Re: [CAD_CAM_EDM_DRO] Geckos. 201 or 210?

John wrote:

>
> Why do servos run on encoders mounted in the belt or rotor instead of
> always using something like a capacitive strip on the ways? That's totally
> limited backlashing down to the accuracy the strip could be read. I mean,
> the encoder gives the position of the rotor but not the table. Does it
> create problems with the machine trying to find the right spot for the table
> to be in? Or is it just more expensive? : )

We go through this every few months. If there is backlash in the drive from
motor to table, you already have 2 big problems. One is that the servo drive will
make the motor bang from one side of the backlash to the other, several
times a second, trying to maintain control of the table position. Second
is that the table cannot be held in any particular position, as the motor takes
finite time to get from one side of the backlash to the other. Both are a
SERIOUS problem in any machining application, if the backlash is more
than a few thousandths of an inch (.1 mm or so is alreay quite bad).

If you have backlash under control, a screw-mounted shaft encoder is a LOT
cheaper than a linear encoder, and is as accurate as the screw. If you
have a low precision screw, accuracy can be improved through software
correction, which is cheaper than the linear encoder. For small machines,
the linear encoder is not much more expensive, but for long axes, especially
over 1 M, they get VERY expensive.

Jon