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

Mariss Freimanis wrote:

>
> 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.

Of course, but in most step/direction CNC software, there is no provision
fro direct feedback.

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

Yes, that notifies you there has been a loss of position, as well as the problem
that caused the fault, but you still have to re-home the system.

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

But, with tach feedback, it IS smooth between the counts.

> 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.

I find this VERY hard to believe. Maybe, with no realistic band-limiting
it is only 25 Db, but with reasonable band limiting (just a simple RC
at the input to the servo amp) I think I have demonstrated a SNR of
over 40 Db on my own system. The tach is still working WELL down
into the tens of uV region, but the finite gain of the amplifiers is eventually
overcome by stick-slip friction somewhere below .01 IPM. Maybe
I'm just picking at rediculous straws, here, but I want it SMOOTH down
as far as I can possibly get it. (The idea is partly that if it is smooth
down to rediculously low velocities, it will also perform well while holding
position against intermittent external loads, ie. cutting forces.)

Jon