The article describes a new, highly accurate, system for measuring filling levels in ball mills by measuring the structure-borne sound directly on the rotating mill shell. Data are transmitted wireless. This direct measurement avoids all the familiar disadvantages of “classical” microphones. There is absolutely no cross-sensitivity to other sources of sound, which means that the filling levels in the first and second grinding chambers can be measured independently of one another. In general, the structure-borne sound provides a great deal more information than emitted airborne sound about the state of a ball mill. A new, non-linear, algorithm for evaluating the structure-borne sound levels can indicate the filling levels in the two grinding chambers with an accuracy of better than 2 % of the maximum value. This has made it possible for the first time to set up control systems that keep the material being ground at a constant level inside the mill. The high precision of the measurement in the two chambers permits new insights into the actual behaviour of a ball mill and also makes it possible to assess the state of the intermediate and outlet diaphragms. If required, the temperature of the cement inside the mill can also be measured by a thermocouple in the intermediate diaphragm and transmitted by a third channel in the wireless link.
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