Re: [CAD_CAM_EDM_DRO] First note-Help Mariss Help us with shielding???

Jon,

I grounded the cable shield to the case of the Gecko 340, but Mariss had
grounded the +5 volt power supply encoder ground signal supplied by the
Gecko to the motor case. (When I started having these problems I sent the
motor to him to test, and he found that the servo brushes had never been
broken in and the motor brushes were producing tremendous amounts of noise
which was interfereing with the system.) I hadn't thought of that. Maybe I
have thus created another ground loop. I will try grounding the cable
shield to the encoder ground supplied by the Gecko, or some other ground
shield combinations.

Thank you for your help.

Tom E..

----- Original Message -----
From: Jon Elson <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, June 30, 2001 12:48 PM
Subject: Re: [CAD_CAM_EDM_DRO] First note-Help Mariss Help us with
shielding???


> Tom Eldredge wrote:
>
> > As far as the encoder noise is concerned, what a puzzling thing. We
used a
> > foil shielded cable for the servo motor drive voltage, and a separate
foil
> > shielded cable for the encoder signals, and kept them separate all the
way.
> > We also put a .1 uf cap at the encoder end of the cable between +5 and
gnd.
> > Cables around 10 feet from drives. We grounded the encoder cable (no
> > twisted pairs inside) at the drive end, and left the motor shield
> > disconnected. The drives faulted constantly. We cut the ground wire
> > connecting to the shield on the encoder cable, and it ran with only an
> > occasional fault. We grounded the servo motor drive cable and the
faults
> > stopped. Never could see much on the scope in the way of noise. Before
we
> > found success, we also built a little amplifier and put it right in the
> > cavity of the servo motor with the encoder. The amplifier was made with
a
> > 74HC04 (per Marris's advice). We paralleled three of the inverters for
A
> > and three for encoder B signal. It still faulted every time until we
> > shielded the servo motor power cable. Jon Elson has pointed out to us
that
> > this foil shielding will not last the life of the cable if the cable
flexes
> > all the time. So, keep that in mind when you select a cable. If I have
> > dragged this out too long to keep up, the bottom line is that the only
> > configuration that works at this time is to have the servo motor drive
cable
> > shielded, and leave the encoder cable unshielded. Don't ask me why. I
> > don't know. I would have thought it better to shield the encoder cable.
If
> > we try to ground the encoder cable sheild, it faults out all the time.
>
> What did you ground the encoder cable to? A ground terminal on the
> drive, the case of the drive, or some other point? If you ground the
shield
> to a ground terminal of the drive, then it is injecting all intercepted
> RFI current INTO the drive's electronics! Don't assume, just because the
> encoder cable is 'causing' the problem, that the encoder's signal is
> being disturbed. It sounds a lot more like the Gecko drive's internal
> circuitry is being disturbed. Your shield was conducting interference
> into it. A better thing to try might be to ground the shield to the
drive's
> case.
>
> > Electrical noise has been a real pain for me in many projects I have
worked
> > on.
>
> Many EE's think grounding, shielding and EMI is black magic, and they are
> not far from the truth. Although the laws of physics are fairly simple
and
> totally predictable, when you have complex systems with noise emitters
> and noise-sensitive circuits, the paths unwanted signals will travel to
> interfere with others can be very hard to figure out. If current can flow
> several different ways from one point to another (a "ground loop") it
> can be nearly impossible to determine which path most of the current
> will follow, and what the effects will be.
>
> There are EMI consultants who charge $250 - 1000 an hour to fly out,
> look over your setup and suggest what to do to make the system work
> reliably. Thay couldn't get away with charging that unless they were
> good at solving problems most engineers couldn't.
>
> Jon
>
>
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