Constructing the largest commercial aircraft in the world involves engineering ingenuity in design of the plane, as well as the tools to assemble it. Up to 111 ft long and weighing more than 4 ton, one wing panel of the Airbus A380 creates assembly challenges. Electroimpact Inc., Mukilteo, Wash., the company charged with designing and installing the assembly automation, used components from MTS Sensors, Bosch Rexroth, and Honeywell to create a state-of-the-art, automated wing assembly system.
Panel-handling techniques often involved constraining the panel form with large strong backs or bringing it into form as it is pulled into position. The process requires the panels to be located, drilled, and pulled apart for cleaning. Sealant is applied as the panel is reassembled to the mating structure. All six panels (per wing) must be positioned, removed, and repositioned for fastening. This labor-intensive, time-consuming process proved ripe for automating, although the size and variability of the panels required engineers to adopt a new process that could handle different panel configurations, jog levels, and panel weights.
Electroimpact needed to design a material handling solution that would establish panel form in the oversized wing panels as they're lifted off the ground and provide a stable control system to allow operators to control the position of the skin assembly as the stringers were engaged into the ribs. The solution includes six telescoping panel loader arms that attach to the wing panels at discrete lift points and enable operators to manage panel position and form. However, such a configuration of the distributed support points creates a system that is statically indeterminate. To overcome this, the design team created a two-point lift system by setting two of the panel loader arms to control position. The other arms are set to seek a predetermined load. Two panel loader arms control position, while remaining arms chase load adjust according to the vertical height of the position arms.
Using a servohydraulic vertical axis, a single controller monitors the load on the wing and the position of the panel loader arms. When the wing panel is stationary, all of the arms provide self-regulating feedback for the fixed position. As the panel is moved, the two position arms continue to provide position feedback, while the other arms provide load feedback, ensuring force is continuously monitored across the length of the panel. The HNC servo controller enables a seamless transition between position and load, regardless of how the panel is moved. Since the system is almost entirely automated, the operator is free to guide the panels as needed.
Controller accuracy and reliability is key to the operation. The immense wing panel size and the number of stringer-to-rib interfaces require the wing to be precisely controlled. Since the entire system is digitally controlled, position is accurate to 1 mm, force to 50 lb, and all with a 4 msec scan time.
Several parts on the panel loader arms guarantee precise and accurate measurements when positioning panels. Parts include a servovalve and Rexroth HNC 100 servohydraulic controller from Bosch Rexroth Corp. that provides closed-loop control; a Honeywell Sensotec Model 41 load cell to read vertical force; and a Temposonics RH SSI linear transducer from MTS Sensors to measure vertical position.
Linear transducers are contained in a large hydraulic cylinder that drives the vertical axis. The servovalve and HNC controller maintain pressure on each side of the cylinder and communicate with the load cell and transducer. Burying the sensor inside the hydraulic cylinder allows it to remain protected. The SSI interface eliminates issues with noise. The package integrates well with the HNC servo hydraulic controller.
Synchronous serial interface (SSI) features synchronized data transfer. In the closed-loop system, the HNC controller provides a means of monitoring position and force. The linear transducer simplifies system synchronization. A clock pulse train from the HNC controller gates out sensor data: One bit of position data is transmitted to the controller per one clock pulse received from the sensor. The absolute position data is continuously updated by the sensor and converted into serial information. Pulses occur milliseconds apart, allowing for real-time data. The HNC was set up to interface with the SSI scales directly, meaning no conversions were needed.
The magnetostrictive technology employed by the linear sensors was well suited for this application because it eliminates wear and guarantees durability and output repeatability. Within the sensing element, a sonic strain pulse is induced in a specially designed magnetostrictive waveguide by the momentary interaction of two magnetic fields. One field comes from a movable permanent magnet that passes along the outside of the sensor. The other field comes from an “interrogation” current pulse applied along the waveguide. The resulting strain pulse travels at ultrasonic speed along the waveguide and is detected at the head of the sensing element. The magnet's position is determined with high precision and speed by accurately measuring elapsed time between application of the interrogation pulse and arrival of the resulting strain pulse with a high-speed counter. Using the elapsed time to determine position of the permanent magnet provides an absolute position reading that never needs recalibration or rehoming after a power loss.