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).

The Decommissioning of the Quadrotor Project

I bet everyone forgot about this thing!

I almost did, anyway. After the last test some time in July ended in one fan cracking its motor mount, fortunately not resulting in catastrophic failure, I lost my enthusiasm for flying objects and literally hid it under my cart of parts and stuff – as in, it fit perfectly between the wheels. I pledged that one day I will once again have a quadrotor boner and return to finish it off, since I did have a spare fan and all.

Well, I figured Thanksgiving weekend was as good as any. This year, MITERS seems to have accumulated a fair number of freshmen and newbies with quadrotors (and trirotors). The pressure was on to just finish it once and for all. It was a roughly 1 hour job to pull the damaged fan casing out and mount the spare, including printing a replacement tailcone.

noise isolation

During the testing of the Z-axis yaw rate control loop last time, I noticed a fair amount of mechanical noise coupling into the gyro readings. The gyros are mounted facing the same axis as the fans, almost in plane with them, and essentially on the end of a long carbon fiber stick. While the filtered readings were clean with stationary fans, they would become very erratic as soon as any power was applied. This was clearly visible in the test videos where the fans are jiggling quickly almost like they’re loose. The noise was of the same order of magnitude as the signal I was trying to get from the gyro – as a result, the filtering was not very effective. That was number one thing I was out to address before I moved onto the other axes of control.

I elected to remount the center Arduino Nano carrier board using memory foam – a tip I picked up from tricopter-nick, which is in fairly wide use.

With all 4 fans functional again, I was able to find out that the noise coupling was significantly lessened. It was still causing some trouble, but only when fan speeds were significantly higher than before – the “barely-touching-the-ground-hover” was mostly erratic jitter-free. Refactoring the yaw rate control code to not pre-scale the gyro readings also made the control less granular (and consequently even less shaky), and I also implemented a 2nd-order filter instead of a 1st-order. As a result, I was able to increase the loop PI gains by 2 times to yield a faster response to stick inputs. A little bit more driving on the ground like a doped-up robot on ice confirmed that the updates worked.

So now I was at the same external functionality I was 4 months ago. Yippee.

The only way I could get more data and test if the filtering and shock mount actually mattered was by continuing with the rest of the stability code.

how i killed the giant quadrotor

I hit the reset button on the Arduino. Actually, to be more specific, I left the controller calibration code in and then absentmindedly hit the reset button on the Arduino.

On occasion, I’d notice the fans peg off to one side and not respond to rudder stick inputs. They would some times recover again, but usually it required a power cycle to fix. I have a sneaking suspicion that my 2.4Ghz FHSS transmitter and the 2.4Ghz FHSS Xbee modules I had as a data link were Frequency Hopping over eachother’s Spread Spectrum at times.  Regardless, my usual rigorous procedure is to completely power down the electronics and then restart them.

The number one mistake was not commenting the calibration code out after the first time. After I temporarily decommissioned it the first time in july, I had “borrowed” one of the Turnigy 100A controllers for melonscooter. With melonscooter having moved onto greener melon vine patches,  I returned that controller to quadrotor duty, but it had to be recalibrated again.

The calibration procedure for these R/C controllers is full-stick within 1-2 seconds of powerup (2000uS pulse width), hold, then neutral stick (1000 or 1500uS, depending on your type of stick). This has to be done within 2 seconds of powerup, or else the ESC initializes and waits for command. The problem is when 2000uS is commanded after the ESC is already on and initialized, that means gun it.

For reasons I can only explain as “meh”, I elected after testing the yaw rate controller for the last time and seeing it lock up to just hit Reset on the Arduino to clear all the readings and start over, without powering down the ESCs. About 1 second after I did so, the stunning realization of idiocy hit me – along with the entire thing.

I only really remember leaping back as it flew upwards and sideways towards my head – keep in mind stabilization code wasn’t even written yet – and batting it out of the air with my left arm before ducking and turning around. It did a powered somersault over my head and crash-landed on top of a substantially more solid vehicle, and consequently shattered into many pieces.

It took me a second or two to realize what had happened, upon which I leapt for the big red key switches of main battery disconnection. The two remaining fans were still spinning – a consequence of me knocking over the radio – but luckily only slowly.

Pieces of fans were being collected from across the room afterwards – this destroyed rotor was found a few feet away with motor still attached.

I didn’t escape without damage. Batting the Deathcopter from the sky, I was grazed by what must have been a ducted fan leading edge:

In what is probably the luckiest near-miss I’ve ever been in, I’m only missing about 3 square inches of skin from my left forearm where the fan scraped it off. There’s 3 very evenly spaced cuts where I presume a rotor dinged me while disintegrating, but none were deep. Bleeding was slight, so the damage was cleaned, bandaged up, and ice-packed. It could have gone way more wrong in many possible alternate scenarios.  I’m still periodically testing my left hand fingers and wrist for full motion just in case.

What’s the lesson for other quadrotor builders who use RC controllers?

Well, besides don’t be a total dumbass and violate your own safety procedure,

calibrate the motor controllers once and cross it out of your code after.

This accident was totally avoidable if I wasn’t trying to rush to get to writing the stability controller.

Nevertheless, with 2 fans completely destroyed and the other two now in unknown condition possibly with structural damage, and the frame in many pieces, I think this project is done. Maybe I will instead concentrate on other flying things, or maybe I’ll build a more normal quadrotor or a tiny and cute one I can swat without worry.