Measuring speed of the shaft

Simple Average Speed Measurement - Using Piezo Sensors
Another idea for measuring Average Speed over a set distance can be stolen from this video about a coil gun:

In the video, the creator uses a small coil to either sense when the arrow passes the coil at the moment of the shot (or he senses the charge from the bigger "shooting coil"). But the real trick we could steal from him is using a simple piezo impact sensor on the target. Then connect both sensors to the audio recorder. Quite simple!

On an airgun, I would even give it a go swapping out the coil sensor for another piezo sensor mounted directly on the muzzle. If the muzzle piezo could pick up the impact of the piston onto the shock absorber then the whole setup would be even simpler.

Also, adding a piezo sensor on the target could be combined with some of the above to provide both initial speeds and average speed in one shot

BTW. I realize, this setup measures from the moment the shaft leaves the gun and not from when the trigger is pressed.
[Addition] I have spent a bit of time just now looking more into piezos and they seem to be pretty sensitive. People are making hydrophones and contact microphones out of them and they are cheap as chips. Perhaps, a piezo could actually pick up the sound of the start of the shot if mounted close by the trigger mech. It should be fairly easy to 3D print a holder of sorts which could do that job. You might even be able to talk into the piezo (out of the water) to record spoken notes before/after the shot.

tromic said:

Interesting! Nice to have one method more.

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Yes, if we get into Arduinos and such, the sky is almost the limit. You could mount a tiny acceleration/gyro sensor on the gun and measure recoil and its force and directions, use lasers and laser detectors and then link that with a force measurement sensor on the target - but while I know it can be done, I have no clue about how to program and set that stuff up.
The beauty of your coil idea and of piezos is that we don't have to even power them - they generate a voltage spike on their own which can be picked up by the recorder - so it really is ultra simple.

tromic said:

How would you measure the initial speed? Initial speed talks about the performance of the speargun itself. Average speed is influenced additionally by the rigging of the shaft: line properties, slider properties...

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True, but I think the latter is more important to me to actually measure those exact things. Also, it seems to be what others care more about so if I want to compare to others, then it will likely be average speed. And most people these days can measure average speed fairly easy with a GoPro on a high frame rate.

Using Inductive Sensors - Another Idea
I thought of another idea of sensing the shaft - you could mount an inductive sensor on the muzzle. Here's a somewhat stolen and rewritten blurb on how those sensors work:
Inductive proximity sensors are used for non-contact detection of metallic objects. Inside the sensor is a coil and oscillator that creates an electromagnetic field in the close surroundings of the sensing surface which allows the sensor to detect metal nearby.

They often look like this and cost just a few dollars:


They should work in water - as opposed to their capacitive cousins which actually trigger when they "see" water. Now, there are a few challenges to this idea but nothing major. First of all, an inductive sensor doesn't see metal very far so it can't be mounted off of the gun. It would need a holder of sorts to attach it to the gun and place it just in front of and fairly close to the shaft tip. The smaller the sensor, the shorter its sensing distance. E.g. the small 5mm sensors (tube size) sense less than 1mm from their tips but I think the M18 sizes can sense up to around 8mm from the face of the sensor. Otherwise, one could use the even large M30 versions which can sense up to 15mm (all these distances are pretty general). The point is, you want one that doesn't have to sit super close to the shaft. As for the holder, I could 3D print it easily. Mount it opposite the line release and make the holder design without any snag points for the shooting like to catch onto and I think it should work.

The second challenge is that even though these sensors don't need a "controller" like an Arduino to work or to be read, they still need to be powered to generate a strong enough magnetic field, I guess. Most of these work on 10-30V but some work from 6V. Depending on their configuration, they will pass or block the voltage do go through as an output signal when they sense the metal nearby. Best for us to use one that only sends through the voltage when the shaft is sensed so we get a spike in the audio recording. (The opposite could work, too - we would just get a drop in the audio when the shaft came through). The last issue is that even 6V is way too high for an audio-in signal, as far as I understand. According to my basic research, mic voltage levels are very low at under 0.3v for a high output mic and up to just 1.3V for pro line level. Remember, the sensor will pass through the voltage level that it is fed, so we would need to lower that with some resistors but I think that should be fairly easy to figure out.
Finally, these sensors also look quite easy to waterproof and even if we have to power them from a 9V batt or, say, two 18650 cells, then that's not a big deal.

Come to think about it, if you want is to sense when the shaft end leaves the muzzle, you could mount the sensor just at the muzzle opening and then use a sensor that sends through the signal when it does not sense any metal. That way, you would get an high input on the recorder the moment the shaft end has passed by the sensor. But on an airgun, if you want to sense that point in time, I still think just mounting an unpowered piezo on the muzzle itself to pick up the sound of the impact on the shock absorber is a much easier proposition.

Diving Gecko said:

Yes, if we get into Arduinos and such, the sky is almost the limit. You could mount a tiny acceleration/gyro sensor on the gun and measure recoil and its force and directions and then link that with a force measurement sensor on the target - but while I know it can be done, I have no clue about how to program and set that stuff up.
The beauty of your coil idea and of piezos is that we don't have to even power them - they generate a voltage spike on their own which can be picked up by the recorder - so it really is ultra simple.

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Hi Tomi,
What would you prefer to use as the start point of measurements and why? Ideally, would you measure from the point in time when the shaft starts moving or from the moment it leaves the piston?

tromic said:

Davide, I would measure from the moment it leaves the piston. Then you'll have the max of the speed, the initial speed. After that moment the shaft starts decelerate, lose the speed over distance.

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Thanks Tomi