Cellulo V2 Mechanics
The changes in the mechanics were mainly fueled by a need to improve the locomotion system as well as to introduce modularity while maintaining the design criteria defined in the main wiki of V2 (cost, scalability, usability, production...). The decisions taken and the results are detailed bellow.
The biggest burden spotted in the previous version involved the locomotion system. In particular, Cellulo is based on a novel technology that enables the robot to move holonomically. A holonomic movement is such that each of the degrees of freedom is directly controllable. When using wheeled robots, this tends to be a problem, since the wheels can only be driven in one direction, and that is the reason why the Omni wheels or Mecano wheels were created since they are active in the rotational direction, but thanks to the small rotational parts in the radius of the wheel, it doesn't block the displacement perpendicular to the rotation. Seeking the same effect, the CHILI team came up with the idea to use magnets to drive the wheels. In a detailed manner, the wheel consists of a metallic ball that is magnetically paired with the shaft of the motor, and when the motor moves it drives the wheel, however, if the ball wants to rotate in a direction that is not the same as the magnet direction, the shear force would not be sufficient to interrupt the movement.
More details about the design can be found in the thesis Cellulo: Tangible Haptic Swarm Robots for Learning.
This design presents multiple advantages, however, it comes with some inconveniences. First of all, the size of the robot requires small parts, hence, the bearings and the magnet are parts that are harder to find out and accommodate in the design, i.e. the bearings are 3mm in diameter, nylon made and allocated directly in the chassis, hence, to provide adequate functionality the tolerances in the bearing hole must have small tolerances and smoothness, which is hard to produce. As for the ball, the initial design was done with metallic balls of 14 mm and a casting of 1mm to provide friction in the movement. Those balls are custom-made, and hence an appropriate supplier must be found, the last supplier used can be found in the following email: 1007785188[at]qq.com. Be aware that it is a Chinese supplier and it has limitations in the language as well as payment methods. Additionally, these design incurs a maintenance cost. Due to the nature of the movement and the design, dust and particles tend to be accumulated in the bearings, which difficult the movement over time. The same movement and friction between the floor, the magnets, and the balls might cause erosion in the coating surface, which deteriorates the movement. Because of those reasons, it is required to do maintenance periodically, and parts substitution.
Alternatively, the use of omniwheels is proposed. This provides trades-off against the ball wheels. On one hand, the omniwheels reduce the number of parts that need to be obtained for the locomotion, simplify the design, and provide locomotion slightly better (subject to the quality of the omniwheels). On the other hand, the production of omniwheels of the initially proposed diameter (14 mm), has had to be done in-house, which is difficult for its scalability and requires specialized tools. Additionally, the use of omniwheels requires some adequations in the design. Either the kinematics of the robot is changed to use the same motors, or it is required to find motors with a 90º shaft that satisfy the requirements in r.p.m. and torque. Ultimately, and depending on its use, the omniwheels could be allocated outside of the robot, which was an initial proposal in version 1.
