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Author Topic: Motors stall when using Goto Z's  (Read 10670 times)

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Offline Hood

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Re: Motors stall when using Goto Z's
« Reply #20 on: November 17, 2011, 09:48:36 AM »
Can you check the voltage supplied to your drives whilst running the three axis independently and then when running together, could be your supply is on the weak side.
Hood

Offline stirling

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Re: Motors stall when using Goto Z's
« Reply #21 on: November 17, 2011, 11:45:00 AM »
Hood - can you clear something up for me that's bugged me for years? I've seen this mentioned loads of times but just don't get it. How does running one axis use less current from the supply than when you run (say) all three? (and hence potentially cause a voltage drop). I can understand how that's the case with servos but with steppers I've always thought they "drew" the same current when stationary as when moving (in fact slightly more when stationary if we want to get picky). Maybe I'm wrong but if so I'm darned if I can understand why.

Ian
« Last Edit: November 17, 2011, 01:46:11 PM by stirling »
Re: Motors stall when using Goto Z's
« Reply #22 on: November 17, 2011, 12:02:12 PM »
http://www.youtube.com/fixittt#p/u/70/8Am69_F93yA

http://www.youtube.com/fixittt#p/u/71/ivDQTeGuduk

http://www.youtube.com/fixittt#p/u/72/9RoOZs9LrYo

Take a look at these, your problem is the drive i bet you.  Find a way to make a dampener for testing.  you will be surprised.

Here is a good thread on dampeners. Resonance is horrid in the xylotex.  The vibration is going into the motor and builds up.  If you lower your accel way down and jog the motor so that it ramps up to speed, you will see that as it reaches full speed, it will stall.  Any setting in mach will not solve it

another thing to keep in mind, you are limited to a 24v power supply, which means that if you have say, the 269 ox steppers,  you are running them at less then half of their rated power.
« Last Edit: November 17, 2011, 12:10:33 PM by fixittt »

Offline Hood

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Re: Motors stall when using Goto Z's
« Reply #23 on: November 17, 2011, 05:06:20 PM »
Ian,
 I dont know a great deal about steppers, well I should say steppers are in a long list of things ;)
I was actually thinking more along the lines of not enough capacitance but really I dont even know if that would be the case.
Hood

Offline RICH

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Re: Motors stall when using Goto Z's
« Reply #24 on: November 17, 2011, 06:58:54 PM »
Bob,
I think your correct on it being resonance. Adding a rotating mass to the motor will change the resonant frequency / speed at which resonance occurs.
It is a combination of the drive electronics and the rotating systems moment of initia of that creates the situation ( simplisticaly said) .


RICH


 
Re: Motors stall when using Goto Z's
« Reply #25 on: November 17, 2011, 11:39:34 PM »
The driver test says "system excellent" up to 65 Khz.
Measuring the voltage showed no difference when running the motors, it stays at 23.5 VDC. I'm not too worried running at half the rated power since the application will have little to no load.
I took the acceleration of the x-axis down to 0.1875 (the minimum it would let me set). That equates to 4 seconds to ramp up to speed. I then jogged the x-axis up to 45 IPM with no problem several times in both directions.
When using goto z's at this slow ramp rate, it usually gets up to speed (not always), but after a little bit at the top speed starts to stall or as it is starting to slow down.
In addition to that test, I tried to squeeze the motor shaft with a cloth while using goto z's (to see if I could change resonance). Still, it stalls.
Note that I've never had trouble jogging with any axis up to 45 IPM. I'm not sure why goto z's is different than jogging. Does anyone have any ideas for why these would be different within mach3? Is there a reason the resonance would be different with either method?

Also, it seems odd to me that the y-axis or z-axis will stall too even though they have different loads due to the design of the taig mill. Shouldn't there be a difference?

I'm curious about the dampers though. Does anyone have a source for purchasing or design? If they are not too expensive (or easy to build) I could give one a try.

Thanks a ton for the input so far. I really want to understand this issue.

Online Tweakie.CNC

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Re: Motors stall when using Goto Z's
« Reply #26 on: November 18, 2011, 03:33:06 AM »
Hi Larry,

The mid-band resonance issue has been covered here and on the zone quite a lot and looking through a previous thread Gerry said that his Xylotex  was practically unusable without the dampners. http://www.machsupport.com/forum/index.php/topic,17670.0.html
Some have had success using just a flywheel but technically they should ‘rattle’ and have the moveable elements which dynamically alter the resonant frequency as soon as the stepper stutters. They are pretty simple to make and from the tests that I have made the one I fitted to my 4th axis makes a big difference to the rapid speeds.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline stirling

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Re: Motors stall when using Goto Z's
« Reply #27 on: November 18, 2011, 04:47:03 AM »
Hi Hood - no probs. I've tried to get a definitive answer to this before with little luck. All I can say is I'm pretty sure that contrary to what seems to be a common belief, steppers actually "draw" the most current from the PS when stationary and this reduces with speed. That's why (very simply put) steppers eventually stall with speed.

lcluff2000

I'm not too worried running at half the rated power since the application will have little to no load.

This is slightly missing the point. Granted, at low speed you won't be "too" far away from the theoretical "optimum" torque (because of your phase current of 2.2A as opposed to the rated value of 2.8A). However as speed increases, your 24V is pitifully low compared to the ideal for your motors of 60V (if my calcs are correct). High speed torque is a direct function of Voltage. Even with "zero" load your motors would stall way below their potential maximum speed.

Also, it seems odd to me that the y-axis or z-axis will stall too even though they have different loads due to the design of the taig mill. Shouldn't there be a difference?
As above, the load isn't the real issue here.

Note that I've never had trouble jogging with any axis up to 45 IPM. I'm not sure why goto z's is different than jogging. Does anyone have any ideas for why these would be different within mach3? Is there a reason the resonance would be different with either method?

This is JMHO but with jogging, the Mach application isn't doing much. It simply tells the driver to jog - the driver's doing all the work independantly. With commanded moves, the Mach app is doing more. This *might* mean that the jitter is marginally worse. In a *good* system, this probably makes no difference. However, given you're "undercurrenting", "undervolting" and sadly your drivers are known to not handle mid band resonance as well as some other drivers, it may be just enough to cause the problems your seeing. Just a guess.

Just a quick comment re "rattlers". They do seem to work well with less than perfect drivers. With decent drivers that handle mid band resonance they (IMHO) bring nothing to the party. Personally I'd swap drivers rather than go the rattler route. As I said - JMHO.

Ian
« Last Edit: November 18, 2011, 05:27:05 AM by stirling »

Offline RICH

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Re: Motors stall when using Goto Z's
« Reply #28 on: November 18, 2011, 06:53:16 AM »
Quote
I really want to understand this issue.
Do an internet search for "Jones on Stepping Motors". The book or course is rather technical but certainly worth a read if interested.

RICH
Re: Motors stall when using Goto Z's
« Reply #29 on: November 18, 2011, 07:18:34 AM »
Hi lcluff2000,
  Here is a basic but very informative article from the GECKO site about stepper motors.
I found it quite interesting.
Russ
 :)