Type synthesis of 6-DOF mobile parallel link mechanisms based on screw theory

Comparing with serial robots, parallel mechanisms have the advantages of higher accuracy, rigidity, velocity, and payload-to-weight ratio (Furqan et al., 2017), so that they are widely used in a wide variety of fields, including medicine, machining, and factory automation. However, one of the main drawbacks of parallel mechanisms is their limited workspace. To solve this problem, various researchers have tried to maximize their available workspace by addressing the nonlinear and complex input-output relationships of those mechanisms. Carretero et al. (2000) applied the quasi-Newton with Hessian update method to find the optimal structural parameters for maximizing workspace volume. Lou et al. (2005) employed a controlled random search method to maximize the effective regular workspace. Hosseini et al. (2011) divided Jacobian entries by units of length to produce a homogeneous twist array for the convenience of workspace optimization. Herrero et al. (2005) proposed a combined geometrical and discretization method for obtaining the most useful workspace containing the largest geometric volume. Che et al. (2020) employed the differential evolution algorithm to solve design problems related to maximizing the effective volume of transmission positional workspace. Shin et al. (2011) applied the Taguchi method to choose the most influential parameters in the optimization of workspace. However, from a macroscopic perspective, the fixed base of a parallel mechanism still physically limits its available workspace. With this issue in mind, several researchers suggested changing the fixed base into moveable bases in order to achieve unlimited workspace, and two types of moveable parallel mechanism types have been proposed. One is a parallel mechanism fixed on a wheeled mobile robot, as shown in Fig. 1(a) (Chong et al., 2020; Fujita and Sugawara, 2014; Moosavian et al., 2009; Yamamoto and Yun, 1996). This type achieves unlimited workspace because the position and orientation of the base on the ground can be changed by moving the mobile robot. In this type, the output platform Type synthesis of 6-DOF mobile parallel link mechanisms based on screw theory