Wiring Question

[The Wizard]

Hi Folks,

I am part way through building a router/engraver (the project log can be found here... http://cnczone.com/forums/showthread.php?t=6244 ) and just came across a potential wiring problem that I hope you guys can clarify for me.

Due to space constraints on the machine I decided to mount the control board and power supply in an old XT desktop computer case I happened to have lying around. I had planned on using a DB25 connector (parallel port type) to connect the controller to the machine using a bi-directional printer cable as the umbilical between the two units. This would allow me break down the machine for storage as I do not have the room to leave it permanently set up.

My concern is whether a standard bi-directional printer cable is capable of handling the current for the motors (they are rated at 1A). Any thoughts? I could of course make up a cable of a suitable size using standard connectors but I was wondering if 'off the shelf' cables were up to the job.

I had planned on using Cat. 5 cable for the 'on machine' wiring with a paired cable (possibly speaker wire) for the home/limit switches. I also planned on incorporating a tool set-up device using a set of 'plug in' wires connected to the limit switch circuit. See the attached wiring diagram for details.

The alligator clip would attach to the cutting tool while the piece of key steel would be held against the edge/top of the part. By either jogging or winding the leadscrews by hand the edge of the key steel could be found when the machine stopped (if jogging) and/or the buzzer sounds.

I also have to think about an E-stop system and I am pondering using a double pole E-stop button and running the 12V and 5V power lines through the switch prior to the contol board. That way I could effectively kill both power sources to the control board (the power to the router would be controlled separately).

Any thoughts about the ability of standard printer cables being able to carry 1A would be apprciated as would any comments about the general set up.

Larry Green

[CJL5585]

Using a cable as you described is NOT recommended for the following reasons:

1. The step and direction signals should not be run in the same cable as the stepping motor power supply voltages.

The reason is induced voltage spikes on the step and direction lines from the current pulses which will be present in the motor supply lines each time the motor is advanced a step. This can totally destroy your stepping motor controller. Also, the motor will see a spike as a step or could see the spike as a momentary change of direction.

2. It is recommended that the voltage to each stepping motor be run seperately, and that the cable conductors have a twist of 6 turns per foot to help neutralize induction into other signal lines. (I personally prefer a shielded twisted cable with the shield being connected to the controller chassis ground only -- motor end not connected)

3. Although the stepping motor is rated at 1 amp, the instant current value of a step could be in theory as high as 20 to 50 Amps.

[Al_The_Man]

I agree with CJL, it is never good to go against approved practices in CNC control wiring, you may be lucky and get away with using exceptions but if you start getting miss-steps or problems due to noise etc, now you have the headache of trying to discover where the problem lies.
I personally only use stranded conductors, Cat5 BTW is 24awg and is rated at 2amps max, I would guess your printer cable would be 24g also rated at 2amps.
AWG Ampacity
30 0.5
28 0.8
26 1.
24 2
22 3
20 5
18 7
16 10
14 15
12 20
10 30
8 40
Al

[The Wizard]

OK.......I can see where this is going

So are you now saying I actually need seven cables between the control unit and the machine? Three cables to carry motor signals, three cables to carry motor voltage plus a cable to carry the home/limit switch signals plus the associated plugs and sockets?

Hmmmmmmm......after giving this a few minutes thought how do you feel about this idea............

The biggest problem appears to be the motor cables as they are the ones carrying the current and the associated spikes, the others are only carrying signals. How about if I still used the DB25 connector to carry the signal info and used a shielded, multi-core, 16 AWG cable to carry the motor power (the biggest I can find to fit an AMP Tyco connector)? That way I would only have two unbilicals to worry about between the controller and the machine.

That means I could also still use Cat 5 cable for the signals on the machine with separate cables for the motor power.

I have attached an updated wiring diagram to show what I am now thinking.

Any comments?

Larry

[Al_The_Man]

Unless I am missing something, does'nt that show the motor cables still going throught the printer cable?
Al

[Bubba]

Another "picky" point, is that you have shown NO switches for the limits. It is general practice to use NC so if a wire breaks (murphy) you will trip out on a limit. If you use the NO switches, and a wire breaks; NO LIMIT!


Bubba

[The Wizard]

Originally Posted by Al_The_Man Unless I am missing something, does'nt that show the motor cables still going throught the printer cable? Al

OK now I am confused and the more I think about this the worse it gets....lol. I am medically disabled with M.E. and one of the 'ailments' is intermittent 'brain fogs' (or 'brain farts' as they are known around here......lol). Having looked at that diagram again even I find it confusing and I drew it!

To set things straight (and I think this may have confused 'CJL' too) the 'printer' cable in question was not carrying the step/direction signals from the computer to the control board, rather it was carrying the 'output' from the control board to the machine (and yes I messed up and included some of the power cables in error thinking that the black/white wires only were carrying the power......DOH!)

I will revisit this tonight and now that my head is clearing I may get it right this time! I am now thinking I need three power cables to the motors as I can't imagine being able to get 15+ core shielded cable heavy enough to carry the amperage plus the sizes of the required connectors might be a problem and a fourth cable (possibly a serial type DB9 cable) to carry the home/limit switch signals.

I will post more later (with a bit of luck)......lol.

Larry

(where would I be without you guys when the M.E. strikes )

[The Wizard]

Originally Posted by Bubba Another "picky" point, is that you have shown NO switches for the limits. It is general practice to use NC so if a wire breaks (murphy) you will trip out on a limit. If you use the NO switches, and a wire breaks; NO LIMIT! Bubba

I had originally intended to use NC limit switches but then decided to include the set-up option of the key strip and alligator clip and could not think of a way of wiring it into a NC circuit (see my other post re. 'brain fogs').

I suppose I could parallel the set-up option off one of the NO 'home' switches but I have a feeling the machine would not stop on contact if I were to approach under power (jogging) as it would do if it was in the limit switch circuit. I am planning on fitting handles to all the leadscrews so I can manually 'touch on' if required.

A second option would be to make the set-up 'tool' self powered (battery) and hand held although I was hoping to just use the power on the machine and a couple of leads.

Larry

[The Wizard]

OK.......I spent the evening re-working the wiring diagram and I think I have my head on straight again now

I decided to go with three cables for the motors and a serial cable to carry the home/limit switch signals between the control box and the machine. Hopefully this will sort out the unbilical problem and I can concentrate on the actual machine wiring after that.

Diagram attached.......again

Larry

[CJL5585]

Grading your latest post on Wiring---- 100 A+

Think you have solved the problem.
Jerry

[The Wizard]

Originally Posted by CJL5585 Grading your latest post on Wiring---- 100 A+ Think you have solved the problem. Jerry

LOL.......Thanks for the high grade Jerry! In one of my former lives I worked as a College Lecturer and would rarely give out a grade as high as that!

I have been trying to find an online source for the connectors, limit switches and cables but, as is usual with my luck, it looks like I can't get everything I want from a single source :frown:

Most of the places I have found only want to sell multi-core cable by the reel too. I think I am going to have to find something locally which may mean a drive into the nearest city.

Larry

[The Wizard]

Hi Folks,

It's been a while since my last post on this thread due to circumstances that prevented me working on the CNC project (I guess Xmas related projects had to come first...lol).

The good news is that the CNC project is on the boil again and I now have my MDF parts cut and once I get a few more holes drilled I can start assembling the beast. In the meantime I have been giving some thoughts to my wiring ideas and made some changes (again!)

I read somewhere that limit switches should be wired via a pull-up resistor so I have included one of those this time and I have also added an E-stop circuit using a latching pushbutton and a DPDT 12V relay. The idea is that the power supply is wired through the NC contacts and when the 'remote' latching pushbutton is pressed it energises the coil which in turn disconnects all power to the control board. Pressing the button again would de-energise the coil and return power to the board. I realise the software would still be running using this method but I could not find a method of killing the machine and the software via the control board.

The control 'box' will be connected to the machine via a set of unbilical cables (three for the motors and a serial cable for the limit/home switches) and the E-stop button will be on a seperate lead so that it is always 'to hand'. I priced commercial E-stop buttons and decided that at $50+ CDN they were out of my budget so I am hoping this system will work at a much lower cost.

If someone could look over the drawings and shout if they spot an error I would appreciate it.

Larry