Sunday, 4 January 2015

Multicopter, Control and Stability basics (Part 4)

As far a flying goes getting the multicopter off the ground is easy. It's just a matter of getting the propellors spinning fast enough so that a sufficient stream of air is deflected downwards. But, as soon as it leaves the ground the two crucial considerations, control and stability, take over. The KK2 flight control board is the prime mover for these two issues.


How does control work?
To take off I increase the power - the left stick on the radio control is pushed forwards. This is transmitted to the copter and passed to the KK2 board. The KK2 takes this increasing throttle command and simultaneously increases the speed demand of all 4 motors. The multicopter leaves the ground and hovers.

If we want to fly forward the radio control right stick is pushed forward a little. What's required is for the angle of the multicopter frame to tilt. The two motors at the rear turn a little faster and the two motors at the front turn a little slower. The front of the airframe lowers and the rear rises. Similarly, if we want to go to the right or left the pair of motors on each side of the frame must change speed when the right stick is pushed either left or right. The right stick is essentially an airframe angle demand control.

This picture from Hobby King, the Hong Kong distributer shows the KK2 essentials. The pins labelled AIL, ELE, THR etc are where the stick demands from the radio enter the system. The outputs to the motor speed controls are on the left of the board.

The KK2 has onboard sensors, solid state accelerometers and gyroscopes to sense motion and angle. The right stick position - which might be called a demanded airframe angle - is compared to the airframe angle as detected by the pitch and roll gyros. When the airframe angle corresponds to the 'demand' from the control stick all 4 motors resume the same speed.  

Stability, maintaining that desired airframe angle in the face of turbulent air and small differences in the performance of the motors is, for the KK2, an ongoing task.  Even when the control sticks are stationary the airframe will be pitching and rolling constantly losing the demanded airframe angle. Using the gyros as a reference the KK2 automatically generates new changes to the motor speeds in order to keep the airframe at the demanded angle.

But the KK2 has a display screen and pushbuttons, what are they for? I'll deal with those in the next blog.

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