## 1 Introduction  
Robots have undoubtedly demonstrated their value in the automotive industry, they have been regarded suitable for aerospace applications such as drilling and fastening due to the obvious low investment costs compared to bespoke industrial solutions [1]. The traditional application areas for industrial robots involve highly repetitive operations such as drilling. Robotic drilling system has made extensive application in both industry and research field. In the aircraft industry, where the drilling of many thousands of holes per aircraft is needed, the usually adopted solution is to stack the parts in a joint position and drill them in a single operation [2; 3]. The use of industrial robots for drilling is gaining an increasing interest due to their flexibility and the comparatively low cost of industrial robot systems [4; 5]. [PERSON] et al. [6] and [PERSON] et al. [7] developed a drilling system for the skin to substructure joint on the F/A-18E/F Super Hornet wing trailing edge flaps (TEF) and for 737 Aileron, which utilizes a mass produced, high capacity industrial robot such as Kuka KR350/2 and KR360-2 as the motion platform for an automated drilling, countersinking, and hole inspection machine. In general, a servo-controlled robotic drilling end-effector is coupled with an industrial robot to complete the drilling process. [PERSON] et al. [8] developed a compact and innovative end-effector which is capable of performing drilling, countersinking, sealing and riveting operations and also contains a significant amount of process monitoring sensors to enable automated in-process checks and quality measurement. [PERSON] et al. [9] developed an end-effector for use on a Kuka KR350 robot. The end-effector pushes up on a wing panel with programmable pressure, drills a hole with a servo drill, inspects the hole with a servo ball-type hole gauge and then drives a pin-tail style lock bolt into the hole. [PERSON] et al. [PERSON] et al. (2016) further designed a drilling end-effector to use for industrial robots. The real-time force feedback can detect dull or broken bits, drill to breakthrough, and plot thrust force while drilling. [PERSON] (2016) also developed a single process end-effector that performs all required functions, including one-sided pressure application, touch probing, barcode scanning, drilling/countersinking, measurement of hole diameter and countersink depth, and face milling. Automatic drilling requires the drilling bit to be perpendicular to the curved surface of fuselage. However, most of these references focus exclusively on all-in-one end-effector structure design and functions such as drilling, countersinking, inspecting or monitoring, and fastening ([PERSON] et al., 2016), and a few on the normal adjustment dynamical model and control. The development of these robotic drilling systems has created a research need for the dynamical models and control of the normal adjustment of robotic drilling end-effector. The dynamic performance of the normal adjustment of the robotic drilling end-effector will affect the drilling quality and productivity. [PERSON] (2016) presented methods and systems for force-controlled robot drilling, based on active suppression of drill sliding through a model-based force control scheme. [PERSON] et al. [PERSON] et al. (2016) proposed a novel approach based on the detection system to detect the normal vector to product surface in real time for the robotic precision drilling system in aircraft component assembly. [PERSON] et al. [PERSON] et al. (2016) proposed a novel in-process robot base frame calibration method with a 2D vision system to measure and compensate the position errors of drilled fastener holes. [PERSON] (2016) presented a multi-sensor measurement system for robotic drilling. The system enables a robot to measure its 6D pose with respect to the work piece and to establish a reference coordinate system for drilling. [PERSON] et al. [PERSON] et al.