The Forcegrid: A Buffer Structure for Haptic Interaction with Virtual Elastic Objects

Today there is considerable interest in surgical simulators for training, operation planning, and outcome prediction. Surgical simulation often involves real-time computation of the deformations of visco-elastic human-body tissues and providing users with both graphic and haptic rendering. As humans are more sensitive to small haptic discrepancies than visual ones, good force rendering requires a refresh rate of 500Hz or more, which is far beyond the capabilities of current simulation techniques. If the simulator runs on a remote server, limited bandwidth, network latency and variance often compromise satisfactory haptic rendering. To address these problems, a new interpolation/extrapolation data structure the Forcegrid is proposed, which makes it possible to approximate forces at rates greater than 500Hz, regardless of the complexity of the model of the simulated object. The forcegrid is designed for haptic interaction during continuous contact. It is a regular grid defined over the virtual workspace, in which every vertex contains a force calculated from the reaction forces computed at previous simulation cycles. The controller of the haptic device exploits the forcegrid to render forces at the appropriate rate. The two key algorithms associated with the forcegrid are an extrapolation algorithm that updates the forces contained in the grid vertices at every simulation cycle (≈ 30Hz) and an interpolation algorithm that computes the force to be rendered on the haptic device (≈ 500Hz). The forcegrid has been implemented and successfully experimented with on numerous virtual deformable objects.