After enjoying nice holidays knowing we had a well working configuration and current value, it was time to get back on it and move further ahead.
Or we thought so. We tried to have it work again with the same values but it did not work. So we needed to some more tests.
Alexis has told us to do some measures : we needed to measure the current rise time in the coil, the overvoltage and the voltage drop.
First of all we measured the overvoltage when brutally disconnecting the coil from the power source. To do so, we needed to disconnect the cable directly from the little breadboard we have and not just shut down the power supply.
And when doing so we measured an important overvoltage up to 260V, that we’ll need to address this problem by adding a diode for current recirculation.
Secondly we measured the rise time at terminals of our coils. We used an oscilloscope (probably much older than I am) on trigger mode.
After several measures, we found that this rise time was consistently under 3,4 ms which is in the bounds of our project although it might prove a little short.
We want to be able to display different images at the rate of 1 image/s. In the worst case scenario (which isn’t likely to happen that much), we would have to flip all marbles and assuming we’re flipping marbles one by one, it would take approximately 900 ms which is still ok.
We then measured the voltage drop at the terminals of our coil. The resistance value given by the constructor is low : roughly 3 Ω. As such, and because the wires’ resistance wasn’t negligible in regard of the coil one, we had a little voltage drop between our DC power source and the coil.
Vg is the voltage at the terminals of our generator. Vg_cons is the value we configured the generator with.
Ig_cons is the current value we used to configure the generator.
Vg_mes and Ig_mes are respectively the voltage and current value we read on the generator when outputting power.
Vmulti is the DC voltage value we read on the multimeter.
Vosc is the value we read on the oscilloscope, it corrresponds to the voltage plateau value after rise.
Vosc_max is the maximum value we read on the oscilloscope due to overshoot.
|Vg_cons (V)||Ig_cons (A)||Vg_mes (V)||Ig_mes (A)||Vmulti (V)||Vosc_max (V)||Vosc (V)||t_10-90% (ms)|
This the table with the mean values we measured for the last configuration of the DC voltage source we used. We tried with lower values of Ig_cons, but after a moment we noticed each time that they are not consistent enough to be acceptable for the project. It works but requires too much precision to be easily scalable for 256 marbles.
We explained on this post why we had a difference between Vg_cons and Vg_mes. These values are in check with the coil resistance given by the constructor.
There’s still something wrong that we’ll need to further investigate, according to these measurements we have a 1V drop between the DC source and the coil, which is unexpectedly high.