Infrared light curtain target conditioning circuit and data acquisition

The light curtain target is a projectile speed measurement and interception device based on LED photoelectric conversion technology. It uses artificial light source, so it is easy to operate, stable, sensitive and accurate. Here, a new infrared light curtain target LED photoelectric detection system and signal conditioning circuit design is proposed. It uses infrared light emitting diodes as the light source, receives signals through photodiode arrays, processes the signals and connects them to the test instrument to obtain the speed of the projectile passing the target.
1 Lighting system

In the design, an infrared light emitting diode array is used to emit infrared light to form a light curtain. The diameter of adjacent light emitting diodes is 5 mm, and the center distance is 5 mm after close arrangement. According to the voltage drop, working current, rated voltage, etc. of the infrared light emitting diode, a circuit is designed to make the light emitting tube meet the requirements of luminous intensity and work stably.

In the past, most light curtain targets used a constant voltage source design method. However, since the main factor affecting the brightness and stability of infrared light-emitting diodes is the current flowing through them, LM317 is used to design a constant current source circuit. The specific circuit diagram is shown in Figure 1. This circuit design uses the standard constant current source circuit connection method of LM317, and its output current Iout is:

2 Photoelectric conversion circuit

Infrared photodiodes are used as photoelectric conversion devices to complete the conversion of optical signals to electrical signals. This device has the advantages of fast response speed and small size, and is widely used in photoelectric detection. The circuit design uses an array composed of 50 photodiodes connected together, as shown in Figure 2. Figure 2 only shows 2 of them, and the connection methods of the other paths are the same. Among them, resistors R1 and R2 are sampling resistors , and the resistance value can be slightly larger, which is conducive to increasing the circuit sensitivity to improve the sensitivity of the entire test system; capacitors C5 and C6 are mainly used for AC coupling.

3 Amplifier circuit

In the test system, when the projectile passes through the target surface, the electrical signal output by the photodiode is relatively weak. If this output signal is directly output to the subsequent circuit, it will often be drowned by noise. Therefore, in order to effectively use the output signal, it must be amplified. The function of the amplifier circuit is to amplify the weak electrical signal output by the photodiode to meet the needs of the subsequent processing circuit. The circuit needs to be amplified by about 1,000 times. The previous light curtain target design uses two-stage amplification (Figure 3). Due to the high amplification factor, the two-stage amplification noise and temperature drift are relatively serious. Therefore, this design uses a three-stage amplifier circuit, as shown in Figure 4. Each stage amplifies 10 times. Because it is a resistor-capacitor coupling, it amplifies about 1,000 times. A low-pass filter circuit is connected after each stage. The circuit is designed to allow signals below 50 kHz to pass.

4 Trigger circuit
In the system, the signal generated when the projectile passes through the target surface will trigger the subsequent timing circuit. The moment of generating the trigger timing pulse should not be affected by the input signal amplitude. Generally, when a gun projectile passes through the target surface vertically, the leading edge of the waveform changes slowly, while the trailing edge of the waveform changes steeply, as shown in Figure 5.

The tip triggering method uses the waveform leading edge of the projectile passing the target signal to trigger the subsequent timing circuit. This triggering method has a small slope of the waveform leading edge of the target signal, and the thickness of the light curtain of different light curtain targets is different, which makes the triggering time consistency of the projectile when passing through different light curtains poor, resulting in a large trigger error; while the bottom triggering method uses the waveform trailing edge of the projectile passing the target signal to trigger the subsequent timing circuit. Conventional target signal processing uses the zero value point of the second differential of the waveform trailing edge signal as the trigger point. In comparison, the bottom triggering method has a strong anti-interference ability. It is suitable for general projectile testing.

When designing the trigger circuit, first use the voltage comparator to convert the projectile passing target signal into a pulse signal. The threshold of the voltage comparator can be adjusted according to the actual situation, and then use the CPLD device to select the projectile tip and the projectile bottom. When the projectile tip trigger is selected, the pulse signal after the voltage comparator is directly input into the subsequent circuit; when the projectile bottom trigger is selected, the pulse signal after the voltage comparator is filtered through the filter circuit, and finally outputs a pulse with a fixed pulse width to the subsequent circuit. Here, the voltage comparator LM311 is used to design the comparison circuit, and the projectile passing target signal is designed to be a pulse signal. The specific circuit schematic is shown in Figure 6.

5 Filter circuit

When the light flux in the target surface changes greatly enough during the operation of the photoelectric target, the photoelectric sensor will respond to this change and generate an electrical signal. In other words, some non-projectile objects will also cause the light flux in the light curtain to change when passing through the target surface, so that the photoelectric sensor will generate an electrical signal. According to the working principle of the photoelectric target, the flying objects passing through the target surface have different speeds and different shielding times, which is manifested in the circuit as different widths of the square wave pulses generated after the comparator.

Where l is the length of the flying object, d is the thickness of the light curtain surface, and v is the speed of the flying object.

The shock wave is calculated based on the speed of sound, v is 340 m/s, d=3 mm, then the pulse width of the square wave signal generated by the shock wave passing through the light curtain is:; if v=330 m/s, then t1≌9.1 μs. The flying speed v of flying objects such as mosquitoes is 20 m/s, and the length of the object l is about 10 mm, then the pulse width of the square wave signal generated by the mosquito flying through the light curtain can be known. The infrared density target test system tests projectile speeds ranging from 200 to 1,200 m/s, and is mainly used for 5.8, 7.62, and 9 mm bullets. The impact of the shock wave is mainly generated by the measurement of 9 mmx19 mm pistol bullets, which have a bullet speed of about 320 m/s. According to the bullet speed and bullet length, the pulse width of the square wave signal generated by the projectile passing through the light curtain can be known.

According to the actual test conditions at the shooting range, the square wave signal pulse width range generated when the projectile passes through the light curtain is 150 to 10 μs. The signal with a pulse width within this range is considered invalid and needs to be eliminated. At this time, the mosquito interference signal and the shock wave signal can be filtered out to achieve the purpose of anti-interference.

6 Experimental Results

At a shooting range in Chongqing, the manufacturer conducted a live-fire acceptance test on the infrared density optoelectronic target. The test data is shown in Table 1.

It can be seen from Table 1 that the data of this system is stable in the live-fire test, and there is no off-target and unreasonable data, which meets the requirements of the shooting range test.

7 Conclusion

Through theoretical analysis and live ammunition tests, it is proved that the photoelectric detection system and signal processing circuit have successfully obtained the projectile passing target signal and can trigger the timer, and can obtain a stable projectile passing target signal with strong anti-interference ability. It is suitable for testing projectiles of various calibers.