Saturday, 3 January 2015

Multicopter, motor control considerations (Part 3)

I've now come to the conclusion that mixer settings for the flight control board can remain as is. The new 'Spyda' motor positions still form a square. I should be able to get the centre of gravity of the machine at, essentially, the centre of that square. (Through careful positioning of the battery and other equipment). The centre of gravity is the centre through which a plane turns around as it pitches and rolls. Thus, the roll and pitch axes are through the centre of gravity. The flight control board with its accelerometers and gyros will be at the centre of gravity.

However, this remains untested. As I've not yet received my consignment from Hobby King which will contain new motors, speed controllers and a programmer to update the flight control firmware. Unless I decide to print more parts construction is temporarily stalled. So a little more on electronic speed controllers and the need to reprogram them for Multicopter use.

When model aeroplane radio controls were first commercially produced, around 1970, most model aeroplanes that could use them were fixed wing and powered by combustion engines. It will be no surprise to readers of this blog that much of the pioneering work on radio control was done during the Second World War, by Germany, and this was subsequently copied and miniaturised for model use in the 1950s. But modern, transistorised, so called 'digital' systems didn't appear until the 1970s.

To control the model's engine speed a small 'servo motor' was used. The servo received a pulse from the radio receiver and examined it's duration. A pulse 1 millisecond long sent the motor to one end of its travel, a 2 millisecond pulse sent the motor to the other end of its travel. Intermediate positions were achieved through pulse durations between 1 and 2 millisecond. Now this was fine in 1970, model aeroplane engines don't need to change speed too quickly.

When electric powered flying models became feasible around the year 2000 (due to battery and motor improvements) the same protocol was adopted to control the speed of electric motors. But the Multicopter uses motor speed changes to affect fine manoeuvring control. Moreover, the multicopter flight control system can potentially update the required speed of a motor more quickly than the refresh rate of a transmitted signal.

Hence the SimonK firmware, which I mentioned in a previous post. Simon K, (Simon Kirby) has adapted the way a number of the popular electronic speed controllers work so that they respond much more quickly to speed change inputs. SimonK software repository. Speed controllers ready flashed with SimonK firmware can be bought, or if you have hardware compatible controllers already you can buy or build a programmer and flash them yourself.

Now factors in China are churning out speed controllers ready programmed with SimonK firmware. This, of course is another aspect of the story, how China is manufacturing the vast bulk of volume electronics - even for this rather esoteric part of the consumer market. But that's a matter for another blog.


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