An example of ‘out-of-sight’ engineering at its best
01 November 2009
Wimbledon’s staff closed the Centre Court’s new retractable roof for the first time in competition on Monday, June 29 2009 after light rain stopped play during the second set of a match on Centre Court. The new feature, which received a great deal of publicity during this year’s tournament, is an excellent example of precision positioning control using electric motors and actuators to move structures weighing a combined 1,100 tonnes
The Wimbledon retractable roof consists of two sections (on the southern and northern sides of the Centre Court), which together support 5,200m2 of translucent, waterproof fabric when deployed. The 1,100 tonne structure, comprising ten trusses, each weighing 110 tonnes, is guided along two sets of parallel rails over a span of 77m with a tolerance of 2 to 3mm - all in less than ten minutes. The retracting mechanism is operated and controlled by a system of servomotors and electric actuators designed and supplied by Moog Industrial Group’s Tewkesbury plant, with special locking actuators designed and supplied by Fraserburg based Power Jacks.
Moog’s system includes 148 axes of control, comprising 108 electric actuators (variously providing up to 35 tonne and up to 14 tonne of thrust) and 40 servomotors, which drive the wheeled carriages that carry the trusses over the supporting rails. As well as the servo drives for each of these axes, there are 20 real-time Moog servo controllers and 20 pre-programmed supervisory PLCs, all of which are housed in 40 control cabinets.
The Moog equipment and bespoke software provides closed-loop control of roof movement, which includes operation of the truss end arms, restraint arms and carriages. All monitoring, including real time position and other critical data acquisition, is carried out by the PLCs.
The end arm assemblies are essentially large inverted hinges, which open from a deep narrow ‘V’ into a wide, shallow ‘V’. During movement, the point of the ‘V’ moves up and the ends move out, thus pushing the top of the deploying truss away from its neighbour. This movement is controlled using four end arm actuators on each side.
The four restraint arm assemblies, each with its own actuator, are located at the centre of each roof section. These help maintain the shape of the truss lengthwise and ensure that the trusses meet squarely when the north and south sections are fully deployed. Together with the end arms, these actuators must be precisely synchronized with respect to the position of the wheeled carriages, and this is an important task of the Moog servo controllers.
Power Jacks’ element included the design and supply of 42 bespoke electrically driven linear actuators to create the locking devices, 36 of which operate when the roof opens and closes. Their design is based on the company’s Type-A Rolaram technology and each of the Wimbledon Rolaram’s has a peak load rating of 160kN – more than sufficient to withstand peak wind loading. They operate at 1,500 mm/min over a 400mm stroke, with the useable stroke set by limit switches integrated into the actuator. Each actuator is mounted in the roof using a trunnion-to-clevis type arrangement that allows it to pivot through an arc during operation.
The structure was designed by Populous (formerly HOK Sport Venue Event) with structural engineering services provided by Capita Symonds. Sheffield based Street Crane Xpress (SCX) designed and installed the mechanical systems and control technology.
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