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

I don't have the figures to hand, but if memory serves me right the
inductance is something like 5 to 8 mH area which is well above the limits
set by many servo drives.

The drive folding back was the Rutex R2020, which has a very slow current
limit response and is easy to blow it's fets if pushed too hard. With the
Rutex drive I actually pushed it quite hard and most of the time it worked.
I could of probably sacrificed a little acceleration and top speed and got
the machine to run with acceptable performance. The worrying bit is most of
the time it was OK. However a non user repairable fault developed on one
card, and they are presently not in production so I had to find something
else. The Rutex does have a good dynamic range for what it is with some
current feedback and dynamic compensation it would appear built in. As a
drive it is almost there, needs the encoder noise issues fixed, slightly
bigger fets and a better faster current limit circuit.



The other drive I tried was Tek20's and Tek10's. These are variable PWM
rate, something like 20KHz or 48KHz, and at 48KHz current tripping is
better. The current circuit has an issue where it uses three fets Rdson
resistance to take the current measurement across, but when you have several
cards, added into the voltage across these fets is all the ground noise so
at full tilt you get 360mV from the fets added to 200mV+ in their ground
terminations so it trips. Peter at CNCteknix has been as helpful as he can
be, and warned me of some of the issues, so much so I have a lot of internal
confidential info. I could cure the sensitive current limit problem, we gave
up on his measurement technique on a current mode bridge circuit for a
totally unrelated product due the same problems, and our fix works very very
well. My problem with CNC teknix is the fact the PID calculation effects a
PWM width, not a torque current command and due to the motor there just
isn't enough range in that method of control. If you can get it to move from
stopped without current issues, and I don't mean false tripping, I mean
without blowing fets, the gain is so low you have serious error lag at
machining speeds, and I mean lag of over 1mm which just isn't viable. The
only way to help is reduce the acceleration or have an S curve acceleration,
but I am down to 25mm/sec/sec which is pretty slow and still can't get it to
move at 400mm/min cutting speed without a massive error tripping the drive.
Simply not enough dynamic range.



I have worked with Rutex to identify many of their problems, similarly I am
working with CNC Teknix to improve theirs, and the problems we presently
have can be overcome, but I don't feel there is anything at this time
adequate on the market. My mill now has been in rebuild for over 12 months,
it has to be operating by July or I will have to go out and buy a new one
and I hate giving those big CNC guys the satisfaction they have beaten me.



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 thecalfees
Sent: 27 May 2007 17:32
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Big servo motors drive recommendations



Mark -

You have shared some interesting observations. 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. In an earlier post you described that
the PWM was "folding back" on one of the drives you tested at large
PWM ratios and that makes me wonder if the issue has more to do with
the PWM response in individual PWM cycles than with the PID algorithm
itself. Ideally the PWM cycle rate is fast compared to the current
rise time in the motor windings such that the motor current during a
move would change over multiple cycles and not hit the trip limit
within a cycle. If you are hitting the drives current limit during
hard acceleration and over several PWM cycles, then this would
probably not be the case. It is an interesting issue; thanks for the
post.

Regards,
Gary

--- In CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups.com, "Mark Vaughan" <mark@...> wrote:

>
> I'll second that.
>
>
>
> 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_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>

DRO@yahoogroups.com

> [mailto:CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>

DRO@yahoogroups.com] On Behalf Of Steve Blackmore

> Sent: 27 May 2007 16:33
> To: CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>

DRO@yahoogroups.com

> Subject: Re: [CAD_CAM_EDM_DRO] Re: Big servo motors drive

recommendations

>
>
>
> On Sun, 27 May 2007 13:27:21 +0200, you wrote:
>
> >Well, time is money and money is time. If converting is sucha a

pita ill

> hold on
> >to this machine a year longer bleeding $$$ for repairs and then buy

a new

> or
> >fairly new instead of messing for countless hours to convert it into
> something
> >slow and sensitive. Rapids in my machine are 12000mm/s, 10000mm/s and
> >7000mm/s. And I need to be able to cut in 400mm/s.
>
> Yep - Often when you take into account all the messing around with time
> and "cheap" parts, you could have gone out and bought commercial parts
> or even a good second hand machine for less...
>
> Steve Blackmore
> --
>
>
>
>
>
> [Non-text portions of this message have been removed]
>

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