Design of a family of multi-DOF drive systems for fewer limb parallel mechanisms

Abstract

Inspired by fewer-limb multi-degree-of-freedom (DOF) parallel mechanisms (PMs), a systematic method that synthesizes multi-DOF drive systems is presented based on Lie group theory. Multi-DOF drive systems can generate more DOFs motion than a pure rotation or a pure translation. The benefit of such multi-DOF drive systems is that all motors are mounted on a fixed base. The topological structures of novel single-loop PMs and multi-DOF drive systems are proposed. A class of 2-DOF drivers are designed. The motion types of the multi-DOF drive systems are systematically synthesized based on Lie group theory. Moreover, a family of 3-DOF drive systems are developed based on the designed 2-DOF drivers and synthesized motion types. Multi-DOF drive systems play a key role in simplifying the structure of PMs to improve the motion characteristics of PMs. A novel single-loop PM is designed to illustrate the application of the designed multi-DOF drive system, and kinematics analysis is performed to reveal the motion characteristics of the single-loop PM with the novel multi-DOF drive system.

Introduction

Parallel mechanisms can offer better accuracy, higher stiffness, larger carrying capacity, faster speed and faster acceleration than serial counterparts [1], [2], [3]. These advantages of PMs have attracted the attention of researchers, which has made PMs a focus in the field of robotics over the past few decades. However, due to the multi-limb and multi-closed-loop structures in PMs, which increase the interference and loop-closure constraints among limbs, and the geometric constraints between the limbs and the moving platform [4,5], the workspace of PMs is smaller than that of serial ones of the same size.

To overcome this drawback, researchers have presented fewer-limb multi-DOF PMs to simplify the structure and extend the workspace. Various three-limb, 6-DOF PMs have been studied [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Additionally, a series of two-limb 4-DOF PMs have also been proposed [15], [16], [17], [18], [19]. Recently, a novel class of 3-DOF single-loop parallel leg mechanism have been presented [20]. Compared with traditional PMs, the number of limbs connected between the fixed base and the moving platform of the fewer-limb multi-DOF PM is reduced, which decreases the interference and geometric constraints among the limbs or between the limbs and the moving platform. As a result, the workspace of the mechanism is extended. Furthermore, fewer limbs corresponds to a simpler structure of the mechanism, fewer required moving parts for the mechanism, and a lower inertia of PMs [4,5].

For fewer-limb multi-DOF PMs, the number of limbs is less than the number of DOFs, which implies that some limbs must contain at least two actuators. When the actuators of a limb are connected in serial [11], [12], [13], the moving mass of the limb will increase, which increases the inertia load. It is preferable to mount all the actuators of a limb on the fixed base to decrease the inertia load of the limb and increase the load-carrying capacity and rigidity [4,19]. Hence, some 2-DOF drivers with all motors mounted on the fixed base have been proposed to drive a limb. Harada et al. [15] designed a novel cylindrical driver (C-driver) that generates 2-DOF cylindrical motion based on a cylindrical differential mechanism. Eskandary and Angeles [18] developed four C-drivers based on the belt-pulley, external gears, planetary gear and strain wave gear design. Eskandary and Angeles [19] also proposed a series of cable-driven C-drivers with an arbitrarily large pitch. In [5,8,10], belt-, cable- and linkage-driven drivers were applied to replace two serial actuated joints with parallel axes on each limb of the PMs, which led to all motors being located on the fixed base. Forgó and Rosca [21] introduced a novel drive module with one translational and one rotational DOFs, and a low (2, 3 and 4) DOF manipulator was systematically presented based on the presented drive module. Both drivers utilized mechanisms to drive each limb of the PMs. In addition, there also are a few multi-DOF motors, such as spherical motors, which can produce multi-DOF rotations in one driving unit [22,23].

In general, all the drivers of the PM are mounted on a fixed base and arranged according to the assembly conditions of the limbs. When the drivers of the limbs for the PM are arranged depending on specific conditions to form a special drive system, the performances of the PMs can be potentially improved. For example, in axis-symmetric PMs [24], [25], [26], [27], [28], all the axes of the revolute drivers are coaxial, which allows the entire arm system to rotate around the base column and extend the position workspace in relation to the footprint of the mechanisms.

To satisfy the requirements of designing novel PMs, this paper presents a systematic method to synthesize multi-DOF drive systems. In this process, an important design criterion is to avoid moving motors. The structure of this paper is organized as follows: Three types of topological structures of single-loop PM and multi-DOF drive systems are presented in Section 2. A class of 2-DOF drivers are designed in Section 3. Section 4 synthesizes the motion types of three types of multi-DOF drive systems based on Lie group theory. Using the designed 2-DOF drivers, a family of novel 3-DOF drive systems are constructed in Section 5. Finally, a novel single-loop PM with three translational DOFs is developed to illustrate the application of multi-DOF drive system and kinematics analysis is performed.