Retrofitting Ultrasonic Immersion Automated Scanners with Phased Array Ultrasonic Testing Instrumentation

Figure 1: Conventional ultrasonic immersion scanner retrofitted with phased array ultrasonic testing (PAUT) technology.

Introduction to Ultrasonic Immersion Testing

Automated ultrasonic immersion testing (AUT) offers a high-speed, high-resolution method to scan primary, formed, and finished materials for material defects. Ultrasonic immersion testing may be performed using full immersion, water jet, and bubbler configurations. While initially developed for steel, aluminum, and other advanced metal inspection applications [1], immersion testing is now integrated across the aerospace composite industry.  AUT first became viable as an inspection method in the mid-1950s through contact testing, with immersion testing eventually becoming automated by the 1970s. The first commercial use of mechanized ultrasonic testing was engineered in 1956 to inspect longitudinal welds in pipe mills [2] and later circumferential welds. In 1962, the first related patent was filed [3] and the pathway towards modern automated ultrasonic immersion testing was paved [4]. At the same time as these advancements, engineers were figuring out how to program scans in laboratory immersion tanks to inspect parts with standardized geometries. This would allow technicians to load a sample and tell a mechanized arm to follow a specific path in the tank as it scans the whole part.

Immersion testing offers unique advantages that can solve problems posed to conventional contact testing. By using water as a coupling medium, immersion testing guarantees consistent coupling quality. This allows for inspection of rough surfaces and curved surfaces that would otherwise require workarounds in contact testing. In the case of an immersion tank, an ultrasonic probe mounted on an arm can follow pre-programmed trajectories. Each pass on an automated route generates incremental data which can be viewed through various formats: A-scans, B-scans, and C-scans. Immersion technology with squirter jets works for pipelines and other on-site inspections, whereas laboratory tanks are used for corrosion mapping of steel sheets and pipes, weld inspection of ex-situ process piping, quality control of automotive parts, and damage characterization of post-impacted composites [5].

AUT provides a cost-efficient and detailed method of nondestructive testing. By pairing automated systems with the benefits of immersion techniques, inspectors can easily map and identify defects in test parts. Through both portable devices that utilize squirter jets for water coupling and stationary tanks with transducers mounted on robotic arms, automated immersion testing is a highly effective solution for weld inspection, corrosion inspection, and material testing.

The Anatomy of Phased Array Ultrasonic Immersion Testing

The advantages of PAUT compared to conventional ultrasonic testing apply also to immersion testing. These include electric scanning of the aperture, dynamic focusing, and multi-angle electronic scanning. Additionally, the basic hardware used in a PAUT immersion system are identical to those used in the conventional UT. This includes the human-machine interface (HMI), programmable logic controller (PLC), motor drives, motors, and encoders. These components work together to ensure seamless operation, accurate positioning, and efficient data acquisition.

The Human-Machine Interface (HMI) is the user interface that connects the operator to the system. It provides a visual representation of the system’s status and allows for manual control and monitoring of the inspection process. This paired with ultrasonic software allows the communication between systems to control not only the automation portion, but also the data acquisition system. Shown below is an HMI developed to control a 6-axis PAUT immersion system using an Olympus Focus PX platform. Some key features of this HMI include PLC and Focus PX communication, activation of servo motor drives, part definition and multi-axis motion control.

Figure 2: PAUT 6-axis immersion scanner Human Machine Interface (HMI) motion control menu.

The Programmable Logic Controller (PLC) for Phased Array Immersion Systems

The PLC is the brain of the automation system that executes the desired scan plan defined in the HMI. Using the parameters transferred to an SQL, or comparable database structure, the PLC controls the immersion scan start, stop, resolution, and scan speeds

Figure 3: Example PLC unit for multi-axis PAUT immersion scanner.

Immersion PAUT Encoders, Motor Drives and Motors

Figure 3: Phased array immersion testing encoder

Encoders provide feedback on the position and movement of the motors, ensuring precise control. They are essential for:

• Position Feedback: Encoders measure the position of the motor shaft and provide feedback to the PLC, enabling accurate control of the transducer and test piece movement.

• Speed Monitoring: Encoders also monitor the speed of the motors, ensuring that they operate within the desired parameters.

• Error Detection: By comparing the actual position and speed with the desired values, encoders help detect and correct errors in the systems.

Motor drives and motors are critical for the precise positioning and movement of the transducer and the test piece. In phased array ultrasonic immersion systems, they ensure:

• Accurate Positioning: Motors move the transducer and the test piece to the exact positions required for inspection, guided by the control signals from the PLC.

• Smooth Operation: Motor drives control the speed and torque of the motors, ensuring smooth and stable movement.

• Dynamic Control: Advanced motor drives can adjust motor performance in real-time based on feedback from the system, enhancing accuracy and responsiveness.

Example Immersion Phased Array Testing Results

Shown below are example phased array L-wave linear scan data on a compressed gas steel calibration cylinder. Prior to inspection, the automated system must detect a series of simulated longitudinal cracks, circumferential cracks, generalized corrosion, and pitting corrosion. Pitting corrosion is simulated via a 0.25” diameter flat bottom hole (FBH). The FBH diameter and depth were sized accurately using the 5 MHz 64 element. The top left scan is the PAUT A-scan with the detection gate positioned on the FBH reflection. The top right is the PAUT B-scan across the transducer index axis. The bottom shows the PAUT C-scan in which the pitting diameter was sized using the 6 dB technique. The FBH was sized at 0.249” versus 0.250”. 

Figure 4: Immersion phased array data example on a steel calibration cylinder.

 References

1. Ixar Group. (2022, December 27). Everything you need to know about Automated Ultrasonic Testing (AUT). Ixar. https://www.ixar.in/everything-you-need-to-know-about-automated-ultrasonic-testing-aut/

2. De Raad, J., & Dijkstra, F. (2006). The History of AUT. 9th European Conference on NDT - September 2006 - Berlin (Germany) . https://www.ndt.net/?id=3774

3. McNulty, J. F. (1966, July 12). Ultrasonic Testing Apparatus and Method.

4. Ginzel, E. A. (2000). Mechanised Ultrasonic Inspections of Pipeline Girth Welds. The E-Journal of Nondestructive Testing & Ultrasonics, 5(3). https://www.ndt.net/article/v05n03/eginzel/eginzel.htm

5. TWI Ltd. (2024). What is ultrasonic immersion testing?. TWI. https://www.twi-global.com/technical-knowledge/faqs/ultrasonic-immersion-testing