There are multiple technologies that combine in perfect sync to give rise to a unique flying machines that until recently were almost unthinkable and today, on the other hand, thanks to platforms such as Arduino and the generous collaboration of developers around the world, are perfectly within reach of any amateur.
Multirotors are similar to helicopters, but with multiple engines with its corresponding propellers. There are various possible configurations with different number of engines and different geometrical arrangement.
In the view of the boom are taking the 250 class quadcopters with FPV for aerobatic flight and racing I decided to build one, trying to minimize costs by using, for example, 3D printing technology, as well as the re-use of parts of previous projects.
With the electronic speed controllers integrated into the structure of fiberglass printed circuit board, powered by batteries 2S and governed by a Multiwii flight controller based on atmel ATmega 328p is a good choice as a first contact with the world of multirotors.
A key element that makes possible the stable flight is the flightcontroller board or FCB which is an microcontroller governed electronic system that reads information from a series of sensors that provide data on the position and current movement of the ship. This set of sensors is often called inertial measurement unit or IMU and usually includes accelerometer, gyroscope, magnetometer or compass and altimeter. With these measurements the software that we introduce in the microcontroller should generate the right signals to control the different engines that will bring each one the right thrust to maintain the horizontality of the aircraft.
Flight Controller board chosen is multiwii 328p of Hobbyking, a very affordable but certainly limited choice. Its specifications are:
- microcontroller Atmel Atmega328p 16MHz.
-3-axis gyroscope ITG3205
-voltage regulator 3.3V
-FTDI USB-serial converter(mini-USB connector)
-dimensions: 50 x 50 mm, mounting holes: 45x45mm.
Given the limited memory of the 328p firmware program, i.e., the code executed by the microcontroller is not very extensive, and therefore the choice more successful is the multiwii software, originally developed to use inertial sensors of the famous Nintendo console along with an arduino based on the 328p.
Communication between the microcontroller and the sensors is done through an I2C bus, synchronous 2-wire bus (SDA, SCL) used for this task, usually where each Member has an address identifies it uniquely to the host that is the microcontroller. The used sensors are usually fed to 3.3V and the MCU (microcontroller) as well as the FTDI USB chip work to 5V, this board incorporates a 3.3V linear voltage regulator, and 3.3V-5V bidirectional logic level converter. Between the outputs of the plate we find this i2c bus and 3.3V power lines that can be used to connect new sensors or other devices to our "flight computer".
We also have a serial / UART port, which we can be used to connect a GPS, a Bluetooth module, LCD display, telemetry via radio link, etc.
Loading the firmware chosen, sometimes manipulated to a greater or lesser extent, takes place via the USB connection, being the firmware normally a "scketch" or program of the Arduino environment. The USB connection also allows using a Setup program or GUI (Graphic User Interface) monitor and configure the functioning of all hardware (later speak of the loop control PID, responsible for the stability and maneuverability of the vehicle).
Since a time engines commonly used in aircraft are brushless type that have no electrical contact between rotor and stator, which lengthens its shelf life and gives them great reliability. They really are three-phase synchronous permanent magnets rotor motors. As its name suggests the rotor is the part that rotates and, although it has traditionally been the inner part (inrunners), is currently more common to be the outer (outrunners).
Leaving aside the construction details of the engine, electrically speak of three-phase Motors which must be fed by three-phase alternating current which can be varied both voltage and frequency to control right over the engine. This typically is the use of DC-AC inverters based on bridges of power semiconductors, in this case MOSFET transistors that frequently appear dual encapsulated forming what is known as "half-bridge". With three half bridges have a three-phase inverter and just only need to think of how to control it.
The inverter control is carried out, as it could not be otherwise, by a microcontroller, whose software reads a PWM signal input which is proportional to the desired speed for the engine and generates, also through PWM (but at higher frequency), the three signals that drive the power semiconductors.
Typically, the electronic speed control or ESC is a standalone device although in this project it is integrated in the printed circuit board which forms the structure of the aircraft.
Structure of carbon fiber of 550 mm, 250W motors and multiwii pro (based on ATmega2560) flight controller is able to use the APM firmware among others.It has camera and transmitter AV 5.8 GHz, OSD as well as datalink at 433 MHz.
I have tried to approach as far as possible to the design of the model that we see in the image. It's the InmersionRC Vortex-285 racing drone. The structure of this model is made of carbon fiber, although it uses some parts of plastic. I'll use printed pieces in plastic ABS except for arms that will be of 12mm (excess of previous projects) carbon fiber tube.
Digging on Thingiverse, we find a clone of this printable model under the name Firefly, well documented with its own web site with Assembly instructions. However, my first problem arises: my carbon fiber tubes are 12 mm and design includes 10mm tubes, the most obvious solution would be to scale up the model to 120% but this makes base not printable because of its size, therefore adopted the commitment of print pieces to scale 1.13 and 1.15 tube clamps as well as the vertical sides.
Also I think that motor mounts are unnecessarily complicated to get an impression with sufficient robustness. So I modified an existing design by adding a foot, with a result that convinces me more.
One of the advantages of this model is its ability to fold (thanks to a few clips or plastic cliplocks that ensure the tubes). In this way transport is much more comfortable.
Although the original size (distance between the axes of two diagonally opposite engines) is 285mm, due to the use of scaling my prototype reaches 330mm
I am going to use 2204 - 2300kv motors controlled by 18A Opto ESCs of very reduced size suitable for 2S-4S batteries. Opto models lack of BEC, so I'll use a 5V/12V dual BEC to feed various electronic components.
I'll try different combinations of power supply (3S/4S) and propeller sizes to experience different behavior. The idea is to use 5045 propellers with 4S and 6045 with 3S.
For stabilization control, I have in principle the multiwii 328p previously used, even though it would be interesting to migrate to a 32-bit system as Naze32 with a relatively low cost currently.
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