My custom hex pixhawk APM 3.1.2 flies like a dream. I didn’t even have to auto-tune or trim-save it.
What I have noticed is that during a rapid descent, it wobbles and can even flip (or almost flip). Can someone tell me exactly what parameter I can set (for stabilize and alt-hold mode) that will prevent it from descending too quickly even if I’m a little overzealous on cutting-back the throttle?
Thank you in advance,
Brian
you will find descend parameters (two) here: copter.ardupilot.com/wiki/arducopter-parameters/
these will not limit stabilize mode, if you need to limit that, the only option is MOT_SPIN_ARMED
Anyway, to feel the turbulent air during vertical descend is normal - but if it’s like you describe it below a sink rate of ~1.5m/s , (you almost flip), then you should check for motor/esc sync problems, or you will crash.
It definitely isn’t a motor sync issue (I know that for a number of reasons).
The vehicle has impressive performance characteristics. It’s basically some T-Motor U3’s, a battery and a little bit of carbon fiber holding the entire thing together.
Here’s what I’ve noticed though: If I take the throttle to zero, the props actually stop spinning – complete stop. So let’s say I’m several hundred feet high and want to lose altitude quickly. If I take the throttle to near zero, I get more than the characteristic wobble that I see in my other vehicles. This one will just start and uncontrolled tumble out of the air.
What I’d like is a parameter that prevents me from actually killing the motors. But I’m also a little nervous about setting a minimum throttle, because this thing hovers at 25%-30% throttle. I could see myself setting the throttle minimum high enough that it could never descend again (and give me a fly-away).
Thoughts?
What is your MOT_SPIN_ARMED parameter? If it is zero it means your props will stop spinning at zero throttle which is what you don’t want according to what you said. Also, you may want to change your throttle mid position to something more reasonable so that the copter hovers at 50% throttle and not lower.
Other than that, I would advise using alt_hold mode and perhaps set a larger descent rate than specified so that you can have a quick but controlled descent without the danger of stopping your props completely.
Finally, I know this will be tricky, but make sure your center of gravity is near the middle of the copter to avoid flips ands so forth when it falls without power. (If you are trimming lots on your pitch/ “aileron” channel it may be one indication of bad balance.)
[quote=“saadman”]What is your MOT_SPIN_ARMED parameter? If it is zero it means your props will stop spinning at zero throttle which is what you don’t want according to what you said. Also, you may want to change your throttle mid position to something more reasonable so that the copter hovers at 50% throttle and not lower.
Other than that, I would advise using alt_hold mode and perhaps set a larger descent rate than specified so that you can have a quick but controlled descent without the danger of stopping your props completely.
Finally, I know this will be tricky, but make sure your center of gravity is near the middle of the copter to avoid flips ands so forth when it falls without power. (If you are trimming lots on your pitch/ “aileron” channel it may be one indication of bad balance.)[/quote]
Thank you for the input. I’ll check MOT_SPIN_ARMED tonight. What is a reasonable value for this parameter? On my Pixhawk quad, the motors turn at minimum throttle, albeit just barely enough to keep the vehicle level on the ground. On the Pixhawk hex, the motors come to a complete stop which is what I’m almost certain is happening in the air.
I’m not concerned about the COG, because I built the entire thing from scratch and designed it to be radially symmetric (except the battery pack, which is bilaterally symmetric and balanced front-to-back).
My MOT_SPIN_ARMED is 70, but my props definitely don’t spin when armed and at minimum throttle.
My throttle_min is 130 (which I believe is the default).
Any ideas?
Also… let’s say I screw-up one (or both) of those values. As long as I keep my RTL switch programmed and handy, will it override those values and land the craft?
Hi, I did have the same issue here …
I’ve just re-tuned my pid settings ( raised P on roll and pitch until it induces a wooble, then raised I and D )
Did a couple of “free-fall” checks and put throttle back then it goes level within a few meters, basically ok if the copter falls due to baroglitch in AltHold mode … just have to check some RTL in the next days …
I wonder if there’s some settings which tells the quad to stay leveled even if it falls ( throttle < 20 % )
or a min_throttle after takeoff setting or a maximal height drop / sec : 3m/s would be also a great value …
I’ve previously encountered some problems while testing RTL, some baro glitches leaded to previous crashes, so I’ve got to insist on PID settings … cause it is the thing I’ve skipped a lot …
I want to set up my quad such that on taking throttle down to zero it starts losing altitude but not too fast to crash and come down to ground like DJI controller does (phantom)