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April, 2011
Sub-Micron Resolution from New Generation of Hexapods
By Jim McMahon, Zebra Communications H
exapods were first introduced to the public in the late 1800s and first employed in an
industrial application in the late 1940s, when one was used on a tire testing machine. Six-axes parallel kinematic posi-
tioning systems (hexapods) used in robotics assembly cells made their debut in 1954. Since then, with the advent of improved technologies in robotics, servo-motors, and control systems, parallel manipulators have been under continual evolution. Within the past decade, with
advances in computer programming capabilities, hexapods have taken a major step forward with increased accuracy and precision. But in the past several years, with the integra- tion of advances in technologies like piezo actuators, virtual programma- ble pivot points and system simula- tion, hexapod performance has taken a significant leap forward, to the extent that electromechanical and piezo electric hexapods are now viewed as more efficient and accurate systems compared to serial linkage and hydraulic kinematic positioning systems, including those commonly used on robotic arms. The latest versions of hexapods
are capable of delivering unprecedent- ed ultra-precision resolution to as low as 0.03µ (33nm) of incremental motion, and some can provide velocity
of several 10mm/s, making them a preferred robotics system for industri- al testing and positioning, microscopy, semiconductor handling, biotechnolo- gy and medical applications. A hexapod, also known as a
Stewart platform or Gough/Stewart
base platform. With six DOF the movable platform is capable of mov- ing in the three linear directions — x, y, z (lateral, longitudinal and verti- cal) — and the three angular direc- tions (pitch, roll and yaw) singularly or in any combination. Because hexa-
and are completely free of backlash. However, load capacity and travel range are rather small compared to cardanic or sphere joints. Cardanic joints with Z-offset
provide the best load characteristics. These joints are much stiffer than gimbal-type joints, however they require more sophisticated control algorithms to take into account the complex geometry of each actuator. Hexapods come in many config-
Square hexapod with large aperture.
platform, is a six-legged parallel mechanical structure. In its most common form, it consists of two plat- forms, one fixed and the other mov- able, which are connected and sup- ported by six actuator legs (struts) that expand and contract, acting in parallel between them. Coordinated motion of these six struts enables the movable platform, and devices mounted to it, to move in any direc- tion, operating with six degrees of freedom (DOF) relative to the other
pods have all six degrees of freedom, they can perform manipulations that cannot be done with any other tradi- tional motion system. In addition to the variable strut
length hexapod of the Gough/Stewart flavor, other designs with fixed strut length are also available.
Cardanic Joints Most hexapod struts are equip -
ped with cardanic joints. Flexure joints provide the highest precision
urations and sizes, capable of han- dling loads from a few pounds to more than two tons. Advanced designs include servo motor-driven systems for moving large optics or mirrors, piezo-based units for nanometer precision control of pro - cesses, and non-magnetic and vacu- um-compatible versions. While most hexapods have a cylindrical shape, square or rectangular versions have also been used. Hexapods have a tremendous
potential for streamlining many manufacturing processes by improv- ing accuracy and speed, and reducing set-up and processing time.
Benefits of Hexapods Parallel-kinematic mechanism
(PKM) motion systems have a num- ber of advantages over standard seri- al kinematic (stacked) positioning systems. It is easy to see most bene-
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