Monday, June 25, 2012

Multicopter Madness

R/C airplane modelling exists for several decades, but with late improvements and reduced cost in microelectronics, energy storage, and RF communications, products in this domain have suffered a significant improvement in quality and innovation.

We can owe it to the general move of the industry towards miniaturization and delivery of hardware for consumer electronics products, industrial tools, and applications in general requiring microcontrollers, sensors, reliable short range RF communications, motor control, etc. As such, the low cost of R/C hobby products can be associated both to the increasing demand (with chinese manufacturers/sellers pushing the standards), and the use of common components and manufacturing processes as much as possible.

This tendency towards low cost have enabled people with constrained budgets like me to have some fun in this domain, and materialize projects that years ago would be prohibitively expensive, if feasible at all.
In previous blog posts I have shown my first adventures in the R/C hobby world with a 400 size electric (brushless) helicopter. It was a Ready to Fly model from Art-Tech (the Falcon 3D model). This little 90º swashplate helicopter, allowed me to understand about what it would be to fly an helicopter with all the control surfaces, just like the real ones.

After having gained more confidence with this family of flying machines, I have more recently decided to build a flying machine from scratch. This led me to the development of a quadcopter.

Quadcopters are by "definition", helicopters with four individual rotors, each one with a set of propellers of fixed or variable pitch (most commonly fixed). Propellers are usually of 2 or 3 blades (most commonly two). Similar helicopters may have more or less rotors, in varying configurations. Tricopters (three rotors) are not so common, and usually require a servo to tilt one of the motors in order to provide yaw movement. Its efficiency and reliability is usually inferior to the quadcopter.

For increased reliability, stability, and payload capacity, six or eight rotor helicopters are chosen. These however tend to be more expensive, require more battery power, and add extra complexity to the control part. It is however easier to survive single motor failure, and probably even double engine failure, in the case of the octocopter (depending on the all up weight). In a quadcopter there is no redundancy whatsoever. If one motor fails, it is impossible to maintain stability (yaw and pitch/roll control is immediately lost). As there is no decoupling between the motor and the propeller, glide is also not a possibility, and as such the helicopter will fall like a brick.

Still, and having budget as a constraint (impossible not to be, in the course of this nearly global money crisis), I decided to go for the quadcopter.

Having chinese sellers more or less as best friends, I went for the goal of building a reasonably good quality 450 size quadcopter while spending as little money as possible (below 200 USD).

My favourite sellers, definitively Hobbyking, and Dealextreme, as I couldn't find anywhere else in the online market the same products for a competing price.

My selection of parts was:

  • Controller - KK Multicopter Board -  $14.99
  • Frame - Q450 Glass Fiber frame - $11.00
  • Motors - 4 x Mystery A2212-15 930KV Outrunner - $12.79 x 4 = $51.16
  • ESC - 4 x Hobbyking SS Series 25-30A ESC - $5.99 x 4 = $23.96
  • Battery - Turnigy 5000mAh 3S 20C Lipo Pack - $23.23
  • Propellers - 10 x 4.5 1045 Rotating Shaft Propellers for Multi Copter - Black (2-Pair) - $4.70 x 2 = $9.4
Later I had to add the ordering of the TX upgrade:
  • FrSky V8HT 2.4Ghz DIY Module - $15.80
  • FrSky V8FR-II 2.4Ghz 8CH Receiver (HV) - $19.99
The list of the main required items accounted for less than 200 USD (including shipping where applicable). This is a pretty reasonable price for such a machine. It includes everything that is necessary to have a working quadcopter: control board with 3 gyros (a KK Multicopter board with an ATMega 168 MCU, opensource firmwares available online), one ESC for each motor, battery, frame, blades, power distribution. The radio was not considered in the budget as I already had one. As customs are a bit tricky in my country (orders above 22 Euros tend to be retained and an extra cost be charged), I took this into account in the calculations and decided to place small orders spaced in time to avoid this problem.

During the construction:

As the X quad configuration seemed my personal favourite, I reflashed the board with the most common XXcontrol_KR_XCopter v2.5 firmware.

I just used the simple parallel port programmer (DAPA) together with AVRDude, just like I did in previous AVR projects:

Reading the instructions was fundamental, together with adjusting everything and testing WITHOUT the propellers attached. It is never too much to remind of this important detail. The untrained user may easily underestimate the damaging power of brushless motors and 10 inch props spinning at several thousands of RPM. These things can cut fingers, so some respect is important. First finished version:

The first flight was not entirely successful. Radio glitches produced nasty instabilities. I soon discovered the receiver was faulty and not picking up the PPM signals properly. I had a second one in my Collective Pitch Helicopter, but it was at home, so I didn't had the chance to replace on the field. Still I managed to figure out that apart from the glitches, the quadcopter seemed pretty stable, and promised to be easy to fly (for someone experienced with RC helicopters) with a proper radio. Anyway for initial training, and for possible indoor testing I found it was important to add a guard for the dangerous propellers, to protect myself, other people and objects, and the helicopter from colisions. As such I designed a styrofoam structure that would offer protection, and add the marginal benefit of the ducts around the propellers (even though the extra weight most likely will cancel the benefit and reduce flight time). Cutting styrofoam is a bit messy using knives or saws. The ideal tool is a hot wire, rendering nearly perfect cuts, depending almost entirely of the steadiness of the user hand. As I didn't had such tool, and buying one would be a little off budget, and hard to find in local stores, I went for another DIY project (which may deserve a separate blog entry later) just to make the tool.

With this tool cutting styrofoam is literally like cutting through soft butter. In a couple of hours I had a nice structure ready to be coupled to the helicopter:

So far I did a small indoor test, and everything seems fine (video still to be added). For a more solid solution for the radio problem I decided to go a little bit further, and while standing below the 200 USD budget, upgrade my crappy art-tech radio (a E-FLY 100C) with a proper TX module. The module have been ordered and for now I'm waiting for it's arrival. It's a FrSky V8HT DIY, and it's designed to fitted into any transmitter that produces a PPM signal. This page shows an example of the module being fitted to a radio similar to mine:

This module is entirely digital, using spread spectrum for radio spectrum management. Is uses great components such as the CC2500 radio chip, and the Texas Instruments MSP430 microcontroller. The module plus the receiver cost me 31 Euros including shipping. It is a great price considering it turns a crappy radio into a very decent device. Moreover it provides glitch free control, a superb range of up to 2 km, and a failsafe feature, for configuring proper model behaviour on signal loss.

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