Gordon Dobie

Time-of-flight measurement techniques for airborne ultrasonic ranging

Airborne ultrasonic ranging is used in a variety of different engineering applications for which other positional metrology techniques cannot be used, for example in closed-cell locations, when optical line of sight is limited, and when multipath effects preclude electromagnetic-based wireless systems. Although subject to fundamental physical limitations, e.g., because of the temperature dependence of acoustic velocity in air, these acoustic techniques often provide a cost-effective solution for applications in mobile robotics, structural inspection, and biomedical imaging. In this article, the different techniques and limitations of a range of airborne ultrasonic ranging approaches are reviewed, with an emphasis on the accuracy and repeatability of the measurements. Simple time-domain approaches are compared with their frequency-domain equivalents, and the use of hybrid models and biologically inspired approaches are discussed.

A Noncontact Ultrasonic Platform for Structural Inspection

Miniature robotic vehicles are receiving increasing attention for use in nondestructive testing (NDE) due to their attractiveness in terms of cost, safety, and their accessibility to areas where manual inspection is not practical. Conventional ultrasonic inspection requires the provision of a suitable coupling liquid between the probe and the structure under test. This necessitates either an on board reservoir or umbilical providing a constant flow of coupling fluid, neither of which are practical for a fleet of miniature robotic inspection vehicles. Air-coupled ultrasound offers the possibility of couplant-free ultrasonic inspection. This paper describes the sensing methodology, hardware platform and algorithms used to integrate an air-coupled ultrasonic inspection payload into a miniature robotic vehicle platform. The work takes account of the robots inherent positional uncertainty when constructing an image of the test specimen from aggregated sensor measurements. This paper concludes with the results of an automatic inspection of a aluminium sample.

Simulation of ultrasonic lamb wave generation, propagation and detection for a reconfigurable air coupled scanner.

A computer simulator, to facilitate the design and assessment of a reconfigurable, air-coupled ultrasonic scanner is described and evaluated. The specific scanning system comprises a team of remote sensing agents, in the form of miniature robotic platforms that can reposition non-contact Lamb wave transducers over a plate type of structure, for the purpose of non-destructive evaluation (NDE). The overall objective is to implement reconfigurable array scanning, where transmission and reception are facilitated by different sensing agents which can be organised in a variety of pulse-echo and pitch-catch configurations, with guided waves used to generate data in the form of 2-D and 3-D images. The ability to reconfigure the scanner adaptively requires an understanding of the ultrasonic wave generation, its propagation and interaction with potential defects and boundaries. Transducer behaviour has been simulated using a linear systems approximation, with wave propagation in the structure modelled using the local interaction simulation approach (LISA). Integration of the linear systems and LISA approaches are validated for use in Lamb wave scanning by comparison with both analytic techniques and more computationally intensive commercial finite element/difference codes. Starting with fundamental dispersion data, the paper goes on to describe the simulation of wave propagation and the subsequent interaction with artificial defects and plate boundaries, before presenting a theoretical image obtained from a team of sensing agents based on the current generation of sensors and instrumentation.

Miniature Mobile Sensor Platforms for Condition Monitoring of Structures

In this paper, a wireless, multisensor inspection system for nondestructive evaluation (NDE) of materials is described. The sensor configuration enables two inspection modes-magnetic (flux leakage and eddy current) and noncontact ultrasound. Each is designed to function in a complementary manner, maximizing the potential for detection of both surface and internal defects. Particular emphasis is placed on the generic architecture of a novel, intelligent sensor platform, and its positioning on the structure under test. The sensor units are capable of wireless communication with a remote host computer, which controls manipulation and data interpretation. Results are presented in the form of automatic scans with different NDE sensors in a series of experiments on thin plate structures. To highlight the advantage of utilizing multiple inspection modalities, data fusion approaches are employed to combine data collected by complementary sensor systems. Fusion of data is shown to demonstrate the potential for improved inspection reliability.

A PROBABILISTIC APPROACH TO ROBOTIC NDE INSPECTION

The application of wireless robotic inspection vehicles equipped with different NDE payloads has been introduced previously, with emphasis placed on inspection applications in hazardous and inaccessible environments. A particular challenge to the practical application of such robotic inspection lies in the localization of the devices. The authors here consider a probabilistic approach to both the positioning and defect problems by using the location of the robot and the NDE measurements (acquired from the onboard transducers) to make inference about defect existence and position. Using a particle filter approach running locally on the robots, the vehicle location is tracked by fusing noisy redundant data sets supplying positional information.

Quantifying and Improving Laser Range Data When Scanning Industrial Materials

This paper presents the procedure and results of a performance study of a miniature laser range scanner, along with a novel error correction calibration. Critically, this paper investigates the accuracy and performance of the ranger sensor when scanning large industrial materials over a range of distances. In addition, this paper investigated the effects of small orientation angle changes of the scanner, in a similar manner to which it would experience when being deployed on a mobile robotic platform. A detailed process of error measurement and visualization was undertaken on a number of parameters, not limited to traditional range data but also received intensity and amplifier gain. This paper highlights that significant range distance errors are introduced when optically laser scanning common industrial materials, such as aluminum and stainless steel. The specular reflective nature of some materials results in large deviation in range data from the true value, with mean root mean square error (RMSE) errors as high as 100.12 mm recorded. The correction algorithm was shown to reduce the RMSE error associated with range estimation on a planar aluminum surface from 6.48% to 1.39% of the true distance range.

Positioning challenges in reconfigurable semi-autonomous robotic NDE inspection

This paper describes work conducted into mobile, wireless, semi-autonomous NDE inspection robots developed at The University of Strathclyde as part of the UK Research Centre for Non Destructive Evaluation (RCNDE). The inspection vehicles can incorporate a number of different NDE payloads including ultrasonic, eddy current, visual and magnetic based payloads, and have been developed to try and improve NDE inspection techniques in challenging inspection areas (for example oil, gas, and nuclear structures). A significant research challenge remains in the accurate positioning and guidance of such vehicles for real inspection tasks. Employing both relative and absolute position measurements, we discuss a number of approaches to position estimation including Kalman and particle filtering. Using probabilistic approaches enables a common mathematical framework to be employed for both positioning and data fusion from different NDE sensors. In this fashion the uncertainties in both position and defect identification and classification can be dealt with using a consistent approach. A number of practical constraints and considerations to different precision positioning techniques are discussed, along with NDE applications and the potential for improved inspection capabilities by utilising the inherent reconfigurable capabilities of the inspection vehicles.

P0-8 Robotic Based Reconfigurable Lamb Wave Scanner for Non-Destructive Evaluation

Robotic vehicles are receiving increasing attention due to their attractiveness in terms of cost, safety and their accessibility to areas where manual inspection is not practical. This paper details work on a reconfigurable Lamb wave scanner using autonomous robotic platforms. The scanner is built from a fleet of miniature robotic agents, each with an air coupled ultrasonic Lamb wave transmitter or receiver. These agents can transmit Lamb waves between each other to interrogate the material and efficiently create a C-scan of the test component. The aggregated nature of the scanner makes it both adaptable and robust - agents can be removed and the system will reconfigure to compensate. The transmission of Lamb waves over distances up to 1m also makes the scanner highly efficient. The authors have successfully demonstrated a reconfigurable Lamb wave scanner that works on both aluminium and steel. The scanner can detect artificial defects of 10% of the samples depth with a positional accuracy of 1 cm.

Automatic ultrasonic robotic array

A novel, autonomous reconfigurable ultrasonic phased array inspection robot for non-destructive evaluation (NDE) is presented. The robotic system significantly will reduce manual labor over current inspection regimes, as well as enabling inspection of inaccessible/hazardous areas such as those found in the nuclear and petrochemical industries. It will offer three quantitative benefits: improved inspection accuracy, improved safety and reduced inspection costs. The current major innovation is in embedding ultrasonic phased array technology into a small form-factor robotic vehicle, overcoming issues in ultrasonic coupling, miniaturized electronics and robot positioning. This paper presents an overview of the robot specification and system architecture along with details of a specific inspection scenario where the robot is required to inspect a saddle weld found in reheat bifurcation. This weld is formed from the intersection of two 60 mm thick steel pipes with diameters 500 and 300 mm. The robot will be capable of tracking the weld from either pipe, projecting an ultrasonic beam normal to the direction of travel. The design of a 2 MHz, 16 element embedded phased array controller is presented. A timing model of the controller details the throughput required to enable the robot to perform ultrasonic inspection while tracking the weld at 20 mm/s. The paper also considers robot positional estimation. The nature of the inspection prohibits the use of external positioning systems so the system is limited to on-board sensors, namely wheels encoders, a six axis inertial sensor and a surface feature tracking camera. The results section focuses on the characterization of inspection performance, driven in part by the ultrasonic phased array controller and robot positional estimation. A-Scans are presented to show the SNR of each array channel which was approximately 24 dB when measuring the back wall echo. It is shown that ultrasonic scan rate is limited by 802.11g wireless transmission from the robot to the host computer.

Active Whisking-Based Remotely Deployable NDE Sensor

The fundamental sensitivity characterization of a novel whisking sensor for applications in nondestructive evaluation is presented. The whisking sensors, originally developed for proximity detection applications in autonomous robotics are evaluated for measurements of surface roughness and surface form change. These surface parameters are the representatives of the typical changes associated with corrosion and surface breaking defects in real structures. The authors demonstrate that the whisking sensor can be used to accurately quantify surface roughness in the range 14-53 μm with excellent correlation to a standard reference. Furthermore, it is shown that that the sensor can detect 14 mm diameter flat bottomed holes with depths ranging from 0.4 to 1.0 mm. In contrast to conventional ultrasonic and eddy current techniques, the sensor is insensitive to surface liftoff, producing an error of only 1.2% for liftoffs of several mm. This liftoff insensitivity is a highly desirable characteristic for real-world deployment of the sensors, and the authors describe how the sensor can be incorporated into autonomous inspection robots.