Gimbal gear ratio theory. Why not using ratios of 2:1 for faster movement?

[Hokeypokey]

Hey guys,

right after finishing the modification of the fast Savox brushless servo (0.06s) I am still having problems with the overall speed of my system. I am using the hoverfly gimbal controller with the Savox SB-2271SG. My university project needs to rotate the camera more than 360°. I am wondering why all professional gimbals are using gear ratios of about 1:3 upwards (servo:camera-axis)? (That is the reason why I chose a gear ration of 1:2.6 for the first test.) Is that just for achieving a smooth rotation of the camera? A smaller or "negative" gear ratio ratio would result in a much faster camera-movement. What about using a ratio of 1:1 or 2:1 (servo:camera-axis)? Why is nobody using these sort of gear ratios? Has anyone ever tried one of these ratios?

Help is greatly appreciated.

Christian

P.S. I wasn't sure where to post this thread, so I decided to use the general discussion board and this aerial photography board. If anyone knows where it belongs, please remove the wrong one.

 

[Soaringclauz]

I started using rc servos with direct drive (1:1) in the first gimbals i built. The result was that they were very unprecise, movement was not smooth and that affected both control and compensation.
With 4/1 and 5/1 (the ratios i use now) the movement, although slower for the same amount of used power, is smooth and precise, it's a matter of resolution that is even more important than torque.
This might turn out in an interesting discussion, i have the idea that 2013 will change many of the things we're used to.

 

[DucktileMedia]

Your weakest links are the potentiometers and the backlash from slop in the servo gears. Personally, I would rather have smooth than fast. If you are getting that kind of drastic motion that you really need ultra fast response i think the problem is more in piloting or the heli. But I guess I am not that fussy with my setup as some are. You're not gonna get a Zenmuse from an AV or cinestar. But if you tune everything as well as you can you will get comparable results. I cant wait to see PH Hero's when/if they ever come out. Would also be interesting to see what evolves with linear motor technology although I think someone, probably Denny, answered why they are not ideal for this application but i cant remember. I would stick with the factory reduction ratios.

 

[Hokeypokey]

Hmm, unfortunately I was expecting what you said.

So what would you do to speed up the overall rotation without getting to much of jitters and stuff that makes the final movie unusable? I have a camera, that only needs to rotate around its lens axis and align with the horizon that way. I am already using a servo that is quite fast and strong (0.06s, 20kg).

I read about the PH hero system. It uses direct drives as far as I can see. What kind of motor are they using. Any ideas?

 

[Bartman]

HP

How did you modify the brushless servo for an external potentiometer? I took a look at those servos and figured they weren't worth the work to get them apart.

As for the gear ratios, part of the benefit is that a 5:1 ratio acts as a mechanical filter of sorts in that any backlash or jitter from the servo end is reduced by 80% at the lens. Quality of the pot is relevant also although a cheaper pot will work fine at first before wearing out from so much rapid movement at such a small spot within its range.

for what we're doing with aerial media where smoothness is everything, reducing a .06 sec servo to 5 times that still seems to be well within what the gimbals need to function well.

bart