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Positioning stages at the frontiers of science

01 May 2009

The Diamond Light Source has taken delivery of a purpose-designed and built, 21-axis positioning system, comprising manual, stepper and ceramic servo motor based positioning elements, which will be used to conduct groundbreaking experiments in the field of structural biology

Scientists at the Diamond Light Source (DLS) in South Oxfordshire are using a custom-engineered 21-axis positioning system to provide beam collimation, optical focusing and optical modification for the newly commissioned I24 Microfocus Macromolecular Crystallography (MX) beam line, which will be used to investigate the link between the structure and function of molecules known as membrane proteins. These proteins prove excellent targets for drug delivery and are increasingly used in drug design to combat major diseases and viruses.

To date, work in structural biology of this kind has been limited by the irregular quality and small size of the crystallised membrane protein samples and the relatively large beam sizes of older synchrotron light sources that together have restricted investigation to just a few of the 7,000 human membrane proteins.

The new DLS uses advanced technologies to produce a much finer and incredibly more intense beam than any other synchrotron in operation today. The I24 Microfocus MX beam line has a highly tuneable X- ray beam size of between 5 and 30 microns, its advanced positioning and modifying capability allowing nanometre level manipulation of the beam around the higher quality areas of the crystallised sample. These factors, combined with advances in crystal preparation techniques and instrumentation systems, has made the collection of more precise measurement possible and opened endless potential for future research.

The challenge to deliver such small focussed beams and modify their characteristics has been solved using a combination of motion control technologies. The I24 Microfocus MX beam line includes manual and stepper motor positioning for some parts of the beam-to-detector area and for less demanding displacements such as polarizing filters. For highly precise beam collimation and optical modification, sub-micron resolution ceramic motors are used.

As each positioning system is closely integrated and essentially shares the same mechanical housing, a part of which is a vacuum chamber, the DLS decided that the best approach was to use a single supplier to provide a customised solution for this entire area of the machine. A design team from Heason Technology worked closely with DLS scientists to develop positioning systems that could be fitted within the available space, as well as a considerable mixture of instrumentation and other key components.

A vacuum chamber was designed and built by Heason as part of the scope of supply. Although a relatively moderate vacuum level is used, components are fully vacuum compatible. The chamber houses a 4-axis beam collimation positioned, which comprises four ‘jaws’ that shape and tune the X-ray beam. Heason has considerable experience of collimation positioners and recently supplied similar, albeit larger, jaws for the ISIS Pulsed Neutron Source at the nearby Rutherford Appleton Laboratory. Each jaw is driven by a separate ceramic servo motor supplied by Nanomotion Inc, one of Heason’s principle high-technology distribution partners.

The ceramic motors provide nanometre level positioning resolution and a travel range of some 5mm through a designed-in linear motion guide bearing system complete with over-travel sensors. Servo position feedback resolution from built-in Renishaw linear encoders is 50 nanometres.

A major feature of the ceramic motor is its ability to hold and lock position completely, with zero position shift when power is removed. This level of stability is an essential requirement of this rig in order to maintain the beam’s characteristics throughout the duration of the experiment.

Also within the vacuum chamber are three open-loop, stepper motor driven discs, each containing a selection of polarizing filters which can be manually or automatically positioned across the beam line to optimise the beam characteristics for target illumination.

The actual crystallised sample is situated in a cryogenic chamber elsewhere within the machine and a complex optical system delivers the beam to an external detector system. The beam line delivery assembly, built by Diamond, is positioned by a relatively simple combination of manual and stepper motor driven axes that Heason has accommodated within a large fabricated frame housing other critical components. The frame itself is mounted on a large granite structure that provides a stable base for the entire machine.

Possibly the most impressive positioning system on I24 takes care of micromanipulation of up to three separate ‘optical modifying’ devices. These are positioned in the exiting beam line path to optimise its characteristics before it reaches the crystallography detector. Each of the three devices is mounted on a separate 3-axis, custom-built stage complete with integral linear motion guide bearings.

Depending upon the type of experiment, these stages may be nested closely and each allowed to manipulate its associated optical modifier device in very close proximity and in line with the exiting beam. Using the same Nanomotion ceramic motor servo motor drives as the jaw axes, a nanometre level positioning resolution is achieved with 50 nanometre resolution linear encoder feedback back to the motion controller. The entire 9-axis positioner and vacuum chamber are mounted on a 2-axis stepper motor driven stage that is used to move the assemblies away from the beam area.

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