Why is Phoebe so manic?

Given that Phoebe and Chloe are so similar, why is their behaviour so different?  Chloe is like a Mum: calm, mellow and thoughtful; Phoebe is like her 3 year old daughter, bouncing around, screaming with delight right to the point she hurts herself and starts crying*.

I’ve been thinking about what could be through cause of this behaviour; the HoG’s are identical, which means hardware.

Motors:

  • Chloe’s has T-motor MN3501-16 12N14P (12 coils, 14 magnets)
  • Phoebe’s has T-motors MT2216-11 12N14P (12 coils, 14 magnets)

Because they have the same coil / magnet configuration, if they have the same ESC with the same PWM pulse width feeding it, they will rotate at the same speed, so this isn’t it.

Props

  • Chloe’s has T-motor 13 x 4.4 CF props – the larger of the two sizes recommended for use with 11.1 batteries and the motors (above) she has
  • Phoebe’s has T-motor 11 x 3.7 CF props – the middle of the 3 sets of props recommended for use with 11.1V batteries, and the motors (above) she has

So assuming they use identical PWM and ESCs and the motors above with identical coil / magnets, how much power to their props generate?  Dunno, but given Chloe weighs more than Phoebe, and has longer arms than Phoebe, there’s some logic that suggests she needs proportionally larger bigger props, which she does.  So this doesn’t seem a likely cause.

ESCs

  • Chloe’s has T-motor 30A opto ESCs
  • Phoebe has DJI 30A opto ESCs

The translation of the PWM pulse to the motors coil switching is very specific to each ESC – the details can only be found from the code running on the microcontroller inside the ESC so I have little to really prove it’s the ESCs fault except DJI have replaced the ESCs I use with new ones – perhaps related to Phoebe’s nutty behaviour – at least that’s plausible.

And last, but not least comes Noise

  • Chloe’s HoG is attached to the rest of her frame with very soft silicone grommets; there is no hard physically connect between her HoG and the motor / prop noise transmitted through the rest of the frame.
  • Phoebe on the other hand is directly connected to the rest of the frame and feels everything.

I already have silicone grommets on order (and they should have arrived today), so I shall be fitting those as soon as they arrive.

I may well also buy new ESCs for Phoebe that match Chloe, but I’ll need to check my bank account first as they’ll be £125 for 4!

Why bother?  Because Phoebe is running through props at probably 10 times the rate Chloe does, and so even a change in ESCs will pay for itself within a month!

P.S. While I was search for what 12N14P meant, I found this article about how to wind the coils of a 12N14P motor – worth a read if only for background knowledge.


*Through experience within earshot right know, the comparison I’ve made between Phoebe and Chloe, and my wife and daughter is absolutely accurate!

Phoebe vs. Chloe

So Chloe is working nigh on perfectly – each flight is as good or better than the recent video.

With the same code other than personal 0g tuning and PID gains, Phoebe is still a danger to herself and all around her.

The only real difference in hardware are the silicone grommets separating Chloe’s HoG from Chloe’s main frame noise.

I’d really like to get Phoebe up to Chloe’s standard, as she’s small and therefore easier to transport. Trouble is those grommet seem to be rare as hens teeth. The first source were out of stock, a US seller sent 3 instead of the 10 I ordered, but I do now have a UK supplier so I’ll have another go on Phoebe.

Wow, what an exciting day!

Despite the bitter temperature, Zoë’s been out lots over the past few days to see what the new temperature control was like regarding vertical climb during hover.  She takes just over a minute to warm the sensors up to 40ºC and for the sensors to stabilize regardless of ambient temperature. And for the first time since I swapped to motion control processing rather than angular control, she just hovered – she didn’t drift up or down during the hover phase:

Zoë in the snow from Andy Baker on Vimeo.

The ‘fix’ to the problems I’d been seeing due to wind chill was solved by a lump of blue-tak covering the MPU6050 to help keep it warm.  Well, that along with a complete rewrite of the start up code that waits for temperature and sensor stabilizationbefore anything else happens.  The transformation is amazing!

I’ve also recalibrated the sensors to be confident as she’d been showing a consistent slight drift to starboard. Clearly that’s still there but much better, though also clearly, I need to work on the landings / shutdown to make sure the ESCs have shutdown completelly before the PWM signal to them is turned off!

You might have spotted the golden foam dome has reappeared on her underside to protect the battery and keep it warm.  I’m not sure I’ve ever mentioned what the landing dome is: it’s a Slazenger Shortex foam tennis ball sliced up with an electric carving knife normally used for the Sunday joint!

I did run a few more flights after this one, but one of the blades had fractured, so they were a lot less good.  As always, it was a right hand blade that got broken, so no more flights until the new ones arrive, probably tomorrow.

Here’s the blade graveyard, just to give you an idea of how many I’ve got through in the last week or so.  These are only the ones where the blade was ripped completely off the prop.  Ones where the prop still has two blades, but with one damaged go into the bin indoors.  Each prop is £15.  Ouch!

Blade graveyard

Blade graveyard

Prototyping Zoë

Zoë is the child of Phoebe and Chloë, taking the best of both and adding a little bit of magic to the mix.  She’s only in a partial build state currently – there’s no point in completing the build until I’m extremely confident in Phoebe’s flights.

At first glance, the finished Zoë will pretty much like Chloë:

Chloë

Chloë

The difference is in the details – and specifically the base plate(s).  Phoebe and Chloë only have a single base; Zoë has two:

Silicone Sandwich

Silicone Sandwich

The lower (base) plate handles power distribution from the LiPo on top out to the ESCs on the arms.  The new intermediate plate lies just a few millimeters above, and carries just the Raspberry Pi flight controller.  This intermediate plate ‘hovers’ above the base plate on a set of silicone grommets.  These are extremely soft, hopefully providing extreme isolation from the power frame’s vibrations, thus allowing the sensors to pick up only real acceleration.  Other than the ESC cables and the micro-USB cable, there is no hard connection between the intermediate plate and the rest of Zoë.

The only slight concern I have is the placement of Zoë’s battery bank.  It can’t sit on the intermediate plate next to the Raspberry Pi as with Phoebe; its too heavy and will squish the dampers on that side.

Power bank placement

Power bank placement

The picture shows it under the bottom plate, but as the arms don’t have hands (see Chloë’s photo at the top), the battery bank would be what hits the floor on landing, and experience tells me it won’t survive many landings that way unless I’m careful!  On the plus side, the way I’ve oriented the bank, the USB B sockets is closest to the base plate given some protection from the USB B plug wrenching the socket off the PSB inside the bank.  I think it’s worth trying as any other solution seems very ugly in comparison.

Last but not least, here’s the top side view; currently, that’s Chloë’s flight controller; Zoë will have her own as there are changes I’d like to make to Phoebe’s controller too.  The nuts and washers attached to the silicone are where Zoë’s arms will connect; they have a rock-solid connection to the base plate right through the centre of the silicone grommets maintaining structural solidity, while the intermediate plate (top in the picture) chills out in her vibration free environment.

Top-side

Top-side