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
Peter Wallace
Mesa Electronics
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