Currently, drones have reached the stage of practical application, business, and social contribution from the stage of research and development. In fact, drones are now opening up a new market not only in the field of aerial photography and agricultural chemical spraying, which had traditionally been on the market, but also in the field of surveying. Furthermore, new markets are expected to be formed within several years in the field of infrastructure inspection and logistics.

Under these circumstances, research on drone application is becoming significant, along with research on drone design and control, which have been done conventionally. However, there are still only a few studies and journals focusing not only on drone development and design but also drone application. Therefore, in this special issue, we invited papers with comprehensive contents, including research focusing on drone application, and created a space to present cutting-edge research results.

By reading this special issue, we hope that readers will understand the latest information about the applications of drones and their cutting-edge technology. Furthermore, we also hope that readers will be able to proactively promote the use of drones in their own research and work, based on the information obtained from this special issue.

The history of drone dates far back. Human in those days must have had a yearn for the sky. This article first gives an overview of the history of research and development of drone including fixed wing and rotary wing, followed by, in particular, the history of development of rotary wing drone with the following four periods: before 1990 when radio control is at its dawn period; 1990 to 2010 when drone is at its dawn period; 2010 to 2015 when hobby-use drone is at its spread period; and 2016 and after when industrial-use drone is at its dawn period to growth period. In addition, this article presents the Japanese government’s roadmap, followed by a five-stage class of drone in terms of flight level and autonomy. In particular, autonomy will be discussed from the points of view of guidance, navigation, and control. Then with reference to an ideal state of drone in future, the article will discuss importance of the guidance system by fault tolerant control and supervisor control with implementation of the AI technology and the like.

Multicopter-type unmanned aerial vehicles, called drones, have been attracting wide attention because of their immense potential for use in various missions such as surveillance, reconnaissance, and delivery service. For the application of drones, however, their noise will be a serious issue especially when operating in urban areas, and to our knowledge, it has not been resolved yet. In this study, inspired by the unique wing structures of insects and birds, we have developed new low-noise-type propellers for drones. The various bio-inspired attachments of drones such as the serrations at the leading edge, velvet-like surface, and fringes at the trailing edge were tested, and their acoustic and aerodynamic performances were evaluated experimentally and numerically. Our results indicate that an attachment at the trailing edge can suppress the noise level while maintaining the aerodynamic efficiency of the proposed propeller close to that of the basic propeller.

The influence of walls on the performance of multiple rotor type drones is numerically simulated. With the current wide-spread applications of autonomously flyable UAVs, there are potential needs to use the drones for inspections and observation near various structures, such as buildings and bridges. The flow fields around multiple rotors are influenced significantly by the existence of an upper or a side wall, so that the thrust, required torque, and other forces and moments are changed. It is found that when a rotor approaches an upper wall, as the distance between the rotor and the upper wall is less than a diameter of the rotor, the thrust suddenly increases, which causes the rotors to collide with the upper wall. When an isolated rotor is operated near a side wall, the thrust decreases and a rolling moment appears to tilt the rotor toward the wall as the distance becomes shorter. For a multiple rotor drone near a side wall, the rotors have different distances from the wall, which causes the whole aircraft tilts toward the wall. From the perspective of safety operations, the multi-rotor drone should be kept away from both the upper wall and the side wall at a distance of at least 1.5 times of the rotor diameter to prevent unexpected motions of the aircraft caused by the wall during hovering flight.

Recently, many studies on unmanned aerial vehicle (UAVs) that perform position control using camera images have been conducted. The measurements of the surrounding environment and position of the mobile robot are important in controlling the UAV. The distance and direction of the optical ray to the object can be obtained from the diameter and coordinates in the image. In these studies, various camera systems using plane cameras, fisheye cameras, or omnidirectional cameras are used. Because these camera systems have different geometrical optics, one simple image position measurement method cannot yield the position and posture. Therefore, we propose a new method that measures the position from the size of three-dimensional landmarks using omnidirectional cameras. Three-dimensional measurements are performed by these omnidirectional cameras using the distance and direction to the object. This method can measure three-dimensional positions from the direction and distance of the ray; therefore, if the optical path such as the reflection or refraction is known, it can perform measurements using a different optical system’s camera. In this study, we construct a method to obtain the relative position and relative posture necessary for the self-position estimation based on an object with an omnidirectional camera; further, we verify this method by experiment.

In recent years, applications of high-speed visual systems have been well developed because of their high environmental recognition ability. These system help to improve the maneuverability of unmanned aerial vehicles (UAVs). Thus, we herein propose a high-speed visual unit for UAVs. The unit is lightweight and compact, consisting of a 500 Hz high-speed camera and a graphic processing unit. We also propose an improved UAV velocity estimation algorithm using optical flows and a continuous homography constraint. By using the high-frequency sampling rate of the high-speed vision unit, the estimation accuracy is improved. The operation of our high-speed visual unit is verified in the experiments.

In this study, a novel robust navigation system for a drone in global positioning system (GPS) and GPS-denied environments is proposed. In general, the drone uses position and velocity information from GPS for guidance and control. However, GPS cannot be used in several environments; for example, GPS exhibits huge errors near buildings and trees, indoor environments. In such GPS-denied environments, a Laser Imaging Detection and Ranging (LIDAR) sensor-based navigation system has generally been used. However, the LIDAR sensor also has a weakness, and it cannot be used in an open outdoor environment where GPS can be used. Therefore, it is advantageous to develop an integrated navigation system that operates seamlessly in both GPS and GPS-denied environments. In this study, an integrated navigation system for the drone using GPS and LIDAR was developed. The design of the navigation system is based on the extended Kalman filter, and the effectiveness of the developed system is verified by numerical simulation and experiment.

The controller parameters of a quadrotor helicopter have to be properly adjusted for the helicopter to fly stably, while considering the helicopter’s dynamics. Typically, there exists uncertainty in helicopter dynamics such as parametric uncertainty and unmodeled dynamics including external disturbances. To cope with uncertainty in the dynamics, it is effective to update controller parameters according to the performance during the flight. This paper proposes the introduction of a simple adaptive control (SAC), which is an adaptive control technique to augment the conventional attitude controller based on the PD controller to attenuate the effect of uncertainty. The introduction of a constraint to the adaptive gain is also proposed to suppress overshoot in the transient response. In this study, it was demonstrated through numerical simulations and experiments that the proposed controller was effective and that its performance was better in comparison to the original PD controller.

Proportional-integral-derivative (PID) controllers are a classical control algorithm that are still widely used owing to their simplicity and accuracy. However, tuning the three parameters is difficult. No methods have been known to determine the exact ideal combination of the P, I, and D gains. Moreover, controlling a system that contains dynamics changes over time using fixed parameters is difficult. A self-tuning neuro-PID controller is applied to a balloon robot for indoor entertainment to enhance its accuracy in following a target trajectory. Our experiment shows the effectiveness of the neuro-PID controller over conventional hand-tuned PID controller.

This study applies a sliding mode control (SMC) strategy for a robust controller of a quad-rotor vehicle. First, a controller combined with a nested control loop and an SMC is introduced, because a quad-rotor vehicle has only four control inputs although the vehicle has six degrees of freedom. The control performance for the feedback gains in the nested loop is investigated in numerical simulations. Subsequently, the effects of practical system limitations (control cycle and rotor dynamics) on the control performance are examined. Finally, the robust performance of the SMC strategy on a quad-rotor vehicle is discussed.

A teleoperation system for a construction robot works effectively in a hazardous environment. However, it has problems with work precision and mental workload. In this study, a moving visual support system using a drone is developed to solve these operability problems. The usability of the system is confirmed through a subjective experiment.

Many social infrastructures and buildings are aging. Therefore, inspections to find deteriorated parts to prevent accidents are important. However, significant costs and inspection time are required for the current technologies. Therefore, inspection methods using robots are attracting attention. As the most appropriate option to traverse freely on concrete bridges and building walls is by traversing with lift force from propellers, a robot called HORNET has been developed that can run along a wall with two rotors. However, the floating-type robot with propellers could drop from a wall if hit by strong wind. Therefore, a gyro sensor was installed in the robot to detect its posture and adopted a simple maneuvering assist control to reduce the above-mentioned issue. In this study, the motion of HORNET was analyzed and a simple control system was designed to realize the maneuvering assist control. It was confirmed that the resistance of HORNET to the crosswind was improved by adding the control system.

We have studied sound source localization, using a microphone array embedded on a UAV (unmanned aerial vehicle), for the purpose of detecting for people to rescue from disaster-stricken areas or other dangerous situations, and we have proposed sound source localization methods for use in outdoor environments. In these methods, noise robustness and real-time processing have a trade-off relationship, which is a problem to be solved for the practical application of the methods. Sound source localization in a disaster area requires both noise robustness and real-time processing. For this we propose a sound source localization method using an active frequency range filter based on the MUSIC (MUltiple Signal Classification) method. Our proposed method can successively create and apply a frequency range filter by simply using the four arithmetic operations, so it can ensure both noise robustness and real-time processing. As numerical simulations carried out to compare the successful localization rate and the processing delay with conventional methods have affirmed the usefulness of the proposed method, we have successfully produced a sound source localization method that has both noise robustness and real-time processing.

In recent years, promotional videos of mountains, seashores, and lakes have been created using drones. Shooting videos of natural landscapes with drones usually requires manual operation, and relies on the skill of the operator. However, if the intention is to shoot videos over a wide geographical area, manual operation is not sufficiently accurate. Therefore to accomplish this, and to take full advantage of the features of a drone, automatic operation is desirable. In this paper, we propose a method of safely modelling a video target and flight route. This includes planning for video shooting on the basis of a model, to realize accurate automatic video shooting of natural landscapes with drones. It was assumed that multiple drones would be operated simultaneously. Therefore, we developed an error verification method to compensate for performance differences between drones. To verify the usefulness of the method, it was used to shoot actual video images of Onuma quasi-national park in the south of Hokkaido Prefecture.

Because of the frequent occurrence of large-scale disasters, such as earthquakes, tsunamis, volcanic eruptions, and river floods, there is an increased demand for emergency response, restoration, and disaster prevention using robotic technology. One such technology involves assessment of the damage status using flying robots, which have undergone rapid development in recent years. In this study, using images of the disaster site obtained from a flying robot and a terrain database consisting of predisaster 3-D data, we aim to detect efficiently the collapse of electric utility poles, which are man-made objects, and the water level difference before and after river flooding, which is part of the natural landscape. By detecting these disaster-related events, we show the validity of the proposed method to assess the damage situation using the terrain database.

In this study, we developed a monocular stereo tracking system to be used as a marker-based, three-dimensional (3-D) motion capture system. This system aims to localize dozens of markers on multiple moving objects in real time by switching five hundred different views in 1 s. The ultrafast mirror-drive active vision used in our catadioptric stereo tracking system can accelerate a series of operations for multithread gaze control with video shooting, computation, and actuation within 2 ms. By switching between five hundred different views in 1 s, with real-time video processing for marker extraction, our system can function as J virtual left and right pan-tilt tracking cameras, operating at 250/J fps to simultaneously capture and process J pairs of 512 × 512 stereo images with different views via the catadioptric mirror system. We conducted several real-time 3-D motion experiments to capture multiple fast-moving objects with markers. The results demonstrated the effectiveness of our monocular 3-D motion tracking system.

For decommissioning of the Fukushima Daiichi Nuclear Power Station, we are developing an “automated construction system for a modularized rail structure.” The intersection module developed in this study produces structure branches and changes robot attitudes. To maintain module functions, assuming the anticipated malfunctions caused by high-radiation environments, remotely controlled robots are used to replace malfunctioning parts of the module. To facilitate maintenance tasks, small positioning errors must be accommodated. This compliant function can be realized by the maintenance robot or the module. To elucidate effective combinations of the compliant mechanisms, experiments are conducted using prototypes. Experimental results confirm that the compliant mechanism of the robot contributes more efficiently than that of the module. This study is expected to be applied to practical distributed robotic systems because it is necessary to replace malfunction parts as simply as possible if they are replaced by remotely controlled robots.

As robot workshops for elementary and junior-high school students are being held across Japan, there is a large variety of teaching-material robots available for them. An increasing number of freshmen at colleges or universities already undertake some basic education on robotics before they enter a college or a university. On the other hand, as research and development of service robots is being promoted as a part of the measures to cope with the declining birthrate and the aging population of Japan, service robots are expected to come into much wider use. Under these circumstances, it is imperative for universities and technical colleges to enrich the contents of their robotics education, especially on service robots. However, there are no robots available as teaching materials. Therefore, we developed an educational service robot and used it to conduct practical trainings with the aim of motivating students to develop robots, as well as enhancing their motivations. This paper reports on the details of this project.

Motion planning of mobile robots for occluded obstacles is a challenge in dynamic environments. The occlusion problem states that if an obstacle suddenly appears from the occluded area, the robot might collide with the obstacle. To overcome this, we propose a novel motion planner, the Velocity Obstacle for occlusion (VOO). The VOO is based on a previous motion planner, the Velocity Obstacle (VO), which is effective for moving obstacles. In the proposed motion planner, information uncertainties about occluded obstacles, such as position, velocity, and moving direction, are quantitatively addressed. Thus, the robot based on the VOO is able to move not only among observed obstacles, but also among the occluded ones. Through simulation experiments, the effectiveness of the VOO for the occlusion problem is demonstrated by comparison with the VO.