servo dac offset

For first time Install brushless servo drive on linuxcnc .... for star I have setting pid on linuxnc to 0,0,0 and on drive from factory. Drive have your one close loop and send feedback encoder signal to linuxcnc. I use mesa 7i77.

When run linuxcnc the encoder show a little running offsett ..... I try to eliminate it on drive whit drive function .... on pncconfig its seems ok .... but I see these offsett only on encoder running. Than I try to use dac offset .... it seems to work fine ... the running offset stop .... , I press ok button and finish the config. But in .ini and .hal file not see any reference about dac offset ....

These offset is very little (these axis is set as angular) but in 60ser become 1 degree .... I see it on mechanical device when running linuxcnc, when not running,

I try to insert it by hand but nothing happens and when launch linuxcnc I see the running offset again .....

I think the problem not is linuxcnc .... but not knows what I see ....

Small velocity offsets are normal in velocity mode servos and cause no harm.
If you exactly zero them, you will find that the drive will still drift at a different temperature...

If the offset is small (say less than .25% of full speed) They don't cause significant
errors once the feedback loop is closed. Because you can never really get 0 velocity
from a analog servo drive, its important that the drive enable is controlled by
linuxcnc so the drive is not enabled when linuxcnc is not running (and the position PID loop closed)

This is also important for safety reasons

ok It's right ... the drive must controll by linuxcnc ..... in my case from linuxcnc and drive there are safety plc (servo on, alarm, etc etc ... i/o from drive to Lcnc or vice versa) but now I'm in test mode .....

I think If unpluged Lcnc from drive and send the right servo on signal, Fw and ccw enable and 0V from system the motor shaft must be in stop ...but thse not happen ..... PCW is really normal these situation? (note that: the servo has a function to compensate these offset and I use it more time .... from display function I see these offset... I think 3 mVolt .... but if servo is in on state the offset start .... ).

Regards
Giorgio

Its absolutely expected (and harmless) that analog velocity modes servos will drift if
enabled but with 0 input

There is really no such thing as 0 input with analog

This never happens in an operating system since the PID loop is closed when running and the drive is
disabled when in machine off or E-Stop states, that is if you see drift with LinuxCNC in the machine-off state,
it means your drive enables are not connected properly (externally and in HAL)

BTW I would _not_ enable the drives anti-drift logic, its unnecessary and will hurt low speed performance
(Some drives will auto disable if the input voltage is below some minimum, you dont want this)

the dac offset entry in the pncconf test is just for assisting while testing.
The open loop test does not use PID so minimizing drift is done by hand.

Chris M

To try and get an idea how important a 3 mV offset is in a +-10V servo system,
lets do a little bit of arithmetic:

For example lets say the servo was driving a linear axis 1:2 with 5 mm pitch ball screw
and further say the servo motor has a 3000 RPM maximum speed (1500 RPM at the ball screw)
(so 125 mm/s or 7500 mm/m rapids)

If we have a 3.333 mV offset this is a velocity offset of 1 RPM at the motor or 1/2 RPM at the ball screw
1/2 RPM is a .042 mm/s speed error. Assuming we have a P gain of 20, it will take a .0021 mm error
to correct for this offset, probably below the error budget by quite a bit and maybe lower
than the encoder resolution

@PCW than if I have these derive ... is all normal .... ok... but if connect Lcnc and try to move these axis in jog mode i have follow error isn't for these fact .... Is only for bad pid regulation on Lcnc.

In these case I have 2 close loop. 1 In drive and the second in Lcnc .... Tuning well in drive I do the tuning in Lcnc ..ok These are some particular rules for make these dual loop? (these are some link with explanation to make these particular tuning ...etc etc)

regards
Giorgio

Getting following errors when first setting up a servo system is expected and normal

John Thornton has a nice velocity mode drive tuning tutorial here:

gnipsel.com/linuxcnc/tuning/servo.html

i think i make a first big error .... in 7i77 with default jumper i connect a-/a--b-/b---z-/z...... but If i understand well ... if jumper is in default setting I should have connect only a b z signal from feedback encoder ... is right these??

Default 7I77 jumpering is for differential encoders (A, /A, B, /B, Z, /Z)

I would _ALWAYS_ use differential mode as single ended is very susceptible
to noise from the drives