1. An apparatus comprising: a vertically-movable robotic arm; and a vertically-movable sensor unit operatively coupled to said robotic arm and having a monitoring mechanism configured to physically contact a surface in a region of interest, wherein said monitoring mechanism includes: at least one rod configured to be placed in physical contact with said surface through vertical motion of said sensor unit, a force sensor coupled to said at lest one rod to generate an output signal based on contact force between said at least one rod and said surface, and a feedback mechanism coupled to said force sensor to receive said output signal and to maintain said output signal substantially constant by adjusting vertical separation between said sensor unit and said surface in the region of interest thereby maintaining said separation substantially constant so as to compensate for vertical movement of said robotic arm during an analysis of said region of interest.