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Old Oct 11, 2010, 03:24 PM
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Joined Oct 2010
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Servo Keeps on Shaking ?

Hi All.


I am trying to make remote controlled Pan/Tilt heads for my video camera.
The camera is a lightweight Sony MemoryStick Camera.

The Tilting is working fine, just when I stop a pan, it keeps on rocking CW and CCW, (like when you overcorrect with your car steering wheel).

I am using a Pic Microcontroller to run the Servos (Which are Futaba S3003 Clones). The whole mechanism is resting on the horn of the pan servo.

This rocking happens whether the camera is on it or not. If I put a bit of extra load, it seems to stop sooner, Also if I help stop it by hand.

The video of the unit is on youtube, with the rocking evident in the video : http://www.youtube.com/watch?v=4SETqYQOpYc

Another point, is it fine to have the mechanism on the horn, or must I use some sort of pivot (bearing or castor), and have the servo next to it, and hooked up with some stiff wire for 'steering' the mechanism.

I am new to using RC Servos, so Please excuse my lack of the proper terminology.

Please could somebody help shed some light on this

Thanks
Peter Kolbe
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Old Oct 11, 2010, 03:29 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,637 Posts
It's quite common for servos to hunt like that. Some of it can be cured as you found by damping the travel. In that case the damper or your finger acts like a shock absorber to slow down and kill the oscillations in the servo's gear train that keeps it overshooting the stop point which is what is making it jiggle like this. If you add a light spring or rubber band to the system to lightly pull it one way likely the jittering will go away.

Also some servos will do this more than others. Try out the rest of them and see which are the least jittery. And finally opening the servos up and cleaning and lubing the feedback pot element with a light silicone electronics oil or lubricating control cleaner can makes things better.

In the end if it keeps doing it the same with all your servos it's likely that your PIC chip is not sending a consistently accurate pulse to the servo. If you can put the pulse output onto an oscilliscope look for a sharp and consistent leading edge where it triggers and a fuzzy trailing edge which indicates that your pulse duration is dithering. If that is happening then nothing you do with the servo will cure the jitters. You need to have a smooth and very consistent pulse first of all.
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Old Oct 11, 2010, 03:44 PM
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Joined Oct 2010
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Wow, Thanks for that. This must be the quickest reply I have had on a discussion board before.

My thoughts were also about damping with some elastic bands, now that I have confirmation I will try it out.

The servos are brand new, so I doubt it can need a cleaning already, (although they are Clones, so you never know - come to think of it, how bad are RC servo clones anyway?)

As I am not having the problem with my tilt servo, and the coding is the same for both the servos (I am not using the PIC's PWM, I am bit bashing the port to generate my own PWM. - and the simulators stopwatch is saying that the periods are fine), I doubt it is the program, but if the damping does not work, then I will definately hunt down a scope.

P
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Old Oct 11, 2010, 06:44 PM
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meshyx's Avatar
Norway
Joined Feb 2010
602 Posts
RC plane servos aren't meant for moving that kind of weight. The wood has a lot of momentum, while a planes control surfaces will have very little. This is why it constantly over-corrects.

Using rubber bands so the servo has something to "work against" might work. Mounting the wood and servo separately and using a pushrod would also introduce some friction into the system, helping to damp it out.
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