Bridging EMC and Router

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20 Dec 2010 04:14 #6178 by atmegaist
Hello, all -

I'll get right to it:

I'm curious to see what I can do as far as constructing a router and am having some trouble understanding what it is that EMC is actually sending through the parallel port.

I understand that the answer to this question probably isn't as simple as others so what I'm really interested in is seeing if someone could point me in the right direction as far as topics for research. I'm comfortable with driving steppers and interfacing custom hardware with computers so I'm ok with the terminology as far as that end goes but as far as the router/EMC-specific topics/terms are concerned, I simply don't know where to start.

I fully expect for this to take some time and reading so whether it's a 2-minute tutorial or a 100-page PDF with bits and pieces of info scattered throughout, send it on!

Thanks in advance!

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20 Dec 2010 10:31 #6180 by BigJohnT
Replied by BigJohnT on topic Re:Bridging EMC and Router
You might start with the EMC manuals.

John

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20 Dec 2010 14:15 #6182 by andypugh
Replied by andypugh on topic Re:Bridging EMC and Router
atmegaist wrote:

I'm curious to see what I can do as far as constructing a router and am having some trouble understanding what it is that EMC is actually sending through the parallel port.


That rather depends on how you set up the system:

1) Possibly the most common, is step and direction signals on 2 parallel port pins per axis, to external drives which handle the power electronics, current limiting and step pattern generation. One pin goes high for one direction and low for the other, and the second pin pulses every time a motor step is needed to keep the motor in the correct position.

2) Rather less common: EMC2 generates the step patten on 4 parallel port pins, this goes to simple power electronics (as simple as a set of power transistors) and drives the motor sem-directly. This mode could in fact drive extra-tiny motors with no extra electronics. This isn't a very good system.

3) The parallel port can be used as a communications port to an external step generator. (Pico PPMC, Mesa 7i43, Motenc or Pluto(just don't)). EMC2 sends a step rate and the external hardware does it. This gives much higher step rates, and frees up a lot of CPU. You still need external step/direction drives though both Mesa and Pico sell hardware that connects directly to the interface card.

4) You can not use steppers at all. You could use servo motors. The parallel port would have a PWM signal to control motor speed (or torque, or voltage, or currrent depending on the drive hardware) and a direction pin (optional, you can use biased PWM but that has certain "issues"). You then need at least 2 input pins on the p-port for an encoder on the motor so close the feedback loop.

5) As 4, but with a Pico or Mesa card generating the PWM with amplitude commands sent digitally. You can use separate PWM and direction drives, or integrated drives from the manufacturer.

6) As 5, but with the interface communicating digitally to the drive (defining PWM as analogue, for the moment)

7) Skip the Parallel port and fit a PCI card.

8) Almost anything else you can imagine. EMC2 is extremely flexible, and if you need something special, it is all open-source so you can write your own bits.

www.mesanet.com
www.pico-systems.com/motion.html

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20 Dec 2010 22:39 #6195 by atmegaist
andypugh -

You've hit the nail right on the head, my friend. This is DEFINITELY the direction I was looking for. I tend to overcomplicate things and didn't even begin to think that it might be as simple (well, conceptually, at least) as your first suggested configuration. I saw that teh setup wizard had a panel in which to assign duties to each of the parallel port pins but, again, had no idea that it was so directly able to send setp pulses and the like.

Apologies, again, for the pedestrian nature of these questions but your post was most helpful in tying things together.

BigJohnT -

The manuals were a great help, indeed. They seemed to be oriented toward a semi-complete setup but will definitely be kept in mind as I progress (hopefully!)

Thanks to both of your for your time!

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21 Dec 2010 05:57 #6197 by atmegaist
Aaaaaaand we're back...

My first inclination in doing this was to use an arduino to handle the driver signals but after a bit of reading I can see that it would be more efficient to connect the parallel port directly to the stepper driver. So - all of my aforementioned comfort with driving stepper motors got up and left when the arduino proved to be inefficient.

I can see, though, that I'm basically working with the single (axis step) lead coming from the parallel port which provides a "high" and "low" to act as the alternating step commands. If I were to use a ULN2003 or the like which allows a 2-wire connection to control steps, how would I turn my "high" and "low" into two separate "high" states to control the two inputs on the ULN2003?

I know this is probably a simple analog electronics issue but any tips would be appreciated.

Many thanks, again!

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21 Dec 2010 09:43 #6198 by andypugh
Replied by andypugh on topic Re:Bridging EMC and Router
What sort of stepper driver are you using?
If it is a step/dir type then all you need is wires.

if you are trying to drive the stepper semi-directly, then you can drive a unipolar stepper motor like this using a ULN2003
www.mycncuk.com/forums/showthread.php/20...r-motor-with-ULN2003

In that case you would use the EMC2 stepgen component in Mode 9
www.linuxcnc.org/docview/html/man/man9/stepgen.9.html

However, this is not a very efficient way to drive steppers, you will get much better performance in Bipolar mode with a current-limiting chopper driver, running at much higher than "rated" voltage. In this setup the drive limits the motor current, not the motor DC resistance. You then use 10x the DC voltage and get about 10x the speed/torque.

I am still not at all clear what drive hardware you have?

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22 Dec 2010 00:49 - 22 Dec 2010 00:50 #6210 by atmegaist
I actually don't have a stepper driver (I thought that was what I would ultimately build and LinuxCNC would act as the controller... Is my thinking correct on this?)

I think I get what you're saying about the chopper circuit. So could I essentially use a 74194 to handle the logic and an L293D H-bridge? If so, the only question I have pertains to the two direction inputs on the 74194 instead of one (isn't this simply a "low" or "high" scenario for direction?)

The chart showing the aforementioned parts can be seen at the link below (it's the 3rd image down the page).

home.cogeco.ca/~rpaisley4/Stepper.html <<< I couldn't get the hyperlink format to work :dry:

If this approach seems correct then I should have enough to get started. One more possibly stupid question: can all unipolar stepper motors be configured to operate in bipolar mode? I understand how to set them up as such but was curious to know if there were any certain exceptions.

Thank you so much for your help.
Last edit: 22 Dec 2010 00:50 by atmegaist.

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22 Dec 2010 11:47 #6211 by andypugh
Replied by andypugh on topic Re:Bridging EMC and Router
atmegaist wrote:

I actually don't have a stepper driver (I thought that was what I would ultimately build and LinuxCNC would act as the controller... Is my thinking correct on this?)

Indeed, that is exactly what you would do.
You might want to consider buying rather than making a driver board, there are many on eBay as direct imports from China for rather less than you can build them for.
As a random example: cgi.ebay.co.uk/3-Axis-CNC-Stepper-Motor-...TB6560-/250733402363
Try a search for "CNC Stepper" on your own local version.

One more possibly stupid question: can all unipolar stepper motors be configured to operate in bipolar mode? I understand how to set them up as such but was curious to know if there were any certain exceptions.

I don't know of any exceptions, nor can I imagine how there could be, but I could be overlooking something.

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23 Dec 2010 06:18 #6223 by atmegaist
Great tip on checking into eBay for drivers - I had no idea they'd be that inexpensive!

Out of sheer, bold ignorance, though, I think I'm going to try to have a go at building one. I've got some time off and was intending to make this more of a learning "experiment" than anything else. Also, pretty much everything I'm using was scavenged from trashed machines and parts so I won't be out anything if it all burns to a crisp. I think I'm just going to shoot for getting a solid, consistent feel for stepper implementation in general and then we'll see about getting into the CNC end of things. So, with that said, I believe I'll take my inquiries to the books and let you folks get back to the more relevant questions out there.

Your help has been MOST appreciated and I hope to be back around soon!

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23 Dec 2010 23:11 #6228 by BigJohnT
Replied by BigJohnT on topic Re:Bridging EMC and Router
Keep us posted on your progress building a drive... it could be interesting.

John

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