Micromech Systems Limited (MSL) has recently completed a £1,100,000 contract to provide control systems for 360 axes of servomotors controlling 72 beam alignment girders at the Diamond Light Source synchrotron installation, Harwell. Each girder weighs in the region of 15 tonnes, and they are positioned around the storage ring of the facility. The storage ring consists of a continuous circular vacuum chamber with a circumference of 561m enclosed by over 450 powerful magnets mounted on the girders, which guide an electron beam around the ring. Synchrotron light is produced each time the electron beam is diverted by the magnetic field, and is emitted at various points around the storage ring into a number of beamlines.
Due to possible settlement of the building it may be necessary to realign the ring by adjusting the girders. It is intended that, as a future development, the electron beam itself will be used as a means of feedback to form a closed loop system for required changes to the girder positions.
The servomotors used for the task are 50mm dc brushed and are coupled via a small worm gearbox with a ratio of 15:1 and a larger planetary gearbox with a ratio of 320:1 giving a final drive ratio of 4800:1. The motor/gearboxes are fitted to cam assemblies (Movers) which in groups of five provide +/-
5mm of motion in ‘sway’, ‘heave’, ‘pitch’, ‘roll’, and ‘yaw’ to within a few microns accuracy. Absolute encoders provide retention of positional data in the event of power failure.
To ensure that the encoders were zeroed relative to the correct cam position MSL designed and built a measurement system, which automatically positions the cam at a precise position from top dead centre. Once in position the encoders can be automatically zeroed.
A major concern for DLS is that if the girders are moved too far, expensive bellows assemblies which join the vacuum tube sections will be damaged. MSL designed and manufactured custom limit switch assemblies to protect the integrity of the vacuum. The switches and can cope with the ‘sway’, ‘heave’, ‘pitch’, ‘roll’, and ‘yaw’ movements of the girders.
24 control panels were designed and built by MSL each containing a 7- and an 8-axis custom built servo rack and a VME crate. The control systems are based on the Oregon Microsystems VME58 motion controller card mounted in VME crates, each card controls up to eight servo axes.
Application software was written which interfaces with DLS’ own high-level software platform (“EPICS” used internationally by the scientific community) to provide operator level screens to control the system.
The installation required in excess of 3.5km of multicore cable, used in the numerous cable assemblies required for the system. MSL installation engineers carried out the termination of the cables at the control rack and storage ring ends. Final commissioning was also carried out by MSL.