> 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.
Yes inductance has little effect but not zero. > Rotational inertia, however, is a BIG issue. ... Agree inertia is often a problem but it depends on gear ratio, then gear ratio become high rotational inertia in motor have big impact. > 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. Agree this is a problem in your machine but do not think this is a big issure in my machine. Maybe you get better accurcay in one end than the other. > 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. With more resistance motor speed will be reduce more with higher load. Some motors may have other effects in particular an ordinary asynchronous induction motor then driven by a certain frequency and voltage. > 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. I spent quite much time on it so maybe it's a little bit hard, otherwise there are standard methods to tune PID controllers, Ziegler-Nichols method come to my mind. _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
