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Re: strange stopping - dealing with noise issues
开云体育?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 From: [email protected] [mailto:[email protected]] On Behalf Of John Dammeyer Sent: Friday, March 27, 2020 6:59 PM To: [email protected] Subject: Re: [MachCNC] strange stopping 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. ? From:
[email protected] [mailto:[email protected]] On Behalf Of Todor
Boshnakov ? 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. On 27 Mar 2020, at 18:32, "spencer@..." <spencer@...> wrote:
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