If you are using step/dir to drive your AC servos, then what are the linear scales for?  The control loop should be closed in the servo drive and not need any scale feedback for a traditional PID control loop. 

If you are wanting backlash compensation with dual feedback, then you really can't do that with digital (step/dir) control on the servo drive, even on a Galil.  It would need to be analog control with the control loop closed on the controller.  The Galil can close the loop with dual position feedback using analog control, but it may require special firmware. 

Some servo drives allow for dual feedback but I do not remember any model or brands right off the bat.  Just that they do exist.  If your servo drives will not allow this, then we are talking about getting new servo drives instead of a new motion controller. 

Steve

Hi,
controllers that close a feedback loop with linear encoders are a fairly rare breed and going the way of the dinosuar.

There are three such controllers that have a Mach4 plugin, Gallil, Hicon Integra by Vital Systems, and CSMIO/A by CSLabs.
CSLabs had an excellent reputation for build quality and support with Mach3 but their Mach4 support is patchy/lacking. Given the
price I find it hard to recommend.

The reason feedback capable controllers are fading away is that servos/servodrives are ever smarter and the servodrive alone closes
the loop.

If you argue that an AC servo cannot enclose a linear scale in a feedback loop only its own encoder....you might have been right a while ago.
Current AC servos from all the leading manfacturers have 'load sensing' in addition to the normal rotary encoder.

For example the Delta A2 series servos. The servos are fitted with 20 bit incremental encoders but can in addition be fed from an external
position sensor, eg a linear scale attached to the load (axis). The previous Delta model, the B2 series has a 17 bit incremental encoder and
did not have load sensing, I bought 3 750W B2 series servos recently....they are fantastic. I did not realise at the time that I could have had
the additional load sensing capability for an extra $50 each. The 750W B2's cost me $500USD each whereas the A2 series would cost about $550USD
each.

If you want to include linear scales in your feedback loop consider load sensing servos. You would use the ESS as you do now but the servo would close
not only on the rotary encoder but the linear scale as well....the best of all possible worlds!

Craig

because there are serious limitations to the accuracy that can be achieved using leadscrews even with ground precision types like I have

I've already got SureServo AC Servos, so it's not economic to change those

Hi,
Mach4 is and cannot be a feedback controller, the PC on which Mach runs IS NOT NOR CAN EVER BE REALTIME.
Its not because no one can be bothered or lack of imagination or anything else, but a limitation of PC hardware
or rather Windows operating software. - I never said that is did...

If you want to stick with Mach4 and use linear scales then you must either use a feedback capable motion
controller, that is Gallil, Hicon Integra or maybe CSMIO/A......OR use load sensing servos like the Delta's.

Even the cheapest feedback motion controller (CSMIO/A) is 600 Euro....so they are not cheap. The CSMIO/A - 600 Euro is fine if it works.
must operate analogue servos, it cant mix and match step/direction and analogue, its one or the other.
Who wants to spend money on old school analogue servos? They are so last century, even the cheapest crappiest - nobody, I have AC servos...

Quote

because there are serious limitations to the accuracy that can be achieved using leadscrews even with ground precision types like I have

I don't know what you are using but my existing C5 20mm dia. 5mm pitch ballscrews suffer NO backlash whatever.
The ballscrews I have bought for my new build, also C5, suffer from no backlash whatever. They are guaranteed within 18um - My friend, your claims about sub-micron accuracy are utter nonsense. I've worked with high precision CNC machines for years, and it's near to impossible to get that sort of accuracy even if you spend hundreds of thousands on machines!
EVERY system ever created has lost motion, and 18microns is NOT 'no backlash whatsoever'! Don't imagine that you get the same amount of lost motion each time you reverse the direction either. It depends on the rate of the reversal and the stiction and load at the time. You simply can't say that you have 18microns of lost motion and then dial in 18microns of backlash compensation and claim sub-micron accuracy. You simply don't know what you're talking about.
If I put my Ball Bar measuring system on your machine, you'd see what you've really got, and it isn't sub-micron!

Quote

I've already got SureServo AC Servos, so it's not economic to change those

How many axes require linear scales? If you have two axes then two load sensing AC servos like the A2 series Delta cost I have three axes that have linear scales, I want to use them in X,Y & Z. I plan to build a 4-th axis, so I need a four axes system even though the 4th axis won't have have one.
$500 each. A Galil controller will cost over $2000, and a Hicon Integra with servo activations $1200.
By my calculation it may be cheaper to sell your existing servos, which no doubt would attract significant buyer interest
and use those funds toward two of the A2 Delta's.

Craig