Re: re:cleaning, lovejoy, black box tach, linear scales, making pulleys

--- In CAD_CAM_EDM_DRO@egroups.com, Jon Elson <jmelson@a...> wrote:

>
> I had trouble following this thread, I hope this is the original

message

> that spurred the question at top.
>
> My feeling, not having tried the Gecko servo system, is that there
> are two differences in the system. One, is that it is not a closed

loop

> system, in that the CNC control has no direct feedback from the

encoders.

> The Gecko Servo unit DOES have feedback to IT, but it can't tell the
> CNC control where it is, all it can do is try to keep the machine

where

> it has been told to put it. If there is a fault (crash or

overload) or an

> emergency stop, the machine position may no longer correspond to
> where the CNC control thinks the machine is. A servo system with
> position feedback to the CNC control will not lose position under

these

> conditions.
>
> The second difference is under extremely slow movement, which ocurrs
> any time you interpolate a circle, or when you are following an

angle that

> is almost parallel to one axis. The stepper-> servo system, or ANY
> system without a DC tach, cannot make smooth movements that leave
> it between encoder pulses for a significant time, because that is

the only

> form of feedback, both for position and velocity. A DC tach allows
> smooth motion even when it is several seconds between encoder

counts!

> If you have a very fine (high resolution) encoder, this may be no
> concern. If the encoders are of relatively low resolution, it

could cause

> wavy lines in machined surfaces.
>
> Jon

Hi,

Jon, sorry, I jumped into this thread at about the 40th "Re:" level
with my 2 cents worth. Point by point:

1) The encoder is free to be used by the PC as well if you wish for
direct feedback.

2) The drive has a "fault" output readable by the PC in the event of
a crash.

3) Smooth movement between encoder edges. In either case the motor is
moving between encoder edges, smooth or not.

4) Seconds between encoder edges. Let's do the math. Assume 1 second
between encoder edges, a modest encoder resolution of 500 lines and
an agressive tachometer Kv of 100V/1,000 RPM.

It would take 2,000 seconds per revolution, for a motor speed of
0.0000083 RPM. The tach feedback voltage would be 0.00000083V, or
less than 1 microvolt! My point here is taking brush noise and other
factors into account, a really good tach has a signal to noise ratio
of no better than 25 db.

Mariss