Abstract: 
    
            
                    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.