**5. Conclusions**

In this paper, two critical issues related to the compliance of industrial robots have been addressed. The first part of the paper has shown that a modal approach based on the selective excitation of some modes of vibration is a valid alternative to static tests for the identification of joint compliances. The second part of the paper has shown that the Mozzi axis of the end-effector of a robot in a certain configuration and with a certain load gives direct and concise information about the location and direction of rotational and linear compliance.

These results have been obtained dealing with a serial six-joint robot with three compliant joints. In the near future, the analysis will be extended to other types of robots with a larger number of joints. The concept of Mozzi axis will be used to explore the possibility of improving the stiffness of a redundant robot in a specific task choosing appropriate configurations of the arm.

**Author Contributions:** Conceptualization, A.D., S.C., M.B., N.C., and A.R.; methodology, A.D., S.C.; software, N.C., M.B.; validation, N.C.; formal analysis, A.D.; investigation, S.C., N.C., M.B.; writing—original draft preparation, S.C.; writing—review and editing, M.B., A.D.; supervision, A.D., A.R.

**Funding:** This research was partially funded by Foundation Aldo Gini (call 2018), and by University of Padova DII project BIRD187930.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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