Re: [CAD_CAM_EDM_DRO] Shielding

In a message dated 31-May-02 17:35:02 Central Daylight Time, js3mc@...
writes:

> There has been discussin on the shielding of the motor cables but I'm
> wondering what else should/needs to be shielded as well AND why?

Jay: The currents flowing in wires to stepper-motors are "chopped" and
generate HUGE quantities of electrical nasties called "EMI". Actually, in
well-done assemblies, were it not for the electrical noise in the motor-drive
conductors, other than just plain good wiring-layout, no other shielding
should be necessary.

However, it is probably NOT a bad idea to used shielded cable for the
printer-port-cable as well, and at least dress the wires to the STEP and
DIRection inputs on your motor-drives next to the chassis-metal, and do NOT
run these "logic-level" input-wiring adjacent/parallel to any power-supply
and motor-cables. Any "ribbon-cable" inside the box containing your drives
and/or motor-power-supply should be laid FLAT on the chassis-metal, and
covering that will adhesive-backed aluminum foil wouldn't be a bad idea, if
you... Nevermind.

You see, the currents in "logic input lines" are very small, particularly
when CMOS logic is involved. That's one reason the Gecko's use OPTO-ISOLATOR
inputs, as the LED-inputs on those require MUCH more current-in (16 mA) than
"logic inputs" such as LS or CMOS gate-inputs. However, the currents in the
motor-lines are not only AMPERES, but this current is "chopped" at tens of
kilohertz, making a "spray" of EMI into the air near these wires. So, if you
were to lay these input-conductors parallel and "touching" the motor-cables,
often, enough electrical energy from this "spray" of EMI can be INDUCED into
the input-conductors so that the drive-inputs "see" these signals as inputs,
and things go nuts!

See Education; Experience; Years of Trial-and-Error Breadboarding; Patience
with "Where's THAT Coming From???; Large Quantities of Aspirin...

Lotsa luck! Jan Rowland, Old Troll


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