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Robot hardware: design and implementation of a new robot hardware (called s-bot) for swarm robotics application

One of the main results of the SWARM-BOTS project is the design and physical implementation of 35 s-bots. The s-bots are mobile autonomous robots with the ability to connect to and to disconnect from each other. An artefact composed of a swarm of physically connected s-bots is referred to as a swarm-bot.

The hardware design of a s-bot is particularly innovative, both as far as it concerns its actuators and its sensing devices. The s-bot is equipped with an innovative traction system which makes use of both tracks and wheels. The wheel and the track on a same side are driven by the same motor, building a differential drive system controlled by two motors. This combination of tracks and wheels is labelled Differential Treels Drive (Treels is a contraction of TRacks and whEELS). Such a combination has two advantages. First, it allows an efficient rotation on the spot due to the larger diameter and position of the wheels. Second, it gives to the traction system a shape close to the cylindrical one of the main body (turret), avoiding in this way the typical rectangular shape of simple tracks and thus improving the s-bot mobility and stability.

The s-bot's traction system can rotate with respect to the main body- i.e., the robot's turret- by means of a motorised joint. The turret holds a gripper for establishing rigid connections between two s-bots or between a s-bot and an object. The gripper is mounted on a horizontal active axis, and it has a very large acceptance area allowing it to realize a secure grasp at a wide angle range. The s-bot gripper can grasp another s-bot on a T-shaped ring placed around the s-bot turret. If it is not completely closed, such a grasp lets the two joined robots free to move with respect to each other while navigating. If the grasp is firm, the gripper ensures a very rigid connection which can even sustain the lifting up of another s-bot.

A s-bot is provided with many sensory systems, useful for the perception of the surrounding environment or for proprioception. Infrared proximity sensors are distributed around the rotating turret, and can be used for detection of obstacles and other s-bots. Four proximity sensors are placed under the chassis, and can be used for perceiving holes or the terrain's roughness. Additionally, a s-bot is provided with eight light sensors, two temperature/humidity sensors, a 3-axes accelerometer and incremental encoders on each degree of freedom. Each s-bot is also equipped with audio and video devices to detect and communicate with other s-bots, such as an omni-directional camera, coloured LEDs around the s-bot's turret, microphones and loudspeakers. Eight groups of three coloured LEDs each- red, green and blue- are mounted around the s-bot's turret, and they can be used to display colours. The colour emitted by a robot can be detected by other s-bots by using an omni-directional camera, which allows to grab panoramic views of the scene surrounding a s-bot.

In addition to a large number of sensors for perceiving the environment, several sensors provide each s-bot with information about physical contacts, efforts, and reactions at the interconnection joints with other s-bots. These include torque sensors on all joints as well as a traction sensor to measure the pulling/pushing forces exerted on the s-bot's turret. The traction sensor is placed at the junction between the turret and the chassis. This sensor measures the direction (i.e., the angle with respect to the chassis orientation) and the intensity of the force of traction (henceforth called "traction") that the turret exerts on the chassis. The traction perceived by one robot can be caused either by the force applied by the robot itself while pulling/pushing an object grasped through the gripper element, or by the mismatch of its movement with respect to the movement of other robots connected to it, or by both the previous circumstances at the same time. The turret of a s-bot physically integrates, through a vector summation, the forces that are applied to it by another s-bot, as well as the force the s-bot itself applies to an object grasped. The traction sensor plays an important role in the context of coordinated movement of a group of physically connected s-bots (i.e., a swarm-bot). In particular, it can be employed to provide a s-bot with an indication of the average direction toward which the swarm-bot is trying to move. More precisely, the traction sensor measures the mismatch between the direction in which the s-bot's own chassis is trying to move and the direction in which the whole group is trying to move.

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