1. Department of Industrial Engineering, School of Mechanical Engineering, Northwestern Polytechnical University, Xi'an 710072, Shaanxi, China;
2. Performance Analysis Center of Production and Operations Systems (PacPos), Northwestern Polytechnical University, Xi'an 710072, Shaanxi, China e-mail:
3. Department of Mechanical and Aerospace Engineering, The University of Virginia, Charlottesville, VA 22904 e-mail: