Drone On

Well, I finally built me a drone so's I could fit in with all the cool kids. What follows is a short description of my experience with helpful links for someone else who would like to build a substantially similar quad. I built basically the cheapest quadcopter you could use for anything more than just crashing into stuff, a SK450 whose parts were primarily sourced from eBay — and usually with the very cheapest parts.

Introduction

SK450 Quadcopter

For those who've somehow avoided experiencing them to date, a quadcopter is a flying vehicle with four horizontally-oriented lifting props, and for a basic quad, no control surfaces. Such a craft achieves every movement by varying the speeds at which the propellers are spun. Every other propeller spins in a different direction, and by increasing the speed of the props rotating one direction while decreasing the speed of the others causes the vehicle to yaw (rotate around the vertical axis). The other types of rotation are more obvious; increasing the speed of the rotors on the left side while decreasing the speed of those on the right will cause it to tilt to the right, and so on. There are fancier quadcopters which can change the angle of some or all of their fans, and even models built with small helicopter blades which can use collective control to do fun tricks like flying inverted, but I didn't build one of those. Such functionality comes with substantial costs in complexity and weight.

The Parts

Part 1: Powertrain

My process started by looking at what motor and ESC combos would be cheapest. By far, the cheapest thing available to me right now, which might be different next week, was XXD knockoffs of Neewer A2212/13 13-turn 1kV motors and 30 AMP ESCs and even two full sets of 10x4.5 ("1045") props. The whole schmeer was around $56 shipped from eBay seller "dodotech". I also later purchased a set of five (in case one is lost) new prop hub adapters from him for $6.66 (yep) because as it turned out, the stock ones included with the motors are a bit garbage. They don't have a centering sleeve, and the prop just sits on the threaded part of the shaft, which is no good. The ESCs have 2A linear BEC, so they can be wired straight to the KK board without worrying about disconnecting wires first.1

Part 2: The Frame

SK450 Quadcopter (bottom)

Picking the most expensive single component first (actually six components, but anyway) informed the rest of the build. 2212-size motors are best suited to a 450 quad, so I went looking for 450 quad frames. The cheapest one turned out to be the Neewer SK450. This is a knockoff of the original SK450 kit sold at Hobby King for twenty bucks; I got it for ten. The SK450 is a sort of evolution of the DJI Flamewheel 450, a nearly-identical quad which features two boards as opposed to three. The Neewer kit is functionally identical to the "real thing": there are three "boards" (circuit boards without metal on them) and four plastic arms, feet for the arms, and a raft of tiny screws. One nice thing about the SK450 is that nearly all the screw holes have pre-threaded inserts so there's no nut to lose, and all the screws are the same size. The only exception to this is the mounting hardware for the top board to the middle board: nylon standoffs with nylon nuts and black-coated metal #2 Philips-head screws.

HobbyKing (hence "HK") sells a solder board for the SK450 for only five dollars, and especially if you are already ordering other stuff from HK, I strongly recommend it. Making a five-way splice between your power supply wires and your ESCs is a real bear,2 and if you let the smoke out of any of them you're going to have to redo it completely! That is not unlikely with these el cheapo parts, but it also seems to be sadly likely with the expensive ones as well.

The other frame option you might want to consider, especially if you intend to hang a camera under your quad, is the "dead cat" or "alien" type. HK offers the "dead cat" kit as an upgrade to the SK450 for twenty bucks, and another twenty or so for the solder board with the power distribution/regulator for FPV. The inexpensive option is to instead go with Neewer's "Alien" frame, which appears to essentially be a dead cat knockoff.3 These quads move the arms out for more stability, and especially space out the front arms to accommodate a broad camera view with no props in the way.

Part 3: Flight Controller

People have strong opinions about all of these parts, but now we come to the really controversial part: the flight controller ("FC"). I wanted a FC with a display on it, and one which would be very small and light in weight, so I went with HK's "KK Mini". HK has now made three LCD-equipped FCs in this series; KK2 was followed by KK2.1 and then KK 2.1.5, which is being sold concurrently with KK2.1.5 Mini. KK2.1 and 2.1.5 Mini are based on the same hardware, including the same AVR chip. The mini has less pins (mostly redundant power and ground pins) and a slightly smaller display. It's suited for basically any size of quad, all the way down to a 250. It's a good choice if you think you might change FCs later, because it is very small and thus useful for a broad range of other copters. It can also be loaded with an Arduino bootloader, so you can use it for Arduino FC software.

Besides the display, what is interesting about the KK FC is the range of available software. It comes with some fairly sophomoric stuff, but steveis' replacement firmware improves it considerably. Besides Arduino, you can also load the KK with Multiwii, thanks to e_lm_70. Multiwii is more advanced FC software than even steveis' KK firmware, and you can use it to add GPS to your drone. e_lm_70's Multiwii for KK includes LCD support, but you can also hook up a bluetooth module and configure it from your phone.

If you choose not to use KK, there are many other options. You could rip apart a Wii MotionPlus, if it's an original one you can even cut the interface logic away from the sensors on the PCB, and then couple it to a $2.50 Chinese Arduino Nano... and load Multiwii on that. You could probably have yourself a FC under $5 that way. If you want altitude hold, you'll need a $2 BMP085 (or BMP180, the modern replacement) or you can spend $15 to get a uBlox NEO-6M GPS/MAG/Baro sensor package with its own antenna, config EEPROM and tiny rechargeable lithium battery for warm starts etc. I've got both on order so that I can try them both out, but the GPS is what I'll be installing on the quad. But I went with the KK, so that's what I'll cover here. There's no compelling reason not to use the KK for this purpose, but there are lots of options.

Part 4: Radio

In order to control your model, you will need a radio transmitter and receiver. The cheapest thing worth buying is the $25 HK-T6A V2, a six-channel controller HK has licensed from FrSky. There are only two real down sides to this radio. The first is that it has no display and not even reversing switches, so you will need the $5 programming cable (usually only available from their HK warehouse) to program your radio, which you need to do before you can use it for most purposes. The second is that the receiver doesn't have PPM output; it used to, but they took that functionality away. That means that if you want to use all six outputs with the KK (which only has five inputs) you need a PPM encoder module. That's only about $5 on eBay, and weighs little and uses little power, but it's still an annoyance. The other popular option is the Turnigy 9X, a nine-channel 2.4 GHz radio. This is literally twice the money, but it has a nice display right on the unit for configuration. It also doesn't have PPM output from the receiver, but it can be hacked to be compatible with receivers that do.

Part 5: Battery

The last component, though no less necessary than any of the others, is the battery pack. I already had a 2650 mAh Zippy Flightmax pack which I bought to use with my E-Flite Apprentice, and that's right about the ideal maximum capacity (given mass) for a 450-size quad, with an XT60 connector on it. I've been using Traxxas connectors, which carry a lot of current and are very cheap, but the XT60s are much nicer. Notably, you don't have to assemble them. Since I buy all my batteries from HK anyway, XT60 makes good sense. Deans always did suck. The battery was on sale from HK a while back, and maybe that was a closeout; you could aim for something like a $20 Zippy Flightmax 2800/30C, although that's backordered.

Construction

SK450 Quadcopter (ESC)

Construction was mostly very straightforward. I fit everything without loc-tite at first, which was good because I did have to take the bottom board off as I figured out how construction works. There's very little to know; connect the FC to the top board first, connect the bottom board last, and put in lots of screws. I used blue Loc-Tite stick, which is seriously wonderful. The only tricky step is prop balancing, which could cover an article in itself.4 You should get either the cheapest possible magnetic balancer, or a DuBro balancer with an extra rod for balancing quad props. A cheap balancer is five bucks, the DuBro will run you about $35 with the extra rod. The cheap balancer needs to be balanced itself (by filing the cones) when you get it, and will never do a fantastic job. I've also recently read about balancing the motors, which is even more fun and it's something I haven't done yet.5 Shock-isolating your upper board helps considerably, and will generally let you fly even with unbalanced props, but for the best results you're going to need to do as much balancing as possible.

I made some minor modifications to my equipment, and more are coming. The kit comes with four 20mm nylon standoffs which isolate the upper board from the middle board. I used those to mount my FC to the upper board, and used four silicone fan "screws" to mount the upper board to the middle board. That way, the entire upper board is vibration-mounted away from the working parts, and any electronics, camera etc. mounted here will be shock-protected. Eventually I'll come up with a plastic dome to cover the upper board. I have right angle header strips on their way, and I'm going to take all the pin connections off of the top of the KK board and move them to the bottom, sticking out the sides. Then I will mount the KK on shorter (~10mm) standoffs, so it doesn't stick up so far. I have plans for FPV, but my current FPV rig uses UHF TV broadcast and is probably not legal even with the radio technician's license I don't have yet.

Configuration

Software and reflashing

Setup was slightly trickier than construction. The first thing you should know is that multiwii for KK currently does not handle motor calibration, so you need to do that on the stock firmware. After binding your receiver6 you hold down buttons 1 and 4 to put the KK with stock (or steveis') firmware into pass-through mode. Then you calibration your ESCs as usual. For my XXD ESCs, and most others, that involves pinning the throttle while you wake them up, then minimizing the throttle within two seconds. After this step, you can reflash your KK to Multiwii. This is easiest done with KKMulticopterFlashtool, a Java application for Win/Mac/Lin which uses the command-line avrdude programmer to do the heavy lifting. It will even download steveis' firmware for you if you'd just like to use that, but you'll have to download Multiwii on your own. If you don't intend to use GPS, you can skip Multiwii, but it has superior PID control as well. In order to reflash, you will need an AVR ISP, or In-System Programmer. The best option is to buy a USBASP with a 10-to-6 pin cable from eBay, which should cost you around only five dollars.

Radio configuration

The HK-T6A v2 transmitter also neds to be configured, especially if you use Multiwii; besides calibration, you also want to set just the first channel to be reversed and all the others to be normal. I use the software Digital Radio to configure my transmitter. If you have a fancier transmitter, it probably has reversing built in. The first screen displayed when powering up the KK with Multiwii shows you sensor and radio input; 1000 is fully one direction and 2000 is fully the other, while the center should be at 1500.

Copter configuration

Finally, you have to actually configure the copter itself. The default PID tuning values will actually let you fly pretty well, but for this setup it looks like turning down the P value is necessary. When you turn down P, it is customary to also reduce I and D. PID tuning is another subject which deserves its own blog post, and preferably by someone more comfortable with mathematics than yours truly. But besides PID tuning, there are some other details to work out. If you are using a non-PPM RX, you will need to set the option "External RX" to yes. In order to get stable flight, you have to configure flight modes. This is really the only other tricky part of configuration.

If you bring up the flight mode screen and play with channel 5 (and if you have them, channels 6-8 as well) you will see the numbers moving around at the top of the grid. For example, I just have one AUX channel since I have no PPM. If I bring up that screen and twiddle the knob at the top right of my TX, I can see the "5" moving between the first three columns. So under these columns, I simply check the features that I want to be active when the knob is in the corresponding position. Features including LEVEL, HORIZON, BARO, RTH (return to home) and so on. If you don't have a GPS, you might reasonably set the leftmost position to LEVEL, the middle position to HORIZON, and the rightmost position to nothing at all, which means ACRO(batic) mode. In ACRO mode, the copter only tries to self-stabilize against external forces (e.g. wind) and it rotates when you move the tilt/pitch stick; on a "Mode 2" transmitter, the type we normally use in the USA, that's the right stick. The left stick controls thrust and rotation. In LEVEL mode (also called "stable") "the board will attempt to set pitch and roll according to the absolute angle of the input sticks". HORIZON mode is actually like ACRO mode, except that it will try to self-level when you let go of the stick. These are your main interactive flight modes. If you have a barometric sensor, you can engage altitude hold (Multiwii on KK calls it BARO) in any of these modes; you can combine it with them in the flight mode config, or if you have PPM and more channels, you can put it on a separate aux input so that you can turn it on and off independently. The last really interesting mode is RTH or Return to Home, which will attempt to return the quadcopter to the home position automatically. This requires GPS and MAG sensors (both included in uBlox NEO-6M) and a good MAG sensor calibration, and that you had a GPS fix at the the time you armed your quad.

Having configured, you can move on to calibration. Note that the quad has both accelerometers and a gyroscope. Accelerometer calibration is performed once (or if problems occur) while gyro calibration is performed before every flight. If you place the model on a flat level surface before plugging in the battery, and don't move it while it's powering up, it will perform the gyro calibration on its own. You'll know if you need to perform calibration because the red light on the top of the KK board will flash incessantly. After you calibrate, and if all is well, the light should stop flashing and you should be able to "arm", or enable the motors. Arm the quad by moving the throttle to minimum and the rudder (yaw) to the full right. Once you land, you can disarm by moving it down and left. Happy flying! Note that this quad is not only heavy enough to hurt if it hits you at speed, but it has plenty of power to cut the living crap out of you, plants, and other stuff. Don't crash into yourself, other people, pets, or anything you care about! But if that should happen, cut the throttle immediately, and do your best to disarm! Many transmitters have a throttle cut switch for just this purpose; if yours is one of them, make sure you know where it is.

Prices

This table is based on current eBay prices as of October 23, 2015. Hopefully, all of this stuff costs this much or less by the time you read this. The stuff above the subtotal line is what you need, assuming you don't already have a transmitter. if you do, then scratch the TX/RX and get a new RX for your TX, preferably with PPM output. This list does not cover minor incidentals like wire from your ESCs to your battery, battery connector, silicone fan standoffs, solder, etc.

ItemNameSourceCost
FrameNeewer SK450eBay$9.99
FCHK KK 2.1.5 MinieBay$19.99
Motors/ESCs/Props4x XXD A2212/13T+30A ESC+4pr 10x4.5 propseBay$56.75
TX/RXHK-T6A V2HK$25.10
BatteryZippy Flightmax 3S1P 2650 mAhHK$22
Subtotal$133.83
other goodies you may want/need
Solder BoardHobbyking SK450 Lower PCB Main FrameHK$5.89
BalancerCheapest possibleeBay$4.89
Isolators8Pcs Silicone Rubber Cooling Fan Screws Mount Anti-Vibration For Cooling PC CaseeBay$1.39
Prop Adaptersno-name, with centering collareBay$6.66
GPSuBlox NEO-6M w/EEPROM and batteryeBay$13.28
CF Props2 pair no-name carbon fiber 1045 propseBay$11.64
Plastic props10 pair no-name plastic 1045 propseBay$12.25
Plastic props4 pair no-name plastic 1045 propseBay$5.88
AVR ISPUSBASP 2.0 w/10-to-6 pin cableeBay$5.99
Fire BagHSP LiPo fireproof charging bageBay$3.69
ChargerIMAX B6eBay$20
  • 1. The KK has two power buses; one is connected to ESC 1 and that powers the KK itself, which passes power on to the receiver. The other is connected to ESC2-8 (I have 4, but there are 8 outputs, so you can run an octocopter with it) and runs only between them. If your ESCs have switching BEC, they can freak one another out with it and you should disconnect the power leads from the servo connectors of ESCs 3 and up. If you have particularly garbage switching BEC in your ESC it can confuse the KK board, and you will then need an external linear BEC.
  • 2. Get two big washers and two big bindery clips. Stack the two washers in the middle of your model on the box the KK came in, then bring your four wires in from the four different directions and clamp with the clips on opposing sides of the washers. If you have to adjust any of the wires' positions, pinch the washers with your fingers on the side opposite that wire and then you can wiggle it around. After you solder those together, bring your battery wire in from the bottom. Solder the battery connector last. Fun!
  • 3. I did see some complaints online about the rigidity of the arms on the Alien frame; one user solved this issue by mounting the arms from a DJI quad, probably a F450.
  • 4. The short story on prop balancing is that you put it on a balancer, then either remove material from the top of the blade on the heavy side, or add thin CA to the light side.
  • 5. You can allegedly either use a smartphone app (e.g. iSeismometer) or you can use a laser; either mount the laser to the arm of the quadcopter or put a mirror on there and reflect the laser. Move a piece of tape around the motor until you find the high point by trial and error, then you can use a Dremel (or similar rotary tool) to remove material from the opposite, heavy side.
  • 6. For HK-T6A v2 binding involves installing the bind plug which grounds the signal pin on output 1 of the receiver, then holding the range test/bind button while switching on the transmitter.

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