RE: [CAD_CAM_EDM_DRO] Re: Big servo motors drive recommendations

Tried, for my motors inductance isn't a problem, but adding resistance will
reduce the target current and help things. This was necessary for the Rutex
cards to build a total resistance up of about 2.2 Ohms, Tek stated it was
not needed for theirs, but adding resistance also helped.

The limiting inductance for these drives is supposed to be 1mH, mine motors
are five times this, and we upped the PWM on the Tek's to 48KHz. All helped
but was still way way short of providing a viable solution.

Part of the dynamic range issue isn't the inductance within one pwm cycle,
it is the fact a big system takes a while to respond, but mean time the
error has grown and the gain in the control is large trying to pump more
current into the system than is viable. So you add resistance, reduce the
gain, reduce the acceleration, but then you get a system that just doesn't
have enough torque for normal cutting operation so lags way behind the
command and trips out on position.

As to the noise issues. With Rutex the problem is that they take a
differential input for the encoders, all good, they decode it to a single
ended signal, then they make this signal available to the outside world to
drive DRO's and to the cards processor. There are no buffers to the outside
world so noise comes back in from the loom attached to that port. If you
look at the card there are also many many other areas that are weak
regarding noise immunity.

For the Teks, the current sensing is resistance in the ground path, but the
0V connection to this is common between the DC control supply and the HV DC
supply. As current build on the HV supply it is injected into the LV side of
the current measuring circuit almost doubling the voltage we need to
measure. They work better on their own, but are not viable in a group, small
motors, low current low noise issue. Big motors big current big noise issue.
Improving the PCB connectors might help, there are quite hi noise
differentials just on these. This noise lifiting the ground last as long as
you are trying to accelerate the motor, several seconds, you never get it
this far it just trips. Even with this disabled though they do not have
enough dynamic range.



Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of David G. LeVine
Sent: 27 May 2007 21:50
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: Big servo motors drive recommendations



At 12:32 PM 5/27/2007, you wrote:

>You mentioned that
>your motors were about 1 ohm resistance. Do you know the inductance?
>The PWM drives usually have a minimum inductance specification that
>will work with the PWM rate.

An interesting thought from an old analog engineer (late 1960s!), why
not put an inductor is series with the motor? It will degrade
performance if too large, but if it changes the problem, it may point
to the answer. Remember that noise is ALWAYS affected by the rate of
rise of signals...

David G. LeVine
Nashua, NH 03060





[Non-text portions of this message have been removed]