When you build a moving ganrty machine utilizing a rack and pinion drive, how do you wire the two stepper motors that drive the x axis or do you have to us an additional driver?
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When you build a moving ganrty machine utilizing a rack and pinion drive, how do you wire the two stepper motors that drive the x axis or do you have to us an additional driver?
hand1,
Each motor is wired to it's own drive. One motor would be the X-Axis and the other would be the A-Axis and be "slaved" to the X-Axis in Mach3.
Randy,
Randy,does one motor have to rotate clockwise.the other counter clockwise?
DIYaholic,
Thanks for the help.
Chris
I run a moving gantry, 61" x 100", rack and pinion drive X and Y, router with two large, 5 amp stepper motors on a 10 amp drive. When I checked with the drive manufacturer's tech, he informed me this was OK. It has run for years perfectly fine like this, and can rapid traverse at 1200 IPM. One issue I discovered was the gantry cross beam was not machined perfectly square, which caused the two motors to have to fight the gantry "square". Once shimmed, it ran much better, and skipped fewer steps. The two motors run opposite directions of each other.
Jim
Good question. I have not built a machine yet, just passing on what little I have learned here in the Zone". I suppose, it actually depends upon how you set up your machine (many non-standard setups have been done), but cheetahcnc posted that they do run opposite directions. I don't know how Mach3 deals with "slaving" motors. Perhaps someone wiser than me can chime in.Quote:
Originally Posted by lgalla
Randy,
RANDY come to think about it,would you reverse the wireing on one motor or would Mach take care of the slave?
Larry
lgalla,
I'll let "The Great CarveOne" answer that:
http://www.cnczone.com/forums/diy-cn..._question.html. I didn't read all of it but the question is answered.
Randy,
as cheeta said, yes, you can run multiple steppers on one drive. if both go same dir, then just wire them in parallel to the drive output. If opposite directions as Igalla said to to look for, then one must be wired to go opposite direction.
If the drives are regulating current I would imagine you would want a drive per motor. Otherwise, one motor may not get the current it requires. You would need two perfectly matched stepper motors if you want to ensure they will both take the same current and generate proper magnetic fields. It is hard to believe you will ever get that. At the same time, however, it is hard to believe differences will be so large that the motors will behave abnormally. Somebody mentioned that's how they are doing it, so in some cases it clearly works!
On the other hand, I don't think you need the A axis to drive the second stepper. You can have the output of the X axis to drive both stepper drivers. They will both react at the same time as STEP impulses are registered. If you ever need to re-synchronize both steppers (say one of them lost steps whereas the other didn't) you may need to connect the stepper drivers to different axis outputs, clear up the error in travel, and then reconnect to the existing setup again. So if you are not using the A Axis at all, then it may be prudent to separate the two motors on a per axis basis as stated before.
I had assume both motors were same part number when you said u wanted to run 2 from 1 drive. Obviously if they are not the same part no then you dont want to run 2 in parallel on 1 drive. If same part no then you too can run mutliple on 1 drive as many others have and do today. Of course if one looses steps they and they need to be in synch you are in trouble. Of course you cannot gaurantee exactly same current into both so one may burn up. But if the idea is to be really cheap and do this you can do it same as many others do today. If you need to be sure no missed steps, if you need to be sure to not overheat one, etc., then you can buy a couple servo motors and drives and do it right with none of these questions, but is will cost you more. Trade offs one and all.Quote:
Originally Posted by avayan
I may be being a purist here, but what I meant about two steppers with a single drive not being matched would still apply if the motors are of same part number. Agreed you may never see the problems. But my point is that as you regulate current, the H Bridge will ITrip once it reaches the current peak the H Bridge has been set to.
Both stepper motors will be different, I can assure you of that. There are some tolerances we can not ignore and to expect the windings to be identical from motor to motor is ill fated. What I expect to see is that when the ITrip takes place, one stepper winding will have more current than the other stepper winding in parallel. Hence, the magnetic field will be stronger in one motor than on the other.
Like I said, you may never see the problem. But what may happen (BIG "MAY" here) is that at some point in time one stepper will start loosing steps because its magnetic field was just not strong enough. I would expect this phenomenon to get exacerbated if you are employing any form of microstepping with higher resolutions making it worst.
The solution could be to increase the current regulation value so that more current goes into the system. Take into consideration that now you will be heating your motors more (current will have to go out as heat, since the motion is still pretty much the same), and your hammering effect at slow speeds will increase as well, making resonance a little bit worst.
BTW, if you are using an LR drive (which I think nobody uses anymore), then you are in good shape. Both steppers should get the current they need provided the resistor is sized accordingly.
So to recap, it will most likely work, but it may not work ;-) And because this is a CNC machine where you are striving for perfection, such a "cheap" method may end up being very EXPENSIVE!
Trip? Having two motors on the same drive will double the current but not cause a trip unless you mis-program it....Quote:
Originally Posted by avayan
Guess I don't understand you Itrip thing - sounds like you are saying something like excessive current draw drive trip fault. But no, in reality the same part no motors will be close enough in resistance and will have the same windings so virtually identical inductance, so they will not cause a problem. Again, there are lots of machines doing this. Heck, we even do it with linear servo motors.Quote:
Both stepper motors will be different, I can assure you of that. There are some tolerances we can not ignore and to expect the windings to be identical from motor to motor is ill fated. What I expect to see is that when the ITrip takes place, one stepper winding will have more current than the other stepper winding in parallel. Hence, the magnetic field will be stronger in one motor than on the other.
Most here on the zone are building machines as economically as possible. If you need the A axis of your control for a 4th axis, rotary etc., you would want to pair the X axis motors on the single drive. If you plan to only use three axes, a separate drive is a good choice.Quote:
Originally Posted by avayan
Jim
If you are using current regulation (microstepping uses this technique most likely) this means there is a current chopper that is continuously monitoring the stepper winding current and disabling the H Bridge to ensure a certain current is not exceeded. Per example, if you set your drive unit to 1A peak, the system will charge up the motor inductance until it reaches the 1A and then disable the H Bridge for a small period of time. This process is carried on and this is how the current is limited electronically. Every time the current reaches said current value, an ITrip event occurs.Quote:
Originally Posted by mike_Kilroy
If you have a stepper motor taking 1A from your drive and now parallel another stepper motor, the 1A will be divided among the two steppers. Obviously we all understand this. What we are apparently not seeing is that there is the possibility of one stepper getting 510 mA while the other gets 490 mA. Such a difference seems negligible and if you are full stepping, that will most likely be the case.
However, when you microstep, current grows exponentially from 0A to whatever your peak current is. In this case, there will be microsteps in which the current is so small, the difference in impedance matching may matter.
Again, this only happens on a current chopper (regulated current) system. LR drives should be fine.
It is perfectly fine that there are other machines doing it. I bet the technique will work most of the time. If you are not trying to do anything fancy, with high resolution you may never see the missed steps, if any.Quote:
Originally Posted by mike_Kilroy
Linear servos, on the other hand, are a completely different beast. These motors are not steppers but DC motors (brushed or brushlless) which take advantage on the back EMF making the current usage to be low and only directly proportional to torque, whereas a stepper will use whatever current you give it until the winding is saturated, or the current is regulated. Whichever comes first.
With a Linear Servo, the system will most likely not need to regulate the current, but will PWM the voltage to control the motor speed and position. Hence, you can have as many DC motors as you want until reaching the power stage cap. For example, if you have a 30A H Bridge, you should be able to have 3 DC motors drawing up to 10A each. But at the same time, since the torque is divided by the three motors, the current flowing through the motor is a third.
And since losses on a DC motor are directly proportional to current (due to the internal resistance and I^2*R factor), the lower the current you pass through a DC motor, the more efficient the system is. This would apply to a stepper if you were only putting the current it needs, but dynamic current control with closed loop is still quite rare on the widely available stepper drivers out there. Unless you want to spend thousands on an industrial drives, but this conversation started with the fact that we want to make it cheap, so the point is mega-moot.
BTW! I am in favor of doing the stepper in parallel thing, although I prefer the parallel driver method. Just wanted to point out where a source of error may be in case somebody sees the issue in a future.
My point is that it is safest not to parallel the motors with a single Drive, but to use two drives receiving the same control signals from a single axis. Drives are not that expensive today.Quote:
Originally Posted by cheetahcnc
Now, I understand the idea is to make the machines as economical as possible. But if that implies a possibility of ruining material, I am already shaking. If you are doing MDF and acrylic, then who cares? That stuff is cheap! But if you are cutting fancy wood or aluminum, that stuff is frigging expensive, even on surplus!
Missing steps could mean you are cutting a line too much into the final part. If you are doing multiple passes, they will not align. That is the EXPENSIVE I was referring to.
If you used a belt and run two motors on one belt, Could you not somehow 'switch off' a second motor when the resolution needs the microsteps, as the motor would then act as a pully and the other motor would run microsteps with full accuracy, you could somehow switch it back on when the 'errors' are overseen? i mean go karts use twin motors running on one axis but they use a clutch. Im thinking of running dual motors because i want a table which can use laser, and router ( not at the same time) with belt drive action that encorporates two nema 17's (i have a few of them so i wana use them up) on one axis, the current on my board takes over two of these motors on one drive. Essentially the current wouldnt trip. They are from the same batch and the coil windings would have a difference of resistance in the nano ohms i would suspect, which in that case the rubber belt would take up some nano miss steps. What do you think?
Why can't 2 motors connected to 2 drivers connected to the same axis work? And or what are the limitations/issues?
That's exactly the reason many people buy the G540, so they can slave two motors on the one axis. I would expect that missed steps on one of the axis won't be an issue when using the same model steppers Even if there's a slight inductance or resistance difference, this should be an insignificant factor so long as a reasonably suited power supply is used.Quote:
Originally Posted by Great-Dragon
If you had insanely high inductance steppers, a low power supply, and was trying to cut at break neck speeds, then yeah, it'll miss steps like crazy. Under normal use, and especially if your using Mach3 and tuned everything fine, I wouldn't expect issues. And there's many such builds in these forums.
cheers, Ian