Recently faced with the following problem: I already balanced my BMPCC with Lumix 12-35 mm lenses on FF 3 axis camera gimbal . Rather tricky… but anyway…. So, in whole process of balancing the camera, my main target was to have camera located at particular point, so, when it is tilted down 90grad (looks exactly down) the center of rotation (pan) of gimbal to be exactly on the same line as the optical axis of the lens. I like the shots, when camera looks 90 grad down and rotates, but this shots are more beautiful when camera rotates around its optical axis. And here comes my problem. Having camera at that particular place I have my whole gimbal CG out of balance. I mean it is bottom heavy. The only way to balance the CG is to move camera forward, but I can't do it, because I will shift the camera's optical axis from the center rotation of the gimbal. So: 1. I can mount the gimbal to CS8 as it is now and let's CS brain to make everything level. And I know, that in that case some of the motors will work with increased load to compensate the issue. 2. I can put some load on the front, so the gimbal's CG will be balanced, but the gimbal will become more heavy and in that case all the motors will work with equal load, but heavier gimbal will also eat my flight time. SO, which case is more acceptable and more correct way. Thank You in advance David
David Answer: Neither First question: Is your tilt balance perfect from flat position to completely down 90 degrees? IF your tilt balance is perfect your CG shoudn't change hardly at all regardless of its position or selection of lens. For the tilt balance of the camera, it should be able to easily stay in any position from straight ahead to straight down. If you can't achieve this then its not optimized and you need to readdress. The adjustable tilt bars allow you to raise or lower the CG of the camera to help get a better balance on the tilt. If the CG of the camera is too low you will notice that the camera's tilt wont stay still regardless of position because it will always want to self level. IF the cameras CG is too high it will stay balanced in straighter positions but as soon as it tilts down more the inertia of the camera will move the camera and it won't stay in balance. The trick is to find the sweet spot for the vertical CG of the camera and you use the height of the adjustable plates to nail it. Once you nail the tilt balance the CG of the gimbal (front to back) should remain pretty much the same regardless of the position of the lens. This is not an easy thing sometimes. it can take me 6-8 hours to completely dial in a camera's balance and CG from starting from scratch on a new combo.
Dave Thank You for the answer, but I am afraid, that You did not understand what exactly is the problem. The tilt balance is perfect ( ok, may be not so perfect as it should be in case with brashness motors, but for servos it's good enough) Also the Roll balance. And when I tilt camera all way down (I mean when I move it using my radio) the optical axis of the lens and the centre of the rotation of the gimbal ( the hole on the mounting cross-form bracket) coincides with each other. This is what I want. Till here everything is ok. But the problem is: while holding the gimbal up with a string with it's one end tied to the center of cross-form bracket the whole gimbal is bends backwards, showing that it's bottom heavy. I try to upload pictures, but do not know how it can be done here. David
g So, here You can see what I mean. I would like when I tilt all down the camera's optical axis coincide with the center of rotation of the gimbal, as You can see in the picture
And here You can see what happens when I try to lift up the gimbal with a string tied to the center of gimbal. It seems obvious to shift camera together with the camera plate and side booms forward, to move CG of the gimbal forward. But I can not do it, because in that case 2 axises will not coincide any more. Does it mean that FF 3 axis gimbal is not compatible with BMPCC? Seems, that implementation of my task on this gimbal is impossible.
Your not bottom heavy, your back heavy on the pan axis. Have a look at the pan balancing tutorial for the MoVI on the freeflysystems page.. Same principle applies.
With the pictures I see (especially the one in post #5) shows the CG is way out. The CG can't go that out of whack with just the camera tilted down. I believe your problem is that you need to move the horizontal bars up front more which will move your camera forward more. You have way too much space behind the rear clamps where it meets the roll bar. You need to slide everything up closer to the front several inches.
At the end of the day just take your time. Maybe setup the camera in the position you want, then using a series of weights being added get it into balance. The balance point is the 3 axis center of gravity. In this case you want the center of the lens for pan. Just work the problem backwards and see if you can get there without adding so much weight that you can't fly.
Gary, Dave, Brad Thanks for reply. I hope You got the main Idea why the problem persists. If I did not want the optical axis of lens ( when it is pointed all way down) to be coincide with the center of rotation of the gimbal, which is the little hole on the center of CineStar-8 Gimbal Vibration Plate, I would move just horizontal bars with camera forward to move CG forward until the gimbal will be level. BUT I can't NOT do it, because I will move the optical axis of the down oriented camera forward so it will be no more coincide with the center of rotation. That is why , as I can imagine , the only solution in this case is to add some weights to both front landing gears until I get equilibrium. Another solution is leave the gimbal as is and let the CS8 make it level during the fly. Thanks again David
I see what you are trying to do. You want the lens directly under the pan axis. You might be better off just balancing it, if the lens is not too far off center. With the Radians it really needs to be balanced well. I have a gimbal for a NEX-5, where the lens is way on one side of the camera. With the camera balanced the lens is not centered on the tilt axis, by about 2 cm, but I still get pretty good results. With you side arms all the way forward like that and your CG that far off I don't think you would have a choice but to add weight. But you might have to add too much for the heli and really effect it's performance.
I think it would take quite a bit of added weight to get that front to back balance to come in. Possibily 400 grams?
Dave, I think even more . I am not sure that addition of that load could be a good idea. So, for the moment the decision is: 1. I will move camera forward and will make the gimbal level "on the air". It will be normal setup. 2. In some cases, when I will need those multi-rotation (several turns 360 grad) with camera faced strictly down coincided with the center of rotation of gimbal, I will put additional weights in front, knowing that I have decreased flight time. Thanks all of You. David
If your more than 10ft from your subject matter then I think your doing a lot of work for nothing. Your not going to notice the optical center being off by 2-3 inches when your up 100ft.
I started to type a very similar comment this afternoon but decided blah...didn't have the energy. David just balance your gimbal as usual and go fly. This is a non issue...really.