Proceedings of 2014 Workshop on Fundamental Issues and Future Research Directions for Parallel Mechanisms and Manipulators July 7–8, 2014, Tianjin, China Design and Analysis of a 2-DOF Compliant Parallel Pan-Tilt Platform Jingjun Yu 1 *, Dengfeng Lu 1 , Guangbo Hao 2 1 Robotics Institute, Beihang University, Beijing, 100191, China 2 School of Engineering, University College Cork, Cork, Ireland *e-mail: jjyu@buaa.edu.cn Abstract: In combination of the advantages of both parallel mechanisms and compliant mechanisms, a compliant parallel mechanism with two rotational DOFs (degrees of freedom) is designed to meet the requirement of a lightweight and compact pan-tilt platform. Firstly, two commonly-used design methods are applied to design the type and structure of the pan-tilt system, and similarities and differences of the two design alternatives are compared. Then kinematic analysis of the candidate mechanism is implemented by using the pseudo-rigid-body model (PRBM). In addition, the mechanism’s maximum stress existing within its workspace is tested by finite element analysis. Finally, the method to determine joint damping of the flexure hinge is presented, which aims at exploring the influence of joint damping on actuator selection and real-time control. Keywords: Compliant parallel mechanism; Pan-tilt; Flexure hinge; Joint damping 1 Introduction With the rapid development of computer and control technology, UAVs (unmanned aerial vehicles) that install visual tracking equipment come into being. They are capable of monitoring the ground or low altitude and are therefore widely used in many fields, such as aerial photography, geodetic survey, highway cruising, and disaster relief. In general, cameras of UAVs are installed upon a pan-tilt device with two or three rotational DOFs (degrees of freedom), which enhance its capability to track the target and monitor the environment. As one of important parts of an UAV, performances of the pan-tilt platform will directly affect the UAV’s ability to execute tasks. In particular, it is necessary to design a simple, universal, lightweight and compact pan-tilt system. Currently, most of pan-tilt systems are essentially a rigid serial universal joint mechanism. They are simple, but have some disadvantages including complex installation, slow-response, large drive torque of joints closer to the base. Considering the fact that compliant mechanisms are free assembly, free lubrication, no backlash [1-2] and parallel mechanisms have fast response, high stiffness, high bearing capacity, and simple reverse analysis [3-4], it is tried to design a compliant parallel mechanism with two rotational DOFs as the pan-tilt device. The basic requirements of the new pan-tilt device include a pitch angle range of ±10°, a phase angle range of 0∼360°, and a simple lightweight structure as well. In this paper, we will present a design of 2-DOF compliant parallel pan-tilt platform. The whole paper is organized as follows: Section 2 elaborates the design approach and process of the compliant parallel system. In Section 3, kinematics of the compliant parallel mechanism is analyzed based on the pseudo-rigid-body model (PRBM). Furthermore, the stress of the mechanism is tested. In Section 4, the method to determine joint damping of the mechanism is introduced and how joint damping affects actuator selection and real-time control is finally explored. 2 Conceptual Design of 2-DOF Compliant Parallel Rotational Mechanisms Many researches have concerned about compliant parallel mechanisms. In general, two methods are usually used to design this kind of mechanism. One is the direct substitution method based on the existing rigid parallel counterparts. For example, A. Hara [5] had designed a compliant micro-positioning platform with six DOFs which is a 6-SPS (S: spherical joint; P: prismatic joint) structure: the same type as the Stewart Platform. The other is the Freedom and Constraint Topology (FACT) approach [6-7] evolved from screw theory. For example, a compliant parallel guiding stage can be composed of two parallel sheets and two rigid bodies. In the following, the two methods are used to design pan-tilt devices with two rotational DOFs and the resulting structures from different methods are compared. 2.1 Direct Substitution Method Direct substitution Method is based on the existing rigid parallel mechanism, in which all kinematic pairs are replaced by flexure hinges. As a result, some practical compliant parallel mechanisms might be found after attempts. By referring to some literatures, some typical parallel mechanisms with two rotational DOFs are acquired, such as spherical 5R (R: revolute) parallel mechanism [8-9], 8R (RRR-RR-RRR) mechanism [10-11], Omni Wrist III [12-13]. Due to its structural simplicity and outstanding performance, RRR-RR-RRR mechanisms (hereinafter referred to 8R mechanism) are chosen as the mechanism that would be substituted. One structure of 8R mechanism is shown in Fig. 1, in which three limbs are identical, the angle between the button joint and the middle joint is 90° and the angle θ 0 between the top joint and the middle joint is 60° within each limb. The orientation of the moving platform is determined by the pitch angle γ and the phase angle α. Then, notch flexure hinges are used to replace the all kinematic joints in 8R mechanism (Fig. 1). There are four kinds of notch flexure hinges which are usually used. 1