Stepper Motor Current Settings

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RocketMagnet
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Stepper Motor Current Settings

Post by RocketMagnet » Fri Jan 02, 2015 9:23 am

I downloaded the latest firmware a couple of months ago, made some UI tweaks and flashed the Rambo board.
What I didn't realise until recently was this reset my stepper motor settings to the defaults so as I was running Kysan motors they were running very hot.

So my question is are there any other settings like this that I've missed that SeeMeCNC define during setup/config over and above the baseline file you can download?

Also I'm seeing different values for the current settings for XYZ&Ext on this site Vs remarks in the code:
viewtopic.php?f=77&t=6024&hilit=Kysan
Which suggests the following:
#define MOTOR_CURRENT {155,155,155,165,0} // Values 0-255 (RAMBO 135 = ~0.75A, 185 = ~1A KYSAN MOTOR CURRENT SETTINGS )

Whereas the code remarks essentially say it should be {140,140,140,160,0} for Kysan mothors
#define MOTOR_CURRENT {175,175,175,200,0} // Values 0-255 (RAMBO 135 = ~0.75A, 185 = ~1A) use 140 for xyz and 160 for the E if using Kysan motors and 175 xyz and 200 if using wantai motors

I've dropped them to 155,155,155,165 but should I really be setting them as the Code suggests to 140,140,140,160,
Also any idea what the last number 0 is after X,Y,Z,E,? .. presumably another extruder if you have one?

Nareikuk
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Re: Stepper Motor Current Settings

Post by Nareikuk » Fri Jan 02, 2015 9:42 am

I did not follow the manual when setting the motor current level and chose to go with the comment values instead. They seem to work fine, although I am considering reducing the extruder current value to somewhat lower than 160 because the extruder motor still gets very hot. Maybe 105 as has been suggested elsewhere on this forum would be better. I don't get any skipping at the moment, even at high print speeds, so it might be worth going lower.

BenTheRighteous
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Re: Stepper Motor Current Settings

Post by BenTheRighteous » Fri Jan 02, 2015 11:18 am

I do believe the 0 is for a second extruder as you suggest, but that is just a guess.

I have the Kysan motors and I am using {155,155,155,165,0} in the firmware. My extruder motor has been getting very hot lately and I tried to see if I could monitor its voltage with a multimeter. Well, I could only watch the voltage bounce around (I really needed an oscilloscope, but I don't own one) but at one point I saw it reach as high as 5.5v. The Kysan datasheet says they're specced for 4.2v so I'm thinking the firmware numbers are probably high by about 20%. I'm going to drop them all to 130 and see how that goes; I'll report back when I have some results.
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Re: Stepper Motor Current Settings

Post by jdurand » Fri Jan 02, 2015 11:38 am

You actually need to monitor current, stepping motors run with constant current drivers. It common to run stepping motors from voltages much higher than the label voltage. The higher the voltage, the faster the current builds until it reaches the set point. At the set point the driver chip starts switching the voltage on and off rapidly to hold the current at the desired level.

I'm running some coils to generate HV pulses more or less the same way. 6V coil run from 24V, when the current reaches the trip point the hardware switches off the voltage.
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Re: Stepper Motor Current Settings

Post by BenTheRighteous » Fri Jan 02, 2015 11:53 am

Ok, I learned something new today. :)

Anyway, I just reran my (perhaps misguided) test and this time around I didn't see it break 3.6v. So I changed my mind, I'm not going to be messing with the firmware numbers.

I'll just mod the printer to have a cooling fan pointed at the extruder; I'm hoping I'll be able to get to that this weekend.
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Re: Stepper Motor Current Settings

Post by Polygonhell » Fri Jan 02, 2015 12:37 pm

You probably want to Google for stepper motor voltage to understand why but, in general repraps run way too little voltage through the drivers for optimal torque.
It's not unusual in other applications to run 15-20x the rated voltage through the driver. The gecko article on the web somewhere has a formula for the optimum voltage for a given stepper based on it's voltage rating and inductance, if you run most common Reprap steppers through the equation, you'll get closer to 30V than the 12 used, some much higher.
But it's all compromises, the pololu drivers aren't exactly the best, and it's all sufficient for the application.

Generally steppers are designed to run hot, most are rated to run continuously somewhere between 50C and 80C, if you think it's hotter than that your only real recourses are add a heat sink, or turn down the current.

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Re: Stepper Motor Current Settings

Post by RocketMagnet » Thu Jan 29, 2015 12:50 pm

Just thought I'd feed back my experience with current settings.

I tried {140,140,140,160,0} as it says in the code...

However 160 wasn't enough power for the Feed Stepper Motor, It was definitely starving the feed and was not delivering enough with obvious feed starvation evident in the skirts printed around objects.
Increasing the flow compensated a little but it was 120%+ so I had to increase the current setting to 170 for it to be comfortable and feed adequately. Much happier printer at 170 on the feed Stepper.

So I would say the code remark advice isn't correct of setting it to 160 for Kysan.. it definitely isn't enough for my printer/feed stepper motor. So yet another thing to consider if your struggling with gaps in your printed items especially on thin walled stuff.

So my final settings that work really well are.... {140,140,140,170,0}

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