Motion control plays part in redefinition of the kilogramme

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Motion control plays part in redefinition of the kilogramme
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Heason Technology has recently completed the design and supply of a UHV compatible and completely non-magnetic 3-axis manipulator, commissioned for use by the German national metrology institute, Physikalisch-Technische Bundesanstalt (PTB), that is being used to microposition a very special one kilogram silicon sphere that could play a significant role in the redefinition of the unit of mass for the International System of Units (SI). With the exception of mass, all of the SI base units can be traced to fundamental constants or to the properties of atoms or molecules. The mass standard, the kilogram, is currently defined as the international prototype kilogram (IPK) which is a 135 year old platinum-iridium artefact that is held at the International Bureau of Weights and Measures (BIPM) in Sevres near Paris with about 80 ‘national prototype’ replicas distributed world-wide to metrology institutes. Periodically the replicas are compared with the original IPK for reference with the increasing realisation that the prototypes have been slowly but inexorably deviating from each other due to reasons such as micro-contamination, surface adsorption and other complex factors that are extremely difficult to counter. These albeit minuscule variations, which amount to nothing more than the mass of a fingerprint over several years, have led scientists to look for other ways to quantify mass and one line of research led to the foundation of the international Avogadro Project – a long term collaboration set up in 2003 between metrology institutes in Germany, Italy, Belgium, Japan, Australia and the USA. The Project aims to redefine the kilogram in terms of a constant on the basis of atomic mass. To achieve their goals, two pure silicon-28 spheres with a roundness delta of less than 50 nanometres over a 93.6 mm diameter – believed to be the roundest objects in the world – have been produced. These have been extensively analysed for their crystal perfection and measured to determine the sphere diameter and resulting volume. From this data, the specific number of a certain type of atom contained in the 1 kg mass of the silicon-28 can be predicted with a degree of certainty. In this way the Avogadro constant, NA, could be established and recognised as the new definition of mass. The ongoing project has returned some very promising results and many milestones have been achieved but more exhaustive work is scheduled well into the future. As surface contamination is still a concern and a barrier to obtaining the levels of measurement certainty required the PTB are analysing, amongst many other aspects, the surface properties of these high purity silicon balls using a synchrotron beam to determine its influence at the atomic level on the sphere diameter and consequent volume.  The Heason 3-axis manipulator is used inside a vacuum chamber to position the sphere relative to the synchrotron radiation beam for surface spectroscopy work. The demanding specification for the positioner covered base and upper rotary axes with 370º and 110º travel respectively with positional repeatability of 0.01º; and with a central horizontal 55 mm travel linear stage with repeatability specification of 10 microns. In order to ensure the optimal stiffness for minimum deviation through the Z- axis of the manipulator, these key specification factors necessitated extremely tight radial and axial runout values for the rotary stages and equally confining pitch, yaw, roll for the linear stage. For each axis, the quadrature encoder feedback resolution needed to be 10 times higher than the positional resolution to enable the servo motion control system to perform optimally. The very restricted volume of the vacuum chamber meant that the overall dimensions available for the manipulator were just 300 mm in diameter and 500 mm in height. Further challenges necessitated a 75 mm clear aperture through each positioning stage for instrumentation feed-through and restricting the axes’ travel with the use of customised hard wired limit switches. The vacuum level was specified to 10-9 mbar and required the stage to be subject to pre-heating to 120º C for outgassing in addition to a completely non-magnetic component inventory to be employed throughout the whole assembly. Heason Technology has developed its own in-house design and build service over several years and has a large portfolio of proven application successes for motion system installations in UHV for synchrotron sources and many other equally demanding research and manufacturing environments. The West Sussex based motion control specialist works with a select number global motion control equipment manufacturing partners for specialised components to undertake such tasks. The choice of piezo-ceramic motors for all axes, from Heason’s distribution partner Nanomotion, provided the perfect solution for the drive and control. These very compact, completely non-magnetic, motors are arranged on the periphery of each rotary stage, creating rotary motion by driving a ceramic ring. Using the same principle, the linear stage is equipped with a ceramic strip. A combination of three 2-element HR-2 motors for each rotary axis are synchronised to provide the very fine positioning resolution required. A single 8-element HR-8 motor is used for the linear stage. Position feedback used compact exposed optical rotary and linear encoders from Numeric-Jena with glass discs and strips integrated into the assembly. All stages have ceramic ball-type caged bearings and the titanium support structure and housings, which have sufficient air gaps and spacing to allow trouble-free vacuum outgassing, were specified to provide maximum stage stiffness. UHV compatible connectors and cables, and other vacuum compatible materials such as Peek were specified throughout with all cabling arranged outside of the manipulator housing with a simple harnessing system. The positioning system has a relatively low duty cycle requirement with slow positioning followed by longer motion breaks for surface measurements where the piezo-ceramic motors inherent locking ability provides complete stability. The 3-axis sphere manipulator was supplied to the German Technical Consultancy firm of Dr.-Ing. Giora Baum (Precision-Motion) who provided the motion control system.  PTB are now busy using the Heason manipulator and the results they obtain will be forthcoming over time – perhaps with the announcement of further encouraging results for the new definition of mass.

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