Re: step/dir inputs to servo controllers

> I am retired and have been slowly improving my home machine shop.
> I am thinking I may buy a set of Camtronics equipment (using Gecko
> drivers) for CNC but have been unclear as to going for "steppers"
> or "servos".
>
> bull2003winkle's information on the way servos are contolled using
> Gecko G320 and G340 drives is welcome.

I am retired also, congratulations.

> But now may I ask questions for whoever wishes to point me down the
> right path. I want to get good finishes and work to .0002 to .0005
> in boring small engine cylinders using a lathe.
>
> I have been thinking that servos (with their necessary encoders)
> would do the job better because the encoder data could be processed
> and produce DRO data. I felt that having the realtime DRO data would
> help me in keeping the process of boring under better control by
> stoping the machine between passes measuring the cylinder and putting
> in new values of Y axis data as necessary. This is sort of manually
> controlling a CNC machine to get the most possible out of it in
> accuracy.
>
> So here are some questions:
>
> # 1 Is there DRO like information available with steppers as well
> as servos?

No, steppers are driven by step pulses and the driver assumes they
follow the step commands. If they are overloaded or run too fast, step
loss can occur and the drive will either lose track or shut down.
Stepper systems are usually open loop, meaning there is no absolute
position feedback to the controller. Buy the way, many servo system
are open loop also. The only way to run a closed loop system is to
have a separate position sensing system to tell the controller where
the tool is.

> #2 If so what is the source and how is it displayed if it is
> displayed? Is the answer different with different software packages?

Some software can keep track of step pulse commands and display where
it "thinks" the tool is.

> #3 Since the DC motors have brushes is their life a lot shorter
> than steppers? Also don't the brushes make a lot of electrical
> noise that must be suppressed and filtered?

Not that much. Motor brushes last a long time and with good motors,
replacing them is very simple. There is less RFI (electrical noise)
with brush DC motors than with most servo controls. I get about 5
years life from DC brushes in 8 hours per day use.

Not all DC motors have brushes. There are brushless DC motors
available but that's a whole different story

> #4 If DC servos are "better" than steppers would it not be even
> better to use brushless servos and avoid the sparks of brushed
> motors? Does Gecko have a version that drives brushless DC motors?

Brushless DC motors are a very different class of motor. They differ
from brush commutated motors in how they are energized. With a brush
motor the commutator controls the communcacton. With a brushless motor
that is done electronically by sensing the rotor position and changing
the field current in the windings. None of the machine control drivers
that I am aware of can do this, including the Gecko's

The argument over steppers vs servos is as much a matter of belief
than fact. Servos or hydraulics are required for very large machines
that have to move very high masses fast over large distances. For the
size machines we usually deal with at home of a small production shop
servos will work just fine.

The problem of losing track of position is real but should never
happen if the steppers are sized correctly and are not driven beyond
their speed range.

A well designed stepper system can position to an accuracy much better
than the machine. The stepper will khow where it is to about 0.0001 or
better, most machines can't do that.

Regards;

Ken