Abstract: Virtual Reality technology, English Virtual Reality, referred to as VR. It is an ideal man-machine communication technology. The three essential characteristics of this technology are immersion, interactivity and imagination. Force-haptic interaction technology is one of the core technologies in virtual reality. Starting from the difference between virtual object and force-haptic expression, this paper summarizes the research status of force-haptic modeling technology, and analyzes the characteristics of force-haptic modeling in various modes. Finally, the development trend of force tactile interaction technology is prospected.

Keywords: virtual reality modeling technology, force tactile interaction, force tactile modeling

1 the concept of virtual reality technology

The most essential characteristics of virtual reality technology can be summarized by three I, namely, Immersion, Interaction and Imagination. Immersion is the degree to which the user feels real in the virtual world. Interactivity refers to the degree to which users can manipulate objects in the virtual world and the real degree to which users can get feedback from the virtual world. Virtual reality technology as a new man-machine communication technology, different from the traditional mode of keyboard and mouse, using digital helmet, digital gloves and other complex sensor equipment, three-dimensional interaction technology, voice recognition, voice input technology has become an important means of human-machine interaction. In recent years, virtual reality technology has achieved rapid development, showing some new characteristics and development trends, mainly: virtual reality modeling technology, real-time THREE-DIMENSIONAL graphics generation and display technology, haptic interaction technology, large-scale network distributed virtual reality research.

2 force tactile interaction technology

Tactile interaction has important implications for virtual reality. Human curiosity determines that when people see objects, they are born with an impulse to touch. In addition, tactile interaction can help people interact with virtual objects in real time when experiencing virtual space, so as to obtain realistic experience, so as to complete training or operation control with high quality. Tactile interaction mainly involves two aspects of tactile reproduction and tactile sensing.

Modeling technology in force tactile representation technology

The force tactile modeling of virtual objects is the most important part of force tactile representation technology, which is essentially a force and deformation model of objects based on physical constraints. At present, most of the researches on force tactile modeling are for simple cases, and there are still many difficulties in calculating the real deformation of objects under force tactile action. According to the different models of objects in virtual environment (rigid or flexible, etc.), the force tactile modeling methods can be divided into two categories: the force calculation method of rigid objects and the relationship between the force and deformation of flexible objects.

3.1 Force calculation method for rigid objects

According to the different avatars of haptic interface devices in virtual environment (such as point, line or body), the force production algorithm of rigid objects can be divided into the following three aspects. [1]

3.1.1 Point-based force generation algorithm

In the point-based force generation algorithm, the end points of interface devices interact with virtual objects, which is similar to touching objects with the end of a probe tool in the real world. This model, which moves only in a certain one-dimensional direction of space, is a 1DOF force generation model, such as the simulation of squeezing scissors to cut paper, and the piston movement of pushing syringes. The virtual avatar in the virtual environment of this model will Pierce into the interior of the virtual object, and the puncture degree information can be used to establish the corresponding force generation algorithm model. This kind of force modeling method establishes the one-to-one relationship between the spatial position of the virtual avatar and the generated force, which is also called vector field method. This algorithm is based on the method of punishment, the force is directly proportional to the penetration distance, calculation is more simple and effective, but it does not record the history of the avatar location information, so that in comparison with some kind of flat or complex 3 d object interaction, uncertain, and the direction of the force should be based on which a surface to calculate the force of the virtual object, Moreover, the vector field method will make the virtual force oscillate and energy leak, which will easily lead to the instability of the system.

3.1.2 Force generation algorithm based on line

Although the point contact model is simple and effective, it cannot simulate the moment generated in the collision process. The interaction between a strip tool and a virtual object is simulated by using a ray segment model (segment with direction). This model enables the operator to simultaneously interact with multiple objects in the virtual environment and obtain the reproduction of torque effects. Some people have studied the force tactile generation of multi-touch grasping virtual objects. Point contact action with friction is used to simulate the torque action near the normal direction of the contact point.

3.1.3 Force generation algorithm based on geometry

The force generation algorithm based on geometry requires more computation than the algorithm based on point and line model, but it has greatly improved the accuracy and fidelity of force generation. Although a single point is not enough to simulate the contact between two 3D polyhedra, a collection of points distributed on the surface of a touching 3D object can calculate force and torque effects. [2] A typical force generation algorithm model between complex mechanical components. A new force rendering method for human body data, which runs directly on voxel data and uses implicit surface representations generated in operation rather than any pre-calculated structure, has been proposed as a good alternative to existing techniques while avoiding most common pitfalls.

3.2 The relationship between force and deformation of flexible objects

The acting force in contact with a flexible body is usually derived from deformation calculations. Therefore, the modeling of contact force mainly studies the relationship between force and deformation. According to whether the force calculation model of flexible object is consistent with the deformation calculation model, it can be roughly divided into the following two categories: one is called open-loop simulation, the other is called closed-loop simulation. [3]

In open-loop simulation, the model used to calculate deformation and the model used to calculate contact force are independent of each other. This is easy to implement, but there is no intrinsic link between deformation and force calculation, and visual and force simulation inconsistencies may occur. In closed-loop simulation, deformation and contact force calculations are correlated. Deformation calculation and virtual force calculation generally adopt the same model based on physical meaning. In this simulation, the contact force and the deformation calculation are consistent, but the deformation calculation and the force calculation form two close closed loops, and there are problems of coordination and stability of visual representation and force perception display.

According to the different requirements of flexible object deformation calculation, it can also be divided into geometrical deformation model and deformation model based on physical meaning. In a geometric deformation model, the deformation of an object is determined only by geometric manipulation, in which the user manipulates vertices or control points on a 3D object to obtain deformation. This deformation method is fast and easy to implement. It is mainly used to realize simple and easily controlled object deformation in visual reproduction.

In the deformation based on physical meaning, the deformation of the object is determined by the physical law and dynamic characteristics in the interaction process, which is mainly used to simulate the real physical behavior characteristics of the object under the action of internal force or external force. At present, commonly used models based on physical meaning include spring particle model and volume element model based on direct construction, finite element model and boundary element model based on continuum mechanics, etc. For different deformation models, there are different force tactile feedback algorithms. Because of the different refresh frequencies required by force representation and visual display, force representation requires a refresh frequency of more than 200 to 500 Hz for force calculation, while the refresh frequency of deformation calculation and visual display is generally only 20 to 30 Hz. [4] If the deformation results are input into the force sensor to reproduce the closed loop for contact force calculation after the visual display calculation is refreshed, the sampling frequency of contact force can only be kept at 20 ~ 30 Hz, resulting in unstable force feedback.

3.3 Stability of force tactile representation

Interaction stability is the premise to ensure high performance and high fidelity in the process of force tactile representation. The unstable factors of the force tactile reproduction system mainly include: the error introduced by the sampling holding link in the force tactile reproduction system of data acquisition system, the inherent error of force tactile equipment (friction force and sensor resolution, etc.), the calculation delay, the interference caused by the displacement differential calculation speed and other errors. These factors can lead to energy leakage in the process of force and touch interaction, thus causing system instability. At present, there are two methods to improve the stability of force tactile representation system: one is to improve the force tactile control algorithm, the other is to improve the refresh rate of force calculation. Improved force tactile control algorithm. [5] A virtual matching method was proposed, in which rigid and damping elements were inserted between the force tactile device and the virtual object, so as to limit the maximum mechanical impedance in the interaction process and ensure the stability of the system. Still have a kind of method based on energy, by using the passive control to consume energy generated by the virtual environment, namely, through the passive observer energy generated by real time observation system, when detected by virtual environment imposed on the operator energy, through passive controller release the excess energy, to ensure that the system of passive resistance. An energy compensation controller is developed to compensate the underwork and overwork in the force tactile representation system and achieve the stability control of the system. The second way to improve the force is to calculate the refresh rate. In order to ensure the refresh rate of force calculation, such methods generally separate force generation algorithm from graphics and image rendering, collision detection and other algorithms with large computational load. Someone put forward the method of using the local model to improve the force refresh rate, namely USES the simple simulation model of the local geometry of the virtual object, and the data of the model with interaction to constantly refresh, this method simplified model can improve the speed of operation and have a wide range of applications, but also reduce the accuracy of the system. There is a buffering model theory that uses dynamic simple structures as models to improve the stability of the system. This method can achieve better results in multi-layer interaction scenarios.

Application of force tactile modeling technology

Tactile representation modeling can allow operators to perceive and manipulate virtual objects, from military simulation to art, education, entertainment, from medicine to manufacturing, from robotics to data visualization are important application fields of tactile modeling. In the field of medicine, it has been widely used to realize the operation training system of doctors by using virtual reality technology. The Virtual reality-based DRE simulation training system, developed by the State University of New Jersey and the University of Medicine and Dentistry of New Jersey, consists of a force-haptic device, an SGI workstation, and a motion limitative plate. The system helps the trainees master DRE diagnostic skills by recreating the index finger of a force-haptic model. Immersion Medical developed CathSim's endovascular catheter insertion simulation system. The system simulates the injection characteristics of a wide variety of patients using AccuTouch's dedicated force tactile interface and computer. In the field of rehabilitation, force tactile modeling can assist doctors in rehabilitation training of patients. In the field of robot operation, especially in the robot remote control operation, the operator can reproduce the multi-channel contact with the robot through vision, hearing and force and touch, to achieve an immersive feeling. Kinesthesia plays a crucial role in this process. NASA's Robonaut system can replace astronauts in harsh or dangerous environments. In addition, in the field of industrial design and manufacturing, force-haptic representation has attracted more and more attention because it can not only provide multi-mode interaction but also provide realism, which can help realize virtual assembly verification and virtual product design, and reduce the development cost. SensAble has developed a FreeForm application that provides operators with a virtual interface for digital clay sculpture.

5 conclusion

The goal of virtual reality technology is multi-sensory human-computer interaction with a sense of presence. As a non-visual interaction in human-computer interaction, force tactile interaction technology determines the importance of force tactile perception to virtual reality technology because of its unique two-way information interaction. The force tactile modeling technology is the core technology of force tactile interaction technology. With the rapid development of computer technology, force tactile modeling technology will have better development, which is of great significance to the future development of human-computer interaction technology in various fields.

reference

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