Case Study | PowerFlex DC Drives

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King County Preserves the Past in New State-of-the-art Drawbridge

When a Seattle drawbridge needed to be replaced, officials wanted to keep not only its familiar look but also the tradition of using DC drives for bascule-type bridges.



  • A new drawbridge replacing Seattle’s iconic South Park Bridge required a modern drive control system to support safe, reliable bridge operations.


  • PowerFlex® DC digital drives precisely control motor velocity and position for raising and lowering bridge draw spans.
  • CENTERLINE® 2100 Motor Control Centers integrate power and control in one centralized platform.
  • ControlLogix® controller manages bridge subsystems and functions.
  • Allen-Bradley industrial computers and FactoryTalk® View software provide graphics-rich displays for bridge operations, maintenance and inspections.


  • The drive controls provide reliable operation to support the flow of ground and marine traffic.
  • Commonality with other Seattle bridge control systems helps maintain consistency in maintenance and inspection procedures. 
Preserving a link to the past

When Seattle’s decades-old South Park Bridge closed, the thousands of commuters who crossed the bridge daily lost a vital connection between the city’s south-end working-class community and the rest of the city.

Officials vowed to replace the iconic drawbridge that had carried traffic over the Duwamish River since 1931. And within a matter of months, funding was indeed secured for a multi-year, total project cost of $175 million for replacement of the bridge.

A critical element of the new bridge was the drive controls used to operate the raising and lowering of the bridge’s draw span. While only a small fraction of the total project cost, the controls were essential to providing safe, reliable bridge operations. At the same time, it was also important that the drive controls share commonality with the controls used in other Seattle bridges to streamline maintenance and inspection activities and ensure that any local bridge operator could easily adjust to the new bridge’s system.

Everything old is new again

During World War II, thousands of workers crossed the South Park Bridge to help produce Boeing’s B-17 Flying Fortress – and later other versions of Boeing’s heavy bombers. Millions of tons of barge and ocean-going vessels also passed through the bridge over the years to help sustain local industries.

These decades of heavy use left the bridge in poor condition, with King County officials spending significant time and resources on maintenance and refurbishment to keep the bridge operational.

“The bridge started settling, or moving, and we had to continuously alter it to allow it to land and align properly,” said Larry Finnick, bridge electrical and mechanical maintenance supervisor for SDOT Roadway Structures who operate and maintain the bridge for King County. “The underwater cross waterway wiring was also failing. Wires were shorting out and causing problems with the operations. And electrical components were outdated and obsolete.”

A 6.8-magnitude earthquake in the Puget Sound region only further compromised the structure. Shortly after the 2001 Nisqually earthquake, King County inspectors gave the bridge a score of 6 out of a possible 100 and eventually closed the bridge to ground traffic in 2010 for safety reasons.

When it came time to replace the bridge, King County, who owns the bridge, sought to create a new, state-of-the-art bridge – while still preserving some of its history. The old South Park Bridge was, after all, on the National Register of Historic Places. Residents also made it clear that they wanted it to keep the bridge’s familiar architecture.

But it wasn’t only the bridge’s look that needed to be preserved.

Planners also decided to continue the tradition of other Seattle drawbridges using DC drive technology for the movement of the structure. While AC drive technology has become the solution of choice for most heavy movable applications, the use of DC drive technology would keep inspection and maintenance activities for the new bridge consistent with other city owned and operated movable bridges.

Drawing on experience

Systems Interface, a systems integrator based in the Seattle area and Solution Partner in the Rockwell Automation PartnerNetwork™ program, was tasked with designing, specifying and installing the new South Park Bridge control system. To do this, the company leveraged its extensive experience delivering control solutions for movable bridges, including drawbridges in the Seattle area.

For the bridge’s DC drives, Systems Interface chose the Rockwell Automation PowerFlex DC digital drives. The drives provide precise speed and current regulation, easy programmability and extensive diagnostics. Allen-Bradley CENTERLINE 2100 Motor Control Centers (MCCs) were also selected for integrated control and power in one package.

For the bridge’s automation control, Systems Interface chose the Allen-Bradley ControlLogix controllers. While various steps of the bridge’s operation would be initiated manually by an operator in the bridge’s control tower, the controller would manage dozens of subsystems and functions, including alarm enunciation, center lock actuation and brake sequencing.

An Allen-Bradley integrated display computer with FactoryTalk View software was chosen to give workers in the control tower access to critical information about bridge operations, maintenance and inspections.

Reliable, familiar bridge operations

King County celebrated the grand opening of the new South Park Bridge four years to the day after the old bridge closed and decommissioned.

Systems Interface supported the on-time opening by taking advantage of the premier integration between the controller and drive. The ability to configure the two devices in one design environment and use integrated drive profiles in that environment helped save development time. Premier integration also provided easy access to drive diagnostics for maintenance purposes.

The control solution that Systems Interface implemented has brought reliability to the South Park Bridge’s operations and consistency with the control systems used in other City of Seattle owned and operated movable bridges, while also providing additional operational benefits.

“We’ve been using Rockwell Automation drives for a while, and one of the reasons we like them so much is because they load share,” Finnick said. “You don’t have a differential. You just have two separate motors on each side of the bridge, and they lift the bridge up and down and share the load.”

Looking to the future, Systems Interface’s movable bridge know-how combined with Rockwell Automation’s product lifecycle support may prove valuable to sustaining the new bridge’s control system for the long-term – the very long-term. The South Park Bridge has an anticipated 100-year lifespan, and over that time, the control system will require migration to new processors, I/O cards and platforms.

“I’ve been doing drive systems for over 20 years, now, and I’ve always been really impressed with everything from Rockwell Automation,” Finnick said. “I love the ease of operation and of reading and manipulating the logic. But I’ve also been really happy with Rockwell Automation’s ability to support migrations. Working with them, I know we’ll always have the support we need.”

The results mentioned above are specific to King County’s use of Rockwell Automation products and services in conjunction with other products. Specific results may vary for other customers.

Trademark Information:

Allen-Bradley, CENTERLINE, ControlLogix, FactoryTalk and PowerFlex are trademarks of Rockwell Automation Inc.


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