  Inertial measurement units (IMUs) are used in medical applications for many
different purposes. However, an IMU's measurement accuracy can degrade over
time, entailing re-calibration. In their 2014 paper, Tedaldi et al. presented
an IMU calibration method that does not require external precision equipment or
complex procedures. This allows end-users or personnel without expert knowledge
of inertial measurement to re-calibrate the sensors by placing them in several
suitable but not precisely defined orientations. In this work, we present
several improvements to Tedaldi's method, both on the algorithmic level and the
calibration procedure: adaptions for low noise accelerometers, a calibration
helper object, and packet loss compensation for wireless calibration. We
applied the modified calibration procedure to our custom-built IMU platform and
verified the consistency of results across multiple calibration runs. In order
to minimize the time needed for re-calibration, we analyzed how the calibration
result accuracy degrades when fewer calibration orientations are used. We found
that N=12 different orientations are sufficient to achieve a very good
calibration, and more orientations yielded only marginal improvements. This is
a significant improvement compared to the 37 to 50 orientations recommended by
Tedaldi. Thus, we were reduced the time required to calibrate a single IMU from
ca. 5 minutes to less than 2 minutes without sacrificing any meaningful
calibration accuracy.
