Parallel-Kinematic Precision
Positioning Systems
Six Axes of Motion with Hexapods and SpaceFAB

Compact Positioning Systemwith Six Degrees of Freedom
Hexapod platforms are used for moving andprecision positioning, aligning and displacing
loads in all six degrees of freedom, i.e. threelinear and three rotational axes.
Hexapods have a parallel-kinematic structure, i.e. the six drives act together on a single moving platform. The length of the single
drives can be changed so that the systemmoves in all six degrees of freedom in space.
This special Hexapod design optimizes theoverall system stiffness and allows for a large
central aperture.
Precise Positioning Even of Heavy Loads
Depending on their design, Hexapods canposition loads from several kg up to several
tons in any spatial orientation, in other words
independently of the mounting orientation andwith submicrometer precision.

Advantages over Serial-Kinematic Design
Hexapods can be designed considerably morecompact than serially stacked multi-axis positioning systems. Since only a single platform,
most often provided with a large aperture, isactuated, the moving mass of the Hexapod issignificantly smaller. This results in improved
dynamics with considerably faster response.
Furthermore, cabling is no issue, so that no
additional forces and torques reduce the accuracy.
In case of stacked systems, the lower axes
not only move the mass of the payload but
also the mass all other following drives. This
reduces the dynamic properties and the totalsystem stiffness. Moreover, the runouts of theindividual axes add up to a lower accuracy andrepeatability.

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Cardanic joints of the H-840
Hexapod model

Matching Components for High Precision

JointsThe basis is a zero-backlash structure and carefully selected and matching components. This
includes first of all the right material selection,
when e.g. thermal effects are to be expected
at the place of operation. The motor, if necessary with gearhead, an integrated guiding, theleadscrew/nut unit, as well as the joints forthe required load range up to high-resolution
position detection in every strut – all these elements determine the achievable precision.

Hexapods for precision positioning often have
Cardanic joints with two orthogonally arranged
axes. This is the optimum combination of two
degrees of freedom and the stiffness of thestructure.

Motors and Drives

Ball-and-socket joints

PI Hexapods are based on electromechanicaldrives and are much more accurate than thehydraulic Hexapods known from flight or driving simulators. Precision leadscrew drives or
piezo linear motors are used. Most systems
are self-locking. Direct-drive Hexapods ensure
higher velocities; for industrial use, brushless
motors (BLDC) are particularly suitable.

Ball-and-socket joints offer more degrees offreedom in a relatively simple design. However, the overall stiffness and precision incase of external loads and torque can suffer.
A compensating preload is recommended but
requires drives with high output forces such as
the NEXLINE® piezomotor drives.
If the highest precision and few linear bending
displacements and angles are required, flexurejoints are recommended. They exhibit neither
friction nor backlash and do not require lubricants.

The application determines the drive technologies: Hexapods with piezoelectric PiezoWalk®
stepping drives are suitable for ultrahigh vacuum applications and can also be operated invery strong magnetic fields.

The positioning accuracy of a precision H-824 Hexapod in Z direction over the complete travel range of 25 mm is afew micrometers, and the repeatability is considerably below ±0.1 μm

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|POSITIONING

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TEC73 Parallel-Kinematic Precision Positioning Systems 10/2017 Subject to change without notice. © Physik Instrumente (PI) GmbH & Co. KG 2017

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