SAMPE Journal - July/August 2022 - 23

the insertion needle stitch pattern depends on whether the curve is convex or
concave as discussed previously.
Each stitch progresses from its starting point to the ending point, which is
the starting point for the next stitch (with the exception of the last stitch that
ends at the stitching path end point). Using the set of calculated stitch points,
a direction vector from the starting point to ending point for each stitch is
calculated and denoted as (Vi
) for the ith
stitch. This direction vector is coincident
with the local tool point coordinate system y-axis during the stitching process
and represents the direction that the stitching head will traverse during the
stitch cycle for each stitch. In addition to the orientation vector for each stitch,
the tool point coordinate system z-axis and x-axis orientations are calculated
for each stitch at the stitch starting point, with the z-axis being coincident with
the surface normal at the stitch starting point. The local tool point coordinate
system axis vectors are defined in the global coordinate system used by the
control system to control the head motion during the stitching process. Given
the stitching path starting point and the local tool point coordinate system for
the first stitch, the translational and rotational positions for the head at the
start of the stitching process for the stitch path are calculated.
Programming begins by writing out the code for the stitching path starting
point calculated translational and rotational positions. Also written out on
each line of code are the cumulative path length and the drive axis distance
that are both set to zero for the starting point. The drive axis distance is defined
as a distance that identifies when full rotations of the drive motor occur, and
it is determined by the stitch length. That is, when the head moves a distance
equal to the stitch length, the drive motor completes a full rotation and
operates the stitching mechanism through an entire stitch formation process.
Programming continues for each stitch by defining the code for the end point
of the stitch (path ending point for the last stitch) and consists of two lines
of code for each stitch. Prior to writing out the two lines of code for a stitch,
the Euler angles required for rotating the local tool point coordinate system
at the starting point of the current stitch to be coincident with the local tool
point coordinate system at the starting point of the next stitch are calculated.
These Euler angles represent the total rotation of the head that occurs from the
start of the stitch to the end of the stitch. Next, the first line of code for a stitch
consists of the global translational position of the stitch end point replacing the
values in the previous line of code. The rotational position values on the first
line of stitch code are identical to the previous line of code, indicating that only
translation of the head occurs with no rotation. The drive axis and the stitching
path distances on the first line of stitch code are incremented by the stitch
length, so that for the nth
distances are defined as n x ls
stitch, the drive axis and stitching path cumulative
. The second line of code is written out with the
translational position values, the drive axis distance and the stitching path
distance equal to those on the first line of stitch code. The rotational positions
of the second line of stitch code are calculated by adjusting the rotational
values on the first line by the calculated Euler angles for the stitch. The second
line of code is purely a rotation of the head after the stitch translation occurs
in order to line up the head for stitching of the next stitch. Therefore, the two
lines of code represent the uncoupling of the stitch translation that occurs in
the first line of stitch code and the total rotation of the stitch that occurs in the
second line of stitch code. Generation of these two lines of code for each stitch
progresses until the final stitch lines of code are generated.
The process described provides the coding necessary to decouple the
translation and rotation for each stitch to ensure that the problems associated
with rotations of the head while the stitching needles are in the preform do
not occur. During the course of stitching trials, it has been seen that if the
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SAMPE Journal - July/August 2022

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