Advances in Remote Visual Inspection

Remote visual inspection (RVI) is a long-established inspection and nondestructive testing technique in the aerospace sector. Although conventional rigid borescopes are still widely used in the industry, using inspection ports built into aero engines...

In the aerospace sector, which often struggles to cope with ever-increasing amounts of inspection information, the archiving data is especially relevant. It is important for software platforms to accept images from any number of LAN-connected or remote workstations and store these using various compression techniques to save storage space without sacrificing image quality. Input and retrieval of information is quick and easy by virtue of the simple DICONDE tagging system. Chosen software should not only store the raw inspection data but also any enhanced images and allow for more efficient data searching, as all information from all workstations in the network is available at one central repository. A similar order of productivity improvement can also be achieved in post-inspection, as only relevant inspection data needs to be sent for further analysis.

Making it simple

Software is also available to standardize inspection procedures to ensure consistency of both inspection and presentation of inspection results. Menu-directed inspection (MDI) is a software solution that provides a guided inspection, where context is added automatically. For example, when inspecting an engine, a drop-down menu will first allow an inspector to select the relevant manufacturer and specific engine. All the identification data relevant to the task (inspector, site, date, etc.) is then inputted before the inspector carries out the inspection, in the manner specified for that engine and component. The data image file is then tagged with annotations and filed within the borescope’s data capture system. Finally, a hard copy report is produced with a convenient “click-to-report” feature.

Measuring what you see

The measurement of flaws, discrepancies, and clearances is nowadays often just as important as their detection and identification. To date, there have been three major measurement systems: comparison measurement, stereo probe measurement, and shadow probe measurement.

Comparison measurement is based on a known reference dimension in the inspection image, which is used to measure other objects in the same view and plane. (The reference dimension is often set in place by the instrument manufacturer or introduced with the probe.)

Stereo probe measurement uses a prism to split images, allowing the camera to capture left and right views with a precise angle of separation. The position of user-place cursors is then analyzed using a computer algorithm and triangulation geometry is applied to obtain accurate measurements.

Shadow probe measurement relies on a shadow triangulation of tip-to-target distance. A shadow probe projects a shadow across an inspection image and the position of the shadow in the image indicates the distance to the object. With this information, the shadow probe system can accurately calculate the size of user-selected features or defects. Typical measurements afforded by these measurement methods are depth, length, area, point-to-line, multi-segment length, and circle gauge.

Toward better accuracy and improved imaging

Even with the current range of measurement techniques, measurement remains the most difficult aspect of using video borescopes. Operators must be highly trained and practiced to obtain reliable and repeatable results. This expertise level has been addressed as RVI is now professionalized as an official NDT discipline and is a module within ASNT’s TC1A Level-III testing and certification process.

However, advances have been made recently in improving the accuracy, repeatability, and ease-of-use of video borescopes with phase measurement.

Phase measurement is based on an existing optical metrology technique and involves projecting line patterns onto a surface, capturing the patterns in a video camera with high-quality viewing optics, and processing the images using proprietary algorithms to produce a point cloud, 3-D map of the entire surface. This is then used in conjunction with measurement to obtain more precise information of the defect or object being viewed. Measurement itself involves the placing of cursors on the full-screen image, with no need for the point matching, shadow identification, or dot selection steps that can be challenging with other techniques.

An innovative feature of this new measurement system is that the 3-D scan can be rotated and zoomed to provide enhanced indication of the indication’s size and shape. Further assistance in assessing an indication’s size and shape is provided by the system’s profile view feature. This is achieved when the user positions cursors on either side of an area of interest and the phase measurement system draws a line between them. Profile view is then selected and a cross section of the part along that line is displayed, helping to visualize the shape of a pit, crack, or corrosion area. At the same time, profile view can also be used to measure depth at points along the cross-section.

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