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Development of a robotic system for the inspection of large steel/aluminium plates in industrial plants.

Exploitable results

Phased array ultrasonics is used to inspect welds for such faults as internal cracking, inclusions and lack of penetration. This is a well established technology, in which an ultrasonic beam may be steered electronically by the sequential firing of an integrated linear piezo-electric array. In this application a specially designed phased array probe is placed on the inside vertical plate of a fillet weld. The UT beam is steered in such a way as to be able to detect toe cracking of the outer fillet weld. Toe cracking is a surface breaking crack on the upper face of the weld by the edge of the crown. It creates a stress riser in the weld that could lead to failure under bending moments created by fatigue loading. During inspections of fillet welds the operator will typically have access to only one side of the weld. If both sides are accessible then there are other simpler methods of locating these cracks, such as ACFM - see result four. The ability to detect toe cracking from the opposite side of the fillet would be of considerable use when inspecting above ground storage tanks, ships, or any other container. The probe has been designed and constructed. Its performance has been experimentally evaluated.
A prototype robotic NDT inspection system for large welded plates was designed, built and evaluated. Steel and aluminium plates are inspected extensively during manufacture and fabrication - this inspection is often done manually in hazardous environments, by skilled personnel. It has been proven that such difficult conditions may reduce the operating ability of such technicians, leading to poor performance and low-quality subjective assessments. There is a need for an autonomous mobile NDT system that is versatile, portable and is able to perform in hazardous environment with little or no human intervention. The major economic objectives of Robot Inspector are a significant cost reduction in inspection of large plates in the factory. The major social and environmental objectives are elimination of labour intensive, monotonous and hazardous inspection tasks, elimination of error due to operator stress and subjective test data and a substantial decrease in leaks from storage tanks. The Robot Inspector system is primarily designed for the following industries, but may be used on any large welded plate: ship building, construction of aboveground storage tanks and steelyards. The need for effective inspection systems has grown in the EU over recent years, due to the increase in the number of ageing aboveground storage tanks, and an increase in the requirements of the end-users of large steel plates. This has been caused by the number of aboveground storage tanks that are near the end of their useful life, and by the expansion of the EU to central and Eastern Europe. The oil and power companies in the former eastern bloc countries have relatively old aboveground storage tanks. The market for such a system is expected to increase over the next ten years, due to the exponential increase in environmental and safety legislation, and the integration of the former eastern bloc countries into the European community. The main specification was to develop a robotic NDT inspection system for large welded plates, capable of detecting corrosion on both sides of plates, laminations and segregation in plates and defects in welded joints.
ACFM sensors induce an alternating magnetic field within the surface of the metallic subject. Small coils are used to measure the disturbance of this filed caused by surface breaking cracks. The Robot Inspector probe consists of an array of ACFM sensors in a curved geometry to 'mould' itself around the weld. The sensors are deployed in a line across the weld and the probe is then scanned along the weld so that the whole weld cap, and an area either side of the weld is inspected in one pass. It is estimated that only one such probe will have to be manufactured in order to accommodate butt, fillet and over lapped welds, by using different parts of the same probe. In order to achieve this, the number of sensors in the probe will have to be limited, as the probe will be designed to size defects in one orientation only (namely parallel to the weld), although it will be capable of detecting transverse defects. This array is used to investigate surface-breaking defects, particularly toe-cracking. This probe was constructed and experimentally evaluated.
The typical ultrasonic probes used for the thickness measurement of steel plates, similar to those used in aboveground storage tanks, use a gel (or water) couplant to carry the UT signal from the transducer to the steel specimen. If there isn't enough couplant to carry the signal, the sensor will fail to gather the information correctly. The transducers are often attached to water irrigation systems that flood the steel plate with copious amounts of water as the transducer is dragged along the plate. This leads to an abnormal amount of wear in the transducer, and leaves the test-piece covered in water or gel couplant. A vacuum based recovery system is typically used to collect the couplant. Sonatest have developed a wheel probe, which consists of a UT sensor housed within a water filled rubber tyre. The tyre material is designed to act as a couplant, without the need to irrigate the sample with couplant. This removes the need to flood the plate, and eliminates the need to use a vacuum recovery system. The wear and tear on the probe is also reduced.
In order to accommodate the needs of the Robot Inspector, specialised software was developed by ZENON to enable the mobile platform to perform the required movements for the inspection task. The software was developed in C++, using the libraries ARIA, ARNL and ArNetworking and structured as a layered software system consisting of 8 layers executing on three CPUs with as many as five concurrent threads per CPU. The bottom layer is hardware specific, the next three layers are generally usable on any mobile robot with holonomic steering and laser baser sensing, the next four layers are NDT testing application specific and are reusable (two fully and two partially) on any mobile platform used for NDT testing while the last layer serves as a test interface. The overall software architecture is designed to be highly flexible and fully reusable at the design level. The specific instance of this architecture that was implemented was tailored specifically to the needs of Robot Inspector. However several individual components are portable to other mobile robot platforms, other mobile robot applications for NDT and other mobile robot applications subject to certain similarities in the robot hardware capabilities.