On the Design of a Low - Force 5 - Dof Force - Feedback Haptic Mechanism

Virtual reality is becoming very important for training medical surgeons in various operations. Interfacing users with a virtual training environment, requires the existence of a properly designed haptic device. This paper presents the design of a new force feedback haptic mechanism with five active degrees of freedom (dof), which is used as part of a training simulator for urological operations. The mechanism consists of a two dof, 5Ðbar linkage and a three dof spherical joint, designed to present low friction, inertia and mass, to be statically balanced, and have a simple mass matrix. The device is suitable for the accurate application of small forces and moments. All five actuators of the haptic device are basemounted DC motors and use a force transmission system based on capstan drives, pulleys and miniropes. The paper describes the chosen design, and its kinematics, dynamics, and control algorithm and hardware employed. INTRODUCTION During the past several years, research on the use of virtual reality in medicine has rapidly increased. Because of the recent development in software and hardware, it is now possible to create realistic simulation environments for educational purposes in medicine, [1]. Training in a simulated environment has significant advantages against the traditional medical training. It is a less expensive and results in a faster way for training and practice in complicated procedures, [2]. The practice on animals becomes an undesired alternative for ethical and economical reasons. Also, the existence of a training simulator yields greater availability of the training environment and allows an easier evaluation of the performance of the trainee. Development of a useful and sufficiently realistic surgical simulator requires the use of two core technologies such as of graphical simulation in a virtual reality context and of haptic technologies including force feedback. While a realistic visual representation of human anatomy and tissue deformation are very important, the ability to command the graphic environment and interact with it through the feel of the forces and torques is also of paramount importance. In order to implement such a feel, haptic devices are used. The earliest haptic devices were of the master Ð slave type, and were used for telemanipulation of hazardous materials, [3]. In the last fifteen years, the availability of powerful computers, have resulted in the proliferation of virtual reality systems. In these systems, the slave has been replaced by a computational model and its motion appears graphically. Today, one can distinguish two trends in the development of medical simulators. The first is described by the use of general-purpose haptic devices, like the Phantom or the Freedom Ð 7, [4, 5, 6]. The Phantom is a five bar parallel mechanism with three or six degrees-of-freedom (dof) which is designed as a general-use haptic interface, [7]. The Phantom 1.5/6DOF and the Phantom Premium 6DOF Prototype have six active dof, but are designed mainly for virtual prototyping,