A computer-controlled robotic platform with a collapsible lifting arm that positions a non-destructive inspection (NDI) sensor for scanning inside tunnel regions of a composite structure such as an integrally stiffened wing box. The lifting arm of a modified scissor lift mechanism can be collapsed to a very low height to pass through narrow sections of the integrally stiffened wing box, and also extended by more than a factor of three to reach the maximum height of the wing box tunnels. The system performs a vertical position sensing and control process that uses inverse kinematics to enable position control using data from a standard rotational encoder on the motor to determine vertical position. The system produces simulated encoder pulses that represent unit vertical displacements of a distal portion of a modified scissor lift mechanism using a forward kinematics equation in which the rotation angle of a lead screw is an input variable.