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| Nerfo meter - measuring the speed of Nerf darts - final setup |
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| HendriXML:
I also did some testing breaking the beam with a nerf dart. The sensor curves look very similar because it is mostly determined by the slowness of the nerf dart. When the dart hits the beam it will gradually block more of the beam and make the exposure on the sensor less. If that takes 100 us than with the speed of 15 m/s the distance at which this happens would be 1500 um thus 1.5mm. This indeed near the diameter of the laser beam/photo diode. This principle will lower the accuracy of one gate measurement, however because it's repeated on the second one the precision might be much better. This will depend on how well the sensors are lit. But measuring distance with 1 mm accuracy would anyways be a bit of a challenge. So the distance travelled would be something like 150mm +/- 3 mm. |
| HendriXML:
Also shooting the dart straight/perpendicular is important. Maybe the setup should of less width, so its more obvious when the aiming was wrong. But we will be shooting different diameters of darts, so that's not a great solution. Having two vertical gates as well to check straightness would be nice, but for now to complex. |
| HendriXML:
So the mounting device is 3D printed. It came out great (17 hours). I had only printed the laser beam holes to 1.5 mm. The idea was to drill them more accurately. Letting the drill follow that path. That worked perfectly for one laserdiode hole (6 mm). The other one is slightly non perpendicular. I really could have used a bench drill. However the lasers are Chinese so they got angles them self. With a bit of matching and turning the sensor will be lit just fine! However the next time I would add larger holes in the 3D model. Going from 1.5 - 2 - 2.5 - 3 - 3.5 - 4 - 4.5 - 5 - 5.5 - 6 and drilling very slowly takes a lot of time, however this is critical not to heat up the drill. I also made a PCB layout just for laying out the used components in order and usable with a breadboard. It takes a little effort, but my son will be soldering one of the PCB's so the session shouldn't take too long. |
| HendriXML:
Yesterday my son did a very good job on soldering the components. Most of them we did from the the top of the perfboard. That makes it more easy to bend the legs to form traces. (Also faster) That part I've done alone. I'll post some pics of the finished product. So I hooked the 2 gates on 2 scope channels for a testing out a script. The script uses SCPI to set things up, retrieve the data and does some calculating. In the script I just took the number of samples between the zero crossings. Because of the 250MSa/s, this is off course very accurate. The accuracy of the experiment is in that regard entirely determined by the distance between the gates/laser beams and the place at which the beam is registered as broken. The distance error will be around 2 mm combining those errors. So that would make the speed calculation around 1% accurate. Because the error will be mostly systematic, this setup can surely be used in calculating the added power to the Nerf shooter when modding with more precision. However this we won't do, because then it would probably loose it's function as a toy. The next step would be to have the time measured without using a scope. Getting data gate A Sample number: 874998 Getting data gate B Sample number: 3334790 Sample difference: 2459792 Time difference: 9,83917 ms Speed: 15,24519 m/s Speed: 54,88269 km/u |
| HendriXML:
This is how the setup looks like. On the back is a battery voltage to 5V regulator https://www.eevblog.com/forum/projects/battery-voltage-to-5v-regulation/msg2280605/?all#lastPost in a 3D printed box. The nice thing of this box is that the air slots are zigzagged, so you can't see inside the box or poke a straight object in it. With this project https://www.eevblog.com/forum/projects/very-stable-temperature-control/msg2514615/#msg2514615 the thermal dissipation capacity of the box and heatsink inside could be determined by measuring the power it takes to keep for instance a stable 60 deg. That is something I'd like to experiment with in the future. As also can be seen the laser is regulated to a very dim light source. |
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