Improvement of the direct kinematic model of a haptic device for medical application in real time using an extra sensor

Abstract

Abstract ² This paper deals with a spherical parallel manipulator developed for a master device, used in a teleoperation system for surgical application. The aim is to improve the calculation of the direct kinematic model (DKM) even when the parallel manipulator is i n a singular configuration by using an extra sensor for a passive joint. A new simple approach for the DKM calculation of the haptic device, in order to meet requirement of real time applications, is developed. The measure of a passive joint's angle reduc es the calculation time and improves the accuracy of the direct model particularly in the vicinity of the singular positions. Experiments were conducted on tele -operation system despite of the presence of singularities in the master workspace. The obtaine d results demonstrate the feasibility of the approach. I. I NTRODUCTION The use of parallel structure as haptic device in tele -operation systems has substantially increased in past years, because parallel mechanisms have the advantage of being accurate and rig id. Tobergte et al. [1] proposed a delta parallel structure as haptic device called sigma.7 to control the DLR lightweight robot MIRO [2]. Ma et al. [3] have experiment a 4 -DOF haptic device in a virtual environment. The spherical parallel manipulator ( SPM ) is class of the parallel robot that has a fixed center of rotation. It was used by Gosselin et al . to control the orientation of a camera [4]. A new SPM is presented in this paper used as a haptic interface in a tele -operation system for a surgical application. The parallel robots suffer from the parallel singularity. This type of singularity, for the case of SPM, appears inside the workspace and generates large actuator torques and a loss of stiffness. The forces (and/or) torques applied to the end -effector, when it is located in parallel singularity, cannot be balanced by the tor ques produced by the actuators . Therefore, uncontrollable movements appear instantly . In the literature, several studies have attempted to remove this type of singularity of the workspace by proposing an optimized structure [5]-[6]. However by improving the kinematic behavior, the parallel robot structure often becomes larger and a problem of interference