Re: Servo hardware

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "timgoldstein" <timg@...> wrote:

>
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "grd750" <grd750@> wrote:
> > Usually if a servo faults it already has lost position. The control software is in charge of knowing where the toolpath is SUPPOSED to be but if the error is great enough to fault, do you really want to restart from the wrong position? The best thing is to determine what caused the fault in the first place (or it will just do it again) and then reestablish table zero and restart the cut from an accurate position.
> >
> > TOM caudle
>
> For me, I could care less about what happens if the servo faults. At that point the part is probably scrap, the tool is ruined, and the setup is likely mucked up. At least this is the result when we fault an axis on the Haas, but it has 2000 lbs of thrust at fault.

We added a servo fault detection to our SYS-3A controllers. If any of the servos fault, 2 thing happen,

1) Motion is suspended on all axis drives
2) The controller is simultaneously placed in Feed hold until the fault is cleared.

Since the DeskCNC controller is in charge instead of a buffer on the parallel port, the feed hold is effective at the step level, even if the fault occurs in the middle of an arc. On a Sherline mill or lathe ( or Taig mill) the servo step resolution is 112,000 steps per inch. A few error steps for this cycle does not measurably affect the position of the machine. (It could build up if it occurred several times without re-homing.)

Depending on what has caused the fault, re-homing will not change the machine coordinates much, and then only on the axis that was faulted ( that is visibly indicated by the driver fault leds). For sure the part will probably still be OK, and only the faulted axis position will be affected, and might need to be re-homed.

To put this in perspective a bit, by design, our servo systems can be faulted by grabbing hold of the machine handles. The same hand action can easily stall the mill spindle by burying a 1/4 or 1/8 tool in metal, (there is only about 1/4-1/3 HP available in the spindle motor). Overpowering a small desktop machine can result in damaged components and premature wear of leadscrews and lead nuts.

Fred Smith - IMService
http://www.imsrv.com