Fastener assembly sensor unit

A sensor unit for a fastener assembly, the sensor unit comprising:

• a mounting arranged to rigidly attach the sensor unit to, or adjacent to, a fastener assembly;
• an optical flow sensor configured to sense a characteristic of the fastener assembly;
• and
• a processor arranged to process the output of the optical flow sensor to detect the characteristic of the fastener assembly and to output a signal representing the status of the fastener assembly;
• wherein:
• the fastener assembly comprises a plurality of parts that include a bolt or screw;
• a longitudinal axis can be defined for the bolt, or screw, such that a rotation about the longitudinal axis of the bolt, or screw, either acts to screw or acts to unscrew the bolt, or screw;
• the optical flow sensor, that is positioned such that its plane of view is parallel to the longitudinal axis, is configured to directly sense relative movement along the longitudinal axis of parts of the fastener assembly towards or apart from each other in a direction that is parallel to the plane of view of the optical flow sensor;
• and
• the characteristic includes at least one of loosening, tension, compression, stress, strain, and/or distortion of the fastener assembly.