Balancing 3-Axis Gimbal with Sony HDR CX760

Discussion in '3 Axis Gimbal' started by Andy Johnson-Laird, Aug 16, 2012.

  1. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Has anyone successfully balanced the gimbal with this camera, please?
    The centre of gravity (CoG) for the camera appears to be right over (or pretty close) to the 1/4" x 20 tripod hole in the base of the camera.

    With the slotted camera plate mounted on the front transverse boom so that the camera platform is behind the forward transverse boom (everyone with me thus far?) the CoG for the tilt axis is way too far back (measured with the tilt servo belt removed). The camera is so tail-heavy it isn't funny.

    On the other hand if the camera plate is positioned so that it sticks forward of the forward transverse boom, I can't get the camera quite far enough back because the knurled plastic nut for the 1/4" x 20 bolt fouls against the forward transverse boom. With a bit of jockeying, I can get the camera just back far enough to balance if, and only if, the camera is horizontal.

    (But the entire gimbal assembly sits tail down if I suspend it by the center of the star-arm vibration isolator mounting plate. I'll come back to this.)

    If I manually displace the camera by manually tilting it up or down, the camera swiftly pendulums back to horizontal.

    If I rotate the camera plate boom clamps around the forward transverse boom so that the camera is tilting down, closer to a normal shooting angle, the CoG of the camera now moves sharply forward of the front transverse boom and the camera nose-dives. So balancing it is not possible, it seems.

    As for the gimbal as a whole being tail heavy, I'm going to try and move the gimbal's vertical boom as far forward as I can until it touches the Y-boom clamp and see if that brings the entire gimbal's CoG far enough forward to balance the gimbal as a whole. I'm only talking about 1/4" of movement, but it might help.

    When it comes to balancing the gimbal/camera in yaw, suspending the C8 by one boom produces a very definite (and that's understatement) yawing rotation -- the Sony camera pulls the entire gimbal around until it's at the low point. Balanced, it ain't!

    Balancing on roll seems a little easier. I got a pretty good balance -- so that's really not part of my question....

    But my original question still stands: Has anyone successfully balanced tilt and yaw with this camera please? And, if so, how did they do it?

    Thanks in advance
    Andy.
     
  2. Gary Haynes

    Gary Haynes Administrator
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    Andy I have the same issue with my Hasselblad. The center of gravity is dead center over the horizontal cross bar. No way to be balanced and a lot more weight than the CX760. My temporary solution was to take some longer 3mm screws, I think they were 40mm, put spacers between the clamps and the mounting plate to raise the mounting plate up just a bit. Rather than use the adjustable tripod screw from QC I used a very short 1/4x20 cap head that I use for mounting tripod plates to a camera body. You might find something locally but if not I get them from Really Right Stuff (http://reallyrightstuff.com/Product...q=&desc=B103:-1/4-20-x-1/2-inch-Captive-Screw). I have to mount the plate to the camera and then mount the camera/plate to the clamps. Bit of a pain but works.

    I also asked Tabb about making another plate that would extend both forward and back, not just one direction. If you fly different cameras often the only way to get close to balance is to completely remove the plate and reverse it as you described.
     
  3. Gary Haynes

    Gary Haynes Administrator
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  4. Nick Kolias

    Nick Kolias Moderator
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    Hi Andy, I'm using a big FV100 battery on a 760 and if I use a socket head cap screw to mount it all the way back as close to the tube as possible it's balanced fairly well. I think it could be balanced perfectly with a FV70 battery.

    All of that Really Right Stuff Gary mentioned is outstanding. Ultra high end craftsmanship. That screw he linked looks ideal as it would allow you to get even closer to the support tube. Great idea, Gary.
     
  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Nick:
    Thanks for the info. I just posted a follow up, but it doesn't seem to have made it to this thread....the second order problem is that the CoG of the CX760 is so low that it cannot be properly balanced because it's well below the pivot point for the tilt axis (the line between the axles of the drive wheel and the potentiometer.

    Thus the CX760 has to act like a pendulum -- you can get it to balance with the camera level, but deflect it from there and it wants to return to level because of the relative positions of the camera CoG and the pivot axis.

    I think I need to raise the camera up about an inch or so to solve that problem.

    Also, seeing as you're shooting with a CX760, are you by chance using Final Cut Pro? Just wondered how you were converting the 1080p60 -- I'm doing some experiments with Brorsoft's MTS converter and setting custom sequence settings in FCP7.

    Regards
    Andy
     
  6. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Nick: I forgot to mention that I'm also using the FV70 battery -- as I mention above, you can indeed "balance" the camera horizontally, but you cannot "park" it in any position because of the pendulum effect caused by the CoG of the camera being below the tilt pivot axis.

    Regards
    Andy
     
  7. Nick Kolias

    Nick Kolias Moderator
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    Andy, the tilt CG is indeed much too low. It's not ideal to have it that way but as a practical matter the BOSS takes care of any small anomalies from the tilt axis that result from that condition. It's really what makes the camera so magical in the air. While I agree that ideally we'd have each axis precisely balanced it's really the roll axis that's most critical with this camera since the optical block does not compensate for that.

    As for editing the footage I'm using Premiere CS6. It handles the MTS files (and all AVCHD content) natively which has been very convenient. What experiments have you been doing with transcoding for FCP?
     
  8. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Nick:
    Yeah -- I was just trying to go "by the book" and balance the camera in tilt as that's the axis I can see a shooter tweaking the most in-flight and I figured it would lighten the load on the tilt servo if it wasn't fighting the weight of the camera all the time. The BOSS works fine -- that wasn't the motivation to balance the camera -- it was that if you take the tilt belt off, you really get a sense of how much force the CX760 exerts against the servo and that seemed grossly inefficient -- especially given that, by lifting the camera up an inch or so, that force would not be required.

    I've been trying to convert the AVCHD MTS files with Brorsoft MTS Converter. It claims to input and output 1920 x 1080 at 59.94 fps ProRes 422 (HQ), but the resulting video clip from the MTS Converter, when placed in FCP's browser, appears as 1908 x 1080 at 59.94. That means you have to wait for FCP to render it all over again. Roughly speaking that means for a 10 minute AVCHD clip, you have to spend two hours -- one hour converting it in Brorsoft MTS converter and about 50 minutes while FCP7 renders it so you can cut it. I have no idea why Brorsoft's converter says it's working at 1920x1080 but then emits 1908x1080. Sigh.

    I have a feeling I'm heading towards Adobe Premiere, at least for shoots done using the CX760. Trouble is I've got so many projects done with FCP7 that I cannot jump ship entirely. (And I don't think I'm quite ready to deal with FCPX -- not to start a flame war on that topic, but...)

    Andy.
     
  9. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Another afterthought: What did you end up setting the pan Radian module gain to? At 250% it goes into advanced spasms...I'm back down to 10-20% to try and avoid it shaking itself apart! Tabb was kind enough to provide a beta set to me too.

    Andy.
     
  10. Nick Kolias

    Nick Kolias Moderator
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    Raising the camera up is the true and proper solution. One of my many problems is that I'm pretty impatient and I just slap stuff on there and go out and fly. :D Then later I realize what an idiot I am and do it right.

    I'm running 220 on my pan axis. I'm having very good performance with it at that gain.

    I'm not a editor. Not even close. But I have friends who are professional editors and they all used FCP7. I don't know any personally who are using X. But several have jumped ship to Premiere and they tell me they miss certain things but love many others that Premiere brings to the party. I was fortunate that, with no prior commitment to any particular workflow, I was advised to start with Premiere and I think it's been a good decision. Two hours to transcode and manipulate AVCHD clips in FC is a doozy. Maybe a different converter would solve your issues?
     
  11. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Raising the camera's the way to go....now need to figure out how best to fabricate something. I'm wondering whether I could use some really long metric bolts and stack a pair of boom clamps above the pair currently holding the camera platform to the front transverse boom...a double-decker boom clamp?

    220 gain? Wow. Hmm. I wonder why I'm not able to use that high a gain. I was setting up the gimbal with it hanging from a lab-clamp without the frameset on top and I think that was inducing a positive feedback oscillation -- it damn near threw itself off the table it was spazzing so violently. I'm just about to bolt the frameset back on, once I've solved the camera balancing. I think I'll need to use M3 threaded rod. (hobby-lobby.com usually carries them, but they're out of stock. Amazon/Drillspot has it: $5.12 for M3 x 1 meter -- and $6.13 shipping. Gotta love that when the shipping's more than the item.)

    Yeah. FCP7 and Avid were the way to go, but Premiere's made great progress. I know one or two shops that have abandoned FCP7 in favor of Premiere. Fortunately I have Premiere CS5, and just ordered Premiere CS6 so I'll give it a whirl with the AVCHD from the CX760. I might end up using Premiere to export a QT .mov file from the AVCHD -- how's that for being perverse -- using Premiere as a transcoder! That might be the different converter to which you're referring!
     
  12. Tabb Firchau

    Tabb Firchau Administrator
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    Hi Andy,

    You need to have the entire CineStar 8 bolted to the 360 gimbal in order to tune Pan axis. It is important as the inertia of the system provided by the CineStar is what allows you to tune the gains up.

    As an experiment try and tune it while holding the star plate and then compare to when it is attached to the multi rotor!

    Best,

    Tabb
     
  13. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Tabb:
    Yeah....the harmonics of the system are completely different with the frameset attached, aren't they? (British understatement.)

    I was just experimenting with the pan module as I was finishing up the gimbal balancing and had it dangling. So much for being lazy and not wanting to bolt the frameset on top. The tilt and roll tuned up nicely, though. I burned my boats and took the FC off (it was really only being used for the Recom SR78-05).

    I did try tuning it with the star plate, btw. The emphasis being on the word "try." (Kids, don't try this at home....)

    Andy.
     
  14. Jeff Scholl

    Jeff Scholl Distributor

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    For my 760 I have a 1/4 20 pan head bolt that I start to thread, slide it all the way foward till it hits the carbon tube, and then tighten down. The bolt has a shallow but wide pan head that I'm able to tighten down with a flathead screwdriver. Also I put gaff tape on the gimbal plate and the bottom of the camera to prevent any sliding. It is a bit low in the CG, but it hasn't been a problem when I review the footage. For the pan on my 360 I have extension tubes moving the camera foward from my FS700 that also put the 760 into a good position.

    Cheers,
    Jeff
     
  15. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Jeff:
    I put thin strips of camper tape on the camera plate. Seems to work pretty well. The other thing I figured out is that if I slide the CX760 forward slightly, with the drive belt removed, it adopts a natural "angle of dangle" position of about 25 - 30 degrees nose down -- so at that angle the servo is not going to have to work. That, of course, can be adjusted for the kind of shoot, but it's a compromise until I come up with a better way of elevating the camera to bring its CoG up to the tilt pivot axis.

    On the other hand I may just take the position "let the damn servos work -- I have to, so why shouldn't they -- they just sit there and spin their wheels all day...." ;)

    Thanks
    Andy
     
  16. Joe Azzarelli

    Joe Azzarelli Active Member

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    I found the only rational thing to do is set aside my frustrations and prejudices and just jump in to FCPX. There is good and bad but nice import of AVCHD is a good thing.

    Best
    Joe
     
  17. Jeff Scholl

    Jeff Scholl Distributor

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    Andy,
    My latest config is rotating the gimbal plate 180, and then mounting the 760 foward of the boom. This configuration with a larger 760 battery balances perfectly with the camera pointed down around 20 degrees. You will also enjoy longer battery duration with the 760.

    Cheers,
    Jeff
     
  18. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    True. But given limited time and the fact that Adobe Premiere (and Media Converter) import AVCHD directly, I'm being indolent and taking the path of least resistance.

    But you're absolutely correct, Joe.
     
  19. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Thanks, Jeff. I intuitively ended up doing that. The gimbal was still a little tail-heavy but that pretty much went away when I moved the Mondo Stinger from the gimbal boom nearest the down-tube on the gimbal out onto the left forward gimbal leg (viewing the gimbal from behind the camera). That shifted the combined CoG forware enough -- and also allowed me to mount the Radian sensor above the roll servo.

    Andy.
     
  20. Rex D Peters

    Rex D Peters New Member

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    Andy
    I use FCPX on my MacBook Pro. I insert the SD Card in the slot and import via import from camera and it goes straight to FCPX.
    Thought you'd like to know
    Rex Peters
     
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