Cheetahs are characterized by large spinal flexion and extension during high-speed running, yet the dynamical role of the phase relationship between spinal motion and limb support remains unclear. We aimed to clarify how this phase relationship affects running performance, focusing on the effect of asymmetric spinal stiffness. Using a simple planar cheetah model with asymmetric torsional spinal stiffness, we numerically searched for periodic bounding solutions over a range of stiffness parameters and compared their ground reaction forces, horizontal velocities, and stability. We obtained both cheetah-like solutions, in which the spine extends after hindlimb liftoff and flexes after forelimb liftoff, and non-cheetah-like solutions, in which the spine flexes after hindlimb liftoff and extends after forelimb liftoff. Under asymmetric spinal stiffness, cheetah-like solutions reduced ground reaction forces while maintaining horizontal velocity more effectively than non-cheetah-like solutions. The phase relationship between spinal motion and stance timing is a key determinant of high-speed running performance. These findings provide a dynamical understanding of cheetah locomotion and suggest design principles for spined legged robots.

Mammals exhibit robust walking across diverse environments, a capability largely attributed to central pattern generators (CPGs) in the spinal cord. Afferent feedback modulates CPG output and plays a critical role in adaptive locomotion, yet its specific contributions remain poorly understood. To investigate this, we used a neuromusculoskeletal model to simulate hindlimb locomotion in spinalized cats encountering a hole and experiencing a sudden loss of ground support, as described in prior experimental studies. The model couples a trunk-and-hindlimb musculoskeletal system to a pair of two-level, half-center CPGs-one for each hindlimb. The model reproduced the observed adaptive interlimb coordination that allows cats to maintain walking after the sudden loss of ground support. Notably, the adaptive response emerged without re-optimizing parameters, which were tuned for steady walking in an environment without holes. Nullcline analysis based on dynamical systems theory revealed that afferent feedback mechanisms controlling the transitions between fast and slow neuronal dynamics facilitated adaptive interlimb coordination. These findings provide mechanistic insight into how spinal feedback circuits support robust locomotion through dynamic interactions between the nervous system, the musculoskeletal system, and the environment.

Although continuum robots have the potential to operate in narrow areas by changing their shapes and propelling their bodies, they easily vibrate under sudden or periodic applications of external forces. Suppressing vibrations is difficult in our jet-actuated continuum robot because the movements of its mobile base cannot be controlled with the same system as the movements of the robot, and mobile base oscillation increases the risk of resonance. In this study, a disturbance rejection was realized for the Dragon Firefighter, a jet-actuated flying continuum robot on a mobile base, for rapid and safe fire extinguishing using a 4-m-long flying fire hose consisting of two nozzle units and flexible hoses. An $\mathcal {H}_\infty$ -based disturbance-rejection controller was designed to suppress the vibration of the head nozzle unit posture against the acceleration of the mobile base. Then, the robot parameters were identified from tensile tests and dynamic excitation experiments. Dynamic simulations confirmed that the controller reduced the peak gain of the frequency response by approximately 2 dB for various robot shapes. Robot experiments confirmed that the proposed method reduced the peak gain of the frequency response by approximately 3 dB, which increased the extra injection range of the nozzle by approximately 16%.

Flexible, continuum-type robots can access narrow spaces in debris areas during search-and-rescue missions. However, their current problem is the lifting capability of the head, which is necessary to surmount the rubble. This study proposes a passive-thrust vectoring method to stabilize head levitation for air-jet-actuated, long, continuum robots, and thus enhance their step-climbing abilities. An air jet can generate a thrust force that is sufficient for head levitation. A critical issue in levitation involves the backward bending of the head without using any air-jet control; however, thrust control is inappropriate because of the delay caused by the long channel. Therefore, the proposed method maintains the thrust direction constant. Sufficient conditions for global stability are derived and confirmed via dynamic simulations. The proposed method is simple and can be mechanically realized with a passive head-bending mechanism, thus contributing to a lightweight design. Experiments demonstrated that the developed air-floating-type, 7-m long robot, can achieve stabilized head levitation, and that the robot can climb a step with a height of 250 mm. Robot demonstrations in rubble justify the robot’s capacity to surmount the rubble. This passive-thrust vectoring method is expected to contribute to the future enhancement of the mobility of continuum robots owing to its simplicity and practicality.

In a fire outbreak, firefighters are expected to rapidly extinguish fires to stop the spread of damage and prevent secondary disasters. We proposed the concept of a dragon firefighter (DFF), which is a flying-hose-type firefighting robot. We developed a 3.6 m long DFF equipped with two nozzle units and achieved stable flight. However, the system was not yet completed because the root of the robot, which should have been operated remotely, was operated manually. In addition, the system’s reliability was insufficient to successfully repeat the demonstration several times. The development of a robot demonstration system is crucial for the practical application of such a firefighting robot. In this study, we developed a demonstration system for a remotely controllable 4 m flying firehose robot for demonstration at the World Robot Summit 2020 (WRS 2020) opening ceremony in Fukushima as a milestone. This paper focuses on the following issues: 1): installation of the remotely controllable mobile base, 2): redesign of the water channels (the sizes of nozzle outlets) to get enough thrusts to fly with a fire engine, 3): development of nozzle units with a larger movable range (1.5 times larger than the conventional nozzle) in addition to waterproofing technique to improve system reliability, and 4): redesign of a passive damping mechanism to ensure better stability. Thus, a firefighting demonstration was successfully conducted at the opening ceremony of the World Robot Summit 2020 in Fukushima, Japan, and we discuss the lessons learned through the demonstration. We found that the developed DFF system incorporating a mobile base could achieve remote fire extinguishing.

Traditionally engaging in search and rescue operations within collapsed structures introduces a range of significant risks that directly impact the safety of the responders involved. Thus, robots have emerged as invaluable assets in search and rescue operations. Their unique capabilities address the challenges faced in locating and rescuing victims in various scenarios One of the remarkable solutions is the utilization of serpent-shaped robots for traversing through confined places of a collapsed building. Serpent robots can navigate over tight gaps depending on what surface is accessible.In this study, a segmented-body and cilia-driven serpent robot was developed. The serpent robot body and head assembly were designed and analyzed. 32 segments of the robot body were made, and it was found that a modularized segmented body is more functional. The single-segment and 32-segment bodies were tested on three different surfaces. The single segment showed higher speed in tiles and plywood since it is lightweight. In contrast, the 32-segment body was faster at 59.865 mm/s on concrete surfaces than on tiles and plywood surfaces.Active control for maneuvering and balancing control was developed. The implemented pneumatic floating head control for the serpent robot head was able to do left and right ma-neuvering by controlling its roll angle and yaw angle. The head is also capable of returning to its original position. It can move up to 245 mm distance from its initial position either during right or left maneuvering. Moreover, the developed segmented -body serpent robot was subjected to different obstacles and effectively traversed 40 mm obstacle height, passed through an 80 mm hole with a 200 mm length, and its head can elevate up to 138 mm height.

Bridge inspection using drones has become considerably more attractive owing to the ability of drones to gather information safely in lesser time and at a lower cost than traditional inspection methods. However, one of the critical issues in drone-based bridge inspection is ensuring stable communication between the operation base and inspection drones. The inspection drone easily loses sight of the operation base because bridges have intricate structures. Therefore, this study developed a radio map-based flight planning method of autonomous repeater drones for bridge inspection in order to ensure stable communication. Based on the inspection scenario, the repeater drone needs to ensure the communication with a minimum movement even under position deviations caused by winds. First, we generated a three-dimensional radio map of the bridge. Then, under the assumption that the inspection drone adopted a predetermined path, we developed a path planning method for a repeater drone based on the radio map. Some path planning examples demonstrated that the generated path reduces the moving distance and is also robust against the position error, in addition to ensuring stable communication.

Flexible continuum robots have considerable potential for use in exploring intricate spaces, and their ability to make large body shape deformations can increase the inspection area. We previously proposed a jet-actuated flying continuum robot for extinguishing fires. The main challenge in implementing large shape deformations is accommodating the twisting of the body that results from the deformation. To address this problem, we proposed a two-dimensional passive rotating nozzle unit that can expand the directionality of net force against torsion; however, it has not yet been tested on a flying robot. In this study, we achieved the large shape deformations of a jet-actuated flying continuum robot using an improved passive rotating nozzle unit that can handle three-dimensional (3D) force. First, we developed a model of the improved nozzle unit and confirmed that the unit can increase the net force direction. Herein, the design strategy for the rotary damper to handle the instability that arises from motor limitations is discussed. The stabilized flight controller was applied to a continuum robot with the nozzle unit. Simulation results showed that the 2 m robot could perform large head bends (from 0° to 135°). Although the previous fixed nozzle unit twisted by approximately 40°, which made the extra movable range of the nozzles effectively zero, the proposed nozzle unit maintained the movable range to avoid twisting. We experimentally confirmed that the nozzle unit can expand the direction of the 3D net force, and that a large shape bending of approximately ±90° can be achieved using a 1.6 m flying robot.

Continuum robots can enter narrow spaces and are useful for search and rescue missions in disaster sites. The exploration efficiency at disaster sites improves if the robots can simultaneously acquire several pieces of information. However, a continuum robot that can simultaneously acquire information to such an extent has not yet been designed. This is because attaching multiple sensors to the robot without compromising its body flexibility is challenging. In this study, we installed multiple small sensors in a distributed manner to develop a continuum-robot system with multiple information-gathering functions. In addition, a field experiment with the robot demonstrated that the gathered multiple information has a potential to improve the searching efficiency. Concretely, we developed an active scope camera with sensory functions, which was equipped with a total of 80 distributed sensors, such as inertial measurement units, microphones, speakers, and vibration sensors. Herein, we consider space-saving, noise reduction, and the ease of maintenance for designing the robot. The developed robot can communicate with all the attached sensors even if it is bent with a minimum bending radius of 250 mm. We also developed an operation interface that integrates search-support technologies using the information gathered via sensors. We demonstrated the survivor search procedure in a simulated rubble environment of the Fukushima Robot Test Field. We confirmed that the information provided through the operation interface is useful for searching and finding survivors. The limitations of the designed system are also discussed. The development of such a continuum robot system, with a great potential for several applications, extends the application of continuum robots to disaster management and will benefit the community at large.

According to the species and situations, multi-legged animals show three wave-like ipsilateral interlimb coordination. The swing leg movements propagate from posterior to anterior (direct-wave), from anterior to posterior (retrograde-wave), and in both directions with a source (source-wave). However, the gait generation mechanism is still unclear because of the complex interaction between neural control and dynamic body systems through sensory information (embodied sensorimotor interaction). Our previous study showed that local sensory feedback has a function to generate the three interlimb coordination observed in multi-legged animals using a simple model. In this study, to further understand the functional role of sensory feedback, we investigate the effect of the sensory feedback on a three-dimensional multi-legged robot model developed. The simulation result with the ten-legged dynamic robot model shows that the sensory feedback also generates various wave gaits in the robot due to the embodied sensorimotor interaction. The generated gaits are not predetermined but emerge in a decentralized manner. Parts of generated gaits are similar to direct and retrograde wave gaits. In addition, sink wave gait, in which the swing movements sink in the center of the robot, is also observed.

In this study, we propose a method to estimate the spring constant k and the bending tendency φ0 between rigid bodies with high accuracy when estimating the parameters of the pseudo rigid-body model of a continuum robot. Since the spring constant k and the bending tendency φ0 are deeply related to each other, it is difficult to estimate them simultaneously and with high accuracy. Therefore, we proposed a method to estimate k first by eliminating the term of φ0, which is not affected by the measured value, and showed that the estimation accuracy can be improved by simulation. It was confirmed that the estimation accuracy of k was within 1%.

Disasters such as earthquakes can cause damage to houses, buildings, and other similar infrastructures, which can pose significant threats to human personnel or lead to human entrapment. Traditionally, human rescuers perform search and rescue operations to look for possible survivors within the vicinity of the disaster. However, partially collapsed buildings pose a threat to the rescuers as aftershocks may endanger them while still searching for a victim. In addition, robots can assist human rescuers in the search and rescue operations of collapsed buildings. Recently, serpent robots have risen in popularity and are developed to help rescuers in search and rescue operations. This study aims to design and create a prototype of an alternative serpent robot that is ciliary-driven and constructed with a segmented body. As mentioned, the system in this study utilizes a ciliary drive mechanism that is operated and is mainly based on cilia and vibration motors. Also, the segmented body is designed to offer ease of restoration in the possibility of damaged components inside the robot and retain the proper placement of components. The paper presents the design, proof of concept, and various tests of the proposed system.

Fluid jet actuation is a potential actuation technique for continuum robots. It can generate and rapidly control a relatively large force using a small and lightweight structure because a significant amount of energy can be transported through its internal channels. Recently, jet-actuated flying continuum robots have been developed using this advantageous characteristic. However, a challenging issue in controlling the robot is the fluid structure interaction between the flexible body and the internal flowing fluid. This interaction often causes instability in the pipe conveying fluid. In this study, as a first step to address this issue, we propose a stabilized controller (vertical position control) for a jet-actuated two-dimensional cantilevered pipe with a nozzle unit at the tip using the damping effect of the internal flowing fluid and verify the controller with a real robot. Specifically, a model is constructed with the net force of the jets as the control input. A simple controller that can constantly decrease the energy function is proposed by utilizing the damping effect of the flowing fluid. Numerical simulations verify the stability of the system regardless of the flow velocity. In particular, fluid damping mainly suppresses the higher-order mode oscillations. Moreover, the stability of the system can be improved by adjusting the controller gains. We also conduct experiments using an actual robot to verify the simulation results. The vibrations can be damped by the fluid effect, and the stability can be improved using the proposed controller.

Multi-legged animals show several types of ipsilateral interlimb coordination. Millipedes use a direct-wave gait, in which the swing leg movements propagate from posterior to anterior. In contrast, centipedes use a retrograde-wave gait, in which the swing leg movements propagate from anterior to posterior. Interestingly, when millipedes walk in a specific way, both direct and retrograde waves of the swing leg movements appear with the waves' source, which we call the source-wave gait. However, the gait generation mechanism is still unclear because of the complex nature of the interaction between neural control and dynamic body systems. The present study used a simple model to understand the mechanism better, primarily how local sensory feedback affects multi-legged locomotion. The model comprises a multi-legged body and its locomotion control system using biologically inspired oscillators with local sensory feedback, phase resetting. Each oscillator controls each leg independently. Our simulation produced the above three types of animal gaits. These gaits are not predesigned but emerge through the interaction between the neural control and dynamic body systems through sensory feedback (embodied sensorimotor interaction) in a decentralized manner. The analytical description of these gaits' solution and stability clarifies the embodied sensorimotor interaction's functional roles in the interlimb coordination.