Why FF1 has no effect in LinuxCNC v2.7.15 ?

Hello,

i make retrofit many machines with analog servos in the past with linuxcnc v2.7.15.
And was not clear why FF1 has no action when u tune the PID. I remember, in the past with other versions of linuxcnc i can use FF1, and it is radically lower the following error in the change of direction of motor, and i really like it.

Now i tested it a little.
I made a new 3axis config with pncconf with v2.7.14 for mesa 5i25 + 7i77. Connect a sanyo denki servo with +/-10V velocity mode and try to tune the PID. In the scope i see the effect of FF1.

But, when i start the same config with v2.7.15, the FF1 has absolutly no effect.

I use this ISO:http://www.linuxcnc.org/iso/linuxcnc-2.7.14-wheezy.iso
And the newer (2.7.15) version is compiled from source.

Can someone help me why FF1 not works in 2.7.15?
And what is the solution?
Has anyone encountered this problem yet? 2.7.15 is released in January this year.



Thanks.

I attached a screenshot of difference between PID component from 2.7.14 and 2.7.15 versions. This is the only difference.
I compiled 2.7.15 with pid.c component from 2.7.14 and now i have working FF1 parameter in pid when i running LinuxCNC 2.7.15.
Please check this problem, maybe it woluld be good to solve in newer versions.

Could you share your hal file? There was a longstanding bug found in the PID that was fixed about then. But fixing the bug had the result that connecting things that shouldn't be in the hal file, can cause the results you describe.

Do you have anything connected to the PIDs pins "pid.N.command-deriv"

I answer with phone, therefore cant attach the hal file now. I am beside machine. Tonight i can attach the file.

But the pid.N.command deriv not connected with other signal.

Here is the line from hal.

net x-vel-cmd pid.x.command-deriv

But x-vel-cmd signal has no source.
What to do to use FF1?

comment out or delete:
net x-vel-cmd pid.x.command-deriv
and FF1 will work

The PID comp can only detect if its pid.N.command-deriv is connected to a signal. It can't tell if that signal is receiving anything. By having it connected to an unused signal it disables the FF1. Like Peter said, comment out or delete that line and FF1 function will be restored by calculating the velocity command internally. Alternatively connect that signal to a valid velocity command, But there may be some issues with using the one from motion.

Thanks for quick answers. I deleted a net x-vel-cmd pid.x.command-deriv line and FF1 works.