Method and system for improved motion robustness during location measurement

A method for determining at least one of a position and an orientation of a portion of a handheld personal care device relative to a user's body, the method comprising the steps of:

• emitting, by at least one energy source arranged at a first position in the handheld personal care device and having a first orientation within the handheld personal care device, energy in a three dimensional space surrounding the handheld personal care device;
• detecting, by at least one detector arranged in the handheld personal care device, a portion of the energy emitted from the at least one energy source which has been reflected or scattered from the user's body;
• generating, by the at least one detector, sensor data based on the detected portion of the energy emitted from the at least one energy source;
• extracting, one or more features which correlate with a position or an orientation of the handheld personal care device in use from the generated sensor data, wherein the features extracted depend on the distance from the user's body to the energy source and the at least one detector, and/or the extent to which the user's body fills one or more three dimensional regions in the three dimensional space surrounding the handheld personal care device;
• and
• estimating at least one of a position and an orientation of a portion of the handheld personal care device relative to the user's body based on the extracted one or more features, and/or characterizing motion of the handheld personal care device relative to the user's body based on the extracted one or more features.