1、MECHANICAL DESIGN AND KINEMATIC OPTIMIZATION OF A NOVEL SIX-DEGREE-OF-FREEDOM PARALLEL MECHANISM Antonio Frisoli, Fabio Salsedo, Diego Ferrazzin, Massimo Bergamasco PERCRO Simultaneous Presence, Telepresence and Virtual PresenceScuola Superiore S. AnnaVia Carducci, 40 56127 PISA, ItalyE-mail: antony
2、sssup.it, bergamascopercro.sssup.itABSTRACT A six-degree-of-freedom hand controller with force feedback capabilities has been designed. The proposed mechanism new design is fully parallel and actuator redundant. Actuator redundancy refers to the addition of more actuators than strictly necessary to
3、control the mechanism without increasing the mobility. A new cable transmission is used to drive each of the six degrees of freedom, allowing all actuators be fixed to ground. Kinematic optimization of the dexterity and redundant actuation analysis of the manipulator has been developed. The mechanic
4、al design of a prototype version is shown.KEYWORDS Haptic Interface, Tendon Transmission, Six Dof Parallel ManipulatorINTRODUCTION Parallel manipulators have been extensively studied for their favorable properties in terms of structural stiffness, position accuracy and good dynamic performance (Merl
5、et 1990). Their well-known counterbalance is lack in workspace dimensions and more complex direct kinematics law. Several parallel manipulators proposed in the literature are based on octahedral geometry kinematics (Fichter 1986, Albus et al. 1993) With the adoption of such a kinematics, the geometr
6、ic coincidence between two joints leads to a lack of the one-to-one correspondence between leg points on platform and base. The legs form a “zigzag triangulated pattern” (Hunt and McHaree 1998) that connect the base to the mobile platform. With this kinematics the manipulator can support external lo
7、ads with increased stiffness and avoid the singularity configurations, with a consequent average improvement in kinematic performance. The novel manipulator, presented in this paper, has been devised to realize a six-degree of freedom haptic interface. Requirements of low friction and no backslash a
8、re critical in the design of force feedback devices (Hayward,1995). Moreover a uniform kinematic behavior of the mechanism over the workspace is required. A six-degree of freedom haptic device can be used to replicate the most of the physical interactions in Virtual Environments (VE). The aim of thi
9、s research has been to design an Haptic Interface for the simulation in VE of all tasks involving dexterous manipulation and precise execution, e.g. Surgery, with the replication of all the components of the interaction wrench. The new manipulator design is composed of a mobile platform connected by
10、 four legs to a fixed platform. Two motors located on the base actuate each leg by a novel tendon drive system. Since eight tendons are used to control six degree-of-freedom, the configuration of the tendon driven system according to (Jacobsen et al. 1989) is redundant of type N+2. The tendon drive
11、modifies the kinematic behavior of the system, so that it becomes statically equivalent to a mobile platform connected to the base by eight pistons, disposed in a triangulated pattern. By means of this static analogy, the mechanical architecture of the system recalls an octahedral like geometry with
12、 two more linear actuators. But with respect to the octahedral parallel manipulator classical designs, the mechanical system is implemented by DC iron-less motors and steel cables, yielding an high fidelity force-feedback desktop device. KINEMATIC DESCRIPTION OF THE MECHANISM The kinematics of the l
13、egs of the parallel manipulator is based on the closed 5-bar mechanism. An innovative tendon transmission has been devised to drive the closed 5-bar mechanism. It is composed of two tendons routed orderly over the pulleys mounted on each joint axis, as shown in figure 1. All the pulleys are idle, ex
14、cept the final driven pulleys of each tendon transmission that are bolted to the base link.Figure 1: Scheme of the closed-loop tendon drive The pulley radii are the same for all the joints, but with different winding directions. So differently from classical tendon transmissions used in serial manip
15、ulators, the final driven pulley is grounded and it is not connected to a moving driven link. This new tendon drive design allows, by properly choosing the tendon routing, to improve the kinematic performance of the closed 5-bar linkage, i.e. avoiding the singularities and improving the kinematic de
16、xterity. The closed-loop tendon drive We shall analyze now the properties of the tendon drive. Since the sum of the internal angles of a triangle is p, it is easy to show that for the angles of figure 1 the following differential relations hold: (1) Since the tendon branch tangent to two consecutive pulleys is constant independently from the close 5 bar posture, the displacement
