## 1 Introduction
The data measured with an accelerometer yielded an accelerometer FRF, which was defined as  
\[A(f)=\frac{\ddot{X}}{F(f)} \tag{1}\]  
where \(\ddot{X}\) is the acceleration response and \(F(f)\) is the input force. The compliance FRF \(\alpha(f)\) is represented in the following equation:  
\[a(f)=\frac{X}{F(f)} \tag{2}\]  
where \(X\) is the displacement. Then, the compliance FRF \(\left|\alpha(f)\right|\) is obtained based on Equations (1) and (2) as  
\[\left|\alpha(f)\right|=\frac{\left|A(f)\right|}{f^{2}} \tag{3}\]  
This equation serves to compare compliances of different frequencies at several robot poses between two robots. The results showed that the LBR is as stiff as or stiffer than the KR5 sixx [8]. [PERSON] and [PERSON] investigated the dynamic behavior of a spindle during milling operations based on the state-space identification of the frequency response functions (FRFs) of the spindle [9]. [PERSON] et al. designed a noncontact excitation test to evaluate the dynamic stiffness of the machine tool spindle. Based on the measured FRFs, dynamic uncertainty and its effect on cutting stability were investigated [10]. [PERSON] et al. used a second-order transfer function to build a dynamic stiffness model of an industrial robot. The robot pose configuration was then optimized for machining applications based on this model [11]. [PERSON] et al. analyzed the dynamic stiffness of a five-axis machine tool by using the finite element method (FEM) and experimental modal analysis. Eigenfrequencies obtained by the two methods showed good agreement and the maximum error between them was less than 10%. However, the dynamic stiffness had different degrees of deviation in the XYZ directions [12]. [PERSON] et al. used experimental modal analysis to analyze the eigenfrequency distribution and mode shapes of an industrial robot within the entire workspace. In all poses, similar mode shapes were recognized. However, the oscillation direction varies. The values of the eigenfrequencies showed linear behavior in parts of the workspace [13]. [PERSON] et al. proposed a Gaussian process regression (GPR)-based approach to model the dynamic properties of a six-axis industrial robot. Experimental modal analysis was used to obtain the modal parameters (modal frequency, modal stiffness and modal damping coefficient) of the robot to develop the GPRmodel. An analytical model derived from robot tool tip dynamics was established to compare with the GPR model. The analytical model was found to overestimate the robot's stiffness, especially in extended arm configuration. The GPR model gave an accurate prediction of the robot's dominant natural frequency, stiffness, and damping coefficient in its measurement space. Milling experiments were planned at two positions to measure the tool tip vibration. The average peak-to-valley tool tip vibration amplitudes from the experiments and the GPR model were compared. The results showed that the average peak-to-valley vibrations predicted by the model follow the experimental trends. The predicted values were smaller than the measurements [14]. Other, different robot poses can be planned to measure sufficient modal parameters and to improve this GPR model. As the robot links are assumed to be more or less stiff, the robot can be modeled as a multibody system (MBS) [15]. Depending on the intended use of the dynamic model, the bodies can be assumed as rigid, elastically coupled, or elastically deformable [16]. [PERSON] et al. presented an industrial robot with elastic joints as an MBS. The simulation packages ADAMS and SimMechanics were adopted to build the model of the MBS. To evaluate this model, high speed milling was implemented with the industrial robot. The simulation and measurement of the Tool Center Point (TCP) displacement in the axial force direction and tangential force direction were compared. The results showed a good congruence in the direction of the tangential force. However, the prediction in the direction of the axial force needed improvement [17]. [PERSON] et al.