RE: [CAD_CAM_EDM_DRO] Losing steps...

If your steppers are losing steps then it is because you are instructing the
motors to do more than what they are rated. I am talking about torque here
of course. Typical design practise with steppers is to oversize the motor
just for this reason. This is an advantage of servo's is they are an
inherent closed loop system (vs stepper which is an open loop). An
instrument I finished working on that had steppers we used some encoders to
know when we lost steps and to correct.

I am not sure what the 1:200 means. Although a stepper motor without gearing
without microstepping will have about 200 steps per shaft revolution. Which
means you have to gear it up (shaft must make large number of revolutions to
move the tool), keep in mind that the stepper motor torque curve and how
fast it is has to move.

Joe

-----Original Message-----
From: Patrick [mailto:alleven@...]
Sent: Monday, July 12, 2004 8:28 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] <sigh> Losing steps...

I've recently discovered my machine is losing steps, as steppers do, and so
I'm trying to figure out if some (small) amount of missed steps is "normal."
The resolution on my machine is quite small (.000098"), and I'm wondering if
it's just too much to ask to expect it not to lose any steps at all. I can
usually re-home the machine after a fairly long program and get less than
.0003" "homing error", (measured with the homing switches, which are
purported to be repeatable to .000039" when new). I'm really not getting
wrapped around the axle with meaninglessly small numbers, but I would like
to know if a properly working stepper system can be expected to miss no
steps? At what point do I declare a problem? 3 missed steps or 10 missed
steps?

Also, I was reading Mariss' white papers on stepper vs. servo, and I need
some clarification on one of the disadvantages of stepper
systems: 3) Low accuracy. 1:200 at full load, 1:2000 at light loads. Can
someone explain exactly what the above statement means.

TIA,

MP





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