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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