Method and apparatus for measuring the degree of fullness of a mill with lifting beams by monitoring variation in power consumption

The degree of fullness of a rotary mill is determined by measuring the variation in power consumption of the elector motor due to the lifting beams of the mill casing striking material in the mill during rotation of the mill casing.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for measuring the 
degree of fullness of an electrically driven mill, which method and 
apparatus make use of power variations affecting the electric motor of the 
mill. 
In accordance with the disclosure in Canadian Patent 1,135,346, the power 
consumed by a mill driven by an electric motor, particularly an autogenous 
mill, varies periodically, and the amplitude of the periodic variation 
depends on the degree of fullness of the mill. Accordingly, the Canadian 
patent discloses a method and apparatus for measuring the degree of 
fullness of such a mill by continuously generating an electric signal 
representing the variation in electric power consumed by the mill within a 
relatively low frequency range 2-25 Hz. The degree of fullness of the mill 
is determined on the basis of the power signal, by utilizing test 
measurements performed earlier with different degrees of fullness. 
SUMMARY OF THE INVENTION 
It has been observed that the variation in power consumption of the 
electric motor of the mill is composed of a succession of power peaks that 
occur at a constant frequency. The peaks in the power consumption of the 
mill's electric motor are created when the lifting beams of the mill 
casing strike the mass being milled. Therefore, the time at which a peak 
is formed, relative to the time at which the mill was at a given angular 
position, is earlier when the degree of fullness in the mill is greater. 
Thus the method of measurement is based on an exact determination of the 
moment of striking. The moment of striking is determined by measuring the 
occurrence of the peak in the mill's power consumption in relation to time 
during a revolution of the mill, starting when the mill is at a 
predetermined angular position. A position sensor is used for timing the 
power measurement. The position sensor generates a pulse each time a 
reference marker on the circumference of the mill passes the position 
sensor. 
When applying the method of the invention, the measuring apparatus first 
generates a calibration curve. During an interval that covers a 
predetermined number of revolutions of the mill, a signal representative 
of the power consumed by the motor is sampled at a selected sampling 
frequency to provide a sequence of power values. The pulse generated by 
the position sensor is used to divide the sequence of power values into 
multiple segments, each of which covers one revolution and starts with the 
mill at the same angular position. On the basis of these segments, there 
is then formed an average power curve, which is a calibration curve of the 
mill and is stored in the measuring processor. The degree of fullness of 
the mill that corresponds to the calibration curve is defined in a known 
fashion. 
After forming the calibration curve for the mill, a measurement curve for 
an unknown degree of fullness is formed in an essentially continuous 
fashion. In forming the measurement curve, the power consumed by the 
mill's electric motor is measured over the same number of revolutions of 
the mill and at the same sampling frequency, so that an equal number of 
sample values is obtained as was the case with the calibration curve. The 
measured power values are combined into a measurement curve, which 
represents the average variation in power over the revolutions covered by 
the measurement and is also stored in the memory of the measuring 
processor. 
The measurement curve and the calibration curve are compared to each other 
in order to determine the amount by which the measurement curve must be 
shifted in order to match the calibration curve. This determination is 
carried out by first subtracting from each curve the average power over 
the interval covered by the curve, so that the offset curve has a zero 
d.c. component, whereafter the offset measurement curve is shifted on the 
time axis to the position such that it optimally overlaps the offset 
calibration curve. The most advantageous matching is determined by using 
the least sum of squares method. From the shift corresponding to the least 
sum of squares, the unknown degree of fullness is calculated on the basis 
of the known degree of fullness and revolution period of the mill. 
In the apparatus of the invention, the measuring processor is a 
microprocessor which can, when necessary, be simultaneously provided with 
power signals generated by several electrically driven mills. 
Advantageously the measuring processor is implemented on a processor card 
and may be based on the Intel 8052 processor. The processor card is 
connected to the power input of the mill and to the position sensor, and 
can be connected to a process control computer. Advantageously the 
processor card carries components to provide all necessary interfaces and 
functions required in the measurement of the degree of fullness. However, 
such components may alternatively be provided on a separate input/output 
card. When the processor card is connected to a process control computer, 
the information on the degree of fullness is advantageously transmitted 
from the processor card to the process control computer as current pulses. 
The method of measuring the degree of fullness of a mill can advantageously 
be applied for instance to autogenous mills, semiautogenous mills and ball 
mills. At least with these mills, the lifting beams of the mill cause such 
a significant variation in the power consumed by the electric motor that 
the power signal contains characteristic features that make it possible to 
determine the degree of fullness of the mill from the shift of the mill's 
power curve. 
The measuring apparatus of the invention can be employed in adjusting and 
optimizing the degree of fullness of the mill. In response to measurement 
of the degree of fullness, the feeding of the mill is advantageously 
controlled. Also, by analyzing the power curves stored in the measuring 
processor with different linings of the same mill, it is possible to 
optimize for example the number and size of the lifting beams used in the 
mill.

DETAILED DESCRIPTION 
FIG. 1 of the drawings illustrates a mill 1 having lifting beams 4. The 
mill is driven by a three-phase electric motor 8 that receives current by 
way of conductors R, S and T. During operation of the mill, the mill 
casing is rotated by the motor 8 at a constant angular speed, and the 
power consumption of the motor fluctuates. Peaks in the power consumption 
occur when a lifting beam strikes the mass of material in the mill. 
Consequently, the position of the peak due to a particular lifting beam 
striking the mass depends on the degree of fullness of the mill. 
A power transmitter 9 senses the power consumed by the electric motor and 
provides an analog power signal representative of the power consumption to 
processing unit 5, which includes a microprocessor 7 and an input/output 
card 6. The processing unit includes an analog to digital converter, which 
samples the analog power signal provided by the power transmitter at a 
selected sampling rate, and therefore provides the microprocessor with a 
sequence of digital values representing variation in power consumption as 
a function of time. 
A metal reference marker 2 is fastened to the casing of the mill 1, and a 
measuring sensor 3 mounted at the exterior of the mill provides a pulse to 
the processing unit 5 each time the reference marker 2 passes the sensor 3 
during rotation of the mill. The pulse allows the processing unit to 
divide the sequence of digital values into multiple segments each 
representing variation in power consumption during one revolution of the 
mill, each containing the same number of samples and each starting with 
the mill at the same angular position. 
It is necessary to calibrate the measurement apparatus with reference to a 
known degree of fullness. The mill is operated with a known degree of 
fullness and calibration data is acquired over a selected number of 
revolutions of the mill. By reference to the pulse provided by the sensor 
3, the calibration data is divided into multiple segments, and the 
segments are averaged to form a power curve representing the variation in 
power consumption of the mill over a single revolution. Further, the 
average power consumption over a single revolution is subtracted from the 
power curve so as to offset the curve so that its DC component is 
substantially zero. This calibration curve is stored in the memory of the 
processing unit. 
During operation of the mill at an unknown degree of fullness, power 
consumption data is acquired over an equal number of revolutions, and the 
power consumption data is processed in the same manner as described above, 
providing a measurement curve that is also stored. The processing unit 
compares the measurement curve and the calibration curve. If the 
measurement curve is generated at a higher degree of fullness, the peaks 
and zero crossings of the measurement curve occur earlier than the 
corresponding features of the calibration curve. The microprocessor 
determines what shift of the measurement curve leads to optimum matching 
of the two curves. Advantageously, the matching of the two curves is 
tested by computing the sum of the squares of the difference between the 
measurement curve and the calibration curve and determining the shift that 
provides a minimum value for this quantity. The degree of fullness of the 
mill can be calculated from the shift, the degree of fullness for the 
calibration curve, and the period of revolution of the mill. The 
calculated value of the degree of fullness of the mill may be transmitted 
from the processing unit 5 to a process control computer 10. 
The curves A-E shown in FIG. 2 represent variation in power consumed by the 
electric motor over five revolutions of the mill at a constant degree of 
fullness. The curve F is the average of the five curves A-E, offset 
vertically from the curves A-E for clarity. The curve G is a calibration 
curve representing variation in power consumption of the electric motor 
when the mill has a known degree of fullness. 
It will be appreciated that the invention is not restricted to the 
particular embodiment that has been described, and that variations may be 
made therein without departing from the scope of the invention as defined 
in the appended claims and equivalents thereof.