> -----Original Message----- > From: Jon Elson [mailto:el...@pico-systems.com] > Sent: July-23-20 9:00 AM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Calculating table acceleration. Was: Need help with > Bostomatic BD18-2 to linuxcnc > > 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
In my case the Motor inertia is 3.4kg/cm^2. And I agree. I still even have alignment issues that result in different sounds depending on where the lead screw and table are. And my acme screw is 1"diameter. (25.4mm) However, I suspect that the energy required to spin the pulleys and lead screw (1000 RPM for the lead screw with 3:1 on the motor) is a small part of moving a 220 lb (100kg) table with a motor rated at 226 oz.in (x3 with reduction = 678 oz-in or 4.8Nm on lead screw). The HP_UHU servo has two comparators for current limit and two trim pots for adjustment. One sets high speed and the signal is filtered with a RC 820 Ohm and 470pF cap. The other sets low speed continuous stall current with 330K and 220uF. I imagine your servo drives have something similar. I guess theoretically you set the high speed for the Peak Current of 40A and the Continuous for 7.8A. Those two settings will probably have far more to do with the MAX_ACCELERATION value than lead screw or pulley inertia. But none of that changes the math I did that suggested the acceleration with this motor could be set at 100x of what actually works. And the magic number in the math is the assumption that the units are G. I have to leave this alone for a while. When I have some room and access with my 4 channel scope I'll take a look at the current limit signals relative to encoder pulses to see what's going on. As I said earlier, my little Lazarus program at the moment is junk because it makes an assumption on units of gravity that I don't think are valid. John Dammeyer > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
