so behold her(mione):
Here’s a quick test flight just to check I’ve not broken anything; more details on Vimeo.
It all started with the props: I’d broken 6 of the CF ones in the last week or two (costing £110 to replace) and on hunting for cheaper alternatives*, I found the white beechwood T-motor props. They are half the price, stronger, and less likely to split on impact – the CF ones are actually a sandwich with a middle layer of what looks like wood fibre and any lateral contact with the ground and the sandwich splits in three.
The new props’ span is an inch shorter but with slightly higher pitch; after an initial test flight, it was clear they were more powerful as a result. They also look nicer, and that’s what triggered the rest of the makeover.
I’ve been looking for a chapeau to cover her beret PCB for ages (the PCB doesn’t conform to the standard HAT definition, hence beret), and over time I’ve built up a collection of yellow and orange plastic salad bowls as a result, but none quite fitted right. But with the new white colour scheme, I found one that fitted nigh on perfectly, just a little dremel trimming required.
Finally, the feet: these are rubber lacrosse balls. They are heavier and stronger than the previous yellow dodgeball foam feet that Hermione forever punched holes through on landing. The increased prop power more than copes with the extra foot weight.
I think the new look added nearly a kilo in total, and measuring her on the scales, she now weighs 4.6kg, so it’s amazing she takes off at all!
*The problem with most more-affordable props is they don’t fit upside-down on the ground facing motors in Hermione’s X8 format frame.
So the spoiler in yesterday’s photo was another quadcopter flight controller to the left of the keyboards, pending the release of a fantastic new range of quadcopter frames which is being launched next month. More on that once I’ve got my grubby mitts on one.
But that A+ controller has now been flushed down the pan with the launch of the Raspberry Pi Zero, smaller than an A+, double the memory and just as powerful in every other way. I really have know idea how they halved the size of the PCB and yet kept all of the function. And it’s £4 (yes, that’s not a typo, four quid for a computer that runs Linux happily!), and free if you buy this month’s copy of the MagPi magazine.
So I now need to redo the design for the beret board, to shrink it down to the Pi Zero size!
If you’re in the neighbourhood, pop down to the Cotswold Raspberry Jam in Cheltenham on Saturday and see the Pi Zero for yourself!
With some code checking for temperature stability, and corresponding PID tuning, I can now get the temperature stable to 0.1ºC and Z axis gravity readings have followed suit, stabilizing to less than < ±0.01g which is quite amazing frankly! On the graph, the temperature reached and maintained stability at the 131s mark.
That’s more than good enough for indoor flights due to the stable ambient temperature, but I currently don’t have the courage yet to risk that. I suspect that level of stability can’t be achieved outside in windy winter weather without wrapping the MPU6050 up nice and cosy. I have a cunning plan for that to try.
I’ve also progressed on the alternate plan for vertical height stability: including a barometer. My problem had been that the DroTek 10DOF IMU pin spacing didn’t fit a standard 0.1″ pitch breadboard / beret board. So rather than waiting until spring for the revised version, I’ve now updated the Beret board layout to incorporate the unusual pin spacing, and posted it up to GitHub. Ragworm are producing a couple of these for me now.
I’ve started to consider adding a barometer, even though it’s anathema to my core target of using just accelerometer and gyro (because consensus is it cannot be done but as yet, I’ve not found a single explanation why not). But if I rigidly stick with just the accelerometer and gyro I have a nasty feeling the project will just grind to a halt and I’ll start getting bored. I don’t do bored at all well.
What’s been holding me adding the barometer up is the pin spacing on my chosen IMU replacement for the MPU6050. There will be an improved version coming in the spring with the pin layout I need for 0.1″ pin spacing, but spring is a very long time away on my getting bored scale. Whereas I could just stop waiting for the world to conform to my needs, and instead, choose to fit in with the rest of the world; in other words, produce a new Raspberry Beret for Zoë that has PCB holes specifically designed to take the current IMU.
The changes are trivial, and I’ve already have a draft layout of the new PCB and components; I’ll be talking to Ragworm tomorrow about a quote.
In the meantime, I’ll start looking at FIR / IIR filters (thanks Phil!) to separate gravity from net-acceleration to cope with the sensor drift.
I also have a niggle that there must be an underlying cause for the sensor drift, and the prime candidate has to be temperature, so I’ll probably do some experimentation there too.
*The worm that turned
So if I can’t get the sampling up to 1kHz to average out vibration, perhaps I can do more to suppress the vibration physically before it gets to the sensors? Currently just 3mm foam tape sticking the Raspberry Pi to the frame is all there is. I’ve tinkered with additional vibration suppression in the past but gave up as there were much bigger problems to deal with.
And then just the other day, I stumbled on this site and a cunning plan hatched.
The idea is that the physical strength of the frame lies between the top plate and the top of each arm with 4 bolts per arm. But the Raspberry Pi and sensor Beret sit on the lower plate with 2 bolts per arm. If I could put vibration damping between the lower plate and the lower side of each arm, vibrations from the motors could be damped before reaching the Beret / sensors.
I already have the upper solution using rubber dampers from my previous attempt at vibration damping, but I prefer the lower solution using the silicone gel grommets.
Just one problem: both rely on M3 bolts, but the DJI Flamewheel F450 frame as used by Phoebe uses M2.5 bolts. ‘Luckily’, the T-motor extension arms as used by Chloe use M3 bolts.
The only downside is that to save cash, I need to splice Phoebe and Chloe together – Chloë’s hardware mind melds with Phoebe’s software and electronics to make a hybrid for which I don’t have a suitable name – any ideas?
P.S. Zoë = Phoebe + Chloë; // my best choice so far.