# A Robust Generator-Harvester for Independent Sensor Systems +
## 2 State of the Research Field and Topics Analyzed  
We discuss the designing and selection of a concept to convert various forms of energy into electromagnetic energy, the central aim being to transform mechanical linear motion into electricity. In this context, a broad range of approaches are examined to yield instantaneously delivered/transmitted electrical power in the range of P = 10-10,000 W. The published articles and papers can be classified into several groups. The first one embraces the area of designing and solving the electrical, electronic, and electromagnetic parts of the projected devices; prominent sources include, above all, [5; 6; 7; 8]. Another group addresses the harvester/generator drive issues, often in relation to combustion engine functions and parts [9; 10; 11; 12]. Yet, another set comprises articles on motion generation, the drifting of the electromagnetic part of the generator [13; 14], models [18; 19; 20], motion principles, and appropriate experiments [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37]. Further categories cover the following methods and processes, respectively: the linear motion of the power unit based on the hydraulic transfer of dynamic energy to the motion element within the linear generator configuration, the generator system (kinetic-to-electrical energy conversion) and the control, modeling, simulation, measurement, and evaluation of relevant model parameters. Detailed insights into the analysis and solution of the generator concept and more serious attempts to create a mathematical model and simulation of the hybrid motor are provided in, for example, articles [38; 39], which focus on designing the purely electromagnetic part, i.e., on electromagnetic conversion using a generator. Another research direction leads to the designing and solution of the combustion power unit in linear engines, namely, an engine with free pistons. This mechanism, relative to a rotary engine with a crank mechanism, has only one or two pistons connected on a common shaft [26] and features precisely the controlled detonation and dynamics of the moving mass. A linear motor with free pistons uses the motion of a common shaft, on which a linear electric generator is mounted [40], or, alternatively, a hydraulic piston mounted on a common shaft is employed to generate hydraulic pressure by moving the piston. Only a limited number of authors [41] have hitherto simulated a setup to control a motor with a hydraulic pump. The publications, to date, on the construction and testing of a linear combustion engine in connection with a generator include articles and theses from West Virginia University (the USA) [2; 4], in which using the engine is also considered for an electric power generator applicable in relatively remote locations, thanks to fewer moving parts and greater reliability, efficiency, and compactness [5] compared to the rotary engine concept. Similarly, the authors of [6; 42] propose employing a linear combustion engine as an electric power generator to replace the current gasoline or diesel generators. By extension, the basic areas that host linear generators in conjunction with a free-piston internal combustion engine subsume automotive technologies and independent power systems. The basic elements of a linear generator are a linear internal combustion or a free-piston engine and a linear electric motor generating electrical energy (linear generator). The requirements of a linear electric generator resulting from a basic analysis of a linearly arranged internal combustion engine with a generator, all in terms of the output power achieved for charging the traction battery \(Q\) = 21 kWh and the electric drive P = 80 kW, are published in [43]. Other published articles or papers discuss, for instance, an approach that relies on deploying a linear generator to convert the mechanical vibrations of a car into electrical energy [44].