Question about units

I have a homebuilt printer using a RAMPS 1.4 board on an Arduino Mega. I have Marlin firmware, and i'm having trouble getting the axis speeds optimized.

I'm confused since the documentation at [marlinfw.org] shows acceleration settings as mm/MINUTE while the comments in configuration.h say mm/SECOND.

Which is right?

My steppers are:
12 V Bipolar
1.8 degree step angle
.4 amp/phase
2 Phase
30 Ohm resistance
32 mH Inductance

8mm/rev. lead screws

I have the max speed set at 40mm/sec and G1 X100 F3000 (50 mm/sec) works well. But using the home command causes the axis to start accelerating then the stepper stops with a loud buzz, Can someone recommend starting points for acceleration and jerk?

Walt

Your stepper motors are a very poor choice, especially if you are only using a 12V supply (most RAMPS boards can't handle 24V). You should have chosen motors rated at around 3V and 1.2 to 1.5A per phase, with much lower resistance and inductance. Additionally, your leadscrews imply a steps/mm of 400 assuming x16 microstepping, which is very high. If you can tell us the holding torque of your motors in Nm or Ncm and the actual current you are running them at, I can calculate the loss of torque you will experience at your top speed of 40mm/sec. Or you can work it out yourself, see [duet3d.com].

Accelerations in 3D printing are normally expressed in mm/sec^2 because the values get stupidly large if you express them in mm/min^2.

Edited 1 time(s). Last edit at 05/29/2017 01:19PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thank you.

The stepper motors were actually the starting point, from a CNC milling machine that uses a Dremel tool for the spindle. That then evolved into a laser cutter. Both used a proprietary driver board from the maker of the mill.
I changed out the 1/4-20 lead screws for 2mm pitch, 4 starts so 8mm/rev. That works out to 400 steps/mm, and the axis morion is within .01mm measured with a vernier caliper.

Thanks for the link to Duet. I'll do the math, but I still don't see the correlation to the acceleration settings. I agree, the program documentation itself (mm/sec^2) is probably more accurate than the WIKI.

I will have to spend some time studying the Marlin code to get a better idea what's going on.

Walt

What I was trying to say is that your acceleration settings are probably not the cause of the problem. The more likely cause is that the high inductance and back emf of those motors require a higher drive voltage than 12V at the movement speed you are trying to achieve. The CNC machine probably used a lower movement speed and/or higher drive voltage.

But you can try reducing the acceleration settings anyway. For the X and Y axes of a Cartesian printer, values of 1000 to 3000 mm/sec^2 are commonly used.

Edited 2 time(s). Last edit at 05/30/2017 01:44AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
What I was trying to say is that your acceleration settings are probably not the cause of the problem. The more likely cause is that the high inductance and back emf of those motors require a higher drive voltage than 12V at the movement speed you are trying to achieve. The CNC machine probably used a lower movement speed and/or higher drive voltage.

But you can try reducing the acceleration settings anyway. For the X and Y axes of a Cartesian printer, values of 1000 to 3000 mm/sec^2 are commonly used.

AH! I missed that. BTW, I probably will replace the X and Y steppers at the first (financial) opportunity. For now I just have to make the best of what I have. I definitely understand the higher torque (current, at constant voltage) to accelerate a motor.

I will continue to experiment with feed rates, acceleration, and jerk. You are correct, the calculations run out to 14.5 volts for back EMF so I'd be looking at a 16-18 volt supply voltage, or a lower speed.

Thanks for the help and the links.

Walt

I suspect you would need somewhat more than 18V. If you are only interested in not skipping steps, then the back emf due to rotation needs to be added to the voltage needed to overcome resistance (which for those motors would be 12V if you run them at full rated current), then the back emf due to inductance needs to be added with a 90 degree phase shift. If you are interested in precise positioning too then the back emf due to rotation is in phase with the back emf due to inductance.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].