Re: Servo hardware

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "imserv1" <imserv@...> wrote:
> 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.

{IMHO, focusing on what happens when an axis faults is just buttering a turd. When an axis faults, that means something major out of the ordinary happened. Could be a broken bit that is now bludgening the part instead of cutting, could be a broken spindle belt, could be a faulty program or the wrong cutter loaded. So to me at this point I always want to rehome and check that everything is where I expected before the fault. Plus of course fix the reason for the fault. If you are getting faults randomly (like pushing the max rates on your motors and windows burps and throws a mis-spaced pulse) you have an unstable and unusable setup that needs to be fixed. So that then leaves faults in the something major happened.

Now feedhold is a different story. We use it a lot especially when proofing a program. Feedhold is just a way to stop everything without losing position so you can look, check, etc. We often use it on the Haas to stop and put tap gel in holes when going deep or in diffcult materials and one off. In this case when we hit feedhold, we don't want a buffer to empty first so the machine knows where it is. We want it to stop about instantly (ok, it does have to decelerate}. Then we want it to restart at that exact position without any loss of accuracy. My personal experience is on an open loop machine I just plain an not confident enough to trust this process and if I have to stop, I rewind the program and use a run from. On a closed loop machine I just blaze on and know I will not get a discontinuity in my part from the stop. Maybe not all correct technically, but have reached this point after 11 years of using open loop systems and 6 years of closed.}

> 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.

{I am 1000% behind what you are saying here Fred. The sizes axis drive motors people think they need has gotten out of control, especially with steppers. The shock sent with each step gets damaging as some point the over powering can damage things in a crash. It is sort of like the kid that put a V-8 in his Pinto. Sure it sounds great and can go real fast in a straigh line. But it has zero handling and nothing else in the car is up to the power so it does not handle and can't stop worth a crap. Balance and properly sized is the only way to go. If the machine seems to be not performing often the answer is to learn a better approach to what you want to do, not to add more power.}

Tim Goldstein
A2Z Corp
A2Z CNC division
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Englewood CO 80110
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