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Moving Views From Chicago’s Skydeck

Recently completed and opened for visitors in 2010, The Ledge at Chicago’s Willis Tower (formerly Sears Tower) is the newest attraction to the Skydeck on the 103rd floor.

Recently completed and opened for visitors in 2010, The Ledge at Chicago’s Willis Tower (formerly Sears Tower) is the newest attraction to the Skydeck on the 103rd floor.

The Ledge is a viewing platform made up of four separate “cubes” of glass where visitors can stand beyond the outside edge of Willis Tower and look out or straight down to Wacker Drive and the Chicago River. You can even look back towards the building and see the outside of it.

According to the Skydeck website, Willis Tower is the tallest building in the western hemisphere and The Ledge extends 4.3 feet out from the Skydeck, which attracts more than 1.3 million visitors annually. The cubes making up The Ledge move individually, yet are fixed in the extended position for visitor viewing. They are retracted to align with the rest of the outside surface for window washing, and can be retracted for maintenance.

U.S. Equities Asset Management, Willis Tower’s management and leasing agent, chose Johnson Controls to design and produce a control system that integrates a number of subsystems to move four glass cubes in and out of the Skydeck.

The cubes, which are designed to withstand up to 125 mph winds, have five surfaces that are made of three-layers of laminate glass panels and are open on the sixth side to the interior of the building. During construction, the building’s original glass windows were removed and replaced with 1,500 pound glass panels that are 1½" thick and hung from a steel frame. The exception is the frame-less bottom that uses small glass-to-glass, stainless steel fasteners to enable an unobstructed view of the 1,353 feet below.

Central to moving a cube in and out is a control system that includes what looks like a giant garage door opener with an inch and a half wide chain. Each of the four systems controls the motion of a cube as well as a pneumatic perimeter seal boot, which extends the full length of the cube. This boot is inflated to seal out exterior air when the cube is extended to keep cold and hot air (depending on the season) outside of the building.

The control system orchestrates the following sequence of events to move a cube:

  • Deflation of boots.
  • Pressure monitoring to verify deflation.
  • Locking pin retraction.
  • Sound an audible alert.
  • Verify locking pin retraction with mechanical limit switch.
  • Start the drive motor.
  • Increase the drive motor rate of rotation gradually to rated speed.
  • Release the motor brake upon reaching minimum speed.
  • Enable the visual warning beacons.
  • Sound 2nd alert continuously during movement.

While the cube moves, it constantly monitors its position. It slows as it nears either of two stopping points, and locks the cube into place. The cube locks in at one of three positions: extended, flush/retracted, and fully retracted/maintenance. This process is reversed when extending the cubes to their usual position beyond the side of the building.

Forty one conditions are monitored by the control system, including: presence of control power, a torque limit switch, drive fault, drive running, locking pin retraction, motor brake open, retracted over travel, retracted stop, flush retracting, flush extending, extended stop, and extended over travel limit switches.

Because of each cube's weight, they are moved slowly to decrease momentum and allow precision stopping. It takes approximately 15 minutes to move a cube from its extended position to fully retracted.

"It was important to be able to accurately position the cubes for locking. A way of knowing the position of the [cube] very accurately was needed," says Johnson Control’s Rich Harter. "We specified position sensors from Novotechnik and they met this requirement well.”

“We needed a high-resolution position sensor that would also have 14 feet of linear travel," adds Rich Nabholz, a systems engineer at Johnson Controls. "This is because the cube is moving and it has to slow gradually to a stopping point within 0.1" so that 1¼" locking pins line up precisely with 1½ inch holes.”

At the heart of the control system are four Novotechnik TLM Series position sensors -- one for each cube. The sensors use touchless magnetostrictive technology, a direct, precise, and absolute process employing a free-floating magnetic measurement marker. The position marker is available in a no-contact version or a guided piece that uses guides incorporated into the sensor housing. For The Ledge, Johnson Controls chose the free-floating marker with 4 to 20 mA current output.

Each marker is attached to a cube, while the sensors are fixed in place. The sensors have a resolution of ?0.01% and repeatability of ?0.02%. While not all applications need 11' sensors, the TLM Series provided the ability to resolve to 0.013" to readily meet the lock-alignment tolerance. Because the TLM Series are absolute sensors, their resolution is the same tight spec, regardless of the stroke length. The 11' stroke length of the position sensors measure a cube travel range of 10' 5-5/16".

The cubes travel at an average speed of about 0.7 feet per minute, so the 16 kHz output update rate could handle the new measurement data that output every 62.5 ?sec.

Johnson Controls selected analog rather than start/stop, synchronous or 48-bit DyMos output options for The Ledge, feeding a 4 to 20 mA output signal to their control module.

The Ledge has been open for more than a year now, offering spectacular views. "The cubes have been a big hit," says Nabholz. "People lined up for several weeks following the opening, each waiting his/her turn to stand on The Ledge."

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