In order to measure the explosive force produced by shell explosion, it is often necessary to measure the flight speed of shell fragments. However, the number of shrapnel produced by shell explosion is uncertain, and the flight direction and speed of each shrapnel are also different. Therefore, measuring the rate of projectile fragments is much more complex than measuring the rate of gun bullets. It is the subject of this paper to design a simple, reasonable and easy to implement test method to measure the fragment rate of shell explosion.
1 test principle
Because there are many uncertain factors during shell explosion, only the average rate can be measured when measuring the fragment rate. The specific principle is as follows: before the test, place a circle of targets around the shell, and the horizontal distance S0 between the target and the shell is 8m. Considering that the shrapnel will fly out of the syncline when exploding, in order to ensure that the shrapnel can hit the target with a high probability, the maximum height skmax of the target (i.e. SK when k is the maximum) shall not be less than 8m (about 10m is appropriate), as shown in Figure 1. If the time t0 when the shell explodes and the time ti when a fragment enters the target can be accurately recorded, the average flight rate of the fragment is
As shown in Figure 2, at the moment of shell explosion, the trigger line around the shell is immediately broken, and the trigger line level immediately rises to vtrg, which is the positive level of constant current. It is used for touch power generation, and its value should be less than VCC. Thus, the data acquisition card is triggered, the acquisition is started, and the output waveform of the signal line on the target is recorded. The starting point of the waveform is the explosion time t0 of the shell. Continue to record the output waveform of the signal line on the target, and determine the target entry time ti of each shrapnel according to its waveform characteristics. As shown in Figure 3, when the shrapnel does not enter the target, the high-level VCC is not connected with the signal line, and the collected data is level 0. VCC is the constant current positive level. When the shrapnel enters the target, the metal shrapnel connects VCC with the signal line, and the collected data jumps to VCC level. When the shrapnel leaves the target, the signal line level returns to 0 level. Therefore, when multiple shrapnel enter the target successively, the ideal waveform of the same bullet hole series should be as shown in Figure 4. Among them, T1, T2 and T3 are the target arrival times of shrapnel 1, shrapnel 2 and shrapnel 3 respectively. T0 is the trigger time, that is, the instant of shell explosion. So far, the time of shell explosion and the time of each shrapnel entering the target have been accurately measured, and the average flight rate of each shrapnel can be calculated by the formula
2 software and hardware design of test system
The hardware part is mainly composed of data acquisition card and target. The key is to select the appropriate data acquisition card and target materials.
The selection of data acquisition card mainly considers its sampling rate and range. In the actual measurement, one channel of the data acquisition card corresponds to one bullet hole series, and one bullet hole series may shoot 0 or more shrapnel. Obviously, when multiple shrapnel are launched, the target entry time interval of each shrapnel will be very short. Therefore, only the data acquisition card with large sampling rate can distinguish the target entry time interval of each shrapnel. Therefore, pci50612 data acquisition card is selected here, and its maximum sampling frequency is 50msps. Because there are many explosive shrapnel and their flight directions are different, there are many test channels for deployment, actually up to dozens. Therefore, it is necessary to expand the test channel in the way of multi card parallel expansion, but this will lead to great overhead of the upper PC. therefore, the higher the sampling rate in the actual measurement, the better. The higher the sampling rate, the larger the amount of data processed by PC, and the more complex the PC processing. The sampling rate of 12.5msps is used in the actual measurement, which basically meets the requirements of the actual measurement resolution. In addition, it is better to choose a large range acquisition card. In the actual measurement, it is better to choose about 10V VCC voltage, so the range of the acquisition card must be greater than 10V.
Target materials are also important. As can be seen from Fig. 3, when the shrapnel is connected with two wires of different levels on the target, LC oscillation is just formed due to the inductance effect of shrapnel and the capacitance effect between wires. The equivalent circuit is shown in Fig. 5, resulting in that the collected waveform is no longer the ideal waveform shown in Fig. 4. In order to reduce the waveform oscillation, it is necessary to select appropriate materials and wiring reasonably to reduce the distributed capacitance of the conductor. The pull-down resistor R in Figure 3 also has capacitance effect, and the equivalent circuit is shown in Figure 6. When one shrapnel has left the target and the next shrapnel has not entered the target, the signal line level does not drop to 0 level, but is stable at a certain value. Therefore, the capacitance effect of R should also be reduced. Affected by the above effects, the waveform collected by the acquisition card is no longer the ideal waveform shown in Fig. 4, but the waveform shown in Fig. 7.
The hardware connection of the system is shown in Figure 8. The circuit diagram of each target area is shown in Figure 3, the waveform collected by each target area is similar to that shown in Figure 7, and the trigger signal circuit area is shown in Figure 2. Obviously, the distance between adjacent targets should not be too large to avoid missing measurement. However, this will have a negative impact, that is, when there are shrapnel in target 1 and no shrapnel in target 2, target 1 will have LC oscillation. Due to resonance, target 2 will also have the same frequency oscillation, but the amplitude is smaller. The mutual interference between these channels often makes people mistakenly believe that at the same time when there are shrapnel in target area 1, there are shrapnel in target area 2. Because the target distance between target 1 and target 2 is different, it is bound to cause inaccurate rate calculation. This illusion can be eliminated by software.
2.2 software design
The main task of the software design is: according to the collected waveform as shown in Figure 7, a suitable algorithm is used to determine the target time of each shrapnel, so as to calculate the average flight rate of each shrapnel. The specific algorithm flow is shown in Figure 9.
According to the existing test statistical law, the maximum time interval between two shrapnel entering the target in the same target area will not exceed a certain threshold value Δ t。 Therefore, the intersection points of the same shrapnel can be combined to distinguish the time when each shrapnel enters the target. As shown in Figure 10, Δ T1 is less than the threshold Δ t. Therefore, the intersection still belongs to the intersection of shrapnel 1, and Δ T2 is greater than the threshold value? Stop at t, then the intersection does not belong to shrapnel 1, but the first intersection of shrapnel 2. Due to the discharge phenomenon, the level has begun to rise at a certain time before the shrapnel enters the target. Therefore, the time when the level rises to 1 / 3 of VCC is selected as the target entering time.
As mentioned earlier, the inter channel resonance caused by LC oscillation will lead to the channel without shrapnel entering the target at a certain time will also have a waveform similar to that of the channel with shrapnel entering the target at that time. Although the waveform amplitude of the channel is relatively small when no shrapnel enters the target, its amplitude occasionally exceeds VCC. In order to avoid the rate calculation error caused by mistaking shrapnel into the target, it can be cleared in the following algorithm. As shown in FIG. 10, for each shrapnel, the time TM2 is shifted to the positive time direction from the first intersection when the shrapnel enters the target. From this time, take TM2 for a long time period in the positive time direction to calculate the average level of the waveform (as shown in FIG. 10), and then compare the average level with the average level before the shrapnel enters the target (as shown in FIG. 10). If the level step difference is greater than a certain threshold value, it is considered that the channel has shrapnel entering the target at this time; If the level step difference is less than this threshold, no shrapnel enters the target at this time. This is because, according to the test statistical law, the level step difference of the channel waveform generated by the interference of other channels in a certain period of time will not be very large. In this way, the calculation error of mistaking shrapnel into the target caused by inter channel resonance caused by LC oscillation can be solved.
In this paper, pci50612 data acquisition card and appropriate missile target signal line are used to form the hardware circuit, and combined with the corresponding digital signal processing technology, a set of projectile explosion fragment flight rate test system is designed for a military shooting range. The experiment shows that the average speed is about 1800m / s, which achieves a good test effect. The principle of this method is simple, the hardware design cost is low, and the algorithm used is not complex. It can be easily used to test the velocity of metal explosives.
Responsible editor: GT