Hi :) Finally, we achieve such a
configuration that our frame could raise for a few centimeters above floor. We
have check additional battery and bought another couples of propellers. It was
caused by not enough thrust – it was too low. We could achieve only ~300 g max
thrust. We were using 2Cell battery and 9x4.7 props (CW and CCW). So we decided
to do some changes ;) we decided to try recommended by producer both battery
and props: 3Cell battery and 10x4.7 propellers. They promised to obtain over
700 g of thrust. In best ways we achieve ~550g, which is not close to that
promises, but also good enough :) From simple calculations followed that theoretically
we should flight:
4*550g = 2200g > 1000 g
Our frame weights below 1000g, but the
propellers weren’t achieve exactly 550g, so approximately it looks fine. We
didn’t want to wait, so we mounted our frame immediately:
We did two tests. At first time we used
cable-communication, at second – wireless communication. The very good news is,
that we have enough thrust to flight :) on above ~40% of full power quad started to increase its altitude. But second start was not so fortunately and I have to visit surgeon:
Warning! Photos are not for sensitive persons
We have to be more careful in next tests ;)
Stay tuned with us!
Hello :). Today
I’ll try to describe how brushless direct current motor (BLDC motor) works, and
how to steer it. At the end I’ll show our movie how its rotating :)
Such a motors contains of some coils, which are connected to each other in
star interconnection. Basically, there’s only 3 coils which are immovable - in
opposition to a magnetos. A drive shaft could rotate because it is attached to
a movable case. Due to low friction it can
rotates freely. If we want to rotate the shaft, we have to deliver current to a
single coil, and it have to leave circuit by another coil (third one should be
Hi-Z, and it doesn’t conduct current). In this situation some magnetic field is
inducing, and it acting with magnetos in nearest area – it moves them. Next,
the current is conducted by another couple of coils. It is shown at below
figure.
Figure 1. Coils
BLDCs are kind of actuators which haven’t got any commutator. It is very
helpful, and it’s caused by fact that it works as current inverter from DC to
(approximately) AC. It is not sinusoidal current. Next figure presents some
BLDC actuators. To our quadrocopter we are using out-runner type.
Figure 2. Types of BLDC motors
There’s still a question – how to steer such a motor? We have to use some
equipment, which acts like inverter – it converts direct current into alternate
current. We are using some Electronic Speed Control (ESC). It delivers current
in some triangle/trapezoid shape. It is shown at next picture.
Figure 3. Plot of currents
That shapes are a bit complicated, so I decided to not describe them ;) In
fact, such a steering is good enough to force motor to rotate. But, ESC would
not act as it wants; we have to use some microcontroller. On below figure is
shown, how to connect uC with ESC, and generally how ESC looks like inside.
Figure 4. How to connect uC and ESC to BLDC motor
As you can see, inside the ESC are some kind of bridge (build from MOSFET
transistors with diodes). It provides properly shift phases and thanks to that,
current looks like in the upper figure. Of course, there’s also some protection
circuits, feedback signals and current measuring. And I have to add, that how
fast the motor is rotating (angular velocity) depends directly by us. We could
steer ESC by PWM signals, and it is translated by some control circuit in ESC
to proper motor steering. In most cases, it looks like on below figure:
Figure 5. PWM steering signal
Our microcontroller send PWM signals with ~50 Hz frequency (every 20 ms).
The angular velocity of BLDC motor depends of pulse width. The lowest case
(lowest rotates) are if pulse is 1 ms width, the greatest – 2 ms. If our ESC
provides rotation in both directions (clockwise and counterclockwise) there we
could divide our pulse range into a two areas – the first 0.5 ms (form 1.0 to
1.5 ms) our motor will rotate clockwise, in second part (from 1.5 to 2.0 ms) it
will rotate in opposite direction. If we have ECS which provide rotates only in
one direction, we are forced to use pulses in range only 1ms – 1.5 ms. But,
direction of rotate depends on how we connect motor – changing two cables we
could obtain rotating in opposite direction.
And there’s a movie which shows how our motor rotates :) please watch it only in high quality ;)
