Critical components in the aerospace and defense industries, such as engine housings, wings and control surfaces, undergo rigorous testing to ensure quality and safety. Complex parts require an innovative testing method to accurately determine size and depth without causing damage or permanent alterations; a process that USA based machine design company Marietta Nondestructive Testing (NDT) Inc. was keen to develop.
Historically, x-ray has been used in non-destructive testing (NDT) to check the strength of welds and look for evidence of defects and faults in both large and small components. However, large, complex shaped carbon fibre parts and other advanced composite materials require a different approach.
“X-ray is limited in determining the size and depth of composite parts; however, they can be accurately inspected using ultrasonic technology,” said Curtis Cooper, Director of Engineering for Marietta NDT.
The majority of automated ultrasound testing machines employ immersion tanks filled with water. These act as a medium through which sound waves travel. However, complex parts make immersion tanks impractical with technicians resorting to scanning each component by hand, resulting in a slow and labor-intensive process. This often leads to overlapping scans, which provide inaccurate or inconsistent test results. The challenge was therefore to create an automated system which would avoid these issues.
The revised ultrasonic inspection equipment developed by Marietta NDT and Bosch Rexroth is custom-designed for each client, based on the unique specifications of the parts being inspected. To overcome the challenges of scalability, reliability and speed, the engineers at Marietta NDT designed a gantry style AG2 Overhead Scanner—a rigid, multi-axis, automated testing machine capable of scanning large, sophisticated parts and intricate shapes, without the use of immersion tanks.
With a scanning envelope of up to 60 x 20 x 16 feet, the machine can easily be configured to test a wider variety of components for each customer, instead of being specifically designed for one part. The scanner’s ultrasonic scanning system utilises two sets of jets that face each other. During inspection, the jets stream water —the medium that the sound wave travels through—around the part.
The precise servo motion control of the system became a critical factor in the final design. In order for the machine to offer up to 12 axes of motion, component synchronisation had to be tightly controlled so the testing would be accurate. “Each nozzle is roughly five inches from the face of the part,” said Cooper. “Since the two nozzles face each other, they have to be lined up. We were able to make streams of water, which are each manipulated by five axes of servo motion, concentric within 0.020 of an inch.”
To ensure precise control, accuracy and reliability which would reduce maintenance requirements, Marietta NDT used drive and control components from Bosch Rexroth. Rexroth’s components—including digital servo drives and controllers, profiled guide rails and pneumatic components— allow the machine to follow intricate path planning for scanning complex, curved objects with tightly controlled motion tolerance. Rexroth Distributor Livingston & Haven provided design and programming expertise for the new line of machines.
“The high-quality controllers are reliable and easy to programme. They can also accommodate the large number of interpolated axes of the machines,” said Cooper. In addition, the controllers generate minimal overall electrical noise to minimise ultrasound interference, producing a higher quality image than was possible in the past. The pneumatic components include a vacuum generator to remove air from the nozzle which also improves the quality of the ultrasonic scans.
According to Ben Strong, automation specialist at Livingston & Haven, the linear guide rails used in the AG2 also contribute to the overall rigidity and accuracy of the machine. “If the machine is not stiff enough to handle the squirter system, it will begin to vibrate, which adversely affects the testing,” Strong said. He added that from a maintenance perspective, the linear guide rails are an ideal solution due to their longer lubrication interval, dual rail datums and interchangeable runner blocks. In addition, the rails are plated with dense chrome to resist rust, a crucial consideration for a machine that incorporates water.
Livingston & Haven also assisted Marietta NDT in adapting the Rexroth control system to communicate with LabVIEW™, a robust platform and development package that design and control engineers use to automate measuring equipment. It’s commonly used for data acquisition, instrument control and industrial automation on a variety of programs, including Microsoft Windows and Mac OSX. “We implemented a solution utilising TCPIP sockets that provides real-time communication of position commands,” said Strong. “As a result, Marietta NDT can present an image of the machine in real time.” This allows for remote viewing of the status of the system and for virtual programming of parts.
“Because of the path planning and motion control, this machine is greatly improved over other inspection equipment,” said Cooper. The new squirter-based machine can perform scans accurately and quickly with complete part coverage. “We can scan parts at about 25 inches per second, which increases our speed and output,” said Cooper.
Because the programming, software and mechanics are configurable, the machine can easily accommodate a variety of parts from the same customer. In the past, customers would have needed to order separate, custom-built systems to inspect each individual part. The AG2’s functionality extends its usefulness and significantly reduces the cost of ownership for customers.
Marietta NDT is currently working on the next set of improvements for the scanner, by designing an interface to simplify the machine’s operation even further. Its engineers are currently integrating a CAD file importing feature to allow for automatic motion path planning. “The machine is specifically designed so users can update new part sets easily,” said Cooper. “We want operators to be able to simply log in and create their own scan plans.”