Mitigating RGB-D camera errors for robust ultrasonic inspections using a force-torque sensor

Robot-based phased array ultrasonic testing is widely used for precise defect detection, particularly in complex geometries and various materials. Compact robots with miniature arms can inspect constrained areas, but payload limitations restrict sensor choice. RGB-D cameras, due to their small size and light weight, capture RGB colour and depth data, creating colourised 3D point clouds for scene representation. These point clouds help estimate surface normals to align the ultrasound transducer on complex surfaces. However, sole reliance on RGB-D cameras can lead to inaccuracies, affecting ultrasonic beam direction and test results. This paper investigates the impact of transducer pose and RGB-D camera limitations on ultrasonic inspections and proposes a novel method using force-torque sensors to mitigate errors from inaccurately estimated normals from the camera. The force-torque sensor, integrated into the robot end effector, provides tactile feedback to the controller, enabling joint angle adjustments to correct errors in the estimated normal. Experimental results show the successful application of ultrasound transducers using this method, even with significant misalignment. Adjustments took approximately 4 seconds to correct deviations from 12.55°, with an additional 4 seconds to ensure the probe was parallel to the surface, enhancing ultrasonic inspection accuracy in complex, constrained environments

Design and manufacture of an optimised side-shifted PPM 2 EMAT array for use in mobile robotic localisation

Guided wave Electro Magnetic Acoustic Transducers (EMATs) offer an elegant method for structural inspection and localisation relative to geometric features, such as welds. This paper presents a Lorentz force EMAT construction framework, where a numerical model has been developed for optimising Printed Circuit Board (PCB) coil parameters as well as a methodology for optimising magnet array parameters to a user’s needs. This framework was validated experimentally to show its effectiveness through comparison to an industry built EMAT. The framework was then used to design and manufacture a Side-Shifted Unidirectional Periodic Permanent Magnet (PPM) EMAT for use on a mobile robotic system, which uses guided waves for ranging to build internal maps of a given subject, identifying welded sections, defects and other structural elements. The unidirectional transducer setup was shown to operate in simulation and was then manufactured to compare to the bidirectional transmitter and two-receiver configurations on a localisation system. The unidirectional setup was shown to have clear benefits over the bidirectional setup for mapping an unknown environment using guided waves as there were no dead spots of mapping where signal direction could not be interpreted. Additionally, overall package size was significantly reduced, which in turn allows more measurements to be taken within confined spaces and increases robotic crawler mobility

The effect of complex corrosion profiles on remaining wall thickness quantification using shear horizontal guided waves

Corrosion of plate and pipe structures is a major concern to many key industries, including power, maritime, and oil and gas. When there is a need to inspect large areas quickly or when access to the structure is limited, guided wave testing is often preferred over bulk wave measurements. In this work, shear horizontal guided waves are employed for wall thickness quantification. Specifically, the interaction of modes SH1 and SH0 with complex corrosion defects is investigated. The guided wave mode are selectively generated using a phased array-based approach. A pair of identical phased array probes are positioned before and after the simulated corrosion profile, to monitor the reflected and transmitted waves. The targeted mode is excited selectively using a 32-element 3 mm pitch array and guided wave modes are decomposed after a 2DFFT is performed. The cut-off frequency technique using mode SH1 is shown to be adequate when smooth wall thinning defects are considered. When sharp pits are present, mode SH0 proved sufficient to determine the pits depth

Non-contact ultrasonic-based Bayesian mapping for robotic structural inspection

The state-of-the-art robotic ultrasonic inspection of large structural assets such as oil and gas storage tanks is carried out on a point-by-point scheme, where an ultrasonic probe with a mobile platform is scanned over every point on the component. Point-by-point inspection, which generates a huge amount of data, is time-consuming for inspecting large structural and industrial assets. One way to achieve a more efficient inspection of large structures is to take advantage of ultrasonic guided waves (UGW) suitable for mid-range inspection. Unlike the point-by-point scheme, guided waves can also be used to inspect and detect defects such as corrosions in inaccessible regions (e.g., corrosion under pipe supports). In this ongoing research project, we are working towards simultaneous localisation and mapping (SLAM) of thick structures (~10mm) under inspection using ultrasonic guided waves, in particular shear horizontal (SH) wave modes generated using electromagnetic acoustic transducers (EMATs). In other words, we use guided waves to simultaneously localise the robot and map the geometrical features such as defects and boundaries. We present results on the sensitivity of different guided wave modes for weld detection. We then demonstrate the application of guided wave robotic occupancy grid mapping (GW-OGM) to map internal defects and the unknown structure's edges/boundaries. Both pseudo-pulse-echo and pitch-catch measurement setups are used for this purpose, in which one transducer acts as a transmitter and the other one as a receiver. The former mode is used to localise welded joints, which can be eventually exploited for robot localisation. The latter mode is used for defect identification and characterisation. Defect information such as the depth of defect can be used for predicting the remaining useful life of the component. Furthermore, to create a rich ultrasonic mapping of structures by characterising defects on the fly as the robot navigates the structural assets, we have taken advantage of a machine learning approach to estimate the depth of the corrosion-like defects. Phase-based handcrafted features are extracted and fed into the Gaussian process regression model to estimate the defects' depths using the calibrated simulated data set

Crawler-based automated non-contact ultrasonic inspection of large structural assets

This paper presents an update on the progress of developing a crawler-based automated non-contact ultrasonic inspection system for the evaluation of large structural assets. The system presented is a significant improvement on current robotic NDT crawlers and aims to greatly reduce the time of inspection by creating an internal feature map of the subject in a Simultaneous Localisation And Mapping (SLAM) style method instead of using a lawnmower scanning style where all areas are scanned regardless if they contain features or are featureless. This map will be generated through rapid automated path planning and scanning and will show the location of potential areas of interest, where then, the appropriate method of inspection can be used for a high detailed evaluation. Current and ongoing work presented is as follows; the use of guided waves as the sensory input of an occupancy grid map; evaluating guided wave modes to find the mode most appropriate for this system; minimum thickness estimation using machine learning; improving the transducer setup using a unidirectional transmitter

Inspection at inaccessible locations using medium-range guided waves

Medium-range guided wave testing is commonly employed for inspection of plate and plate-like structures. The method is attractive for crack imaging and wall loss quantification, especially in hidden locations where direct access is limited. Lamb wave excitation at high-frequency-thickness products offers a potential solution for high-resolution guided wave testing, especially sensitive to vertical cracks and sharps pits. The technique usually works in pulse echo mode and at high frequency-thickness products, around 20 MHz⋅mm, offering good sensitivity and resolution. Defect sizing is based on the reflection amplitude of the received mode(s). However, the scattering of guided waves is complex, and the amplitude of the reflected modes does not provide sufficient information for defect sizing. This work aims to overcome this limitation using a focusing technique based on Lamb waves. Specifically, multiple Lamb wave modes are excited individually and superimposed to form a new mode with a desired through-thickness energy distribution. This way, energy is focused on a single point in the structure. Using weighting functions, the location of the focal point is swept across the thickness of the sample. The technique allows for accurate sizing of flaws, such as cracks and wall loss. In contrast to abrupt thickness changes, corrosion scrabs can also appear as gradual wall thinning areas. The main objective is to determine the remaining wall thickness in the affected area in order to decide further actions. For this reason, wall loss quantification is performed utilising the cut-off frequency of mode SH1. The approach requires the excitation of SH1 across a range of frequencies. For this reason, a novel excitation technique using guided wave phased array steering is developed. Specifically, an array generating shear horizontal waves is employed. The influence of the array’s length, pitch, element width, and mode excitability on excitation is investigated. By appropriately phasing the elements of the array, mode SH1 is targeted and dynamically excited over a wide frequency-wavelength range. The directionality of SH1 is also studied, as in certain conditions, this can be critical for the success of the quantification. Simulation results show the technique can accurately quantify a 65% wall thinning defect, offering a 15% increase compared to established techniques. This is critical, as wall loss defects above 50% are considered severe. Additionally, using electronic steering, rapid quantification can be achieved. Experiments using an electromagnetic acoustic transducer and synthetic steering on an intact area and an artificially machined corrosion-like defect validate the technique.Medium-range guided wave testing is commonly employed for inspection of plate and plate-like structures. The method is attractive for crack imaging and wall loss quantification, especially in hidden locations where direct access is limited. Lamb wave excitation at high-frequency-thickness products offers a potential solution for high-resolution guided wave testing, especially sensitive to vertical cracks and sharps pits. The technique usually works in pulse echo mode and at high frequency-thickness products, around 20 MHz⋅mm, offering good sensitivity and resolution. Defect sizing is based on the reflection amplitude of the received mode(s). However, the scattering of guided waves is complex, and the amplitude of the reflected modes does not provide sufficient information for defect sizing. This work aims to overcome this limitation using a focusing technique based on Lamb waves. Specifically, multiple Lamb wave modes are excited individually and superimposed to form a new mode with a desired through-thickness energy distribution. This way, energy is focused on a single point in the structure. Using weighting functions, the location of the focal point is swept across the thickness of the sample. The technique allows for accurate sizing of flaws, such as cracks and wall loss. In contrast to abrupt thickness changes, corrosion scrabs can also appear as gradual wall thinning areas. The main objective is to determine the remaining wall thickness in the affected area in order to decide further actions. For this reason, wall loss quantification is performed utilising the cut-off frequency of mode SH1. The approach requires the excitation of SH1 across a range of frequencies. For this reason, a novel excitation technique using guided wave phased array steering is developed. Specifically, an array generating shear horizontal waves is employed. The influence of the array’s length, pitch, element width, and mode excitability on excitation is investigated. By appropriately phasing the elements of the array, mode SH1 is targeted and dynamically excited over a wide frequency-wavelength range. The directionality of SH1 is also studied, as in certain conditions, this can be critical for the success of the quantification. Simulation results show the technique can accurately quantify a 65% wall thinning defect, offering a 15% increase compared to established techniques. This is critical, as wall loss defects above 50% are considered severe. Additionally, using electronic steering, rapid quantification can be achieved. Experiments using an electromagnetic acoustic transducer and synthetic steering on an intact area and an artificially machined corrosion-like defect validate the technique

A technique for medium-range through-thickness focusing using Lamb waves

Medium-range guided wave testing is commonly employed for inspection of areas with restricted access. The technique usually works in pulse echo mode and at high frequency-thickness products, around 20 MHz ⋅ mm, offering good sensitivity and resolution. Defect sizing is based on the reflection amplitude of the received mode(s). However, the scattering of guided waves is complex, and the amplitude of the reflected modes does not provide sufficient information for defect sizing. This work aims to overcome this limitation using a focusing technique based on Lamb waves. Specifically, multiple Lamb wave modes are excited individually and superimposed to form a new mode with a desired through-thickness energy distribution. This way, energy is focused on a single point in the structure. Using weighting functions, the location of the focal point is swept across the thickness of the sample. The technique allows for accurate sizing of flaws, such as cracks and wall loss

Single mode Lamb wave excitation at high frequency thickness products using a conventional linear array transducer

Lamb wave excitation at high frequency-thickness products offers a potential solution for high-resolution guided wave testing. The method is attractive for corrosion mapping and crack imaging, in particular in locations with limited access. However, multiple modes may propagate, complicating signal interpretation, which is undesirable. In this work, a systematic approach is presented, in an effort to determine the influence of the key parameters related to single higher order Lamb wave mode excitation with a conventional linear array transducer. Specifically, a linear time delay law is used to enhance a targeted mode, while the array's length, pitch and apodisation profile remain to be optimally selected. First, an analytical solution is derived based on modal analysis. This provides a natural decomposition of the amplitudes of a guided wave mode to the product of the response of a single element and the excitation spectrum, which is related to properties of the array. Then, a key observation is made, associating the spectrum to the directivity function for bulk wave phased array steering. This allows the application of well established phased array analysis tools to guided wave phased array excitation. In light of this fact, minimisation of the spectrum’s bandwidth, elimination of the grating lobes and derivation of an apodisation profile are performed, to enhance the purity of the targeted mode. Finally, experiments conducted on an aluminium plate verify the above theoretical results. The Full Matrix is acquired, and all signals are reconstructed synthetically

Dual mode inspection using guided waves and phased array ultrasonics from a single transducer

Asset inspection of large structures such as storage tanks in the oil, gas and petrochemical industry is a challenging and time-consuming process. Compared to the state-of-the-art inspection methods of these structures using ultrasonic bulk wave phased array techniques, guided waves provide a mechanism for monitoring inaccessible areas and increasing the testing range to speed up the inspection process. Combining these two approaches, an efficient inspection scheme can be achieved. For this purpose, in this work, the possibility of exciting a single higher order guided wave mode at a low dispersion region is examined, using a conventional linear phased array on a 10 mm thick sample. An analytical model based on modal analysis is derived. Then, the time delays across each element of the array are optimally selected and a necessary condition on the pitch is provided, to enhance the purity of the desired mode. The results are promising, illustrating that single mode excitation is possible at high frequency-thickness products, greater or equal than 15 MHz⋅mm. Furthermore, the phased array transducer allows for dynamic mode selection capabilities; that is, feeding the appropriate excitation signals on each channel, different modes can be excited, without having to physically alter the transducer configuration

Dual mode inspection using guided waves and phased array ultrasonics from a single transducer

Asset inspection of large structures such as storage tanks in the oil, gas and petrochemical industry is a challenging and time consuming process. Compared to the state-of-the-art inspection methods of these structures using ultrasonic bulk wave phased array techniques, guided waves provide a mechanism for monitoring inaccessible areas and increasing the testing range to speed up the inspection process. Combining these two approaches, an efficient inspection scheme can be achieved. For this purpose, in this work, the possibility of exciting a single higher order guided wave mode at a low dispersion region is examined, using a conventional linear phased array on a 10 mm thick sample. An analytical model based on modal analysis is derived. Then, the time delays across each element of the array are optimally selected and a necessary condition on the pitch is provided, to enhance the purity of the desired mode. The results are promising, illustrating that single mode excitation is possible at high frequency-thickness products, greater or equal than 15 MHz·mm. Furthermore, the phased array transducer allows for dynamic mode selection capabilities; that is, feeding the appropriate excitation signals on each channel, different modes can be excited, without having to physically alter the transducer configuration