تجزیه و تحلیل حساسیت از 3-PRS پلت فرم دوک نخ ریسی حرکتی موازی از ابزار ماشین موازی زنجیره ای

The special configuration of a serial-parallel type machine tool possesses five degrees of freedom and provides more flexibility in NC machining. The parallel spindle platform plays the key role in manipulating three directions of movement. In this study, the inverse kinematics analysis of the platform is derived first. A sensitivity model of the spindle platform subject to the structure parameters is then proposed and analyzed. All the parameters influencing the position and orientation of the spindle platform are discussed based on the sensitivity model and the simulated examples. Critical parameters to the accuracy of the cutter location can then be found, which are the slider position, the strut length, and the tool length.

To obtain higher accuracy and greater dexterity, machine tool manufacturers are developing parallel type structures for the next generation of machine tools. It is always a goal to pursue an interesting development in the machine tool industry that holds great promise for improving accuracy. Machine tools with their high rigidity and accuracy are ideally suited for precision applications. Parallel manipulator offers a radically different type of machine structure relative to the traditional serial type machines. It is believed that the inherent mechanical structure of the parallel type machines provides high dexterity, stiffness, accuracy and speed compared to the conventional multi-axis structure [1] and [2]. In the past, many comprehensive studies and works have been made in the area of parallel manipulators. The kinematic behavior of the parallel mechanism with extensible strut has been discussed in many literatures [3], [4] and [5]. Some reports dealt with the kinematics of three-legged parallel manipulators [6] and [7]. Most of these articles focused on the discussion of both the analytical and the numerical methods to solve the kinematics of pure-parallel mechanisms [6], [7] and [8]. Some papers also discussed the accuracy analysis of the Stewart platform manipulators [9], [10] and [11]. A new type of serial-parallel machine tool has been developed by the Mechanical Laboratory of ITRI in Taiwan. Simulation analysis and experimental tests have shown the necessity of its accuracy enhancement [12], [13] and [14]. Even though many researches have made contributions to the theory and design of the parallel kinematic mechanism, however, commercial applications of this potential parallel machine have not been widespread yet. The main obstacle to the applications of this kind of machine tool is their unsatisfactory accuracy. Similar to the traditional machine tool, there are several major factors, such as the structural imperfection, the elastic deformation and the thermal deformation, that will degrade the machine accuracy. In order to find out the essential error sources, sensitivity analysis of the spindle platform movement and error identification are necessary for the purpose of accuracy enhancement of the machine tool. In this paper, the kinematics model of parallel mechanism is derived in the first part. Sensitivity model of the spindle platform subject to structural and kinematic parameters is then proposed. All parameters influencing the position and orientation of the spindle platform are analytically estimated. Simulated example are given to identify the most critical error sources.

Inverse kinematics model of the parallel mechanism has been derived to compute the positions of the rotational joints Ri and the ball joints bi (i=1 to 3) at given values of α, β and zT. The sensitivity analysis of the accuracy of the cutter location with respect to some structure parameters is also proposed in this study. From simulated examples it shows that the sensitivity of structure parameters will change as the platform varies its orientations. Among all parameters, the Z-coordinate parameters have larger influence on the position and the orientation of the spindle platform in different poses. The sensitivity analyses of platform’s position with respect to the structure parameters have demonstrated that the positioning accuracy of the parallel sliders, the original strut length, and the tool length are the most sensitive parameters influencing the position and orientation of the spindle platform, or the cutter location, in practice.