Bad vibrations…

After a number of flights in stabilize mode with a very stable vehicle it was decided to test the rest of the flight modes. Our goal was to build this into a measurement platform which necessitates the use of the auto modes. Therefore, we went to an open test field near Stellenbosch where we switched the quad copter, while in a stable hover, to loiter mode. The latter action caused the quad copter to ascend at full throttle with no sign of stopping. After landing and trying again the same thing happened.

After this slightly unsuccessful test I learned that in the newer APM firmware versions vibration can be an issue and can cause the observed behavior. I decided to start eliminating all the vibration I could find. Starting with the motors I measured the vibration with a app called “vibration on my iPhone”. Spinning each motor independently while strapping the phone onto the motor arm I could monitor the intensity of the vibration and start balancing the motors in the same manner as the video I found below.

After some experimentation I realized that the motors without any nut and spinner was perfectly balanced, however, as soon as the spinners are added it would put the motor way off balance. This leaded me to balance each spinner with a piece of tape much like they balance the motors in the video. This meant I now had 4 non vibrating motors. At this stage I did not have a prop balancer and had to hope my cheap colorful gem-fan propellers were reasonably balanced.

To actually test if there were any relative improvement I enabled the IMU data logging onboard the apm. I downloaded the log after each change to relatively measure what performance difference the modification had.

It can be seen in the comparison that there was initially also some experimentation with different apm mountings such as being screwed down, double sided tape, moon gel pads and lastly, the configuration that is still being used, dampening pads below the whole electronics platform. Remember that these modifications were done before balancing the spinners, note that it almost seems as if the vibration got worse with the different platforms except the moon gel, it should also be stated that with the new dampening pads the whole layout of the vehicle was changed causing the vibration to increase dramatically.  A motor which was slightly damaged in the previous crash was also replaced with more negative results. However, balancing the prop spinners and propellers showed a slight decrease in vibrations as seen in the graph below. However something weird was noticed when balancing the gemfan propellers, it seemed that they would be heavier on 1 side of the hub. It was here that I decided to invest in more expensive apc propellers which as you can see made a huge difference which in the end allowed loiter and the auto modes to work flawlessly.

Moon gel pads

Moon gel pads

Relative vibration comparison

Relative vibration comparison

Motors and final assembly…

After a long break moving to another city I restarted my efforts on the quadcopter build. This time adding the motors and finalizing the construction to the point of a maiden flight. Some changes to the previous thought process were incorporated in terms of channeling the high current ESC wires through the aluminum tubes. The latter meant disassembling the whole quad copter leaving only lessons learned.

The motors are 800kV tiger motors which will each in turn need to drive a 10×4.5inch prop. The motors can be seen mounted on the edges of the quad copter with their respective cabling running through the aluminium tube into the center plate cavity.

Assembled frame and motors

Assembled frame and motors

Next, the power distribution board was fastened on top of the center plate with the help of plastic screws. This allowed the esc`s to be installed connecting all four motors allowing some initial testing. To test the motors the battery and PWM lines were connected to the distribution board and receiver module respectively. After confirming all four motors are spinning their directions were set to motor 3 and motor 4 in a CW direction and motor 1 and motor 2 in a CCW direction as illustrated in the APM wiki. The connections for the power distribution board can be seen below. The ESC PWM wires will definitely need to be shortened, in the current configuration they will cause horrific RF pickup loops.

ESC’s and PDB mounted

With power and connections to the motors now established the remaining electronics can be added to the build. The APM, telemetry radio and receiver are all now mounted on a platform and connected. This platform is part of the frame and will fit right above the distribution board. The electronics platform can be seen below.

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Assembled electronics platform

After the electronics platform has been fitted the shell containing the GPS along with the landing gear was attached. Before mounting the props the ESC’s needed to be calibrated which on stock settings were extremely different from one another. Due to some physical restrictions the quad is currently configured in a plus orientation which might be changed in the future. The quad copter with its props can be seen below next to its 9x FrSky remote.

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Full Multicopter Build