Haptic rendering and control

Carsten Preusche, Thomas Hulin and Gerd Hirzinger


In haptic simulations a human operator is coupled to a virtual environment, in such a way that the user is able to perceive the scene with his/her sense of touch. In Figure 1 the interaction paths between user and virtual world are shown. To obtain a good level of immersion, at least two interaction paths must be provided for haptic simulations, namely the visual and the haptic path. In the upper part the visual path is shown, in which a visual rendering algorithm computes images from a virtual model. These images are displayed to the user by a visual display. The user can modify the viewpoint by moving the virtual camera, e.g., by using a spacemouse or, if a tracking system is used by moving his/her head. The lower part presents the haptic path, which is in focus of the present book. Similar to the visual path, this path contains a software and hardware module, i.e., a haptic rendering algorithm and a haptic interface. As both these modules are bilateral, no additional device is needed for human interaction.
For exploration tasks, the visual path is of great importance, but not exclusively. Haptic information can increase immersion in virtual reality setups and contribute to intuitive operations. Especially manipulation tasks are much more intuitive if haptic feedback is provided besides visualisation. For several tasks – including virtual assembly verification and training of mechanics – the required time can be considerably decreased if haptic feedback is provided. Haptic interaction with virtual objects is based on a feedback loop, which is minimally composed of:

- digital models of the virtual scene
- a haptic rendering module
- a haptic interface
- control of the haptic interface
- a human operator

Except the digital model, which is loaded by the haptic rendering module, all interactions between the haptic subsystems are bidirectional, i.e., information flows from and to each module. This also means that the human operator closes the control loop with the haptic interface and the rendering module. The parameterisation of the elements within the haptic path depends on two main factors; the human perception of haptic information and the technical limitations of the haptic interface itself (see Chapter 29 and Chapter 32). Immersive simulations with realistic force feedback require stability of the haptic system, and moreover its transparency. As broadly accepted in the scientific community, the update rate of the loop must be at least 1 kHz for immersive haptic simulations. In a fully transparent system the user cannot distinguish whether he/she interacts with a real or a virtual environment. In the field of control for haptic rendering, the concept of passivity is very common despite the fact that it yields in conservative controllers [1, 2].