A swarm system is a branch of a collection of robotic systems composed of a lot of "simple" robots. In general, group cooperative behaviors require a considerable amount of information, numerous sensors and complicated networks, which makes the designing of their control system much difficult; therefore, a more natural and sensorless-based interaction is expected to aid the building of an easy-to-understand control scheme for elaborate swarm systems. This paper inspired by the collective motions of army ants focuses on a group behavior in which individuals can cooperate with each other using its own body. Army ants cooperate with one another to overcome large gaps, such as the gap between the tree and its leaf, wherein some ants form a pillar by connecting with one another, such that other ants can walk on it to cross the gap. This paper presents a sensorless swarm system in which the mechanical structure and the physical interaction of the system is only utilized. The collaborative obstacle-crossing task is considered, and the robotic swarm system, comprising the individual robot along with the mechanical interaction mechanism, is constructed. Through an obstacle climbing experiment and the passing of a gap, the validity of the mechanical interaction approach is examined.

Cooperative swarming behavior of multiple robots is advantageous for various disaster response activities, such as search and rescue. This study proposes an idea of communication of information between swarm robots, especially for estimating the orientation and direction of each robot, to realize decentralized group behavior. Unlike the conventional camera-based systems, we developed robots equipped with a speaker array system and a microphone system to utilize the time difference of arrival (TDoA). Sound waves outputted by each robot was used to estimate the relative direction and orientation. In addition, we attempt to utilize two characteristics of sound waves in our experiments, namely, diffraction and superposition. This paper also investigates the accuracy of state estimation in cases where the robots output sounds simultaneously and are not visible to each other. Finally, we applied our method to achieve behavioral control of a swarm of five robots, and demonstrated that the leader robot and follower robots exhibit good alignment behavior. Our methodology is useful in scenarios where steps or obstacles are present, in which cases camera-based systems are rendered unusable because they require each robot to be visible to each other in order to collect or share information. Furthermore, camera-based systems require expensive devices and necessitate high-speed image processing. Moreover, our method is applicable for behavioral control of swarm robots in water.

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In general, small robots can operate in narrow spaces, and large robots can traverse over rough terrains. Although a few robots with adjustable wheel diameter have been developed, they require multiple actuators for rotating the wheels and changing the wheel diameter. This paper proposes a wheel robot that can adjust the wheel diameter according to the wheel rotation. The robot uses a centrifugal force-aided diameter adjustable mechanism; the wheel diameter is expanded/contracted by a circular pantograph via the centrifugal force. Thus, the wheel diameter can be changed depending on the drive of the wheel. We adopt the circular pantograph mechanism to maintain the round shape of the wheel during expansion and contraction. We conducted experiments with the developed robot and found that the wheel diameter appropriately changes with the wheel acceleration and deceleration. The results show that the proposed mechanism improves the performance when climbing a step and operating in a narrow space. Further, the robot also had mechanical flexibility, which helps reduce the impact when facing a step. These characteristics could be achieved because the wheel was passively and adaptively deformed according to the running environment.