For the last while, Phoebe’s drift in non-windy conditions has always been backwards. Until yesterday, I’d written this off to poorly calibrated X-axis accelerometer leading to slight horizontal acceleration while Phoebe thought she was stationary. This horizontal acceleration is only limited by friction through the air so can soon get out for control.
But yesterday, a conversation outside the blog with Phil, combined with a dislodged battery got me thinking about another possible cause..
Now the position of the battery in her frame shouldn’t cause a problem; the PIDs should handle any physical imbalance and resultant drift. But what if the props produce a small lateral force in certain situations in addition to the significant vertical one, or perhaps the vertical force is not equal in some way…
Bear with me now while I start some unbounded speculation based upon the theory this lateral force does exist…
Individually, when spinning in free space, this lateral force is balanced across a propeller blade pair having zero net effect.
But put an object near the single spinning propeller, and there is a reduced amount of lift as the blade passes the object. This is why helicopters always crash into buildings in the movies. However, across four props spinning at the same speed, at the same distance from the object, the net force is still zero. In this example, that object is Phoebe’s perpex dome protecting her and her circuitry.
There may also be a similar effect due to one pair of prop neighbours spinning in opposite directions, producing an imbalanced lateral force in the zone at the closest point the props pass each other.
Now imagine the quad is slightly unbalanced, perhaps due to a battery being set back slightly from centre making it arse-heavy. To acquire and maintain horizontal hover, the rear blade pair spins faster compared to the front pair to lift the extra weight of the battery.
But in doing so, they increase the lateral force of the back pair of blades. Depending on the blade rotation direction this lateral force could either over correct the unbalance, or counteract the balance. Either way, the quad accelerates laterally, and only friction limits the resultant drift speed.
In a normal quad with a human feedback loop, this can be corrected by setting a fixed horizontal velocity compensation in the transmitter, but autonomously it cannot meaning the only solution is to make sure the quad is physically well balanced.
So on reseating the battery yesterday, I took a little extra care to ensure the battery was seated as central as possible. And as a result, the drift ceased to be as significant, and now was more in a left-right direction (the reseated battery was still skewed that way).
And here’s where the blu-tack and pound coin came in. There’s enough space on the flange of Phoebe’s dome to add small weights – in this case a £1 coin. And things improved further. Not perfect but for me to tune the weight balance more, I need to sort out the scaling problem between the accelerometer axes first, as Phoebe is climbing to 2m+. Thank heavens for her yellow sponge ball!