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Let’s Build an Open Source Quadcopter – Part 2

Let’s Build an Open Source Quadcopter – Part 2

Vibration test

The gyroscope and the accelerometer are very sensitive to the vibrations, but for our application it is necessary that the data corresponding to their readings are free from alterations and falses due to the noise, and particularly to the ones induced by mechanical vibrations, otherwise it will be impossible to stabilize the flight; if there are too many vibrations, they will be transmitted to the gyroscope, that will not supply reliable measures any more. It is not possible to minimize the vibrations problem, therefore it is necessary to face the problem even before any flight test.

The solution for reducing the vibrations is to have a very rigid structure: aluminium, fiberglass, carbon-fiber chassis are generally much less prone to vibrations in comparison to those in printed plastics or Lexan (this material is a polycarbonate). The coupling of the board to the chassis is very important and it must be carried out so that the gyroscope is capable of detecting the small movements of the chassis; however it must not be too rigid, otherwise the vibrations created by the motors will be transmitted to the gyroscope with their full intensity.

When constructing our prototype, we opted for using nylon screws that have rubber washers, when fixing the board to the chassis; moreover a covering has been placed over it, so to prevent that the turbulence of the air induced from the propellers may bring further vibrations on the board.

The vibrations are essentially due to mechanic imprecisions, that is to say, to the bad balancing of the propellers and the motors; in this case, the usage of quality products makes a difference. In order to verify the state of the vibrations it is possible to activate a particular section of the MultiWii software: when in the usual config.h file please remove the comments from the #define DYNBALANCE line and load the sketch on the board; in the upper part in the MultiWiiConf software, please click on MOTORS and a new section will open, that allows to check each motor individually, by means of a pointer activated via the mouse.

fig_6



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You may carry out the first tests without the propellers and verify the presence of possible vibrations due to loosen joints or unbalanced motors. On the main panel please select the visualization of the accelerometer’s data only, so to have a diagram that is easier to read. As for the balancing, we find it valid to use some small pieces of insulating tape, that are slightly thicker than the typical scotch tape used in the offices; the insulating tape must be applied to the rotor’s external part. Try to apply it to various positions (after all it is easily removed) until you find the point in which the vibrations are minimal and then try to work on the quantities, so to eliminate them.

 

fig_7

 

Once the best balancing possible has been found, you may verify the correct rotation of the motors: the one in front and the rear one will have to rotate in a clockwise direction, while the right and left one will have to rotate in a counterclockwise direction. The next step is the installation of the propellers, an important step given the intrisic danger of having four propellers, sharp as razors, that spin at ten thousands revolutions per minute! The front and the rear propellers have to rotate in a clockwise direction (CW) while the right and the left ones will have to rotate in a counterclockwise direction (CCW); please remember that when the propeller rotates, it must push the air downwards. Please activate one motor at the time, and bring  the stick to about halfway; please pay the highest attention since the quadcopter will start to take off and therefore you will have to try to somehow hold it down in order to avoid it causing damages or, worse, to hurt yourself! In the telemetry settings, please leave the only visualization of the accelerometers and possibly zoom on the diagram’s scale, so to better see its details. In this case, the insulating tape must be applied to the back of the propeller. By clicking up in the SETTINGS section, you will be able to see the settings related to the battery alarm and the setup related to the motors’ command.

 

fig_8

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