Re: [CAD_CAM_EDM_DRO] bridgeport series 1 retrofit

Bob Japundza wrote:

>Hello All,
>
>I've been lurking on this list for sometime now, as I have been
>rebuilding my bridgeport series 1 cnc. I am getting close to the
>stage now that I need to start thinking about the controller. The
>bridgeport came without stepper motors and I got a great deal a
>while back on some new nema42 steppers (1125oz/in, $99.00ea). I am
>planning on using EMC with this setup (I was a unix admin in a
>former life) so I feel comfortable with working with emc being a
>newbie. I have a couple of questions I am hoping someone may answer:
>
>1. Stupid question: how can I determine if my motors are capable of
>microstepping? This will obviously determine my choice of drives
>between the G201 or the G210.
>
>

All stepper motors are capable of microstepping. Don't confuse microsteps
with actual resolution of movement, however. Just because a driver has
1000:1 microstepping doesn't mean there really are 200,000 positions the
motor can be placed in.

The difference between the G201 and the G210 is not the amount of
microstepping. Both drives are 10:1 microstepping drives all the time.
The G210 has an INPUT step multiplier, that makes each step from the
computer 1,2 5 or 10 steps into the x10 microstepping drive. (Note that
a G210 is actually a G201 with a step multiplier board plugged inside.)

The only reason to use a G210 is in cases where the computer cannot
produce step pulses fast enough for the speed you want to move at.
(There is another reason, if your computer produces a very ragged
train of step pulses, the G210 smoothes them out somewhat.)

>2. Would a 77vdc power supply with some large caps on the output
>sufficiently filter out ripple and not damage the Gecko's? I'm
>concerned that my power supply may be a wee bit too close to the 80v
>rating for the Gecko's.
>
>

77 is getting up there, 72 might be a better goal. But, unless your
power lines
jump up to higher voltages on occasion, you probably will be OK with that.
Is the 77 V measured at no load? Put the Gecko drives in there, and maybe a
bleeder resistor, and it will bring the voltage down somewhat.

>2. Is it possible to use EMC (or any other cnc package for the
>matter) to drive steppers with a DRO serving as a closed-loop
>system? Some of the DRO's I've looked at have a RS-232 output, and
>am wondering if there's a 'standard' for DRO outputs so they could
>be integrated in a closed-loop system. I'm thinking of building one
>of Scott Shumate's DRO-350's.
>
>

The standard DRO boxes update very slowly, 2-10 updates per second.
This is too slow for a CNC control to do closed-loop motion control.
EMC CAN use encoders to close the positioning loop, although the
usefulness of this with steppers is debatable. It will allow the CNC
control
to go to e-stop whenever a motor misses steps and stalls, so any further
damage to the part will be prevented. But, once the motor is losing
steps, the computer can't do much to help it "catch up".

There are several ways to get the information from an encoder into EMC.
There is the Dan Mauch DRO card, or the Pico Systems Universal
Stepper Controller, which contains encoder counters. EMC is able
to read position from these cards.

If you really want closed-loop operation, you should think about a real
servo system, though.

Jon