Tuning a flybarless system (like vbar, ikon, etc.) it’s much easier seams that the results are better (Heli behave more accurate and stable), so I’m thinking to use one of those flybarless systems with the Pixhawk because Pixhawk tuning is frustrating in Heli mode (PIDs, tail and other tuning).
My questions are:
What is the downside of this setup?
Did someone try it and can advise?
In this setup, is the pixhawk still relay on his sensors for stabilized the Heli or just command the the flybarless system (of course it will use the gyro and GPS but it will use accelerometers?)
I’ve flown one with downstream FBL just to get experience with it and it can be problematic depending on the FBL unit. The FBL unit has to do rate-only control for it to work. And there is no advantage in making the helicopter more stable or accurate. The main problem making helicopters “jittery” is vibration and the inability of the IMU’s to damp it very well. If the IMU’s are not accurate, the heli is not going to fly right.
This is a demo I did with a controller mounted direct to the FBL unit tray on a 766 gas with the only vibration isolation being two strips of velcro. It is every bit as stable and glitch-free as any FBL unit I’ve ever seen. I have since about 6.5 hours of flight time with this controller, flying survey flights that last up to an hour, and the two strips of Velcro are all I’m using for a mount: No fancy engineered vibration isolation etc… - and it flies par excellence.
Also we now have parameter files you can drop into your heli that will fly it “out of the box” with only some minor tweaks required for how you want it to handle, and the particular tail servo you are using. For Copter 3.6 beta the parameter file is built in and can load it with Mission Planner.
For either Copter 3.5 or 3.6 you can download it from here for either Mission Planner or APM Planner2 for Copter 3.5. Or if you use QGroundControl (recommended as the daily build of QGC has a new heli setup page that works really nice) there is one you can load with QGC for Copter 3.6.
I don’t know how you are doing it… my Pixhawk 2.1 shows lots of vibrations and cliping issues (sometimes even at landing).
The Heli crash during landing at Loiter mode so I decided to start all over again (Firmware, remote setup, etc.), hope it will change something in the Heli behavior.
What type of mount are you using for your PH2.1? Both the PH2.1 and Pixhack V3 have internally damped IMU’s. If the mount allows too much movement of the controller the internal damping does not work right. Like those rubber ball “jello plate” mounts used to work pretty good with the old Pixhawk 1’s, I’ve found it requires a more rigid mount with the newer controllers with internal damping. The PH2.1 has four holes in the bottom. A mount that I’ve found works well with those is to make an aluminum plate about the same size as the carrier board and about 3-4mm thickness. Use the holes to screw the controller direct to the aluminum plate. This adds mass to the controller. Then stick it on the frame mount with velcro. And install it level - mounting the PH2.1 on its side will cause the internal damping to not work as well. The Pixhack V3 or V5 can be mounted on its side and it doesn’t affect it. It uses an independent silicone damped IMU board instead of foam.
I’ve also found the two-piece controllers like the PH2.1 or Pixhack V5 are a little more sensitive to vibration than the flat controllers like the Pixhack V3. The V3 is not much bigger than a FBL unit. I experimented with a V3 and a V5 on the same helicopter, same mount, and the V3 registers lower vibration than V5. My theory is that the IMU’s in the two-piece controllers are further from the mount (the source of the vibration). Since the IMU’s are basically sensing accelerations, if the controller can move at all the IMU’s will sense higher accelerations if it is further from the source.
Maybe try my “trick” with attaching an aluminum plate to the bottom of the Pixhawk2.1 and then put velcro between the plate and the frame mount. I use two strips of velco about 15mm wide on both ends of the plate and it works pretty good. Also allows easy removal of the controller for access to wiring connectors, etc…
The differences between the Pixhack and Pixhawk are pretty significant. Both the Pixhack V3 and Pixhawk2.1 are PX4 FMU-v3. The new Pixhack V5 (slated for release sometime this month I think) is a FMU-v5 design with double the RAM, more powerful processor, hardware double-precision Floating Point Unit. The IMU damping between the two is significantly different as well.
I think the controllers manufacturer should separate the the sensors board from the carier board, that way you need much more smaller dumping system and much less cables connected (cables prevent dumping and passing vibrations), like in the Futaba CGY750.
I’m still thinking to use a good reliable external FBL system (like VBAR, Futaba, Skookum) and use small FC (Pixracer or Pixhack V3) and get reed of EKF switching and other IMUs conflicts.
I’m not familiar with the Drotek one. But I got a Pixhack V3 on my 766 Gas and it is an excellent controller. I like the design of these as any sort of stiff wiring connected to the face of the controller is a bad idea for vibration transfer. This one is mounted the same way - aluminum plate underneath it with velcro between the plate and the FBL unit tray. It has three IMU’s, dual baro’s, dual mags, etc… Which I think is ridiculous. Throwing all those extra sensors in there doesn’t achieve anything except for looking good on paper for supposed “redundancy”. In the real world it don’t work.
In the V5 they went back to just dual IMU’s, single compass, baro, etc… Which makes a lot more sense and makes the controller more reliable IMO. Because the EKF system is designed to cope with a failed sensor anyway. If a mag or baro goes on vacation it can use the GPS.
The Pixhack V3 is better at isolating vibration than the V5. But it’s a difference of 4-6 vs 6-8 in this heli. Which is minor, and the design of the IMU board in the V5 keeps the IMU’s consistent so there is no aliasing. Then the EKF can switch and it don’t make any difference. I tested the V5 by banging the heli into the ground at 5G, and subjecting it to vibes of 50-70 and it never affected it as far as IMU consistency. It will be my “go-to” controller for helicopters when it becomes available.
