How many motors can run on the same step and direction signal?

[johannes]

I'm planning a 40foot long production line that will have 4 identical gantry routers run the same permanently fixed program.
Since all 4 routers will essentially be identical copies of eachother, I am curious if I can simply run them all from a single Acorn controller?
The question is can a single step and direction output on the Acorn be split multiple times? (4x X motors, 4x Z motors, 8x Y motors (dual gantry)).

Johannes

[centroid467]

Personally, I think one of your bigger problems with that will be homing. How will you handle homing with that kind of setup? Or guaranteeing that each machine starts in the same spot all the time.

As for running many drives, I'd use something that works well with the open collector step/dir outputs.

However, I don't Centroid is going to recommend this kind of configuration at all.

[johannes]

Personally, I think one of your bigger problems with that will be homing. How will you handle homing with that kind of setup? Or guaranteeing that each machine starts in the same spot all the time.

I have used Clearpath motors with the Precision Homing feature on previous builds. So that should allow each machine to home to itself without needing the Acorn controller to manage that part. I would do the same even if each router had its own Acorn controller, so that doesn't make much difference I think.

[Richards]

The Acorn spec sheet shows that the DB-25 LOW or HIGH current is 20mA max and 3mA typical. The 24V Open-Collector current is 50mA max and 10mA typical. I usually allow 10mA per opto-coupler. Opto-couplers are commonly used in stepper driver step/direction inputs.

IMPORTANT: Part numbers are for reference only. Select parts that can safely handle 3X the current required.

If I were to drive four machines from one Acorn board, I would use a bi-polar transistor or a MOSFET to amplify the current. Here's a simple circuit that shows one method for DB-25 signals:

You would need one transistor/MOSFET per DB-25 signal.

The "H" connectors are Open-Collector, so a PNP transistor or P-type MOSFET should be used:

[attachment=0]BS170_p-type.png[/attachment

[johannes]

Thanks Mike,

I'm planning on using clearpath servos which have a minimum current requirement of 8mA at 5volts. So with 50 mA max current available, it sounds like it should be possible to drive 4 motors from one axis on Acorn.
I also got a response from the cnc4pc team, where they say:

Hello, Johannes

YES! The C86ACCP converts the open collector outputs of the ACORN to high-power TTL. it can deliver 86mA per pin. YES! you can connect the 4 servos to one axis.

So using that board you get more available current as well.

[suntravel]

I do not think this a good idea...

If one drive faults all machines are out of production.

I would use 4 Acorns and additional common Cycle Start / Stop / Feed Hold buttons.

Uwe

[johannes]

Thanks Uwe,
Yes, I agree the complexity is not ideal.

The line will process 1 single aluminum profile at 12meter/36 feet total length, and the initial idea was to split this work into smaller sections.
One option was multiple spindles on the same gantry, or this idea which uses multiple carriages along the track.
I can certainly build them with independent Acorns, but they will never do any sort of independent work.

[suntravel]

Ok, but I still think 4x3 Axis CNC or one 12 axis is the better solution...

Easy to dial in deviations on the 4 stations
Easy to redo one operation if a tool break

Uwe

[johannes]

Ok, but I still think 4x3 Axis CNC or one 12 axis is the better solution...

Easy to dial in deviations on the 4 stations
Easy to redo one operation if a tool break

Uwe

Yes, good points. I do agree it'll be better and easier to manage.
Thanks for the advice, appreciate everyone's input.

[Richards]

Sometimes we don't understand why a circuit is rated with "typical" or "average" current and also "maximum" current. A chip is rated to handle a maximum current flow. While each pin on that chip may be able to handle 30% or even 50% of the chip's maximum current, if ALL pins conducted at or near the pin''s maximum rated current, the chip would overheat. That is one reason that I normally use some type of discreet transistor if I need to run multiple devices from one pin. Back when I designed process control computers, the rule of thumb was that an IC had a "fan out" capacity of 10 unit loads. That meant that one output pin on an IC could drive 10 input pins on other ICs. Those input pins drew very little current. But, it that output pin was connected to anything except other ICs, it could only be connected to one load. An opto-coupler draws 10x as much current as a typical IC input pin, so, without amplification, it's still best to limit one output to one input.