Re: [CAD_CAM_EDM_DRO] EMC2 Servo Loop

Phil@Yahoo wrote:

> This is a question mainly for Jon Elson, being the EMC2 guru-
>
> You mentioned in a recent post that EMC2 can run the servo loop in the PC,
> which I presume means the PC is reading the encoder outputs. My question is:
> how can the PC read the encoder fast enough? I know there are some fairly
> significant delays in the parallel port speed, at least on output. Maybe
> that's not an issue on input? Also, where do you get enough inputs?
>
>

The PC CAN read the encoder inputs fairly quickly, and in some
applications, this works
fine. I make hardware that has encoder counters that update at 1 MHz or
better, so that
you are sure not to miss any counts. I have motion interfaces that
control analog (+/-10 V)
servo amps, PWM (digital)-input servo amps and step/direction input
drives. All of these
can read encoder position and report it to the computer. These boards
also solve the
limited number of pins problem. For instance, the Universal Stepper
Controller and Universal
PWM Controller boards have 16 digital inputs (one dedicated to E-stop),
plus 8 Solid State
Relays for output, plus the encoder inputs (with index pulse handling),
motion command
outputs, and an extra output usually used for spindle speed control.
This is all performed
through ONE parallel port connection. The IEEE-1284 protocol (usually
called EPP mode)
allows you to address up to 256 device registers through the parallel
port. Input and output
functions are about 600-800 ns per byte transferred in EPP mode. This
makes it possible
to easily control 4- or 8-axes with a 1 or 2 KHz servo update cycle.

The analog servo interface is a set of boards that plug into a
motherboard, so it is easy to
add additional boards as needed to get as much I/O as your particular
machine needs.

The USC and UPC boards mentioned above put all their functions on one
board, but they
can be "daisy-chained" on the same parallel port to add capacity.

Without hardware assist, you would want to limit encoder counting to
about 40 KHz, and you
always want to keep a conservative reserve, otherwise there will be a
loss of position and
velocity information if the machine aver moves too fast, causing a servo
runaway.

Jon