Multi-axis motion can be accomplished using classical stacks of individual linear and rotary stages (serial kinematics) or with a more compact and elegant parallel kinematics approach. Two basic designs are common: Hexapod positioners (Greek for “six-legged”) and planar parallel-positioners (SpaceFabs). Hexapods are also known as Stewart Gough platforms.
A particularly demanding application is low-temperature spectroscopy utilizing the ultra-short-wavelength probe beam provided by large synchrotron accelerators. These are the most brilliant light sources on the planet and are instrumental in delving ever deeper into the structural intricacies of micro-electronic materials and biological specimens. Often significant travels in multiple degrees of freedom are necessary. A popular approach to providing stable, high-resolution, 6-DOF positioning in these and other applications is the novel planar parallel positioner (SpaceFab) mechanism manufactured by PImiCos of Eschbach, Germany. These are tri-strut assemblies with fixed-length struts actuated in a common plane by three XY stacks of ultraprecision stages. This assembly provides long transverse travel in the XY plane together with precision adjustability in the three angular axes and Z via differential actuation of the stage stacks. A noteworthy capability of this mechanism is the option to place the rotation centerpoint anywhere in space, such as a focal point or other optical sweet spot. With this flexible, modular and extensible principle, a wide variety of motion technologies can be utilized, including UHV-compatible magnetic and piezo stepper motors.
Hexapods offer less planar XY travel than a SpaceFab but even higher stiffness, greater load capacity, larger angular range and a smaller surface area. Benefits in common with SpaceFab include the virtual rotational centerpoint. Some manufacturers integrate especially tidy cable management that eliminates moving/sweeping cables. An example is PI’s M-824 Vacuum hexapod, a 6-Axis micropositioning and alignment system which provides high resolution submicron motion in six degrees of freedom. It is designed to work in a vacuum and provides up to 45 mm (linear) and 25 degrees (rotation).
The greatest hexapod advantage is their compact size compared to conventional six-axis stage stacks, a fact that is especially beneficial in applications in vacuum chambers, where space is at a premium. Many have through-apertures as well, which facilitates optical designs. Typical applications focus on semiconductor technology, multi-axis alignment of optics, X-ray microscopy, and X-ray monochromators.
Thermal Management and Outgassing
Thermal management is key in vacuum systems because they cannot rely on convection or fans for the removal of heat from the motors, electronic subsystems and bearings. Without careful thermal management, stage performance, reliability and life can be reduced significantly. Development of thermal isolation methods and passive and active cooling techniques help to maximize conduction modes of cooling and reduce or eliminate heat sources inside the chamber.
Similarly, outgassing is a challenge to creating and maintaining clean high-vacuum environments. NASA maintains a list of low-outgassing materials (http://outgassing.nasa.gov) to be used for spacecraft, as outgassing products can condense onto optical elements, thermal radiators, or solar cells and obscure them. Materials not normally considered absorbent can release enough light-weight molecules to interfere with industrial or scientific vacuum processes. Even metals and glass can release gases from cracks or impurities, but moisture, sealants, lubricants, and adhesives are the most common sources. The rate of outgassing increases at higher temperatures because the vapor pressure and rate of chemical reaction increases. For most solid materials, the method of manufacture and preparation can reduce the level of outgassing significantly. Cleaning surfaces, or baking individual components or the entire assembly before use, can drive off volatiles.