A sensor assembly for a passenger conveying device , wherein the sensor assembly comprises: an optical fiber disposed along a length of a skirt board of the passenger conveying device ; characterised in that the sensor assembly further comprises: a light source disposed at a first end of the optical fiber , incident light emitted by the light source being incident into the first end of the optical fiber , through an optical circulator , and the incident light being incident into the optical fiber substantially along a length direction of the optical fiber ; and an optical receiver disposed at the first end of the optical fiber , the optical receiver receiving backwards scattered light from the optical fiber and being capable of sensing a signal indication of the backwards scattered light, wherein the optical fiber cooperates with a sensing element disposed along the skirt board such that the sensing element causes deformation of the optical fiber when the sensing element is subjected to a pressure (P), and the optical receiver is capable of sensing a change in the signal indication of the backward scattered light caused by the deformation of the optical fiber . an optical fiber disposed along a length of a skirt board of the passenger conveying device ; characterised in that the sensor assembly further comprises: a light source disposed at a first end of the optical fiber , incident light emitted by the light source being incident into the first end of the optical fiber , through an optical circulator , and the incident light being incident into the optical fiber substantially along a length direction of the optical fiber ; and an optical receiver disposed at the first end of the optical fiber , the optical receiver receiving backwards scattered light from the optical fiber and being capable of sensing a signal indication of the backwards scattered light, wherein the optical fiber cooperates with a sensing element disposed along the skirt board such that the sensing element causes deformation of the optical fiber when the sensing element is subjected to a pressure (P), and the optical receiver is capable of sensing a change in the signal indication of the backward scattered light caused by the deformation of the optical fiber . a light source disposed at a first end of the optical fiber , incident light emitted by the light source being incident into the first end of the optical fiber , through an optical circulator , and the incident light being incident into the optical fiber substantially along a length direction of the optical fiber ; and an optical receiver disposed at the first end of the optical fiber , the optical receiver receiving backwards scattered light from the optical fiber and being capable of sensing a signal indication of the backwards scattered light, wherein the optical fiber cooperates with a sensing element disposed along the skirt board such that the sensing element causes deformation of the optical fiber when the sensing element is subjected to a pressure (P), and the optical receiver is capable of sensing a change in the signal indication of the backward scattered light caused by the deformation of the optical fiber . <EOS>