Hi,

Q. What determines the positional accuracy of a servo, and what are typical values?

I'm not very experienced with running servos, although I think I understand their basic operational theory.

The new digital servos mention 'increased precision.' Presumably the positional accuracy is detemined by 1) the bit-resolution of the circuitry that receives the PWM signal and 2) the precision of the feedback potentiometer.

Is that right, and do you know of any relevant specs. I looked at Hitec servo specs online and couldn't see anything about positional accuracy. Currently I'm using stepper motors to precisely turn a camera platform, but it's difficult to add a useful feedback element, like a shaft encoder, to something that already rotates so slowly.

What I'd really like is a simple replacement for the stepper - something with feedback so i know exactly where I'm pointing. If i replaced the (1.8degree) stepper with a (360 degree limit) servo, do you think I could get the same 1.8 degree (i.e 200 positions per revolution) resolution out of the servo?

Servocity.com, for example, sell a variety of servos,some of which have 360' or multiturn capabilities. Some with 'milspec' pots. They even have continuous rotation 'servos' without the feedback pot.

Thanks for the magnetic encoder pointer, I was looking at optical ones on ebay and didn't realise you could get encoders with such fine resolution.

What I'm after ultimately is something really fast but really accurate. Like this,

CamBLOCK Teaser

I've actually made a somewhat less sexy 2-axis rig (the controller uses a PIC uC, naturally) for the same purpose (motion timelapses), and although my stepper is fairly precise it's also very slow. I thought a servo type motor (which is what camblock uses) with its feedback would be faster and (potentially more) precise.

Thanks for the replies.

but it's difficult to add a useful feedback element, like a shaft encoder, to something that already rotates so slowly.

not about servos, but I've seen a cheap and accurate way of measuring rotation used in printers, it involves an emitter/sensor in a single piece and a clear wheel of plastic sheet with radial lines printed on it. Its accurate enough that the printer usually doesnt mess up the line spacing. And you could even get the needed wheel and sensor from a printer, people throw those out all the time.