Force-feedback rendering of virtual tools in contact with virtual surfaces is an open research problem. A common strategy is to define an integrable implicit equation that represents the surface and to use the penalty method to haptically render the corresponding force. Nevertheless, some surfaces cannot be described this way, e. g., soft tissues, thus making real-time force rendering inaccurate. In this work, we propose a bilateral teleoperation scheme where the interaction between a virtual tool and a surface is represented by a virtual slave robot subject to holonomic and nonholonomic constraints. This approach allows to define a position-force controller that, on the one hand, guarantees position tracking between the virtual tool and the robot end-effector by projecting the operator's movements on a computer screen via the simulated robotic dynamic model, while on the other hand the felt reaction force is rendered by considering the forces generated by the master robot as references for the control system. To the best of the authors' knowledge this is the first time that nonholonomic constraints have been used to reproduce haptically deformable surfaces in a virtual reality system. Since such tactile sensations are subjective, a set of experiments are carried out to validate the proposed approach by comparing the force responses when using holonomic and nonholonomic constraints. Special emphasis is made on describing how both representations can be used to reproduce some tactile phenomena presented in medical training simulators.
For medical training aims, tele-operation systems have inspired virtual reality systems. Since force sensors placed on the robotic arms provide interaction force information that is transmitted to the human operator, such force produces a tactile sensation that allows feeling some remote or virtual environment properties. However, in the last two decades, researchers have focused on visually simulating the virtual environments present in a surgical environment. This implies that methods that cannot reproduce some characteristics of virtual surfaces, such as the case of penetrable objects, generate the force response. To solve this problem, we study a virtual reality system with haptic feedback using a tele-operation approach. By defining the operator-manipulated interface as the master robot and the virtual environment as the slave robot, we have, by addressing the virtual environment as a restricted motion problem, the force response. Therefore, we implement a control algorithm, based on a tele-operation system, to feedback the corresponding force to the operator. We achieve this through the design of a virtual environment using the dynamic model of the robot in contact with holonomic and non-holonomic constraints. In addition, according to the medical training simulator, before contact, there is always a free movement stage.
In the last decade, the design and implementation of robots have taken a turnaround in areas such as haptics and virtual reality. Unlike the big and heavy industrial robots, haptic ones must be light and suitable to be easily handled by an operator. Moreover, they must have enough actuators to allow a realistic haptic interaction with a complex virtual environment. In this work, we present the two phases Research and Development process of a six-degrees-of-freedom haptic robot. In the first phase, we build on our previous work to design a spherical wrist, improving both the mechanics and electronics of an old one. In the second phase, we design a virtual environment consisting of a ball and beam system with which the operator interacts visually and haptically. The basis of our development is the well-known Novint Falcon parallel robot that acts as the first 3-DOF of the resulting device. The rest is completed with our improved spherical wrist, in which a 6-DOF force sensor that measures the interaction forces between the virtual environment and the operator was mounted. Our ultimate goal is to evaluate the usability of the obtained haptic robot and present it as a viable alternative to current commercial devices.
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