Method of detecting activity with a motion sensor, and corresponding device and computer program

A device for detecting activity of a physical system, the device comprising:

• a motion sensor configured to be carried by the physical system to provide a time sequence of measurements;
• a memory configured to store a statistical model associating possible values of components of a time observation sequence with a plurality of possible predetermined states of the physical system;
• and
• processing circuitry, connected to the motion sensor and to the memory, programmed to
• obtain a reference value ({circumflex over (λ)}) of a vector representing a reference axis that indicates a preferred direction,
• calculate a time observation sequence on the basis of the time sequence of measurements, at least one component of the time observation sequence being calculated from the time sequence of measurements using the obtained reference value ({circumflex over (λ)}),
• determine, using the statistical model, the activity of the physical system in the form of a time sequence of states corresponding to the time observation sequence,
• calculate a likelihood value for the time observation sequence in view of said statistical model,
• compare the likelihood value with a predetermined threshold (δ), and
• perform a calibration of the device by updating the obtained reference value ({circumflex over (λ)}) on the basis of the comparison.

A non-transitory computer-readable storage medium including computer executable instructions, wherein the instructions, when executed by a computer, cause the computer to perform a method for detecting activity of a physical system carrying a motion sensor, the method comprising:

• obtaining a reference value ({circumflex over (λ)}) of a vector representing a reference axis that indicates a preferred direction;
• extracting a time sequence of measurements provided by the motion sensor;
• deducing therefrom a time observation sequence calculated using the time sequence of measurements, at least one component of the time observation sequence being calculated from the time sequence of measurements using the obtained reference value ({circumflex over (λ)});
• using a statistical model of components of the time observation sequence in view of a plurality of possible predetermined states of the physical system, determining the activity of the physical system in the form of a time sequence of states corresponding to the time observation sequence;
• determining whether a calibration is needed by
• calculating a likelihood value for the time observation sequence in view of the statistical model, and
• comparing the likelihood value with a predetermined threshold (δ), and
• on the basis of the comparison, performing the calibration, in response to determining that the calibration is needed, by updating the obtained reference value ({circumflex over (λ)}).