At the heart of a synchrotron is its storage ring; a vacuum chamber through which electrons hurtle at nearly the speed of light. As these electrons circle through specially designed magnets positioned around the ring, they lose energy, which emerges as beams of very bright, highly-focused light of different wavelengths. It is this light that scientists use to drive their experiments. The efficacy of its focussing and handling equipment relies on advanced sub-micron accuracy motion controls from Delta Tau.

Diamond Light Source is the largest scientific facility to be built in the UK for 30 years. The light beams it generates are smaller than the width of a human hair and billions of times brighter than hospital X-rays. Electrons are guided (accelerated) around the storage ring by bending-magnets.

Charged particles emit radiation (photons) when they are accelerated. These beams are carried along beamlines to discrete research stations located around the perimeter of the synchrotron ring. Beamlines can be of two types:insertion device beamlines and bending-magnet beamlines. Bending-magnet beamlines use the x-rays emitted by the electrons as they are accelerated by a bending-magnet. Insertion device beamlines use the x-rays emitted by the electrons as they are accelerated by an insertion device. Insertion devices are essentially a series of alternating magnets that cause the electron beam to undergo a short period oscillation (or a wiggle, as it's known in the business). Insertion devices come in two main types, undulators and wigglers. The extra acceleration the electrons undergo passing through an insertion device leads to a more intense x-ray beam.

The light is directed to the beamlines and accelerated using bending magnets located at the insertion points around the storage ring – one at the mouth of each beamline. Diamond currently has funding for 22 beamlines and this number is likely to grow during the lifetime of the facility.

Once the light enters the beamline, it is guided, intensified or deintesified using series of slits, lenses and crystals; the angles and positions of which are dictated using electric motors whose motion commands are generated by Delta Tau controls. While most of the electric motors used are stepping types, there are also AC and brushed DC servo motors together with some piezo electric motors for fine adjustments. The same Delta Tau motion controllers handle all these mixed motor technologies. While each beamline type will have varying motion requirements, a typical line might have 60 axes of motion. Many of these axes are controlled discretely, but in the workholding and manipulation of specimens, for example, multiple axis interpolation is required.

In addition to the beam adjustment mechanisms, there are workholding and manipulation tools whose motion is controlled. These can include hexapod positioning stages whose six axis motion control uses complicated inverse kinematics calculations carried out a breathtaking speed within the Delta Tau PMAC centralised controllers.

Synchrotron specialist Oxford Danfysik, a company that uses Delta Tau motion controllers, is supplying the first beamline equipment destined for Diamond. This company’s system uses central PMAC2-VMW Ultralite 32-axis controllers networking to Delta Tau UMAC as distributed motion controls. The networking of the motion controllers is via Delta Tau Macro Ring optical fibre fieldbus. Macro Ring has a high performance and robust fieldbus architecture that enables high bandwidth and fast data communications between the PMAC and UMAC controllers.

The motor specialists McLennan completed the integration of the motion controls and the mechanics for Oxford Danphysik. The systems make use of micro stepping motors and McLennan was able to ensure maximum accuracy prior to installation in the beamlines. Vic Sawyer of McLennan explained, “it was important to investigate all the variables in the system, such as resonance, mechanical stiffness and so forth, as well as to prove the system overall before fitting”.

The UMAC (Universal Motion and Automation Controller) is a modular PMAC system built with a set of 3U-format Eurocards. The configuration of any UMAC system starts with the selection of the PMAC CPU or MACRO optical fibre interface and continues with the addition of the necessary axes boards, I/O boards, communication interfaces (USB, Ethernet, etc.) and any other interface boards selected from the rich variety of available accessories. For example, accessory boards interface with virtually any kind of feedback sensor or to implement almost any kind of communication method with the host computer or external devices. The devices are therefore sufficiently flexible to accommodate any of the requirements encountered within each beamline.

Andy Joslin of Delta Tau explained the reasoning behind Diamond Light Source’s decision to use his company’s products, “packaged motion controllers often lack the flexibility and openness needed to accommodate non-standard motion programs”. He added, “with Delta Tau there’s always an extra layer one can use to optimise the motion software”.

One example of this is in the manipulation of a double crystal monochromic system. Here, the Delta Tau PMAC enables a virtual axis to be created to enable the two crystals to position in relation to each other. This is achieved in such a way as to maintain a straight beam even when the first crystal is positioned to deflect the beam – such as when monochromatically filtering white light by Bragg diffraction for specific inspection tasks. The PMAC recognises the virtual axis as theoretical and is able to calculate the real life motions required for each crystal’s axis.

Andy Joslin highlighted the speed and robustness of the Macro Ring network. “Originally it was planned to distribute UMAC controllers remote from the main PMAC control”, explained Joslin, “but in practicality, radiation issues determined that the UMACs are best located adjacent to the PMAC on the first lines we’re building. That being said, the Macro Ring is still advantageous”.

The operator interface to the PMAC is being written in Java by Diamond’s Data Acquisitions Group, in collaboration with Generica Data Acquisition software from Daresbury. Supervisory software is being created by a programming team using the industry standard EPICS with EDM as its graphical user interface.



For further information, contact: Andy Joslin,
Delta Tau UK Limited, Holland Cottage, Kirby Road, Great Holland, Essex CO13 0HZ.
Telephone: 01255 670196 Fax: 01255 850768
e-mail: ajoslin@deltatau.com

Press enquiries to: John Houston
Houston Associates, Media House,181-185 Kirkdale, London, SE26 4QH
Telephone: +44 (0) 20 8778 1900 Fax: +44 (0) 20 8659 9191
e-mail: john@houston-associates.com
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