Flying a helicopter with the HoloLens as head-mounted display

This paper describes the flight testing and the integration process of the Microsoft HoloLens 2 as head-mounted display with DLR's research helicopter ACT/FHS. In previous work, the HoloLens was integrated into a helicopter simulator. Now, while migrating the HoloLens into a real helicopter, the main challenge was the head tracking of the HoloLens, because it is not designed to operate on moving vehicles. Therefore, the internal head tracking is operated in a limited rotation-only mode and resulting drift errors are compensated with an external tracker, various of which have been tested in advance. The fusion is done with a Kalman filter, which contains a nonlinear weighting. Internal tracking errors of the HoloLens caused by vehicle accelerations are mitigated with a system identification approach. For calibration, the virtual world is manually aligned using the helicopter's noseboom. The external head tracker is largely automatically calibrated using an optimization approach, and therefore works for all trackers and regardless of its mounting positions on vehicle and head. Most of the pre-tests were carried out in a car, which indicates the flexibility in terms of vehicle type. The flight tests have shown that the overall quality of this head-mounted display solution is very good. The conformal holograms are jitter-free, there is no latency and errors of lower frequencies are small enough, which greatly improves immersion. Profiting from almost all features of the HoloLens 2 is a major advantage, especially for rapid research and development.