The article proposes a lower limb rehabilitation system based on a passive orthosis and an active parallel robot 3-PRRR. Numerical algorithms have been developed to determine the workspace of the parallel mechanism, taking into account the interference of the links. The optimization of the geometrical parameters of the mechanism has been carried out, and the configuration of the robot has been selected based on the criterion of maximizing the workspace. The analysis of the workspace and the influence of various geometric parameters on it is carried out. The simulation results of the workspaces, as well as the positions of the robot at which interference occur, are rendered using the transformation of a set of 3D boxes to an STL file. © 2022, The Author(s), under exclusive license to Springer Nature Switzerland AG.