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
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
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 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
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
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.