safety place and as high aspossible. IV. COMMUNICATION SOFTWARE DESIGN FOR
SUBSTATION AND WIRELESS RECEIVER
A. Time tuning
Real time collaboration in communication between nodes is required, so time synchronization is needed. Time tuning process: when system is started, substations will receive the instruction of tuning time. Then send it to NO.1 receiver. After it receives acknowledge from NO.1, then send it to No.2 receiver, and so on, until the last one. If there is no acknowledge from one receiver, substation will wait for a period of time, then go on to the next. If one receiver continues with no acknowledge for 3 times, it will be marked “malfunction” and displayed on LCD of the substation. After tuning time, substation detects receivers in turn and collects minersidentification information. Themeanwhile substation transmits acknowledge to corresponding
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