Comprehensive workspace calibration for visuo-haptic augmented reality

Visuo-haptic augmented reality systems enable users to see and touch digital information that is embedded in the real world. Precise co-location of computer graphics and the haptic stylus is necessary to provide a realistic user experience. PHANToM haptic devices are often used in such systems to provide haptic feedback. They consist of two interlinked joints, whose angles define the position of the haptic stylus and three sensors at the gimbal to sense its orientation. Previous work has focused on calibration procedures that align the haptic workspace within a global reference coordinate system and developing algorithms that compensate the non-linear position error, caused by inaccuracies in the joint angle sensors. In this paper, we present an improved workspace calibration that additionally compensates for errors in the gimbal sensors. This enables us to also align the orientation of the haptic stylus with high precision. To reduce the required time for calibration and to increase the sampling coverage, we utilize time-delay estimation to temporally align external sensor readings. This enables users to continuously move the haptic stylus during the calibration process, as opposed to commonly used point and hold processes. We conducted an evaluation of the calibration procedure for visuo-haptic augmented reality setups with two different PHANToMs and two different optical trackers. Our results show a significant improvement of orientation alignment for both setups over the previous state of the art calibration procedure. Improved position and orientation accuracy results in higher fidelity visual and haptic augmentations, which is crucial for fine-motor tasks in areas including medical training simulators, assembly planning tools, or rapid prototyping applications. A user friendly calibration procedure is essential for real-world applications of VHAR.