Automating the World's Largest NFL Stadium

As the saying goes, "everything is bigger in Texas." Cowboys Stadium, the new home of the Dallas Cowboys, is certainly no exception. At a cost of almost $1.3 billion, this enormous structure encompasses more than three million sq ft, and seats up to 100,000 fans. Currently, it is the largest enclosed NFL stadium, has the world's longest single-span roof structure, and the world's largest high-definition video display.

While football might be what this iconic structure is best known for, the engineering that went into designing it is no less impressive.

Retractable Roof  

The retractable roof is comprised of two moveable panels - each weighing 1.68 million lb. Supporting that enormous load, a full 3.5 percent of the entire roof weight, are two box trusses that span the length of the stadium (1,225 ft). Sitting on top of that truss is a steel rail similar to that used by train cars. The panels roll freely along this rail, but are anchored in place by a gear rail, or rack and pinion system. This is a critical component that
allows a team of 128 7.5 hp motors with planetary gear reducers to pull the panels up the inclined roof.

The slope of the incline varies, up to 24 degrees when the panels are fully open. To accommodate this, multiple gear motors were chosen to provide redundancy and manage the safety risk created by the steep travel path. The multiple motor brakes and gear teeth engaged with the gear rack prevents the failure of any single component from allowing the roof panels to literally roll off the roof and fall into the parking lots. This redundant design also allows the retractable roof to be operated with up to five of the 32 motors in each quadrant off line.

The steep incline of the roof created an additional problem for the design engineers. When the panels are fully open and a command is given to close, the motors must start under an enormous load. This high starting torque demand normally requires high-performance motor control to be provided by a closed-loop vector drive. This approach, however, was not preferred due to the high cost and complexity associated with so many motor encoders. Instead, the engineers at Uni-Systems opted to use ABB's Direct Torque Control (DTC), which allows an almost identical level of performance without the use of encoders. Using a 100MHz digital signal processor, the DTC algorithm calculates the current state of the motor 40,000 times per second and determines the best IGBT switching pattern to produce the needed torque.

The ACS800 drives are also line-regenerative drives. This allows the drive to decelerate the motors without the use of a brake resistor. As the panels move from an almost level and fully closed position to a fully open position on a downward slope, they cross a point where the motors transition from motoring to braking. It is during this braking phase that the drives are called upon to slow the motors and keep the opening speed under control.

This braking technique is accomplished by converting the kinetic energy of motion to electrical energy inside the drive - in a process called dynamic braking. Normally the drive dissipates this excess energy as heat, using a brake resistor in much the same way a car braking down a hill causes the brakes to get hot. This heat, or thermal energy, is essentially wasted. In a line regenerative drive, this energy is sent back to the utility instead. While the amount of energy recovered is small, about $14 per opening cycle (128 motors * 7.5hp each * 746 Watts/hp * 96 percent efficiency * 12/60 hours * $0.10/kwh)/1,000 = $13.75), the benefits of not having to install brake resistors were significant, and justified the additional cost of line-regenerative drives.

Load Sharing

Coordinating the motion of 128 gear motors to ensure they all work together is no small feat. To handle this, a total of 32 drives, divided into four groups of eight, are used to drive the 128 gear motors (four motors per drive). Each group of eight drives has one speed-controlled "master" and seven torque-controlled "followers." This so-called master/follower network allows these individual motors to work together as a team. The master drive runs at a speed directed by the PLC (Siemens Simatic S7-300) over Profibus and, using the DTC algorithm, calculates the actual torque needed to maintain this speed before sending this value over a fiber-optic link to the follower drives as a torque reference.

The follower drives run at whatever speed is necessary (subject to a speed window) to achieve this torque value. This type of arrangement ensures that all of the motors share the load equally. And with a master/follower update time of 2ms, the system responds to changes in load very quickly - ensuring that the roof panels stay well away from the parking lots.

While the Profibus network that connects each drive to the PLC allows both control and monitoring, it can also be used to implement one of many safety features critical to the retractable roof system. One such feature is called "torque proving," which ensures that each motor is online and generating torque before allowing the brakes to be released. For example, whenever a roof panel is moved, the PLC sends all master drives a run command at a very slow speed set point. Since the brakes have not yet been released, this causes the drives to generate torque.

Each drive reports its actual torque back to the PLC, where it is compared with a minimum value. Once the PLC sees that all drives are generating at least this amount of torque, it gives the final command to release the brakes. At
that moment, the drives assume control over the panel movement and begin
accelerating.

Making the Video Board Move

The video board at Cowboys Stadium, manufactured by Mitsubishi Electric Diamond Vision, features two 72-ft-high x 160-ft-long displays that span from one 20-yard line to the other. Each is a true high-definition display, capable of 1,920 x 1,080 resolution in a 16:9 format. On the ends are two smaller displays, measuring 27 ft high x 48 ft long.

Initially, the video board was suspended from the roof at a fixed height of 90 ft. However, it soon became apparent that some method of raising and lowering it was needed. An upcoming concert by the band U2 required that the board be raised another 10 ft to accommodate their stage equipment. The Cowboys once again turned to Uni-Systems to design and install a hoist system for the video board. By using a cable drum design borrowed from previous stadium projects, they were able to complete the job with minimal cost and in a very short amount of time. The plan involved distributing the weight of the 600-ton board over 16 hoist drums, using four motors per drum for redundancy and load sharing.

Each end of the video board is supported by a group of eight hoist drums, giving the ability to move each side independently should the board need to be leveled. When working together, they can raise or lower the board from 25 to 115 ft above the field. Four 5hp motors per drum are used and fed from a single ACS800 line regenerative drive. With one master and seven followers, the load-sharing arrangement is identical to that used on the roof. The torque-proving safety feature also was implemented here as one of several layers within the overall safety system.

An additional eight drums were installed, but not for lifting purposes. These so-called "stay drums" use cables anchored to the four corners of the video board. This prevents any undesired back-and-forth swaying motion due to air currents passing through the stadium when the roof and end-zone doors are open. These drums are torque controlled and maintain a fixed tension on the stay cables any time the main hoist is operating. A speed-windowing feature in the drive also was used to place a limit on the speed the drive can run, while trying to maintain its torque setpoint. This important setting prevents the drives from moving cable in or out too quickly, and thus provides a degree of damping which serves to resist any abrupt video board movements. The tuning involved to make this degree of control possible was done with DriveWindow software from ABB, which is used to monitor the movements of several drums at once. With so many motors and drives required to operate in harmony, this real-time monitoring ability was essential.

Brad Cobo is regional application engineer, low voltage drives, ABB Inc.