December 2008

Factory Automation

Rollout and retractable in the desert

The Arizona Cardinals are moving more than goalposts at their new stadium; higher-tech motor starters and drives tackle new adversaries out West

Fast Forward

  • Motion control includes retractable roof and retractable playing surface.
  • Check the drive torque before each roof motion; measure characteristics from the motor at 0 Hz.
  • Parallel, fiber-optic communications network each group of eight VFDs.
By Brian Johnson and Steve Weingarth

As professional sports venues become bigger and more complex in design and scale, architects and engineers are looking for bold new ideas.

Retractable roofs have become a popular feature in these structures because they provide the ability to control the stadium's interior environment more effectively. Minneapolis based-Uni-Systems is one of a few companies that mechanize and control these immense structures, which, themselves, become architectural feats.

With its curved roof track, the Arizona Cardinals' project presented interesting challenges in designing the retractable roof mechanism.

Take the grass outside

When the National Football League's Arizona Cardinals made the decision to construct a new arena several years ago, the desert climate was a major consideration in the design plan.

The heat can take its toll on fans and players, alike, and can be detrimental to the playing surface, as well especially if it's natural grass. On the other hand, in cooler months, the world-famous climate is perfect for hosting outdoor activities.

The exterior design of the stadium resembles the basic form of the barrel cactus and is the creation of renowned architect Peter Eisenman and HOK Sport.

The retractable roof closes so the facility can be air conditioned in the hot months, and then opened in the cooler months. The roof's panels consist of a polytetrafluoroethylene (PTFE) coated woven fiberglass fabric and are much lighter than a traditional, clad roof. 

The stadium design includes not only a retractable roof, but also a retractable playing surface.

The innovative, rollout field will save an estimated $50 million in operating costs, as it is more economical to move the field outside, rather than having the entire roof retract to allow the necessary sunshine to reach the grass.

The retractable, natural-grass playing surface is contained in a 16.9-million pound tray that is 234 ft. wide by 400 ft. long, and it is the first of its kind in North America.

Motor starters activate the motors that retract the field. Overall, the design of the stadium is unique enough that it has been a feature story on "The Discovery Channel."

Direct torque control

This roof was different from any the contractor had previously constructed, according to Lennart Nielsen, Danish master electrician and senior electrical designer with Uni-Systems.

One of the most important decisions was selecting motor drives to control the roof's movement.

"An important factor in choosing the drives was the inherent risks associated with running a roof on a sloped track," said Nielsen. "This caused us to look for a Variable Frequency Drive (VFD) that would allow us to test the drive torque before each roof motion, to ensure that each drive was operational and capable of a 100% torque output.

"Before each motion, the Programmable Logic Controller (PLC) checks the torque output from the VFDs at 0 Hz, before committing to opening the motor brakes." Nielsen also said the drives could work without the need for closed-loop encoders, which was a cost-effective option that helped the company meet budget requirements.

Nielsen said that for other roofs that his company has designed, if the brakes were to release and the motors did not start up for some reason, the roof would simply remain in place. However, at the Cardinals' new stadium, and with its sloped roof track, if the operators released the brakes and the motors did not start up, the roof sections would slide right off and fall into the parking lot.

"With the motor system we used and its direct torque control, it can measure the feedback from the motor much more accurately than a standard drive," said Nielsen. "And its control/output algorithms can measure the characteristics coming back from a motor at 0 Hz (meaning that they start the energy field but don't rotate it), and that's a big reason why we chose them.

"We wanted to have this capability all the way down to 0 Hz, and none of the systems except this particular could guarantee that," Nielsen added.

Critical roof functionality

The retractable roof consists of two moveable panels suspended between two parallel tracks running along the east and west sides of the structure. The tracks curve to follow the roof's slightly domed profile, which slopes down from its apex at the 50-yard line toward the north and south ends of the building.

Each roof panel rests on eight, two-wheeled carriers-four along the west and four along the east side of the roof panel. Each set of four carriers forms one quadrant of the entire retractable roof system.

Conventional techniques-such as the powered traction wheels that Uni-Systems used on previous stadium projects-were not an option for the Arizona stadium's sloped roof. Instead, the company designed a system in which each roof panel tethers to four 1.5 inch-diameter steel cables on each side.

factory 3

There are two cables running on each side of the roof rail, and each wind up on its own 48-inch-diameter cable drum. The cable drums are on each side of the two upper carriers for each roof panel quadrant, and the two lower carriers in each quadrant do not use power.

Each cable drum is equipped with a bull-gear along the outer rim, driven by four, geared 7.5 HP, 480 VAC motors with spring-set brakes. The four motors per drum are controlled by two, 20 HP VFDs, meaning each roof quadrant is powered by 16 motors controlled by eight VFDs.

Accurate control of the VFDs was essential to distribute the load evenly to the roof cables. "It was deemed inadequate to let the roof PLC act as referee for each individual drive via the Profibus that was to handle the regular data communications between the PLC, VFDs, and remote I/O," said Nielsen. "Instead, we used a parallel, fiber-optic communications network between each group of eight VFDs, where one VFD was designated as Master and the other seven as Followers."

Once the roof is moving, it is extremely important to keep a very tight torque-and-speed envelope around each Follower drive in relation to each roof quadrant's Master VFD," Nielsen said. "The direct torque control system provides the means for doing this via the fast intra-VFD fiber-optic network."

The PLC issues a speed (frequency) command to each of the two Master VFDs (one per side), and the seven Follower drives then match the torque output of the Master drive.

Each roof panel's PLC handles the position alignment between the two quadrants of each roof panel, which receives position feedback from an absolute encoder in each quadrant and from incremental encoders on each cable drum.

If a roof side gets more than two inches ahead of the other-the rails are 257 feet apart-the PLC will signal the Master of the leading side to slow down until the two sides are again in alignment.

Since the roof rails are curved, the actual cable load increases as the roof panel moves toward the fully open position and the steeper sections of rail. For optimum motor torque, the VFDs output 60 Hz at the lower half of the rails, and to decrease operating times, 85 Hz on the upper half.

Motors operate in motoring and generating modes, generating when the panels lower down, and motoring to lift and close the panels.

The drives require no maintenance. They reside in air-conditioned enclosures on the carriers.

Cable oscillation tests drives

During initial testing at the Cardinals' stadium, technicians found natural frequencies in the drive cables caused some oscillation or whipping in the cables as the roof opened. "And the faster we ran, the more pronounced it was," said Nielsen. "We saw that the drives actually made it worse. As each cable oscillated, the anchor points of the cables would see a varying torque. The master drive would then react to those changes by increasing or decreasing its torque output. And its torque profile would then be transmitted to the other drives that reacted to it, causing the whole cable system to start into harder and harder oscillations."

The challenge then, was to get the drives tuned in to deliver optimum control and eliminate the bounce in the cable as the roof opened.

"Our natural reaction would have been to just open up the tolerances more to allow a larger window around the optimal speed and torque to allow a little bounce without the drives reacting to it. But the manufacturer's application engineer Steve Boren, went the opposite way and actually made that window extremely small so as not to allow it to react harshly enough to cause the oscillation. We got everything to smooth out and work extremely well," Nielsen said.

"We simply needed to utilize a standard software feature, which allows for Loadshare (torque) followers to have, also, an over-riding speed window about the Master drive's coordinated speed reference," said ABB's Boren.

"Because of the uneven cable stretch, which can be viewed as slip between the driven cable drums, it's tough to make the drums share the load evenly. But by activating the unit's Speed Window capability in the Torque follower drives, and limiting the window (slip) to only 2 rpm on each motor, the cable drums have no choice but to evenly share the load of the immense roof."

More stadium projects are in the pipeline.

In Indianapolis, the Colts football team is replacing the RCA Dome with a state-of-the-art retractable roof stadium. "The system required for that roof is much more complex," said Nielsen. "Instead of a nearly one-to-one, width-to-length ratio, the Colts' panels are around five-to-one. This has resulted in a five-rail design, rather than a two-rail as in the Cardinals stadium, and twice as many cables and drives.

Also in progress is a design for the new Dallas Cowboys' stadium. Both stadiums will use the newer regenerative drive, which was not available when the Cardinals stadium was designed. The Cardinals stadium design uses stand-alone regenerative drives working with the VFDs.


Brian Johnson ( is principal owner of Industrial Drives & Controls a systems integrator and manufacturers' representative in New Berlin, Wisconsin. Steve Weingarth ( has a BS in Applied Science and 30 years of experience in the drive business. He manages the low-voltage drive application-engineering group for ABB.

Facts: Up on the roof

  • Two retractable roof panels at 1,100,000 lb. each (550 tons)
  • Each roof panel's dimensions: 185 ft. long by 285 ft. wide, 16 ft. deep
  • Eight cables (1.5 in. diameter) connect each retractable roof panel to the stadium structure at the 50-yard line (over ½ mile of cable used)
  • 32 motors (7.5 HP) power each roof panel (480 total horsepower for stadium)
  • 16 crane wheels (36 in. diameter) support each retractable roof panel
  • 595,000 lb. of mechanization equipment used to make the roof move
  • 257.5 ft. span of retractable roof over stadium bowl
  • Two rail lines with 175 lb./yd. crane rail (over ¼ mile of rail used)
  • Maximum travel speed: 25 ft./min. or ¼ mph
  • Total travel time: 11.5 min. (10 min. run time, 1.5 min. of final positioning)



PLC (Programmable Logic Controller) is an electronic microprocessor device that stores and executes automatically a series of programmed commands that produce a machine's sequence of operation.

Torque (physics) is a vector that measures the tendency of a force to rotate an object about some axis. Just as a force is a push, or a pull, we can think of a torque as a twist.

Encoder: An encoder comes in two architectures. The first architecture is linear. The second architecture is rotary. Both types sense mechanical motion and translate the information (velocity, position, or acceleration) into useful electrical data.

Closed loop: In a closed-loop control system, a sensor monitors the output (velocity, position, or acceleration) and feeds the data to a computer which continuously adjusts the control input as necessary to keep the control error to a minimum (to maintain the desired velocity, position, or acceleration).

Regenerative drives (AC or DC) have the capability of taking DC power produced by the motor during braking, converting it to AC power, and putting it back into the power lines

Variable frequency drive (VFD) is a specific type of adjustable-speed drive that controls the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. VFDs are also list as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, or inverter drives.