Drone Adventures - Part 1Tuesday, Aug 15, 2017 · 1600 words · approx 8 mins to read
Following the last big crash of my drone, and the subsequent repairs to the point where I think it’s ready for some fresh test flights again tomorrow, I’ve learned a few new things. So, I figured I might as well write down all of the big drone assembly, setup and flying lessons that I’ve learned so far, for future me and any other interested parties.
Receiver antenna wires are a certain length for a reason. Turns out they need to be 1⁄4 wavelength, which for 2.4GHz is 299792458/240000000/4, or 3.1228cm. I accidentally chopped both receiver and ground wires in the accident and both were the wrong length. Expose 31.23mm of the antenna by carefully cutting off the rubber jacket, then also remove the conducting shield to expose the insulated core.
Make sure both exposed antenna sections are the same length, and get them out of the way of your props! This time I strengthened them a bit too, by heatshrinking over the shielded antenna and a section of semi-stiff cable tie.
Thanks very much to Simon for helping me debug this long-standing, enthusiasm-sapping, might-throw-it-all-in-the-bin problem!
Check your wiring and solder points
The large current through the power delivery system to the motors will potentially affect your solder joints for your wiring. Things get hot if your motors are powerful! On my frame, I’d done a poor job of the soldering the wires feeding the battery eliminator circuit (BEC) that provides 5V to things like the flight controller and LEDs on my particular build.
I didn’t have a short, but it was close, and I couldn’t see it because I’d taped over it for moisture protection. Every now and again just check all your wiring.
BEC output point fixing
If you have an outboard BEC because your in-frame or separate power delivery board (PDB) doesn’t have one integrated, the one you have might be variable. Mine had been knocked off of 5V slightly. A blob of glue when you’ve set it will do the job of stopping that happening again.
Make sure your multimeter has good resolution for tuning your BEC, has fine test points for probing the smaller components while debugging, and supports a path test mode. I got a nice Fluke on eBay a while ago (a 179, without thermocouples). They’re expensive when new, but eBay seems like a good source of well looked after second hand ones at good prices.
Mark, thanks for pointing me at eBay so I could join 179 club!
You’ll crash a lot when learning to fly, so spares for most things are a good idea. That especially applies to props. I’ve gone through more cable ties, heat shrink and insulating tape than I care to mention when building and rebuilding (twice now since the initial build!), and I’ve needed a spare BEC, spare FC and wiring harness, spare charger for the radio which I lost while moving house, and I almost needed spare antennas (see above).
You’ll definitely need more than one battery, and many spare sets of props and locking nuts. Take at least the props to the field when you fly, unless you want to go home early. I bent a prop in under 60 seconds once and didn’t carry spares. Don’t be like me.
Carbon fibre frames can affect RF performance. Be savvy where you site your antennas. Googling and forums tell me that 90 degrees between receiving and ground antenna is good, and you can get mounting hardware for that, but I’ve never bothered. Having them touch or be very near a carbon fibre frame is a bad idea though.
FC USB ports
Don’t be like me and leave micro-USB to USB-A OTG adapters connected to the FC when you fly, to make it easy to get telemetry recordings off when you get home. You will crash and that will want to come loose, ripping your FC’s micro-USB port off.
Plan ahead before you solder any connectors to anything, or wires to pads. My first rebuild was almost entirely down to making terrible choices with initial wiring. Pins need to be easily accessible. Remember component and pin heights are non-zero.
Your transmitter radio (Tx) controls will drift a bit over time, affecting flight. Every time you have the FC connected to your computer for tuning or telemetry download, get into the habit of recalibrating the limits of your Tx controls, so that centre means centre and zero means zero.
Building my drone was also my first adventure into soldering. I started with a horrible Maplin iron. Do yourself a favour and get yourself a really good one, since you’ll use it a lot and it’ll really come in handy for other electronics projects. If you’re anything like me and haven’t really soldered anything before starting with a drone, the combination of battery-powered tiny computers and wiring will open a great many doors in your brain to other hobby electronics projects.
I got a Hakko FX-888D. It’s amazing and I highly recommend it, and it’ll probably be the only iron that I ever need to buy. Thanks for the recommendation, Simon!
Propellers on a quadcoptor are motor-specific, because of how they generate lift and the FC controls them and the motors during flight. Google for the correct placement of left-handed and right-handed props on your quad, and then take a note of that for future times when you have to replace the props during a rebuild or because of a crash. Get it wrong and your craft won’t fly too well. That’s very bad when each motor can have upwards of 1.5kg of thrust with the right props.
The motors you’ll get for a racing quad are incredibly powerful. Never ever bench test your quad with props attached unless you absolutely know what you’re doing (and even then, really just don’t do that). Flight controller configurators have warnings all over the motor test controls for a reason. You can seriously hurt yourself and those around you firing propped motors, especially on the bench as you build and tweak. Props off unless you’re flying.
Talking to the FC
It’s highly likely that your flight controller exposes a virtual serial interface over USB, for any configurator software to talk to. You’ll almost certainly need USB drivers for that. Read the manual for your FC closely to figure out where to get them.
Most FCs for quads and other multi-rotor craft run a firmware called Cleanflight, so you’ll use the Cleanflight configurator to configure the firmware and flash it to the FC. See the item above about the virtual serial port drivers you’ll undoubtedly need.
The Cleanflight configurator is a Chrome app, so you’ll need a copy of Chrome installed to use it.
Cleanflight isn’t the only multi-rotor flight controller firmware. There’s also betaflight, forked from Cleanflight, which also has a Chrome-based configurator.
Because they’re Chrome apps and need serial port access to talk to your flight controller, programming it in the field will most likely need a laptop.
If you’re all fancy and your craft has Wi-Fi or BLE connectivity, there’s an iOS app called Mobile Flight that can configure and monitor Cleanflight, betaflight and other compatible Cleanflight forks.
To control the motors, your flight controller sends the motor’s ESC (electronic speed controller) commands that control motor direction and power. In order to map your transmitter inputs to ESC commands, the FC reads inputs from the Rx and translates them to the right motor controls as part of the firmware’s main loop.
To disambiguate each input (throttle, pitch, yaw and roll) in the data sent from Tx to Rx, each one is assigned a channel. Those channels are pretty arbitrary, so when configuring your FC to control your motors, make sure that you can read each input channel correctly and separately (you can see all of the channels in Cleanflight) and then map the channels appropriately so that they line up.
This is where making sure your FC can talk to your Rx properly is key. Choose the right bus and channel setup. If you run an FrSky Taranis radio and receiver like me, you probably want SBUS.
Configure a switch on your radio to arm the motors. If your FC runs Cleanflight, it supports that functionality. Then on your radio, configure a failsafe for that switch too, so that the radio moans at you if you start it up with the failsafe arm switch in the wrong position and the throttle not at zero.
That means you can never arm your motors without explicitly doing so, before or after you power on your radio. Surprise motor operation is a terrible idea. See the sections about props and motors above to remind yourself why operating 4 or more motors with huge thrust by mistake or in the wrong way can be the ultimate health hazard.
These days you can get software for PC and Mac that let you connect your real radio controls and fly a simulated quad around a practice field! Perfect for rainy days or when you want to try a new aerial move without the inevitable crash. I use FPV Freerider, but there are others (droneSim Pro comes to mind). FPV Freerider is just $4.99.
Like the FC, you’ll probably need USB drivers before your radio connects properly to your PC or Mac.
If I learn any more lessons or big stuff you’ll get a part 2!