PLCs protect UK £250 million particle accelerator machine

A super-microscope the size of several football pitches is using PLCs to control the critical interlocking systems needed to protect the machine. Dean Palmer reports Currently under construction at the Chilton/Harwell Science Campus in Oxfordshire, the Diamond Synchrotron will provide the UK's first, third generation light source, superseding the current facility in Daresbury. A doughnut-shaped super-microscope the size of several football pitches, the machine will produce intense light - as X-rays, infra-red and ultraviolet - that will help researchers in the development of new medicines, high tech materials and in the investigation of environmental issues such as climate change. By 2007, phase one of the project will have been completed at a cost of £235m and up to eight synchrotron beam lines will be available to researchers. By the end of phase two in 2012, 22 beam lines will be operational, with the scope to add 18 more according to demand. On a machine this size and complexity, safety is paramount and this is where PLCs are playing a critical role. At the heart of the machine is a linear particle accelerator that fires electrons into a booster synchrotron ring. With powerful magnets steering the electrons around the ring, RF fields accelerate them close to the speed of light. From the booster ring, the electrons pass into a larger storage ring, again guided by magnetic fields and accelerated by RF fields. As the electrons are bent via the dipole magnets, they emit synchrotron light which is channelled into beam lines, where researchers can select the light, at the frequencies or energies they require to perform their experiments. Built up from 24 individual cells, the storage ring has a circumference of 561.1m, storing electrons at 3.0GeV, as a particle stream, that have a half-life of around 10 hours. To ensure maximum efficiency, the machine operates at a very high vacuum of 0.00000001mbar. With all this energy and such a high vacuum, machine protection is therefore vital. Overall control of the facility is by an EPICS distributed control system, but PLCs were chosen to provide the critical interlocking systems needed to protect the machine. A combination of dedicated high speed machine protection, realised in proprietary hardware, and a PLC subsystem was considered the most effective means of managing the protection functionality. Electrical project engineer Simon Lay explained the reasons behind the choice of control system: "It was clear we needed a control system that was modular in design, and distributed. That was the best way to assure the overall reliability of the system. Our target is to achieve greater than 99% availability for the machine for operation in excess of 6,000 hours per year. Single point failure systems take too long to diagnose and too long to repair." He continued: "At the same time, we also recognised a need for the control system to be scalable. The facility is expected to have a 30-year life and any necessary expansion of the control system in tandem with the evolution of the facility, must not impact on the performance. Again, because of the long lifespan, it was key that we built the control system on open standards and an open architecture. The control system had to have the ability to evolve to incorporate upgrades and new technology and had to ensure seamless integration at all levels." The actual machine protection concept is built around a series of interlocks on each of the 24 cells of the storage ring and the four quadrants of the booster ring, the idea being to protect the machine by isolating any cell as quickly as possible. Parameters being monitored include the flow of water that cools the magnets, critical temperatures to prevent thermal shocks, the presence of obstructions in the beam vessel such as vacuum valves or shutters, vacuum system pressures and vacuum leaks within the cells. The PLCs, supplied by Omron Electronics, also control all of the vacuum valves, preventing a vacuum valve being opened without there being good vacuum on both sides and closing to protect in the event of pressure variations. In all, there are 29 machine protection PLCs, 28 four-valve controls and at least two, 6-valve control PLCs per beam line/front end combination. All the controllers are networked over a fibre optic star. Lay concluded: "We wanted to separate the machine protection functions from the valve control for a number of reasons. Building up such a high vacuum takes a long time. We needed to isolate this process from that of machine protection for increased reliability, so using separate PLC systems helps to maintain machine availability. Also, breaking down the functionality onto a number of discrete Omron CJ1 PLCs means we can build and test the machine in blocks." Omron's CJ1 PLC is now the site standard for all subsystem control.