Abstract: According to the characteristics of the molten iron transportation system, a spread spectrum communication wireless data communication network is designed. Its ability to transmit vehicle position data in real time is much better than that of a narrowband radio station and a GPS vehicle monitoring system that uses GSM public telephone network to transmit position data. The designed wireless communication network can manage more vehicles and has the capacity of further expansion.
Baoshan Iron and Steel Group Corporation is a large-scale steel enterprise in China. With the implementation of the third-phase project of Baosteel, the traditional hot metal transportation dispatching system has been unable to meet the needs of expanding production scale. It is very important to establish a new dynamic monitoring system for molten iron transportation to improve the safety, efficiency, automation and economic benefits of production scheduling.
The hot metal transportation dynamic monitoring system adopts today's advanced DGPS positioning technology, 
Combined positioning technology, map matching technology, spread spectrum communication technology, computer and network technology, geographic information system technology, electronic large-screen technology, etc., to solve the positioning tracking of hot metal transportation vehicles on the railway line in the harsh industrial environment of the steel plant The problem enables dispatch management personnel to observe the dynamic position and status of hot metal transportation vehicles and other relevant information on the large electronic screen of the central station in real time, which is convenient for timely and reasonable production and transportation scheduling.
As an important part of the hot metal transportation dynamic monitoring system-wireless communication network, it mainly completes the information transmission between the central station and the vehicle, and its performance directly affects the normal work of the entire system. The wireless communication network introduced in this article has been applied in the Baosteel water transportation dynamic monitoring system. The communication system works normally, is stable and reliable, and has a good effect.
1 Wireless communication network design
1.1 System requirements
(1) In the area of ​​1.8km & TImes; 2.3km, ensure real-time communication between the vehicles inside and outside the plant and the central station;
(2) Monitor 85 vehicles;
(3) The update rate of each vehicle information can reach up to once per second;
(4) Vehicle equipment is powered by batteries. To extend the battery replacement cycle, low-power consumption equipment is required;
(5) It has good electromagnetic compatibility and cannot interfere with other communication equipment currently in use;
(6) Strong anti-interference ability, which can ensure reliable communication in the harsh industrial environment of the steel plant, and the bit error rate is less than 10 -6.
(7) The two-way communication mode between the central station and the vehicle. 
1.2 Scheme selection
In the current GPS vehicle positioning and tracking system, the wireless data transmission usually adopts an analog radio station plus a modem to form a network by itself or to complete the data exchange through a public mobile phone. In the data transmission system of the analogue station's own network, due to the long transmission and reception switching time of the analogue station (about 200ms), the low data transmission rate (usually 1200bps), and the transmission of less vehicle position data per unit time, therefore, when the vehicle When increased, the real-time nature of vehicle location updates will be greatly reduced. The GSM public mobile phone network is used to transmit data. Although the system capacity can be expanded and the data transmission rate is increased, the operating cost is higher, and it is affected by the working state of the public mobile phone network. It is difficult to meet security, reliability, and continuity. , Economic requirements. If the location data is sent by GSM short message, the real-time nature of the data transmission will not be guaranteed.
According to the actual situation on site and system requirements, spread spectrum communication technology is an ideal solution. The main features of spread spectrum communication are: (1) Strong anti-interference, it can suppress the interference of single-frequency and multi-frequency carrier signals, the interference of other pseudo-random modulation signals and the interference of pulsed sinusoidal signals, etc. Signal-to-noise ratio. (2) The transmission power is small, generally less than 1W, and the power consumption of the device is low, so it will not interfere with other communication systems. (3) Code division multiple access can be realized, and the frequency band utilization rate is very high. (4) Anti-multipath interference can overcome the possible impact of severe multipath interference on wireless data communication in the steel plant environment. (5) The wireless data transmission rate is high, which can be as high as 19200bps, and the bit error rate is less than 10 -6, which has the advantages of fast and reliable information transmission.
1.3 Composition of wireless communication network
The hot metal transportation dynamic monitoring system covers a range of about 1.8km & TImes; 2.3km. There are tall steel buildings distributed in the area, which makes it difficult to guarantee line-of-sight communication. A considerable part of the vehicle working area is in the plant, which is greatly affected by communication shielding, and some plants cannot even directly communicate with the outside world. In addition, in order to ensure low power consumption of vehicle-mounted equipment and prolong the battery power supply time of vehicle-mounted equipment, vehicle-mounted equipment uses a smaller transmission power for communication. In order to ensure reliable wireless data communication between the system central station and the vehicle, there are five interruption stations in the work area, which are responsible for the information forwarding between the vehicle and the central station. There are three transfer platforms in the three communication shielded factories, which are responsible for the information forwarding between the vehicles in the factory and the interruption station.
The central station is located in the production control center. The height of the communication antenna is about 45m, and the omnidirectional high gain antenna is used. The communication between the five interruption stations and the central station uses a high-gain directional antenna, and the direction of the antenna points to the central station. 
The communication between the relay station and the vehicle uses a high gain omnidirectional antenna. 3dB omnidirectional antenna is used for vehicle-mounted equipment and turntable.
The system wireless communication network is composed of a central station, a relay station, a relay station, and vehicle-mounted equipment, as shown in Figure 1.
2 Equipment selection and design
2.1 Spread spectrum communication machine selection
In the hot metal transportation dynamic monitoring system, two types of spread-spectrum communication devices, AirLink and WIT915, are used to form a wireless communication network according to different usage methods. The AirLink communicator is used for communication between the central station and the relay station, and the WIT915 communicator is used to complete the communication between the relay station and the mobile vehicle.
AIRLINK 19MP is an L-band wireless spread-spectrum MODEM data transmission communicator of American CYLINK company. It can work in point-to-point and point-to-multipoint working modes, and can also be used as a repeater or hub. Its main technical indicators are as follows:
(1) Working frequency band: 902 ～ 928MHz, 16 channels are optional;
(2) Adopt direct sequence extended working mode, PN sequence length is 32bit;
(3) The modulation method is BPSK (Bi-Phase Shift Keying), the data rate can reach 38400bps, and the channel bandwidth is 1.5MHz;
(4) The system gain (excluding antenna gain) is 130dB. Among them, the spreading gain is 12dB;
(5) The maximum transmission power is 800mW (29dBm), and different transmission power can be selected through the dip switch;
(6) Using time division duplex technology (TDD), full-duplex communication can be achieved, and the line-of-sight transmission can reach 50km.
WIT915 is the spread spectrum communication transceiver of American DIGTAL WIRELESS company. WIT915 adopts combined spread spectrum technology, which can resist noise and multipath fading, and supports CSMA communication protocol and point-to-point communication at the same time. The low power consumption and small size of the WIT915 spread spectrum communication machine are very suitable for use in vehicle-mounted stations. Its technical indicators are as follows:
(1) Working frequency band: 903 ~ 907MHz, 21 channels, with the ability to automatically find clean channels;
(2) 4 levels of adjustable transmit power, from 1mW to 1W, the maximum power requirements comply with US FCC standards, and the power can be adjusted adaptively;
(3) Full-duplex data rate can reach 19200bps, half-duplex data rate can reach 51200bps;
(4) Radio frequency bandwidth: 700kHz, channel spacing 1.2MHz;
(5) Adopt 0dB antenna, under line-of-sight conditions, the transmission distance can reach 1.8km;
(6) In the case of half-duplex, the data transmission and reception conversion time is less than 0.5ms. 
2.2 Communication controller design
In the design of wireless communication data transmission network, the design of communication controller is very important. Because the control of the spread spectrum communication machine and the execution of the communication protocol of the wireless communication network must be realized by the communication controller. In the hot metal transportation dynamic monitoring system, PC / 104 is used as the communication controller. Compared with the communication controller using the single-chip microcomputer as the core, it can reduce product development costs, reduce development risks, shorten development cycles, and improve product performance. PC / 104 has ultra-small size (90mm & TImes; 96mm), low power consumption (typically 1 ~ 2W / module), and the unique stacking bus eliminates the cost and space of the backplane and socket. PC / 104 CPU series products provide highly integrated modules for embedded applications and are compatible with IBM PC / AT systems. Programs debugged on the PC can be directly ported to the PC / 104 for use.
The selected PC / 104 CPU module is CoreModule CM / 486-2. The CM / 486 module provides all the functions of the PC / AT motherboard and the function of the first-line add-on card. The module has standard hardware and software resources that are fully compatible with CP / AT and MS-DOS. The main indicators are as follows:
(1) The CPU is CX486SLC-2, 50MHz internal clock frequency;
(2) 2M, 4M or 16M bytes can be selected on board memory;
(3) 7 DMA channels (equivalent to 8237);
(4) 15 interrupt channels (equivalent to 8259);
(5) Three programmable count / timers;
(6) 16-bit expansion bus;
(7) Two serial ports and one parallel port fully compatible with PC;
(8) Solid state disk with bootable system;
(9) With a watchdog timer that PC does not have.
2.3 Connection relationship between communication controller and spread spectrum communicator
The external data control interface of the spread spectrum communication machine AirLink and WIT915 is an asynchronous serial RS-232 interface compatible with PC. Therefore, the hardware connection between the communication controller composed of PC / 104 and the spread spectrum communicator AirLink and WIT915 is very simple and convenient. Just connect the external data control interface of the spread spectrum communicator directly to the serial port of the PC / 104. Yes, the software running in the PC / 104 can control the data transmission and reception of the spread spectrum communicator.
Figures 2 and 3 show the two connection modes of the communication controller and the spread spectrum communicator. FIG. 2 is the connection mode of the relay station, and FIG. 2 is the connection mode of the mobile vehicle.
3 Design and implementation of wireless communication
3.1 Relay station and central station
A Star network communication mode is formed between the AirLink spread spectrum communicator at the central station and the AirLink spread spectrum communicator at each relay station. The AirLink spread spectrum communicator at the central station is set as the main mode. The AirLink spread spectrum communicator of the relay station is set to the slave mode and adopts the half-duplex communication mode. The communication controller of the central station controls the AirLink spread spectrum communicator and each relay station to perform data transmission exchange by polling. The central station collects the vehicle information received from each relay station from each interruption station, and then broadcasts the differential data required for vehicle DGPS positioning to all relay stations at a certain interval. Each interrupt station is set with a different code. After receiving the information from the central station, the communication controller of each relay station first judges whether the central station fetches the vehicle information. If it is, then determine whether the station code issued by the central station is consistent with the preset station code; if it is consistent, the vehicle position data received by the interruption station will be sent to the central station; if not, it will not be processed. If the communication controller of the interruption station judges that the central station sends out broadcast differential data, it forwards this data to the vehicle-mounted equipment through the WIT915 spread spectrum communication machine. Because the reception level of the AirLink spread spectrum communicator of each relay station and central station has been adjusted to the level required by the AirLink spread spectrum communicator manual to transmit data at a bit error rate of 10 -8, the central station and relay station use ARQ mode and CRC check can guarantee the reliable transmission and exchange of data.
The data format that the central station communication controller sends to the relay station through the AirLink spread spectrum communication machine is as follows:
Query information format:
| Sync header | Start mark | Station code | End sign | CRC check code |
Broadcast DGPS differential information format:
| Sync header | Start mark | Broadcast code | DGPS differential data | CRC check code | End sign |
Relay station response message format:
| Sync header | Start mark | Station code | Vehicle Information | CRC check code | End sign |
The data transmission rate of the central station and the relay station is 19200bps.
3.2 Relay stations and mobile vehicles
The communication controller of the relay station exchanges data through the WIT915 spread spectrum communicator of the relay station and the WINT915 spread spectrum communicator of the vehicle-mounted device. If the relay station communication controller and the mobile on-board equipment communication controller use the query method to exchange vehicle position data, because the hot metal transportation dynamic monitoring system monitors more vehicles (about 85 vehicles), querying all vehicle position data once takes more time long. Secondly, in the process of hot metal transportation, there are fewer moving vehicles and more stopped vehicles at the same time, and the position of the stopped vehicle has not changed. The control center only needs to retain the vehicle position data transmitted from the last time, and does not need to perform vehicle position Update. In order to transmit effective position data in a limited channel, a communication method that dynamically controls the time interval of vehicle information reporting according to the vehicle running speed is adopted, that is, the transmission frequency of vehicle information is adjusted according to the dynamic state of the vehicle. When the vehicle is in a stopped state, the vehicle information is sent every minute to maintain data contact with the control center. When the vehicle is moving, the frequency of vehicle information reports increases with the increase of speed, and the latest vehicle information is sent to the relay station in time. The transmission time of vehicle information is completely determined by the vehicle-mounted communication controller according to the operation of the vehicle, eliminating the transmission time occupied by the downlink data link in the query mode, which can improve the transmission efficiency of the effective information of the vehicle and the real-time nature of the information.
In order to ensure that there is no collision of data transmission when the vehicle information is sent autonomously, the data is transmitted using the CSMA communication protocol of the WIT915 spread spectrum communicator in half-duplex mode. The CSMA communication protocol is a data transmission method in the IEEE802.3 protocol, which is widely used in computer local area networks for carrier sensing and multiple access in data transmission. When the position data of the vehicle needs to be sent, the on-board communication controller first reads the carrier detection DCD level indication sent by the WIT915 spread spectrum communication machine. When the DCD level of the carrier detection is high, it means that there is another communication device in the current channel sending data. At this time, the vehicle communication controller randomly delays for several milliseconds, and reads the carrier detection DCD level of the communication machine again. If the carrier detection DCD level is low at this time, it means that there is no WIT915 spread spectrum communicator in the channel to send data, and the channel is idle, and data can be sent, then the vehicle-mounted communication controller raises the RIT level of the WIT915 spread spectrum communicator. At this time, the WIT915 spread spectrum communicator switches to the transmission state, and at the same time sends out carrier information to occupy the channel, and the vehicle-mounted communication controller then sends the data through the WIT915 spread spectrum communicator. When the vehicle's position data is sent, the vehicle-mounted communication controller sets the RTS level of the WIT915 spread spectrum communicator to a low level, so that the communicator stops sending carrier waves and data, and gives up the channel for other WIT915 spread spectrum communicators to send data.
Using the CSMA communication protocol to send vehicle position data can enable only one WIT915 spread spectrum communication machine to be in the transmitting state at any time, so as to avoid collision interference as much as possible and make the vehicle information transmission reliable. The transmission and reception switching time of the WIT915 spread spectrum communication machine is very short, the maximum is not more than 400μs, and the data transmission rate of the WIT915 spread spectrum communication machine can be as high as 38400bps, and the compressed vehicle information is very short (about 40bit), so each vehicle passes The time that the controller occupies the channel when sending vehicle position data is very short, which can ensure the real-time nature of the data transmission. Of course, in extreme cases, it is possible that two WIT915 spread spectrum communicators simultaneously detect the empty channel and send data at the same time, causing a collision. However, due to the small amount of vehicle information sent, the data transmission rate is high, and the probability of a collision is very low. Even if a collision occurs, in the spread spectrum communication, the communicator may still demodulate the correct data. If the demodulation error of the spread spectrum communicator is eliminated by CRC check, the vehicle information is updated by the next vehicle information transmission.
The DGPS differential data of the central station forwarded by the relay station is also broadcasted and transmitted by the communication controller of the relay station to each vehicle-mounted device in the CSMA communication mode through the WIT915 spread spectrum communication machine of the relay station. The CRC check is used in the CSMA communication protocol to ensure the reliability of the data.
The flow chart of the communication controller sending data in CSMA mode is shown in Figure 4.
When data is sent, the channel is detected. If the channel is busy, it will randomly delay for a period of time, and increase the counter by 1, and then detect the channel. In this cycle, when the counter is accumulated to M times, the channel detection loop is exited. At this time, the channel is considered busy, and the channel busy flag is set, and the data transmission is abandoned this time. After the channel busy flag is set, the interval at which the vehicle sends data when stopped is increased from 1 minute to 10 seconds. This is to ensure that the update time of all vehicle positions does not exceed 10 seconds after the channel blocking interference disappears.
3.3 Data transfer of relay station
In the process of hot metal transportation, vehicles sometimes enter the steel structure of the plant. In order to enable vehicles to send vehicle information to the relay station even when entering the plant, a relay station was installed in the plant. Communication forwarding is shown in Figure 5.
The relay station is equipped with two WIT915 spread spectrum communicators. One communicator is placed in the plant, and the other communicator is placed outside the plant. The forwarding communication controller receives the information sent by the vehicles in the plant through the WIT915 spread-spectrum communication machine in the plant, and then forwards it out in a CSMA manner through the WIT915 spread-spectrum communication machine outside the plant. When the communication controller forwards data, the WIT915 spread spectrum communication machine in the plant should be in a state where data reception is prohibited, to prevent the data forwarded by the WIT915 spread spectrum communication machine outside the plant from being received by the WIT915 spread spectrum communication machine in the plant, forming a circular forward status. 
4 Performance analysis
In the hot metal dynamic monitoring system, the maximum running speed of the vehicle does not exceed 15 kilometers per hour, that is, the fastest movement is 4.1 meters per second (it can be estimated by 5 meters). Every time the vehicle's position changes by 5 meters, the updated position data is sent once, and the vehicle's position update rate is the fastest once per second. The vehicle position data together with the synchronization code and check code is a total of 10 characters (each character is an eight-bit binary number). If the vehicle communication controller sends data to the relay station asynchronously at a rate of 19200bps (one start bit, one stop bit, eight-bit data), the required time is 100/19200 = 5.2ms; if the CSMA communication protocol is used to communicate in the same channel In an ideal situation (not considering waiting and collision when blinking), the number of vehicles that can transmit different position data in one second is 1000 / 5.2 = 192 vehicles. If you consider various disadvantages such as delays, and double the time, the number of vehicles that can transmit different position data in one second is 192/2 = 96 vehicles. Because the data exchange rate between the relay station and the central station is asynchronous at 19200bps, it can be guaranteed that the position data of all vehicles can be transmitted to the central station within one second. Therefore, the designed wireless communication network has the ability to transmit and update the information of 96 vehicles in real time per second, meeting the requirement of the hot metal transportation dynamic monitoring system to manage 85 vehicles. Due to the use of CRC check, the bit error rate of the entire system reaches below 10 -6, which meets the bit error rate requirements for vehicle information transmission.
In the actual operation of the hot metal transportation dynamic monitoring system, all the on-board WIT915 spread-spectrum communicators and relay stations' WIT915 spread-spectrum communicators are working in the same channel, the position of the vehicle can be updated in time, there is no channel congestion, and the vehicle position cannot be transmitted in real time The phenomenon has achieved the design purpose. If the number of managed vehicles increases, simply set the WIT915 spread spectrum communicator of the relay station to a different channel according to the communication range of the relay station, and the on-board communication controller will automatically set the on-board WIT915 spread spectrum communicator in the non-moving area according to the position of the vehicle The channel of is switched to the same channel as the relay station in this area, which can double the number of managed vehicles.
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