Redecorating the lounge ceiling

I don’t like the props Zoe uses that come with the T-motor Air Gear 200 kit; they’re flimsy plastic, they bend and the stay bent.

A quick ebay browse over the weekend turned up some 3 blade carbon props, with the required 5mm bore hole, and 6″ span compared to the plastics’ 6.5″.  Perfect.  They arrived in the post this morning.

So I gave them a quick try prior to packing up Zoe for the work engineering conference tomorrow; the result?  I’m redecorating the lounge ceiling.

Swapping back to the flippy floppy props and normal service was resumed. Phew!

I’d seen this in the past with Phoebe and that triggered the new custom PCB for her, after which she no longer had the I2C errors, and I was able to swap her to the 1kHz sampling FIFO code.  Yet while testing this outdoors yesterday, she still lept into the air until I turned the alpf filter down from 0 (460Hz) down to at least 2 (96Hz).

The fact Zoe now does this too with the CF props is scary and fascinating.  Once I get back from the conference, I’ll be switching her over to the CF props and flying her outside with diagnostics enable to try to track down why such a simple swap of props yields such radically different behavior!

But for now, I need to get back to repainting the lounge ceiling before the wife and kids get home and spot the distruction!

Bored, bored, bored, bored, bored.

to quote Vyvyan from the Young Ones.

So I’ve contradicted everything I’d previously said and balanced my blades.  Only took about 5 minutes per blade courtesy of a 10 year old Dremmel drill, and a buffing attachment. With carbon blades, essentially all you’re doing is skimming some of the gloss finish off the underside of the heavier blade as per standard beliefs.  However that in itself does raise a different concern – aircraft wings are deliberately matt to improve airflow by preventing the air ‘sticking’ to a glossy surface increasing the size of the boundary layer and hence drag.  All my ‘balanced’ blades now have 3 gloss surfaces and one matt leading to unequal drag between the blade pair on a single propeller and hence another potential noise source!  I just hope I never get bored enough to actually ‘matt finish’ the whole of each propeller and then rebalance them – that could be terminal!

I did do a couple of test flights this morning prior to the buffing, and there was a variety of subtle problem, all of which had accelerometer readings at its heart.  This affects both vertical take-off speed, and critically, the measurement of tilt angles.

As a quick reminder, measuring angles is a combination of integrating the gyro for short term changes + calculating Euler angles from the accelerometer for long term stability of measurement.  Both the gyro and accelerometer outputs pass through a digital low pass filter in the MPU6050, and finally the complementary filter merges the results together.

The accelerometer is noisy due to the rotation of the motors, and this morning’s couple of quick tests did show the noise clearly in the accelerometer diagnostics and the resultant calculated angles.  However, I must make it absolutely clear that I’ve not balanced the blades through belief this is the correct fix, but there’s no point spoiling tomorrow’s testing of the complementary filter’s tau versus the MPU6050 dlpf by not ruling out the impossible first, is there? 😉

P.S. – it’s the same matt surface idea that’s been taken to the extreme with golf balls and their dimples – the turbulent boundary layer around dimples reduce drag of the ball making it fly faster and straighter, as wikipedia so eruditely explains

To balance, or not to balance, that is the question

With new blades comes the question of whether to balance them?  Balancing is the process of ensuring each blade of a propeller has the same angular inertia (weight distribution along its length).  Balancing blades is stated as mandatory for reduction of noise, but no corresponding proof of necessity is ever presented;  belief alone supports the theory.  Personally, I’d rather be a heretic until proven wrong.

Balancing involves suspending the propeller axis between 2 fixed points such that if the blades are not equal in angular inertia, the propeller rolls on its axis towards the heavier side.

Then either the underside of the heavier blade is lightly sanded, or the lighter side has some tape added to the leading edge, and the test is repeated until there’s no difference in angular intertia, and the propeller does not rotate from the position it was place at.

I have no doubt it is possible to achieve a high quality balance – there are balances like the one pictured which suspend the propeller central axis very well aligned to the balancing tool central axle.  The two ends of the axle are sharp points.  The axel is shorter than the frame it’s suspended between – strong neodymium magnets set into the frame are used to suspend the axel.  Even with very light carbon blades, this provides a very sensitive and accurate method of balancing the propeller blades angular inertia.

Balancing blades

Balancing blades

But I have interlinked concerns as to the validity of the process

  1. what’s the distribution of mass along the length of the blade – it’s possible to balance the angular intertia while having very different mass distribution along each blade – imagine a long thin blade and a short fat blade, inertially balanced but absolutely no guarantee of equal mass nor mass balanced along the length?
  2. Even with the blades angular inertia and mass statically balanced, what about the efficiency of the aerofoils in motion?  Once you start adding tape or sanding, the aerodynamics of the blade changes – probably only subtly, but a change nonetheless, meaning one blade produces more lift than the other introducing noise that the original blade probably did not possess.
  3. The balancing happens per propeller, but each of a quads 4 propellers could have different mass, angular inertia and lift – while the PIDs can cope with that, it could lead to yaw.

So for the moment, I’m going to leave the new carbon blades alone, assuming that the manufacturing process is sufficiently accurate – I’ll only proceed with balancing if testing shows an definite requirement.

There also the factor that it’s boring to do, and carbon blades have much lower angular intertia anyway compared to plastic or wooden ones, so I’ll see if I can get away with not bothering!

Swapping motors, swapping problems

I’ve swapped the standard DJI motors + plastic blades back to the T-Motor + Carbon fiber blades; sure enough, no rocketting up into the sky, and once more I was able to complete a flight.  But…

all the old problems I had returned – excessive drift and yaw.

That suggests

  • the DJI motors are not powerful enough to manage the angular momentum of their heavier plastic blades
  • the T-motors are not all equal  – from the direction of the drift, I would guess the right-hand pair are damaged leading to drift and yaw.

So I have no choice but to buy new motors T-motors, and see what happens.  Hopefully that will sort things out and I can get back to real testing of my control software PIDs rather than diagnosing bizarre behaviour due to dodgy hardware.

So frustrating wasting my very limited testing time on something that turns out to be not my fault!

Propeller / motor overshoot

As usual, having a pause for thought has resulted in a plausible cause for the very different tests results between yesterday and today.

Both days I was using the cheaper, lower powered motors with plastic props. The only difference was yesterday the battery was charged to perhaps 11.5V – today nearer to 12.2V.

Imagine that the combination of heavier plastic blades (rather than carbon) combined with higher battery charge level creates sufficient angular momentum in the blades that the motor rotor overshoots the motor stator coils it’s supposed to just reach and instead it continues on to the next set. Depending on the number of coils, that would yield a signifiant increase in RPM and therefore lift.

I’d never had problems with the carbon blades; being so light they simply don’t carry anything like as much angular momentum. And the motors they’re attached to have higher Kv value (980 vs 920) and much stronger magnets meaning they had a tighter grip on these lighter blades. The result: lower angular momentum and closer control leading to much reduced risk of overshoot beyond the expected stator coil.

But in these tests, with heavier props and lower powered motors, it seems the motors simply do not have quite enough oomph to restrain the props angular momentum.

Convincing enough for me; time to move back to the better motors and carbon blades.

All I could do was watch in horror…

All I could do is watch in horror! from Andy Baker on Vimeo.

Net result, 3 broken blades (one in a previous flight and two in this).

Prior to that, I’d had several good flights, but somehow taking video and a good flying were out of sync.  Even in the one above, I’d turned off the “3rd person” video by accident as I saw the crash approaching, so only Phoebe’s point of view remains.

The drift into the railings is the breeze’ fault – in the hour I was testing, it went from still to a few miles an hour coming from behind Phoebe – hence the crash into the railings – completely the opposite directions to where she normally drifts.  She is clearly very wind-sensitive, and that definitely raises the game on sorting out the lateral drift code to compensate.  At the same time, it does suggest that if I am extremely careful to make sure the take-off happens with the motor tips as horizontal as possible, there is a good chance of no drift and a vertical takeoff – and that raises the option of moving the testing into the garage during winter.  Still scares me a bit though!

One thing that struck me (metaphorically, thank heavens), is that a vast amount of power is used for each flight.  First flight on a full battery levelled at 5 feet, next at 4 and the next at 3.  So I recharged the battery for the next flight, which is the one on video.

I have one set of blades left, but have just purchased some more – the current ones at 11 x 5 (11 inches total blade length x 0.5 inches pitch); the new ones are 10 x 4.5 – I’m hoping this will reduce the friction imposed on the blades and hence the power they take from the battery.  The flip side is they’ll need to spin faster to take off, so I’ll need to do some more minor tinkering on take-off.  They should work fine though as it’s this style blade that comes in the DJI F450 that was my starting point for this project.

Speedy blade balancing…

As the new test rig is working, I’m going to swap the quad blades from the slightly damaged set back to nigh-on perfect ones.  Nigh-on because they need balancing.  So I thought I’d share with you how I’m doing it, as it’s now taking just a minute or two per blade (compared to when I did my first few, which took up to 20 minutes each and, as it turned out, still weren’t actually balanced as well as they could be).

You need

The key part is the balancer itself.  The axle has sharp ends and is attached by super strong neodymium magnets at both ends; this gives virtually zero friction, and yet makes it simple to remove the blade from the jig for sanding.

Even the subtlest blade imbalance results in the propeller rotating until one blade taps on the work surface.  Once you’re sure which blade it is, then you can use the dremel to take a small amount from the underside of the heavier blade.  Then wipe it with a slightly damp cloth to remove the dust, dry it, pop it back on the balancer and check again.

You’ll soon develop a gut feel for how much sanding needs doing depending on how quickly one side of the blade hits the floor.

An here’s what you end up with:

Balanced blade

Balanced blade

One final thought: this time when I attach the new blades to the drone, I’ll be using nut-lock – although the blades have never unscrewed themselves (except when scrapping with the gravel), I’d hate for my first experience of it to be when they are 10m above my head!

Another afterthought: I’ve just balanced the stockpile of 10 blades I’d bought in preparation for many more crashes.  They’re all the same as the above picture, all sourced from China originally, although bought from ebay, and, yet each has subtle differences: some have been pre-balanced – you can spot this as one of the wings has signs of sanding; others have had the axles sanded too to ensure they are completely flat (a good thing).  So look carefully at any you have to make sure they are all the same once you’ve done balancing them.

No Raspberry Pis were hurt in the making of this film…

but once more the blades and the gravel had a major fight, which again, the blades lost!

I think there’s more work to be done here! from Andy Baker on Vimeo.

On the plus side, the blades didn’t slice the LiPo battery power cable, so no risk of explosion – you can see me in protective glasses and gloves this time just in case.

That then means I have diagnostics to interpret, but even without those, just the video itself suggests the pitch PID has too high a gain, meaning it overshoots it’s correction leading to ever increasing front / back rocking.  My code won’t cope with > 90 degree tilt so when the rocking goes over 90 degrees, the drone flips onto it’s head and accelerates hard towards the gravel.

Two obvious solutions spring to mind:

  • reduce the pitch, roll and yaw PIDs’ P gains and / or add a negative D gain to each
  • add support for > 90 swings – this is harder and unnecessary at this level of flight control if fixing the gains works.

Still not sure about the yaw though – far higher than expected still – I think I need to use the yaw angle PID’s I output to infuence the motors much as I do already with the vertical speed PID. But I’ll leave that for another day.


Balancing act…

So here’s a live run with balanced blades; still noisy, but nowhere as bad as yesterday’s unbalanced run. It seems I’m on the right track. I do suspect some pid tinkering is necessary too, but I need to check the stats in more detail first.

Balanced blades

Balanced blades

The observant amongst you will have also noticed the different blades. These are lighter and stiffer, so I hope the cause less vibration, and better lift, and survive impacts better. Time will tell. This balancer is very low friction, but it’ll still be more that the maglev ones I have on the way from ebay, so I’ll see whether those will show any further fine tuning I can do.

Blade balancer

Blade balancer in action