  In motion tracking of connected multi-body systems Inertial Measurement Units
(IMUs) are used in a wide variety of applications, since they provide a
low-cost easy-to-use method for orientation estimation. However, in indoor
environments or near ferromagnetic material the magnetic field is inhomogeneous
which limits the accuracy of tracking algorithms using magnetometers. Methods
that use only accelerometers and gyroscopes on the other hand yield no
information on the absolute heading of the tracked object. For objects
connected by rotational joints with range of motion constraints we propose a
method that provides a magnetometer-free, long-term stable relative orientation
estimate based on a non-linear, window-based cost function. The method can be
used for real-time estimation as well as post-processing. It is validated
experimentally with a mechanical joint and compared to other methods that are
used in motion tracking. It is shown that for the used test object, the
proposed methods yields the best results with a total angle error of less than
4 degrees for all experiments.
