FPGA-based design photoelectric anti-interference circuit (1)

The basic principles of optical target is when the light curtain in luminous flux of change occurred is large enough, the photoelectric sensor will respond to this change and produce an electrical signal.

This means that some non-projectile body in through the light curtain will make light curtain of light flux change and photoelectric sensor produces an electrical signal. From the principle that it is not the exception, and the test run is interference. In the specific range test, when a disturbance of serious will lead to a test cannot proceed. So, how do I exclude interferences, ensure system uptime, is a must solve.

Infrared photoelectric target test system is a new extension for measuring low ballistic weapons fire intensity test system, the test shall not be liable for any special treatment of the metal projectiles and test non-metallic projectile, better reflect the sensitive, high accuracy and stability, simple operation, easy maintenance, etc., have been put to use in many ranges.

Theory and analysis

Photoelectric target as you work, photoelectric sensor response screens in luminous flux of change, transforming it into a faint signal, after zooming in, enter the voltage comparator, when its amplitude than scheduled benchmarks, voltage comparator flip, trigger pulse.

As the accompanying shot through the light curtain for small objects and external light changes in signal amplitude is relatively small, through the voltage comparator set appropriate comparison threshold will filter out the signal.

In the target plant actual tests, this kind of interference signal amplitude in general less than 0.8V circuit as long as the voltage comparator threshold level is set to 0.8V can eliminate such interference.

According to the electro-optical targeting works, through the light curtain of flying objects at different speeds, blocking the light curtain time is different, in the circuit performance as comparator arising after the width of the square wave pulse.

Compared with projectiles, mosquito flight speed is much lower, when it passes through the light curtain, the resulting square wave pulse width than the projectile is wide; produced in subsonic bomb tests, sound velocity projectile velocity below, caused by the Sonic pulse width less than the projectile square wave pulse width of. Therefore, from the principle in the comparator using filter circuit filtering after interference signals is possible.

Use FPGA implementation of filtering and anti-jamming

The primary function of the entire circuit is resistant to shock and mosquito noise, and effective projectile signal into a pulse width of 50 μ s signal output to the lower processing circuit.

Design of chips is MAX7000 family EPM7128SLC84-15 chips. Here's how to implement filtering and anti-jamming for details.

1 circuit

When the body passes through the light curtain, the resulting square wave pulse width: T = (L + D)/V.

Type, the length of the L-flying objects, D-light curtain surface thickness, velocity V for flying objects. Shock wave to the velocity, V, s for 340m/D = 3mm, you know the shock wave through the light curtain of square wave signal pulse width: T1 = D/V ≌ 8.8 μ s; if V = 330m/s, then T1 ≌ 9.1 μ s. Mosquitoes and other flying objects flying speed v is 20m/s, the body length of about 10 mm L, you know the mosquito flew across the screens of square wave signal pulse width is T2 ≌ 650 μ s. Infrared intensity of the fire test system test shot-velocity range is 200 ~ 1200m/s, mainly used for 5.8mm, 7.62mm, three bombs 9mm; shock waves of chiefly on 9mm×19mm pistol rounds of measurement, the projectile projectile velocity of about 320m/s. According to the projectile velocity and plays Ministers, we shot through the light curtain of square wave signal pulse width range of T3 = 37.5 μ s.

According to the actual test range, shot through the light curtain at square wave signal pulse width is less than 150 μ s and greater than 10 μ s, you can view the pulse width is greater than 150 μ s is less than 10 μ s signal is not a valid signal, can be removed, then the mosquito interference signal and shock wave signal filtering, anti-jamming.

2 resistance to shock wave circuit

Filtering out shock wave circuit shown in Figure 1.

When a client is PULSE_IN jump, jump on the D1 along that trigger output produces high level signal, then the boot counter starts counting. When the counter meter is full, the counter output produces a are hopping, and the rising edge of the D3 allows a trigger output produces a high level signal, the signal and get output signal PULSE_OUT1. When the arrival of the falling edge PULSE_IN will counter and three triggers at zero, wait for next signal. Figure 2 shows that, when the pulse width less than PULSE_IN set timing width that interference signal PULSE_OUT1; when PULSE_IN pulse wider than the width and set the timing is still high, that signal is valid, is higher and in PULSE_OUT1 PULSE_IN-drop along into a low level, wait for next signal.

Figure 1 filter out shock wave circuit

Figure 2 simulation of shock wave circuit waveforms

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