Bridging the motors/drives gap

08 February, 2013

When the Road Traffic Authority (RTA) of NSW decided to optimise the functionality of the inspection and maintenance gantries on the underside of the Sydney Harbour Bridge, it turned to drive solutions expert, SEW-EURODRIVE to supply a complete end-to-end drives package.

When the Road Traffic Authority (RTA) of NSW decided to optimise the functionality of the inspection and maintenance gantries on the underside of the Sydney Harbour Bridge, it turned to drive solutions expert, SEW-EURODRIVE to supply a complete end-to-end drives package.
Connecting the city of Sydney to North Shore at Milsons Point, the Sydney Harbour Bridge provides a key arterial connection for the city. It incorporates eight traffic lanes, two railway lines, a pedestrian pathway, and a cycle lane. Completed in 1932, the Sydney Harbour Bridge requires regular inspections and maintenance programs to sustain its engineering integrity and preserve its appearance.
To facilitate these upkeep programs, the bridge is equipped with a number of moveable gantries located beneath the road and rail deck—some 50 metres above the Port Jackson waterline. Running on rails, these gantries are designed to provide safe access for repair and maintenance teams to the otherwise inaccessible underside areas of the bridge deck.
As part of an improvement and upgrade programme, the Road Traffic Authority (RTA) of NSW decided to upgrade these gantries from the existing pneumatic/hydraulic air-drive system to a new cutting-edge electric-motor-operated variable speed drive (VSD) design. The RTA engaged Australia's largest leading independent automation and control systems integration company, SAGE Automation, as system integrator for the project. SAGE made the decision to incorporate a complete drive solutions package from drive solutions expert, SEW-EURODRIVE.
Perfect platform

According to SAGE Group Business Manager, Robert Craig, many of the existing air-driven gantries had experienced operational difficulties, and needed to be refurbished to optimise their functionality. Initially, four of the underdeck gantries were refurbished—two 'pylon' gantries, and two 'wing' gantries on the eastern side of the bridge. "Rather than try to restore the outdated air-driven system, the decision was made to replace them with electric-motor/VSD solutions," he says. "This would deliver more reliable systems and more accurate levels of control."
To allow autonomous operation, each upgraded gantry is entirely self-contained—complete with diesel generator, and all control and drive equipment. "The choice of drive control solution had to be as compact as possible," says Craig. "By choosing a complete drive package—including motion controller, VSDs, geared brakemotors, encoders and HMI—from SEW-EURODRIVE, we were able to substantially minimise the equipment footprint on each gantry."
Autonomous control

The pylon gantries are equipped with four SEW-EURODRIVE KA57 helical-bevel geared brakemotors, rated at 0.55kW and 470Nm. The wing gantries use two identical KA57 brakemotors, plus a third larger KA67R37 unit, rated at 0.75kW and 990Nm. These deliver efficient, high-torque and wear-free operation, and give a low maintenance service life. Each brakemotor features a Hiperface encoder installed in each motor's rotor. An SEW-EURODRIVE MOVIDRIVE 'B' variable speed frequency inverter drives each brakemotor, and these are supervised by an SEW-EURODRIVE MOVI-PLC motion controller on each gantry. This design provides crucial single-point configurable supervisory control of the drive applications.
The MOVI-PLC features a 'T1' SD-card which generates a virtual encoder pulse, to which each of the Hiperface encoders on the motors synchronise. "The T1 card enables special pre-written and tested motion function blocks within the MOVI-PLC to be used, and this permits the creation of a virtual encoder," says SEW-EURODRIVE NSW Sales Manager, Martin Broglia. "In effect, the virtual encoder is a 'master' unit, and the actual Hiperface encoders are 'slave' units. This set-up ensures that if any motor or encoder fails, the master is not affected and the remaining slaves are still provided with direction. This facilitates synchronisation of the drives to avoid slewing of the gantry when in motion."
Each gearmotor features a BMK1.5 rail mount brake rectifier and a manual release. "The MOVIDRIVE 'B' sends an output to the brake to activate at zero speed," says Broglia. "The intelligence of the system ensures the motor doesn't drive through the brake. This means that the end-user saves on having to build their own logics to make the brake engage and disengage."
Minimal disruption

Connectivity for the various system components is achieved through a combination of S-Bus and Ethernet. S-Bus is used to connect the MOVIDRIVE to the MOVI-PLC. Ethernet provides connectivity from the MOVI-PLC to the SEW-EURODRIVE drive operator panel (DOP) colour touchscreen HMI. "The DOP shows information regarding status, alarms, speed and position," says Craig. "This allows an operator on the gantry to control the gantry movement easily via a joystick, with all relevant information at his fingertips."
The main challenges experienced on the project involved access to the bridge itself. To alleviate this restriction, the majority of the drive system assembly was completed off-site. All controllers and drive components were delivered to SAGE's facility, where the electrical cabinets were assembled and configured before being brought to site for installations. The geared-brakemotors were transported straight to site.
Interestingly, all work was completed in a 'live-bridge' environment. "Due to the essential transport link that the Sydney Harbour Bridge provides, it was not possible to close the bridge at any stage," says Craig. "To accomplish the installation, strategic lane closures—coordinated by the RTA—were kept to an absolute minimum, both in number and duration, and were only used to facilitate the lifting into place of equipment. In this way, impacts to the traffic were effectively mitigated."
Despite these limitations, the installation of the up-rated drive systems for the gantries was accomplished extremely smoothly, and the entire project was completed on schedule. The RTA was so pleased with the finished results that it requested a follow-on project for the remaining two west wing gantries, using exactly the same drive solutions.
Assured future

The success of the project, in Craig's opinion, has been due in no small part to the SEW-EURODRIVE drive solution used, and the company's ability to deliver a complete end-to-end drive package. "Being able to source all components from the one company has simplified matters enormously," says Craig.

"All the components are designed to work together, and this delivers a more compact, efficient and robust drive solution than we'd have been able to source from any other drive solutions provider. Moreover, we have the surety from past experience that SEW follows this up with great levels of service and backup."
The four gantries are now in operation, with the remaining two west wing gantries scheduled for completion shortly. Together, the refurbished gantries will provide complete maintenance access to all areas of the bridge underdeck. This will facilitate the optimisation of regular inspection and repair work of the structure—both safely and efficiently.
The Sydney Harbour Bridge has provided Sydneysiders with a key transport link and a world-renowned landmark for almost 80 years. With the renewed and enhanced ability to inspect, maintain, paint and repair crucial structural elements in the hard-to-access underdeck area, its future looks assured for at least another 80 years.