Cannot drive both Z steppers from the single Z servo driver


#1

The steppers just won’t go when in parallel. My steppers are rated at 2A, which would imply 1.6V for the drivers I have. I can’t get that high. My power supply is running at 12V. For the moment, I’ve put one on the second extruder. So, 3 questions.

  1. Has anyone run steppers in serial? Have any problems with it?
  2. Doesn’t look like any of the Pololui drivers can handle these steppers I have. Would any drivers solve this problem?
  3. Has anyone (i.e. Martin) thought of driving the right lead screw with a belt from the top of the left lead screw? It seems like that may work, then there would not be any synchronization problems, and 1 less motor needed (I could use it for a second extruder!) I have a couple of inches of lead screw above my top Z carriage, maybe I’ll see what I can butcher up. I write software, I’m not a ME… Maybe just attach a gear right above the top bearing, which would minimize the cantilevered axial load.

#2

@LogicalChaos just a thought but, rated at 2A doesn’t mean you have to drive them at 2A, right? I’ve not had any problems driving two motors in parallel with the typical A4988 stepper drivers. What happens when you try?


#3

Unfortunately, it does, almost, I think. The voltage setup of the driver chip configures the current limit. Setting it at .8V (2A limit) for my motors they run great individually. I’ve not tried reducing the current limit to see where it stops driving correctly. The Pololu site has great information on setting up the drivers correctly for your motors, and the spec sheets go into the partial steps and how those affect the driver/current load.

When I hook up both motors to the single driver, regardless of the tuned driver voltage, they have seizures.


#4

Sometimes, the A4988 boards don’t have .05 Ohm sense resistors like Pololu uses, so keep in mind that if yours is different then you may be setting the current limit lower than you think. On the pololu site, they say that the current limit is 2.5xVref, but that’s only if Rsense=0.05 Ohms (the full equation is IMAX = VREF/(8*RSENSE). It is common to see 0.1 Ohm resistors on the A4988 boards, which as an example would mean you need to have Vref=1.6V. Can you tell from looking what value of Rsense you have? (It is probably printed on the parts but you’ll need a magnifying glass to see them)


#5

I wonder if getting a real Polou board would make a difference…

Yes, but I did max out the current limit (according to Vref), and it didn’t behave. I’ll try again to make sure.

Maybe. I’ll take a look tonight.


#6

They are .1 Ohm resisters. Had to remove the heat sync.

Sigh, It works much better at 400 steps per unit, vs the default 4000 (I’m using Marlin 1.1 RC code). Although at .7V, the right one would skip (longer lead). I maxed the driver out at .92V and it’s running clean.


#7

On a repstrap I built using 2.7 volt nema23 motors I initially used two driver chips (one per each). Both Marlin and Repetier FW allow this configuration. However, I now have the motors wired in series to one driver, and it works fine. BTW ANY of the driver modules that you buy from China WILL have 0.1 ohm sense resistors!


#8

How do you like the serial configuration? Notice any difference vs parallel? Similar to dual drivers?