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MOTION CONTROL | ARTICLE a b


<< Figure 1: Example of (a) the ALIO HR2 hexapod model, and (b) the six link parallel kinematic layout common to hexapod motion systems. >>


be mounted to the top plate and be manipulated to be in any location and orientation in the available range of travel.


Hexapods are commonly used because, compared to stacking six individual single-axis stages and the associated stackup of errors, alignment difficulties, and cable management issues, the hexapod will have less error and a higher level of precision in a clean, compact form. Additionally, hexapods have high Z stiffness due to six links/actuators oriented in near vertical orientations.


While hexapod manufacturers are always developing advancements in their products, such as new complex ways to calibrate hexapods or creative alternative position feedback systems, none of those advancements will be able to truly overcome inherent limitations of the hexapod design. Additionally, hexapods are characterised with vague specifications and present hazy situations for engineers, who are told what they want to hear and not the full picture of the actual hexapod precision. These weaknesses, discussed and compared fully below, are not inadequacies of one manufacturer or design compared to another, but are weaknesses and limitations of the hexapod concept in general, and truly present micrometer order limitations on the performance that can be achieved with a hexapod.


Introducing the Hybrid Hexapod and its Advantages The Hybrid Hexapod was developed by ALIO Industries to address the critical weaknesses and limitations of conventional hexapods. The goal was to achieve nanometer order accuracy, nanometer order bidirectional repeatability, and high-integrity flatness and straightness of motion in a six degree of freedom


motion system. The Hybrid Hexapod concept overview is presented, and then its advantages are discussed in detail.


The name Hybrid Hexapod is indicative that the system supplies all the known six degree of freedom motion and existing functionality of a hexapod, but is a hybrid serial kinematic and parallel kinematic structure. It includes a parallel kinematic tripod constructed of three links/actuators generating the Z, pitch, and roll motion. The tripod is integrated with a monolithic serial kinematic XY motion stage and a rotary (yaw) axis mounted into the top of the tripod (or underneath it depending on application needs). In this hybrid design, individual axes can be customised to provide flexible configurations and travel ranges from millimeters to over one meter, while maintaining nanometer levels of precision. The changes did not stop there, however, as the links and structure of the tripod parallel kinematic system were completely redesigned relative to existing hexapod links to enable nanometer order performance of the system.


This hybrid concept remains a simple and elegant solution that draws on the best features of serial and parallel kinematic systems to enable a significant performance improvement over hexapods. The following sections break down some of the specific differences and how they help improve the overall system performance relative to traditional hexapods.


Link Design First, traditional hexapod link or actuator designs are generally designed primarily to support the load against gravity, with precision of the link motion a secondary priority. The motors are


22 | commercial micro manufacturing international Vol 7 No.2


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