Observability of the relative motion from inertial data in kinematic chains

Real-time motion tracking of kinematic chains is a key prerequisite in the control of, e.g., robotic actuators and autonomous vehicles also has numerous biomechanical applications. In recent years, it been shown that, by placing inertial sensors on segments that are connected rotational joints, chain can be tracked accurately. These methods specifically avoid using magnetometer measurements, which known to unreliable since magnetic field at different sensor locations typically different. They rely assumption sufficiently rich assure observability relative pose. However, formal investigation this crucial requirement not yet presented, no specific conditions for have so far given. work, we present an analysis show pose body indeed observable under very mild condition motion. We support our results simulation studies, employ state estimator neither uses measurements nor additional does impose assumptions accelerometer measure only direction gravity, range or degrees freedom joints. investigate effect amount excitation stationary periods data accuracy estimates. then use experimental from two mechanical joints as well human gait experiment validate criterion practice small levels sufficient obtaining accurate estimates even presence time during observable.