?
Almost all PCs
with a three prong plug have the DC signal ground connected internally to the AC
mains safety ground. This makes it important to consider the potential for
ground loops. USB signal cables will unavoidably have their?signal ground reference and shield connected to
the AC mains safety ground because of this.
?
You do not need to
provide an external shield over a USB cable in most situations. The data signals
are differential and carried by a twisted pair. Twisted pair differential
signals inherently resist common mode
induced noise which couples into both conductors at about the same amplitude.
Sometimes the induced signal can exceed the "common mode range" of the receiving
device. This is?when a ferrite choke
over the cable can help. PMDX has put a tiny common mode choke on all of our USB
based boards, but even so some host computers suffer from this problem at the
host end of the cable and a choke there can sometimes help.
?
All USB cables
have a shield which may also serve as the negative side of the power delivery
and the signal ground reference. More commonly there is a separate ground wire
to serve as the power and reference connection. Even so, it is common for the
shield of a USB cable to be connected to?signal ground at both ends. This is why I
recommend a redundant larger wire be connected between the computer chassis and
the signal ground at the controller so that most of the current caused by a
ground loop flows through this wire instead of the shield.
?
It is possible to
defeat ground loops by the use?od USB
isolators or to design USB interfaces that are electrically isolated (example
the PMDX-424 SmartBOB), though this is not common because USB was originally
intended for human interface devices such as keyboards and mice that do not
suffer from ground loops. USB isolators can be purchase on eBay for less that
US$20 each. Twisted pair Ethernet also provides ground loop isolation because
the signals are?usually
transformer?coupled.
?
The PMDX-126 is
designed to allow isolation between the PCs logic (DC) ground and the machine
frame ground. Success of this isolation requires proper attention to ground
points and avoiding things such as encoders whose body connects their signal
ground to the machine frame.
?
Coaxial shielding
of a cable protects only against electrostatic fields. It does nothing to block
magnetic fields. That requires a magnetically permable material such as iron,
nickle, ferrite, or mu-metal. Mu-metal alternates layers of copper and nickle
and has properties that shield both electrostatic and magnetic fields. Nice as
this sounds, it is not very flexible mechanically and it tends to empty your
wallet quickly.? 8-)
?
Electrostatic
shielding is provided by a coaxial shield that is grounded at either end or both
ends.?For signal cables, grounding at
only the end that is receiving the signal is preferred.
?
Grounding both
ends of a coaxial shield that covers signal conductors risks allowing current to
flow through the shield. When this happens, the cable will act as a one turn
transformer and couple the magnetic field generated by the shield current into
the signal conductors.
?
If the cable is
delivering power (stepper motor drive current or spindle motor power) then it is
desirable to ground both ends as there is no worry about corrupting the signals
and the grounding provides safety features.
?
USB, twisted pair
Ethernet, and many encoder signals are differential mode signals which help to
cancel common mode noise induced either electrostatically or magnetically. The
operative word is HELP, not eliminate.
?
Motor drive
current to bipolar stepper motors is essentially differential and this helps
reduce the interferance they generate. If ground loops are a concern, then these
cables, if shielded, should have the shield grounded at the driver end. The best
ground for this purpose is the negative terminal of the power source feeding the
stepper drivers.
?
Power cables
between a VFD and a spindle motor are a potent generator of noise due to the
high voltages, and fast rise time of the PWM modulation of the simulated sine
waves generated by the VFD. Ferrite chokes and/or "reactors" can be used to
reduce the noise level somewhat. Because this is a power cable and there is no
signal information to be compromised, you can and should ground the shield at
both ends for safety reasons.
?
As was mentioned
by John D. separation distance is your friend when trying to reduce the amount
of noise induced by one conductor into another. It is also helpful to have noise
generating cables cross signal carrying cables at right angles. Lastly, if
cables are longer than needed, shorten them or fold them in a Z pattern. Coiling
them up results in a better target for noise induced by magnetic
fields.
?
With all of this
being said, sometimes eliminating noise faults is more art than science.
Overthinking and analyzing things does not always trump simply trying things in
the real world.
?
Regards,
Steve
Stallings
PMDX
Electrical
noise can be a bear.? Steve
Spallings mentioned PC grounds and system or motor grounds.? I have his PMDX-126 and unfortunately
the PC I'm using internally somewhere has the DC ground connected to the system
frame which is the third earth prong on the power cord which then connects to
the motor frames through the machine metal. The CUI encoders on the DC servos
serve to further muck things up where their DC ground is connected to the
frame.? Which is of course connected
to the 3 prong and so DC ground earth ground are connected in several places.
?Shielding has no rhyme or reason
here but it's one place where a single ended connection for a shield is a bit
more logical.
?
My EE book on
electrical noise states that the signal grounds should never be connected to the
DC power grounds or frame. ?The
whole point of shielding is that it isolates the two.
?
For your system
also look at magnetic coupling.? The
way a transformer works is a changing magnetic field introduces current in
windings that are also in the magnetic field.? So the spindle motor current flowing
through the wires can generate a field picked up encoder or limit switch wires.
?Short of what is called mu metal
shielding the only solution to electromagnetic interference is distance.? Route them far
apart.
?
Electrostatic
shielding is different.? Twisted
pairs help to null out electric field noise. Shielding can be grounded at one
end or both of the frame (third prong) earth.? The point is you only want the noise
current flowing back through the shield into the frame.? Not through your signal wires which
includes the DC 0V wires.? People
tend to forget what goes out a signal wire is still a signal as it comes back
through what is commonly called the ground or common wire.? (Some manufacturers have called the +5V
to +24V 'common' for limit switches)
?
John
Dammeyer.
?
?
You are trying to fix a wrong
schematic with cosmetic changes. You sure have a ground loop somewhere. Check
your shields, they should be connected one side only with heavy short wire to
good ground. Preferably star connection.
?
i
forgot to mention that, i have ferrites everywhere. that is one of the first
things i do.? i have them on the signal leads to each stepper and even on
the usb cable to the pokeys although the mfg suggested it would not do
anything. i tried a usb signal conditioner thing but it did not work, meaning
that most things plugged into it stopped working.
anyone know of a signal
conditioned for usb that does work?
Best
regards, Spencer Chase
67550 Bell Springs Rd.
Garberville, CA 95542
Postal service only.
Laytonville, CA 95454 UPS only.
Spencer@...
Spencer@...
Spencer@...
(425)
791-0309
(707) 223-8212
------ Original Message
------
Sent: 3/27/2020 4:03:12
AM
Subject: Re: [MachCNC]
strange stopping
I would probably try
adding ferrite beads to the leads.
I have my router
working really well with the following configuration. Solid state tiny
computer running win 10. Mach 3 latest version Pokeys USB CNC thing
connected to a PDMX.
I am finally getting
smooth motion no lost steps etc etc. Everything works
except.
Occasionally Mach3 just
stops and is totally unresponsive. Can not reset, jog etc. The diag screen
shows all ins and outs off. I have to kill Mach 3 and start
again.
I am pretty sure that
every time it does this, when I restart the 4th axis DRO shows
4099.xxx? the 4th axix is angular.?
I am running a gcode
with a subroutine that repeats over and over with a tool change to allow
inserting a new part for machining. i am also using the subroutine to make
it possible to count parts.?
when this failure
happens is always close to the spindle motor going on or off. it is a
three phase servo run by a high quality US inverter. Forgot the name right
now but it is a good inverter not Chinese no name
stuff.
I am pretty sure the
spindle motor is causing noise that kills the Pokeys. I have proper
grounding and shielding.?
I changed the inverter
carrier frequency from the default of 5Khz to 2khz and have not had the
problem yet but it some times does not happen for a considerable
time.?
any ideas? does the
4099 signify some overflow?? does it sound like it is inverter noise?
might the lower carrier frequency be the fix?
Best
regards, Spencer Chase
67550 Bell Springs Rd.
Garberville, CA 95542
Postal service only.
Laytonville, CA 95454 UPS only.
Spencer@...
Spencer@...
Spencer@...
(425)
791-0309
(707) 223-8212
