Problem- Positioning Problem (axis with brake)

[Stevie Bramwell]

Hi,

I've got an issue on a machine with a Fanuc 18i control.

The Z axis has a brake.

When the door opens the drives go off and the brake energises. The axis drops approximately 1mm before the brake catches it.

It also has a counterbalance cylinder which I have checked out and appears to be working o.k.

The problem we have is that when the machine is in cycle the program stops to allow the operator to open the door to check the component. After he shuts the door and restarts the cycle the machine goes into cut but the axis has dropped and the cut starts out of position.

The machine position shows that the axis has moved position, should the control not correct the position once the drives are powered back on?

Are there parameters that I could check?

Any help would be much appreciated.

Thanks in advance.

Stevie

[John_B]

The machine really shouldn't go into a de-energized mode - sounds like it essentially is being e-stopped.

Does the machine drop power to the drives when the program has ended and reset, then the door is opened?

Rgds,
John B

(more info on the machine brand and model, year and so on will help others chime in...)

[tc429]

make sure you are programming in absolute- if its programmed in incremental (g91 or u/w), its just moving from wherever it is, by that programmed distance...
if using a g92/g50 coord preset at the start in absolute, it wont work either as the preset will be wherever the heck its sitting...at restart use g91g28x0z0, or g28u0w0 depending on which gcode system you use, then do your preset, then go...

I'm guessing they just have it programmed in incremental and thats all that will need changed.

just program in absolute, use geometry offset instead of g50/g92, and at start of program, just g28 or g30, call your t-code and go...much more crashproof than g50/92

personally I would look into keeping the servo on- especially if they are cycling the contactors as that is a drive killer...servo-off dont cycle the contactors, its pretty safe, but if the interlock is doing servo-off function and cycling the contactors, its not good- we have a LOT of R-J robots, they had $srv_brk_enable true by default- I wasnt at that plant till a couple years ago, but they blew hunderds of servo amps...after changing a couple myself, looked into why they failed, here the silver contacts simply wore out on the cheap little fuji relays built into the drives...I reset the variables to either keep them hot in standby or at least cycle slower than machine cycle times so they only shut off on backshifts once, drive failures simply quit happening...

[Dan Fritz]

Sounds like something isn't wired right. There are a couple of input signals that can cause the CNC to lose position, but those should never be wired to the axis brake. A brake is mechanical, and the servo can always move a bit when the brake is energized or de-energized.

When the brake is applied, it would be normal to disable the servo so it can't overheat or overload while just sitting there a wee bit out of position. That signal should not, however, cause the CNC to stop reading the feedback signals from the pulse coder.

Check to see if the ABSOLUTE position is shifting. tc429 might be right because if you were programming in G91, then you would see a shift in your program. After you close the door and the servo becomes enabled again, put a G00G90Z..... command in the program to be sure the axis is at the correct absolute position. Then you can make all the G91 moves you want without shifting the program.

[hitachibos]

look on the control for a push button that says "MAN/ABS" if this is set to MAN then you can stop the machine and reposition it go back to memory and start program and the machine will not correct itself. if it is set to ABS then it should correct is position like you are thinking it should.

If you do not have a push button on the control then look in the electrical cabinets for a toggle switch with the same lable. "MAN/ABS"

[tc429]

Originally Posted by hitachibos look on the control for a push button that says "MAN/ABS" if this is set to MAN then you can stop the machine and reposition it go back to memory and start program and the machine will not correct itself. if it is set to ABS then it should correct is position like you are thinking it should. If you do not have a push button on the control then look in the electrical cabinets for a toggle switch with the same lable. "MAN/ABS"

good call

we had that exact issue years back, cut them out of everything in the shop- I'd forgotten all about it! Daewoo had them in the cabinet(???) somebody musta bumped one one day not long after, crash... I never really understood WHY somebody woud want to use that 'feature', nor what would possess a OEM to hide it in back of the machine...