I know that it is taken as common knowledge that a hexacopter with a single motor failure cannot be flown beyond a controlled crash landing, but this is just plain wrong with our ArduPilot Mega hardware and if we can't do it, it is because of inadequate software.
I have a KK X525 quadcopter which I have flown very successfully for some time with the APM2.
One ESC on it is now progressively failing, continuously reducing power to it's motor after about 2 minutes flight, but all you notice immediately is the need to feed in a little more throttle and eventually it starts descending even at high throttle.
Once it touches the ground you need to switch out of any auto mode to prevent it flipping over (usually too late).
But the moral is that even with one motor failing it tries to maintain level and flight is still controllable.
On a hex if one motor fails all you should have to do is shut off the opposite motor and it should still be completely controllable as long as you have sufficient thrust in the remaining 4 engines to hover the copter.
Auto shutdown of an opposite motor could be handled entirely in software on the APM and control should be fully maintained. as you still have 2 motors turning each way and the lefit points are still symmetrical.
No spinning, no loss of control, no nothing.
If the Arducopter doesn't do this it should and I would definitely like to know what you think.
This could be a tremendous safety and long term cost advantage for a hex and not require an Octo as previously thought.
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There is no reason I can think of that you can't reverse the direction of motors. Maybe you need new esc's but they are already doing it with 3D racing controllers. So you would just determine which motor failed, turn off th eopposite and reconfig for a quadcopter. So this shouldn't be a problem at all.
Still have a lot of trouble sending a RED up in one of these.
Probably what those morons in Aukland New Zealand were using.
shoot = chute! sorry.
Hi Alberto, some time ago I had an idea for an instant deploying parachute where you basically used 4 fiberglass rods sticking up from 4 corners of your copter (still inside of the prop shadow area) which you bent completely over into a small box on the top middle of your multicopter which had a lid on it which could be released by a servo.
At the ends, the rods would attach to the corners of a large square shoot.
When the lid was released the rods would snap out straight and immediately inflate the chute.
No way to get caught in the props because the rods would be attached to the chute itself and their would be no cords.
If it were 10 or 20 feet or more above the ground it would even be self righting.
Could work attached to save only the camera if the camera also had a release.
I've been looking at the hex setup and if you used the motor opposite the dead motor at very low RPM to compensate for undesired yaw it would work fine for one direction of rotation cw or ccw depending on which motor went out, but would not work for the opposite rotation.
To correct for that direction of rotation you would have to use a combination of pitch which would introduce undesired motion in the direction of the pitch, but it seems to me that a short bobbing motion back and forth in pitch could establish both a stable yaw (compensating with the 5th motor on the off side) and allow for fairly good control.
Of course, even if you couldn't get enough control this way "Simple" or Super Simple mode could permit a controlled recovery even if the copter was yawing continuously.
I thought I remembered that being in the source somewhere, thank you Robert.
A (auto stabilized limp home mode) for a single motor (failure) certainly seems possible for a hex.
I think the key issue is a flight mode where the APM is the primary control for maintaining stability and orientation for the copter and transmitter inputs are not allowed to override the APM stability maintenance.
The off side motor can be used at low RPM to assist in maintaining orientation and pitch stability.
I know motor reversing would make it easier, but with a little twitchiness, it should be possible to maintain stability and yaw without reversing which would also require that all ESCs be reversible, actually increasing failure proneness and expense considerably.
It doesn't need to be pretty, but a truly functional limp home mode could greatly increase the safety and utility of these things.
Little quads like the Parrot and Blade UCMx get over the safety issue by being light and foamy and don't even have a very high terminal velocity.
Our bigger multicopters have 2 choices, redundant safety measures or some kind of a really fast deploying parachute.
A stabilized motor out Hex represents one of the cheapest and most satisfactory solutions at this stage of development.
No sensors are required for the ESC to detect the motor has stopped. They already monitor the rotation, and know the RPM. Yes, they do this through the backEMF. It's just a matter of getting that info the APM.
Re your last input Robert, The people with RED cameras are the ones who can LEAST afford to have them fall out of the sky.
Personally I was thinking more on the GoPro end of things, where it is easy to end up with a net high thrust to weight.
The biggest problem we have with these things is zero recovery potential, although as I said, facing progressive single motor failure, the APM has let both myself and a friend with the same KK X525 quads with the same HobbyKing ESC failure land successfully because the APM was doing such a great job of trying to keep itself balanced while the power was going out on one motor.
Hi Robert Chris and Jonathan, I really appreciate your jumping in on this, I no it is not an insignificant issue, but it's solution could have tremendous benefits.
Robert, I do actually grasp the pitch-yaw coupling issue and why it isn't really like a regular quad (basically the tricopter problem but without the servo), but for the sake of APM based hexacopters, I beleive that solutions could be implemented in at least 2 ways.
Once you detect an engine failure or problem the APM could elect to cut down both motors as I originally suggested, then it could go into a sort of balanced jockying mode where it's first priority would be to maintain stability and where it also gave weight to transmitter input for yaw, pitch and rollbasically job 1 would be keep it level and permit stick overrides to influence direction, orientation and thrust. It might be a little (lot) jerky, but it could get you home under control to a safe landing.
Alternatively shut down the failed motor but don't shut down the opposite one.
Use the opposite motor to only provide sufficient thrust to produce desired pitch or yaw or for compensating for undesired pitching or yaw effects.
On reflection, the second approach seems like it would be most effective and is certainly an example of where the APM's automatic control would make recovery feasible where without it, no way.
Chris and Jonatha, as for detecting motor failure, given the gyros and accelerometers, the APM already has a built in method of detecting an improperly performing motor, albeit the software involved for determining whether it is a gust of wind or a motor problem is not insignificant. That having been said closed loop control would be a lot easier to determine and a wider variety of motor problems could be quickly detected and an APM descision made on how to respond. Pretty much any problem could be detected from over speeding, to slowing, to failure.
I would guess that detection of the power pulses from the motor is what as required and their may actually be enough back EMF from the ESC's to make this possible without adding individual sensors from each motor.
Just a thought.
Thanks again for your considerations about this.
Oh, and if you look around, you'll see that DJI has hexa motor failure handling in their code now. There's a test video where they show they can kill a motor, and it will not simply flip over and die. However, you'll notice that they cannot control the yaw. It starts flying in a circle.
Also note that again, all of this is assuming you have enough reserve power for the loss of 2 motors. Not all Multi-rotors do. Particularly heavy lifters. People carrying RED Epic cameras, they're going to fall no matter how smart the code is.
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