Non-orthogonal rotary axes featured in automated fiber placement machine

  • 22-Feb-2013 10:17 EST
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Automated fiber placement machines typically consist of three linear and three rotary axes of motion in order to manufacture complex shapes.

Automated fiber placement (AFP) machines place composite fiber where structural designers need it, and in the orientation required. They produce high-quality tow consolidation while achieving high lay-down rates. For that reason, aerospace manufacturers have increasingly used AFP machines to produce larger and more complex structures.

Large machines with a large working envelope and a wide range of angular motion are required to make large structures. Such machines are expensive and complex, and their shear mass limits the available accelerations needed to change axis positions in the time required to stay on part path during high-speed complex motion.

Machine stiffness and natural frequency, along with mechanical backlash in drive systems and mechanical error during manufacture, contribute to final machine accuracy. Controlling the effects of these parameters during high-speed layup is not difficult on flat or low-contour parts, but it can be quite challenging on high-contour parts.

Traditional AFP kinematic arrangements have the compaction roller attached to three rotary axes (ABC), allowing for any angle to be achieved within the required range of motion. The C-axis provides tow steering while A and B account for compaction axis orientation. These rotary axes are then attached to three linear axes, which translate the compaction roller throughout the working envelope. The standard naming convention is to have the ABC axes rotate about XYZ, respectively.

The compaction roller is where the carbon fiber is applied to the part surface and is considered the tool center point (TCP). As the compaction axis orientation-angle range is increased, it becomes more and more difficult to pass the rotary axes through the TCP. When the rotary axes do not pass through the TCP, they cause a translation of the TCP when they are rotated. This translation occurs regardless of whether the axis of rotation is ahead of or behind the TCP and must be compensated for by moving one or more of the linear axes. This requires the machine to sweep out a larger envelope than the specified TCP operating envelope.

The additional travel increases the machine cost and mass. If one is going to maximize machine stiffness, speed, and acceleration, the worst possible place to add mass is out near the tool point. Any increase in travel of an axis adds mass, increases cantilevers, increases moment loads, and requires all supporting structures and axis drive systems between that axis and the foundation to increase accordingly.

An even more critical component of machine controllability is the kinematics required to keep the compaction roller on path and normal to the part surface. When the rotary axes do not coincide with the TCP, the machine will be required to reverse direction as it traverses various part contours. These reversals prevent smooth high-speed machine motion.

Presented here by engineers from Electroimpact is a kinematic machine design unique in the AFP industry where the orthogonal rotary axes are replaced with three TCP-intersecting coupled-axes which decouple the linear axes from the rotary axes. The three coupled non-orthogonal rotary axes all pass through the TPC. To differentiate these axes from the standard orthogonal axes, the engineers named them J1, J2, and J3.

Manipulating any or all of these rotary axes changes only the steering direction and/or compaction axis angle while having zero effect on the TCP location. Therefore, the linear XYZ axes do not have to move during a rotational axis change. It is very desirable for the linear axes to be decoupled from the rotary axes for smooth machine kinematics. With this arrangement, the lay-down rate can remain high over high-contour surfaces because the linear axes never reverse direction during a change in compaction axis normality.

The rotary axes linkage of this machine was designed to attain any angle between ±20° of true “A” rotation and ±45° of true “B” rotation. The individual angles were also designed to remove a kinematic singularity from the working range of motion. A rotary axis kinematic singularity will cause unsmooth machine motion much in the same way as a linear axis reversal.

Another consideration in the design of this linkage was the mass and stiffness of each member. Electroimpact uses a quick-change fully contained modular process head on their AFP machines. The linkage was designed around the process head to maintain part clearance, and great effort was put into reducing the size and mass of each member. In addition, the angles of linkage design shifted the combined moving-mass as far back as possible toward the machine foundation.

Kinematic compensation was applied to the finished machine and a laser tracker used to measure machine accuracy throughout the working envelope. Then forward kinematic predictions were compared with the laser tracker measured values, and a multivariable iterative solver was used to minimize the difference. The results are an accurate AFP machine for the entire range of motion.

Electroimpact’s latest AFP machine allows for high-speed composite fiber lay-down rates over highly contoured complex part molds by utilizing unique machine kinematics, which eliminates major machine reversals and allows for smooth on-part motion.

This article is based on SAE International technical paper 2012-01-1862 by Guy Faubion and Todd Rudberg of Electroimpact Inc.

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