Can Mesa do dual feedback?

Hi, I am just wondering if a MESA board with LinuxCNC can effectively do what the Galil boards do which feature - "Dual-loop
The dual-loop (DV) feature enables the controller to compensate for mechanical backlash. Typically, dual-loop systems use a rotary encoder on the motor and a linear encoder on the load (Galil controllers accept inputs from two encoders per axis as a standard feature). Dual-loop control changes the standard PID control and closes the position loop with the load encoder (“PI”) and derives the damping terms (“D”) from the motor encoder. This method provides smooth and accurate control along the motion path regardless of backlash.". This is commonly referred to as something else? My searches didn't yield a lot.

I am considering CNC'ing my bench top mill (RF-45 clone) and I am just trying to price options before I dive into purchasing and much more research / reading. Ideally I would like to CNC it in this manner with hybrid steppers (good price point). Is this possible?

The backlash compensation seems like a smart move compared to very expensive ball screws.

If anybody could also point me in the direction of a suitable board or setup that would be much appreciated.

Thanks.

I don't actually know how its done, but if you've added a DRO to your mill, you can interface their glass scales into LinuxCNC and the are unaffected by backlash. I've wondered about CNCing my mill or lathe this way.

Isn't this normally done with the velocity loop closed within the motor drives, and the position loop closed in LinuxCNC? This is the way MAHO did it with Indramats, and the way you normally see it done in LinuxCNC.

This is the block diagram from the Indramat 3TRM2 manual:
.
Mark

It's been done:
wiki.linuxcnc.org/cgi-bin/wiki.pl?Combin..._Devices_On_One_Axis

This was done on a very large mill. (I didn't do it though)
Incidentally this is done regardless of what cards are used.

When using glass scales on the mill as another form of feedback, I have read that the servos should be in velocity set mode rather than step dir mode for this process. Can all the MESA boards do this, or do I want those that can only do analog? Bit confused as to analog vs digital.

Also, can anyone recommend a motion control basics document/book/web page series to learn about it? I don't even really know what step direction is, let alone the myriad of terms used when trying to work out which motion controller to buy from the linuxcnc site. Like, this description - store.mesanet.com/index.php?route=produc...oduct&product_id=290
Do I want an ethernet interface board? I read parallel was dinosaur technology but it seems a lot of mesa board setups in the plug and play sets use parallel.
Thanks.

All of the Mesa fpga cards can do what ever form of command that you need. But the form of command signal you choose to use will determine the daughter card that you need to use, because most of them are specific to what ever command interface you need to use.

Step/dir, is one form of position command interface. It is a digital step signal and a direction signal that tells the drive to move the motor one "step" in either direction. It is commonly used with step-motors. There are also other forms of position commands such as CW/CCW, quadrature, and serial command options.

It is also possible to treat a step/dir interface more like a velocity command by sending velocity commands to the step generator instead of position commands, and it doesn't make much difference to Linuxcnc which way it is done. In fact with Mesa hardware that is normally how it is done.

OK so my goals are:
- 3 axis now, 5 axis much later on
- Servo used as a spindle (1.8kw)
- Glass scales used as feedback later in the year
- Mapping of ballscrew since I'm going for C7
- Intro to industry standard software

I am thinking:
6I25+7I76
5I25+7I76
Depending on the computer I obtain, PCI or PCI-E.

Should I perhaps go for the 7i76e or ethernet solutions? I guess this is my other big question. Skyfire used it, and they generally make good component choices. My goal is to get a Tormach / Skyfire quality machine going eventually.

Pretty much ready to order I think. The 1.8kw servo spindle motor/driver got here today, it sure is pretty.
Also if anyone knows of a good book that starts with "hello world" in motion controller electronics and progress up to understanding all the communication going on that would be great. PID maths / block diagrams and stuff is ok, I can find stuff on that and I've done it long ago. It's actually going to be a good opportunity to review it all. The electronic side gets me. Thanks for all the advice.
These are my x,y,z servos if it makes a difference - tinyurl.com/ycw8xsym

Oh, one other requirement / aim - I intend to add a tool changer.