Re: [CAD_CAM_EDM_DRO] Re: stepper vs. servo

turbulatordude wrote:

>
>But that opens the question of how others can get 256 microsteps or
>more. And THAT really is a more general motion control questions as
>even though we tend to think our interests are 98% of the universe,
>CNC machining applications of motion control are probably more in the
>2% range. I'll bet Hewlett Packard (one supplier) sells more Laser
>Printers in one year using motion control than all the home brew CNC
>stuff ever built.
>
>

If you look at the deflection vs. torque curves for stepper motors, it
becomes
obvious that for most motors, the friction in the motors OWN bearings, only,
is enough that you can't move the rotor in increments smaller than about
1/10th of a full step. Any moves smaller than this don't produce any motion
until you have accumulated some deflection between commanded and
actual position, then the motor jumps. Thinking that a steper motor can
actually
move in 200 * 256= 51200 microsteps/rev is ludicrous. That comes out to 25
arc seconds, I think.

Some of the problem in this stepper/servo debate is caused by the makers of
microstepping drives who make drivers that do microstepping down to
thousandths of steps, without declaring that no motor can be expected to
follow such fine steps. There is some maker that has a drive that
divides down
to 144,000 steps/rev, or is it 144,000 microsteps per full step? Either
way,
there is no motor that can take advantage of such fine microstepping, and
anyone who thinks their machine can be commanded to move in nanometer
increments because of this fine microstepping is, I'm sorry to have to
say this,
a fool. Everything is flexible, of course, and the torsional spring
rate and
compressibility of the leadscrew would make this impossible, even IF the
motor could actually do it. I get a bit fed up with people saying their
steppers
are more accurate than any servo system, because they have microsteps equal
to microinches. But, of course, they have NOT measured the performance, and
in fact they may have .010" of backlash, .002" of cyclical leadscrew error,
.003"/Foot of leadscrew pitch error, .001" of static friction, and a
table that
is .025"/foot out of square with the supposedly orthogonal axis! And, they
have absolutely NO IDEA that any of these huge errors are present in their
machine.

I KNOW what these errors in my machine are, because I have measured
them with the best equipment I have available (no Renishaw ball-bar
testers or
laser interferometers, just gauge blocks, precision squares, dial test
indicators,
etc.)

Jon