Tuesday, December 29, 2020

More probe testing

The probe testing with the "kite" proved somewhat disappointing, I'm afraid. The problem is that the airflow near the hood of the car is curved, so the tail feathers of the "kite" fly in one direction, while its nose is pointed in another direction. I did some testing and verified the angle difference was on the order of 5 degrees nose-down. There may be ways to fix this, and the "kite" does give us good natural stability, but for now I decided to set it aside.

So now we just have the probe mounted on the PVC pipe proboscis using two servo motors. Here it is looking a tiny bit sad and droopy:

It turns out the 12VDC in the car is actually a little over 14V, which over-volts the Dynamixel servos. I decided to instead use a 3S LiPo from my RC airplane days. The long wire to the Dynamixel servos caused intermittent data errors -- I'm suspecting I need to use a twisted pair, since I recall the protocol is basically RS-485 plus power. But no matters. I am getting data. Here's what's on my lap while Melissa drives the car:

My plan is to run a "find zero" routine to zero out the probe, then do a series of sweeps at various speeds with cruise control on. I got a tiny bit of data and it looks reasonable but noisy. I will do some more programming to make my data acquisition stuff resilient in the face of comms failures, and then go out for some more test runs when the roads are quiet. Stay tuned.

For now, this is what the probe looks like when it's "flying":

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