In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

Thanks.

I'd check the manual. Signal ground is that - not safety or earth.
It might be elevated e.g. 5v logic driving 11V logic. Or 3 into 5.

You might blow all of the expensive boxes if you don't know for sure.

Gunner might know - if you tell him the model number...

Martin

Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
"Our Republic and the Press will Rise or Fall Together": Joseph Pulitzer
TSRA: Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Charter Founder
IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com/

On 7/3/2010 8:33 PM, Ignoramus31310 wrote:
In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

Thanks.

On 2010-07-04, Ignoramus31310 wrote:
In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

I think that the servo drives may have separate power and signal
grounds. I would tend to tie all the derived grounds (isolated from the
incoming power but referencing power) to the safety ground, and keep a
separate signal ground bus.

Neutral can go a bit too high over safety ground under certain
conditions of loading.

Your servo amplifiers (drives) are all being powered from a
single power supply -- right? If you had a separate one for each
amplifier, I would alternately connect them to one half or or the other
of the incoming power (presuming 120 VAC input to the power supplies) so
the load is balanced a bit better.

The old Bridgeport BOSS-3 took a *lot* of power for the
steppers, and drove each axis from a different phase of the incoming
three phase to balance that load.

Here, you have:

The VFD (across the full 240V)

The servo amplifiers (drives) which may be 120 VAC loads or 240
VAC loads. If 240 VAC -- no problem, otherwise split between
120 VAC halves of the incoming 240 VAC.

The computer (balancing one of the servo amps if split power
supplies).

Lighting. (120 VAC)

Coolant pump (120 VAC?)

Tool changer? (perhaps another servo axis?) I don't have a tool
changer -- sigh!.

Anything that I've forgotten?

Good Luck,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

that would certainly NOT be the way I would do it - signal ground should go
to its own ground with no current carrying motors, solenoids, etc. Power
ground is some thing else - they could be tied together, but don't put them
all on the same ground stud

"Ignoramus31310" wrote in message
...
In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

Thanks.

"Bill Noble" wrote in message
...
that would certainly NOT be the way I would do it - signal ground should
go to its own ground with no current carrying motors, solenoids, etc.
Power ground is some thing else - they could be tied together, but don't
put them all on the same ground stud

You'll have a boatload of signal ground wires by the time you're done. I
always take a section of terminal strip and bond several together with
little loops of white wire. Then one green wire to the common ground bond
point on the machine. Do NOT put AC nuetral to this bond point. Also don't
daisy chain your signal grounds, each device has a wire back to this strip.
I always use white wire here and then don't bother with wire numbers.

Karl

Iggy,
The purpose of all those signal grounds is to assure that the signal reference is quiet, eliminating voltage reflections from
other device return currents, which can be seen as noise . Bonding them all together defeats that purpose. Obviously at some
point, all those returns will tie to DC return (never AC neutral). Those tying connections must be very low impedance, typically
6/8 gauge wire. To simplify the statement, all the signal returns within an electronic chassis could bond to an insulated from
chassis bus bar. That bus bas bar is then tied to power supply DC return with a heavy gauge wire as short as possible.Repeat that
for every electronic chassis. Never, never, ever connect neutral to safety earth in ANY machine. This connection can only be made
at the equalization bar at the power entry point for the building. DC return can be connected to the green safety earth wire at
the machine power entry, but often is connected through a 10K ohm resister. Under no circumstances should DC return ever be
connected to chassis. These simple rules eliminate noisy ground loops.
Steve

"Ignoramus31310" wrote in message ...
In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

Thanks.

On Sat, 3 Jul 2010 21:54:27 -0700, "Bill Noble"
wrote:

that would certainly NOT be the way I would do it - signal ground should go
to its own ground with no current carrying motors, solenoids, etc. Power
ground is some thing else - they could be tied together, but don't put them
all on the same ground stud

"Ignoramus31310" wrote in message
m...
In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

Thanks.




When using an oscilloscope to trouble shoot there are lots of
occasions where there are conflicts between signal ground and
power ground connections. For safety's sake scope metal work must
be connected to power ground, but for examination of low level
signal circuits scope ground must be to signal ground.

Temporarily breaking the power ground and floating the scope
metalwork is expedient but dangerous. A much safer solution is to
power ground the scope through back to back diodes.

My scope has a 30Amp bridge rectifier mounted on its three pin
power plug. The + & - rectifier pins are connected to each other.
The AC pins are series connected between scope metalwork ground
lead and the plug power ground.

For either polarity of current there are now two diodes in series
beween scope metalwork and power ground. For up to a few hundred
mV difference, the scope is now essentialy floated but it is
still safely anchored to no more than a Volt or two from power
ground.

Jim

On Jul 4, 7:01*am, wrote:
...
For either polarity of current there are now two diodes in series
beween scope metalwork and power ground. For up to a few hundred
mV *difference, the scope is now essentialy floated but it is
still safely anchored to no more than a Volt or two from power
ground.
Jim-

In the 80's I worked on semiconductor production test equipment. In
those days the computers that controlled them drew very substantial
power, ours had about 100A of 5V from switching supplies. The
measurement sensitivity was microvolts and picoamps; without all the
shielding the machine could detect a person several meters away by the
increased noise floor.

There were separate ground blocks for all digital supplies and all
analog supplies, and we had to be careful to return as much of the
digital supply current as possible to the digital return, likewise for
the analog.

The measurement ground, HQC or High Quality Common, was partly
isolated as Jim Pentagrid mentioned, with two parallel diodes and a
470 Ohm resistor connecting it to analog common. We had to account for
any unbalanced current in or out of HQC. IIRC it was considerably less
than 5mA.

It turned out that the biggest problems were the control signals from
digital to analog or the measurement section. The current for heavy
loads like the 2102 memory flowed in well defined loops whose
radiating area could be minimized by twisting the power leads.
Controls passed between sections and had to be handled individually,
to trade speed for impedance. If necessary we used optical isolators
for digital or reed relays for analog controls.

Be thankful you aren't designing the grounding for a digital radio
that combines millivolt analog-to-digital converter sensitivity with
high powered ECL logic. I've seen 3 Volt spikes between two points on
a ground plane that someone else had designed to the DC-appropriate
star topology. The data bus crossed the plane break in that area but
its return path was about three inches away through the star's
center.

As a general rule I was told to treat under 10KHz as single-point,
over a Megahertz (100KHz?) as a common plane, in between takes more
careful analysis, mostly knowing where the currents flow out and
back.

In theoretically-oriented night school classes they taught voltage,
the very practical Army Signal Corps taught us to pay attention to
current flow.

jsw

In article ,
Ignoramus31310 wrote:

In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

First, as many others have said, do NOT use the power neutral (the white wire)
for anything other than power neutral.


I often deal with this exact signal grounding issue in large electronic systems,
where "large" means that the equipment is spread out in a large building.
Another example is a large computer room with raised floor, such as those for
Air Traffic Control systems, where there is always a substantial ground grid
under the floor.

The standard solution is to bond the equipment racks and cases together with a
grid made of heavy copper wire. This grid is called the "signal reference
ground system" or the like, and is an extension of the ordinary safety (green or
bare wire) ground system. Except during faults, neither green nor signal
reference grounds carry significant current. During faults (like a power cross
to an equipment case), thousands of amps may flow, at least until the power
breaker opens. So, the reference ground must be built as heavy as the safety
ground system, because it may serve as a safety ground as well.

One other thing to keep in mind. The National Electrical Code (NEC) requirement
on safety ground impedance is that the resistance can be as high as five ohms,
and no mention is made of a minimum test voltage. This makes perfect sense, as
the NEC's intent is to ensure that the breaker will pop if there is a fault.

However, five ohms and oxide films requiring 50-100 volts to puncture, while
acceptable to the NEC, make such a ground useless as a signal reference. So,
the reference ground system must be made of copper and/or its alloys, with
mechanical connection hardware optionally plated with silver and/or tin, and
must not be made of aluminum or steel alloys of any kind (unless all connections
are made by fusion welding, soft soldering, or brazing).

Another way to look at this is to observe that the signal reference ground
system more than meets the NEC requirements for a safety ground, and so can
serve both safety and signal-reference roles.

For far too much information, see TM 5-690 (Grounding and Bonding in ...
Facilities), downloadable from http://www.army.mil/usapa/eng/index.html.


War Story: In the late 1980s or early 1990s, I was working on the computers of
an Air Traffic Control system in Canada. (We were *not* working on the system
running real-world ATC, so no danger to air safety.)

The computers were VMEbus cards carrying 68000-series processors running at ~10
or ~20 MHz. I had a computer board out on a board extender, chasing some now
long-forgotten problem.

When I went to clip the oscilloscope ground to the VMEbus ground, there was a
tiny spark, and the ATC system crashed. Followed by my heart -- What did I do?
What could I have done?? ..... Umm.... Spark? Between grounds?? This cannot
be. Again touched the scope ground clip to the VME ground pin, watching
closely. Spark! What the hell??? Cannot be...Try it again. Spark!!....
Repeat until eyes are believed.

Turned out that there was a few volts between the two grounds, at 180 Hz, with a
source impedance of perhaps an ohm. Yes, 180 Hz. That turned out to be the key
- it was the charging pulses from all those DC power supplies fed from the three
legs of three phase power lines adding up.

The cause turned out to be that on a single cabinet (out of many tens of
cabinets), the white and green ground leads were reversed, injecting sufficient
noise into the reference ground system to cause many kinds of flaky and hard to
diagnose behaviour. Probably including the now forgotten problem that prompted
me to be poking around with a scope in the first case.


Joe Gwinn

On 2010-07-04, wrote:
On Sat, 3 Jul 2010 21:54:27 -0700, "Bill Noble"
wrote:

that would certainly NOT be the way I would do it - signal ground should go
to its own ground with no current carrying motors, solenoids, etc. Power
ground is some thing else - they could be tied together, but don't put them
all on the same ground stud

Take a bar of copper -- say 3/8" square, drill and tap a bunch
of holes along its length (probably best with a thread-forming tap), put
in screws, and use these to attach ring or fork terminals to a common
signal ground -- while isolating this ground bus from the chassis.

Or -- just go to the electrical department of your local Home
Depot or whatever and pick up one of the ground busses there (intended
to go in power distribution boxes) -- holes drilled through a bar, and
screws at right angle to clamp them. Not as nice for the stranded wire
as proper crimp ring or fork terminals would be, but not too bad either,
and less work.

[ ... ]

When using an oscilloscope to trouble shoot there are lots of
occasions where there are conflicts between signal ground and
power ground connections. For safety's sake scope metal work must
be connected to power ground, but for examination of low level
signal circuits scope ground must be to signal ground.

Temporarily breaking the power ground and floating the scope
metalwork is expedient but dangerous. A much safer solution is to
power ground the scope through back to back diodes.

Another alternative, if you have two (or more) inputs, with an
A-B switch position is to run two identical probes to the two inputs,
and let the scope display the difference between those two probes. One
connects to the logic ground, and the other to the signal being checked.
No need to float or semi-float the scope then.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

Ignoramus31310 wrote:
In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

No. The PPMC encoder board provides power to the encoders. For best
noise immunity,
ALL encoder wires should be connected to the PPMC encoder board, only.
The only exception
would be a cable shield, which might best be connected to the enclosure
the PPMC boards go
in.

The DAC board has individual grounds for the velocity command out. If
your servo amps
have instrumentation amps for the command input, then you want just a
shielded twisted pair from
the DAC board to the servo amp input. Again, connecting the ground of
the DAC board to other
grounds may introduce additional noise into the signal. These are 16
bit DACS, so the least
significant bit is 20 V/65536 = 0.3 mV, so a little noise really can
matter. If your servo amps DON'T
have fully differential (instrumentation amp) inputs, then it matters
less, as they already have some
noise contamination from the drive system, but still, better to leave
out extraneous grounds.
The PPMC should be grounded at one point to the common ground. For ESD
protection, the
PPMC backplane is already grounded to the DB25 shell. You can tap off
the single-point ground
from one of the encoder ground pins, or the motherboard power connector.

This is not critical, but once you have PWM drives switching tens of
amps at 80 V, with rise times
about 100 ns, the noise issues can become significant.

Jon

On Sat, 03 Jul 2010 21:34:48 -0500, "Martin H. Eastburn"
wrote:

I'd check the manual. Signal ground is that - not safety or earth.
It might be elevated e.g. 5v logic driving 11V logic. Or 3 into 5.

You might blow all of the expensive boxes if you don't know for sure.

Gunner might know - if you tell him the model number...

Let me know and Ill check my manuals

Gunner


Martin

Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
"Our Republic and the Press will Rise or Fall Together": Joseph Pulitzer
TSRA: Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Charter Founder
IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com/

On 7/3/2010 8:33 PM, Ignoramus31310 wrote:
In this mill, there will be many devices with "signal ground", such as
encoders, ppmc control box, servo drives etc. I think that I should I
tie all of those signal grounds, and the minus of the isolated DC
from the servo power supply, to neutral (in a star configuration).

Is that a good way to go, or am I way off base with my thinking.

Thanks.

One could not be a successful Leftwinger without realizing that,
in contrast to the popular conception supported by newspapers
and mothers of Leftwingers, a goodly number of Leftwingers are
not only narrow-minded and dull, but also just stupid.
Gunner Asch