# Laser sensor for measuring velocity of very fast moving projectile

#### Gasgunguy

Joined Jun 18, 2022
5
I am looking for laser sensor to measure velocity of fast moving bullet/projectile traveling through barrel.

I have a bullet of 5mm length and it would cut the laser for just 5- 20 microseconds. The sensor need to be fast enough to give trigger to oscilloscope with this kind of small intervals disturbance.

I am looking for suggestions on possible sensors for this kind of measurement to be possible.

Bullet details: 5mm dia , 5mm length , can move in velocity 10m/s to 1000m/s.

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Joined Jan 15, 2015
7,543
What you are describing is commonly called a Chronograph. This is how we typically measure the velocity of a bullet but we generally measure bullet velocity after the bullet exits the barrel. I have worked with plenty of 5.56 mm (.224" Dia) diameter bullets and 4.4mm (.177" Dia) bullets but never a 5mm bullet with a 5mm length. That is strange.

Problem being measuring a bullet velocity inside a barrel is not easy. The bullet is being pushed by a high pressure. This begins with "chamber pressure" so it's not as simple as drawing a picture with a start / stop. You just do not drill holes through a barrel, mount a few fast photo transistors and be done. Starting simple with Velocity = Distance / Time so your start and stop sensors need to be placed an accurate distance apart. With a barrel to measure velocity inside the barrel a light beam be it laser or other light would need to be placed across the barrel. The way this works is a projectile breaks the beam and starts a counter counting pulses of a known frequency. At a known distance the bullet breaks the second beam and stops the counter.

Normally bullet velocity is measured about 4.572 meters (15 feet) from the barrel muzzle. The reason being when the bullet exits the barrel there will be hot gasses (ejecta particles) following it. You do not want anything false triggering your sensors. You are pointing at what is commonly called a ballistic chronograph. I gave a very brief overview of how they work. Most use an optical sensor like a photo transistor. They are plenty fast for what you describe. Using a LASER for a simple proof of concept simple 5 volt laser source and simple 5 volt laser receiver. Your Start pulse opens a gate circuit allowing clock pulses to be counted and your stop pulse closes the gate stopping your pulse train. Your sensors are a set distance and your clock frequency is a known accurate frequency. You apply the formula for velocity. Bullet diameter of bullet length matter not. I regularly measure bullet velocities of 1219.2 meters/sec and this is how it is done. A Google of "ballistic chronograph circuits" should get you started.

In your drawing you are just using the scope sweep speed as a clock. If you are pushing a projectile of a given weight 1,000 meters/sec you will have a high pressure pushing the projectile and need to figure that into your design. You also do not show sensor distance?

Ron

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#### Gasgunguy

Joined Jun 18, 2022
5
What you are describing is commonly called a Chronograph. This is how we typically measure the velocity of a bullet but we generally measure bullet velocity after the bullet exits the barrel. I have worked with plenty of 5.56 mm (.224" Dia) diameter bullets and 4.4mm (.177" Dia) bullets but never a 5mm bullet with a 5mm length. That is strange.

Problem being measuring a bullet velocity inside a barrel is not easy. The bullet is being pushed by a high pressure. This begins with "chamber pressure" so it's not as simple as drawing a picture with a start / stop. You just do not drill holes through a barrel, mount a few fast photo transistors and be done. Starting simple with Velocity = Distance / Time so your start and stop sensors need to be placed an accurate distance apart. With a barrel to measure velocity inside the barrel a light beam be it laser or other light would need to be placed across the barrel. The way this works is a projectile breaks the beam and starts a counter counting pulses of a known frequency. At a known distance the bullet breaks the second beam and stops the counter.

Normally bullet velocity is measured about 4.572 meters (15 feet) from the barrel muzzle. The reason being when the bullet exits the barrel there will be hot gasses (ejecta particles) following it. You do not want anything false triggering your sensors. You are pointing at what is commonly called a ballistic chronograph. I gave a very brief overview of how they work. Most use an optical sensor like a photo transistor. They are plenty fast for what you describe. Using a LASER for a simple proof of concept simple 5 volt laser source and simple 5 volt laser receiver. Your Start pulse opens a gate circuit allowing clock pulses to be counted and your stop pulse closes the gate stopping your pulse train. Your sensors are a set distance and your clock frequency is a known accurate frequency. You apply the formula for velocity. Bullet diameter of bullet length matter not. I regularly measure bullet velocities of 1219.2 meters/sec and this is how it is done. A Google of "ballistic chronograph circuits" should get you started.

In your drawing you are just using the scope sweep speed as a clock. If you are pushing a projectile of a given weight 1,000 meters/sec you will have a high pressure pushing the projectile and need to figure that into your design. You also do not show sensor distance?

Ron

As you pointed out I missed some points to mention in the post.
1) I am using a Gas gun and not a normal gun where the propellent is used. But however, the pressure will be high as you mentioned the design has to withstand the pressure.
2) the distance I am looking for is 100mm between sensors.
3) you have mentioned that bullet length does not matter, but the interval the pulse is cut depends on velocity and length of Projectile. I was using IR module and it is not detecting the projectile. (https://www.electronicscomp.com/ir-...MIuYfUuuS4-AIVptlMAh3Pqw4WEAQYASABEgIv1vD_BwE)

I was looking if a laser or any other sensor that can detect the 5-microsecond level disturbance. As you mentioned I will try out the 5 volt laser and 5 volt laser receiver.

Thank you very much for the answer.

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#### Gasgunguy

Joined Jun 18, 2022
5
the other thing I want to mention is that I am using a high-speed camera some distance ahead of the barrel.
the lighting I have to use for the high-speed camera is intense, this light should not disturb my measurements.
I have to shield the sensors from these lights. the laser receiver you mentioned is non-modulated, are there any modulated receivers and can you direct me to one?

#### ericgibbs

Joined Jan 29, 2010
18,987
Hi Gguy,
Does the sensor pair have to be mounted along the barrel length, or could the sensor pair be mounted as an adapter on the end of the barrel?
Measuring at the barrel exit would give a more accurate indication of the actual projectiles barrel exit velocity.

If you measured the projectile velocity along the barrel length, how would you determine the location of the sensors to allow for the projectile acceleration.?
E

#### Gasgunguy

Joined Jun 18, 2022
5
Hi Gguy,
Does the sensor pair have to be mounted along the barrel length, or could the sensor pair be mounted as an adapter on the end of the barrel?
Measuring at the barrel exit would give a more accurate indication of the actual projectiles barrel exit velocity.

If you measured the projectile velocity along the barrel length, how would you determine the location of the sensors to allow for the projectile acceleration.?
E
The plan is to mount as an adaptor on the end.

#### ericgibbs

Joined Jan 29, 2010
18,987
sensor to measure velocity of fast moving bullet/projectile traveling through barrel.
hi Gguy,
Adapter makes more sense, your original post was a little misleading.

There are photo interrupters that work in the nano-Sec response range, will suggest a type.

E

Update:
This is the type I have in mind, the only limitation is the 5mm intergap.!

Check the 2nd PDF 8mm slotted
@Gasgunguy

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#### Halfpint786

Joined Feb 19, 2018
109
What about two thin sheets of paper a known distance apart with a microphone or piezo attached to each sheet connected to left and right channels of a stereo line-in on a computer and record the sound. A sound card should be plenty fast enough to get a good measurement between the sheets being punctured. You would need the microphone elements very close to the point of impact though.

Traditionally, this problem was solved with a ballistic pendulum...

#### Ya’akov

Joined Jan 27, 2019
9,229
What about two thin sheets of paper a known distance apart with a microphone or piezo attached to each sheet connected to left and right channels of a stereo line-in on a computer and record the sound. A sound card should be plenty fast enough to get a good measurement between the sheets being punctured. You would need the microphone elements very close to the point of impact though.

Traditionally, this problem was solved with a ballistic pendulum...
I don’t know if your scheme would work but why go out of the way to avoid using the effective and highly refined principles of the modern chronograph? The optical electronic chronograph is relatively simple and thoroughly proven.

Also, I don’t think this problem was solved with a ballistic pendulum which was for mea using the terminal ballistics, not the muzzle velocity which would be a very scary thing to do with it.

Joined Jan 15, 2015
7,543
Well alrighty then and many thanks to Eric.

My thinking based on many experiments with ballistic chronographs goes like this.

You want to use a scope as your time base (clock) which is fine.

You can place your sensors at the muzzle (barrel exit point) which makes things much easier.

Your source is pneumatic which is fine. I can share this much on pressure. You will want a high pressure solenoid. Using a 5.56 mm slug weighing about 45 to 50 grains a pressure burst of 344737.86 Kilopascal (50,000 PSI) will net you about 877.824 meters/sec (2880 FPS). All of this subject to barrel bore diameter verse projectile diameter. There is quite a bit involved. Anyway you will need some high pressure tubing and fittings as well as the solenoid. Projectile material composition is also a consideration as well as the barrel.

Yes, I did say projectile length did not matter. Here is my thinking on that and why I say this.

Using a scope for example. I would use a two channel scope displaying two traces in an alternate mode. I would use two sensors of the slotted type as Eric mentions. The pulse with, as you mention will be a function of projectile length and velocity. If your sensors output for example a negative going pulse when beam id broken I would set your scope to trigger (One Shot) channel 1 negative trigger with a level of about 2.5 volts for a 5.0 to 0.0 pulse. That is the start pulse. The second sensor of known distance will be channel two. Now just observe the time interval between when channel 1 goes negative (start) to where channel two goes negative (stop). A more accurate way would be to use a counter set up for time interval A to B. Just adjust triggering accordingly.

Halfpint also mentions a good point. I never tried it but a Piezo sensor could be glued to sheets of paper. The earliest ballistic sensors used a fine wire matrix on sheets of cardboard. Problem was after a shot broke the wire the matrix needed repaired. This worked well until the sensor was so shot up it needed thrown away.

Here is an example of my own ballistic chronograph. It uses photo transistors.

It uses three sensors just so I can meke sure my data is good.

On the range it looks like this.

All it takes is an interupt of ambient light. The sensors are called sky screens.

Anyway I would consider Eric's suggestion of a slooted optical sensor. I would not consider the reflective type you are trying.

Edit: I forgot to mention barrel length just like pressure duration figures into all of this. A Google of ballistic chamber pressure curves will net you some images even though you are not using smokeless powder in an actual cartridge.

Ron

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#### Gasgunguy

Joined Jun 18, 2022
5
The 5Volt
What you are describing is commonly called a Chronograph. This is how we typically measure the velocity of a bullet but we generally measure bullet velocity after the bullet exits the barrel. I have worked with plenty of 5.56 mm (.224" Dia) diameter bullets and 4.4mm (.177" Dia) bullets but never a 5mm bullet with a 5mm length. That is strange.

Problem being measuring a bullet velocity inside a barrel is not easy. The bullet is being pushed by a high pressure. This begins with "chamber pressure" so it's not as simple as drawing a picture with a start / stop. You just do not drill holes through a barrel, mount a few fast photo transistors and be done. Starting simple with Velocity = Distance / Time so your start and stop sensors need to be placed an accurate distance apart. With a barrel to measure velocity inside the barrel a light beam be it laser or other light would need to be placed across the barrel. The way this works is a projectile breaks the beam and starts a counter counting pulses of a known frequency. At a known distance the bullet breaks the second beam and stops the counter.

Normally bullet velocity is measured about 4.572 meters (15 feet) from the barrel muzzle. The reason being when the bullet exits the barrel there will be hot gasses (ejecta particles) following it. You do not want anything false triggering your sensors. You are pointing at what is commonly called a ballistic chronograph. I gave a very brief overview of how they work. Most use an optical sensor like a photo transistor. They are plenty fast for what you describe. Using a LASER for a simple proof of concept simple 5 volt laser source and simple 5 volt laser receiver. Your Start pulse opens a gate circuit allowing clock pulses to be counted and your stop pulse closes the gate stopping your pulse train. Your sensors are a set distance and your clock frequency is a known accurate frequency. You apply the formula for velocity. Bullet diameter of bullet length matter not. I regularly measure bullet velocities of 1219.2 meters/sec and this is how it is done. A Google of "ballistic chronograph circuits" should get you started.

In your drawing you are just using the scope sweep speed as a clock. If you are pushing a projectile of a given weight 1,000 meters/sec you will have a high pressure pushing the projectile and need to figure that into your design. You also do not show sensor distance?

Ron
I have tried the 5 Volt laser source and laser receiver, It is catching the projectile and gives a 5-volt signal to the oscilloscope. I am using this as a trigger to high speed camera.
Thank you @ Reloadron for the suggestion.