Re: [CAD_CAM_EDM_DRO] Re: servo board

mikezcnc wrote:

>If you look at iicosystems site then you will notice that he is
>showing a schematic how to connect a board from the other website you
>have mentioned...
>
>However, $800 for a servo? I see no point of spending that much when
>you can get a Gecko for less. I would say that if the ISA servo
>controller guy were smart he would drop his price to $400 and that
>would put Gecko next day in receivership...
>gecko costs around $150 per axis.
>Oh, another thing, if you have to have more than 3 axis than it is a
>different calculation.
>
>
>

I'm not sure what you are comparing, here. Each system has advantages
and features that others don't. The Pico Systems PPMC card set is an
expandable, flexible, buy-only-what-you-need interface for analog
servo systems. There's no need for 16-bit DACs in a hobby machine.
But, note, the $650 price for the standard 4-axis package does NOT
include the servo amps! It is only the INTERFACE to the servo amps.

ISA slots are pretty scarce in new computers, that's why I went with the
parallel port.

One advantage that you don't get when hooking Gecko or other step/direction
drives to the parallel port is actual position feedback to the computer.
The computer sends step pulses to the Gecko drive, but has no way to
know the steps have really been executed, or when. With the PPMC
interface, the computer can keep a continuous watch on position, and
detect whenever the actual position is too far away. A sliding scale
based on velocity could be used, for instance. Also, for machines with
handwheels, you can switch back and forth between CNC and manual
movement, with the CNC program acting as a DRO when in manual.

One other difference is smotthness of motion. With a step/direction
drive, the steps are the finest granularity of movement, and below some
rate, the drive will move in discrete steps, stopping in between.

With analog servo amps operating as a velocity servo with a tachometer,
the amp will continue giving smooth motion to incredibly slow velocity.
My own servo system on a Bridgeport conversion can go to .01 IPM
before stick-slip friction takes over. This is 1 inch every 100 minutes!
At this rate, even with high-resolution encoders (20,000 counts/inch)
there is only 3 encoder counts per second going by. A gecko drive would
move in very discrete steps. My drives are still moving smoothly.
This can make a difference in fine contouring work, boring with a large
radius, etc.

Jon