Before revision:  To solve these problems , we propose a simple and effective stiffness estimation method that estimates a complete end-effector stiffness matrix from the variation of demonstrations. To that end, Gaussian Mixture Regression (GMR) is applied to extract the reference pose trajectory and the variability. Quaternion logarithmic map is integrated to generate complete rotational stiffness . Besides, the Dynamic Movement Primitives (DMPs) model is further developed to encode and schedule both the movement trajectory and stiffness profiles in task space simultaneously.
After revision:  To solve these problems , we propose a simple and effective stiffness estimation method that estimates a complete end-effector ) and inherits the efficiency of demonstration variance-based stiffness estimation method . Besides, the Dynamic Movement Primitives (DMPs) model is further developed to encode and schedule both the movement trajectory and stiffness profiles in task space simultaneously.