Unmanned Aerial Vehicles in Non-Destructive Testing

Ferdinand Große-Dunker, February 2019

Introduction

Unmanned aerial vehicles are widely applied in research and commercial applications. They are used to take over repetitive or sometimes dangerous tasks which can be more effectively or safely be performed by a machine.^[1] Different methods of non-destructive testing can be realized with such devices.

Terminology UAV

An unmanned or pilotless aircraft is known as an unmanned aerial vehicle (UAV). There are other different terms for the same concept. The term drone is more widely used by the public. A UAV is part of an unmanned aircraft system (UAS). These systems consist of ground control stations, communication links, launch and retrieval systems in addition to the aircraft itself. UAV’s can be operated autonomously or remotely. The different terms often came from the different requirements and concepts between military and civilian systems or from regulatory and legal considerations. The European Commission classifies remotely piloted aviation systems (RPAS) for drones with a responsible human pilot.^[1] Completely autonomous flying drones without any pilot are not yet authorised for use, either by ICAO (International Civil Aviation Organization) or under EU rules.^[1]

Multirotor UAV design

Body

The most favored civil drone design is the multirotor UAV design. A cockpit area with windows is not needed. The body or airframe structure supports the arms with their motors and propellers. The only moving parts are the fixed horizontally oriented rotors. A body with four motors/propellers is called quadrocopter, with six motors a hexacopter and with eight motors an octocopter. The multirotor principle of flight gives the pilot a very good control over the drone. Multirotor UAVs have high flight stability and good maneuverability. To maneuver in the air, individual rotor speeds can be adjusted with digital electronic speed controllers to orient the UAV as intended. Even an inexperienced pilot can take-off and land most consumer-market drones with ease. Drones come in many shapes and sizes with different controllers and auto-pilot functions. Another UAV design is the fixed-wing drone design, like normal manned airplanes. They are much more energy efficient than the multirotor drones and can reach higher speeds and flight time. Fixed-wing airplanes are often used in military applications for carrying payload. A launchpad or runway for take-off and landing is essential as they are not able to be launched straight from the hand. The maneuverability is limited and fixed-wing UAVs are therefore harder to control and need an experienced pilot.^[2]

Power supply

Most UAV’s are electric powered with rechargeable lithium-polymer (Li-Po-) or lithium-ion (Li-Ion-) batteries. These battery types offer good power density, handling and charging times for moderate costs. Air time correlates to payload and flying conditions, but often 30-40 minutes of flight time is sufficient time for most applications. The durability of batteries depends on usage and temperature conditions. For drones with higher safety conditions a redundancy of batteries should be considered. So called tethered drones are power supplied with cables but therefore are restricted in range and height.

Communications

Typical communication to ground station works by radio frequency and GPS. The drone as well as the ground station controller use radio antennas. Live signal transmission for control and video data limit the flying range. A common frequency range for control is the 2,4 GHz band. Walls or buildings, antenna alignment or other electrical components can have disturbing affects and limit the radio range. The transmission of video data causes little latency and therefore limits flying ranges to a few kilometers for regular UAV devices. High end devices offer live video data encryption for more security against hacker abuse.

Security features

Drones can be a significant risk of injury. To avoid collision with humans or objects many UAVs are equipped with some security features:

• With the use of GPS technology, the Geofencing feature creates a virtual geographic boundary for no-flight zones. This limits the drones flying range from entering to particular areas.^[2]
• The Fail-Safe feature ensures safe landing at the starting point in case of low battery level or communication breakup. Some UAVs can also be recalled manually with a return-to-home or coming-home button.^[2]
• With high end UAVs a redundancy of flight controller cards, batteries or navigation sensors assures save landing in case of malfunction of one component.^[2]
• Anti-Collision functions are very important for future drone permissions. Many functions are based on 3D-camera-technology for visual detection of obstacles. Ultrasound technology is also used for obstacle detection in closer distances. Sometimes Light Detection AND Ranging (LiDAR) Systems are combined with UAVs. This Laser-based technology suits well for mapping and 3D-modelling tasks. Some drone manufacturers even apply plastic cages or grids around the propellers as a mechanical protection in case of physical contact.^[2]

Applications in NDT

UAVs work well for inspection methods with difficult accessibility. They are taking advantage of carrying light-weight remote sensors without physical coupling to the tested material. Large spatial coverage has been applied to some fields, for instance, topographic mapping, natural disaster surveying, precision agriculture, routine industrial inspections and traffic monitoring. ^[3]

Visual Inspection

Most techniques use well advanced camera technology for visual testing. A research study in Xinjiang, China, used a UAV in order to improve the efficiency of pavement inspection. A six-wing UAV equipped with a multispectral camera microminiature multiple camera array system flied along the road at the height of 25 m above the ground level. Multispectral pavement images were acquired and used to distinguish between the normal pavement and pavement damages (e.g., cracks and potholes) using Machine Learning algorithms. This study indicates that a UAV remote sensing system offers a new tool for monitoring asphalt road pavement condition, which can be used as decision support for road maintenance practice.^[3]

In order to reduce inspection time, cost, and workload of aircraft wing panels, a UAV is used in a study in Cranfield, UK. This non-destructive inpsection method usually is a time-consuming post-production and maintenance process. A small off-the-shelf quadrocopter equipped with a wide field-angle camera and an ultraviolet torch is programmed for a path following method of inspection. The extra payload of the UV-light reduced the the flight endurance to approximate 17 minutes. The onboard 14-megapixel camera with digital image stabilisation captured high-definition videos, which were transmitted in real-time to a ground station. A six-meter long vertically mounted wing panel can be examined in about two minutes. The UAV is following a path of simulated waypoints with 1 m separation distance from the wing panel. Little defects can succesfully be detected after applying an image processing algorithm.^[4]

Infrared Thermography

UAVs provide an excellent investigative tool for detecting heat leakages and their surveillance using high-resolution thermal cameras. A research team in Izmir, Turkey, developed a low cost optimal hexacopter for surveillance and heat leakage detection. With analyzing Infrared Thermography images by an image processing toolkit and illustrating the heat leakages in the pictures, a solution for important needs in civil applications like search and rescue, surveillance and heat loss mapping for buildings can be found. Also the total waste of money due to heat leakages of buildings can be calculatet.^[5]

Radar

There are many applications for Ground Penetrating Radar like locating pipelines or cables or investigating sediment and soil structures. A research team in Spain introduced a novel airborne system for subsurface sensing and imaging applications for applications such as landmine detection or archeological surveys. The system consists of a Ground Penetrating Radar (GPR) mounted on an Unmanned Aerial Vehicle, including a high-accuracy positioning system. Metallic and dielectric targets were buried in a sand-soil mix and radar measurements with the UAV were conducted. The results of these flight tests prove the feasibility of the system to detect both metallic and dielectric targets. Furthermore these measurements could be coherently combined to improve the radar image resolution.^[6]

Laser Vibrometry

Another noncontact inspection method is using acoustic irradiation induced vibration and Laser Vibrometry. This method is used for surface inspection of buildings or structures. With mounting a sound source on an UAV, the sound source itself can be brought close to the measured surface without the problems of angle dependence. A study in Japan confirmed the feasability of this method. A planar speaker, a sighting laser and a laser range finder were mounted on an UAV underside. Delamination defects of outer wall tiles could be detected even if the UAV itself was swinging at flight.^[7]

Regulations in Germany

There are a few essential regulations for UAV using in Germany. The most important are following:

• Labeling requirements: all UAVs weighting more than 0,25 kilograms must be labelled with a fireproof badge, which includes the name and the address of the UAV owner. In case of damage or loss the owner can be quickly addressed.^[8]

• Proof of knowledge: for operating UAVs and model aircrafts over 2 kilograms of weight a proof of knowledge is necessary. This can be a valid pilot’s license, a certification by the german aviation federal office or a certification by the german air sports federation (for sports and civil UAVs only). These certifications are valid for 5 years only. On model flight terrains no proof of knowledge is needed.^[8]

• Permit of freedom: UAVs weighting under 5 kilograms do not need a permit of freedom. Public authorities or safety organizations (fire department, THW, DRK) generally do not need a permission for task operations.^[8]

• Permit requirements: a permit is necessary for UAVs weighting over 5 kilograms and operations at night, as well as for operations inside of restricted areas.^[8]

• In certain situations, operating UAVs is prohibited: UAVs weighting under 5kg can not be operated out of visibility. Operations are banned in and over operating sites of police and rescue, hospitals, prisons, gathering of people, industrial facilities, nature reserve areas, certain transport routes and airport control zones and airport areas. Flight heights over 100 meters are prohibited, except for permitted areas. Flying over residential areas is prohibited for UAVs weighting over 0,25 kg and equipped with optical, acoustical or radio signal receiving and recording functions. As an exception UAV operations over residential areas are allowed with the explicit agreement of the property owner. UAVs weighting over 25 kilograms are generally forbidden.^[8]

• Evasive duty: UAVs and model aircrafts are committed to evasion for manned aircrafts.^[8]

• Use of video glasses: some UAVs can be steered with video glasses (VR googles). Permits are valid for up to 30m height and UAV weights under 0,25 kilograms. For UAVs heavier than 0,25 kilograms an additional person is needed for steady drone visibility and caution warning.^[8]

Literature

1. European Commision, MEMO/14/384 Remotely Piloted Aviation Systems (RPAS). Retrieved from Remotely Piloted Aviation Systems (RPAS) on 05.02.2019.
2. Landrock, H.,Baumgärtel, A.:"Die Industriedrohne - der fliegende Roboter", Springer Verlag, 2018.
3. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Vol. 11, No. 10,: Detection of Asphalt Pavement Potholes and Cracks Based on the Unmanned Aerial Vehicle Multispectral Imagery, October 2018.
4. 5th IEEE International Workshop on Metrology for AeroSpace, MetroAeroSpace 2018, Proceedings: Inspection of Aircraft Wing Panels Using Unmanned Aerial Vehicles, 31 August 2018, Article number 8453598, Pages 56-61.
5. 26th IEEE Signal Processing and Communications Applications Conference: Heat leakage detection and surveiallance using aerial thermography drone, 5th July 2018, Pages 1-4.
6. 15th European Radar Conference, EuRAD 2018, Article number 8546594: UAV-mounted GPR for NDT applications, 26 November 2018, Pages 2-5.
7. IEEE International Ultrasonics Symposium, IUS Volume 2018-October: Outer Wall Inspection Using Acoustic Irradiation Induced Vibration from UAV for Noncontact Acoustic Inspection Method, 17 December 2018, Article number 8579757.
8. Bundesministerium für Verkehr und digitale Infrastruktur."Verordnung zur Regelung des Betriebs von unbemannten Fluggeräten." 2017.