Re: [CAD_CAM_EDM_DRO] EMC2 Servo Loop

Phil@Yahoo wrote:

> Yes, I get the idea. I just wonder what the main advantage is of having the
> servo loop run through the PC if not to reduce the external hardware. If you
> have to add external encoder counters it doesn't seem the cost would be much
> different from having a self-contained servo controller that simply takes
> step-direction commands and reports errors, etc. Cheap microcontrollers can
> easily handle that computing load.
>

The step/dir driver can only report errors that it sees. There are two
directions that information flows in a control loop. In the case of
step/dir drives, the computer outputs a set of step pulses, but has no
indication from the drive that those pulses have been received. You can
disconnect the step line from the computer, and it will still report
that the machine is moving as directed. The drive won't fault because
it doesn't see any step commands, so it just keeps the motor where it is.

If you add a cheap microcontroller or FPGA card, you either have to put
the motion controller into the card (which means it has to know about
everything you want to do: electronic gearing, threading, jogging, feed
override, etc.), or you need a way of monitoring the actual motion vs.
commanded motion. This is necessary in the simplest case just because
you might be a step off on every command due to timing jitter in the PC.

- Steve