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Old Jul 13, 2014, 01:59 AM
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Norway, Ostfold, Askim
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Originally Posted by Stbrightman View Post
Looks like prop wash causing it to me.. what is your setup: quad, X, spider? Is the camera/gimbal in the prop wash?( keeping in mind that the direction of flight (relative wind) can move the prop wash over the camera and cause turbulence.) I see it happens badly during forward flight, and less severe in reverse flight.
its a 1070mm quad with a front mounted gimbal, and the gimbal is mounted level with the props
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Old Jul 13, 2014, 10:01 AM
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Canada, QC, Blainville
Joined Aug 2009
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Anyone tried to stop propwash (on the gimbal) by adding a plate on top of the gimbal to protect it from the wind ?

i guess this would not be too good for the effectiveness of the props but might make better video.
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Old Jul 15, 2014, 03:42 PM
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Joined Jan 2013
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Hi guys
it is important how is 3rd axis expansion board turned?

my 3rd axis when i connect battery does not come in ZERO its stay where i turn it before connect power

how i can correct this (firmware ver is 2.2b2)

TY
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Last edited by Z303; Jul 15, 2014 at 03:48 PM.
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Old Jul 20, 2014, 04:32 AM
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Quote:
Originally Posted by ivers View Post
its a 1070mm quad with a front mounted gimbal, and the gimbal is mounted level with the props
You say the gimbal is mounted level with the props, but where is the camera in relation to them? Is it level with the props? Maybe a pic would help because I'm assuming you're running a traditionally configured gimbal where the camera hangs vertically from the mount, and it seems improbable that the camera would NOT be below the props if that is the case. Look at the propeller disc of those two front props and if they even come close to overlapping the camera (looking from top down) in this case, you will get turbulence and shake in the footage. A better place to mount the gimbal is obviously going to be under the center of the quad if possible.
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Old Jul 20, 2014, 04:34 AM
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Quote:
Originally Posted by bakchos View Post
Anyone tried to stop propwash (on the gimbal) by adding a plate on top of the gimbal to protect it from the wind ?

i guess this would not be too good for the effectiveness of the props but might make better video.
I would think adding anything to the gimbal itself would only catch more air and cause more shake. If the plate were attached to the frame instead, it might help.
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Old Jul 20, 2014, 06:59 AM
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Canada, QC, Blainville
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Quote:
Originally Posted by Stbrightman View Post
I would think adding anything to the gimbal itself would only catch more air and cause more shake. If the plate were attached to the frame instead, it might help.
Yes that what I mean, on top of the gimbal but attached to the frame arms
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Old Yesterday, 03:20 AM
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Romania, Bucharest
Joined Jul 2010
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I have a big annoying issue with an Alexmos 2axis + 3rd axis extension and a quad with NAZA v2, the gimbal works perfectly on the ground but when up in the air it starts oscillating on roll axis. It is not a typical "too-much-D-gain-oscillation" (as far as I tested) it's different, the gimbal controller is fighting with the NAZA controller of the quadcopter for some reason I don't fully understand. I know that I should present detailed specs for my setup so somebody could give a precise possible solution... the point is that I the quadcopter belong to a friend of mine and I took care of the gimbal. We just "joined" the 2 assemblies but the result was not as expected.
So, until in the evening when I hope I could come back with some pictures and video, here is a list of what we tried so far:
- I can't tell much about the copter, (my friend was in charged with building it), it is a Tarot carbon frame (45-50 cm I think), some quality motors, NAZA v2 FC., 5000mAh 4S top mounted. It flies beautifully, very stable, precise, it can lift more a few kilos... on the bottom line the exact flying platform for aerial video. My intention was the have an independent 3 axis gimbal with a dedicated operator.
- the gimbal was purchased last year when 3 axis gimbals weren't very popular yet, so I transformed it into a 3 axis one by adding some CNC machined hardware parts and iPower4008 motor for yaw axis. For suspending the gimbal by the frame I used 4 rubber dampers (during the tests I tried different styles too).
- the modification I made caused an elongation of the gimbal (I stacked the Alexmos board, the 2S battery, separate RC receiver, vTX. The total weight of the gimbal, with everything attached is about 500gr.
- another add-on I made was to add some retractable legs attached to the copter frame.
- the gimbal performs excellent on the ground, even up in the air is working almost all the time but sometimes, after I raise the retractable legs (they are moving on the roll axis too) or if my friend puts the quad into some sudden extremely roll positions, the copter starts to oscillate. It is like Alexmos board and NAZA are summing up their compensations resulting this unwanted roll oscillation. The amount of oscillation is increasing up (in a few seconds) to a certain point and after this is stays constant. I could stop this oscillation in midair just by hand (touch the GoPro) or after putting the retractable legs into landing position (so I changed the CG of the frame I guess). But not everytime, sometimes I had to land.
What I tried:
- different values for ROLL axis PID gain in Alexmoss
- smalled values for I parameter
What I intend to try:
- different PID gains for NAZA
- change the rubber dumpers with a more softer ones
- shortening the total length of the gimbal (I noticed all the GoPro 3 Axis gimbals are very short in height)
- change the yaw engine with a more robust one, maybe a slip ring type to have a stiffer gimbal.
- move the quadcopter battery below the frame

Has anyone experienced this behavior with the gimbal drastically influencing the flying platform?

Thank you for the patience to read my long post.
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Old Yesterday, 06:31 AM
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Yes it could, but it appears that this platform is really big compared to the gimbal, so not very likely in this case.

What do you mean "after I raise the retractable legs (they are moving on the roll axis too)" I suppose they are attached to the airframe anyway?

My guess would be that the retractable legs affect the airflow, causing all kind of problems. Maybe also the vibration dampening still needs some work. I have noted that if the dampening plate (above yaw) is very light, the gimbal motors actually moving the dampening plate and this causes very low frequency oscillations. If you attach the batteries to the gimbal assembly on the dampening plate and make the dampers stiffer, that could help, maybe...
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Old Today, 12:36 AM
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Romania, Bucharest
Joined Jul 2010
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Yes, the retractable were attached to the airframe. They "were" because yesterday, after so many hours of frustration, when we finally had founded the right dampers and "I" parameter combination and the nasty oscillation finally disappeared, my friend (too happy because we finally had made it) ran the quad straight into some cables in the area we were making the tests. It could have been total loss, but the main parts survived the impact: Alexmoss board, the gimbal, GoPro3 and the airframe but not the iPower gimbal motor for yaw, the retractable legs and some hardware parts.
After 12h of redesignig(Solid Works) and machining the gimbal connecting parts, we managed to assemble everything, this time with a new solution for the landing gear, we attached it to the gimbal. We finished at 2.00 in the night so today there will be the tests. I am a little worried about the 3rd axis as the gimbal has now some legs attached to it and a bigger motor but I hope it will work properly. As you can see in the video, if this won't work, we will have a nice wheel of fortune
My new 3axis gimbal (0 min 13 sec)

Oh, I've forgot to tell the most important part: we have to provide an aerial video shooting on Saturday`

Anyway, as this post is about Alexmoss... I can conclude my little adventure that when the gimbal and the FC are influencing each other, the "I" parameter is the one to tweak, so the 2 inertial systems are not on the same compensation speed.
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Old Today, 02:09 AM
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Interesting wheel of fortune :-)

However I am afraid that will not be good, too much inertia especial when/if the legs are up and aerodynamic forces, but pioneering work takes trial and error and I always learn from it, but who knows, lets see how it works. The legs could be much smaller now and I guess no need to lift them up.
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Last edited by GARUG; Today at 02:15 AM.
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Old Today, 02:39 AM
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Romania, Bucharest
Joined Jul 2010
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Indeed, the legs are oversized (though they are not so heavy), they were from another bird, we were just lazy to cut them (we also have a plan B which means we will remake the old solution with a smaller yaw axis motor but no landing gear at all, taking of and landing will occur from my hands). And yes, I am afraid of too much inertia too with this new setup. Anyway, AlexMos based board is a great and flexible system, if offers many tweaks and in my case the possibility to safely take off. How is that? Simple, when the copter is landed, the yaw axis will struggle to compensate by rotating the airframe (a normal behavior). This way the taking off could be tricky but luckily Simple BGC is stuffed with so many useful features and Motor ON/OFF toggle is one of them . So after powering up the gimbal I'll turn OFF the motors by the CMD channel on the radio. After the quad will be in the air I'll switch on the motors.
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Old Today, 03:05 AM
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Germany, NRW, Dusseldorf
Joined Jun 2010
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We use the wheel-of-fortune solution on a heavy lifter for quite some time with the 3rd axis driven by a servo, directly attached to the camopīs radio.
Works very well, if the weight/masses of the gimbal/landing gear construction are distributed around the COG.

Otherwise when you turn the gimbal fast, the craft could start some kind of small toilet bowl.
In reality you rather do slow&smooth turns for pans and roundshots.
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Old Today, 03:07 AM
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Looking the picture: making the distance between roll and yaw motor as short as possible would be good.
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Old Today, 03:27 AM
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Romania, Bucharest
Joined Jul 2010
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Yes, this was another thing that yesterday brought the (short) success, I shortened the height of the gimbal so it was less prone to roll oscillations. Combined with low "I" values those oscillations disappeared. Another concern with shortening very much the gimbal height is that the propellers would get into the image, although I prefer narrow mode with GoPro 3 and after all, aerial video means more shooting towards ground not to the sky.
Today I'll find the answers to all my questions, I hope.
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