Keywords: data acquisition, wireless communication, spread spectrum, anti-interference
Weapon hit accuracy and explosive power are the two most important tactical technical indicators of weapons and equipment. The underwater explosion pressure telemetry system is developed to assess the underwater explosive power of underwater weapons and their damage to ships. It can be used against Simultaneous measurement of the peak explosion pressure, shock wave and secondary wave waveforms, the distance between the explosion center and the measurement point at multiple measurement points underwater.
1 System design scheme The underwater explosion pressure telemetry system consists of two parts: the measuring station and the main control station. The measuring station is composed of sensor group, data collector, single chip microcomputer control system, digital radio station, etc .; the main control station is composed of digital radio station, communication adapter, main control machine and its input and output devices. As shown in Figure 1.
The working process is as follows: After the explosion, the pressure sensor group of the measuring station converts the single transient explosion pressure wave signal into the corresponding electrical signal, sends it to the data collector for collection, and then sends the sampling data through the digital radio station; After the radio station receives the sampled data, it establishes communication with the host computer through the adapter. The host computer receives and processes the data and displays the peak value of the shock wave, shock wave and secondary wave waveform.
2 Key technology?
2.1 Spread spectrum communication technology?
Motorola's GM950I radio station adds a wireless data transmission Modem and becomes a digital radio station with spread spectrum function. The principle is that spread spectrum coding is used at the sending end to spread the signal spectrum, and the same code is used at the receiving end to despread to restore it to the original signal. The specific method is: the signal at the sending end is modulated by a modulo-2 adder, and then a spreading signal is generated by a pseudo-random code generator, added to the input of the carrier modulator, and modulated into a carrier signal by QPSK, and the broadband spreading signal obtained after modulation It is transmitted after amplification; at the receiving end, the RF signal is amplified and RF wideband filtered, and the synchronization signal is picked up from the phase of the spread spectrum code sent by the synchronization circuit, and the original signal is restored after despreading.
The wireless communication adopts spread spectrum technology. Because the spread spectrum signal is hidden in white noise and is difficult to detect, it has better communication confidentiality, and at the same time makes the system have a higher signal-to-noise ratio, good anti-interference performance, and easy to implement code division multiple access .
The communication protocol between the digital radio and the host computer is:
· Communication interface: standard serial RS? 232 interface, 9-wire full duplex mode;
· Communication word format: 1 start bit, 8 data bits, 1 stop bit;
· Baud rate: 4800bau / s. ?
2.2 Data collection technology?
Data collection generally refers to the process of converting continuously changing analog quantities into discrete digital quantities and sending them to a processor for processing. The core of data acquisition is the selection of A / D conversion chip and its interface method with the processor. The circuit implementation of the data collector is shown in Figure 2.
2.2.1 Selection of A / D conversion chip?
The data collector of the underwater explosion pressure telemetry system uses MAXIM's MAX153 chip. This is a high-speed 8-bit ADC based on half-flash technology. Its main features are as follows:
· Conversion time is 660ns;
· The sampling rate is 1Msps;
· Internal sample / hold;
· Single / dual analog input;
· Single + 5V or ± 5V power supply;
· 8-bit data output in parallel. ?
The main reason for choosing MAX153 is: the acquisition of explosive pressure wave signals requires a higher sampling rate, but the accuracy requirements are not high; the MAX153 peripheral interface circuit is simple; + 5V power supply, 8-bit data parallel output, making it 8-bit The interface of the single chip AT89C52 becomes very simple.
2.2.2 Sampling rate conversion and sampling control?
The shock wave shape of the underwater explosion signal is sharp, and the secondary wave shape is gentle. In order to save storage space, the shock wave sampling rate is 1Msps (quick sampling), and the secondary wave sampling rate is 40ksps (slow sampling). Fast and slow sampling clock circuits are designed on the hardware, and the fast and slow sampling start and end times are set by the timer. Therefore, the fast and slow sampling clock circuits are selected to provide periods for the / WR and / RD pins of the MAX153. 1MHz and 40kHz, negative pulse width of 300 ns (/ WR) and 250 ns (/ RD) read and write control signals.
2.2.3 Collection data storage?
The data collector has a high-speed non-volatile RAM (HK1235) with a capacity of 32K, which stores 28K of collected data, including 12K of shock wave data, stored in 0000H ~ 2FFFH unit; 16K of secondary wave data, stored in 3000H ~ 6FFFH unit After sampling 28K, sampling will stop automatically. A 16-bit counter composed of four SN74F161 (only the lower 15 bits) is used as a memory address generator.
2.3 Dual MCU control technology?
The measuring station is powered by batteries. In order to reduce energy consumption and extend working hours, dual-single-chip control technology is adopted. That is, one piece of 89C52 is used as the value-changing CPU and one piece of 89C52 is used as the main CPU. When the system is turned on, the CPU changes its value. It receives, executes, and responds to the remote control commands from the main control station. When it is ready for measurement, the main control station sends remote control startup commands. The relay operates to power on the main CPU. The communication of the master control station, if the value is changed, the CPU will exit the communication state, waiting to receive the command of the main CPU. The value-changing CPU mainly completes functions such as power on / off, measurement system self-checking, serial communication, and power supply monitoring; the main CPU mainly performs functions such as address decoding, peak detection, data transmission control, keyboard, and display control. Its functional block diagram is shown in Figure 3.
2.4 Anti-interference technology?
2.4.1 Long-distance wireless communication technology under strong interference environment?
The measurement station is placed on a floating body at sea level. The main control station is located on a shore-based or offshore power ship with a communication distance of 5km-10km. Since seawater absorbs electromagnetic waves, and the electromagnetic interference around the ship is relatively serious, the communication conditions are bad, and necessary anti-interference measures must be taken.
(1) Antenna The height of the antenna and its gain directly affect the communication distance and communication quality. In view of the fact that seawater absorbs electromagnetic waves and there are many sources of interference on board, a special retractable omnidirectional antenna is designed to prevent lightning, strong electromagnetic interference and multipath interference. The erection height is 10m-20m, the gain is 6dB ~ 10dB, and the communication quality is very good.
(2) Redundancy design Two sets of digital radio stations with different frequency points are used as the redundancy design, avoiding the main interference frequency band of the local environment in the selection of frequency points.
2.4.2 Power supply anti-interference technology?
Using power supply voltage monitor to resist power supply interference is an effective method. The system uses the ADM696 chip to ensure power-on reset to prevent code errors when the power is turned off or when the voltage is low; whenever the power supply voltage is lower than the reset threshold, the reset terminal can be reliably reset, and the interference of the power supply's instantaneous voltage drop can be detected. Pulse and generate an execution signal to make the system reset reliably.
2.4.3 Software anti-jamming technology?
The anti-interference measures used in software design are:
(1) Watchdog, also known as program running monitor, can effectively prevent the program execution disorder under the influence of unpredictable interference of the single-chip system. ?
(2) Software traps and instruction redundancy. By artificially inserting some single-byte instructions in key places, the program that has already run away is pulled back. Such as the following three-byte instruction:?
NOP?
NOP?
LJMP ERROR?
(3) Keyword resend technology. In the case of correct execution of the system program, some peripheral chips of the single-chip system are easy to lose control words after being interfered, and by retransmitting these control words in the appropriate position of the program, this situation can be effectively avoided. In addition, the system also adopts a series of anti-interference measures such as photoelectric isolation, the use of multi-layer circuit boards, single-point star connection of digital ground and analog ground, and proximity to the power supply, thickened power line and ground line.
3 System software design The system software includes measurement and control software and processing software.
3.1 Measurement and control software (1) The main tasks of the measurement and control software are: initializing the system, self-checking, presetting various parameters, processing keyboard input and LCD screen display, storing and transmitting sampling data, real-time communication control, and anti-interference design. ?
(2) The measurement and control software is written in assembly language, and its functional modules are shown in Figure 4. ?
(3) Communication control method The communication control module in the measurement and control software is very important. Once the communication program has a problem, the entire system will be paralyzed. It is very important to adopt error control and fault tolerance technology.
The command sent by the master control station contains a certain number of synchronization characters FFH and three-byte passwords. The measurement station performs password verification after receiving 5 synchronization symbols in succession, and officially receives the command bytes after verification; if it fails, Then the measuring station sends a signal for the master control station to retransmit, but the command is stopped after three verifications; the sending of the measuring station / receiving of the master control station is the same. ?
· A command consists of three bytes, the second byte is equal to the first byte plus 15H, the third byte is equal to the second byte plus 16H, and the number of remaining bytes is marked after each byte, if If the received command does not meet the increment rule or the number of bytes does not match, the command will be retransmitted, and it will stop sending if it fails three times in a row.
Each time the master control station sends a command, the measuring station sends back a response signal, which contains the original command sample.
3.2 Processing software The processing software uses Power Builder and Visual C ++ as development tools, and the software design of the data management part uses Windows 9X as the operating environment, using Windows technology, friendly interface, and intuitive and simple operation. Processing software functional modules are shown in Figure 5.
The system software adopts a modular design and an effective communication control method. When receiving / transmitting data, there is rarely a phenomenon of low reception error reception, low bit error rate, the system has good anti-interference performance, and the communication has real-time, reliability and safety Sex.
4 Conclusion The underwater explosion pressure telemetry system provides a more advanced measurement method for the assessment of the underwater explosive power of underwater weapons and their damage to ships. The system uses more advanced spread spectrum communication technology in the wireless communication field, more complete software and hardware design and anti-interference measures to ensure the safety and reliability of the system. The system is also suitable for the measurement of the explosion pressure of other underwater explosives, and has a great value in promotion.
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