Design of wireless communication network for dynamic monitoring system of molten iron transportation

In the design of wireless communication data transmission network, the design of communication controller is very important. Because the control of spread spectrum communication machine and the execution of wireless communication network communication protocol are realized through communication controller. In the dynamic monitoring system of molten iron transportation, the use of PC/104 as communication controller can reduce product development costs, development risks, shorten development cycle and improve product performance compared with the communication controller using single chip as the core. PC/104 has ultra-small size (90mm×96mm), low power consumption (typically 1~2W/module), and unique stack bus eliminates the cost and space of the baseboard and socket. PC/104 CPU series products provide highly integrated modules for embedded applications and are compatible with IBM PC/AT system. Programs debugged on PC can be directly transplanted to PC/104 for use.

The PC/104 CPU module selected is CoreModule CM/486-2. The CM/486 module provides all the functions of the PC/AT motherboard and the functions 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. Its main indicators are as follows:

(1) CPU is CX486SLC-2, 50MHz internal clock frequency;

(2) The on-board memory can be selected as 2M, 4M or 16M bytes;

(3) 7 DMA channels (equivalent to 8237);

(4) 15 interrupt channels (equivalent to 8259);

(5) Three programmable counters/timers;

(6) 16-bit expansion bus;

(7) Two serial ports and one parallel port fully compatible with PC;

(8) A solid-state drive with a bootable system;

(9) It has a watchdog timer which is not available on PC.

2.3 Connection between communication controller and spread spectrum communication machine

The external data control interface of the spread spectrum communicator 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. You only need to connect the external data control interface of the spread spectrum communicator directly to the serial port of PC/104, and the data transmission and reception of the spread spectrum communicator can be controlled by the software running in PC/104.

Figures 2 and 3 show two connection modes of the communication controller and the spread spectrum communication machine, wherein Figure 2 shows the connection mode of the relay station and Figure 3 shows the connection mode of the mobile vehicle.

3 Design and implementation of wireless communication methods

3.1 Relay Station and Central Station

The AirLink spread spectrum communicator of the central station and the AirLink spread spectrum communicator of each relay station form a star network communication mode. The AirLink spread spectrum communicator of the central station is set to the master mode. The AirLink spread spectrum communicator of the relay station is set to the slave mode, using the half-duplex communication mode. The communication controller of the central station controls the AirLink spread spectrum communicator and each relay station to exchange data by polling. The central station collects the vehicle information received by each relay station from each interrupt 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 sent by the central station, the communication controller of each relay station first determines whether the central station retrieves the vehicle information. If so, it determines whether the station code sent by the central station is consistent with the pre-set station code; if it is consistent, the vehicle position data received by the interrupt station is sent to the central station; if it is inconsistent, it is not processed. If the communication controller of the interrupt station determines that the central station sends broadcast differential data, it forwards this data to the vehicle-mounted equipment through the WIT915 spread spectrum communicator. Because the receiving level of the AirLink spread spectrum communication machine at each relay station and central station has been adjusted to the level required by the AirLink spread spectrum communication machine manual to transmit data at a bit error rate of 10 -8, the central station and relay station can use a simple ARQ method and CRC check to ensure reliable data transmission and exchange. [page]

The data format sent by the central station communication controller to the relay station via the AirLink spread spectrum communicator is as follows:

Query information format:

Broadcast DGPS differential information format:

Relay station response information format:

The data transmission rate of the central station and relay station is 19200bps.

3.2 Relay Station and Mobile Vehicle

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 on-board equipment. If the communication controller of the relay station and the communication controller of the mobile on-board equipment use the query method to exchange vehicle position data, since the dynamic monitoring system for molten iron transportation monitors a large number of vehicles (about 85 vehicles), it takes a long time to query all the vehicle position data. Secondly, during the molten iron transportation process, there are fewer moving vehicles at the same time and more stopped vehicles. The position of the stopped vehicles has not changed. The control center only needs to retain the vehicle position data transmitted last time, and there is no need to update the vehicle position. In order to transmit effective position data within a limited channel, a communication method is adopted to dynamically control the vehicle information reporting time interval according to the vehicle running speed, that is, to adjust the frequency of vehicle information transmission according to the dynamic state of the vehicle. When the vehicle is in a stopped state, the vehicle information is sent every one minute to maintain data contact with the control center. When the vehicle is in a moving state, the vehicle information reporting frequency increases with the increase in 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 on-board communication controller according to the vehicle's operating conditions, eliminating the transmission time occupied by the downlink data link under the query mode, which can improve the transmission efficiency of vehicle effective information and the real-time nature of information.

In order to ensure that there is no collision of data transmission when the vehicle information is sent autonomously, the CSMA communication protocol of the WIT915 spread spectrum communicator in half-duplex mode is used to transmit data. The CSMA communication protocol is a data transmission method in the IEEE802.3 protocol, which is widely used in computer local area networks. Carrier sensing and multiple access are performed in data transmission. When the vehicle's location data needs to be sent, the on-board communication controller first reads the carrier detection DCD level indication sent by the WIT915 spread spectrum communicator. When the carrier detection DCD level is high, it means that another communicator is currently sending data in the channel. At this time, the on-board communication controller randomly delays and waits for a few milliseconds, and reads the carrier detection DCD level of the communicator again. If the carrier detection DCD level is low at this time, it means that there is no WIT915 spread spectrum communicator sending data in the channel at this time, the channel is idle, and data can be sent, then the on-board communication controller raises the RTS level of the WIT915 spread spectrum communicator. At this time, the WIT915 spread spectrum communicator switches to the sending state and sends out carrier information to occupy the channel. The on-board communication controller then sends the data through the WIT915 spread spectrum communicator. When the vehicle's position data is sent, the on-board communication controller sets the RTS level of the WIT915 spread spectrum communicator to a low level, causing the communicator to stop sending carriers and data, and give up the channel for other WIT915 spread spectrum communicators to send data.

The use of CSMA communication protocol to send vehicle location data can ensure that only one WIT915 spread spectrum communicator is in the transmitting state at any time, thereby avoiding collision interference as much as possible and making the vehicle information transmission reliable. The WIT915 spread spectrum communicator has a very short switching time for sending and receiving, with a maximum of no more than 400μs, and the data transmission rate of the WIT915 spread spectrum communicator can be as high as 38400bps. The compressed vehicle information is also very short (about 40bit), so the time each vehicle occupies the channel when sending vehicle location data through the controller is very short, which can ensure the real-time data transmission. Of course, in extreme cases, it is possible that two WIT915 spread spectrum communicators detect that the channel is empty at the same time and send data at the same time, resulting in a collision. However, due to the small amount of vehicle information sent and the high data transmission rate, the probability of collision is very low. Even if a collision occurs, the communicator may still demodulate the correct data in the spread spectrum communication. If the spread spectrum communicator demodulates incorrectly, it will be eliminated through CRC check, and the vehicle information will be updated through the next vehicle information transmission.

The DGPS differential data of the central station forwarded by the relay station is also broadcasted to each vehicle-mounted device by the communication controller of the relay station through the WIT915 spread spectrum communicator of the relay station in the CSMA communication mode. CRC check is used in the CSMA communication protocol to ensure the reliability of the data.

The flowchart of the communication controller sending data in CSMA mode is shown in Figure 4.

When there is data to be sent, the channel is detected. If the channel is busy, a random delay is performed, and the counter is incremented by 1, and the channel is detected again. This cycle is repeated. When the counter is incremented to M times, the channel detection cycle is exited. At this time, the channel is considered busy, and the channel busy flag is set, and the data transmission is abandoned. After the channel busy flag is set, the interval for the vehicle to send data when it stops is increased from 1 minute to 10 seconds. This is done to ensure that after the channel blocking interference disappears, the update time of all vehicle positions does not exceed 10 seconds.

3.3 Repeater Data Transmission

During the transportation of molten iron, vehicles sometimes enter the steel structure workshop. In order to enable the vehicle information to be sent to the relay station even when entering the workshop, a relay station is set up in the workshop. Communication forwarding is shown in Figure 5.

The relay station is equipped with two WIT915 spread spectrum communicators, one of which is placed inside the factory and the other outside the factory. The forwarding communication controller receives the information sent by the vehicles inside the factory through the WIT915 spread spectrum communicator inside the factory, and then forwards it through the WIT915 spread spectrum communicator outside the factory in CSMA mode. When forwarding data, the communication controller must put the WIT915 spread spectrum communicator inside the factory in a state where data reception is prohibited, in order to prevent the data forwarded by the WIT915 spread spectrum communicator outside the factory from being received by the WIT915 spread spectrum communicator inside the factory, forming a circular forwarding state. 4 Performance Analysis

In the dynamic monitoring system for molten iron transportation, 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 (which can be estimated as 5 meters). The vehicle sends updated position data every time the position changes by 5 meters, so the fastest update rate of the vehicle's position is once per second. The vehicle position data, including the synchronization code and the check code, is a total of 10 characters (each character is an eight-bit binary number). If the on-board communication controller sends data to the relay station in an asynchronous manner at a rate of 19200bps (one start bit, one stop bit, and eight bits of data), the time required is 100/19200=5.2ms; if the CSMA communication protocol is used to communicate in the same channel, under ideal conditions (without considering blinking waiting and collision), the number of vehicles that can transmit different position data in one second is 1000/5.2=192 vehicles. If various disadvantages such as delay are considered, and the time consumption is doubled, 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 19200bps, it can be guaranteed that the location 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 that the dynamic monitoring system for molten iron transportation can manage 85 vehicles. Due to the use of CRC check, the bit error rate of the entire system is less than 10-6, meeting the bit error rate requirements for vehicle information transmission.

In the actual operation of the dynamic monitoring system for molten iron transportation, all vehicle-mounted WIT915 spread spectrum communicators and the WIT915 spread spectrum communicators of the relay station work in the same channel, and the vehicle position can be updated in time, without channel congestion and the inability to transmit the vehicle position in real time, thus achieving the design purpose. If the number of managed vehicles increases, it is only necessary to 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 vehicle-mounted communication controller will automatically switch the channel of the vehicle-mounted WIT915 spread spectrum communicator to the channel consistent with the relay station in this area according to the position of the vehicle in the non-moving area, so that the number of managed vehicles can be doubled.