Calibrating different stepper drivers

Hi,

I've just about finished my Prusa i3 build and the Z axis steppers seem to need a little more juice when they did in my Prusa i1.
In the past I just slowly turned the pot until it no longer had issues then a little more for good luck, however I've seen a few posts and guides on using the vref to calculate a good starting point.

The trouble is I'm running a few different drivers, I've got an official Pololu A4988 with the vref, 2 stepstick 1/16 without the vref and a stepstick A4984 without vref (the one controlling the Z axis).

Is there any calculator or guide I can follow for this motley collection? Or is my old method the best with this variety?

Thanks

---

My Reprap blog

jds-reprap.blogspot.com

Allegro site would have datasheets for each chip and datasheet will give a relation between Vref pin and I(peak) and Rs, where Rs is the sense resistors used on the board. The boards could have Rs=0.05ohms or maybe 0.20ohms and that would change the Vref to I(peak) relation, so you need to know exact Rs for each board.

The calculation is good for the purpose to understand the setting and have a good starting point. If you would have a starting point taken too high, could perhaps get chips to overheat, damage, etc. Not necessarily - just "perhaps". On the other hand if you start only from 0V and then up, you will not have a starting point too high, and so you avoid the above.

To manually adjust the Vref (and hence Ipeak) is a good thing to do, should do that anyway (imo). That is to have the drivers work appropriately for the given mechanics. But that is also being efficient, e.g. if you would set the current any higher than that, the current would just be consumed in excess without any additional benefit, sort of speaking more current after that point is kind of wasted, which is not good to have either.

Also as a note, all those do have Vref, where on the IC driver Vref is an input pin that takes in a voltage set by external components. Usually should be a potentiometer for the purpose to allow to vary the voltage. If there is no potentiometer, then probably there are some fixed resistors, to do the same thing e.g. create a voltage except this way the user cant change that voltage. Normally that should be adjustable for the purpose to be tweaked by user in a manner to provide appropriate current for the mechanics and not waste much more than that.

On the boards without the vref point it can be taken from the pot correct? You just answered this.
Also is there an easy way to identify the Rs on the board?
The information for the calculation seems pretty easy to get from the Allegro site which is nice.

Thanks for your help and sorry if I'm asking a lot of questions. I want to do the whole 3d printer thing right this time, learning as much about everything as I can (well within reason).

Edited 1 time(s). Last edit at 03/27/2014 11:08AM by JoeDaStudd.

---

My Reprap blog

jds-reprap.blogspot.com

Quote
JoeDaStudd
On the boards without the vref point it can be taken from the pot correct?

This is where its nice to be in open source environment, if you can locate sources for what you have and you can open them you will be able to see exact point(s) where Vref can be measured. Some times if the chip pins are exposed (not bga ones), some stuff could be measured on the chip pin itself, i usually do that but its not advisable in this case because the board is under voltage and if you have a hand slipping you make a short with other pins and kill the chip (did that more than once, after which point started to appreciate a probe with a very sharp nose). Without sources i think try to locate Vref pin from chip datasheet and see where it leads to and find a better and safer place to measure it.

Quote
JoeDaStudd
Also is there an easy way to identify the Rs on the board?

If you can locate them perhaps its written on them, otherwise you can not directly measure them because a typical multimeter will not measure confidently under 0.3-0.2 ohms. Even if it says it measures to like 0.1ohms resolution, that is for higher values, the error is % and plus 1-2 last digits, and in this case the last digit is probably all there is. Again if you locate sources you could find the value written on the schematic file, or part list. Otherwise there are multimeters that measure resistance in Si, if you have one of those, or maybe some other meter specifically for low resistances you could measure it directly.

Quote
JoeDaStudd
Thanks for your help and sorry if I'm asking a lot of questions. I want to do the whole 3d printer thing right this time, learning as much about everything as I can (well within reason).

Dont be sorry about asking questions, or learning, we are all here for that. The only thing worth to be sorry would be NOT doing these