Contact inspection: from drones that look to drones that touch and measure

Some things in inspection can be seen, and some can only be found by touch. Cracks and rust show up in photos. But how many millimetres a steel plate has lost to corrosion, whether concrete is delaminated inside, or whether rust is creeping under a coating cannot be seen from the surface. These checks are non-destructive testing (NDT), and most of them require the sensor to touch the surface.

Today, scaffolding, aerial lifts and rope access are used just to make that contact. The measurement takes seconds; getting there takes days. That is exactly what contact drones aim at.

The closest to routine use is ultrasonic thickness measurement (UT). The probe must be held perpendicular to the surface, with steady force, for a few seconds. Pushing while hovering makes the surface push back, so aircraft use several tricks.

In Japanese infrastructure inspection, hammer sounding is essential for finding delaminated concrete. Sound concrete rings clearly; a hollow spot gives a dull thud. To do this by drone, a hammer or rolling device must strike the surface while microphones pick up the sound and separate it from rotor noise.

• Mostly research and trials: many prototypes and demonstrations by universities, contractors and manufacturers, but not yet a general substitute for close visual inspection and sounding.
• Separating sound: rotor noise can overlap the frequencies of the tap. Microphone placement and signal processing are key.
• Contact and positioning: under decks and tunnel crowns the device presses upwards, where ceiling effect (flight principles) also comes into play.
• Other methods: thermal imaging can narrow down candidate delamination over large areas, with sounding used only on suspicious spots.

An aerial manipulator is a drone with a robot arm. Research targets tasks such as placing sensors, checking bolts or attaching and removing small parts. Moving the arm shifts the centre of mass and every push produces a reaction, so flight control and arm control must be designed together.

Perching lets the aircraft land on the structure like a bird on a branch, then stop or throttle back its rotors. Magnets, suction or grippers hold it to steel members or beams. Because it no longer has to hover, battery life stretches and measurements are free of vibration — useful for long-duration monitoring or installing sensors.

A tethered drone flies while connected to a ground power supply by cable. With continuous power it can stay up for hours, and data can be sent through the wire at high bandwidth. Its range is limited by the cable, but the risk of it flying away is small.

• Good fits: long-duration monitoring from a fixed point, overhead cameras in disasters, work inside vertical shafts and pipes such as sewers and chimneys, and relaying where radio is weak.
• Watch out for: snagging and tangling, cable weight and wind drag, ground crew and power supply.
• Regulation: since a September 2021 amendment of Japan’s aviation ordinance, flights tethered by a sufficiently strong line of 30 m or less, with third-party access controlled, need no permission or approval for flights over populated areas, at night, beyond visual line of sight, within 30 m of people or property, or for dropping objects. Check world regulations and the official guidance.

In confined spaces, however skilled the pilot, the aircraft will eventually touch something. So design it to be touched — that is the idea of collision tolerance. The aircraft is enclosed in a spherical or lattice cage so it can bounce off walls and keep flying (e.g. Flyability Elios series).

These spaces often combine confined spaces, darkness and oxygen deficiency & toxic gas. Not sending a person in is the biggest safety gain of all.

The central question in contact inspection is whether the measurement can be accepted into the inspection record. Inspections of pressure vessels, piping and tanks prescribe methods, equipment calibration and inspector qualifications (e.g. ISO 9712 or national NDT certifications). To be used in records under industry codes (e.g. API 510, 570, 653), drone-taken values must meet the same conditions.