NI motion servo jerk/jump on enable

[Guzz]

we're having an issue with our servo motion control

PCI-7354 + UMI

Labview RT 8.6

nimotion 73xx controller support 7.7.0

we've got a mechanical safety switch that kills power to our servo amps (via the inhibit line)

if we engage the safety switch while the motor is moving the motor stops ok thou when we reenagage it the motor "jumps" sometimes by hundreds of counts

s/w process:

as soon as the safety switch is detected we issue a kill stop command to both motors then disable the limit switches (as the motor carriage can fall onto the reverse switch)

when a reset safety is requested the limit switches are reenabled, the 2 axis are reenabled (configure inhibit i/o - triggers the amp inhibit line) then a kill stop and halt stop are called to reenable the ni motion axis

any thoughts as to why the motor would jump on re-enable?

[Brian_Beal]

What motor and drive are you using?  What is the resolution of the encoder?

[Guzz]

hi

each system uses 2 baldor flex+drive amps with 2 baldor servo motors driven by the ni motion cards voltage output (speed mode) where 10v = 3000rpm at the amp stage

thanks

[Guzz]

we've just noticed the "jump" occurs on axis that have the a lowpass filter enabled in the PID control loop!

[Brian_Beal]

Not sure about the jumping issue exactly, but you should have a brake on the vertical axis.  The Flex+ has an output that can be configured for a brake, and hence have timing adjustments.  The jumping issue may have to do with the analog.  There is an offset in the Flex+, and it can be auto adjusted.  You also shouldn't have to disable the limit switches.  You should be able to move off of the switch without any problems.

[Guzz]

alas we can't have a brake on the axis as it needs to be movable when not enabled, we have to disable the limit switches as the motion driver will re-enable the drive if it rolls onto a limit switch.

we think the jump/jerk is down to the lowpass filter in the PID thou we can't find a way to "flush" it (disabling then enabling the filter didnt help)

[sahpe]

Hello Guzz,

I am not sure if this will help, but have you checked to confirm that you are using valid PID values? Even at startup?

Have a look at this article that might help:

http://digital.ni.com/public.nsf/allkb/C0C521CD425A253B862574400069237A

Kind Regards,

Michael S.
Applications Engineer
NI UK & Ireland

[Laurencej]

Hi,

I don't know if you have solved this yet - so here are my thoughts:

We used to use the Baldor NextMove ISA cards in one of our systems. We found that if the motion was disabled (in our case by opening the instrument's door), and the axis was subsequently moved (in your case it drops under gravity), upon re-enabling the controller, the controller would try to close the position loop to return the axis to the position it was at, when the system was disabled. It essentially did this by applying maximum DAC volts to the drive (dumb drive - voltage to current converter). On a long axis (approximately 1m, in our case), this would make you jump!!!! 🙂

Could it be a similar phenomonen in your case?

Regards,

Laurence