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Author Topic: Steppers Stop When Driven Below .8 IPM?  (Read 3472 times)

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Steppers Stop When Driven Below .8 IPM?
« on: November 14, 2014, 03:50:43 AM »
Alright, having a pretty annoying problem, and it seems I'm the only one to have this problem. 
Got my CNC running great for the most part, but the if ran too slowly, around .8 IPM or less, steppers go into standbye, as if getting no commands. Basically on slight diagonal angles and slow curves, the slower axis will stop and lose steps. I'm running the gecko g540 on a 48v 7.3 amp PSU, pushing three 3.5 amp steppers, at 20,000 steps per inch.
I just wish Mach3 had a minimum speed setting.
Anyone have any suggestions?

Offline Tweakie.CNC

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Re: Steppers Stop When Driven Below .8 IPM?
« Reply #1 on: November 14, 2014, 04:35:34 AM »
You could try changing the Step Pulse width in motor tuning and see if that makes any difference to the situation.

Tweakie.
KEEP SAFE !

Offline RICH

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Re: Steppers Stop When Driven Below .8 IPM?
« Reply #2 on: November 14, 2014, 06:16:17 AM »
Your power supply should be capable of taking care of the three axes. I will assume that you have the amp setting reduced, and when you do that, the motor torque is reduced, and additionaly when going into standby the torque is reduced.
Not sure if you have option to NOT have standby active for G540. If the standby  can be disabbled and the motors don't get hot you may want try that.

BTW, I have run my steppers at a lot less than .8 ipm ( .1 and below) and have no problem so it's not a MACH thing.

RICH
Re: Steppers Stop When Driven Below .8 IPM?
« Reply #3 on: November 14, 2014, 02:28:55 PM »
My steppers are all built for 3.5 amps, and I am using the current resistors at 3.48 K ohm. You have to remove the resistor to get out of standbye mode correct?

And I will check my steps pulse width settings, and see what I can do.

It does seem nobody else has this problem lol.

And on the power supply note, I do have a 4th stepper I use for 3d printing it's like 2 or 2.5 amp,  sometimes that is used.

Offline RICH

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Re: Steppers Stop When Driven Below .8 IPM?
« Reply #4 on: November 14, 2014, 08:15:48 PM »
Quote
You have to remove the resistor to get out of standbye mode correct?

Not sure about that since normaly you use a resistor to set the desired amp rating for a drive.
I have different drives and you can enable or disable the standby.
Read your drive info...........I am to lazy to look it up ;)

RICH
Re: Steppers Stop When Driven Below .8 IPM?
« Reply #5 on: November 15, 2014, 06:38:14 PM »
Ya I read somewhere that you don't need resistors for 3.5 amp steppers on the G540, because it's max is 3.5 amps. But they also said it will not go into standbye that way as well. But I'm going to see if I can find something else on it.

Offline RICH

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Re: Steppers Stop When Driven Below .8 IPM?
« Reply #6 on: November 16, 2014, 06:42:21 AM »
I would allow for some headroom on the power supply , say 5% then you would have aprox 7 amps available.

7 amps / 3 axis= 2.3 amps available per drive when all in use.
Use a 2.2k resistor for each.

Per Gecko, the step pulse should be at least 2us and direction signals Active High.
You can also adjust for smoothness using the trimpots.

Tune you steppers by trial and error for max velocity and accel. Reduce max velocity by 30% from where they would skip.
You can refine the velocity and accel more later on.

RICH
Re: Steppers Stop When Driven Below .8 IPM?
« Reply #7 on: November 18, 2014, 03:22:56 AM »
The step pulse suggestion worked perfectly,  Changed it from 2uS to 3uS, and bam, problem fixed. Steppers run all the way down to .01 IPM now. Thank you guys for the help so much, machine is so much better now.

 How is an underpowered power supply finally going to bite me? What symptoms should I be on the look out for? I haven't noticed any problems. I choose the 7.3 amp supply based on being cheap and planning for only 3 axis, but now I'm running the three 3.5 amp steppers plus the fourth 2.5 amp stepper for hours straight on that power supply. Given that when all four axis are being used, it's for 3d printing, so the Z axis only see's a small blip every now and then, I hoped it would OK.

Offline RICH

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Re: Steppers Stop When Driven Below .8 IPM?
« Reply #8 on: November 18, 2014, 06:55:58 AM »
 
Quote
How is an underpowered power supply finally going to bite me?

You may experience skipping. 3D plotting dosen't required the amount of torque that actual machining would require
and also the  drive may not use the max current for all 4 motors. So you may get away with the current configuration.
Thus you only need capability to move the axis reliably all the time based on some feedrate.

So why not drop the current setting for all four steppers such that the total power supply is adequate. You want power which is speed x torque with an appropriate / max accel. Since you could have very small moves at some velocity, accel becomes important.

If all's working to your satisfaction leave it alone. If not, try what is noted above. Only time will tell.

RICH


Re: Steppers Stop When Driven Below .8 IPM?
« Reply #9 on: November 18, 2014, 02:28:02 PM »
Yes sir, thank you very much for your info. I will have to keep an eye on things, maybe do a stress test where all 4 motors are zinging around and do my tuning around that. Thanks again!