On Sunday 05 November 2017 15:16:09 Peter C. Wallace wrote:
> On Sun, 5 Nov 2017, Gene Heskett wrote:
> > It makes sense to me that for a velocity servo, FF0 should be
> > brought up to a zero or slightly negative error, with the spindle
> > running just a few percent faster than the commanded setpoint. This
> > might be close to 1.0 in high gear, but close to 2.0 when the
> > backgear is engaged. Then Pgain brought up from zero until it
> > responds quickly to a load disturbance, holding the error as close
> > to the setpoint as it can and still stabilize. Then the mass of the
> > chuck on the mill is far less important than the mass of the motors
> > spinning armature, which in backgear, if if follows the E=mv2 law,
> > is at least 4x what it is in high gear since its turning 2x faster
> > in low for the same spindle rpms.
> >
> > The idea is that setpoints won't change during a given operation,
> > other than the reversals for G33.1's, but loads can change, a lot.
> > And its loads that need the compensation of lots of Pgain. This is
> > made difficult by the differences in load we see when the tap
> > actually starts cutting, and is IMO much more important. I had an
> > Igain windup so bad that when I finally broke it loose so it would
> > reverse back out of the hole, it over sped z and threw a following
> > error. With the smaller, much lower inductance motor on the mills Z,
> > it can now move at 70 ipm upwards.
>
> If you have a velocity mode drive then FF0 and little bit of I should
> be sufficient, but if you have a voltage mode drive (PWM or SCR) then
> you will need P and I
Its one of Jon's pwm-servo amps. The 20 amp 160 volt rated version, so
yes, the pid output gets scaled and limited to stay within the 98% max
duty cycle range of its input, using a PDM mode signal. Control
bandwidth is more than just fast enough. The PDM is generated by a 5i25.
My code reboots it every time the motor is started or reversed.
Does that change things?
Thanks Peter.
> Peter Wallace
> Mesa Electronics
>
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Cheers, Gene Heskett
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