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Thursday, June 28, 2012

Letting it hover...

While the building phase can be an enjoyable experience, grabbing the sticks, putting the chopper on the ground, and gently increasing the thrust until that custom build apparatus starts to appear light on its stands is where the real fun begins. Would we ignore the noise of the struggling motors, and this styrofoam-shielded quadcopter would seem like a magical object for which gravity would be opening an exception everytime we wanted to. There is no magic, just technology, but still there is a sense of victory in tricking gravity in a brute force manner. Not in a magnificent scale as Howard Hughes H-4 Hercules, or lighter than air as Bartolomeu de Gusmão's flying bird, but still admirable for mimicking birds in the size and ability to fly (in a cumbersome way however), and for hovering like few of these animals can.

Here is the inspiring video, that hopefully shall represent the first of many flight ordeals:

In spite of the 1.26 Kg of All Up Weight (AUW), at about 40 % throttle the propellers would start to support the entire weight of the quadcopter. Some rudder trimming to the left was required, as it slowly started to yaw to the right initially. Minor corrections on the elevator and aileron trims were also applied
to ensure proper stability in the neutral position of the sticks. No gyro gain adjustments were performed so far, as at least for hovering the settings seemed ok (about 50% for yaw, pitch and roll). An outdoor test may allow for a better assessment of this.

The styrofoam strucuture weights approximately 130 grams. An improvement of this will probably be attempted by cutting some styrofoam in some structurally insensitive areas. Maybe a 15-20% weight reduction is feasible.

For outdoor flying, the structure is not so important (unless flying close to people and important objects is necessary), so that it may be detached, resulting in a significant weight reduction, and consequent increase in flight time. Probably stability is improved as well, because of the decrease in angular inertia around the rotor areas, resulting in lower motor output to correct attitude errors, and faster response to increase in motor power for correcting these errors.

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:

Later I had to add the ordering of the TX upgrade:
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.