Techniques > Misc

Servos, is it me?

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IceWind:
I hate that on servos too.
Despite some radios have a function like sub-trimm why don't they center the arm so it can be alwways in a 90degree angle perpendicular to the servo body. Most of the time that is the way the servo is going to work.

A solution could be also a variable resistor inside the servo for fine tuning.

Ron:
Hi Icewind,

It's annoying isn't it?

I can see me stripping a servo apart....
Maybe I'll start with an old Futaba one.

Cheers,
Ron

cvanscho:
Guys, remember that some cheaper servos do not "centre" electronically very well, some are better than others.  With "bad" ones, the servo never seems to return to the same spot every time..  So, even if the servo is mechanically spot-on, you might still find problems.. :!:  Only one thing to do with one of those: step outside, haul back into a nice grenade launch position, and send them into orbit :evil:  !
 
Chris

Ron:
Yes that's right.
Having had a look inside a Futaba one, the pot is directly on the output shaft.
I know from experience in the electronics industry that pots are very poorly made. They have to be to keep the price down.
Normally the accuracy of the track resistance to position isn't any worry. No-one is concerned about that with a volume control on a radio.
But use the same sort of pot on a servo and you get position problems.

Also the gain of the servo amplifier is a big influence on the 'return' position.
But too much gain and you can get 'hunting' back and forth with the inertia of the motor etc.
Not enough and the servo will stop in a different position coming from one direction to coming from the other.

The good news is, as the pot is directly on the output shaft, fitting another 'preset' type pot on one end of the position pot, will give a mid-point adjustment.
In the one I took apart there are a couple of fixed resistors either side of the position pot, so replacing one of those with a small preset, perhaps with an access hole in the case, will give some adjustment.

Not sure if it's all worth the effort though :!:

Anyone know about digital servos?
Do they use a digitally coded output shaft system?
If so they should be more accurate.

That's my lot for now.
(Everyone breaths sigh of relief)
Back to taking the visitors out for the day... :roll:

Ron

Fred:
Hi Ron,

Yeah, the pot lways was the problem, cheap servos or not.. A bit of dust  or xxx hours of uses and that's it...

I use a few digital servos now... I am not a specialist in that sort of things, but the movement is piloted by a chip, a bit like a sensorless esc, counting the number of steps as far as I understand.
You know you are using a digital servo by the noise of it... Very high pitch noise in idle as the microprocessor  is permanently correcting the center position, even to a few 0.0xx degrees and you touching nothing.

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