f-error

Is it normal to have more of a following error during a positive direction move to a negative direction move? Or should the following error be the same for each directional move? This is a servo position-velocity loop.

It depends on the mechanics
A non-counterbalanced vertical Z might have a higher error in one direction due to gravity

Otherwise I would not expect any difference unless there ware a motor/drive issue

Worm gear reduction tends to have a slight difference in forces from one direction to the other. High quality ones, it is barely noticeable, on cheap ones it is quite noticeable.
If you get that with ballscrews, misalignment or end bearings are due for a check.
Regards

Also I wanted to know the old system had a parameter for "in position servo loop gain" and a parameter for VFF velocity feed forward given as a percentage? Where would these values correspond in LCNC? Thanks!

I think VFF would be most analogous to FF1 in Linuxcnc. But there is not an equivalent to the "in position servo loop gain".

Like most things with LinuxCNC, features like "in position servo loop gain".
can be built in HAL. All of the PID comps tuning variables are HAL pins which
means they can be connected to hal signals with net commands and changed
dynamically.

so as a first approximation:

Xcmd and Xfb .--> near comp --> mux comp --> PID P term

edit:
(commanded velocity near 0 might be a better "in position" flag as it would not change during a move)
In fact if you wanted to get fancy you could feed the commanded velocity into the lincurve comp to
set the P (or D or I) value based on velocity

Often these sorts of PID gain scheduling are done to get good high speed tracking without excessive buzzyness when static

Most anything is possible if your smart and clever enough to figure it out yourself. But there still isn't a ready made equivalent suitable for the unwashed masses to use.

PCW wrote:
Often these sorts of PID gain scheduling are done to get good high speed tracking without excessive buzzyness when static

This is the issue I have currently, I have a lot of hunting/buzzing going on while static or in position/stand still. Peter always a great help, thanks as always!

Todd Zuercher wrote: Most anything is possible if your smart and clever enough to figure it out yourself. But there still isn't a ready made equivalent suitable for the unwashed masses to use.

Yeah, there's always a tension between providing specific ready made features or providing
the tools to build almost any feature

LinuxCNC and HAL definitely tilt in the toolkit direction

pmcstoneinc wrote: This is the issue I have currently, I have a lot of hunting/buzzing going on while static or in position/stand still. Peter always a great help, thanks as always!

"lincurve" was written mainly to allow you to vary PID parameters based on velocity or error.[strike] (this is why it has the -io output pins, as the gain pins in PID are (for no good reason) bidirectional pins. [/strike](Fixed in versions 2.6 onwards)

linuxcnc.org/docs/2.7/html/man/man9/lincurve.9.html