Re: [CAD_CAM_EDM_DRO] isolation, current and velocity, encoders

jmw@... wrote:

> (Rated Continuous Current)*(Bus Voltage)*(# of Axes)
> So for example a UM4-100-2 rated at 15 continuous Amps with a 100VDC
> buss voltage controlling three axes would require:
> (15A)*(100VDC)*(3 Axes) = 4.5kVA.
> This method of transformer sizing is safe and it may be a little
> oversized."
>
> Well, OK; I guess I'll go hunt down a 4.5kVA isolation transformer;
> it would seem that "ungrounded AC power source" is the key here.

Well, this will be overkill, I'm sure. Besides, I think your amps were
only 6 or 7 continuous amps - or am I confusing two different people?
It would be a rare condition that all servo amps were delivering full
continuous power all the time. Just put in a slow-blow fuse rated
for the full transformer rating. I think a transformer rated at 1/2
this size should be fine for most home machines. A high speed router
that was run in commercial service might be a different story.

>
> ------------------------------------------------------------------
>
> I know I've raised the question of torque vs velocity control mode
> before, but I'd like to nail it down. The docs claim the amp can be
> operated in current (torque) mode or velocity mode, and seem to say
> that in torque mode no tach is required.

Right - but if you run in torque mode, then the CNC control is
expected to send a torque command, not a velocity command.
If your CNC control sends a velocity command, the amp needs to
be configured to receive that and therefore needs a velocity feedback
signal, ie. a tach.

<snip>

> So do you read this as saying I don't need no stinkin' tach so long
> as the motion controller doesn't require one?

No. Only if your motion controller is sending a torque command signal
can you run the amp in torque mode.

> Jon, does it sound like your planned card(s) can substitute for this
> SMCC device?

No, not at all. My card set is designed to connect to a PC, and
provide velocity command signals and read encoder signals. If you
provide the code to do the velocity loop in software, then the amps
could be run in torque mode. EMC as it stands does not do this
calculation, and I have some good reasons to think it is not too great
an idea to do that. As I have stated before, if you have a higher
servo loop sampling rate and a high resolution encoder, this can
work OK, but a DC tach may be a much cheaper solution to get
the same fine performance.

Jon