| Electronics > Projects, Designs, and Technical Stuff |
| Nerfo meter - measuring the speed of Nerf darts - final setup |
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| HendriXML:
Today we had fun creating the laser and sensor mounting model in Fusion 360. It's not finished, the "beam tunnel" will be subtracted, but only a small diameter. It will be drilled trough to get a better result. I keep my fingers crossed that the alignments will be ok. I will drill the laser tunnel first and see where the red dot goes after installing the laser. The PCB will be vertically mounted, the stands however won't be printed. Because the laserdiode has very short leads to the op amp, I hope the EM interference is minimal. In another setup with longer leads turning devices on and off influenced too much. The length will be 200 mm, the distance between the gates about 150 mm. |
| HendriXML:
The model had to loose some weight, there's still a lot to improve in that regard, but everything under 200 gr of PLA is fine by me. |
| HendriXML:
I mounted the circuit on a breadboard. The laser regulation is oscillating at 240 kHz measuring SensorVolt. The PID regulation is probably to fast. I'll try to finetune that. It may also that the TIA might need some a little capacitor as feedback (but that would slow down the response). When the laser is driven by a 90 Hz AWG signal the SensorVolt falls in about 2 us. The 10% fall will happen a bit faster. If a Nerf dart would travel at 15 m/s this would correspond to 30 um (that very little compared to the 150000 um between the sensors and even small relative to the beam/sensor diameter). |
| HendriXML:
After some problems with a LM311, which wasn't a LM311 the circuit is now working. I slowed down the laser power response at Q1, so the output is regulated nicely. But the PID will to be tuned a bit more to have "the slowness" evenly spread. Also I changed the voltage at which the comparator collapses. One of the LM211 inputs needs to be 1.5V from the positive rail. In the previous schematic it was very close to that with no real benefits. Potmeter RV1 is now replaced by R13, with a value of 5K (If I remember correctly). Will do some final tuning and benchmarking later. From what I saw today the circuit works fast enough for the purpose. |
| HendriXML:
I did some testing of the sensing response times using incrementing the laser diode current. At some point the TIA op amp will saturate. Lower power means less response time. This is a good reason to automatically control the laser power like the circuit does. Red is input square wave Blue is amplified sensor signal Yellow is comparator output. |
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