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    All rights reserved © 2012-2015 The Cement Grinding Office
      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 high-efficiency
          separators and pre-grinding 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 (ON-OFF)        
          * 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 X-axis, we have the time and the moment when a disturbance occurs.    
        - In the Y-axis, 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 ON-OFF 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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