A shock wave overpressure test system based on multiple triggers

ZHANG Jin-wen, WANG Wen-lian, ZHANG Zhi-jie

(1. Key Laboratory of Instrumentation Science & Dynamic Measurement (North University of China),Ministry of Education, Taiyuan 030051, China；2. Science and Technology on Electronic Test & Measurement Laboratory, North University of China, Taiyuan 030051， China)



A shock wave overpressure test system based on multiple triggers

ZHANG Jin-wen1,2, WANG Wen-lian1,2, ZHANG Zhi-jie1,2

(1.KeyLaboratoryofInstrumentationScience&DynamicMeasurement(NorthUniversityofChina),MinistryofEducation,Taiyuan030051,China；2.ScienceandTechnologyonElectronicTest&MeasurementLaboratory,NorthUniversityofChina,Taiyuan030051，China)

Because single trigger system is unreliable for shock wave overpressure test, this paper presents a multi-trigger overpressure test system. The large memory capacity is divided into parts to achieve data acquisition and storage with multiple triggers. Compared with conventional single-shot storage test system, this system can prevent false trigger and improve reliability of the test. By using explosion time to extract valid signal segments, it improves the efficiency of data recovery. These characteristics of the system contribute to multi-point test. After the dynamic characteristics of the system are calibrated, the valid data can be obtained in explosion experiments. The results show that the multi-trigger test system has higher reliability than single trigger test system.

explosion field; overpressure test； multiple triggers; explosion time extraction

0 Introduction

Shock wave overpressure test is an important method to evaluate destructive power of weapons. Recently, the effective methods of shock wave overpressure test include cable-connection test method and storage test method. The storage test method is applied to most test systems, which can avoid signal attenuation and poor reliability caused by the cables when using cable-connection test method. However, these systems almost adopt a single trigger to collect signals, which means that the system can be triggered only one time[1-9]. It is unreliable because the system is easy to be falsely triggered by uncontrollable environmental factors in explosion field. Because the testers must evacuate for security and thus they cannot monitor the system. If falsely triggered, the test will stop in a few seconds and the obtained data will be stored and read. Consequently, when real shock waves arrive, the system will not be triggered and fail to store the data. To overcome the shortcomings of single trigger, a new method using multiple triggers is presented to improve the reliability of the existing test systems, which can acquire signals and store data after each trigger.

1 Design ideas

In this paper, a multi-trigger system is proposed that can be triggered many times for shock wave overpressure test. If falsely triggered, the system can reenter the un-triggered state. When the shock wave arrives, it can be triggered again to acquire the shock wave signals.

In this way, the obtained data are transmitted repeatedly into storage space, which is divided into parts for data transmission. In case of increasing test nodes to meet the test requirements, the overall data will increase inevitably as a result of false triggers. And the testers need to distinguish valid data from a large amount of confused data while observing the waveform of the signals in the process of collecting data, which will decrease data transmission and processing efficiency. Therefore, an effective method, the first judgment is used to extract data segment.

While the collected analog signals are converted to the digital signals by analog-to-digital (A/D) converter, the system continually monitors digital signals and judge from the mutation of signal amplitude and width. If the data accords with the predefined standard of judgment, the system will label it as valid data and records the absolute time. When multiple valid data segments occur, the matching data segment is accurately extracted by means of the absolute time. The testers can read and collect data quickly and effectively. The flow chart of the system is shown in Fig.1.

Fig.1 Flow chart of test system

2 System design

The system hardware consists of several subsystems, including test nodes, PC and time nodes. The test nodes are controlled by field-programmable gate array (FPGA) which can support multiple tasks and perform first-in-first-out (FIFO) function for internal random access memory (RAM). Besides, it has many advantages such as automation, low power consumption, high speed, etc. The hardware architecture of the test system is shown in Fig.2.

Fig.2 Hardware architecture of test system

The working process of the test system is as follows. The upper computer are connected with the test nodes and it sets parameters such as gain magnification and trigger level through universal serial bus (USB) before the experiment is conducted. Then the test nodes convert stored data circularly and wait to be triggered after power on. When the digital signal detected by FPGA is greater than the trigger level and increases continuously, the test nodes begin to store the data sequentially. It records the message of coordinated universal time (UTC) sent by global positioning system (GPS) through universal asynchronous receiver/transmitter (UART) port at the rising edge of pulse per second(PPS). If the storage capacity reaches 8 MB, the data will not be stored sequentially, but be stored circularly. The system waits for being triggered again, otherwise signal sampling and data storage are completed and the data waits for being read.

The time node is used to acquire the moment of explosion. Fig.3 shows the composition of the time node. The working process of the time node is as follows.

Strong light occurs at the moment of explosion, which makes the photodiode produce photocurrent. The photocurrent becomes a pulse through a trans-impedance amplifier and a comparator. The pulse causes microcontroller to generate interrupt signal to record UTC at the rising edge of PPS.

Fig.3 Hardware structure of time node

3 Key technologies

3.1 Analog circuit design

The shock wave overpressure signal has the characteristics of fast rising speed, short time of positive pressure, wide bandwidth, etc. Therefore, the sensor should have high dynamic response when testing. 113B internal electronics piezoelectric (ICP) sensor[10]is integrated in the system with rise time less than 1 ms and resonant frequency greater than 500 kHz. Its internal acceleration compensation can minimize shock and vibration sensitivity. The measurement range of sensors is 345-68 950 kPa. The test nodes can be equipped with different sensors according to test requirements. The system is battery-powered, and power management circuit converts the battery voltage of 8.4 V to an accuracy output voltage of 5 V, which is responsible for the analog circuits. The sensor should be powered by excitation voltage in the range of 20-30 V, and constant current exaction of 2-20 mA. The voltage booster achieves DC-DC conversion from constant voltage source to constant-current source and boosts the voltage of 8.4 V to a voltage of 24 V, which is for the sensors. When ICP sensor is stressed by the overpressure of the shock wave, charge is produced. Inner microelectronic amplifier converts the charge into a voltage signal. And then the signal is amplified by signal amplifying circuit that consists of operational amplifiers and analog switch. The analog switch chooses different resistances to implement different gains. The gain is programmable through universal serial bus (USB) contact. The amplified signal is filtered by the filter of signal conditioning circuit, and then is transmitted to A/D converter. The filter is a low pass filter with an upper cut-off frequency of 250 kHz.

3.2 Data storage and extraction

The capacity of storage space is averagely divided into two parts: one is to store data and address of bad blocks, the other is for backup storage space when the first part has bad blocks. These two parts are all equally divided by 8 MB and each 1 MB in 8 MB is for the circular storage. The data must be written into NAND flash by page write mode, thus, the data is written quickly. Moreover, FIFO is also generated as data cache in FPGA. When the test nodes start to work, the system circularly records the data into the circular storage space of 1 MB before being triggered and always keeps the latest data of 1 MB. When being triggered, the data is stored sequentially in 7 MB of 8 MB. Then the system repeats the previous process and enters the un-triggered state. The data can be stored in FIFO before the system is triggered by the shock wave. If uncontrollable factors cause the test nodes to be falsely triggered, multiple data will be accumulated in the storage space. To improve the transmission efficiency, it is necessary to extract valid data, namely, shock wave signal.

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The data is extracted through two steps. Firstly, the system analyzes data whether it is the shock wave signal, where the rising time and positive phase duration of the data are the main basis of judgment. To make the system distinguish the shock wave signal obtained in different distances from the explosion center or in different explosive equivalents, the criterion for judgment is set in a broad range. If the rise time is less than 15 μs and positive phase duration is longer than 120 μs, the data is judged as valid data segment. However, the data caused by striking the sensor or other factors may be ruled as valid data segment. Therefore, it needs to be further judged. Secondly, the valid data segment can be extracted by using UTC time recorded by test nodes and time node. If the difference value of the time is less than or equal to 1 s, the valid data segment is extracted.

Fig.4 Configuration of storage capacity

4 Dynamic calibration

This paper describes the dynamic calibration of one test node with a 345 kPa sensor. As shown in Fig.5, shock tube is made of straight metal pipe with sleek inner wall, including a high-pressure chamber, a low-pressure chamber and a diaphragm. The pressure of low-pressure chamber is atmospheric pressure at room temperature, and the high-pressure chamber is filled with high-pressure gas. The low-pressure chamber wall is equipped with two pressure sensors for testing shock wave speed. The calibrated test system is installed in the face of low-pressure chamber.

Fig.5 Experimental set-up of shock tube

When calibrating, high-pressure gas goes into the shock tube of high-pressure chamber and the diaphragm will burst under certain pressure. Consequently, high-pressure gas is ballooned into low-pressure chamber and a shock wave forms. The shock wave successively passes through two pressure sensors at supersonic speed. The delay timetcan be obtained by means of the rising edges of the signals captured by two pressure sensors，

Shock Mach is obtained as

whereP1is the initial pressure of the low-pressure chamber. In order that the platform of shock wave pressure is relatively flat after reflection, the shock Mach must be about 1.3. Therefore, the thickness of the diaphragm should be appropriate. Calibration experiments are conducted three times, and the typical calibration curve is shown in Fig.6.

Fig.6 Typical calibration curve

Table 1 Dynamic calibration results

It can be seen that the tested pressure value for test system isPand the system error can be calculated from Eq.(4),

The system error is less than 5%, which indicates that the test system has good stability and high reliability. In dynamic calibration, test system calibration can be performed repeatedly to accelerate the calibration process by using multi-trigger technology.

5 Experiment results and analysis

To verify the reliability of the system, the explosion experiment was conducted. The explosive to be tested was fixed at the circle center at the height of 0.8 m. The test nodes along two directions from the center were placed in circles of radius 3 m and 4 m. In 15 experiments, the test nodes acquired valid data. Since false trigger does not happen often in the experiment, there were two times that the system was triggered artificially by striking on sensors, which simulated interference of environment factors. Using multiple triggers, the test nodes also obtained valid data. The results show that the multi-trigger technology can improve system reliability. The shock wave signal could be accurately extracted through UTC time. The test curves are shown in Figs.7 and 8.

Fig.7 Test curve in 3 m

Fig.8 Test curve in 4 m

6 Conclusion

In this paper, a detailed description of hardware system to improve the reliability of test system is presented. A shock wave pressure test experiment with multi-trigger technology is carried out. The experiment results show that the system is a powerful tool for overpressure test. It provides reliable ways of extracting data and testing different explosives. After dynamic calibration, the system error is less than 5%. The field experiments verify the system is reliable and stable, and it can be used move than once in a row shock test.

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基于可多次触发技术的冲击波超压测试系统设计

张晋文1,2， 王文廉1,2， 张志杰1,2

(1. 中北大学 仪器科学与动态测试教育部重点实验室， 山西 太原 030051；2. 中北大学 电子测试技术重点实验室， 山西 太原 030051)

在冲击波测试中， 单触发系统具有不可靠性。 为此， 本文提出了一种基于多次触发技术的冲击波测试系统。 它将存储空间分割成多个存储数据段， 以实现多次触发的数据采集与存储， 提高了测试系统的可靠性。 对测试系统在误触发后存在的多个数据段， 采用爆炸时刻来提取有用数据段， 提高了数据回收效率， 适合于多点分布式测试。 该系统经动态特性标定后， 通过爆炸试验采集了有效数据。 结果表明， 该测试系统较单触发系统有较高的可靠性。

爆炸场； 超压测试； 多次触发技术； 爆炸时刻提取

ZHANG Jin-wen, WANG Wen-lian, ZHANG Zhi-jie. A shock wave overpressure test system based on multiple triggers. Journal of Measurement Science and Instrumentation, 2015, 6(1)： 19-24.

10.3969/j.issn.1674-8042.2015.01.004

ZHANG Jin-wen (zjwzwq1989@163.com)

1674-8042(2015)01-0019-06 doi： 10.3969/j.issn.1674-8042.2015.01.004

Received date： 2014-10-02

CLD number： TP274 Document code： A