Thinking about wires

by rich on May 21, 2012

There’s quite a lot of wiring in a CNC Router:

  •  A cable from the PC’s parallel port, to the controller board (a Gecko G540 in my case),
  • Motor driver wires,
  • Power supply wiring,
  • Limit switches on each axis – a switch at each end,
  • Home switches,
  • Emergency Stop switch,
  • Possibly a control pendant,
  • Power for the router, with associated relays, and possibly a separate power supply for this relay,
  • And, possibly encoders – to ensure that the stepper motors accurately reflect the commands sent to them.

And, I’m sure there are many more…

Some of these wires contain low voltage control signals – not much current, but still, you don’t want spurious signals interfering… while other wires are actually driving quite a bit of current – the power supply lines, and the motor driver wires.

In my first iteration, I noticed that the Gecko driver uses DB9 connectors, so I simply ordered some 15 foot DB9 Male – DB9 Female cables to drive the motors. What I found in my initial tests is that these cables got very warm – not hot, but warm, as the machine was running, or even sitting idle. And, I’m experiencing some issues where the stepper motors seem to be missing some pulses – getting out of sync with where they should be. This is very obvious for the Y axis drive – which has a motor on each side driving the whole gantry back and forth on rack and pinion gears. Sometimes, the gear appears to skip a step – with a really loud thunk.

I think the DB9 serial cables I’m using actually have very small wires within the cable. The whole cable, with 9 conductors, and the sheath, is no more than1/8 or maybe 3/16 of an inch in diameter! So, the wires inside there must be 28 or 30 gauge! And, I’m trying to drive 3.5 amps of current through here! No wonder the wires are warming up!!

So, to rectify this, I’ve ordered some cable from Digikey – 4 conductor, 20 gauge, stranded wire (A5054C-100-ND). I’m planning to use this wire for the motors, as well as the limit and home switches, Emergency Stop.

Some other things I’m looking at:

  • Shortening the parallel port cable – which currently is 20 feet long,
  • Better grounding and shielding,
  • Better wiring from the power supply to the Gecko G540.

I’ll update this post once I’ve got this wire installed and working – I’m expecting the wire to arrive tomorrow.

Update: After updating all the wiring, all the issues I’ve observed previously have disappeared. The two stepper motors moving each side of the gantry are now in perfect sync, and I’ve seen no “skipping” where the gear drive gets out of sync with a loud clunk. Also, the wires are not warm at all, handling the current properly.


Wiring terminating at the Gecko G540

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