On 07/23/2020 05:09 AM, andy pugh wrote:
On Thu, 23 Jul 2020 at 09:22, John Dammeyer <jo...@autoartisans.com> wrote:
Not ignoring you at all. Just waiting for an idea for the math that leads to
calculating MAX_ACCELERATION in the ini file given the parameters I've
mentioned before.
I think that the problem is treating the motor as an ideal torque
source. It has inertia, it has inductance, it has a non-linear
current-torque graph and it has back-emf.
While the inductance is visible at PWM frequencies, for the
size motors we are dealing with, it really has little effect
at the frequencies and bandwidth involved in CNC motion control.
Rotational inertia, however, is a BIG issue. The leadscrew
can be thought of as a torsion spring, the length of the
active spring changes as the ball nut progresses down the
screw, and the motor angular momentum turns it all into a
spring and mass system. I have this issue on my Bridgeport,
the leadscrews are too small diameter for such a massive
system, and I get a resonance that varies between 20 - 40 Hz.
Applying a voltage to a motor leads to reduced current as
the motor speed increases, due to the back EMF generated.
That's why it would be good to put a resistance in the
circuit to get more constant current if trying to test
practical acceleration limits. Motor torque will be quite
linear with respect to current over a wide range of currents
within the motor's rating envelope.
Your extensive calculations are appreciated, but without
knowing a few key parameters it can be hard to get started.
I have the gear here to measure inductance, but not
everybody does. I guess with an encoder connected to a
motor, and a constant-current source, rotor inertia could be
obtained.
Jon
_______________________________________________
Emc-users mailing list
Emc-users@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/emc-users
