What
is Noise:
"Noise" or "Interference" consists of unwanted
electrical signals which superimposes on and masks the desired
signal. Designing a control system is challenging enough, but
designing a control system that has noise immunity adds a whole
other dimension. Ideally, you want the noise-to-signal ratio to be
as small as possible. Noise is always present in a system that
involves high power and small signal circuitry. The key is to
manage the noise so that it does not interfere with the
performance of the system at hand.
Sources
of Noise:
Sources of noise can be external to the stepper motor system as
well as within. The most common external sources are relays and
motors. Internally, the relatively high current motor drivers are
the source. All bi-polar stepper motor drivers apply a chopping
function to the applied voltage of each phase. This chopping
enables use of higher voltages than the motor is rated for,
achieving higher speeds while keeping the motor from getting too
hot. The combination of the chopping and inductance of the motor
creates noise on the ground plane. This [ground plane noise] can
be introduced into nearby external systems if proper wiring and
shielding precautions are not taken. The result can be
intermittent failures of the system as a whole.
Components
of Noise & How to Manage Them:
In order to manage noise it is important to understand its
components. Noise [Interference] is categorized into two groups:
radiated and conducted. Radiated interference is transmitted by
electromagnetic fields and picked up by the antenna effect of
other equipment. If it were always possible to isolate susceptible
receivers and radiating sources from one another, radiated
interference would be more manageable. As distance increases,
radiation fields become weaker thus energy becomes dampened along
a conduction path. Unfortunately with today's limited system
real-estate, distance isn't usually an option. Reducing the
antenna effect and adding shielding, controls this type of
interference. (Improper shielding can cause more problems than no
shielding at all.(See Rules of
Shielding.)
Conducted interference
is that which is introduced into a circuit by either direct or
indirect coupling. Both direct and indirect coupling are
classified into three specific types: Resistive, Capacitive, and
Inductive. These types of coupling are most frequent where common
return circuits and power supply grounds exist. Conducted
interference can originate from a variety of sources, such as
relay and switch contacts, fan motors, power switching or digital
devices with short rise and fall times. The effect of conducted
interference cannot be eliminated as easily as shielding
eliminates the effect of radiated interference. Good wiring
practices are necessary to minimize Conducted interference. Give
close consideration to connections to and from power supplies.
Give particular attention to common grounds. Ultimately, the whole
system must be referenced to them. (See
Rules of Wiring.)
How
to Detect Noise:
The first step in troubleshooting a noise problem is acquiring the
right tools for the task. An isolated Oscilloscope is the chosen
tool for detecting noise. A battery powered scope [if one is
available] achieves the best circuit isolation, however a scope
with an isolated ground will still be an effective tool. Also,
keep in mind that a Digital scope may mask the noise depending on
it's sample rate and frequency response. Therefore, an Analog
Oscilloscope is better than a Digital scope for detecting
asynchronous signals of high frequency such as noise. Along with
the scope a wiring diagram and a basic knowledge of the systems
operation are the best tools. The next step is to simplify the
system. Start by removing power then disconnect all system
components from the Stepper Driver that are not absolutely
necessary for basic motion. Keep an open mind, even experiment a
little by using a jumper wire to introduce noise and simulate the
failure mode you are experiencing. Remember there may be more than
one noise source.
IMS
encourages our customers to ask questions and take advantage of
our Application Support Team
early in your design. We can review your system and make
suggestions on the interfacing and wiring practices. We may
suggest other tips not listed below that are application specific;
but as a starting point refer to "Good Wiring and Shielding
Practices" for the basic rules.
Good
Wiring and Shielding Practices
-
Find
the Noise
-
Disconnect
all unnecessary I/O.
-
Connect
I/O one at a time until problem reoccurs.
-
Shut
down any other equipment that may be emitting noise.
Rules
of Wiring
-
Power Supply and
Motor wiring should be shielded twisted pairs, and run
separately from signal-carrying wires.
-
A minimum of one
twist per inch is recommended.
-
Motor wiring should
be shielded twisted pairs using 20 gauge, or for distances of
more than 5 feet, 18 gauge or better.
-
Power ground return
should be as short as possible to established ground.
-
Power supply wiring
should be shielded twisted pairs of 18 gauge for less than 4
amps DC and 16 gauge for more than 4 amps DC.
-
The shield must be
tied to zero-signal reference potential. It is necessary that
the signal be earthed or grounded, for the shield to become
earthed or grounded. Earthing or grounding the shield is not
effective if the signal is not earthed or grounded.
-
Do not assume that
Earth ground is a true Earth ground. Depending on the distance
from the main power cabinet, it may be necessary to sink a
ground rod at the critical location.
-
The shield must be
connected so that shield currents drain to signal-earth
connections.
-
The number of
separate shields required in a system is equal to the number of
independent signals being processed plus one for each power
entrance.
-
The shield should be
tied to a single point to prevent ground loops.
-
A second shield can
be used over the primary shield, however the second shield is
tied to ground at both ends.
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Questions? Contact IMS applications support:
Ph:
(860) 295-6102, E-mail: etech@imshome.com
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