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|NDT.net Issue - 2014-03 - NEWS |
A new development by Ultrasonic Sciences heralds a step change in the application of industrial robots for NDT applications. Although commercial off the shelf (COTS) robots have been used by USL and others in the past, the scanning speeds were restricted by some inherent limitations of the robot controllers.|
For ultrasonic imaging applications at scanning speeds which are meaningful in industry, it is a requirement to monitor the probe position in real time, typically at a rate of 2000 readings per second, so that image pixels can be accurately placed on a C scan image, with data points every 0.5mm. This is necessary to ensure that the size and position of defects and features can be precisely recorded and measured. Unfortunately standard robot controllers can provide this position data at only around one tenth of this rate, which means that scanning speeds have to be reduced.
The main thrust of the USL improvement is the development of a robot motion controller which achieves the necessary position feedback rate, so that scanning speeds can match those used in traditional scanning systems based on Cartesian mechanisms.
For over 25 years USL have been designing and manufacturing bespoke ultrasonic testing systems. One major application is the inspection of aerospace composite components with a complex shape, using through transmission and pulse echo methods. Multi-axis scanning mechanisms are built, with the ultrasound transmitted through water jets onto the part being tested. The water jets must be maintained normal to the part surface during the scan and, crucially for through transmission inspection, the two jets must be maintained precisely coaxial with one another. All this whilst the probes are scanning the surfaces at around 700mms/second. In the last 6 years USL has installed more than 20 of these complex squirter systems around the world, with the largest being 18 x 2 x 3 metres in size. With the new development, the way is now open to utilize industrial robots for these applications, to avoid the often long and expensive procedure of manufacturing a custom multi-axis scanner.
The system has been developed using dual Stäubli 6 axis robots with the UNIval drive interface and the USL controller.The Stäubli robots have the necessary accuracy, rigidity and robustness for the application, with the added advantage of being inherently waterproof to IP65/67 and with the cabling being fed inside the arm. The combined system has sufficient capacity to control not only the 12 robot axes, but also the linear tracks which are required for long travel applications and the water pumps which supply the squirter heads. Control of the water flow is an important factor, to compensate for the position of the squirters as they follow the contour and for gravitational effects when operating at a high level.
The robotic systems maintain the standard characteristics of existing USL ultrasonic systems, such as simultaneous through transmission and pulse echo testing using logarithmic and linear amplifiers, multi gate data acquisition and full waveform capture.
The manufacturing time for robot based systems can be significantly reduced in comparison with bespoke manufacture and there are also benefits in terms of capital cost. In addition, the fact that well established, standard parts are used means that reliability, servicing and maintenance are improved.
The first application of this new system will be installed in the USA for a fuselage inspection, in conjunction with USL’s partners in that project, Arcadia Aerospace. Other applications are expected in UK and elsewhere.
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