Tennis in the rain thanks to engineering feat

31 July 2009

Rain couldn’t stop play on Wimbledon’s centre court this year as a new closing roof protected the matches below from the inclement British weather. Sick provided linear encoders for each of the ten 77m long, 100 tonne roof trusses.

‘The linear encoders enable 1000 tonnes of roof to be moved with machine tool precision, positioning to an accuracy of 1mm,’ said Darren Pratt, encoder specialist for Sick (UK). ‘This accuracy is essential to ensure its smooth operation, every time it’s used.’

Wimbledon’s opening roof structure was engineered, manufactured and assembled by specialist hoist and crane designers, Street CraneXpress for the All England Lawn Tennis Club and their main contractor Galliford Try. The roof trusses run on 80m tracks either side of the court roof.

Each truss has a Sick KH53 encoder read head mounted at either end, which monitors the truss’ position and progresses across the roof from the KH53’s magnetic measurement scale alongside the running track. They are moved by electric motors on either end of each truss, powered from a bus bar system running the length of the track.

In addition, to the linear encoders, Sick supplied SRM50 Hiperface absolute rotary encoders to monitor the rotational positioning of each motor. The motors move the ten trusses across the roof by combining wheel driven motion along the track with ballscrew actuated opening of the roof panels via restraint and end arms.

Dan Salthouse, project manager for Street Crane Xpress, who oversaw the trialling and installation of the mechanical and control systems, said: ‘The high degree of position accuracy provided by the Sick encoders is essential as each truss has to be driven at the same speed and finish in the same position to avoid crabbing. Any misalignment and the trusses will slew sideways and jam up.

‘For each truss, the linear and rotary encoders at each end provide signals via a Digitronic splitter to a Moog motor controller and two PLCs, a control system developed with Fairfield Control Systems. The PLCs co-ordinate the motor operation at each end of the truss to maintain perfectly synchronised operation of the truss and the roof opening. In this way, we avoid any undue stress on individual components and reduce the energy load on the motors.’

In their turn, the 20 PLCs are integrated from a roof control room where, when the referee gives the rain signal, the operative presses the button that starts the roof operation.

There are four positions for the roof trusses: Parked with all ten locked at the North end of the stand; Championships with five locked at each end of the stand to reduce coverage delays; Deployed, i.e. totally closed in a time of under 10 minutes; Sunshade, which is designed to shade the Royal Box with a two minute deployment.

The development of the roof, originally designed by Populous (formerly HOK), took five years, with two years spent on a trial site in Sheffield, building up its operation truss by truss and proving the bespoke control software to ensure the massive structure could operate without a hitch on Centre Court.

‘The KH53 is highly suited to rugged external operation,’ states Darren Pratt. ‘It operates through the reading of the unique coded spacings of the permanent magnets in the measurement scale, which can be up to 1700 metres long. It is unaffected by wet or dirt and is highly reliable; Street CraneXpress has been using it for years on a number of prestigious projects. The alternative was laser position sensing, but there were potential line of sight, alignment and response time drawbacks.’

The roof panels are covered with Gore Tenara architectural fabric which, although waterproof, allows some light through to create a unique ambience within the stadium. Supplementary direct and indirect lighting will allow prolonged play, although there is no intention of holding specific night-time matches. A very high power air-handling and dehumidifying system maintains the atmospheric conditions suitable for play, and avoid condensation under the roof.

‘Extensive trialling of the roof and trusses was an essential part of the QC process with this type of pioneering engineering design,’ concludes Dan Salthouse. ‘This meant that installation and commissioning on the Centre Court roof went relatively smoothly.

‘The reward for me came when Andy Murray played his epic match against Wawrinka in the Wimbledon fourth round. It made history when the roof was closed, enabling the 15,000-strong crowd, not to mention a gripped nation, to watch a riveting match uninterrupted until it reached its conclusion at 10.30pm.’

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