Custom control system incorporates ceramic motors

The STFC's ISIS Neutron Source recently awarded a contract to Heason Technology Limited to provide neutron beam modifying Jaw Packages for the Wish and nimrod instruments on its new Second Target Station, which is due to be fully operational by October 2008.

ISIS uses neutron scattering as a powerful non-destructive tool that can penetrate and reveal the molecular structure of materials using beams of neutrons and extremely sensitive detectors. Wish and nimrod are two of several instruments in TS2 (the second Target Station project) that use a variety of techniques to allow scientists to understand the atomic structure of materials by measuring the spacing of atoms and the forces between them. Building upon the huge contribution the facility has made to science over several decades, the ISIS Second Target Station will create new opportunities with particular emphasis on soft matter, bio-molecular science and advanced materials.

Both Wish and nimrod use a technique known as magnetic diffraction but are suited to different types of materials. Wish is aimed at the study of the large crystaline structures of solid materials and nimrod is for liquids and surface studies. For these instruments, the Jaw Packages are essentially motorised blades that modify the face area of the neutron beam perpendicular to the target to tune its resolution and divergence to the optimal frequency required for the materials under test - which varies considerably between solids and liquids as well as for differing test temperatures and environments. Several sets of large and small Jaw Packages, each with four motorised iron boron/nimonic steel blades are typically used depending upon the characteristics of the material to be tested.

Heason's challenge

While the objective of these instruments is highly technical and the environmental restrictions - which include low vacuum, zero tolerance of magnetic materials and extremes of temperature - are somewhat demanding, Heason accepts such challenges as part of its customised design and engineering service. Working closely with global partners and with its own team of specialist engineers, the company has developed a specialist service to provide the enabling motion system technologies for a broad base of applications in aerospace, defence, printing, packaging, semiconductors, photonics, simulation and robotics.

The design specification for both applications called for the construction materials to be aluminium, non magnetic stainless steel or plastic. Ceramic servo motors were a natural choice for positioning each of the four blades in the Jaw package assembly. Supplied by NanoMotion, an exclusive distribution partner of Heason's and with a legacy of demanding nanometre-scale positioning applications, the technology is completely non-magnetic and suits vacuum environments. Each blade was driven with two synchronised ceramic motors, configured with either four or eight elements depending upon the load and force required for the Jaw Package size. A further benefit of the ceramic motor is its ability to hold and lock position with zero position shift when power is removed.

As part of the Variohm Holdings Group, Heason's designers called upon its sister company, Variohm EuroSensor, to provide linear potentiometers for jaw position feedback. Swiss manufacturer Novotechnik was selected for its experience of conductive plastic technology potentiometers used in similar harsh environments. Novotechnik has pioneered the development of this materials technology, which has proven to be critical to the development of the modern linear displacement transducer. These transducers employ conductive plastic resistance and collector tracks to provide direct, absolute measurement of displacement or length in control, regulation, measuring, and laboratory applications. With the resistance output providing absolute position measurement, there was no need to include datum switches to home each axis, which reduced set-up time and simplified control.

Each blade was supported and retained in a special holder with a quick-release mechanism to allow for simple in-field blade replacement. To provide support for each jaw axis, special polymer bearings with a PTFE liner offered a non-magnetic alternative to standard steel bushings.

Dynamic load calculations and compound axis motor sizing were carried out to ensure that the motors and complete assembly were able to meet and exceed the bearing friction and stage loading for the life and duty cycle requirements for the application. This also involved development work carried out at Heason's manufacturing and assembly facility to evaluate materials and designs.

For more information about Heason's customisation services, visit www.heasonmotionsolutions.com/.