2.3.5. Control Algorithm

In this section, we focus on the tuning control as implemented in the system-level model of Figure 2. A control algorithm, as visualized in Figure 12, maintains maximum voltage output even under varying excitation frequency. We consider this as a realistic use case. The time variable and DC voltage output are used as inputs for the control algorithm. The control scheme analyzes the voltage levels at a given frequency and selects one of the two tuning mechanisms.

**Figure 12.** Tuning control algorithm scheme based on maximum-voltage tracking.

In order to demonstrate the self-adaption of our harvester, we excite the structure with a stepwise frequency-varying harmonic excitation, as illustrated in Figure 13. The results demonstrate that the system is able to choose the adequate tuning mechanism. This choice is based on the current voltage level. The maximum voltage is achieved within 15 s. In contrast to our previous work [33], where we implemented an adaptive step size, here the tuning scheme employs a constant step size. This results in less tuning steps, which improve the energy efficiency of the tuning mechanism.

**Figure 13.** A control algorithm, which is based on maximum-voltage tracking, chooses the most effective tuning actuator.
