Dual Motors per axis
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I'm trying to get my head around why 2 motors per axis isn't a thing (ignore the fact for the need of more stepper drivers and higher cost).
With all the nitpicking of low interia, high torque motors I'm wondering what the downside of using 2 motors per axis would be?
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Let's say on a delta tower: what's the downside to mounting a motor on top and on bottom of the tower? Aren't they synchronized 'per default' due to the belt?
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Imagine a Makerbot style cartesian (on quality linear rails instead of rods and a sturdy frame). What's keeping me from just mounting 2 motors on Y and dragging around another motor directly on the X axis? Shouldn't the sturdyness of the frame and the precision of the rails kind of equalize the added weight and make the racking force insignificant? If you synchronize the Y motors via the rod shouldn't that also erase any concerns about the gantry becoming skewed?
The first one on the Delta is purely out of curiosity because of the achievable speeds.
The second one however is what currently keeps me up at night.. I'm not too much into CoreXY as I have my delta for real speedy stuff.
However I'd like another (400x400ish) printer that can drag around a direct drive extruder, preferrably with the option for IDEX later on (print + support material).I studied the Annex Engineering K1/K2 a bit that basically is a linear rail cross gantry. However - why would I drag a 350g linear rail around when I could just drag a 300g (or less) Nema 17 around?
I hope someone can explain to me what I'm missing here..
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Any time you use more than one motor per axis you have to do something to fix the inevitable shifting of the axis when power is cycled. It's isn't necessarily hard to do, but begs the question: what is the advantage of using two motors to do the job that one motor can do?
9 motors in a printer to do the work of 4 is certainly different. More isn't always better. In that particular design I think print quality would be better served by building a more rigid frame than by adding extra motors. What advantage do the extra motors, cables, drivers etc. confer that justifies their cost and trouble to install?
Linear guides are very intolerant of misalignment. Using 4 linear guides in Z is at least two more than needed and will be a nightmare to align and keep aligned when the frame warms up during a long print.
If you consider the machine itself a work of geek art/sculpture, and your "thing" is to unnecessarily complicate things as much as possible, why not throw in a bunch of extra motors?
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As I said, on the delta example it's just out of curiosity if it could be done reliably with any benefit.
On the second one however we're talking about moving mass on that axis. Like 1kg+ worth of mass if you'd factor in an dual direct drive IDEX.
That mass wants to get accelerated and decelerated. That may be something that could actually need some additional oomph.
Not talking about just throwing in additional motors just for the sake of it - I'm trying to figure out why basically nobody is moving a gantry (except on Z) with more than 1 on a cartesian cube.
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@bberger said in Dual Motors per axis:
........................ - I'm trying to figure out why basically nobody is moving a gantry (except on Z) with more than 1 on a cartesian cube.
Some people are using multiple motors per axis - you wouldn't be the first.
Mark has already pointed out the downsides of cost and complexity as well as the fact that motors will jump to some multiple of full steps whenever power is appied, which might be either forward or backward. This in turn means that once you start using multiple motors per axis, unless they are belted together then you have to add multiple end stops and go through some extra homing moves before you can start a print. Which adds more cost and complexity. Connecting motors by belts doesn't necessarily eliminate all the problems either.
But there are instances where some people use multiple motors on an X or Y axis. One that comes to mind is where the head moves in Y (rather than the bed). A single motor dragging one end of the X rails might cause them to twist if the joint between the X rails and the carriage isn't rigid enough. This can be difficult to achieve with very long X rails so putting a motor at either end might be a solution. But as stated above, because the motors can get out of sync, if you go this route then it becomes necessary to have end stops at either end and "square" the axes every time you cycle power.
It's one of the reasons why CoreXY is such an elegant design because for anything other than 45 degree moves, two motors are always employed. And on those 45 degree moves when only one motor is employed, the other side of the gantry is held in place by the "static" motor and belts.
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@bberger said in Dual Motors per axis:
I'm not too much into CoreXY
It's become popular because it solves a lot of mechanical problems in an elegant way.
Perhaps dual motors would be more common if we used closed loop motor control so that the motors getting out of sync wouldn't be as much of an issue.
In very large moving gantry style printers (think room sized) multiple motors are common because of the mass being moved, but in those cases the downsides are overcome by the necessity. In a desktop sized printer there are more efficient ways of doing things now compared to the makerbot/ultimaker style cartesian.
CoreXY survives occam's razor much better.
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@bberger said in Dual Motors per axis:
As I said, on the delta example it's just out of curiosity if it could be done reliably with any benefit.
On the second one however we're talking about moving mass on that axis. Like 1kg+ worth of mass if you'd factor in an dual direct drive IDEX.
That mass wants to get accelerated and decelerated. That may be something that could actually need some additional oomph.
Not talking about just throwing in additional motors just for the sake of it - I'm trying to figure out why basically nobody is moving a gantry (except on Z) with more than 1 on a cartesian cube.
On Z at least 3 motors allows for true bed leveling. It seems like more than 1 motor for Y and Z is over constraint.
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@tekstyle said in Dual Motors per axis:
more than 1 motor for Y and Z is over constraint.
Y and
ZXYES.
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@tekstyle What is "true" leveling? Is it a bed that (needs to) adjusts itself every time you power up the printer, or is it a bed that doesn't go out of level in the first place? I think a lot of people think they want autoleveling, but what they really want is to not have to level the bed themselves before every print. If you use multiple motors, you have to use some autoleveling scheme because you used multiple motors. If you use a single motor to drive multiple screws or belts, they will remain in sync and once the bed is leveled, it will remain level without requiring extra motors, cables, drivers, sensors, etc.
