**5. Conclusions**

The proposed 2-DOF nano-stepping motor offers the advantage of endless rotation and 6-mm translational stroke, at the cost of providing a consistent step size, making it impossible to use an open loop controller. The direct visual measurement is the most practical method to sense 360◦ rotation precisely and without contact. As a result, this article proposes a visual servo controller for the 2-DOF nano-stepping motor. The experiments show that the proposed controller can precisely navigate the rotor along various trajectories. In the case of following a 45◦ straight line, the best straightness is 0.291 μm at a speed of 0.273 μm/s. A higher speed could be achieved if a slightly greater tracking error (i.e., closeness) were acceptable. The trade-off between accuracy and speed remains inevitable.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2072-666X/10/12/811/s1, **Video:** The time-lapse microscopic video of the pentagram trajectory experiment.

**Author Contributions:** Conceptualization: S.-K.H.; methodology: S.-K.H. and C.-L.C.; software: C.-L.C.; validation: S.-K.H. and C.-L.C.; formal analysis: C.-L.C.; investigation: S.-K.H. and C.-L.C.; resources: S.-K.H.; data curation: C.-L.C.; writing—original draft preparation: S.-K.H.; writing—review and editing: S.-K.H.; visualization: C.-L.C.; supervision: S.-K.H.; project administration: S.-K.H. funding acquisition: S.-K.H.

**Funding:** This research was funded by Ministry of Science and Technology, Taiwan, gran<sup>t</sup> number 108-2221-E-009- 084-MY2.

**Acknowledgments:** The authors would like to thank Force Precision Instrument Co. Ltd., Taiwan for technical support.

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