With gravity camera stabilisers like below, how come counterweights are needed to stabilise the load (camera)?

enter image description here

Is it to ensure that the handle joins at the centre of gravity? If so, why does it have to be at the centre of gravity?


The mass at the bottom is actually to make sure that the center of gravity of the whole assembly is directly below the pivot (ball joint) at the top of the handle. Otherwise, it will tilt or try to flip over.

The mass at the bend (at the same level as the handle) increases the moment of inertia in the horizontal plane to add stability to the pointing direction.

The two masses together also increase the total "sprung mass" to reduce the resonant frequency of the entire assembly and reduce vibration. (Your arm provides the "spring" and "damper" for the system.)

  • $\begingroup$ Right. Why does the CoG have to be directly below the joint? The mass at the bend sits below the camera platform, so how does it increase the moment of inertia in the horizontal plane? Why does adding moment of inertia there add stability? $\endgroup$
    – John M.
    Apr 23 '16 at 13:39
  • 3
    $\begingroup$ Let me put it this way: The COG will be directly below the pivot -- that's the nature of a ball joint. The mass needs to be distributed so that the camera is level under this condition. The mass at the bend is as far away from the vertical axis through the ball joint as it can be, so it has the greatest effect to resist any rotation about that axis. $\endgroup$
    – Dave Tweed
    Apr 23 '16 at 13:41
  • $\begingroup$ @DaveTweed How do you tell that the COG is directly under the ball joint rather than at or on the ball joint? $\endgroup$
    – Kar
    Apr 23 '16 at 17:20
  • $\begingroup$ @Kar In a system that can freely rotate at the ball joint, all we can say is that in equilibrum, the CoG is on the same vertical line as the ball joint. Tilt the system out of vertical position slightly, e.g., by pullng the bottom counterweight. If the system destabilzes and topples over, the CoG was initially above the ball joint; if nothing happens, the CoG as at the ball joint; if it swings back into the original position, it was below the ball joint. As per the intended purpose of the construction, the latter is the desired behaviour. $\endgroup$ Apr 23 '16 at 18:02
  • $\begingroup$ @Kar if it's above the joint, then unless its locked int place via an explicit lock or high friction any vibration will shift the CoG slightly to the side of the ball joint at which point gravity will flip it 180* so that CoG is below it. WIt the CoG below, any perturbation will be damped out by the CoG shifting back underneath it. If the CoG is at the ball joint there's no stable orientation at all because there's nothing to shift it back into place. $\endgroup$ Apr 23 '16 at 18:50

A camera system is more sensitive to rotational shake than any translation. That is because translation causes a shift proportional to the noise but rotation causes an error proportional to the distance to the target.


Image 1: Example of noise in a plane, note that in 3 dimensions there are more rotations.

Conservation of momentum, means that more mass is slower to move around. When it comes to rotation the distribution of this mass affects the way something resists rotation. We call this moment of inertia.

Inertia brings "slowness" to the system. Net effect is that the harder it is to rotate, the more force you need for the rotation to happen. Thus the system rotates less from the noise.

By putting the weight far away from the joint you make the moment of inertia bigger. Meaning its harder to rotate (around the axis that has furthest distance to weight) thus damping the system more. At the same time the overall weight of the system does not need to grow as much.

For similar reasons, you want the center of gravity be below the ball joint so that the camera stays upright. Or more exactly, the system will orient so that the center of gravity is under the ball joint, so you want to design it so that this benefits you. Also you want to avoid having a lot of weight around the ball joint on a horizontal plane so that turning the camera horizontally is not too heavy. This way, the system prefers motions that are more natural for your camera work.

  • $\begingroup$ Sorry, i had to write this post on the run and appologize for all the typos. I hate onscreen keyboards, especially multilingual autocorrection just does not work. Feel free to correct this. PS image from a old post on how to stabilize a go pro camera $\endgroup$
    – joojaa
    Apr 23 '16 at 19:20

When the pivot point is right at the center of gravity, the camera will not tilt if you move the support left or right - there will be no net torque on the camera. If you have the COG just below the pivot (ball joint), you make the system stable (that is, it will naturally want to be upright) but lateral movement will result in a tilt.

The closer to the pivot your center of gravity is, the less the camera will tilt - but also, the longer it will take to come back to vertical if you tilt it.

  • $\begingroup$ Isn't the purpose of the construction to not let the camera tilt when the support (handle) is moved? If so, shouldn't the ball joint be at COG rather than above COG? $\endgroup$
    – John M.
    Apr 23 '16 at 18:46
  • $\begingroup$ @JohnMunroe the point of my post was to point out what happens when you choose the relationship of COG and ball joint. I agree that "at the joint" results in "no tilt when moving". Whether that is the desired effect depends on how you want to use it. $\endgroup$
    – Floris
    Apr 23 '16 at 19:25

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