In the Interest of Future Flying Objects: XBypass

Despite losing the Spruce Goose of Quadrotors to an unfortunate semi-controlled flight into terrain where I was the terrain (from which I’m still recovering… the square area of skin has recovered by more than 75%), I haven’t quite given up on the prospect of building a flying thing yet – but for now, they will be smaller and more easily batted from the sky. For instance, I’ve figured out why Ballcopter never worked properly. The very last update left it in a state of limbo, which I can sum up in one sentence: No, it didn’t work, but more on that later.

Eventually, Ballcopter was scrapped as its foam structure was slowly eroded away by virtue of existing in MITERS.

But there will be a day.

In the mean time, I wanted to satisfy one of my persistent self-stabilizing-flying-thing itches: how to pass control inputs from a handheld  radio to the flying thing in question without having it process several consecutive servo pulse lengths. For the most common & simplest Arduino-compatible balancing algorithms, this limits the maximum control update frequency to 50hz, since the microcontroller has to actually measure 10 milliseconds of pulse commands (for, say, up to 5 or 6 channels) out of every 20 milliseconds. The remaining time is often taken up by software floating point multiplication and division.

The reason it takes so much of the loop time is because the abomination that is Arduino’s pulseIn() command. It’s a “busy wait” loop that sits and increments a counter while the pulse is being detected. It’s neither interrupt based nor uses the hardware timers (the loop is timed empirically). So while taking in a servo pulsewidth, the microcontroller can literally do nothing else. A little inefficient, in my mind.

More sophisticated ATmega based flight controllers like the Ardupilot use the chip’s hardware timers and internally generated interrupts. For instance, as far as I can tell in the Ardupilot, Timer 1 is constantly running (counting) and overflowing (resetting to count more), and pin change interrupts on the input pins are used to capture snapshots of the counter status, and the resultant pulse width is just the difference between the values when the pin changed from low to high (pulse started) and from high to low again (pulse ended). And if the counter overflowed while the pulse was high, no problem: just add the max possible value of the counter to the pulse width variable and keep counting. Yes, I spent a few hours reading Ardupilot source code to figure that out.

And no, I’m not going to steal it and use it, because it’s written all in straight Atmel C, and so it might as well be gibberish to me. I can read and  understand it, but hell if I’m going to write it.

One method around the onboard servo processing problem is completely bypassing hobby radios as an interface medium, which people often do with joysticks (with or without tethering to a computer). For example, there is the Arbotix Commander, which is essentially an XBee and Arduino in a completely unergonomic, overpriced package which seems to have functionality problems based on a few reports from my peers – I had the displeasure of dealing with the first version personally. Shane tethers his USB Playstation controller to a laptop, but that means the laptop has to be deployed whenever something has to be operated (The upside is infinite debugging and telemetry ability, which a plain R/C radio doesn’t give you).

So really what I would like is the ability to replace the transmitter-air-receiver-servo_pulses interface with something much more direct – like serial over wireless; something that just takes the transmitter inputs and pipes them directly to my vehicle, where it can be buffered and read at will. XBees make the serial over wireless problem very easy to solve. The result would be an XBee using a cheap R/C radio as a brainless “trainer box”: most radios have a “trainer port” over which servo pulses can be sent (over a cable) to a master radio, for situations like when you’re actually training someone who is utterly clueless about flying things (me) and want to take command of the vehicle if anything bad should happen. If this pulsetrain is read and parsed at the transmitter, then I effectively offload the overhead of processing servo pulses from the vehicle – allowing it to process much faster (hundreds or thousands of Hz) and making closed loop control less granular. My inputs would still be updated at 50hz, of course, but I doubt I can react to an event as fast as 0.02 seconds anyway. The vehicle can keep itself stable much faster.

Anyways, that’s exactly what I built. An Arduino Nano, reading the pulsetrain from a transmitter trainer port, and piping the read values as bytes over to an XBee.

It’s not nearly as epic as I made it seem to be, seriously.

I have two cheap 2.4ghz transmitters, one of which I got ages ago for Cold Arbor, and another one I picked up over the summer for Ballcopter. Which really means I have 2 of the same radio. They are both made by FlySky, but the right one is a Hobbyking rebadge. These things cost $28 brand new, with receiver, and some of the consequences of that cheapness show through – for instance, they don’t come with external servo reversing switches like every other radio, but the holes and pads are present internally to use them and if you mount your own switches the functionality is even there in the microcontroller, but no we will absolutely not spent 20 more cents to install those damned switches.  They have a 4-pin mini-DIN (also known as S-Video) connector on the back which is a trainer port.  And as usual with small Chinese gadgets, there is a community devoted to hacking them. It took me all of 10 seconds of Googling to find the pinout of that trainer port.

A little scope probing of pin 1:


The PPM pulsetrain


And zoomed in further.

The biggest difference between this “PPM” signal and the servo-style “PWM” (which isn’t even “PWM” but pulse-duration modulation… which collection of idiots agreed on these terms?) is that

  • It is falling edge driven – the time between falling edges of the square wave is actually the duration of the “PWM” pulse. When the receiver detects one of these falling edges, it will stop outputting the pulse on one channel and immediately start it on the next.
  • As a result, the actual “High” time of the pulse only goes between approx. 600 to 1600 microseconds on my radio
  • The low time is fixed duration at 400us. Makes sense – the minimum pulse lengths on the servo side, then is roughly 1000 to 2000us as it should be.
  • It “idles high” – meaning the gap between signal is +5v.

So I’d have to come up with a way to distinguish a signal pulse from the long idle pulse. Arduino’s pulseIn() doesn’t even allow in-pulse timeouts. It allows pre-detection timeouts, so you don’t wait forever for a pulse if it never comes, but it does not have “hey, this pulse is over the length I care about… let’s move on with life”. I’d have to get around this by throwing away the first update and “syncing” to the rest of the pulses, since I know how many there are and about how long they should be.

The first step of the hardware hacking was disabling the internal radio. As I found out while building the Deathcopter, 2.4ghz radios will tend to step on eachother. I didn’t want to permanently take this radio module out, however. Leaving myself the chance to return this radio to ‘normal’ duty if I ever figured out something better, I just added a pin header connection to the power supply wire. It would be left disconnected when I’m using my hack.

Next was throwing the hardware together on a perfboard. This little arrangement costs way more than the transmitter ($17-34 for an Arduino Nano and $22-25 for an XBee regular 1mW), so I really hope this isn’t the terminal solution. It’s also way underutilizing the Arduino right now, since I only have Tx and Rx pins connected, and one other digital pin hooked up to the PPM output. The transmitter provides 5 volts from the trainer port, so this board has no other power supply.

The XBee is mounted on an Adafruit XBee Adapter.

And that’s it.

Yeah, I didn’t even bother trying to source a 4-miniDIN for this. At least, not until my Digikey order arrives – until then, it’s wires jammed into sockets.

I took a few minutes hours to write the single pin read, serial write software for the Nano. The hard part was, again, thinking my code does one thing but really having it do something different – just logic errors, all caused by having to differentiate the long idle pulse from the servo pulses. But at the end of it:

There we go. I’m only piping back 5 channels out of 6 here, but this was successfully done over the air between 2 XBees. This is all done using Serial.print(), which converts everything to ASCII and thus has unnecessary overhead. When commanding a vehicle, I’d just send the raw byte values because I’m fairly sure my vehicles are not literate.

So how do I test this contraption? With someone elses’s flying object, of course.

4pcb became the subject of a few transmission tests and a whole lot of floor whomping, as seen in the test video:

While not the most economical option, it does work, and it does solve the problem I wanted to solve. I’ll probably look to refine this board or permanently mounting it inside the transmitter, possibly with a plain ATmega chip by itself so I at least don’t have to buy even more Arduini. Otherwise, creating a dongle using Arduino USB host shields and an XBee would open the floor up to using just about any gamepad or joystick as a remote.

The “XBypass” sketche as used on 4pcb is here. The general idea was to wait for the first time the 9ms long timing pulse was received, then immediately start taking in n channels, storing them, and then processing them into single-byte values (for transmission convenience as well as because 4pcb takes such single byte commands).

Hub Motors on Everything, Part V: Hub Motors on Even More Things

A month ago, I received 12 skatemotor-sized motor cans that I had commissioned through mfg.com by a semirandom Chinese machine shop. It was mostly an excercise of curiosity, but with 12 motor cans and no other motor parts, I decided to keep going with it – I went back to the semirandom Chinese machine shop and had them fabricate 12 sets of skatemotor parts: both endcaps, the center shaft, and the big ring nut thing that mounts the wheel.

That’s alot of little aluminum bits. What I have now learned is that someone else will actually hold the tolerances I indicate on my drawings, whereas I usually would just ignore them. The result is that the endcaps fit very tightly in the can (one is supposed to be looser) and the shaft-bearing fit is also a little tight, but it’s within sandpapering reach, not second lathe operations. The next time I do this, I have to specify a different tolerance band… though for now I do not mind making a single lathe pass and freeing up said endcap, or heating up the bearing and freezing the shaft for a real bearing fit.

Before I sent the dimensioned drawings off for quoting, I did a little re-engineering of the motor in the interest of easier assembly.

The shaft was remade as a straight pass-through, single-diameter part with key slot to clear wires. I actually designed it to mimic a 15mm metric keyed shaft such that it can form the starting stock, and the only operations done to it would be drilling and grooving.

Otherwise, the method of axial alignment has now switched to retaining rings. Yes, I know – snap rings are terrible, and I hated them the most when I was younger and eagerly disassembling appliances which always seemed to use them and I had no retaining ring pliers to cleanly remove them with and and… well, normal circular snap rings didn’t fit in this design anyway. I designed in two “low clearance” retaining rings (e.g. McMaster 97414A670) on each side of the stator. The inner ones keep the stator aligned, and the outer ones both space the bearings out and act as a restraint to prevent wire insulation from grinding on the bearings. I think this is a nice and simple system that is easy to assemble.

The stator is still a 50mm copier stator mounted on a custom 3d printed adapter hub –  there is no more “stator bore” shoulder on the shaft. I did try getting quotes for fully custom punched and stacked laminations designed to a slightly different geometry with thicker teeth to maximize torque production, but it would have cost way more than I wanted to spend (or had) – $1200 for tooling alone. While the stators would be cheaper per unit afterwards,  I think my best bet is still to find a supplier of completed (and epoxy coated) stators. The problem is that most places want minimum orders of 1000 or 10000 – small quantities for this kind of value added item is always difficult, and even if they cost a dollar each after that I still can’t dump that much.

The endcaps also got a little fatter. To fit the extra width of the retaining ring and wire-clearing gap and to give myself more room to put windings, I pushed out the interior width by 6 millimeters, necessitating protruding the endcaps out on each side. This makes the motors 42mm wide – while that’s just as wide as the old version 3 RazEr wheelmotor upon which this design is based, it does make them incompatible with the existing RazErBlades frame.

Gee, my stuff is always so much shinier when someone else makes it. Here, I’ve arbor pressed the static endcap into the can and pressed bearings into both. The bearing bore is actually a somewhat tight slip fit – again, another fiddle factor to adjust – so I dropped them in with a dose of 609 Loctite.

I have 2 sets of custom curved magnets remaining from the ‘blades project which didn’t see use in the final vehicle because I ran against a deadline. This set installed perfectly – the final one was a not-too-tight press fit into the bore, so this set doesn’t even need any glue to hold it in. However, it IS a little close for my comfort – there’s no guarantee this diameter will be the same across all the cans; so again, a dimension to change very slightly (on the order of 0.03-0.05mm or less).

I only have 1 50mm stator (the one shown), so either I’m gonna need to buy some more copier motors, ravage some more recycled office equipment, or find a source for them Yo Real Quick. I’m not keen on selling them to strangers over the internet just yet due to these sourcing issues, but it’s either that or build infinite hub motor powered vehicles…