Mesa board with support for DROmagnetic encoders

Hi everyone,

I just picked up a used LMS3990 and it came with a DROPros EL403-M kit: www.dropros.com/Digital_Readout_DRO_Displays.htm

I want to convert my LMS3990 to CNC and would like to retain the DRO functionality. Is it possible to perform closed loop drive using steppers / stepper drives and the magnetic encoders? If so, what Mesa boards should I purchase for this? I believe the encoders use the typical 5V TTL, DB15 pinout.

Any advice would be greatly appreciated! Thanks!

Yes, it's possible to use those scales to close the position loop back to LCNC.

The 403M appears to come with 5 micron resolution, not sure if that's appropriate for your machine - others with more experience may share an opinion on that.

As far as appropriate Mesa boards for a step & direction control, it gets confusing because there are so many options.  Basically, you will need either:

• A single board which has both an FPGA chip AND encoder inputs
  • Example: 7I95T
    • 6 axes of step/direction outputs
    • 6 encoder inputs (TTL or differential)
    • 24 general inputs
    • 6 general outputs
    • 1 expansion port
    • 2 serial expansion ports
• An FPGA board, plus a daughterboard with encoder inputs
  • Example: 7I96S + 7i85
    • 9 axes of step/direction outputs (5-7i96S, 4-7i85S)
    • 5 encoder inputs (1-7i96S, 4-7i85S)
    • 1 analog out for spindle (VFD)
    • 11 general inputs
    • 6 general outputs
    • 2 serial expansion ports (1 on 7i96S, 1 on 7i85S)

These are just two examples.  Other DB25 daughterboards can be used with the 7i96S which provide different capabilities.  And the 7I95T can take daughterboards as well.

Main thing you need to keep in mind is you need an FPGA board, enough S&D outputs for your system, and enough encoder inputs for the axes you want to close the loop on.  Plus whatever you need to control a spindle, if any.

Make a spreadsheet with everything you have - all axes, encoders, sensors (limit switches, etc.), operator panel physical buttons, motorized disco balls.  Then you can figure out how to use the Mesa boards to accomplish what you want.

Got it, thank you, that's really helpful. I will get a 7I96S + 7i85 and see if I can figure out how to get all that set up. Do you have any recommendations on figuring out closed-loop drive using linear encoders and stepper motors?

SLOW DOWN!

First - there's a 7I85 and a 7i85S.  The difference is:

• 7i85
  • 4 encoder inputs
  • 5 serial expansion ports
• 7i85S
  • 4 step & direction outputs (more axes)
  • 4 encoder inputs
  • 1 serial expansion port

In my previous example, I was referring to the "S" version, but I think I left that off the first line of my explanation.  Before slamming the "TAKE MY MONEY" button, figure out what you need.

• Does the 7i96S and one of the two 7i85's have all the IO you need?
• What's your spindle?
• How many IO points do you have (or want)?
• What sensors do you have?  PNP? NPN? Switches?  What voltage?
• Do you want a tool changer?
• Do you plan on a couple - or a bunch - of physical control buttons for the operator?
• How much room do you have in your electrical cabinet?

You can usually expand Mesa boards rather extensively, but its better to plan out and make sure you have what you need.

The 7i96S + 7i85S is a great combo... it's what I'm using in my current lathe project.  But I had to add more IO (lots) and now I'm out of serial expansion ports.  No regrets, but I'm not expanding any further without swapping out a board.

Second - there are numerous threads in this forum about closing the loop back to LCNC with steppers.  The problem is searching the forum is a bit difficult... but get used to it.  Read as much as you can.  Maybe search for "loop" and limit it to search titles only.

One of the users here (Hakan) posted a number of nice videos about closing the loop with steppers (and later servos) on you tube.  Search YT for some good overviews.

More importantly... dealing with closed loop is WAY down the road you're about to travel.  The LCNC learning curve is steep, especially if you aren't a 'computer person' (like me).  There are multiple concepts you'll need to become familiar with to get an open loop system sorted out.  Once it's happy, then you can close the loop and sort out the tuning.

Read the LCNC online manual.
Read about HAL and INI files.
Read about user interfaces (GUI's)
Search YT for "Feral Engineer" and watch EVERY linuxcnc video he published.

Come back when your brain hurts.

tommylight wrote:

 You do not need closed loop drives, you need the simple stepper drives (even crappy china ones will do) and simple stepper motors.
 

Thank you Tommy.

Regarding closed-loop drives... I sort-of disagree with you here.  Crappy open loop drives will be ok with linear scales, but I've found that closed-loop drive & motor combos are so cheap these days they're worth the slight premium.

Given enough voltage, decent closed loop drives can drive stepper motors way faster than the open loop ones before stalling, and they tend to have better circuitry (i.e. faster optos & differential step/dir inputs) inside.  Some have 24v capable IO - better than 5v for noise resistance.

I'd leave the position error fault at a fairly high setting inside the drive and let LCNC close the position loop tighter - no fighting. But you get fast, quiet drives for a very slight cost increase over garbage components.

Stepperonline is selling the Y-series closed loop drives - 8A, 110vdc(!) - for $35 right now. And the 'upgraded' T-series are about $60-$75.

On the other hand, they're also selling 1kw AC servos - rebranded Leadshines - for about $300 shipped. No more steppers for me...