This paper is devoted to the kinematic design of a new six degree-of-freedom haptic device using two parallel mechanisms. The first one, called orthoglide, provides the translation motions and the second one, called agile eye, produces the rotational motions. These two motions are decoupled to simplify the direct and inverse kinematics, as it is needed for real-time control. To reduce the inertial load, the motors are fixed on the base and a transmission with two universal joints is used to transmit the rotational motions from the base to the end-effector. Two alternative wrists are proposed (i), the agile eye with three degrees of freedom or (ii) a hybrid wrist made by the assembly of a two-dof agile eye with a rotary motor. The last one is optimized to increase its stiffness and to decrease the number of moving parts.
The aim of this paper is the kinematic design of a new haptic device that can be used as a new interface in a Virtual Reality system. Virtual Reality, characterized by real-time simulation, allows a human to interact with a Virtual 3D world and to understand its behavior by sensory feedback [1]. Associated with this definition, four key elements are usually considered: (i) virtual world, (ii) immersion (of human), (iii) interactive, and (iv) sensory feedback. Most often, Virtual Reality systems provide only visual sensory feedback to the user (mono or stereo vision). However, aural or haptic interfaces exist that allow the user to hear or to feel the virtual environment. We want our device to be able to feel the virtual environment. The haptic feedback is associated with the sense of touch. For a user, the existence of an object in a virtual world is verified by coming into physical contact or touch. Usually, a haptic device enables input and output communication between the computer and the user.
Haptic devices play important roles in the recognition of virtual objects. With these devices, the user can feel the rigidity or weight of virtual objects. Most robotic manipulators have largescale and high-cost hardware, which inhibits their application to human-computer interaction. Our device was specifically developed for desktop use. It provides haptic feedback, which strongly enhances human capabilities in the major application areas of virtual reality, such as scientific visualization and 3D shape modeling. The six-dof Orthoglide features two parallel mechanisms mounted serially with a suitable system, which makes it possible to know and control the orientation of a stylus, even though all the motors are fixed. These features contribute to a significant reduction of the inertia when compared to serial architectures commonly used in haptic devices [2]. Thus, the inertia of the manipulator’s moving parts is so small that compensation is not needed. This hardware design is compact, and it has the ability to carry a relatively large payload. Furthermore, the parallel kinematic architecture increases drastically the stiffness of the structure.
Basically, haptic devices that enable the user to touch an object can be classified into two main types: the first one provides only one single point of contact and the second one provides a single point of contact with a torque or multiple points of contact.
Many haptic devices have only three degrees of freedom, such as the Phantom Desktop [2] depicted in Fig. 1. With such a device, the user drives a stylus in space and can feel a single point of contact with an object. The force feedback provides stimuli to fingertip but no torque is provided.
To provide high stiffness and a low inertia, parallel mechanisms can be also experimented as depicted in Fig. 2. This design has been optimized to avoid the singular configuration inside the workspace by Leguay-Durand and Reboulet [3].
With a three-dof haptic device, the user can only manage the motion of one point. However, in virtual environment, he may also want to handle objects. For example, when one assembles a screw in a hole, it is necessary to produce a feedback by forces and torques to feel the contact. This feature is related to the problem of grasping. Only a six-dof device can provide forces and torques at the same time and there are three types of mechanisms: -Serial: The main problem of the serial type is the lack of stiffness. Therefore, the position and the rotation are coupled. An example of the first type, depicted in Fig. 3(a), was originally designed at McGill University by Hayward [4] and is now commercialized by MPB [5]. The Phantom Premium sixdof device is also of the serial type but with a closed loop in the serial chain. -Parallel: An example of this type, depicted in Fig. 3(b), was designed at Tsukuba University [6]. The main advantage of this device is that all the actuators are fixed on the base. However, its rotation motions are limited and are function of the position. The computation of the inverse and direct kinematic models is not simple, which is not compatible with real time applications. -Hybrid: An example of this type, depicted in Fig. 3(c), was designed at École Fédérale de Lausanne and is now commercialized by Force Dimension [7]. A parallel architecture provides high stiffness but the shape of its Cartesian workspace is generally not simple and few of them have an isotropic configuration. made of a Delta [9] a two-dof agile eye [10] mounted serially with a revolute joint. Such a design enables to decouple the translation motions and the rotation motions. The kinematics models are simple but its main drawbacks are the actuated joints of the wrist, which increase the inertia in motion and its volume. The aim of our paper is to propose a design with the same advantage but without these drawbacks.
The main desirable characteristics are quick motion and a regular workspace shape. In addit
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