## 3 Measured Parameters and Signal
### Direct and Indirect Measurements
Similarly to the measurement of working chamber fulfilment, indirect measurement of the solid particles in air transporting system bases on the energy index. Despite that the vibration method can indirectly measure granulation (see Figure 9), its resolution and disturbances sensitivity much reduces its applicability. Interesting trial was undertaken in [14] where presence of water in the material was used to evaluate its granulation. The grinding process in the electromagnetic mill is carried out on dry material. Significant improvement the product quality and efficiency of the milling process can be achieved by appropriate control of the moisture content of the grinded material. A model describing the changes of moisture content of bulk material in the installation of the electromagnetic mill has been developed. This model allows for the evaluation relation of the drying process in the path of material classification and its separation from the transport air, which significantly depends on granulation. Figure 8: Example of waveforms acquired in two axes of accelerometer for fine and coarse grains transported with two speeds. The most promising method of indirect granulation measurement is described in [16], where a novel method for grain size determination using real-time digital images processing is proposed. The method uses a probe (sampling) unit mounted on the product path (see Figure 4a). Vision camera (FLIR, Wilsonville, OR, USA) with lightning mounted at two specially determined angles is used to pick up the image. Detection of the grains bases on an adaptive segmentation algorithm, improved with distance transform to enhance grains detection. Figure 10 presents five stages of image processing. Each localized grain in the final image is parameterized with 11 factors: centre of mass X, centre of mass Y, perimeter, max ferret diameter, waddle disk diameter, area, orientation, Elongation factor, compactness factor, Heywood factor, and type factor. The set of parameters that were established for a certain probe can be used in a few scenarios. Firstly, as a tracking algorithm of assumed one granularity i.e., particle size. In this case, the algorithm acquires images with constant frequency and as result of processing each time, it produces a percent of particles in the sample with assumed granularity. Secondly, as a typical distribution of the particle sizes classes for one taken sample. The most important part in this case are the changes in time between next succeeding distributions. Thirdly, the method recognizes problem of milling media reduction, which may be observed in the Heywood factor value and area. Figure 10: Stages of image processing: (**a**) original image; (**b**) after histogram equalization; (**c**) after extraction; (**d**) after segmentation; (**e**) after refining; (**f**) final result. Figure 9: Example of indirect flow measurement of solid particles: (**a**) power spectral densities, (**b**) energy index. These two parameters are not the only which point out the necessity of grinding media supplement (the grinding media are constantly wear out during operation). If the image processing concludes the necessity of the grinding media supplement, then it would be necessary to confirm such a conclusion because there is no possibility to reduce the mass of the grinding media in the working chamber during operation. On the other hand, there is no way to measure the mass of the grinding media directly. Dependence of active power consumed by electromagnets on mass of the grinding media inside the working chamber makes indirect measurement possible [13; 20]. Figure 11 presents respective results.