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stepper or drive problem
« on: June 27, 2010, 08:45:56 AM »
I had an incident yesterday where my X axis stepper lost it's "grip" so to speak. That is it lost its orientation by a number of steps and it wasn't "locked" when not in motion. The ball rod could be turned somewhat easily when the motor should have been holding it in place. I have Superior Elect. SLO-SYN stepper motors with Superior MD808 drives. This happened after the  CNC had been running for a while and my Z had dropped down a couple of thousands. I had to stop the program and re-wind the code to start over, when doing this I noticed the X had also lost...or gained a few hundredths. I have occasionally had this problem with my Z dropping a couple of thousands during a large file which puts a line across my work piece. I have also had the same problem with my X on rare occasion and only on large files. Could this be the drives or steppers? Could heat be a factor? I have been meaning to build a temp controlled, positive pressure box to house my computer, power supply and drives but still have them sitting on top of my CNC in the heat...and dust of my shop. Any advice or opinions would be appreciated. Anyone also know of a good source for these drives and steppers? I can't afford down time and would like to keep some on hand as spares. Thanks.

--Rick

Offline RICH

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Re: stepper or drive problem
« Reply #1 on: June 27, 2010, 10:20:14 AM »
Could this be the drives or steppers?
-Steppers can get finiky when run at too hiigh curent setting or when they get hot.

Sounds like you are running near the limit of what the x axis will do, so if the Z drops down ( for whatever reason ) you
get additional demands on the X axis and it skips. Maybe lower the velocity and accel some.
Long files means longer running  time which means increase in stepper temp depending on amp and volts provided to them.

RICH
Re: stepper or drive problem
« Reply #2 on: June 27, 2010, 10:46:12 AM »
Thanks for the reply Rich...and yes, I am running almost to the limit of my x axis. I'm impressed. How did you know this and what effect does that have on losing steps as I am not "to the limit"? Other than obvious, with the problem I am having. I'm also doing a raster cut along the x axis so there is that "jolt" each time it changes direction. I've considered this and thought slowing the speed would help.

--Rick

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Re: stepper or drive problem
« Reply #3 on: June 27, 2010, 03:29:28 PM »
Quote
How did you know this and what effect does that have on losing steps as I am not "to the limit"?

- Just experience Rick ;), could be wrong ;D and testing to find what i can do with my  machines such that they will be reliable when i machine something.
You only need to have things go wrong once......when you have invested a lot of time into something.......to convince yourself that speed
alone is not the only consideration.

For a stepper, the torque will decrease as rpm increases. So if you don't have some "headroom" ie; running at velocity and accel below the max, you will stall the motor / skip steps. SO,  should a machining operation,  or maybe lubrication / gib adjustment or anything change and require more power, you can end up with a problem.

Power is basically speed x torque and there is a point on the motor curve ( some value of pulses per second / rpm and torque ) where
you will have the most power available. Power is what you want not necessarily speed, but, you can rationalize that depending on what you are doing.

There is something called exact stop and constant velocity and acceleration can become important. Have a read of the manual or search 
for more info. When i do engraving work i use exact stop at a reduced feedrate to make sure  the steppers won't skip. The last thing i want to
have happen, on some finished part which i invested 15 hours into, because i am trying to save some minutes from an hour of machine time.

RICH









 

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Re: stepper or drive problem
« Reply #4 on: June 28, 2010, 06:08:25 AM »
Just going back to your first para.

I had an incident yesterday where my X axis stepper lost it's "grip" so to speak. That is it lost its orientation by a number of steps and it wasn't "locked" when not in motion. The ball rod could be turned somewhat easily when the motor should have been holding it in place.

When a stepper is "holding" i.e. powered but not being rotated by the driver and IF you supply enough external torque, the holding torque will be overcome and the motor will then spin freely i.e. even more freely than an unconnected motor. It won't even have that light cogging you get from the permanent magnets. This is one of the few ways you CAN actually spin a stepper "smoothly" with virtually no resistance. The problem is that a stepper has it's maximum torque when stationary (hence why the holding torque is allways quoted) so IF you can "break" that relatively easilly it's all down hill from there I'm afraid. i.e. if the motor is already being driven correctly in terms of current, voltage etc. no matter what you do you will never get more torque than you had when it was stationary. If this is a new issue with your system I'd be tending to think hardware issue more than software - could be anything - but I'd look first at an overheated motor being temporarilly or permanently damaged.

Don't know if this'll help but just what I'd be thinking.

Ian
Re: stepper or drive problem
« Reply #5 on: June 28, 2010, 08:24:24 AM »
Thanks for the replies, I found a wire to the motor that come disconnected which caused the problem with the motor hold torque. Hopefully I caught it in time before any damage was done to either the motor or drive. It seems to be working fine now. I still have the occasional problem of my Z axis dropping a few thousands during a large file and on rare occasion my X as well. I'm going to contribute this to motors heating up and am going to change the way I run these files...not after the machine has been running for a while but when first starting up. I do wood carvings and signs and try to do a finish cut immediately after a rough with a larger tool as too not loose my zeros (do not have limits connected) It is extremely difficult to re-zero after a rough and get it exact although I will look into some methods I've read here posted by Rich. (doing math in the DRO from the edges of workpieces). I've been using Mach for a while but need to become more familiar with some of the other features. Thanks!
--Rick

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Re: stepper or drive problem
« Reply #6 on: June 28, 2010, 08:38:23 AM »
Quote
It is extremely difficult to re-zero after a rough and get it exact


Even though you don't have limit switches you could create a work offset and have a home position. So after the rough you would be able to
go home, change or set the finish tool. Then do the finish pass.

RICH

Offline stirling

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Re: stepper or drive problem
« Reply #7 on: June 28, 2010, 09:22:15 AM »
I found a wire to the motor that come disconnected which caused the problem with the motor hold torque.
::)

Hopefully I caught it in time before any damage was done to either the motor or drive. It seems to be working fine now.
You're lucky - this is one of the most efficient ways to let the smoke out of a driver  :)

I'm going to contribute this to motors heating up
It depends on what you mean by heating up. Stepper motors are designed to withstand quite a bit of heat - some people say that intentionally running them hot is the way to get the best bang for your buck - I'll reserve comment - but if you can hold them without burning your hands, as I understand it - that's not too hot.

Ian

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Re: stepper or drive problem
« Reply #8 on: June 28, 2010, 10:32:14 AM »
Simply put, if no heat is generated from the motor, then your not working them very hard.
If you can hold them, your below 140 deg F and that's what i go by. The winding insulator has a max rating and
some will state a max temperature rise. Don't mean a hoot if you find that the stepper is acting funny due to
it's temperture. Seems like, here anyway, that the old style can types as compared to the hybrid type are not as sensitive
to the heat, but that may just be a figment of my imagination. Just use common sense.
RICH

Offline stirling

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Re: stepper or drive problem
« Reply #9 on: June 28, 2010, 10:39:07 AM »
Simply put...
And that's why I'm not going there  ;D