Research On System Stability For Haptic Interaction Based On Virtual Reality

Virtual surgery training is an important application of virtual reality technology in the medical field, it plays an important role in the improvement of medical education and training mode. At present, the means of interaction based on virtual reality technology are concentrated in the visual and auditory. The introduction of force feedback in virtual surgery system can make the surgeons not only see the organs and tissues, but also feel the reflecting force and the hardness degree of different muscles and tissues. Enhancing operators’ performance as they manipulate scalpel in contact with the real human organs.This paper put the virtual brain surgery simulation as pointcut, and deeply studied the working principle, mathematical model and force feedback control algorithm of haptic interaction system. The system model has been established which include operator, haptic devices and virtual environments. The geometric model of human brain tissue was drawn and rendered by the OpenGL 3d graphics tool, which is based on the real magnetic resonance and CT data of patients. And the physical model was used spring-damper model, this model is simple for calculation of deformation and force feedback and can well meet the requirements of real-time interaction. According to the established model, the paper also analyzed the cause of leading the haptic interaction system to be unstable. The main factors are the friction of haptic device and the impedance of virtual environment. Especially, quantitative discussed the effect of virtual environment impedance on stability.The control algorithm design of haptic interaction system is the effective method to improve the stability of the system. Firstly, the paper designed a PO/PC control algorithm based on passivity theorem which is the mainstream of current research direction. The algorithm designed passivity observer to calculate the energy of each two-port network, so as to get the energy dissipation of the whole system. Then,designed passivity controller to control the time of active and consume the excess energy in the haptic system, so as to achieve the stability. Experimental results showed that using this algorithm can effectively inhibit the active phenomenon andensure stable of whole process. At the same time, the algorithm can meet the requirement of stability for the project “virtual brain surgery simulation and training system”. Secondly, for the problem of that haptic interaction system is to be unstable in large impedance and nonlinear virtual environments, this paper presents a sliding mode adaptive control algorithm based on Lyapunov theory to realize stable operation for virtual surgery which is never seen in previous research. The algorithm can guarantee the stability of haptic interaction system on the premise of not destroy the virtual environment model and not limit the output of force feedback. The simulation results show that 1) the haptic interaction can remain stable when interacting with both small and large impedance tissues virtual environments; 2) the haptic interaction can remain stable even when haptic devices interact with nonlinear impedance virtual environments.