## 3 Measured Parameters and Signal  
### Structure of the Plant  
The electromagnetic mill installation comprises a plant to be controlled with many control goals defined for each subsystem. The most important are parameters of the product concerning grain size, shape, physical, and chemical properties. To meet these requirements, every subsystem of the installation needs to perform properly in the nominal conditions, which means that after start-up procedures, a separate group of tasks arise and need to be performed by the control system. Control of the ground material flow should be singled out. Vertical arrangement of the mill causes the control system of air streams fed to the mill system is designed to keep the ground material in the working chamber. A single pulling fan is a major source of the air flow, which is split into four coupled paths. Control system uses flaps to regulate the air flow. Pneumatic transport subsystem is then multi-input and multi-output plant, which realizes different objectives: the main air stream keeps ground material in the working chamber, recycle stream transports recycling material back to the working chamber, additional air stream parameterizes cut-off grain size of the main classifier. Any changes of one air stream causes changes in another two. If the objective in all of the paths would be the same then the control system could be designed similarly for all paths. Multi-objective case compels special design. The pneumatic transport system is a dynamical plant what follows from significant volume of the coupled pipes and units. Therefore, pressure plays the crucial role in the control system design and serves as disturbance that needs compensation. Figure 5: Cumulated mass curves of copper ore (black line—feed, blue line—product obtained for the feed volumetric flow \(F\!f=0.307\) Mg/h, grey line—product with \(F\!f=0.154\) Mg/h and orange line—product with \(F\!f=0.077\) Mg/h). The final but not least structural element of the plant is energy management. There are two main energy consumers: the pulling fun and the electromagnetic mill. The first is not adjusted and can be dropped if control issues are concerned. The electromagnetic mill is supplied with three-phase current by an inverter generating set frequency. The electromagnets create a circulating field and grinding media exchange movement energy into crushing of the grains and produce heat. Part of this heat is taken away by the grinded material and transporting air stream. Another part is dissipated or recovered by cooling fans. Three aspects are met here: proper frequency of the rotating field, heat recovery, and mass of grinding media in the working chamber. The entire energy management system should take into account different objectives: to keep required output and quality spending as little energy as possible (optimization task); use additional subsystems to save or recover energy (design task); and, keep proper ratio of grinding media mass in the working chamber to set flow of the material (control system goal). The structure of the plan can be illustrated by a set of signals that influences plant states. Figure 6 presents general structure of the circuit as a controlled system. All three transporting air flows: main (\(F_{g}\)), recycle (\(F_{r}\)), and additional (\(F_{d}\)) are enforced by the blower (compare Figure 3). All of the streams are set by their own flaps. Changes of the position of one flap influence the flow ratio of all streams in the circuit and the output air stream parameters (flow--\(F_{o}\), temperature--\(Tl_{o}\), and pressure--\(Pl_{o}\)). It also influences the solids flow (\(W_{p}\)), fulfilment level (\(LI\)), and product temperature (\(TI\)). Finally, the most important influence concerns the product quality (\(QI\)). The circuit reacts similarly on the fresh feed flow changes (\(F_{n}\)), rotating electromagnetic field frequency changes (\(\omega\)), and volume of the mass flow of the grinding media inserted into the working chamber (\(F_{m}\)).