Flight Controller

Controllers

The controller is the brain of a multirotor. It takes input from the receiver and tells the speed controllers to translate that input into movement. The difficult part of its job is to keep the copter stable. To do that it needs all kinds of sensors to learn about its orientation and position.

The keyword here is "degrees of freedom" (DOF):

• 3DOF
  A three axis accelerometer or gyroscope measures changes in speed or rotation.


• 6DOF
  The combination of a three axis accelerometer and gyroscope allows to measure precise changes in a copters orientation. For a stable flight a multirotor requires at least 6DOF.


• 9DOF
  In addition to the two acceleration and orientation sensors a compass (magnetometer) is used to determine the orientation of the multirotor.


• 10DOF
  To maintain the height an additional sensor is required. Usually that's a barometer to measure air pressure, but sometimes ultrasound is used to measure the distance to the ground.


• 11DOF
  Finally an additional GPS is used to find the exact position of the machine.

Simple (and cheap) controllers usually only support 6 DOF. Some can be extended using external modules.

More complete controllers have more complex software and provide a lot of additional features. This way they can support a lot of different flight modes. For example the Ardupilot Mega supports the following:

• Manual
  Direct transfer of transmitter input to the multicopter. If you let go of the sticks the copter keeps flying in the direction you pointed it at.


• Stabilized
  Let go of the sticks and the copter stops and maintains a stable position


• Fly-By-Wire, Training
  Some flight-characteristics are maintained by the controller. You can define a min and max yaw, roll and pitch rate.


• Automatic
  The multirotor flies a pre-programmed path. You can intervene by using your controls.


• Return to Launch (RTL)
  The copter flies back to its starting position and tries to stay there at a programmed height. GPS is required for this feature.


• Loiter
  As long as you don't give any control input the copter tries to maintain the current position.


• Automated starting and landing
  Perhaps the best way to avoid horrific crashes when getting closer to the ground

Other FCs have different features or use other terms. What you need depends on the intended use.

What flight controllers are there?

A lot. Too much to get a good overview. It is best to distinguish commercial modify the complete programming of those - or at least make a feature request and hope someone in the community is able to program it.

Controllers around at the moment are:

• HobbyKing KK2.1.5, KK2.1 or older KK2
  A very simple commercial 6DOF controller with integrated display and buttons that allow you to change all parameters directly. It's a very basic system but very popular and great to learn on. There are different firmwares out there which make setup easier and result in better flying characteristics. There is no way to to add more sensors, so you can not extend or upgrade the features. Even though it's considered large by todays standards it is a great controller for beginners.


• Naze32
  A small lightweight board that has a very good reputation of flying small multi's very well. 32bit is a plus. The full version has a barometer, compass and can be hooked up to a GPS but the later is still experimental. The acro Naze does not have these extra features. These are not as easy to setup with no inbuilt screen and some computer knowledge is required.


• MultiWii
  This has a lot of different versions from basic 6DOF to full GPS capability. The simpler acro boards like the flip boards can give great results for simple setups buts can be fiddly to setup. The fullhouse boards with GPS etc. require patience to setup and even then the results are mixed. These boards are cheap which is a plus and are good quality when bought through RTFQ who can make the initial setup for you.


• ArduPilot
: APM 2.5 / APM 2.6 / Pixhawk
  Open-source project that has a huge amount of features with many add on features like GPS. They are very powerful boards that have the usual magnetometer, barometer and GPS functions plus a whole host of others giving you an amazing suite of options including full autonomous capabilities and control via computer. But this also means a complicated setup. A fair bit of work and knowledge is required to tune it. Also can be expensive but this is what commercial operators use.


• HKPilot Mega
  A cheap ArduPilot Mega clone made by HobbyKing.


• OpenPilot
 CC3D
  Another open-source based project. The hardware is difficult to get hold of because it's not manufactured commercially. Very similar to the acro Naze.


• DJI Naza
  A commercial controller that is quite popular. Can have a full list of features but being a commercial system is more restrictive on what you can do. Easy to use and setup. Known for good GPS lock and stable hover but not much else. It is very expensive and limited on its capabilities especially considering the high purchase price.

A much bigger list can be found at Oddcopter.

Remember that simplicity is the key when starting out. Extra features like GPS may just get you into more trouble early on.

For a beginner you would be wise to get a KK board or risk a very steep learning curve.