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>Knowledge Center>Grinding Machine>Measure the Fill Level of the Ball Mill Gri
Measure the Fill Level of the Ball Mill Grinding Process
The exact measurementof the fill level is the key and basic problem for automatic control and optimized operation of the coal pulverizing system. Because the ball mill pulverizing system is nonlinearity, long time delay and timevarying, the reliable and effective method for measuring the fill level was lacked at present. In order to reduce the influence by various factors on measuring the fill level and improve the precision of the fill level measurement, a novel characteristic variable is proposed. A set of wireless transmission device was designed to record vibration signals, and an accelerometer with high accuracy and large measuring range was mounted directly on the mill shell to pick up the vibration signals from the mill shell. A series of data acquisition experiments under various ball load and water content ofcoal conditions were conducted in an industrial mill to investigatethe relationshipbetweenthe fill level and the angular position of the maximum vibration point of the mill shell through the analysis of the vibration signals. The analytical result of test data clearly show that the angular position of the maximum vibration point on the mill shell decreases as the fill level increases. At the same time, comparing with the traditional characteristic variable, the feature variable of the fill level proposed in this paper is not subject to the effect of the ball load and water content of coal, which provides a new solution and reliable basis for the accurate measurement ofthe fill level.
As the rotating of the mill, the steel balls and coal particles in the ball mill are brought to a certain height by the centrifugal force of the mill and the friction, and then fall alonga parabolic trajectory under the effect of gravity. In the course of falling, the part of impact energy of the steel balls is absorbed by the coal particles to realize the grinding and comminuting of the coal. Another part of impact energy is delivered to the mill shell and causes the vibration of the shell, this part of the impact energy is further transmitted to the front and rear bearing housings causing vibration. Thehigher the coal load in the mill, the more impact energy of steel balls is absorbed by the coal powder, and the lower impact energy reaches the mill shell and bearing housings to cause relatively small vibration, and viceversa. So the vibration strength of the mill shell and the bearing housings to some extent can reflect the fill level information. Because the accelerometer can be mounted easily on the front and rear bearing housing, the traditional vibration methods can be used to measure the fill level based on the vibration signals of the bearing housings. At present among these methods, some characteristic values of the fill level, such as amplitude, energy, power and root mean square (RMS) of vibration signals, were extracted from the vibration signal of the bearing housings to monitor the fill level.
However, besides the impact of steel balls, there are many reasons and vibration sources, such as transmission system vibration, foundation vibration, vibration caused by the moment of asymmetry and installation error, which can also cause the vibration of bearing housings. These vibration sources will affect the characteristics of the time and frequency domain of the vibration signals of the bearing housings. Furthermore, vibration transmission path has also influence on the vibration signals of bearing housings. So the vibration signals of the bearing housings cannot accurately reflect the collision condition of the steel balls and the fill level information in the mill. Due to the difficulties in accelerometer installation and vibration data transmission, in addition to the high cost of the vibration data acquisition system, to date there has been little work on measuring the fill level by processing the vibration signals of the mill shell but the work of GUGEL is noteworthy. In their work, two vibration sensors, 180 degrees apart, were used to collect signal information from the mill shell, so that the instantaneous readings can be averaged. The averaged data were then fed into a dynamic neural network and a fill level measurement from 0%to 100%percent was output.
In this research, a series of experiments werecarried out at an industrial ball mill in coalfired power plant, and the vibration signals were collected directly from the mill shell. The main purpose of this paper is to investigate the correlation between the angular position of maximum vibration point on the mill shell and the fill level by analyzing and processing the vibration signals from the mill shell.
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