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Discussion Starter #1
Hi. I used parts from an Actobotics assortment (including a servo block) to mount a HiTec HS-485HB servo in a skull for side-to-side rotation. It works fine except that it has a tendency to quiver for a few seconds before stabilizing.

I picked this servo because I read that others have used them in their 3-axis skulls... though I suspect that my skull is heavier than theirs (typically Lindberg).

I’m guessing that the skull’s inertia is a bit much for the servo’s holding torque - causing oscillation in the feedback control loop.

I’m not sure if the best fix is to get a higher torque servo or to try to dampen the movement (e.g. compressing a felt pad in the servo block). I suppose I might ultimately be able to decelerate the movement in software as the skull approaches its next stop position. I’m not sure how to do that.

I’d appreciate any suggestions. The quivering skull might actually look kinda cool in the final prop (a cauldron creep), but I’d like to at least know how to stabilize the motion.

I couldn’t figure out how to attach a video to this post using my iPhone.

Thanks!
 

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Discussion Starter #3
Good question. For test purposes I’m using an Actobotics Manual Dual Servo Driver.

Update: Compressing a felt pad (a stick-on furniture foot pad I’d cut a hole in) between the servo block’s rotating spindle flange and the bearing frame seems to have dampened out almost all of the servo chatter without noticeably impeding rotation. As long as the felt’s friction properties don’t wane much over time, I’m cautiously optimistic that this will fix my problem.

That said, I’d appreciate suggestions for resolving this problem in other (probably better) ways.

Thanks for your quick reply!
 

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That servo should be fine for Pan.
If you are controlling the servo manually, that's probably the cause. Make sure you're not overdriving the servo, that is, don't try to push it to the max that the linkage will allow. When you have a program controlling it, slow the speed down and make sure your end points are well short of maximum movement. You don't wanna rely on something to physically stop or dampen the travel, in time that will destroy the servo.
 

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If you use a controller like the Pololu Micro Maestro it allows you to set the acceleration for each servo (and change it on the fly). This allows you to slow the servo speed as it approaches the target position and may help the quivering.
 

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Discussion Starter #7
MCR, you and all the other swell folks who respond to the questions of other, less-experienced “haunters” like me are the reason this forum is so valuable.

Never in a million years would I have thought of using a magnet to generate an eddy current as an electric servo brake!

I’ll definitely give this approach some thought!

Thanks!
 
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