Modern control for Tower Bridge

The implementation of a state of the art control system and networking strategy will keep London's 108 year old Tower Bridge operating reliably for many years to come

Of London's 19 major road or rail bridges, 108 year old Tower Bridge is closest to the sea and the best known. Owned and managed by the Corporation of London, the bridge is raised several times a day and in a typical year opens between 800 and 900 times. When it was built, Tower Bridge was the largest and most sophisticated bascule bridge ever built ('bascule' comes from the French for see-saw). The two 1000 tonne balanced bascules are operated hydraulically and take only about a minute to be raised to their maximum 86 degrees.

Two 11kV incoming supplies and a standby generator guarantee Tower Bridge's electrical supply, with on-site transformers stepping the voltage down to three phase 400V for the eight 37kW motors, which drive the hydraulic pumps. The lifting gear is housed in four machinery rooms under the piers on either side of the main towers and, although all pumps are normally running during a lifting operation, each bascule can be raised and lowered using just one pump if necessary. Originally hydraulic power was maintained using steam engines, but since 1976 modern hydraulic pumps have been used. By 1998, a hard-wired electrical control system installed was becoming unreliable and poor control over the bascules was causing high shock loads on the structure. Options were considered for a more modern replacement and part of this £1.7 million (euro2.5 million) refurbishment was a state of the art control system.

The mechanical contractor was Bosch Rexroth, and Nottingham-based Systems Integrator Fairfield Control Systems was chosen to design, install and maintain a new control system. This is based around an Allen-Bradley ControlLogix PLC, linked to Rockwell Software RSView operator stations via a ControlNet high-speed fibre optic network and to the hydraulic systems by Allen-Bradley Flex I/O modules. The PLC system is duplicated on either side of the bridge.

Easy programming ControlLogix PLCs simplify programming by using a tag-based address structure that makes the design future-proof. The processor understands named tags so there is no need to maintain a cross-reference table between tag names and physical addresses. A maintenance engineer can therefore read the program even if documentation is not available.

"The control system has been designed for a minimum life of 25 years, so we needed a modern PLC and SCADA package," says Fairfield applications engineer Nigel Montgomery. "The ControlLogix PLC makes maintenance easier because an engineer only needs basic configuration software on a laptop to troubleshoot and maintain the system. It is also possible to dial into the PLC and RSView systems and diagnose many faults remotely."

Compared with the old analogue system the ControlLogix-based system results in far smoother bridge operation that reduces the stress on major components. Proportional control cards ensure a smooth 'creep' start to the bridge lift, which can then be speeded up to minimise the opening and closing cycle time. The PLC also incorporates integrated control of hydraulically operated active resting blocks and pawls which are automatically adjusted to take the weight of the bascules as the bridge closes. This helps prevent excessive wear on the shafts.

"The shafts and bearings are now 108 years old, and mechanical sympathy is very important," says Tower Bridge senior technical officer Charlie Harrison. "The use of active resting blocks reduces component wear, while improved control of bridge operation greatly reduces shock loading, especially at the beginning of the lift and at closure.".

Maximum flexibility In order to avoid disruption to river and road traffic if the bridge failed, the operating system has been designed to give maximum flexibility and reliability, with dual redundant communications between the North and South Towers. A single operator from either of two control cabins on each side of the river can open the bridge using a simple joystick lever on his control desk. Pushbuttons are used to start and stop pumps and operate ancillary systems such as the pedestrian and road barriers across the bridge. Each cabin is equipped with a standard desktop PC running Rockwell Software RSView SCADA software. As well as providing a graphical indication of system status and alarms, it acts as a second human machine interface to operate the systems.

The ControlLogix PLC is located in the northwest pier machinery room and duplicated on the other side of the river. Communication with the control cabins is by dual redundant, 5Mb digital ControlNet fibre optic data network which runs in the main electrical trunking within the fixed walkways high above the opening sections. Fibre optic cables were chosen because of their immunity to electrical and radio frequency interference and to minimise the space occupied in already crowded cabling space.

The PLC is also linked using standard coaxial cable and remote Flex I/O stations to more than 800 I/O points on the hydraulic pump motors and active resting block system. The pumps are able to vary their output in response to analogue signals from the variable control card in the PLC by moving a swash plate in the pump to achieve the required hydraulic pressure.

Eight active resting blocks and four eccentric pawls at the rear of the bascules are hydraulically driven and automatically engage as the bridge closes, to support the weight of the lifting sections. ABB Pressductor load cells and analogue position transducers signal the PLC so that the position of the resting blocks can be adjusted with hydraulic rams until the load is shared evenly. The PLC also controls nose bolts which are used to lock the bascules together when the bridge is down.

"ControlLogix, RSView, and ControlNet have been used together to provide a robust distributed control system that has improved the operation and reliability of Tower Bridge," says Montgomery. "It was fitted without difficulty into existing control cabinets and will be simple to maintain for many years to come."

Fairfield Control Systems
m101@industrialnetworking.co.uk

Rockwell Automation
m102@industrialnetworking.co.uk

Bosch Rexroth
m103@industrialnetworking.co.uk