DeviceNet in the London Eye

Source: CAN Newsletter March 2003

The British Airways London Eye is the largest observation wheel ever built, and the fourth tallest structure in London. The design and realization for the wheel has drawn on many areas and disciplines of engineering. Fairfield Control Systems designed, implemented and commissioned the PLC control system for the “Dynamic Boarding Platform”. During the simulation stage of the wheel design, it was determined that due to environmental conditions and manufacturing tolerances imposed on the wheel, the boarding platform would have to be a dynamically moving structure relative to each of the 32 capsules around the wheel rim. In essence the platform would have to adjust its horizontal and vertical position to maintain a constant step for each passenger embarking or disembarking from each capsule. The boarding platform is made up from eighteen “finger” units, each providing 800 mm of horizontal movement. The eighteen “finger” units are mounted on a steel structure that can be tilted, to provide up to 200 mm of vertical movement at the “finger” end. Both horizontal and vertical movement is achieved through the use of a network of electrical servo drives and screw rod actuators. Through measurement of wheel angle and wheel rim vertical and horizontal displacement, combined with known manufacturing data on the wheel Fairfield was able to dynamically generate a profile for the position of the wheel rim in all three axes. The PLC then uses this displacement profile to command the boarding platform actuators to move, and so maintain a 50 mm gap between the edge of the platform and the passenger capsule as the wheel rotates.

The boarding platform system is controlled by two main PLCs, one in each of the North/South switch rooms. All PLC I/O modules communicate on DeviceNet with main processors. By having duplicated I/O signals each receives the same field/operating information and therefore either PLC can be the master PLC issuing instructions to the drives network and Operator Interface. Arbitrarily the South PLC will normally be selected as master. Selection of master PLC will be available via key-switch in the south panel Both main PLCs will perform their own calculations as to where the fingers should be positioned for the capsules within and approaching the platform area. The results of these positional calculations will be cross-checked between the main PLCs and, if within a set tolerance of each other, the Master PLC will be enabled. However no change in outputs will be made after this first cross-check. Should this first cross check fail then an emergency condition routine will operate to retract the fingers to a safe position.