diamond: RE: [DIY-CNC] What kind of wire are you using to

If you have a sensor the screened case of which is isolated from the rest of
the machine then it is good practice to connected the screen at both ends,
this was probably the case in your machine. I some across this a lot on
ships steering systems, sometimes the metal rudder position sensors have an
earthed case on a metals ship, sometimes not on a fibre glass ship, trying
to get installers to understand this is another issue. On the sensitive
systems I have to use isolated sensors within the main sensor case so I can
internally connect a screen. Obviously an encoder sensor on a CNC machine is
usually mounted on the motor and therefore bonded to the motor so a
connection at one end only is viable

The other possibility with you machine is there may have been a missing
earth bond at the sensor end. Some sensors connect their internal
electronics to their case, the machine may have been leaking through a
signal conductor, when you attached the screen at both ends the noise ran in
the screen rather than signal cable so the signal to noise ratio improved. A
separate earth bond would have been a better option.

As many have said screening and earth bonding gets to be a complex subject,
the more we discuss it here the more people will get to grips with it.

Regs Mark

Dr. Mark Vaughan Ph'D., B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd., reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH UK.

Phone/Fax 44 (0) 1872 561288

RSGB DRM111 (Cornwall)

_____

From: DIY-CNC@yahoogroups.com [mailto:DIY-CNC@yahoogroups.com] On Behalf Of
Jim Peck
Sent: 24 February 2008 19:48
To: DIY-CNC@yahoogroups.com
Subject: [DIY-CNC] What kind of wire are you using to

A number of years ago I inherited responsibility for some machines with
punch list items that had to be resolved. One were Reaction Injection
Molding presses that were seeing a fluctuation in chemical pressure when
chemical was being stroked by a moving hydraulic cylinder. All the cable
shields had been connected to a common ground at the main electrical panel
and were picking up noise like an antenna. When we grounded the cable at the
pressure transducer end, the noise went away. Reliance Electric used to use
24 twist per foot twisted pair instead of shielded cable for tachometer
feedback and had test data showing it substantially self-shielded.

[ Harko Schwartz]If there is one thing that is always a point of discussion
it is proper grounding. I always thought that grounding the shield at one
end was the best way to reduce the noise on a encoder line. Allen Bradley
has a DVD which covers electrical noise and proper grounding it is free. It
shows the amount of electrical noise with one end grounded and both ends
grounded with both ends grounded there is less noise. The main issue is that
the motors are properly grounded to begin with both with a ground wire from
the drive and a ground wire attached to the motor direct to your single
point ground. In all cases you should not have power and signal wires in
your sealtite they should be run separate and not near any main power
transformers or parallel to power wire inside the cabinet.

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