What did I do wrong?

[clough42]

Wish there was a way to adjust it without it changing so drastic. Any upgrade or anything I can do to get the bed up or the head down like just a tiny big? Any tricks?

If you've got the i3v (the one with the head-scratcher Z limit design) I've got you covered. Here's one that uses a screw to adjust: http://www.thingiverse.com/thing:336665

Or if you want, you can use a micrometer barrel: http://www.thingiverse.com/thing:356819

Or you can go with Roxy's suggestion and use auto-bed leveling. Here's my interpretation: http://www.thingiverse.com/thing:335632

I'm in the auto bed leveling camp. If you use my probe design, you will definitely need to deal with getting the printer to raise the Z axis before retracting the probe so it doesn't strike the bed. I hacked the firmware myself, but it looks like Roxy has a thread with code changes that do something similar.

[printbus]

Keep in mind that I'm entirely self-taught here, so if someone with more experience or formal education wants to shed light on this, I'm all ears.

Well, you are correct that inductance is resistant to current change, much like capacitance is resistant to voltage change.

It would be helpful to know whether the mH value you have is right. The type numbers on the stickers differ - ours apparently being a newer part number than the type number on the CS page. What appears to be the motor manufacturer doesn't cross-reference an old part number to our new one (42BHH8-050-24A) in their new part number listing. Assuming 32 mH is correct, and applying the math from this reprap thread, the impedance from the inductance at a 125 Hz step rate for 100mm/sec movement on an X/Y motor would be 25 ohms. That is considerable. EDIT: Yeah, the numbers for the X/Y motor may not apply, but the other thread gave those to work with. I retired from engineering so I didn't have to fret over this kind of stuff.

[clough42]

Well, you are correct that inductance is resistant to current change, much like capacitance is resistant to voltage change.

It would be helpful to know whether the mH value you have is right. The type numbers on the stickers differ - ours apparently being a newer part number than the type number on the CS page. What appears to be the motor manufacturer doesn't cross-reference an old part number to our new one (42BHH8-050-24A) in their new part number listing. Assuming 32 mH is correct, and applying the math from this reprap thread, the impedance from the inductance at a 125 Hz step rate for 100mm/sec movement on an X/Y motor would be 25 ohms. That is considerable. EDIT: Yeah, the numbers for the X/Y motor may not apply, but the other thread gave those to work with. I retired from engineering so I didn't have to fret over this kind of stuff.

IIRC, the steps/mm on the extruder is around 860, which is 1/16 microsteps. This means it's about 54 full steps/mm. If you're trying to retract at 25mm/s, this is 1350Hz.

That seems like a lot. Let's try calculating it another way. The effective diameter of the hobbed bolt is about 6mm, so the circumference is about 19mm. 200 steps per motor revolution times 5.2:1 gear ratio divided by 19mm effective circumference equals 55 steps/mm. That agrees, so it's likely about right.

How does this compare to an X/Y motor? A typical 20-tooth pulley has a 40mm circumference, which means 100mm/sec would be 2.5rev/sec. Time 200 steps/rev = 500Hz. So the extruder is moving almost three times as fast during a 25mm/s retract.

Check my math.

We still have the question about whether I have identified the correct motor.

[clough42]

I checked the math in the thread you referenced, and I'm not following exactly how they got to the numbers, but I get the same numbers. 500Hz step rate (100mm/s X/Y motor) is 125Hz to any one coil in a bipolar motor. 125Hz through 32mH gives the 25 ohms you quoted according to this calculator: http://www.electronics2000.co.uk/cal...calculator.php

For the extruder, 1350Hz step rate is 337Hz electrically through the coil. At 32mH, this is 68 ohms.

Still working on identifying the motor. It looks like other people have been asking Colin about them for a while, without apparent success. I'll see if I can get a direct measurement.

[printbus]

I haven't studied the numbers, but I agree that the extruder motor is moving faster than X/Y/Z so a higher impedance seems logical. After pondering this for a while, I now somewhat regret referencing the reprap thread. I think its concept of applying the z=2*pi*frequency*inductance formula to stepper motors is debatable. The formula applies to sine wave signals. For square wave PWM signals like we're dealing with here, the frequency content is really in the rise and fall times of the PWM signal more than the pulse rate. ie, sharper square waves have more higher frequency content. But what do I know - my expertise was in digital circuit design, not motors!

[clough42]

I haven't studied the numbers, but I agree that the extruder motor is moving faster than X/Y/Z so a higher impedance seems logical. After pondering this for a while, I now somewhat regret referencing the reprap thread. I think its concept of applying the z=2*pi*frequency*inductance formula to stepper motors is debatable. The formula applies to sine wave signals. For square wave PWM signals like we're dealing with here, the frequency content is really in the rise and fall times of the PWM signal more than the pulse rate. ie, sharper square waves have more higher frequency content. But what do I know - my expertise was in digital circuit design, not motors!

Actually, I think it's fine. In this scenario, the microstepping of the driver is attempting to create a sine (or triangle) current waveform in the motor coil. The inductor smooths out the current waveform and the driver uses feedback to drive the PWM to get the desired current waveform. It's perfectly valid to analyze the fundamental step signal using the normal inductor equations. I ran this by our motor guy at work (designs laser mirror drive systems) and his position is that the combination of the large inductor and the feedback in the drive pretty much makes the PWM a no-op for this kind of analysis.

I also got some real numbers for the MakerFarm motors--both from Colin and from direct measurement. The DC resistance of the coils is about 18.5 ohms and the inductance is about 35mH.

I'll start a new thread on the MakerFarm motors, just to make this info easier for other people to find.

[clough42]

The new thread is here: http://3dprintboard.com/showthread.p...Specifications