## 4 Structure of the Control System
### Chosen Aspects of the Direct Control  
Functional structure of the direct control is designed according to the leading variable which is inlet of the raw material to be grinded (the inlet flow is determined by management layer). Main task of the direct layer control is disturbances counteracting. Figure 13 presents example of functional structure of control systems in the direct layer namely the working chamber fulfilment control. Leading variable is inlet of the ground material, and thus the fulfilment level (measured indirectly as LI) is controlled by transporting air stream from the bottom side of the working chamber. The structure is of cascade type. Inner loop (FC) controls the air flow with the set point put out by the outer level control (LC). Leading variable stands for disturbances of the outer loop (slow disturbances), while changes of the air flow (fast disturbances) are regulated by the inner loop, which is the standard motivation for cascade structure application. To improve the inner loop performance, additional pressure compensation (PT) is applied. To improve the outer loop response, additional compensation (ET) of power supply of the electromagnetic mill is applied (slow disturbances as well). The crucial role in the presented structure plays the flow controller FC. As mentioned in the previous sections, pneumatic transporting system is strongly inter-correlated. It can be seen in Figure 13 that changes of the air flow/pressure in the recycle path change immediately LI, which in turns, involves the reaction of the controller LC and FC. As the consequence, one obtains changes in air flow/pressure in the recycle. Controller FC needs proper tuning. It is shown that the modelling of control path can significantly improve tuning. The most important, however, seems intervention of the supervisory control layer, which influences controllers set-points WN (see Section 4.3). Figure 14a [10] presents example of control system response (LI). The auger feeder frequency was changed in the range 20 to 60 Hz, according to the management layer statement. The mill frequency was set to 30 Hz. Mass of the grinding media was 600 g. The experiment was carried out on the mill of 200 mm diameter. All of the above parameters have been established for copper ore of granulation 1-2 mm. The main air stream was controlled with the set point LI\({}_{\text{SP}}\) = 3.5 (the value of LI is out of the unit and in this case may be positioned in the range 1-10). In this experiment, the working chamber  
Figure 13: Functional structure of working chamber fulfilment control (measurement points are: F—flow, TI—temperature, PI—pressure, HI—humidity, X—position, LI—level, S—speed, E—voltage, FI—flow of the grains). was empty at the beginning. It can be seen how the system reaches a steady state while the working chamber if filled. In the experiment No. 1, at 90 s the feeder was switched off. In the experiment No. 2, the feeder was switched off at 65 s. In both of the experiments, the frequency of the feeder was increased linearly from 20 Hz up to 60 Hz starting from 20 s up to 50 s. It can be seen in Figure 14 that the measured signal did not drop below value 3, even for maximal mill throughput when auger feeder was operating at 60 Hz. Figure 14b illustrates the performance of the inner loop of the control system that is presented in Figure 13. Red line represents the output signal that is generated by the controller FC. As can be seen, flap position (blue line) follows the control signal almost perfectly. It results in air flow value represented with black line. Note nonlinearity of static behaviour of the actuator (flap), which causes special attention when the controller is tuned. Another example of direct control is the frequency of the electromagnetic mill. The actuator is the inverter. Figure 15 presents an example of control system response. As can be seen, set point is followed almost perfectly. The only drawback is delay. It results from specific communication with the inverter.