Smoothie - onboard drivers in parallel?

Hi all,

In a nutshell, I'm trying to decide if I want to drive my gantry with a single stepper or 2 separate steppers.

Question: If I choose to use a single stepper, the smoothieboard (individual drivers) won't handle the current I expect to need. Their website makes mention of slaving one of the drivers so that 2 steppers can be driven. BUT, can I use this method, but also parallel the driver outputs so that I can double the current to a single larger stepper motor?

It makes sense to me, but I'm not getting any hits on my searches, only hits re: 2 steppers on one driver, or slaved steppers.

Thanks everyone

Use separate drivers for each stepper.

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Are you saying it's not possible to take the output from 2 drivers and wire them in parallel?

Quote:

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Originally Posted by Blackened

Are you saying it's not possible to take the output from 2 drivers and wire them in parallel?

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In general you cannot do this as the output Hbridges will not switch at the same times
so you will either have poor current control or simply destroy the driver chips
( depending on the drivers decay mode )

Quote:

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Originally Posted by PCW_MESA

In general you cannot do this as the output Hbridges will not switch at the same times
so you will either have poor current control or simply destroy the driver chips
( depending on the drivers decay mode )

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Thanks PCW_MESA. I hoped the two drivers would be synced by the jumpers on the board. Electronics was never my strong suit though. What time scale are we talking here? Shorter time than the steppers would be effected by, in regards to one stepper lagging or leading the other (in the case of 2 steppers working from 2 separate drivers that have been slaved) ? If you traced the 2 outputs on a scope, would they be miles apart or just a degree or so? I suppose I don't even know if that would be ok lol so it's not very important. I'm just curious.

If the output stages are not perfectly synchronized you will likely get high side and low side drivers on at the same time,
causing what is called "shoot-through" meaning a dead short across the stepper power supply, this can kill the output devices
in microseconds

Ok, that makes sense. I got a shorter, but similar reaction to my question on the smoothieboard forum. So I'll assume that option is off the table.

Hang on a second. I've just had a further thought. What if I had an eight wire stepper? Could I split the phases/wiring to drive half with one driver, and the other half with the other driver? In effect, treating the single large stepper as if it's 2 smaller 4 wire ones? This is getting a bit complicated though. I think I'm leaning towards the 2 stepper per axis idea. Maybe once I have a simpler, working machine I'll explore this idea further. But until then, probably best that I go with a tried and tested method.

its a pity the stepper drivers are not plug in modules like the Pololu A4988 stepper motor drivers

at first sight connecting an 8 wire motor to 2 drivers is a better idea but the motor will act as a 1 to 1 transformer
so the two driver outputs will still interact and probably damage each other

the stepper drivers will need to work in sync and each IC set up to provide half the motor current

it will not work without modifying one driver
so they share a common set of current sense resistors , a common clock for the " chopper "
current control and care full adjustment of the potentiometer to set the motor current
even then its not going to be easy to ensure they share the load current

If the smoothie board has the connector making the signals available
It would be better to re-route the step & direction signals going to one axis and connect them to a high current driver

John

Thanks John,

Very valuable info there. I've discarded the whole idea, and will be using 2 steppers, 2 drivers (one slaved). The smoothieboard does have connectors to make the signals available to external drivers, I was trying to make use of the 2 unused onboard drivers rather than have an external one with greater current handling capacity. Oh well, if I fry a driver I have 2 more I can swap to lol