Re: someone told me... (question)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Earl" <colin@v...> wrote:

> I am looking at buying some linear motion units. The specs on them
> are:
>
> 16" travel, 20mm pitch ballscrew, 200W Panasonic DC brush motor

with

> 500 C/T encoder.
>
> The seller claims positioning accuracy of 0.0005" with a Gecko

drive.

> I am confused by this. from what I understand is 20mm (per turn
> ballscrew) divided into 500 positions for that turn is only 0.04mm,
> or ~.00157" resolution.
> I asked him and he replied with this:
>
> The 500c/t encorder. The Gecko will count 1 pulse x 4 = 2000 pulse
> per turn 20 mm / 2000 counts = .01 mm ( .0003937" )
>
> something is not right. am I confused or is he.. or are we

both!!? :)

> Thanks for any insite yall. I have been running a stepper system

and

> am looming to jump into the servo world.
>
> Colin

Colin, encoder resolution is usually spec'd as quadrant so a
500 quadrature count encoder will resolve 2000 positions per
360 degr. of rotation.
20mm = 0.787401575 inch per turn
So the theoretical resolution would be
0.797401575/2000 = 0.000393701 inch per step or 0.01 mm.
This is called the resolution of this setup.
The accuracy will be the actual positioning precision after
all electronic and mechanical deviations are counted in and
are likely to be be much more then this.
Good Luck