User guide

112 6250 Servo Controller User Guide
control computer is using RS-232C communication.
Symptoms like garbled transmissions and intermittent
operation are typical.
The problem occurs in systems where multiple Earth
ground connections exist, particularly when these
connections are far apart.
Ground Loops—Noise Scenario
Suppose a 6250 is controlling an axis, and the limit
switches use an external power supply. The 6250 is
controlled by a computer in another room. If the power
supply Common is connected to Earth, the potential
exists for ground loop problems. This is because most
computers have their RS-232C signal common tied to
Earth. The loop starts at the 6250 system limit switch
ground, goes to Earth through the drive, and on to Earth
at the computer. From there, the loop returns to the 6250
system through RS-232C signal ground. If a voltage
potential exists between drive Earth and remote
computer Earth, ground current will flow through the
RS-232C ground, creating unpredictable results.
The way to test for and ultimately eliminate a ground loop
is to lift or
cheat Earth ground connections in the system
until the symptoms disappear.
Defeating Noise
The best time to handle electrical noise problems is
before they occur. When a motion system is in the
design process, the designer should consider the
following set of guidelines for system wiring (in order of
importance):
➀ Put surge suppression components on all electrical
coils: Resistor/capacitor filters, MOVs, Zener and
clamping diodes.
➁ Shield all remote connections, use twisted pairs.
Shields should be tied to Earth at one end.
➂ Put all microelectronic components in an enclosure.
Keep noisy devices outside. Watch internal
temperature.
➃ Ground signal common wiring at one point. Float this
ground from Earth if possible.
➄ Tie all mechanical grounds to Earth at one point. Run
chassis and motor grounds to the frame, and the
frame to Earth.
➅ Isolate remote signals. Solid state relays or opto
isolators are recommended.
➆ Filter the power line. Use common RF filters, and use
an isolation transformer for worst case.
A noise problem must be identified before it can be
solved. The obvious way to approach a problem situation
is to eliminate potential noise sources until the symptoms
disappear, as in the case of ground loops. When this is
not practical, use the above guidelines to
shotgun the
installation.
References
Information about the equipment referred to may be
obtained by calling the numbers listed below.
❏ Corcom line filters,
[phone]
❏ Opto-22 optically isolated relays, [phone]
❏ Crydom optically isolated relays, [phone]
❏ Potter Brumfield optically isolated relays, [phone]
❏ Teal power line isolation filters, [phone]