Marietta Sounds Off on Ultrasonic Nondestructive Testing
Large, complex aero parts inspire completely automated inspection system.
Machine design companies like Marietta Nondestructive Testing LLC (MNDT; Marietta, GA)—in partnership with drive and control experts from Bosch Rexroth (Charlotte, NC) and its automation distributor, Livingston & Haven (Charlotte, NC)—needed to get creative in their approach to developing testing equipment.
Large Parts Present Big Problems
Large parts made of carbon fiber are difficult to inspect with X-ray technology, which had been used for other aerospace components. “X-ray is limited in determining the defect size and depth in composite parts; however, it can be accurately inspected using ultrasonic technology,” said Curtis Cooper, director of engineering for MNDT.
The most common automated ultrasound testing machines employ immersion tanks filled with water as a medium through which the sound waves travel. As parts requiring this level of inspection became more complex, the immersion tanks became impractical. Instead, technicians began to scan complex parts by hand, a slow and labor-intensive process that resulted in overlapping scans, which could lead to inaccurate or inconsistent test results.
Complex Challenges Require Innovative Solutions
MNDT’s new ultrasonic inspection equipment 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 MNDT designed the gantry style AG2 Overhead Scanner—a rigid, multiaxis, 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 × 20 × 16' (18.3 × 6.1 × 4.9 m), the machine can easily be configured to test a wide range of parts for each customer, instead of being designed for one specific part. The scanner’s ultrasonic scanning system utilizes two sets of squirter 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 design. In order for the machine to offer multiple axes of motion, component synchronization had to be tightly controlled so the testing would be accurate. “Each nozzle is roughly five inches [127 mm] 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" [0.508 mm].”
Select Components for Precision and Reliability
To ensure precise control, accuracy and reliability, MNDT used drive and control components from Bosch Rexroth. Rexroth’s components—including digital servodrives and controllers, profiled guiderails 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 program. 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 minimize ultrasound interference, which can distort the image quality of the scan. 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 guiderails 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 guiderails are an ideal solution due to their longer lubrication interval, dual rail datums and interchangeable runner blocks. In addition, the rails are plated with thin, dense chrome to resist rust, a crucial consideration for a machine that incorporates water.
Livingston & Haven also assisted MNDT 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. “Using TCPIP sockets, we implemented a solution that provides real-time communication of position commands,” said Strong. “As a result, MNDT 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.
Scalable Machine with Consistent, Reliable Testing Capabilities
“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 ips [635 mm/sec], 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; before, customers had 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.
MNDT is currently working on the next set of improvements for the scanner, by designing an interface to simplify the machine’s operation even further. MNDT 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.” ✈
Article edited by Yearbook Editor Michael Anderson from information provided by Bosch Rexroth.
This article was first published in the 2013 edition of the Aerospace & Defense Manufacturing Yearbook.
Published Date : 12/10/2013