parallel vs serial z motors

  • Hi,
    I am looking to use duel z motors in parallel mode, other than buying a spliter from aliexpress, would there be a way of doing this directly on the duet i.e. how would i go about wiring z axis for parallel mode rather than serial mode.

    Would this improve bed levelling in any way i.e. stop the motors from going out of sync.

    Kind Regards

  • Would depend on motors and supply voltage (if its a good idea). To wire it, just crimp one wire from each motor into the same pin on the duet.

  • Moderator

    @npm1 said in parallel vs serial z motors:

    Would this improve bed levelling in any way i.e. stop the motors from going out of sync.

    Not really. Any time the motors are powered off they will snap to nearest full step, which may not be in the same direction. Being on the same driver serial or parallel won't help that. Better to use a seperate driver entirely and use active leveling to get it synced, or use a belt to mechanically link them together.

  • @phaedrux Actually they do stay in sync better in parallel in some situations, although I run mine in series.

    I found in series if you stall one the other still spins. In parallel stalling one tends to stall the other. Something to do with the coupling because if you turn one it will act as a generator and turn the other. So I also think when it powers up they will both snap the same way.

    The issue is if you power up on a phase that is exactly two whole steps away from where the motors stopped. In that case they could snap in different directions in series but more likely the same way in parallel. As long as your home position is not such a phase you should be OK assuming you only power off when homed.

    I put pointers on my Z-axis lead screws that I can rotate to point the same way when homed and see of they get out of step.

  • I'd concur with @Phaedrux. There is no way to force the motors to snap in the same direction once powered off. In either parallel or series, when they're connected to the same driver, they will behave in a similar manner. Series actually provides more reliability against problems that may arise from slight variations in the physical characteristics of the two motors as well, since there is no way for the motors to draw different amounts of current.

    Once powered off, there is no way for the driver to control what happens with the motor.

    Since the duet makes it so easy, and I'm only using a single extruder, it made so much more sense for me to use 2 drivers for the Z axis. I really can't think of a good reason to leave a stepper driver on-board unused.

    @nophead DC motors are also always generators, but if you spin a DC motor one way, another DC motor in parallel with it will spin in the opposite direction, so the (hopefully) more normal situation is more likely to have the motors spin in opposite directions when powered down. In series, it needs a path for current to flow, so the motors will find it more difficult to influence each other on an open circuit. IN parallel, each motor closes the circuit of each other. Steppers aren't exactly normal DC motors, so the effect isn't quite the same.

    For stalling, yes. Because a stalled motor coil acts something like a short, one motor stalling will current starve the other. This isn't really good for the stalled motor, because it's getting twice the current that you want to normally give it, and it far from guarantees a stall. More likely, it will still be almost correct, but will still require some correction, and will almost certainly be one or more full steps out when powered down. Personally, I'd prefer a more obvious mismatch so that I know to re-level the motors. It's not like it's that much more work. Parallel really only gets used when you've got a lower voltage power source, like most of the Arduino/RAMPS boards that only tolerate 12V.

  • @supraguy Steppers are nothing like DC motors. They don't take more current when stalled because the drivers deliver constant current regardless of the back EMF.

    Have you tried connecting two steppers in parallel and series and stalling them and rotating them with power off?

    I have and stand by what I said.

    Assuming they don't move with power off, other than to their detents, they will snap the same way unless you are unlucky to apply power two full phases from their rest position. I.e. one in sixteen chance of an indeterminate position with x 16 microstepping, which is why two motors on one drive is feasible. If you power off in the home position and the home position is not the unlucky one they will power up in sync. This is why I can run my machines for years without having to re-sync them.

  • Yeah, I get that they're different, and said as much, but they're still DC coils and still provide back EMF when turned, which the other motor provides a path for and it will apply opposing force within that one step.

    In parallel they WILL take more current.

    Say you have 2 motors, and want to deliver 1A to each of them. In series, you set your stepper driver to 1A current. No more than that will leave the driver. In parallel, you set your stepper driver to deliver 2A current, which each motor will draw from, and get 1A each. When the one motor stalls, its coil does not have the correct control inductance, since it did not move through the magnetic field as anticipated. It accordingly reduces its resistance, and draws more than its share of the 2A available, thus starving the current for the other motor. This is what stalls the second motor, as you observed happens. While stalled, the one motor will draw the majority of the 2A available. In addition, it becomes harder to detect that stall, if you are trying to do that.

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