Getting a quadcopter stable in the air isn’t trivial. Stability of a quadcopter relies on the harmonious working of all of it’s parts.
An unbalanced propeller produces excessive vibration. This vibration travels through the entire airframe affecting the handling of the aircraft, produces inaccurate readings by the sensors, and creates premature failure of motor bearings and parts. A balanced propeller is paramount to a stable aircraft. A balanced propeller produces less vibration and draws less current, which results in greater stability and extended flight times. You should balance any propeller before installing it on your aircraft. Balancing a propeller requires the use of a special tool, you guessed it a propeller balancer. The propeller balancer that I use is the Top Flite Propeller balance. It is essentially a shaft held by two magnets. The magnets create a frictionless surface for the shaft to spin freely. Read More…
For Scout to auto-stabilize, it needs sensors to find its orientation in space. No one sensor can do this on its own. It takes several different sensors working together to calculate Scout’s orientation. The sensors used to calculate orientation are gyroscopes and accelerometers. More recently, magnetometers are being incorporated to calculate direction.
Conveniently, there are boards that incorporate all of these sensors together. These boards are called inertial measurement unit, or IMU. An IMU uses accelerometers and gyroscopes to measure and report the aircrafts velocity, orientation and gravitational forces on multiples axis.
Scout uses the ArduPilot Mega IMU shield. I chose this board because it was developed to work with the ArduPilot Mega board.
The ArduPilot Mega supports serial communication between the quadcopter and a computer. This serial communication is used to upload the software, change tuning settings and to update the flight path. The ideal wireless solution is XBee. XBee modules are available at different frequencies and power outputs. I am using the XBee 2.4GHz 1mW with the wire antenna modules. The XBee 2.4GHz modules are more than sufficient for initial setup, tuning and short range use. The XBee 2.4GHz modules operate at the same frequency as microwave ovens, WiFi routers and just about any other wireless consumer device. These modules have to contend with radio interference and this why they are not ideal as a robust long range solution. For applications requiring longer range and robust communication, using the XBee Pro modules is a better solution. The XBee Pro’s operates at 900MHz the lower frequency allowing for a longer transmission range for the same power output as the 2.4GHz modules and are less susceptible to interference. The XBee Pro’s are offered in different power outputs. The higher the power output, the longer the transmission range.
There are several communities developing software for the ArduPilot Mega platform. Currently, the communities supporting ArduPilot Mega are ArduCopter, ArduPirates and AeroQuad.
The Arducopter and ArduPirates share the same code base. The ArduPirates community has branched off and is developing the ArduCopter software for their own needs. I initially chose ArduPirates but recently switched to the ArduCopter code because the code is well commented, simpler and easier to understand. Both code bases work well, but I find Arducopter code easier to modify.
The only reason why I did not chose AeroQuad is because I could not get wireless communication with Scout working as I could with both ArduPirates and ArduCopter.
Wireless communication allows the freedom for on-the-fly tuning and reading sensor data from Scout without having to connect a USB cable to it. If I had GPS installed, wireless communication would also allow for real-time updating of the flight path without having to bring Scout back to the base station for programming.