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Mills controls systems














1


Introduction

















In
the past, when the cement mills were in open circuit, the conduct of the
installation was almost always




operated manually,
without any control system.








In
open circuit, the task of the operator in the control room was reduced to a
minimum, i.e. increase or decrease




the mill output according
to the fineness of the finished product.








In
reality, it's a little more complicated than that, but there was no
imperative need to develop control tools




as it is the case today.









In
effect, grinding plants in closed circuit have become more and more complex
with the addition of highefficiency




separators and pregrinding systems.









The measured values are
numerous and the conduct in control room has become tedious.






It is not uncommon for
operators decrease the output of their
mill to avoid a big problem.





It is a human attitude!









Therefore more and more
sophisticated control systems have been developed to enable:





* to achieve complex or
sensitive operations that can not be entrusted to humans





* to replace the operator
for repetitive tasks







* to increase the precision of the system








* to improve the
stability of the grinding circuit







* to ensure the quality
of the finished product







* to achieve and maintain
optimum capacity of the installation








And ultimately, increase
the production of the mill and reduce its specific consumption.






A diagram among many
others of control system:













The advantages of
automation of a grinding circuit are the following:







Security: Keeping the output within a range that ensures the security
of the system.






Stability: Keep the output at a
constant value, despite the disturbances that can affect the process.





Optimization: Keep the output to a higher value than the output without
automation.






Quality: Optimize the output
precisely up to the desired value to ensure the quality of the final product.





Repetitiveness: Enable to perform repetitive tasks at regular intervals.







Reproducibility: Perform a sequence of operations without requiring human
intervention.











2


Concept of open loop (OL)
and closed loop (CL)
















An
open loop system is a system which does not include feedback between the
output and the input.





Typically,
it consists of the physical process, a sensor to measure the output and an
actuator to act on the system




input variable.









This
solution is adopted in the case where the system is well known and/or in the
case where obtaining a measure




of the output is not economically feasible.









Diagram of an open loop:















The open loop has the
following disadvantages:







* One cannot regulate unstable systems








* The disturbances have
uncompensated undesirable effects







* It is difficult to
obtain an output with the desired value quickly and accurately







One
greatly improves the situation with a closed loop because one can operate on
the system by correcting it




following the measurement
of the controlled variable.








Here is a general
flowsheet of a closed loop:













With the closed loop one
introduces the notion of control system.








The regulation of the
grinding circuit is in closed loop.








Open loop vs closed loop comparison:





















3


Notions of transfer
functions  Laplace Transform
















Before
starting the chapters describing the different types of controllers, it is
necessary to speak a little bit of the




transfer functions used
in order to define it mathematically.








We
call transfer function of a system, the ratio of the Laplace transform of the
output signal to the one of the input.





The
transfer function thus characterizes the dynamics of the system and depends
only on its physical characteristics.





Thus from now, a system
will be described by its transfer function.








Then, let's talk to the Laplace transform.









The Laplace transform is
an integral transformation.








Laplace
transformation is particularly adapted for the study of dynamic systems
assumed to be in a known state at




a certain time.









The Laplace transform of
a function f(t), where t is the time, is:






























The result is a function of s and not of t.









The operator s is the
inverse of time and represents a frequency.








Laplace transform allows
to transform the problem of domain of time to a frequency domain.






The
main advantage to analyze systems in this manner is that the calculations are
easier in the Laplace domain.





In
the Laplace domain, integral and derivative are combined using simple
algebraic operations, there is no need




of differential equations.









Here is a table with some usual transforms:











4


Types of control systems

















Different types of systems are:








* Controllers All or Nothing (ONOFF)








* PID controllers (P, PI, PD and PID)








* Controllers called fuzzy logic (Fuzzy logic)








* Expert Systems









All
these systems have different efficiency of course, according to their
complexity and thus their market value.





We can see on the chart
below a first comparison between the different types.














In the Xaxis, we have
the time and the moment when a disturbance occurs.







In
the Yaxis, we have the value of the controlled variable and the set point
(in this case, the mill output in t/h).





So
we see that the ONOFF controller is the worst and the fuzzy logic the best
controller on this diagram.





Expert systems are not
shown on this diagram.








These systems will be
explained in the following chapters.












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