Re: [CAD_CAM_EDM_DRO] Re: Relation of a DRO to a CNC modification of a machine?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, erie wrote:
> There you go again Tom, just throw reality in the mix....;)
>
> erie
>
> (ducking)

What's all this fuss about animals (deers and ducks). Hey THIS IS A
FORUM ABOUT CNC! (geez)

Now for the layman's explaination of the last paragraph:

Used to be that there was a large difference in price between servo
and stepper systems because of the more advanced electronics you
needed to build a servo system that didn't shake like a cold dog.
The term "delta" in math is the same as "difference" or the spread
between to two numbers. In today's hobby CNC market you have low
cost solutions to the servo "delta" issue. While some argue that
it's not TRUE servo since the software does not know about the error,
you can take a Gecko 320/340 and hook it to a source of DC and to a
brushed DC motor with a common two phase(quadrature) , 5V rotary
encoder and the Gecko will keep track of any error between the
commanded postion (sent from the PC as step and direction pulses) and
the sensed position by the encoder and adjust the psotion
accordingly. You can use the same low cost controller software you
did with your steppers.

For that level of improved accuracy you will pay a little more for
the motor controller (maybe not) and most likely more for the
servo/encoder but it won't be a huge delta. Here are a few facts:

Servo motors develop their torque across a wider range of RPM than a
stepper. The torque does not tend to drop off with RPM.

It's easier to spin a servo at 1500 RPM than a stepper.

You can usually get by with smaller servo motors than a stepper for
the same application because you don't have to derate the motor's
torque when you are running at normal cutting speeds. In additon the
higher RPM of the servo lends itself to belt reduction thereby
trading speed for torque.

The servo motor will run smoother since there are no distinct steps.
The new micro stepping drives have done a lot to really smooth out
the stepper motors but you can get some interesting sounds from a
stepper system as it changes speed and load.

At this point I have all of the stepper guys screaming and breaking
glass. I know, I know! You can get stepper motors cheaper and for
smaller one's, there are really low cost drivers available. Steppers
can be used to build some really good equipment. I have several
machines with steppers. Some of my best friends use steppers.

Tom C
too dumb to duck (or deer).

Thank you for a very articulate explanation of the last paragraph. I'm aware of the differences between Steppers and Servos as far as performance. I struggled to find suitable drives for some big unipolar skip-o-matic Slo-syns for my Bridgeport and it was a dead-end everywhere I turned and experimenting can get expensive. So, I 86'd them and went with big servos instead. Does Mach2 perform the way you outline above with a Gecko 320? Someone mentioned that they thought Geckos monitor the return from the encoder and will fault after a 128 step discrepancy. Will they actually correct themselves? I know in playing with the motor tuning in Mach2 I've adjusted the acceleration and velocity way high and the Z axis driving the knee will start and stall at like 80 ipm. To my knowledge the Gecko didnt fault. Unless it did and it reset itself as I was resetting the acceleration and velocity. I attributed the stall to the10 or 12 amp Power supply not keeping up with the 30 amp stall torque Servo
(40 in-lb)

Ron

Ron





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