Robust machine vision based displacement analysis for tunnel boring machines

Underground construction projects require accurate techniques for machine guidance control. This paper presents an advanced design of a fully integrated active laser target (ALT), which is employed as an optical displacement and orientation sensor. The instrument is mounted on a tunnel boring machine (TBM) and delivers the TBM´s pitch and yaw angles to a remote host system. The optical arrangement of the device includes a semitransparent glass target and an opaque aluminum target, whereby measures are taken to increase the sensitivity of red light. A red reference laser beam is projected onto both parallel targets; the positions of the laser spots are observed by two Power over Ethernet (PoE) enabled Gigabit Ethernet (GigE) cameras. A robust plane-to-plane mapping using projective transformation is presented together with a cross validation procedure, which evaluates the quality of the calibration. An a-priori estimation of measurement uncertainty can be given. The system level calibration process yields two sets of transformation coefficients, such that the distortion associated with the optical components and the inexactness of the mechanical construction are effectively canceled out. Multiple images are acquired for a single measurement and analyzed statistically to deliver a statement of measurement uncertainty in order to compensate for mechanical vibrations during the machine´s operation.