## 1 Introduction  
Limiting electrical energy consumption is one of the most important tasks for researchers and engineers, especially given its increasing costs. That is why many researchers are working on and proposing solutions to improve the operation and energy efficiency of machines and devices [1, 2]. In recent decades, there has been an increasingly strong need to develop technologies that enable the extraction of energy from the environment to power components such as wireless networks or micro electromechanical systems (MEMS) [3, 4, 5, 6, 7]. For this reason, such technologies are being developed and continually miniaturised, which in turn has made them increasingly attractive (in terms of price and practicality) relative to replaceable battery power [8]. The use of these technologies also allows wireless sensors to be used in areas where access is difficult for various reasons, such as embedded systems, security monitoring devices or medical implants [7]. Energy harvesting (EH) devices work by converting other types of energy into electrical energy. There are many types of conversion of different types of energy into electrical energy. Mechanical energy, chemical energy, solar energy, thermal energy or radio waves can be converted in this way. Wind energy recovery is also mentioned in the literature. Mechanical energy recovery can be carried out in many ways. The most common is vibration energy recovery. The use of energy extracted from vibrations is also popular due to the prevalence of vibrations in the human environment. Vibrations can be generatedby the operation of motors (for example, motors for processing machinery, windmills, compressors) and the passage of vehicles over bridges or viaducts. Vibrations can also be produced by human beings breathing or making movements while walking, running, or swimming [8]. For this reason, transducers can even be used in devices that are normally difficult to install and repair, as long as vibrations are present at the location. Vibration energy recovery can be carried out on the basis of capacitive, inductive, or piezoelectric transducers. The operation of capacitive transducers is based on the relative movement of two capacitor covers, which cause a flow of charges to accumulate on their surfaces, resulting in the generation of electrical energy. Their use, however, requires the initial polarisation of the capacitor covers, and the charge stored in them disappears over time. Their high impedance also means that the currents obtained are small and the voltages can be higher than 100 V, which can cause problems regarding the effective use of the obtained energy. Transducers of this type have the lowest received energy density compared to other types [9]. Inductive transducers use permanent magnets that move relative to the coil, which creates a voltage in the coil. They are simple to make and the components needed are readily available; however, the voltage obtained from them is often less than 1 V, requiring the use of sophisticated circuits to prepare the obtained electrical energy for use. There is also the difficulty of applying them to electromechanical microcircuits. The obtained energy density is higher than that of capacitive transducers but lower than that of piezoelectric transducers [9]. Piezoelectric transducers benefit from the stresses that act on the piezoelectric material, thus producing a charge and, subsequently, electrical energy. They have the broadest applications of all the cases analysed as their use does not require additional complex components. This is because the appropriately polarised piezoelectric material is able to convert vibrations into electrical energy on its own. Transducers of this type can be very small in size and theoretically of any geometry, thus they are suitable for MEMS applications. The electrical voltage obtained can be high (even at low excitation); however, the currents obtained are most often relatively low. Many common piezoelectric materials are also characterised by brittleness.