Fault tolerance and style are key factors in the design of roof for Arthur Ashe Stadium
The world’s largest tennis venue, the Arthur Ashe Stadium, has seen many rain delays since its opening in 1997. Adding a roof was a daunting challenge, one that was finally completed nearly 20 years after the stadium opened.
After New York’s Premier Grand Slam Tournament was delayed five straight times by rain from 2008-2012, the board of the US Tennis Association’s Billie Jean King National Tennis Center decided to put an end to those delays by adding a roof to the stadium in Queens, New York. Morgan Automation Systems Inc. of Alliance, Ohio, was the key contractor. Creating a structure that could quickly open and close in a region known for strong winds was only part of the requirements list for Morgan’s engineering staff.
“There are a lot of technical challenges, but the roof also needed to be aesthetically pleasing,” says Mark Sharamitaro, President, Morgan Automation Systems, Inc. “They wanted it to be more architectural, not industrial.”
The industrial side of those challenges loomed large, which is a fitting term for the mechanical and electrical designs. The two sections of the roof are each 125 feet long, so the full roof has a 250-foot span. Though the roof was designed to keep weight down by using a PTFE wrap to cover the frame, each still weighs about one million pounds.
If the challenges of moving those huge spans wasn’t enough, the strong winds coming off the Long Island Sound added another dimension for engineers to deal with. Whipping winds could turn the panels into wings, forcing Morgan to design roof modules that wouldn’t head towards nearby airports.
That was accomplished by clamping the structures to the rails that guide them along their paths. Additionally, engineers shaped the roof so wind loading pushes the modules down – so long as gusts don’t get too strong.
“A 25-mph wind generates a significant amount of downward force. We can operate when winds go up to 45 mph for a three-second gust. If gusts get up to 50 mph, we need to stop and clamp the roof down,” Sharamitaro says.
It takes some powerful motors to move a million-pound structure in strong winds. Four winches, each with five 30 horsepower motor/brake/gearboxes, move the roofs from fully closed to fully open in six minutes. Closing the roof is literally an uphill battle.
“One of the biggest challenges is that the roof is not flat, it’s on an arc with a 782-foot radius,” Sharamitaro says. “That means we’re pulling the roof up a hill or backing down a hill. While it’s moving, it’s also a challenge to know the position of the roof.”
The curved path poses a major challenge for measuring roof panel positions, which must be monitored closely to facilitate smooth movement. Two encoders are installed on wheels, with duplicates serving to provide redundancy.
Further positioning redundancy is provided by encoders on the winches, which turn a drum that is attached to redundant wire ropes. This drum has dual encoders to protect against failures. Since the roof moves on a curved path, it took some effort to write the software that determines position by measuring drum movement. “The track of the roof follows a curve, so one foot of movement on the drum is not one foot of movement for the roof,” Sharamitaro says.
The system also has redundant anemometers to measure wing speed. All this duplication is mandatory for a system that’s central to the timely start and finish of tennis matches. All components are designed to ensure that rain delays are a thing of the past even if some of the components fail.
“We oversized everything by at least 20%,” Sharamitaro says. “For instance, the winch has five motors. If one fails, any four can meet the requirements and move the roof at full speed. All the communications are sent using PROFINET. The network is tied together with a redundant fiber optic ring. If one fiber has a problem, it won’t’ effect the system.”
The control systems are also designed for both efficient operation and fault tolerance. Each winch drive system has fully regenerative S120 smart line modules with dual redundant feeds. The PLC is an S7-1500 Fail Safe unit that’s combined with an ET-200S Remote IO module with Safety IO. All drives have safety integrated via PROFIsafe. All 16 rail clamps are included in the safety system to make sure the roofs are securely held in place.
Dual safety zones for emergency stops and rope break switches are among the many other elements that help make sure that problems don’t arise. These systems all get continuous workouts, the stadium roof is closed every night. It’s opened many days per year for a number of events other than major tournaments.
Operators monitor the activity using 15-inch Comfort Panel HMIs, which are also duplicated to avoid problems due to failures. When operators initiate a sequence, rail clamps are all released so the roof panels can move. The inflatable roof seals that rainproof the space between roof panels and stadium walls are included in this sequence. Once movement is completed, rail clamps are reactivated to hold the panels in place.
All this happens with no impact on the 23,000 fans who can fill the stadium. That’s important to the staff at Morgan Automation as well as all those who work in the large tennis facility. Like many large technical programs, one important measure of success is smooth operations that happen routinely without fail.
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