Monday, December 30, 2013

Gathering with fellow pilots and Multicopter improvements

Most of the time I have been a lone rider in this hobby, having started from scratch on my own, since the traditional helicopters back in 2009. I would occasionally go to local air fields and try to mix in with seasoned RC airplane and helicopter pilots. But the environment would not always be too friendly, with the fixed wing guys not enjoying much the flying grass cutters.

Since the quadcopter I tried going to the local airfield, and was received with curiosity and enthusiasm by some, for showing a uncommon flying apparatus for those accustomed to the traditional RC gear, and apathy of the more senior club representatives who just care about figuring out if we are in a club and paying a annual fee.

Today I just look for a safe and calm place to fly. Unlike a fixed wing aircraft, the ground requirements for an helicopter are rather modest.

After having found that my colleague at work Luis Sismeiro (check out in his blog the post about this encounter, including photos and filming of the quadcopters: is also an enthusiast for this hobby, we have managed to do a couple of encounters, and also manage to stimulate the interest of a third colleague (Nuno Padriano) into this hobby. The later already comes to the air field with his tiny Hubsan quadcopter to be pushed down by the propeller wash of his giant counterparts :)

In my previous post I have detailed the main changes to the quadcopter setup, which included a new flight controller (HK Megapirates board), and addition of OSD and a camera.

After this, most of the work have been about tuning the platform, both in hardware and software aspects. Even though very interesting improvements have been achieved, I still consider this a work in progress.

Main achievements:
  • Removal of video noise with the motors running and under load: added dedicated voltage regulator and ripple filter for the camera, OSD and transmitter. Changed the power supply to the OSD, by no longer using the internal switching regulator and using only an external, clean 5 V supply;
  • Fixed the bluetooth module, which was operating with erratic serial communication: instead of using a pair of bipolar transistors for the step-up of the TX signal from 3.3 V to 5 V, replaced by a 74LS05 open collector NOT buffers. By using correctly sized pull-up resistors I managed to obtain a decent rise time (below 300 ns) for the RS-232 signals, completely eliminating the errors. Initially I had rise times in excess of 3 us, which was unacceptable;

  • Calibrated motors, to reduce vibrations affecting video quality and flight controller performance, not to mention motor bearings wear: used the laser beam and mirror procedure, which proved to work great;
  • Upgraded to Arducopter 3.0.1 RC4: decided to take the risk, after the hazardous RC3 version. Given the positive feedback of users I found this version could be trusted and went ahead and flashed the board. More that a couple of flying hours so far,  and no crashes to account for;
  • Found the sweet spot of PID values: after playing with the relevant parameters such as Stabilize P values (for both pitch and roll), and Rate P and I, the correct values rendering good stability results were finally achieved. No more overshooting or wobbling detected;

Things to be fixed/improved:

  • Get the position hold mode to work: after having identified inaccuracy problem with the compass as motor load increases, it is not worth getting any further into getting this mode to work as the compass is vital to allow the quadcopter to make position corrections in the right direction. As the helicopter begins to hover, a 90º drift in the compass reading can be observed. After landing, the heading slowly returns to the correct value (the gradual variation is possibly due to Kalman filtering performed in the board). To solve this I have ordered an external compass which I will look forward to place the furthest away from strong currents as possible;
  • Flight efficiency: the current setup is quite nice, with a new 3S 5000 mAh Turnigy LiPo battery rendering about 14m50s of maximum attained flight time at an all up weight of about 1.35 Kg. However, not entirely satisfied with the results, I am still inclined to improve this figure and us such found some great 800 Kv, 260 Watt pancake motors at a fairly low price and as such decided to order these. I expect to couple these with  some 12x4.5 propellers and obtain in the vicinity of 18 mins of flight for each charge. Furthermore this accounts for over a 1000 Watts of theoretical maximum power, which for these propellers translate into over 4 Kg of maximum static thrust. Of course, tuning PIDs will be an exercise to repeat again.

  • Install the already ordered video recorder next to the receiver, in order to keep a record of the flights;
  • Build the PPM sum module, to allow GPIO pins to be freed for other purposes such as ADC inputs to measure battery voltage and current. The same module should also integrate the voltage divider and the current sensor;
  • Think about adding an HD camera and a stabilized gymbal for aerial photography and footage.

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