Canon XH-A1, Glidecam 4000Pro and Dynamic Balance

[Julian Frost]

Maybe someone can explain this to me, because no matter how I try, I can't get my head around it!

My Glidecam 4000Pro and XH-A1 have great static balance. But, if I hold the Glidecam so that its Central Post is horizontal, and balance the sled end on my finger and allow the system to find its own resting point, the camera is always either front or rear heavy. This means that the system can't be in perfect dynamic balance because one end of the camera is "heavier" than the other and if I spin my body, the camera will not remain pointing in the same direction, but will drift. The question is, how would I correct this? Let's give an example...

The Glidecam manual says to find the camera's center of gravity (COG) and attach the camera (at its COG) to the center of the Head Plate. After everything has been assembled, you move the camera forwards, backwards and side to side to achieve perfect static balance.

Suppose now that we have perfect static balance. In achieving static balance, we had to move the head plate around and the camera will not be sitting directly over the absolute center of the Central Post. Suppose we now hold the Central Post horizontal, and the camera causes the Central Post to rotate, resulting in the front of the camera pointing down. There are two ways I can see of correcting this:

1) Move the Head Plate back to balance the camera until the camera remains horizontal

2) Move the weights on the front of the sled towards the Central Post until the camera remains horizontal

As far as I can tell, if I do EITHER of these, it will change the system's STATIC balance, and the camera will tilt up! To correct that, I could move the camera forward, or add weight to the front of the sled, but both methods would take the system out of dynamic balance again!

To me, it looks like a vicious circle!

Is my thinking wrong?

[Charles Papert]

If your spin results in the nose of your camera pointing down, move the camera forward and the lower weights backwards until you regain static balance. The next spin should be better. Just keep adjusting until you find the situation improving--if it worsens, go back the other way.

[Julian Frost]

Hi Charles,

Just to make sure I'm getting what you're saying...

If, during the "horizontal spin test", the camera is nose heavy, I move the camera Further forward, and then move the FRONT weights on the Base Plate back towards the Central Post, until I achieve Static Balance again? Repeat the horizontal spin test to see if that helped.

Wouldn't moving the camera forward make it even more nose heavy? Why wouldn't I move the camera *back*, and adjust the front Base Plate weights forward?

I hate to ask these confirmation questions, especially as I know that you're certainly The Man, when it comes to Steadycam operation! Thanks again for your time.

[Charles Papert]

It does seem weird, but with a nose-heavy rig you drive the camera back for static balance but forward for dynamic balance.

I refer you to the dynamic balance primer at the Steadicam.com site, but here's my quickie explanation: when the CG of the camera is behind the post (and the CG of the base is in front of it, which puts the rig into static balance), draw an imaginary diagonal line between the two CG's. When you spin the rig, that line will become vertical (while the actual center post now becomes a diagonal). By sliding the camera forward, you are bringing the CG of the camera closer to the post and assuming you do the complementary action on the base, you are effectively bringing the two lines (center post and imaginary line) into one, then the rig will spin flat.

[Pascal Jolink]

Assuming you are using the on-board LCD screen, dynamic balance on a Glidecam 4000 is as easy as it gets.

Because the CG of the weights on the base plate are on the same horizontal plane, you can mount the camera with its CG directly on top of the center line of the center post. Use an equal amount of discs at an equal distance from the center post. Balance by sliding the camera side to side and fore and aft, and you're done.

If this doesn't work your gimbal might be off. If so send it in to Glidecam for maintenance.

Also, quoting Jerry Holway: there is no static test for dynamic balance. If it seems out of balance when you're holding the sled horizontal and balancing it on your finger, it doesn't tell you anything, because DB affects the way the sled behaves when it's in motion i.e. when panning...

[Julian Frost]

Originally Posted by Charles Papert

...you are effectively bringing the two lines (center post and imaginary line) into one, then the rig will spin flat.

Ok, that makes perfect sense.

I just took another look at my set up, and noticed that my camera is also "right side" heavy. When I do the "horizontal spin test", the rig will always favor the right edge being down. If I turn the camera so that the left edge is down, it'll rotate back. Now, the Glidecam's base weights don't have a whole lot of side-to-side play in them, so is there a way to correct for this, or am I just expecting too much from a Glidecam?

Thanks again for your help.

[Julian Frost]

Another concern, is that the Glidecam's telescoping section is free to rotate inside the the Central Post. During initial set up, or while adjusting the length of the Central Post, it's quite possible to mis-align the base plate and the central axis of the camera. If this happens, wouldn't this introduce an instability too?

Is there a good method of ensuring the alignment of the base and the head unit?

I'm asking a lot of questions, but in reality, I think my Glidecam is actually balanced pretty well!

[Charles Papert]

If the spin returns repeatable issues such as pitching to the right every time, then yes, it may be demonstrating something more devious. I would try rotating the telescoping section slightly to see what happens. No, there is no keyway that assures you of proper alignment (again, cost-cutting measures).

Pascal's points are all excellent ones. The tricky thing with dynamic balance is that even if you follow all of the rules, things can be off a little bit somehow--we call these "gremlins". This is why it's good that all users of stabilizers understand the concepts of balancing their rigs thoroughly rather than relying on "cookbook" settings because you need to know what to do when the gremlins materialize, or if you change cameras or even add accessories.

And yes, there is a possibility that your gimbal is non-linear as this is often the case with the Glidecam units as reported by users. The bottom line once again is that you get what you pay for.

[Julian Frost]

Originally Posted by Charles Papert

If the spin returns repeatable issues such as pitching to the right every time, then yes, it may be demonstrating something more devious.

Just a thought... maybe I'm not describing this issue correctly. What I mean is, if I hold the Glidecam HORIZONTALLY (as at the beginning of the Arc Drop Test), and balance the Base Plate end over my extended finger and keep it there, the camera will rotate until it's right side is parallel to the ground. I'm not actually spinning the camera, just seeing where the camera comes to rest. Ideally, the camera should remain pointing in whatever direction i was left (pointing down, up, or any angle in-between), right?

Charles, for a quick, down and dirty COG initial set up, what do you think of this...

For small camera setups (like my Canon XH-A1 and Glicecam 4000 Pro), put the camera on the Head Plate, but remove ALL of the weights on the base plate. Attach the Glidecam's handle to a mount and carefully allow the entire system to flip upside down. From this position, you can see if the Central Post is pointing *up* vertically. If not, move the camera around until it is. Now the camera's COG should be directly in line with the center of the Central Post! From there, flip the system back over, and add the base weights to achieve correct static balance. This would be pretty much what Pascal is saying... "just put the COG over the Central Post". By doing it inverted, though, you've removed the base weights from the equation and are concerning yourself only with the camera. Maybe this would be an easier and more accurate way of determining the camera's COG than rolling it on a pen barrel?

I'm sure this has been tried, tested and dismissed by people way smarter than me, so I'd be interested in any comments, for or against.

[Allen Plowman]

I am brand new to the glidecam 2000, and my unit had misaligned gimbals that I had to correct. I tried to balance the camera both vertically and horizontally without spinning the camera. I learned several things. you arm gets really really tired, get some sort of static support. I wasted a lot of fruitless hours trying to statically balance in two planes. I am not sure it could ever be achieved that way. I followed Charles posted in this thread http://www.dvinfo.net/conf/showthread.php?t=91524 and what really worked easily was to center the weights on the platform, as far out as possible. set the drop time. let it hang, and statically balance it by moving the camera. I then did the spin test, and watch what way the camera dipped, and adjusted accordingly as per Charles instructions. it took me about 2 hours to figure out how to get it to spin flat. after it spun flat, I can hold it horizontal, and it is still slightly heavy on the LCD side of the camera, but not nearly as bad as it had been previously. also, if I hold it horizontal, and put the LCD side up, it will stay in the LCD up position. The reason for this must be that to balance perfectly in all planes, horizontal, veritcal, and upside down vertical, the weights on the base plate would HAVE to be proportioned exactly as the camera weight is proportioned in all directions.
on the other hand, once it is dynamically spin balanced, it is very close in all planes, and as Charles said, feels much more comfortable. Now all I have to do is figure out how to walk with it....

[Julian Frost]

Originally Posted by Allen Plowman

I am brand new to the glidecam 2000, and my unit had misaligned gimbals that I had to correct.

Allen, just out of interest, how did you correct the gimbal problem? Did you use the shim method? I saw that described somewhere, maybe on the Glidecam forums, but don't really see how adding a shim would correct the problem.

I find myself wondering why we go through the effort of finding the camera's COG, when the next phase in balancing the camera is to put it on the Head Plate and move it around! It might be just as "quick" to slap the camera on the mounting plate and have at it!

Like you, I struggled with achieving static balance while hand-holding the rig. After I got it pretty much dialed in, I realized that I had a mic stand with a rather large and heavy base -- and the Glidecam handle fit almost perfectly over the top! From then on, dialing in the static balance became pretty easy, by doing exactly what you described (moving the weights out as far as possible, setting the drop time and going from there).

When my A1 comes back from the Canon service center, I'll be trying all the suggestions Charles gave and I'm sure I'll soon be reporting perfect static and dynamic balance! Even now, I can't see an appreciable dip when spinning the camera when the Glidecam is vertical (panning). If I hold the Glidecam horizontally and let the camera rotate under its own weight, it'll always turn LCD up.

[Allen Plowman]

I got a good deal on a glidecam due to the gimbal being inaccurate. I learned several things: to discover if the gimbal is inaccurate you simply do a static balance, and very slowly rotate the camera, or just check it at 0, 90, 180, 270 degrees of rotation. if the lens is always pointed down, forinstance, you did not balance it properly. if at one postion the lens points up, and another postion it points down, your gimbal is out of whack.
It is a three way gimbal. the rotation of the bearing is one, the two screws on the side are one, and the pivot for the handle is one. the bearing can not be adjusted, however, the other two were both out on mine. i added a washer to one side to correct one of them, and to correct the other, I *gasp* took a 15" crescent wrench and fine tuned the angularity of the steel U-bracket. this was all tedious trial and error, make a change, see if it improved. once the camera could be rotated in a full circle extremely slow without dipping in any direction, I started pulling my hair out looking for dynamic balance.
yes, bumping anything will throw off the balance. however, once it is balanced and working proplerly, it is much easier to re-balance. If you bump the weights, and not the camera, you simply static balance it, and only adjust the weights, you should then be dynamic again. If you bump the camera, microphone, etc. then you adjust the camera to get static balance again. I found everything to be much more forgiving once it was all balanced properly. and I could not have figured out dynamic balance without Charles advice.

[Charles Papert]

Julian:

Your upside down concept seems flawed on paper as the rig will be exceptionally bottom-heavy and thus might not demonstrate subtleties of balance. I say "on paper" because I can't quite work out the physics in my head and it might just work...! Even still, I think that if you do the pencil test but on the bottom of your sled rather than the camera you can get the sucker centered.

Keep in mind, folks, that the only time you want the CG of the camera centered over the post is when the base is symmetrical fore and aft, i.e. same mass. As soon as you have a different mass on one side versus the other, it becomes three separate masses vs two and this affects how the components relate to each other.

[Julian Frost]

Being, as you put it, exceptionally bottom heavy, was the drawback I saw. When the camera is above the Central Post, there's no hiding whether it in balance or not, as even a tiny bit of mis-alignment results in a huge swing.

You would think that the base *would* be symmetrical, fore and aft, if there are the same number of weights on it and they are the same distance from the Central Post. It didn't work out that way for me, unfortunately! Still, like I said, after some tweaking, I have achieved pretty good static balance, and can get there again quite quickly (though not as quickly as you guys with your Steadicam thumb screws!). I've yet to try your dynamic balance techniques though, as the camera is out for repair, but it was spinning pretty flat before the camera was removed from the Head Plate earlier today.