Big wing spars demand accurate squirters

Citation

(2006), "Big wing spars demand accurate squirters", Aircraft Engineering and Aerospace Technology, Vol. 78 No. 4. https://doi.org/10.1108/aeat.2006.12778daf.001

Big wing spars demand accurate squirters

Big wing spars demand accurate squirters

Keywords: Aircraft, Transport engineering, Aircraft components

Inspecting components for the new Airbus A400M aircraft presents many challenges, not the least of which is achieving accurate ultrasonic evaluation of components with complex geometries. Precision motion control products from Delta Tau are the key to an innovative solution developed by Midas NDT Systems.

Precise co-ordination between seven axes of motion, operating speeds of up to 1,200 mm per second and the ready availability of synchronising signals to fire ultrasonic test pulses: these are just a few of the requirements addressed by the Delta Tau PMAC motion control system selected by non-destructive testing expert, Midas NDT, for an ultrasonic inspection installation which the company recently supplied to GKN Aerospace.

The primary role of this innovative installation is to automatically inspect wing spars for the new Airbus A400M passenger aircraft. Like the aeroplane itself, the spars are fabricated on a big scale, with some of them up to 15 m long. The spars are made from carbon- fibre based composites, and this is the first time that materials of this type have been used for key primary structures in a large aircraft.

Dependable inspection is, therefore, a prime requirement. The solution developed by Midas NDT is an automatic scanning multi-axis ultrasonic inspection installation. This provides a test volume 24 m long by 3.5 m high by 1m wide, allowing it to accommodate even the largest of the Airbus wing spars with ease. Available test modes include through transmission, pulse echo and time of flight. Full three-dimensional scanning throughout the test volume means that fast dependable inspection is achieved irrespective of the complexity of the component geometry.

Inspection is carried out by passing signals between two ultrasonic transducers, one of which is positioned on each side of the component under test. The transducers move at high speed to scan the component, and must remain aligned with accuracy better than 1 mm throughout the inspection process. Spacing between the transducers must also be accurately controlled.

The ultrasonic signals are coupled to the component under test via narrow jets of water, as air attenuates and spread ultrasonic beams, preventing the required inspection performance from being achieved. Because of the water jets, inspection systems of this type are known colloquially as “squirters”.

Midas NDT has long experience in the design and development of squirters and, in their early work on these machines, the company's engineers quickly discovered that by no means every motion control system could meet their requirements. In particular, the limited bandwidth of some systems evaluated meant that accurate alignment between the transducers could not be guaranteed.

Work on a previous project had unequivocally demonstrated that the performance of Delta Tau products easily exceeds Midas NDT's requirements in this respect. This is, however, far from the only reason that the company chose a Delta Tau PMAC motion control system for the demanding Airbus project.

“A big challenge that everyone faces in designing ultrasonic inspection systems is overcoming electromagnetic compatibility (EMC) problems,” said Iain Clarke, senior systems engineer at Midas NDT.

“Pulses of up to 700 V are applied to the ultrasonic transmitter, and they have the potential to interfere with other parts of the installation. Conversely, signals from the ultrasonic receivers are in the microvolt range, and they need a quiet electromagnetic environment if they are to be evaluated correctly.”

“The Delta Tau PMAC system, however, has built-in provision for the use of fibre-optic connections between the drives and the controller,” continued Clarke. “This provides us with a convenient, cost-effective and totally dependable solution for our EMC problems.”

Flexibility was another reason that Midas NDT chose PMAC. Porting the specialised software used for controlling squirters from one platform to another is a complicated and time consuming process, as is the exhaustive testing needed after porting to prove that full functionality has been maintained.

Standardising on a single supplier for motion control products is, therefore, highly desirable. An important benefit is Delta Tau's ability to provide fully compatible, cost-effective products which can be readily enhanced by the addition of accessories, for systems needing anything from four to 32 axes.

Other factors that influenced Midas NDT's decision to use Delta Tau included expert technical support together with the ease with which the motion controllers can be linked to Midas NDT's own control software, using Delta Tau's software libraries. With Midas NDT exporting globally, Delta Tau's world-wide technical support for its equipment was also seen as a plus factor.

A feature of the PMAC system which is said to have proved particularly useful in this application was its ability to produce synchronising signals. During the inspection process, the ultrasonic transmitter is required to produce a pulse of ultrasound for every millimetre that it moves. With other motion control systems it was not easy to produce the signals needed to trigger these pulses, but suitable synchronising signals are available as standard with PMAC products.

As finally implemented, the motion control installation for the Airbus wing spar inspection system comprises a compact PMAC controller from the Delta Tau PMAC range, which is installed in the operator's control desk. This communicates with the machine- mounted drives via an optical link configured as a ring.

Delta Tau Macro Ring high-speed optical links are specifically designed to have ample bandwidth to allow the remote closing of control loops, even in demanding applications, and this made it possible for Midas NDT to implement a true distributed control solution.

According to Delta Tau compared with traditional cabled links, this arrangement not only provides greatly enhanced EMC performance, but also substantially reduces wiring costs by eliminating the need for complicated and expensive flexible cable looms.

The adoption of a ring topology has the added bonus of allowing communication to be maintained with all devices in the event of any single break in the optical link. This means that, should a break occur, full control is maintained while the machine is brought to a controlled stop.

The operator interface is provided by intuitive graphics-based software developed by Midas NDT, which provides full teach-and-learn functionality. This software, which integrates tightly with Delta Tau's motion control software, allows users to easily create shape files to match the components that are being inspected. Shape information can also be imported from component CATIA data, an option which is particular useful for complex shapes.

All parameters, including ultrasonic settings, are stored for examination at a later date, and for use as the basis of repeat inspections.

“We enjoyed excellent support from Delta Tau throughout this project,” said Midas NDT's Iain Clarke, “with fast response whenever we had a query or problem. In addition, the motion control systems were easy to commission and, in service, they are performing flawlessly. Delta Tau will certainly be our motion control supplier of choice for future projects.”

Delta Tau PMAC motion control products have provided Midas NDT with a technically elegant and cost effective motion control solution for the Airbus A400M wing spar inspection system which it has developed for GKN Aerospace.

The company believes that versatility, ease of use and reliability of its products, coupled with their outstanding performance and the expert technical support which backs them, have proved an ideal match for the challenges of this demanding application.

Details available from: Delta Tau UK Limited, Tel: 44 (0)1255 670196, Fax: +44 (0) 1255 850768, E-mail: ajoslin@deltatau.co.uk