**8. Conclusions**

The purpose of this research was a real-time implementation and experimental study of the nonlinear optimal-based vibration control for a scaled wind turbine tower-nacelle model with the H-MR-TVA system, using the simultaneous operation of the small electric servo drive and the MR damper. With the use of commercially available hardware, the previously developed approach was successfully implemented and validated. The combined operation of the electric actuator and the MR damper in the H-MR-TVA *baseline* system led to a 35% maximum structure deflection reduction in relation to H-MR-TVA operation with MR damper in the passive mode (with zero current) and a 52% reduction compared to the previous research [41]. Both *GH* and *OPT* solutions guaranteed maximum deflection values close to 1.1 mm (the 57% reduction with regard to the best passive TVA configuration), using the MR damper of 32 N maximum force and actuator of 12.5 N nominal force and 0.76 W nominal power. This proved the effectiveness and validity of the adopted concept. No MR damper/actuator force tracking nor online vibration frequency determination were required, which decreased the control adequacy, especially during the multi-frequency and random vibration phases. The MR damper and actuator force constraints did not compromise the control quality. No offline calculation nor disturbance presumption were required for proper system operation.

An interesting alternative is the *OPT* system with the actuator power weight *g*23 = 1010. It provided a 30% maximum structure deflection attenuation, regarding the passive configuration, while using a passive damper of 3.3 N maximum force and electric drive of

0.17 W nominal power, which was 22% of the *baseline* system nominal actuator power. This also makes evident the advantage of the properly tuned *OPT* solution (over the simple *GH* control) with its various optimisation fields, covered by the optimal control task quality index—it yielded significantly better energy efficiency marks compared to the *GH* approach exhibiting the same vibration attenuation properties. The *GH* control is devoted to the case when only the protected structure deflection has to be minimised, while the *OPT* concept coped well with different system constraints as, e.g., the TVA working space, the MR damper/actuator nonlinearities (including stroke and force limitations), the actuator power and energy demand, etc., which makes the proposed method a valuable real-time vibration control solution. However, the used electric drive response time is a substantial drawback of the regarded system; the actuator linear dynamics were not considered in this study, and this turned out to yield elevated vibration levels at higher frequencies (which may not be the case for other actuator types to be used, e.g., the electro-hydraulic one). Thus, the nearest research involves a (model) predictive optimal control with the actuator linear dynamics implemented to cope with this problem. Moreover, the possible increase in the sampling rate will benefit the *OPT* control.

**Funding:** This work was supported by AGH University of Science and Technology (research program no. 16.16.130.942).

**Data Availability Statement:** The data presented in this study are available on request.

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