## 2 Materials and Methods
### Prototype and Physical Experiment  
The prototype of the energy harvester with a rotational pendulum is based on electromagnetic induction phenomena. To rid it of magnetic coupling, the system's housing and pendulum's housing are made of ABS (Acrylonitrile Butadiene Styrene) in 3D printing technique (Figure 1). Many steps are taken to miniaturize the EH systems, because in case of electromagnetic systems this is hard to do. Our goal was not to exceed 10 cm of system's design in any direction, what was succeeded. The system consists of 12 coils connected in series mounted symmetrically on both sides (6 on one side) of the energy harvester. By such a connection we wanted to obtain the biggest possible voltage/power response from the system. The system's design provide to apply versatile coils connection, exemplary connecting 6 coils mounted on one wall of the system. Inside 4 of them, additional adjustable permanent magnets are installed, two of them on each side of the system (Figure 2). They will have an influence on electromagnetic induction of the system and movement of the pendulum by creating double potential well. Because the initial design of the pendulum was too light, additional inertial masses had to be mounted on the system's sides to increase its moment of inertia. The total weight of entire system was equal to ca. 900 g. In the system, both mechanical and electrical damping will occur. Mechanicaldamping was in form of friction inside bearings, and an electrical one from additional resistors. The first one was only considered later in the formulation of the equation of motion. In the prototype presented above it is possible to adjust a few factors such as the position of additional magnets, additional inertial masses of pendulum and the resistance load attached to the electric circuit. The aforementioned variables will have an influence on output results: voltage, power, magnetic force and magnetic induction in the system. Change of magnetic force and electromagnetic induction were simulated in Finite Element Method Magnetics (FEMM) software. The principle of calculations will be explained in next sections. Because we used the strongest available magnets made of neodymium N37 for the magnetic forces acting between them, in the experiment, results were only obtained for the furthest distance between them. Then, the additional load was regulated and output  
Figure 1: (**a**) Solid model and real system of the electromagnetic energy harvester: (1) coil, (2) ball bearing, (3) longitudinal, cylindrical free space in form channels for additional magnets, (**b**) system made using 3D printing technique, (**c**) cross-section of system: (4) main pendulum with magnet, (5) exemplary additional magnet mounted inside coil, (**d**) system with additional inertial masses: (6) additional external inertial masses of the pendulum. Figure 2: Principle of operation and alignment of coils and additional magnets. Interior view of the system in a plane perpendicular to the axis of rotation of the pendulum. The system could be fixed to the moving frame with an additional constant tilting angle. voltage and power were measured. Such measurements were performed for different frequencies of excitation to obtain chart output voltage/power in the frequency of excitation domain. The scheme of measuring setup is presented in Figure 3. The measurement system consisted of the following equipment: signal generator--KZ 1406 (ZOPAN, Warsaw, Poland), signal amplifier--BAA 120 (TIRA GmbH, Schalkau, Germany), vibration exciter--S 513 (TIRA, Schalkau, Germany), oscilloscope--MDO3024 (Tektronix, Beaverton, OR, USA), accelerometer--AltIMU-10 v3 (Pololu, Las Vegas, NV, USA). Entire measuring system starts with signal generator, where periodic sinusoidal signal was generated with specific frequency matched to the operational frequency of the energy harvester, where the rotational movement of the pendulum could be sustained.