That presented a significant challenge for Airbus' manufacturing team in Broughton, U.K., and Electroimpact Inc., Mukilteo, Wash., the prime contractor for wing-assembly automation tools.
Wings are a structural framework of spars and ribs covered with metal panels. The panels consist of a curved, aluminumalloy skin reinforced by stringers that ensure shape and strength. The tricky part is moving assembled panels into four-story high jigs holding the ribs, spars, and leading and trailing edges, for subsequent assembly and fastening.
The huge size and flexible nature of a completed panel up to 111-ft long and weighing 8,818 lb creates a motion-control nightmare. According to Electroimpact's Ted Karagias, handling a wing panel with multiple support points isn't easy. Cranes won't work because the panels distort when suspended, he explains. "Basically you have a statically indeterminate system. The panels twist, bend, and kick as they react to the forces introduced by lifting equipment," he says.
To overcome this problem, Electroimpact devised a manipulator with six coordinated servohydraulic arms to maintain the panel's proper form and precisely control position. "Two of the six arms control the panel's vertical position," he says. "The other four act as slaves imparting a constant programmed force on the wing panel. That way, when the positioning arms are commanded to move up or down, the load-seeking arms follow along to maintain the panel's form."
Each panel-loader arm has four hydraulic-driven axes plus one passive axis, requiring the simultaneous coordination of 24 axes. Each arm has a servocylinder driving the vertical axis. A servovalve and Rexroth HNC 100 servohydraulic controller from Bosch Rexroth Corp., Hoffman Estates, Ill., provide close-loop control. A Temposonics RH SSI linear transducer from MTS Systems measures vertical position, and a Honeywell Sensotec Model 41 load cell reads vertical force.
The servovalve and HNC controller constantly manage pressure on each side of the main cylinder, explains Electroimpact's Charles Hopper. The transducer and load cell couple directly to the HNC for high resolution and minimal communication delays. With a 4-msec scan time, the HNC continuously monitors position and force, and seamlessly transitions between load and position control. It proved to be ideal for the application, says Hopper, controlling position to 0.5 mm and force to about 50 lb.
According to Karagias, the Rexroth HNC controller imparts several important system benefits, namely:
- Reducing the statically indeterminate problem to a determinate one, letting flexible wing panels move as if they were a rigid part.
- Controlling distribution of force on the wing panel to control the panel's shape and how it is presented to the wing structure.
- Simplifying system-level PLC logic and position-control instructions.
- Allowing direct access to all critical system components and providing servocontrol regardless of PLC scan rates or network speeds.
The complete handling system for upper and lower, port and starboard panels has 22 arms (smaller panels require only five arms). In addition to the HNCs and servovalves, it contains 154 directional valves plus associated pressure regulators, check valves, pressure sensors, and flow restrictors.
With 22 HNCs controlling the critical axes, PLC speed is not an issue, says Hopper. Thus, a single Siemens CPU315-2DP PLC runs the entire system. The HNCs and other I/O connect to the PLC via a 400-m long Profibus network running at 500 kHz. The unit also has a redundant, hardwired E-stop.
All the HNC's run virtually the same program for simplicity, with any variations handled in the PLC. The software includes a calibration routine for every axis on every arm, with calibration data stored in the PLC. The program also stores configuration data for eight different panels per face, with three different setups for each panel.
The operator uses a wireless HBC pendant or Siemens MP270B touch screen. After selecting the required face and panel, movements are, for the most part, automatic. All buttons are hold-torun for safety, and the operator may jog any axis at one of two speeds or set it to incremental mode for finer adjustment.
Thanks to a high level of automation, the Broughton plant can produce four pairs of wings a month, making it the largest and most productive wing-assembly plant in the industry, according to company officials.