four-rotor aircraft mostly use I2C digital communication. It put all
brushless motor speed controller connection in series on the bus through
two data lines (one for the clock SCL, one for data SDA), and send the
control signals to brushless motor speed controller at regular time. MK
products used this communication mode, as well as our early X500D X600D.
I2C communication mode looks perfect, you can save the number of
communication cable, moreover does not need to modify the hardware and
interface when extended the four-rotor to six or eight rotor, just
directly attached to the existing communications bus. This is a economy
design that easy to extend.
However, there are some problem in actual use:
1. As long as one of brushless motor speed controller broken down, will affect all the other brushless motor speed controller on I2C bus, this failure is fatal. Although we can change the software to make the all brushless motor speed controller to enter the normal working condition after broken down. But the I2C communication mode has greatly increased the probability of failure, for expensive camera equipment and x650 aircraft, this is clearly not the best choice.
2. ESC is working in Series on the I2C, the more ESC, the more time cost in communication. The maximum communication rate of the standard I2C is 400kbps, for reliability and stability, the general four-rotor will use the communication speed 100kbps, means it can only send and receive data only 10 byte per 1ms. No enough for high-speed and high-precision motor control, as 500Hz and 1000 control level with the increase in the number of motors.
3. There are many ESC in in the field of model airplane before four-rotor aircraft invention, after years of development they are become very mature and reliable and has a very rich type and power output. But I2C are incompatible with these traditional ESC interface, and nothing much in the type, inconvenient to extend.
In order to solve these problems, we used a more stable, reliable interface solution that compatible with current brushless motor speed controller, we called it UltraPWM technology:
All ESC are connect parallel to flight controller, any one of them brokecn down will not affect the work of the entire system, aircraft still able to landed safely via the remaining seven motor. This is essential for multi-rotor aircraft to meet high security requirement.
Motor control frequency 500Hz, 1000 control level. The the control pulse width error between two brushless motor less than 1%.
Compatible with the existing ESC interface, without requiring high speed and high precision control, instead, it can work with the general speed controller too.
Due to most of xaircraft X650 documents are chinese that hard to get the details of it, for convenience I will translate(poor translation) all article on their website and post it in www.xaircraft.org, also this forum.
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