Servers by jDrones

Dual-motor tailsitters

(Mark Whitehorn) #1813

I’m afraid that without a log, we won’t be able to know for certain what caused the bad transition. My first guess would be that the PID gains for that axis are too high. Tailsitter body-frame yaw control authority is very high, since it is provided by the wing motors. Unless I’m confused again :slight_smile: that’s the copter roll axis, so the relevant params for Q modes are q_a_rat_rll*.

With a dual-motor tailsitter there’s also the possibility of the roll and yaw controllers interacting in a bad way; were the elevons flapping about a lot too?

@iampete any thoughts on this?

(Peter Hall) #1814

As you say could be some interaction between roll and yaw controllers.

Hard to draw any conclusions without a log or vid.

(letpi) #1815

No elevons flapping but the impression that one motor acceleration was faster than the other. Transition from FBWA are still very reliable. I will investigate more the transition from qacro with a SD card this time. We had a lot of wind last week so I hardly found at least 10 minutes to fly. I hope coming days will be better.

(Cuong le) #1816

Hello everyone!
I’m new to vtol tailsitter! I am planning to mod my Zohd Orbit kit to duo tailsitter, if anyone had done with this kit and successful?

(letpi) #1817

I made a flight this evening with my test wing quad motor to compare back transition from FBWA or qacro. In fact the answer seems very simple and there is no bug or bad parameter.

This is a normal transition from FBWA, all four motors output is set to q_m_thst_hover until the transition angle is reached.

This is the transition from qacro, no output set to q_m_thst_hover but the bottom motor works very hard to get the wing vertical and it works very nicely. Of course no transition angle is involved because we switch from a qmode to an other qmode.

But for a dual motors tailsitter at low speed it is very hard or impossible to make a successful transition because it needs airflow from motors and from speed. So the code is very OK, maybe a warning would be useful.

the log

(Mark Whitehorn) #1818

Yes, this is a major disadvantage of a non-vectored dual-motor tailsitter. I had tried to write code to automatically boost the throttles when in this situation, but didn’t have much success (although the PX4 code has a feature to recover using roll/yaw gymnastics, that takes a fair amount of space).

So I agree that a warning should be in the wiki somewhere; would you be willing to suggest some text?

(letpi) #1819

Hello,
I don’t think the Zohd Orbit has been modified to dual motor tailsitter. You have the choice to make it vectored or not. I strongly advise to begin with vectored motors because the zohd has small elevon surface and is not designed to be a tailsitter. Without vectored thrust the pitch axis will be very hard to control. To begin you don’t need specific servo or a complex mechanism, a 20g servo and ±30° will be good enough. Rotation axle should be in front of the CG.
An other possibility is to make a quad motors tailsitter, this is probably the most stable configuration and you avoid the complexity of tilt motors.

(Pilot) #1820

Hi, what a great topic you have here!
Took me a while to read through all posts, and it did provoke some thought process. I’d like to share some ideas and observations.
If main problem of non-vectored tailsitters is different requirement for CG an in forward flight and hover, maybe it’s possible to change location of CG by moving battery inside the plane by means of servo actuated by estimated airspeed. Another option is to move center of pressure by moving tail/canard type surface back and forth (less feasible).
Dozens of tunable parameters are intimidating, and their number is continuing to grow. Maybe some kind of analysis can be done on logs to automatically find better set of parameters, and then load them into aircraft and fly again, then repeat.
Existence of two sets of PID parameters (plane/quad modes) complicate things two. In reality it’s the same aircraft, so only one controller should exist, (obviously dependent on the airspeed).
Also there is no technical reasons (other than operator convinience) to have speed/pitch angle limited in qstabilize mode. Transition can be seamless (in theory).
On note of transition - if you need rapid transition from hover to FF, it can be done via yaw/roll combination (sort of knife edge).
About quad tailsitters without control surfaces - should be possible, do not see why not. But the frame should be aerodynamically stable in forward flight.
Overall, I think we should rethink controller hierarchy. Airframe types have a lot in common (some combination of (potentially vectored) motors, fixed and articulated aerodynamic surfaces). Given the configuration, there should be a way to know what frame can do (max angular acceleration vector, max acceleration (thrust + lift) and how (servo/motor outputs given current state, including airspeed vector)). It can probably lead to unification of plane/quadplane/quad/tailsitter codebases.
Modes, on the other side, should concern only translating user input/waypoints to desired linear and angular acceleration vectors. Maybe it’s how it works now, I just had a brief look at the codebase.
I was thinking, vectored tailsitter with control surfaces can probably move without roll if motor tilt and ailerons with not work together, but against each other, giving zero pitch change (but very slowly, because vectored motors have much greater pitch authority then ailerons).
Overall, I am very excited because I think tailsitters with allow for very efficient flight in terms of range - very high wing loads become possible without concern of takeoff/landing speeds.

(letpi) #1821

congratulation for reading this long long topic. This not easy to answer your long post with so many thought Moving Cg is a good idea, tailsitter concept is interesting because it is open to innovative design like the amazon delivery drone shown recently by @iampete.
You are right, there is two set of PID but the work recently done by @kd0aij and @iampete about qacro and several gain attenuation method allow to fly with only one set. Stabilize mode with limited max angle is very useful because most airframe are not able to recover at high lean angle and low speed, specially is they are not perfectly tuned.
Probably a lot more can be done, I cant say, my skill is more about building and tuning. But lets go.

(letpi) #1822

This is not an exercise I am not familiar with but I can not escape :innocent: So the warning could be:
Ttransition from qacro mode to any qstabilize mode should be avoided as no throttle boost is involved. This is critical when airspeed is needed for attitude control, eg dual motor tailsitter.

(Mark Whitehorn) #1823

Thanks! I started a PR for the wiki here: https://github.com/ArduPilot/ardupilot_wiki/pull/1799

I tried to expand the explanation a little bit; let me know what you think.

(Mark Whitehorn) #1824

Welcome @bojxkimq and thanks for your comments.

As @losawing pointed out, we have made some progress toward using a single attitude controller in all flight regimes of a tailsitter, but there are very few people in the world that would qualify as test pilots :slight_smile:

One thing that has been encouraging is the success which @losawing has shown with the gain scaling option which needs no airspeed sensor. That allows 3D acrobatic flight over a large speed range with a very simple quad tailsitter.

(Pilot) #1825

I think the problem of lack of the test pilots is because setting up ardupilot for tailsitter is very involving (dozens of test flights and half as many crashes). And even after that you will just hope EKF will not freak out or you will not experience instability is some corner of flight envelope.
Regarding EKF, how feasible do you think is to have 6DOF + baro only, but including control outputs (motors + servos) as inputs of EKF for estimation of wind speeds?

(lorbass) #1826

Yes, that makes it interesting and it is a challenge to solve all problems.
The only area where creativity can still be lived compared whith unboxing and play.
There the only challenge is to be able to charge the battery.

I’m not a developer but as I understand it is possible to get a branch and to follow another way in programming. http://ardupilot.org/dev/index.html

(Mark Whitehorn) #1827

The ArduPilot EKF is extremely robust now. I have installed a PixRacer/GPS as a blackbox logger in an RC pattern plane which was then flown by an expert pilot; lots of inverted flight and aerobatics including snap rolls. The EKF attitude estimate has never diverged in these flights, so I think it is quite capable of providing the attitude estimate for a tailsitter: https://drive.google.com/open?id=1rWynvwDezjSUxcP3tYw3upWoesd6r1FR
Not only is it stable, but the accuracy of its results will be better than that using gyro/accel/baro alone.

(Pilot) #1828

I am quite sure it’s very robust for conventional plane dynamics, and with some altitude where GPS has good fix (no multipath, no obstructed satellites).
I understand having GPS is better than not, but it would be nice to be able to do tests indoors, where GPS is not available, and cope with GPS problems if they arise.

(Sam Salway) #1829

Hi all. Long time reader, first time posting. Thanks to everyone who contributes so much time to making tailsitters possible. I’ve learnt a lot from reading this thread.

I’ve managed to modify an FX-61 as a tailsitter. It hovers pretty well after manual tuning (zeigler nichols method as someone above suggested). I’ve just done flight tuning with autotune, it’s flying fine. However, transition back to hover is doing some weird stuff. Desired pitch seems to oscillate, then throttle gets cut. It looks pretty wild in the air, but eventually it hits the throttle again and gets right. Anyone have any suggestions? I’m hoping to get it a bit smoother than this.

Edit: log https://drive.google.com/open?id=1DmQmVAD1B3-l6N2KQPL35EIuo8J-jsUT

(letpi) #1830

Glad to see a new tailsitter user.
Is the log the right one ? I can not find any transition, just auto and FBW.
A photo of your FX61 would be great too.

(Mark Whitehorn) #1831

I’m not sure what’s going on with desired_pitch, but it looks like autotune may have set your pitch gain too high. Did it increase significantly from your manual tuning value?

(Sam Salway) #1832

I just downloaded the log from the link to make sure, yeah it’s the right log. I did an auto mission twice. The mission was vtol takeoff, fly through 5 waypoints, vtol landing. It’s after the last waypoint when it goes to vtol landing when it transitions.