and so on to obtain a relationship between a two dimensional plane coordinate of the reference point on the backpack frame 7 and a three dimension space coordinate thereof the cameras 3 and 5 need to be calibrated which comprises collecting moving sequence images on a calibration plate as shown in fig7 by the cameras then processing the moving sequence images according to a calibration program guideline of the cameras to obtain parameters of the cameras and finally saving the parameters to a data file for further calculation as shown in fig2 a backpack frame 7 comprises a plurality of bellows 13 a plurality of supporting plates 14 a plurality of reference points 7 u2032 a shoulder armor plate 16 a plurality of aluminum pipes 17 a haunch armor plate 18 a plurality of steel bars 19 a plurality of rivets 20 a waistband 21 a plurality of welding spots 22 and a plurality of connectors 32 the bellows 13 are jointly connected to the aluminum pipes 17 via the shoulder armor plate 16 and the haunch armor plate 18 is connected to the steel bars 19 via the rivets 20 the aluminum pipe 17 is connected to the bellows 13 via the connector 32 and to the supporting plates 14 and the reference point 7 u2032 via the connector 32 with a length of about 20 cm a surface of the supporting plate 14 is coated with low reflectivity materials and the supporting plate 14 is fixed to the connector 31 via a screw thread the reference point 29 is a standard ping pong ball and is fixed to the connector 31 with glue the shoulder armor plate 16 operates to fix the backpack frame to a shoulder of the person wearing the life.