Dallas Cowboys Unveil 600 Ton Video Board Hoist

Automation and motion control are playing a pivotal role in raising and lowering the massive video board at the Dallas Cowboys' new stadium. The hoist system uses 24 cable drums, 80 7.5 HP motors, 40 ABB ACS800 drives and two master/follower networks to safely lift and position the 600-ton board.

Brad Cobo, ABB's Dallas district regional application engineer for low voltage drives, says the drives on the video board are split into two groups. Thirty-two hoist drives lift the board, and eight stay cable drives keep the board from swaying back and forth when the end zone doors and retractable roof are opened and create significant air movement in the stadium. 

ABB worked on the project with Uni-Systems, the OEM supplier of both the video board hoist and retractable roof with significant experience with other stadium roofs in the U.S. including the Arizona Cardinals Stadium, Minute Maid Park in Houston and others.   

The hoist is split into two groups of 16 drives to independently lift either end of the board and automatically keep the board level. Within each group of drives, one drive serves as the speed master and the other 15 drives operate as torque followers. Cobo says using a master-follower fiber optic network allows the system to share the load equally between all of the drives. "The fiber optic cable passes a torque reference from the master to the follower drives to make sure that each hoist drum lifts its share of the load," he says. 

The stay cable drives all operate in torque control mode to create and hold a specified amount of tension on the stay cables as the board moves up and down. The Siemens PLC in the system uses Profibus networking to provide each of the drives the same torque reference.

Cobo says the biggest challenge with the project proved to be the PLC programming done by Alex Krueger of Uni-Systems, and making sure that every drive is ready before the final command is given to release the brakes. Strict sequencing controls in the software guarantee that each drive is ready before releasing the brakes. To properly operate the hoist drives, the software sets a low speed reference of just a few hertz which causes the drives to build torque. 

Because actual torque values are being communicated back to the PLC, the program can confirm that the hoist drives are generating torque before releasing the brakes and increasing the speed reference. At this point, everything is moving and the system continuously monitors all speeds and torques to see if anything is out of range. If any one part of the system should malfunction or do something unexpected, it automatically shuts down and sets the brakes.

"The sequencing algorithms make the system safe and proved to be the most difficult part of the project," says Cobo.

One of the reasons Uni-Systems selected ABB for this project is the direct torque control feature of the ACS800 drives. Direct torque control (DTC) enables a high level of motor performance without using encoder feedback, similar to the way a vector drive maintains a motor model or mathematical simulation of motor operation. The motor model in the ACS800 drives that updates at 40,000/sec allows the system to compensate very quickly for changes in load torque.

Additional drives control the stadium's retractable roof that consists of two panels, one on each side, that weigh about 1.68 million lb each and travel 215 ft on what is essentially railroad tracks. Cobo says the roof can open and close in just under 12 minutes. Each panel uses 64 7.5HP motors to generate about 800,000 lb of pulling force and move the panels up the 24 degree incline.

The motors are powered by ABB ACS800-U11 line regen drives, and a total of 32 motors driven by eight separate drives on each side of the panel. The line regen drives allow regenerative braking energy to be fed back to the utility instead of being wasted as heat through traditional dynamic braking resistors. The cost of the energy being regenerated is actually small, compared to the savings from not having to purchase and install brake resistors.

The drives are connected together using Profibus and a fiber-optic network architecture to form a master/follower system similar to the hoist. The master drive receives a speed setpoint from the PLC, and the other seven drives get a torque setpoint from the master.

Cobo says this approach allows all of the drives run at the same torque and share the load equally. To open or close the roof, the PLC sends the speed signal and start command to each master. Progress is monitored and, if either side of the panel gets ahead of the other by more than 0.25 inch, the leading side is commanded to slow down.  Direct torque control allows the system to operate open loop, eliminating additional wiring and the cost of encoders on the motors.