I have used both and personally prefer a single torque loop. My lathe
runs a 7i80 in torque mode. The router uses velocity mode analog Bosch
drives with tachos. The mill runs Yaskawa drives that were configured in
velocity but now I run in torque mode.
It is easier to tune the outer loop in velocity mode however you then
also need to tune the inner loop (if that isn't already configured in
the drive). Additionally with velocity mode using analog control you
nearly always have dither. The machine will hunt between encoder counts.
With a single torque loop you can configure one encoder count of
deadband. Once the axis reaches position the motor just stops.
On the mill I originally used velocity mode as that is what it used
before the retrofit. However the dither drove me mad. In the end I
switched to torque mode and achieved far lower following errors at speed
combined with zero dither.
Velocity mode used to be pretty much essential on older controls as they
did not have a very fast servo loop. LinuxCNC's 1kHz loop is good enough
to run torque mode for nearly any normal machine.
Les
On 25/11/2020 13:48, Todd Zuercher wrote:
I think I'll disagree with Andy a bit on this. While the minimum following
error I could achieve with a single torque vs dual loops in Linuxcnc was about
the same. The tuning was much easier, and I believe that my drives are running
much cooler (not working as hard dithering) using the dual loops. It is a bit
like comparing balancing a pencil on end on your finger vs a broom handle, the
longer one is much easier to balance.
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