Development of a mini-robot based on parallel kinematic architecture

In this work, a mini-robot based on parallel kinematic architecture is presented. It consists on a mobile platform, which can translate vertically along the axis normal to the base plane and rotates alternatively around the two other orthogonally axes. The structure has overall dimensions of few centimetres with promising applications in the optical field. As the miniaturisation increases, the requirement of high precision becomes one of the most critical point. For this aim, the application of Parallel Kinematic architecture at millimetre and sub-millimetre range offers new solutions, since they can achieve very high positioning accuracy and stiffness, also at high dynamics working conditions. Moreover, all types of architectures that have single plane motion drivers and that use constant length legs are particularly attractive for MEMS applications thanks their production feasibility based on IC-technologies [1] [2] [3]. Furthermore, characteristic limitations reducing their employment in industry, as the dimension of work space with respect to the total volume occupied by the device, become less important at small scale and can be avoided with appropriated design. The present work describes the design and development of the mini PKM. Firstly, different kinematics configurations were analysed and then the feasibility study of the robot - manufactured using silicon based technologies - was performed. These results are reported in previous publications [4] [5]. In particular, two interesting configurations were selected. The first one has 3 DoFs, the kinematic chain is P-R-S (Prismatic, Rotational, Spherical joints), and the legs are arranged symmetrically around the central platform in order to guarantee a better stress distribution. The second one has 2 DoFs, the kinematic chain is P-R-U (Universal joint) and the legs are arranged at T-shape in order to optimise the mobility of the mechanism with respect to the overall stiffness. Although the latter configuration doesn't distribute stress equally and introduces a mutual constraint between the rotational DoFs, it offers other valuable advantages: such as a simple control system due to the decoupling of the rotations and an easier manufacture because of the intrinsic difficulty of realizing spherical joints at small scale. In this paper, the feasibility study of this second kinematic configuration at millimetre range will be explained more in details. The components have been re-designed in order to be manufactured by traditional precision machine tools, like turning, milling and drilling, thus the dimensions were scaled to millimetre range. The design of proper clamping units is presented and the assembly strategy is discussed, as well.