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21/07/2009
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Tom Shelley reports on the technology and skill that has gone into providing a folding roof for a British landmark
 The skills of an extraordinary list of blue chip companies lie behind the success of the complex folding roof constructed for Wimbledon's Centre Court, whose 1000 tonnes can be positioned in minutes to an accuracy of 0.1mm.
Designed by Populous, Capita Symonds and Street CraneXpress, the roof is made in the form of a concertina in which actuators lift the apexes of the folds while coordinated bogie motors move the bases. Other actuators are made to control shape and further sets of actuators secure final lock positions.
Despite the complexity of the multitude of operations that have to be undertaken together in order not to damage the roof, everything is controlled by single button presses and a completely automatic system exercises all motions at regular intervals in order to ensure that nothing seizes up.
Ian Bartlett, project manager with Moog Controls, which was responsible for the main actuation system, explained that: "Part of the problem was how to get the roof onto a structured designed in 1923." He explained that, while the original plan was to move everything using hydraulics, Moog proposed using an entirely electromechanical system in order to prevent the possibility of drips of hydraulic oil falling on Centre Court's 'sacred turf' or on the all important spectators.
The roof consists of ten 100 tonne trusses – the folded sections – five of which are moved to each end of the stadium when it is in 'Championship mode'. But there are other modes: in 'parked' mode, all ten sections are at one end of the stadium, 'sunshade' mode sees a section partially deployed at the Southern end, and 'deployed' means it's raining and the roof is fully closed. All four modes are selected by single button presses. A complete opening or closure takes eight minutes – two minutes less than the target set originally.
According to Bartlett, each truss has four bogie motors – one at each corner – plus eight 35 tonne force end arm actuators and four 14 tonne force restraint arm actuators. These work on large stainless steel rods on the tops of the folds in order to maintain their exact shape so that they match up to each other. There are two Moog Servo Controllers (MSC) on each truss, one at each end. Each MSC is commanded by a Schneider Electric PLC. Bartlett said the motors were made in Italy, the MSC drives were made in Germany and the software, integration and testing— 'equivalent to 12 years in service' – was undertaken in Tewkesbury. In addition, 42 bespoke electric linear actuators were supplied by Power Jacks to create locking devices. Of these, 36 operate when the roof opens and closes.
Actuator and motor shaft positions are all monitored by encoders, which are used to infer actuator extensions. Truss position is monitored by Serial Synchronous Interface technology supplied by Sick Stegmann.
This system makes use of 2m long measuring elements incorporating a number of permanent magnets placed end to end along the full 90m length of the track.
Magnets within the measuring elements are fixed in place in such a way that the separation between them is unique and never repeated. When installed in a specific order, the magnet separations form an absolute code, which is read by a read head. The read heads, mounted on the bogies, can detect at least three magnets at any position along the track. This means that it can therefore detect at least two unique magnet separations and thus identify its absolute position.
Bogie position is controlled to within 2mm when moving, and to 0.1mm when stopped.
The 300 spherical roller bearings for the hinges, as described in Eureka's June edition were made by Schaeffler UK.
Mike Bridges, a director of construction company Galliford Try, described the development as: "An R&D project. It is unique. The nearest design is that of the Toyota Stadium in Toyota City in Japan and it is not the same. We sent somebody to look at every other moving roof anyone else had designed. Our brief was to allow the stand to be made bigger, two floors higher with a moving roof, but to allow the same amount of ultra violet light for the grass."
The fabric used for the roof is Tenara, a woven expanded PTFE fibre that transmits about 30% of incident light. According to Bridges, Tenara was 'the only fabric that will resist constant manipulation and not crack'.
The consortium of companies involved in the design process included Sheffield based Street Crane Xpress, whose role was to focus on the mechanisation and control of the roof.
The overall management system was set up by Fairfield Control Systems using Rockwell software and a central PLC that supervises the other PLCs in the system.
As well as coordinating the system, this PLC also manages safety issues and regular exercising and test of the equipment. John Tooley of Fairfield Control Systems noted that the SCADA system monitors all drive currents in order to detect possible incipient faults. In addition, the system 'sets going a pre-test movement' every 14 hours in which each actuator is made to move 1mm and each motor 3mm. This ensures that all brakes come off and nothing is given a change to seize up.
Safety issues include organising safe stops. As Barlett explained: "If somebody does an emergency stop from maximum speed of 214mm/s, we don't want 500 tonnes of half roof to stop dead". If this was to happen, the structure was liable to be damaged.
In addition to the machinery needed to open and close the roof, the system has a ventilation system with chiller units that can handle 143,000 litres of air per second.
Pointers
* The roof is made up of two 500 tonne sections made up of five concertina elements
* Motors and actuators are used to lift the apexes of the concertina, to move the bases into position, to adjust the exact shape of each folded element and to lock everything in place when stopped.
* Positioning accuracy is to 2mm when moving and 0.1 mm when stopped
* The automatic control system exercises it by an almost imperceptible amount at regular intervals in order to ensure that nothing seizes up.
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Author
Tom Shelley
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