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

Jeff Barlow wrote:

> I'll second that motion. Also, I would like to hear from Jon Elson (or
> someone else with a clue) on how this differs from the EMC, Servo to Go,
> tach gen, conventional servo amp, approach. In the later case there seem
> to be multiple servo loops (hard and soft) interconnected.

Mariss Freimanis wrote :

> A tachometer is used to sense motor speed. In the G320, motor speed
> is not sensed so neither tachometer or pulse averaging is used.

> The G320 is a position servo. The PID loop adjusts torque to maintain
> a "zero" difference between the commanded position and the motor's
> actual position. If the command position itself is moving, such as
> when STEP pulses are issued, then the drive will follow the moving
> position with "zero" error. The speed will exactly equal the STEP
> pulse rate.

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