27 Jul
2015
27 Jul
'15
10:58 p.m.
On Sun, Jul 26, 2015 at 10:03:32PM -0400, Gene Heskett wrote:
High linear or angular speed does not by itself imply high bandwidth.
The required control loop BW (and hence acceptable latency) depends
on the highest frequency that the motion as a function of time is
supposed to contain, not on its amplitude or speed.
Anyway, isn't the whole point of stepper motors that they can be
used without feedback - that every step corresponds to a defined
motion ? Of course you need to plan the trajectory in terms of
acceleration and speed so that the combination of motor and load
stays within its limits and no steps are skipped. But you need
to do that anyway. And if any steps are missed you're in trouble
even with feedback.
If for any reason (e.g. precision) you do need feedback then DC
motors are a lot easier to control, and any advantage a stepper
would have seems to be lost.
On Sunday 26 July 2015 16:33:37 Fons Adriaensen wrote:
And anyway,
what is the BW of any heavy piece of machinery ?
Probably at most 10 Hz or so, and that you can control using
a feedabck loop even with 5 or 10 ms of latency.
That depends on how fast its moving, Fons. A 25k rpm spindle can move
and cut alu at 250 IPM if the coolant can carry away the chips. That
sort of motion is tracked and corrected in a 25 to 100 microsecond loop.
But we don't do floating point in that loop for obvious reasons. Fellow in Cinci picked up one of the biggest mills
Cinci ever made about
4 years back, some 220,000 lbs of scrap because the controller had died,
and so had the guy that designed it. So it got a retrofit to linuxcnc.
While tuning the servos, it got the Z axis into an oscillation at about
20 hz, with about a 1/2" motion either way. It took about 20 seconds to
lower the P in the PID module for that axis & stop it. The moving part
of the Z axis weighs 22 tons and carries a 50 horse motor. That 20
seconds was recorded on the seizmograph stuff across town at the UNI.
For a machine of that size I would *never* let a PC connected via a USB
cable control the motors directly. Just for safety's sake... This is a
job for an embedded controller running somewhat more reliable system
software and interfaces, and designed to be fail-safe.
I once worked on a cutter with a beam that could accelerate at 5g.
That means it could move a meter from stop to stop in 0.3 seconds.
It was not a really heavy machine and it didn't look dangerous.
But it could hit you without warning in a very nasty way...
Ciao,
--
FA
A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)