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MOTION CONTROL | ARTICLE << Figure 2: Example of (a) the ALIO Hybrid


Hexapod, and (b) the six degrees of freedom of motion capable with the Hybrid Hexapod. >>


b


a


The tripod parallel kinematic structure offers both excellent Z stiffness from motors aligned in each link vertically, and excellent XY stiffness from mechanical stiffness of the tripod link joints. Specifically, the tripod joint between the link and base plate has only one rotational degree of freedom (i.e. hinge). This design allows the link rotation in one direction only, but provides mechanical stiffness in other rotational loading directions. Figure 3b illustrates the links’ ability to withstand lateral loading on a Hybrid Hexapod tripod link. With three mechanically rigid joints positioned 120 degrees apart, the tripod can provide equivalent XY stiffness in any vector direction in the XY plane, even without servo-power.


Accuracy and Repeatability In a traditional hexapod, all six links move for any motion command. Therefore for any single axis move, whether it is a rotation or linear move, the error of the top ring is a summation of the error sources from all six links. This includes errors due to miscalibration of each joint location, backlash, link translation errors, and even servo dither. Furthermore, the complex kinematic structure of the hexapod makes these errors hard to isolate or calibrate, and thus, a hexapod’s accuracy is limited to the 10s of micrometers and repeatability limited to several micrometers.


While the new Hybrid Hexapod link design presented above improves accuracy and repeatability significantly, the hybrid serial and parallel kinematic concept enables motion in each degree of freedom to be performed with the minimal amount of error sources affecting its precision. The three link tripod kinematic structure is simpler and symmetric, and therefore, provides simple methods of calibration and compensation to ensure the Z, pitch and roll motion degrees of freedom can be performed with sub- micron accuracy. The hybrid concept, joining the tripod with monolithic XY and rotary stages, decouples error sources of other axes from affecting the XY and yaw motions. Furthermore, with this simplified hybrid approach, all axes, both linear and rotational, can


be easily calibrated for accuracy and orthogonality to optimise performance in three-dimensional space. As a result, multi-axis motion will also be more precise, because the error sources from each axis, orthogonality, will have all been minimised.


Motion Trajectory/Straightness/Flatness Continuing the discussion above, motion trajectory, or straightness and flatness of motion performance, is relatively poor for hexapods due to the multitude of error sources and difficulty of calibration. In fact, a quick review of hexapod manufacturer specifications will show that virtually none mention straightness or flatness at all in specifications for their hexapods. Specifically, many standard precision hexapods will have straightness on the order of 100 micrometers per 100 millimeters of travel.


Again, the hybrid serial and kinematic approach of the Hybrid Hexapod enables optimised geometric (flatness and straightness) motion errors for all axes. In many applications, the Z, pitch and roll are utilised for alignment of a device or substrate, and a process (such as a raster scan) is performed in the XY plane. The precision XY stage, which is designed specifically for accurate and straight planar motion, can perform the raster scan with straightness error of less than ±1 micron per 100 mm of linear travel — two orders of magnitude better than typical hexapod performance. Non-linear or multi-axis motion trajectories (i.e. circles) are also performed with single-digit or sub-micrometer precision.


System Flexibility and Ranges of Travel Lastly, standard traditional hexapods provide a limited range of travel for all six degrees of freedom for any given design. If an end user requires any more travel in any one axis, an entire new hexapod model or design is required. Additionally, yaw rotation is typically limited to ±45 degrees maximum.


24 | commercial micro manufacturing international Vol 7 No.2


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