Hi,
there are quite a few questions in your post.
If I need to feed these new controllers with +/- 10 V surely I would still need something like the Galil card to supply that.
You are correct you would need a Gallil or something like it IF you persist in feeding the servo amps with an analogue signal.
The AC servos I'm recommending are step/direction, not analogue at all. You can thereby avoid the need for a feedback capable
controller.
So if the position loop is closed in the controller how does it know where to stop, if it is not sent a position signal.
That's what the linear scales are about. They not only drive the DROs but also supply the controller with the current actual
machine position and the controller will compare that to the commanded position and cause the motors to drive in a direction
and speed to match the commanded position exactly.
The AC servos I'm recommending don't need linear scales because it has a built in encoder. Thus if the position is commanded
to move 10mm to the right and the machine has a 5mm pitch ballscrew the ballscrew must rotate two times. If the encoder
has a 10,000 count per rev encoder (entry level these days) then the trajectory planner/motion controller will issue 20,000
pluses and the servo drive will follow that command and monitor the encoder so that the commanded position is achieved
exactly. In this instance its the servo drive which 'closes the loop' rather than the controller.
Modern servo drives do a very VERY
VERY good job of closing the loop, easily as good and in many cases better than
Gallil simply because the manufacturer is making a drive for his servo and he knows them both extremely well. Galil on the other
hand have to accommodate such a wide variety of motors and can't be great at all of them.
If I replace the motors and drivers, as you suggest, then is a PC based system the best way to control them?
This is a bit of a loaded question. I think it has less to do with whether you replace the servos or not but rather is 'PC the right
way to control a CNC machine?'. The shortest answer is NO. The best way is with a dedicated hardware controller like
a late model Fanuc or Seimens 840. These controllers are worth $20,000 plus. The question is whether you have the budget
to buy a dedicated controller or you can afford a much more humble PC.
A suitable but low powered PC for Mach4, an Ethernet SmoothStepper and a MB03 breakout board, and a Mac4Hobby
license would cost under $2000 new. That leaves quite a bit of budget for controller panels and pendants etc.
I run my mill on a Mini-ITX dual core Atom board without a graphics card, as the saying goes, 'it couldn't pull the
skin off a rice pudding' but it runs Mach fine. Its 32 bit by the way, Mach4 runs on either 32 or 64 bit without demur.
If you have budget for a dedicated controller like Fanuc or Siemens and a motion controller like Gallil then why mess about
Mach4 or in fact any PC based solution? That you are on the forum suggests that you don't have that sort of budget.
Mach4 is one of the Windows PC based software solutions that can achieve good, surprisingly good, performance relative
to dedicated hardware controllers worth ten and more times as much.
Craig