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Connecting braided/foil shield on motor wiring cable — Parallax Forums

Connecting braided/foil shield on motor wiring cable

ZootZoot Posts: 2,227
edited 2006-10-24 17:37 in Robotics
I'm rewiring the power and motor distribution in my 'bot (see http://forums.parallax.com/showthread.php?p=610598)...

Since I've got the whole thing torn apart, I'm trying to do it right and use shielded cable for my drive motors. I have some nice 16 gauge 2-wire cable that has a braided shield over a foil shield.

My question is this -- should I connect one end of the shield to my motor supply gnd? I've read elsewhere in the forums that doing this can sometimes make the shield act as an antenna. Or should I leave the shielding to float? Or just try both ways and see what happens?

Related question -- this cable (cut from a 16' USB extension cable) actually has 4 wires plus shielding -- two 16 gauge power wires, and two 28 gauge signal wires. I'm not using the signal wires -- can I just leave them disconnected at both ends since they're both inside the shielding anyway? Or should I tie the ends to the shielding?

Thanks in advance.

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When the going gets weird, the weird turn pro. -- HST

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2006-10-24 16:11
    If you connect BOTH ends of the shield you MAY create a 'ground loop', where two 'grounds' actually sitting at different potentials start conducting current through a connecting shield.

    If you connect the shield to one side's signal ground, that should work well, coupling noise from the environment into the signal ground. If you connect the shield to the MOTOR supply ground -- well, that ground does tend to be noisy (from the current surges the motor creates while operating) and that noise could be coupled onto the shield.

    If you don't connect the shield to anything, then it 'floats', and has no EMI shielding effect. You may not need any shielding effect, though.

    I'd tend to connect the shield to the 'signal ground' on the end away from the motor, personally.
  • ZootZoot Posts: 2,227
    edited 2006-10-24 16:17
    By signal ground, you mean my logic ground?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    When the going gets weird, the weird turn pro. -- HST
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-10-24 16:30
    Yup, "signal" ground == "logic" ground. As opposed to "motor ground". You COULD tie it to a "chassis ground" if you have one.
  • ZootZoot Posts: 2,227
    edited 2006-10-24 16:37
    Actually, you bring up a good point that I had been thinking about -- advantages/disadvantages to a chassis ground. This is a big bot, high amps, etc. I'm a bit nervous about a chassis ground. I've actually tried out this bot with chassis ground and without -- neither made much of a difference in overall noise.

    That said, since I've got separate + and GND for logic, servos, motors (see link to wiring discussion above), if I *were* to ground the chassis, which ground? Or just tie the chassis right to my negative battery terminal? If the latter, then I think the shielding would be the only actual component/wiring tied to the chassis ground.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    When the going gets weird, the weird turn pro. -- HST
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-10-24 17:29
    If it was an AC powered robot, you'd tie the chassis to some sort of 'safety-ground' -- typically the third post of the AC connector. Classic 'chassis grounds' are to make sure a user can touch the case without getting a shock.

    Since it's a battery powered robot, probably you should let the chassis 'float' and provide isolation for the user from all inside voltages. This is a safety issue more than a signal one.

    And having said that, in an automobile the chassis IS tied to the negative terminal of the battery.· On a car, that simplifies wiring, since you only have to run a + wire, and use the car chassis for current return.· But it's a good example that approach can be safe.
    ·
  • ZootZoot Posts: 2,227
    edited 2006-10-24 17:37
    Thanks. I'd prefer not to ground the chassis:

    - I already dropped a screwdriver inside one day and shorted the chassis to + causing sparks and big fun (it's fused, but still...). That's when I disconnected the chassis smile.gif

    - battlebot specs disallow chassis grounding (for safety) -- this isn't a battlebot, but it's the same power and weight class and safe is always good

    - it doesn't simplify wiring since I'm running separate supplies and grounds for each of the 3 major systems anyway

    Thanks so much for the advice.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    When the going gets weird, the weird turn pro. -- HST
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