Research on unmanned aerial vehicles (UAVs) has been conducted for quite some time, even though experiments were rather difficult to carry out. However, recent years have seen a proliferation of published reports in this field. This is most likely due to the spread of multicopters, which are easier and safer to operate than fixed-wing aircraft and single-rotor helicopters.

Other factors that have made it easier to conduct research on UAVs and thus contributed to the increased number of studies include the wide availability of high-performance flight controllers that are either low cost or offered on an open-source basis, such as ArduPilot and MultiWii.

Although this has minimized technological hurdles in conducting research, it has become more difficult to conduct research safely in a social and legal context. Prompted by a 2015 drone incident, Japan’s civil aeronautics law was revised to control UAV flights under stricter regulations. Even so, these regulations are still considered less restrictive than those in most other countries.

UAV research includes obvious risks and dangers of operating airborne devices, but also makes it more interesting as a scientific inquiry, generates high expectations about practical utility, and makes a highly significant area of investigation.

Placing a high priority on safety will hopefully lead to further research in this area.

This special issue covers the latest in UAV research, including

• UAV control system design,

• Studies on flight characteristics of aircraft equipped with special mechanisms,

• UAV applications,

• Studies on operational UAVs.

Readers will find it interesting and rewarding to explore the latest UAV research trends presented in this issue.

The year 2015 marked the beginning of a new phase in the history of drones in which drones came into use for business purposes in addition to the conventional ones flown for recreational purposes, and the amended Civil Aeronautic Act became effective in the same year. In 2016, full-scale drone businesses including the inspection of infrastructures, measurements, security, and disaster responses are expected to begin. An overview of the rapidly expanding drone business and the attempt to deliver objects using drones in Chiba, which is called the industrial revolution in the sky, are presented in this paper. Next, the technological characteristics of mini-surveyors using domestic, production-model drones are introduced, and the use of mini-surveyors is outlined. Cutting-edge research and overseas trends are also discussed. In addition, the approach of the mini-surveyor consortium and the amended Civil Aeronautic Act are introduced. Finally, the tough robotics challenge of an innovative research and development promotion program ImPACT involving the entire Japan and the future society where drones are spreading are described.

The airfoil is often used as the elemental device for flying/swimming robots, determining its basic performances. However, most of the aerodynamic characteristics of the airfoil have been investigated at Reynolds numbers Re’s more than 10⁶. On the other hand, our knowledge is not enough in low Reynolds-number ranges, in spite of the recent miniaturisation of robots. In the present study, referring to our previous findings (Hirata et al., 2011), we numerically examine three kinds of high-performance airfoils proposed for very-low Reynolds numbers; namely, an iNACA0015 (the NACA0015 placed back to front), an FPBi (a flat plate blended with iNACA0015 as its upper half) and an FPBN (a flat plate blended with the NACA0015 as its upper half), in comparison with such basic airfoils as a NACA0015 and an FP (a flat plate), at a Reynolds number Re=1.0×10² using two- and three-dimensional computations. As a result, the FPBi shows the best performance among the five kinds of airfoils.

In this paper, an autonomous flight control method of multicopter using only Web camera is proposed. Autonomous multicopter flight control has attracted much attention. Previously proposed methods have measured the position and posture of multicopter, GPS, motion capture system, and laser range finder are often applied. These methods are effective under certain conditions, but practical use may be difficult in some real environments. To achieve an autonomous flight control of multicopter with simple equipments, we propose an autonomous flight control method using Web camera and simple monitoring display window operation. Colored markers are used to measure the position of multicopter with Cartesian coordinate in three dimensional, then its position connects with elevator, aileron, and throttle operation. For each operation, a simple PI-D control method is used and the controller gains are individually adjusted. To show the effectiveness of our proposed method, experimental results are given.

The UAV flight control method we propose uses LED panels and a video camera on the UAV. Specifically, the LED panel displays patterns related to the UAV commands and blinking patterns for panel detection. A panel detection process based in UAV video camera images uses the frequency of green blinking as a cue for panel detection, then command patterns are distinguished and the UAV performs tasks based on this pattern (command). In experiments we performed for panel detection and discrimination using the UAV, we confirm the effectiveness of the proposed method for autoflight control.

The reduced-order observer design we present estimates the velocity states of a quadrotor helicopter, or quadcopter, based on sampled measurements of position and attitude states. This observer is based on the forward-differentiation Euler model. The observer is robust enough against observation noise that the gain of a closed-loop controller is high enough to improve control performance. A sliding-mode controller stabilizes and implements quadcopter tracking control effectively, as is verified experimentally when compared to a conventional backward-difference method.

Trial production has begun on tricopter-based new-concept vertical take-off and landing aircraft (VTOL). The VTOL features three fixed-angle motors on a reversed-T shape body and a fixed wing with an elevon and a rudder. The proposed craft uses fewer components than quadcopter-based VTOL craft. Continuous transition from rotorcraft to fixed-wing craft is completed with the craft’s 90° tilt.

During the high economic growth period in Japan, the development of new infrastructures was promoted and numerous bridges and buildings were constructed. Currently, the walls of bridges and buildings are inspected manually. This manual inspection process is expensive and time-consuming, and inspectors may be placed in dangerous situation. In this study, a robot that moves stably on a wall while maintaining a distance from the wall was developed to enable low-cost, safe inspection. Several characteristics of the robot were measured, and the possibility of using the robot in practical applications was assessed based on the measurement results.

The quad tilt rotor helicopter (QTRH), a tilt-rotor aircraft prototype, has fixed wings for long-range high-speed flight. Its rotor-tilt mechanism controls 4 rotor-tilt angles independently and controls its roll and pitch angles similarly to a multirotor helicopter. It controls yaw angle by thrust vectoring and moves forward and backward by tilting its rotors. Rotor-tilting maneuvers are the initial stage of flight-mode transition between helicopter and fixed-wing modes.

A quad rotor helicopter (QRH) is a radio controlled (RC) aircraft that tilts its attitude to generate a horizontal force component to move in a certain direction. Using autonomous control, the attitude control system tilts the airframe against disturbances, such as wind. Thus, the attitude of a flying QRH is always slanted. In this study, three types of deflecting thruster were compared to maintain the position and horizontal attitude of the QRH. The extra thrusters are tilted to generate a thrust against disturbances without causing the airframe to incline. It is suitable for precise measurements for which the airframe posture should remain horizontal.

The formation control algorithm we propose for a multi-UAV uses LMI conditions. Starting with a linearized model of UAVs such as a quadrotor, we introduce a formation control algorithm based on a consensus algorithm, a leader-follower structure, graph theory and the Lyapunov stability theorem for a liner system. We propose the control algorithm using the Lyapunov theorem and LMI conditions for intermittent communication. We demonstrate the proposed control algorithm’s stability even when network links are disconnected. Numerical simulation and experimental validation show the proposed control’s effectiveness.

In this study, a cable-suspended transportation system using a small-size helicopter was investigated. For a secure flight, it is necessary to suppress the swing of the suspended load. For this purpose, a cable angle feedback system was adopted because it is easy to mount the corresponding measuring device on a helicopter. Delayed feedback control was applied for efficient swing damping. The control parameters were obtained using a simple planar double pendulum model that takes into consideration the coupled dynamics of the helicopter and the load. To build the model, the system parameters were identified through frequency response flight tests. In this paper, an appropriate design strategy is reported for a trade-off relationship between vibration damping and piloted handling qualities; the robustness of the control method against disturbances and signal noise was verified by comparing the delayed feedback control method with the real-time angular velocity feedback control method. The damping effect was verified by performing flight tests with three cable lengths.

In this paper, a sensory feedback device was developed to improve the operability of a myoelectric prosthetic hand. The device is worn on the user’s upper arm and provides object hardness feedback to by winding a belt onto the upper arm using a motor. When the finger of the myoelectric prosthetic hand grabs the object, the contact force on the object is detected by a pressure sensor attached to a finger cushion on the myoelectric prosthetic hand. Based on the sensor’s input, the hardness of the object is calculated. According to the hardness of the object, a reference input to realize the corresponding winding speed of the belt is generated by a reference input generator. Then, the motor of the feedback device is controlled to track the reference input by using the self-tuning PID control technique, taking parameter variation into account. Thus, the belt of the feedback device is wound by the motor and tightens the user’s upper arm, thereby enabling the user to receive tactile feedback. Finally, confirmation tests are conducted based on a psychophysical method to verify the effectiveness of the feedback device and its control system. As a result, the difference threshold of the sensory feedback device was 0.59 N/mm.

Vectored underwater vehicles (VUVs) are receiving increasing research attention, in part for their maneuverability. In our work, we apply a novel vectored thruster based on a spherical parallel mechanism to an underwater vehicle. We present and calculate the scaling factor based on the vectored thruster’s configuration parameters and set up a six DOF kinematic model. We construct a nonlinear dynamic model of the VUV without appendages using the Newton-Euler method. To demonstrate the VUV’s transverse maneuverability, we set up a perturbation model in a complex domain using Laplacian transformation, and propose the stability margin of vectored propulsion as a maneuverability index. Many numerical examples are provided to verify the maneuverability of the VUV.

Using INASTAMOR pressure-conductive rubber as the sensing material, we developed a flexible tactile sensor array to measure three-axis contact force and slip. The sensor array has 9 (3×3) sensing units, each consisting of three layers, i.e., a bottom electrode, conductive rubber chips, a top polydimethylsiloxane (PDMS) bump. We detailed the array’s structural design, working principle, and fabrication process. We also characterize the array’s three-axis force measurement performance. The full-scale force measurement ranges and sensitivities in x-, y-, and z-axes are characterized as 5, 5, 20 N and 0.675, 0.677, 0.251 V/N, respectively. The array is mounted on a prosthetic hand for detecting contact force and slip occurrence in grasping. Results showed that the array measures three-axis contact force and detects slippage by using discrete wavelet transformation. The tactile sensor array has potential applications in robot-hand grasping that require simultaneous slip detection and three-axis contact force measurement.

In abalone fishing with spearing, size of abalone which was allowed to catch is strictly controlled in Japanese law. Therefore, fisherman usually judge size by observing from a boat. However, this is difficult, due to aging. Therefore, we have been developing an underwater robot to assist fisherman in determining abalone size by using a stereo camera. The robot is small size and uses 6 thrusters to achieve 5 DOF motion in an underwater environment.

This paper proposes a new method of solving the kinematic problems for parallel robots. The paper content aims to solve nonlinear optimization problems with constraints rather than to directly solve high-order nonlinear systems of equations. The nonlinear optimization problems shall be efficiently solved by applying the Generalized Reduced Gradient algorithm and appropriate downgrade techniques. This new method can be able to find exact kinematic solutions by assigning constraints onto the parameters. The procedure can be done without filtering control results from mathematical solution, from which the control time of manipulators can be reduced. The numerical simulation results in this paper shall prove that the method can be applied to solve kinematic problems for a variety of parallel robots regardless of its structures and degree of freedom (DOF). There are several advantages of the proposed method including its simplicity leading to a shorter computing time as well as achieving high accuracy, high reliability, and quick convergence in final results. Hence, the applicability of this method in solving kinematic problems for parallel manipulators is remarkably high.

The support vector machine (SVM) we propose for automated gait and posture recognition is based on acceleration. Acceleration data are obtained from four accelerators attached to the human thigh and lower leg. In the experiment, volunteers take part in four gaits and postures, i.e., sitting, standing, walking and ascending stairs. Acceleration data that are preprocessed include normalization, a wavelet filter and dimension reduction. We used the SVM and a neural network to analyze the data processed. Simulation results indicate that SVM parameters C and g selected by a genetic algorithm (GA) are more effective for gait and posture analysis when compared to the parameter C and g selected by a grid search. The overall classification precision of the four gaits and postures exceeds 90.0%, and neural network simulation results indicate that the SVM using the GA is preferable for use in analysis.