## 1 Introduction
[PERSON] and [PERSON] [31] studied a 3-RRPRR parallel mechanism with two passively offset RR joints at each end of its legs. The two axes of rotation of the offset joints intersected at one point and lay perpendicular to each other with an offset. The lengths of the three legs were varied using an active moving pair. [PERSON] et al. [32; 33; 34] studied a new 6-RRCRR parallel platform that used biased joints and established a kinematics model for this platform. However, because of the existence of joint offset, the inverse kinematics solution could not be determined using analytical methods. The Newton-Raphson numerical iteration method is used to solve kinematics problems. To improve the motion accuracy, working space, and stiffness characteristics of parallel mechanisms, the German researchers [PERSON] and [PERSON] [35] studied three different forms of offset joints and discussed the feasibility of using these joints to replace traditional Hooke joints. Kinematics models of the traditional 6-UCU Hexopod platform and the 6-UCRR platform with offset joints were established and their kinematics solution processes were compared. The 6-P-RR-R-RR parallel platform studied takes stiffness as an important performance, and both longitudinal stiffness and lateral stiffness should be taken into account. In this parallel platform, an offset RR joint replaces the traditional Hook joint. When compared with a traditional Hooke joint, the offset RR joint [35; 36] has characteristics including large carrying capacity and easy processing and adjustment, effectively reducing the risk of interference with the joint during rotation and increasing the working space of the entire machine. This offset joint structure is beneficial in improving the stiffness of the entire platform. All revolute joints in the subchain use a zero-gap dense ball shaft to replace the traditional bearing and the installation datum is designed to improve the accuracy and the stiffness of the kinematic chain. The effectiveness of the proposed design in improving the stiffness is verified via experiments and simulation analyses. There is an offset between the two axes of rotation of the offset joint and additional variables are introduced, thus making the platform kinematics more complex. To solve the complex kinematics problem of the platform effectively, an inverse kinematics and forward kinematics model of the 6-P-RR-R-RR parallel platform was established using the Denavit-Hartenberg(D-H) parameter method for series mechanisms. The kinematics problem was solved using numerical iterative methods and the correctness of the kinematics model was verified through simulations. The remainder of this paper is divided into four sections. The second section mainly introduces the structural design of the parallel platform, including the platform composition and the revolute joint design. The third section mainly studies the method used to establish the kinematics model of the platform, along with the inverse kinematics and forward kinematics solutions. The fourth section mainly describes the experimental research, including the static and dynamic stiffness tests and the motion performance testing. Finally, the fifth section provides an overall summary of the research work.