NI Motion Arcs in X, Y plane with different reduction

[yle]

Hello all I'm using a 7356 motion board to control a CNC milling machine. While implementing the arc move, I noticed that the radius parameter is passed to the Load Circular Arc VI in counts (steps). I was wondering what would be the result of an arc move, in an orthogonal XY plane, if reduction for the X and Y axes is not the same. Wouldn't this create an elliptic move instead? Does this restriction imply that the arc move may not be performed unless the XY plane use the same steps/rev and the same reduction for both axes? Thank you Fares

[Adam_H]

Hi yle,

If you are using motors with different steps/rev then yes this would cause an elliptical pattern if you send them the same number of steps to move.  I am not entirely sure if this answers all of your question however.  If it does not could you please explain a little more what you mean?

[yle]

Hello Adam

Thank you for your reply.

You did answer my question.

This, however, does not qualify as a normal behavior for a motion controller...

Most milling machines do not meet this criteria, i.e. identical reduction ratios for the X and Y axes.

Obviously, this means that NI's motion controller line of products will not drive a CNC mill-style machine in most cases, which is awkward to say the least, knowing that this functionality is usually native, even in any low-end CNC controller

Thanks again
Fares

[Adam_H]

Hi yle,

Before we say that for sure let me make sure that I understood your question.  I took you question to mean that you have two motors with different steps/rev ratings.  If you send these two motors the same number of steps they will obviously move a different number of revolutions.  For example if you have motor A that is 1000 steps/rev and motor B that is 2000 steps/rev.  If the controller tells both motors to move 1000 steps then motor A will move one revolution and motor B will move half a revolution.  This is the basis that I used to answer your question.  I am not entirely sure what you mean by reduction.   Could you clarify exactly what you mean by that. 

[yle]

Hello Adam

Different reduction ratio is equivalent to different steps per revolution.
In short, the same counts on X and Y will translate to different travel distances.

In NI-Motion, arc moves are programmed as starting angle, target angle, and radius.
The arc will start at the current position and figure out where the center is based on the starting angle and radius.
Since there is no way of specifying the circle in terms of physical units (e.g. mm) , the controller will create the circle based on the input in counts.
And since counts in the X direction are different than counts in the Y direction, we will end up with an ellipse instead of a circle.
In other terms, there is no way of tracing a circle unless we have the exact same configuration for the X and Y axes.

If the above is correct, NI's motion controllers cannot draw a simple arc, unless both directions of travel are the same, i.e. same motors, same encoders, same gearbox, same leadscrew, same pulleys etc... That's what I find odd, because this criteria is not met in many machines, especially mill-style machines.

Regards
Fares

[Adam_H]

You are correct this is how the arc moves work.  I think the best work around for you would be to program it as an ellipse move taking into account the difference between the two motor systems to produce a circle.   You can take a look at the Two-Axis Ellipse example.

[yle]

Hello Adam

Yes this would be a workaround to produce the arc.
Well, unless you're limited on the number of axes.
Also in case you're executing a series of blends within the same vector space, where you'll have to stop motion, reconfigure the vector space and the gear ratio before executing the arc move and restore the previous configuration to continue .

Thanks again
Fares

[Adam_H]

Good point.  That would be a limitation of it for sure.  Sorry there is not a better solution for you.