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driven by mechanical contact mechanisms, such as a micrometer leadscrew paired with a rotary motor or a friction drive motor (typically piezoelectric motor), which are not considered high precision motor solutions and introduce heat gradients and vibration into the links. Additionally, screw driven actuators are characterised by backlash and employ rotary encoders from which linear position is calculated, not measured. Furthermore, as any hexapod moves, the forces on each joint of the actuator will vary greatly in magnitude and angular direction. The link designs, some of which are simply off-the-shelf linear actuators, are not designed to maintain the location of the end joint of the cantilevered actuator with high precision under such varied loading. The varying loads can also cause motor forces, and thus motor heat generation, to vary drastically, adversely affecting precision. Most hexapod links or actuators would not be used in a precision single axis stage, for the reasons mentioned, but when coupled together in the form of a hexapod they are billed as a precise six degree of freedom motion system.

The new Hybrid Hexapod links were designed from the ground up with optimum precision as the primary focus. The links utilise brushless, non-contact, linear servo-motors oriented along the link axis eliminating any mechanical coupling. There is no friction or wear to adversely affect precision or create backlash, while minimising vibration and heat generation. Heat variation is minimised by coupling the motor with near frictionless pneumatic cylinders (or non-contact magnetic springs) in each link to counter

balance (i.e. zero-out) the strut load against gravity. This allows for high payload capabilities. Aside from the inertia of the payload, there is very little force (and thus little heat) that the motor needs to generate to move the system because the counterbalance supports the mass. Lastly, all axes of the Hybrid Hexapod use non- contact optical linear encoders that eliminate backlash and rotary encoder errors. Optimised sensor locations ensure the position feedback reflects the actual position of each link/axis. The Hybrid Hexapod link design provides <50 nm repeatability and sub- micron accuracy with higher speed and payload capabilities, making it the ideal link design for a tripod (not hexapod) parallel kinematic structure.

Stiffness The tripod structure was selected for its stiffness benefits compared to traditional hexapods. Traditional hexapod’s high Z stiffness is well publicised, and is a result of all six links being oriented in the near vertical angle. However, the XY (horizontal) stiffness is relatively weak. A review of hexapod manufacturers’ websites will show XY to Z stiffness ratios ranging from 1:10 to 1:32. Poor horizontal stiffness, combined with links that carry varying loads at varying angles, correlates to poor repeatability and positioning performance of the common hexapod top ring. A motion system in three-dimensional space needs to have high and equivalent stiffness in all directions to be able to supply precise motion in all directions.

>> Continued on page 24

23 | commercial micro manufacturing international Vol 7 No.2

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