Begnagh Lift Bridge is the first moveable bridge to be
completed on the Shannon Canal in the republic of Ireland. It has been designed
and constructed to be a completely unmanned installation, which has provided
many challenges to the design team.
The deck is a concrete structure, designed by the client, which is
lifted by a series of four hydraulic cylinders acting at each of the four
corners. The deck is of trapezoidal shape, which means that each cylinder has a
slightly different load as the deck lifts. As the deck is of concrete
construction, there is little margin for error in deck twist, otherwise cracking
of the deck could occur.
In order to achieve unmanned operation, the bridge detects
approaching crafts automatically. This is achieved by a pair of laser scanners
mounted on each approach to the bridge. The scanners, and associated software,
detect large objects moving towards the bridge and notify the bridge PLC
controller.
Upon receiving notification, the control PLC notifies the boats that they are
detected using matrix signs. It also switches on the approach wig wags to notify
road traffic that the bridge is about to lift. In sequence, the road is closed,
barriers lowered and the bridge raised. When raised, the boats are allowed to
proceed by navigation traffic lights. After the boats have passed, the bridge is
lowered and the road barriers lifted.
To operate without a bridge operator, many safety functions, with
SIL ratings between SIL 1 and SIL3, have to be carried out automatically. To
achieve this, safety laser scanners are positioned at each side of the bridge to
monitor objects approaching the structure, and to check that the bridge is clear
before starting to lift. Likewise, barrier proximity sensors and a light curtain
under the bridge protect these zones.
These input devices are processed by a safety PLC to achieve all
safety functions. The use of a safety PLC greatly simplifies the installation,
particularly as all of the sensors have to be muted at various points in the
process.
The safety PLC also monitors the position of all four cylinders,
and uses these measurements to mute the other input devices, and also check for
twist in the bridge. Four twist criteria are set up within the bridge
specification. If any one of these four are breached then the bridge stops with
a safety function integrity of SIL 2.
The use of a safety PLC rather than hard wired logic also produced
a secondary advantage. As all of the control systems are under the control of
the PLC, then logic can be used through each form of control, even manual
pendant control. This proved useful as the hydraulic powerpack is located
approximately 50m away from the bridge. The resulting oil compressibility of
approximately 30mm is significantly greater than the allowable misalignment of
the two master cylinders of 7mm. This problem was overcome by using feedback
control on the cylinders, but also ensuring that the system was pre-pressurised
before attempting any movement to allow the feedback systems to work. Keen
observers will therefore see a slight lift of the bridge before any lowering
cycle. However, the cylinders will always remain totally aligned as a result.
For more information, contact us on 01709-373782.
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