Motion Assistant and contouring mode issue

[Tulio]

We need to control an XY table like machine that has a third angular axis. The XY axes are orthogonal.
Attached to this third axis is a cutting blade. The side of the blade must always be positioned tangentially to the XY trajectory. The input data are XY coordinates and we calculate the angle for each one of those XY coordinates. We are using Motion Assistant to generate data points (from those XY coordinates and angle) to use with PCI 7344 contouring mode.
In some shapes (like the ones that have a round corner with small radius), the data points obtained from Motion Assistant results in a shape that is deformed in those corners. If the angle is not used in the coordinates passed to Motion Assistant the shape is ok.

Is the spl
ine algorithm implemented in Motion Assistant is adequate to this task (use to interpolate two orthogonal axes and one angular axis)?

If not, could you give us some directions on how to accomplish this?

Thanks!

[Scott_R]

Hello,

What do you mean when you say that the data points are "deformed"? What version of NI-Motion are you using? If you haven't already, I suggest upgrading your version of Motion to 6.1.5. To do this, first upgrade to 6.1.2 and then run the upgrade to 6.1.5. Version 6.1.5 repairs an error in contouring for NI-Motion version 6.1.2. You can find both of these upgrades at:

Motion Control - Support
http://digital.ni.com/softlib.nsf/webcategories/85256410006C055586256BE60070AC71?opendocument&node=132060_US

Try running those upgrades and let me know if you have any additional questions on this issue.

Regards,
Scott R.
Applications Engineer
National Instruments

[Tulio]

Hello Scott,

I have upgraded to Motion 6.1.5 but there is no change.

Please look at the attached file FigA.jpg. FigA is the output of the machine. The marked area is the "deformation" that I mention. FigB.jpg is an example of what it should be. FigB is a top view and was made in a CAD software that support cubic spline drawing using the same data points that goes to NI 7344 buffer.

In FigC.jpg there is a side view. The green line is that same cubic spline in FigB, but here we can view the "deformation". The red line is an linear interpolation of the same data points. In those pictures, the Z axis is represented as the height, but in the machine this is an angular axis (or a rotary axis) that is fixed perpendicular to XY axes.

The first problem is that the green line should have the same shape of the red line.
Second problem is that the deformation is affecting the XY axes (as seen in FigA). Since the Z axis is an angular axis, it should be not happening. I think that this deformation may be caused by the NI 7344 contouring mode because I can´t verify this deformation in the top view drawings.

Based on this:

1) Can I use Motion Assintant spline algorithm to my needs ?
2) Why I'm getting those deformations ?
3) Any alternatives ?

Thanks !

[MarcoPolo5]

Tulio –

The deformations you are seeing might be a result of not having sufficient data points for the cubic spline algorithm that Contouring uses. The cubic spline algorithm will try to create the smoothest curve through a number of given points. Evenly distributed points tend to give you the best outcome (unwanted peaks and valleys occur with unevenly spaced and sparse data points), but a higher density of points will be needed in tight corners.

You can see this behavior with the Java app at the following website:
http://www.cse.unsw.edu.au/~lambert/splines/natcubic.html
Notice how when trying to make a tight corner, you must place more points to get the desired curve.

Double check that you CAD program is exporting a large enough array of points to produce the correct behavior. This also might be the same problem you are seeing with your green wavy line that should match up with the red linear interpolation line of your Z-axis picture.

Let me know if you are still seeing problems with this behavior.

[Tulio]

Maybe a I haven't explained the problem clearly ... Please forgive my poor english !

I'm not using a CAD software to export the data points. I'm just using the CAD to visualize the data points (as I tried to explain in the previous messages of these thread). The data points that are plotted in CAD and the ones that goes to PCI 7344 are the same. They are generated using Motion Assistant. I'm just importing those data point in my CAD software.

The lack of points in my original curve (before Motion Assistant spline interpolation) doesn't explain the deformation completely. I fully understand your explanation. But the lack of points only explain de curve in the Z axis (FigC, green line), not in the XY plane (FigB, top view of XY plane. Here, there is no deformation in the XY coordinates.) Somehow, the curve in the Z axis is causing a deformation in the X and Y axis, but only in the output of the machine (I mean, the PCI 7344 contouring mode). There is no deformation in Motion Assistant visualization neither in CAD.

I've done some more trials ... In one of them I only replace the Z coordinate with zero in the PCI 7344 contouring mode buffer and the deformation in XY axes disappear !

About introducing additional points in tight corners of the curve we are doing this for some time. In these particular problem it only minimizes the deformation in the XY axes (but the remaining deformation is still unacceptable). We tried new points that are apart 1 millimeter from another and the deformation is still unacceptable.

I hope that this time I could explain better our problem.

[MarcoPolo5]

Tulio –

Is your system use steppers or servos? Open or closed loop? How are you calculating the tangential direction of your cutter? Are you importing a text file of all these points into a 3D contour move? Do you see the deformation in Measurement and Automation Explorer under your 2D Interactive Position Plot tab while the system is running?

In your Contoured Move, have you tried checking the “Do not apply move constraints to the curve”? (located in the ‘Contouring Parameters’ tab of the Contoured Move setup) This will cause Motion Assistant to not change any position points that it would normally change in order to meet the velocity/accel/decel constraints that are set.