Motion controls take circus for a spin

150-ton stage rises 70 ft at 2 fps and rotates 360°. And, oh yeah, the performers go along for the ride.

Jeffry Schatz
Senior R&D Engineer
Delta Tau Data Systems Inc.
Chatsworth, Calif.

Cirque du Soleil (Circus of the Sun) was conceived in 1984, in Montreal. Unlike traditional circuses, there are no animals. But what Cirque lacks in carnivorous cats, it more than makes up for in acrobatic artistry.

Cirque's production of K¡ at the MGM Grand in Las Vegas is the theatrical interpretation of the epic saga of Imperial twins who embark on a perilous journey to fulfill their destinies. The show's title, K¡, is inspired by the ancient Egyptian belief in the "ka," an invisible spiritual duplicate of the body that accompanies every human being throughout this life and into the next.

Danger lies in wait at every turn. Archers and spearmen hunt the twins relentlessly. Their quest takes them through a succession of landscapes — from mysterious seashore to mountains and forests. But what makes this production unusual (not to mention challenging) is the massive stage that rises, spins, and tilts while performers seemingly defy the laws of physics.

The stage, called the Sand Cliff Deck, accompanies the twins throughout the 90-min performance. It begins as an ancient ship emerging from the depths of the theatre, transforms into a vertical battleground, and then an ice glacier. Projected images bring the stage to life. The deck, weighing approximately 80,000 lb, is the largest of several moving performance spaces in the show. Add the weight of the support structure for a total of 350,000 lb. The deck measures 25 X 50 ft with a 6-ft depth.

In a scene called "Climb," artists ascend the vertical surface untethered, scaling to the top with the aid of 80 pegs hidden in the deck. With artists positioned on the deck, it rotates a full 360°. The automation-board operator retracts pegs that the artists are holding onto to clear the path for their fall into the abyss, beyond the view of the audience. In "Battle," another scene, the artists are suspended by handheld rappel lines.

Behind the scenes, technology directs the action. Industrial hydraulics and electromechanical lifts raise the two main stages and five smaller platforms from a basement several floors deep to a height of 70 ft. An inverted gantry crane moves the 150-ton deck at 2 fps. And the deck tilts up to 100° at 2°/sec. The hydraulic system was designed by Atlantic Industrial Technologies, Islandia, N.Y.

Another performance, this one courtesy of Delta Tau, Chatsworth, Calif., and Tisfoon Ulterior Systems, Raleigh, takes place simultaneously and without fanfare. The two companies provide the 24-axis motion-control technology for the production.

Amir Pirzadeh, president of Tisfoon Ulterior Systems, the company responsible for the motion-control systems, says Macro Ring fiber optic was selected because it allows up to 1,500 ft between nodes on the ring, important given the size of the K¡ stages and moving components. Also, the fiber-optic fieldbus is unaffected by electrical noise.

Four 70-ft-stroke hydraulic cylinders, manufactured by Parker Hannifin's custom cylinder division in Eugene, Oreg., lift the gigantic stage vertically. Two Macro nodes control each of the two hydraulic cylinders that lift the stage. All axes are controlled via closed loop using the Macro fiber-optic communication system between nodes.

To hit 2 fps, five 250-hp hydraulic motors feed 2,200 gpm at 1,700 psi. The entire I/O for the pump room is controlled by one Macro node. The Delta Tau UMAC (Universal Motion and Automation Controller) simultaneously delivers motion profiles to the four proportional valves, which create synchronized motion. The UMAC controller maintains system variances between the four vertical cylinders at less than 0.125 in. Another Macro node controls the four-hydraulic cylinders that control the tilt axis.

Macro Ring transmits at a true data rate of 100 Mbps. Raw data is transmitted at 125 Mbps, with 2 of every 10 bits redundant for low-level error checking. Packets contain 72 bits of transmittable data and 24 bits of supporting data (a header byte, a ring-address byte, and a checksum byte for a second level of error checking) for a total of 96 bits. This permits packet transmission within a microsecond. One packet contains all the data necessary for closed-loop axis control. The clock signal on the ring's master controller drives the timing of the ring transmissions and fully synchronizes the hardware and software.

The UMAC is a modular system built with a set of 3U-format Eurocards mounted inside 3U racks. The system is extensible with power supplies, 3U servoamplifiers, I/O boards, and PC104 processor.

The 5-in.-diameter hydraulic cylinders were mounted rod side down so the stage rides on an increasingly larger column of oil as it travels to the top of its 70-ft stroke. During the climb, the physical properties of the column of oil change. The larger column of oil behaves less like a solid and more like a sponge, even at 1,700 psi. As a result, the proportional gain was set especially low. To compensate, integral gain was used during motion. However, fixed P and I values were incompatible with the changing physical qualities over the travel span.

The controller was flexible enough to dynamically change the P and I values based on the height of the stage. The controller also acts as a dynamic brake, boosting the I value during the deceleration portion of the profile. The dynamic gains overcome the 1.6-Hz natural frequency of the structure and provide smooth motion with accurate (0.1-in.) positioning.

The four hydraulic cylinders are split into two pairs separated by 60 ft. One pair controls stage left and the other, stage right. Each pair is further split into an upstage and a downstage component. Design criteria require that the load be equally balanced between the upstage and downstage cylinders.

"The UMAC controller let us write a PLC program that closed the load-cell loop and dynamically delayed or accelerated the upstage motion profile to balance the load-cell reading of the downstage cylinder," says Pirzadeh.

The control system is kept in open loop when the stage rests on its mechanical brakes. Pilot-activated hydraulic blocking valves are used as a safety backup. Transitioning the massive 150-ton stage from open-loop brakes applied to closed-loop motion control was a challenge overcome by writing a custom algorithm for the closed-loop charge-up of the cylinder.

The algorithm running in a PLC applies hydraulic pressure to each cylinder while monitoring the cylinder's pressure transducer as feedback. When all four cylinders indicate enough pressure for holding up the stage, the mechanical and hydraulic brakes release and closedloop control commences. Fine-tuning the transition at different heights with different columns of hydraulic oil was especially challenging.

The size of the stage and its 2-fps top speed make this an atypical motion-control application. For example, such a large structure cannot be stopped without decelerating or serious injury to the actors and mechanical structure could result. A fail-safe emergency stop is able to bring the massive stage to a halt in 1.8 sec from its top speed. In a traditional following error or amplifier fault condition, motion controllers immediately cancel control and put out zero volts. The virtual axis technique developed by Delta Tau sends input (position feedback) to a virtual-axis (logical motor) register.

"By programming the virtual-axis register to send output as a velocity command to a high-speed PLC, the analog signal was ramped down to zero," says Pirzadeh.

During the show, performers intentionally fall from heights up to 60 ft into two safety nets controlled by 18 hydraulic winches. According to Pirzadeh, winch programming used the template feature of Tisfoons' Code Generator, which created an object-oriented framework within the Delta Tau programming language. To deploy the safety nets, the operator first moves the winches close to the final position. Here they are placed in a tension mode that brings the nets to the required final tension.

Stage Technologies' Nomad Motion Control System controls overall show automation. The Nomad controller is the show operator's direct interface and broadcasts position, velocity, acceleration, and timing messages over Ethernet in UDP (user datagram protocol) format. Traditionally, the Nomad sends these messages to a Siemens PLC. In the K¡ application, an HMI developed by Tisfoon and running on its 1.6-GHz PC104 computer with dual RAID hard drives receives commands from Nomad, then translates and sends them to the UMAC controller transparent to Nomad.

The Tisfoon HMI records diagnostic information every 0.1 sec. The information goes on VCR tapes archived and saved for every 24-hr period, which includes two shows. If a system malfunction were to occur, there's a sequential record of the event leading up to and including the malfunction itself. The events can be played back for analysis to determine appropriate corrective actions.

"The use of the Macro Ring fiber-optic fieldbus and UMAC controller gives the flexibility we need to maintain precision positioning of K¡'s main stage, from the time it rises from the basement until the end of the performance," says Pirzadeh. "While this is an unusual application for an industrial motion-control system, it demonstrates many of the issues that conventional motion-control solutions must address."