As explained in this post, Xavier and I worked on a setup to measure the motor speed. We had to laser-print a drilled plastic plate and some fixation for the dev-board with the optical sensor we’ve borrowed from LitSpin (TCUT1600X01).
The plastic disk is thin enough to slide between the emitter and the receptors of the optical sensor, and has a small aperture on the edge so that once per rotation the receptors see the emitter and their output goes high.
The optical sensor, which we thought we damaged on Friday, actually works very well. We had some bugs when integrating the dshot code to control the motor’s speed, but very quickly we were able to measure the speed, and to determine the correspondence between a dshot command and an actual speed at a given voltage.
However, even with the very small load on the motor, we are a little worried about the supply voltage. We noticed that if the voltage was too low, the motor couldn’t accelerate past a certain point. To be more precise, if we accelerate to a higher speed using a higher voltage, then drop the generated voltage, we can still remain at a relatively high speed. But if we then drop the throttle too low, we can’t accelerate back up. The lower-limit voltage is about 13V for the little laser-printed plastic plate, and reaches 16V with the grey PVC plate. It will probably be even more with all the PCBs.
That’s why we will likely have to change our battery from 4S to 6S, and closely monitor that the supply voltage of the motor doesn’t drop under the threshold that we will determine as soon as we get the PCBs and the whole 3D-printed structure. Our ESC only supports up to a 6S battery, so hopefully it’ll be enough.
We will keep you updated !