Abstract:
A system  10  for controlling an industrial plant  12  comprises automatic control equipment  14  comprising a plurality of measurement sensors  16  for sensing predetermined variables associated with components of the industrial plant  12 . The sensors  16  generate measured data relating to operation of the components of the industrial plant  12 . A database  20  contains operational data, including observational data, regarding the industrial plant  12 . A processor  18  is in communication with the automatic control equipment  14  and the database  20  for receiving the measured data from the sensors  16  of the automatic control equipment  14  and the operational data from the database  20 . The processor  18  manipulates the measured and operational data to provide an evolving description of a process condition of each component over time, along with output information relating to operational control of the industrial plant  12  and for updating the database  20.

Description:
RELATED APPLICATIONS 
     This application is a Continuation application under 35 U.S.C. 111(a) of U.S. non-provisional application Ser. No. 12/376,760 filed on Jul. 9, 2010, which is a National Stage Entry application of PCT/NZ2007/000211 Filed on Aug. 6, 2007. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to process control of an industrial plant. More particularly, the invention relates to a system for, and a method of, controlling an industrial plant particularly, but not necessarily exclusively, a smelting plant. 
     BACKGROUND TO THE INVENTION 
     The present demand for an increasingly rapid financial return in industrial plants such as smelting operations has driven operating parameters beyond their current performance limits. This has resulted in reduction in the lives of operating components of the plant, reduced operating efficiencies and reduction in product quality. The ever present need to reduce carbon and/or other greenhouse gas emissions is adding additional pressure to the situation. In the case of smelting operations, the control systems that are in use were implemented in the early 1980s whereas productivity, raw materials supply, energy price and environmental issues associated with the industry have intensified considerably since that time. Furthermore, the flexibility of pot line electricity usage is an increasingly important issue for smelters because of country and continental electricity grids and variation in availability and price which connection to such grids can impose. 
     Generally, control of processes has evolved in different ways depending on the type of system under consideration. The desire to maintain a process and its operating conditions at the optimum operating parameters for which it was designed, or subsequently retrofitted for the purpose of increased production and minimal capital investment, is a common requirement since these parameters determine the quality of the product and the efficiency and cost of the process. In an attempt to maintain operation at such optimum parameters, control systems have involved some form of compensatory control loop or feedback loop in order to maintain steady operating conditions for the industrial plant. 
     Thus, using a smelting operation as an example once again, a normal control strategy has fixed or specified operating targets for the key process variables associated with the smelting operation. These key variables are adjusted in a compensatory fashion using other control inputs. A problem with this approach is that this may produce greater variation over time and compound the initial causes of the variation. In fact, the initial causes of the variation may not be addressed at all due to the reliance on manipulation of control inputs not necessarily related to the cause, allowing the causes of the variation to remain embedded in the process and increase in number over time. 
     Further, in order to reduce complexity, assessment of the process condition in smelting cells has been characterised by a limited set of measurements performed, at different intervals, on each cell. The last data point for each routinely measured variable is usually the one used in assessment of cell state. 
     With the above arrangements, inadequate information is provided to enable comprehensive operational or automatic control of the smelting operation to be effected. 
     SUMMARY OF THE INVENTION 
     Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
     According to a first aspect of the invention, there is provided a system for controlling an industrial plant, the system comprising: 
     automatic control equipment comprising a plurality of measurement sensors for sensing predetermined variables associated with components of the industrial plant, the sensors generating measured data relating to operation of the components of the industrial plant; 
     a database containing operational data, including observational data, regarding the industrial plant; and 
     a processor in communication with the automatic control equipment and the database for receiving the measured data from the sensors of the automatic control equipment and the operational data from the database, the processor manipulating the measured and the operational data to provide an evolving description of a process condition of each component over time, along with output information relating to operational control of the industrial plant and for updating the database. 
     The automatic control equipment may constitute a first system level, the processor and database constitute a second system level with the system including a third system level, being a management level. The management level may use the information output from the processor for effecting control of the industrial plant. The levels may be configured to achieve an improvement in a number of operating variables of the plant. 
     The system and method are intended particularly, but not necessarily exclusively, for use in an aluminium smelting operation. For ease of explanation, the invention will be described below with reference to this application. Those skilled in the art will, however, appreciate that the system and method are suitable for use in other applications. In particular, the method described here is generally applicable to any complex industrial process involving elements or sub-processes which interact in a non-linear and/or unpredictable way and in which the state of the industrial process has low observability for reasons of sensing or other difficulties, and low controllability because of the interactive nature of inputs and outputs and the varying and unpredictable time scales of their response to a control input. 
     A non-exhaustive list of industrial processes with the characteristics referred to above include: alumina refineries where a multiplicity of interacting caustic liquor circuits exist, each with a different dissolved sodium aluminate concentration and degree of super-saturation, and some streams with precipitating aluminium trihydroxide as well; steel plants where the iron ore thermal reduction step, iron to steel making furnaces and continuous casting processes are closely linked through steel temperature, composition and heat transfer from the transporting and holding vessels; steel or aluminium rolling and annealing/coating lines where coil gauge and width profiles are hard to measure a priori but have a profound effect on the heat transfer to the strip as it is annealed and the correct velocity of the strip through the annealing furnace for metallurgical quality. 
     In an aluminium smelting operation, the smelter contains a plurality of individual cells in which the smelting of aluminium oxide, or alumina, occurs. The cells of the smelting operation are arranged in lines, commonly referred to as pot lines. As indicated above, the system levels are provided to achieve improvements in a number of operational aspects of the smelting plant, more particularly, feed control to achieve good alumina dissolution; feed control to detect when good dissolution is not occurring and to correct this to inhibit periods of sludge accumulation; compositional control to maintain the mass of aluminium fluoride at an approximately constant level in a bath in each cell and to allow reduced compositional and temperature variation over time as alumina feed control and energy balance control are improved; energy balance control to maintain both sufficient superheat and actual bath temperature for alumina dissolution; energy balance control to inhibit periods of excessive superheat over time; statistical and causal analysis to continuously reduce variations across pot lines; and enterprise level management to assess actual pot line capabilities cell by cell to organise and prioritise improvement actions to improve smelter capability over time. 
     The system may be operable within a range for each variable, as determined by variability within the process, and may act to reduce variation within each variable and other key process variables through identifying abnormal or systemic, damaging patterns of variation which can be related to a single dominant cause. 
     The system may include a classifier module in communication with the processor for classifying variations of operating variables of the plant into one of a predetermined number of classes of variations. The classifier module may classify variations in a process variable into one of three classes being: common cause or natural variation, special cause variation or structural variation. 
     Common cause or natural variation may occur where no dominant cause is acting and a mix of causes results in a basically random pattern of variation. This class of variation may not be responded to automatically but may be the subject of process investigations if certain circumstances are present such as if the magnitude of the variation is still high or if there are safety implications. 
     Special cause variation may be one where a statistically significant, rarely encountered pattern of variation indicates that a dominant cause is influencing the process at any one time and that this cause is not part of the way the process is normally run. This class of variation may be signalled or alarmed by the automatic control equipment for investigation by operations staff. The operations staff may use the processor to determine the cause and, ideally, where possible, eliminate or correct the cause. 
     Structural variation may be one where non-random variation occurs often or routinely through the action of physical and chemical laws and the way the process is operated. Corrective automatic control actions may be possible if undesirable structural variations are detected by the sensors of the automatic control equipment. This may require identifying, or “finger printing”, the structural variation and observing corresponding changes in process condition over time. 
     In assessing the cell state, the present system may be operable to take into account information about the total process condition including process variable trajectory over a preceding period of time. In addition, when the plant is an aluminium smelting plant, the automatic control equipment may include bath superheat sensors, bath resistivity sensors, sensors for monitoring and noting electrical current variation and characteristic frequencies, cell off-gas temperature and flow rate sensors, and other control inputs such as the number of alumina shots fed to the cells in different feeding modes, for example, underfeed or overfeed modes and the degree of reduction in cell electrical resistance which occurs when such feeding modes are executed. 
     The observational data may relate to the operational state of the individual cells, the operational state being formally monitored and integrated into individual cell process conditions and including:—
         anode condition including red carbon, airburnt anodes, red stubs, spikes, cracked anodes;   bath condition including carbon dust, gap between bath and crust, bubble generation and location of evolution of the bubbles in the cell, bath level;   metal level and the projected metal tap history;   cover condition (remaining thickness and height on the anode connectors/stubs), crust damage, fume escape from superstructure;   alumina and bath spillage on electrical conductors (rods, beams, bus bars);   control action history over previous weeks including aluminium fluoride addition, alumina addition, extra voltage, excessive, unplanned anode beam movements, metals and bath transfers, etc;   cathode condition including cathode voltage drop (CVD) history, collector bar current density, instability history, anode changing observations, anode effect frequency, etc;   shell condition, including red plates, shell deformation and excessive heat rising to the catwalk from a certain shell location;   hooding condition—gaps, damage, fitment, door and quarter shield sealing;   bus bar and flexible damage, collector bars cut;   lack of duct gas suction as observed through fume escape into the pot room;   feeder operation, feeder chutes, feeder holes blocked, alumina not entering feeder holes;   side wall ledge condition, silicon carbide mass loss, history of silicon level in metal;   excessive liquid bath output from cells or from a pot room, indicating a change in heat balance causing melting of ledge, crust or dissolution of bottom sludge;   iron level in metal which is an indicator of bath level and anode condition;   trace elements in the metal which is indicative of trends in current efficiency over time;   flame colour, including blue flames, lazy yellow flames (sludge), bright yellow (sodium) shooting flames which may indicate some anode to metal direct contact in a cell; and   general housekeeping around each cell.       

     Each operational state may be monitored automatically by the sensors, using regular cell observations or both by the sensors and by observation, information obtained from the monitoring process being integrated with state variable measurements to build a description of the cell process condition of each individual cell and its evolution over time. It will be appreciated that in any industrial process there will be a set of equivalent observational data representative of the operational state of the process. 
     The processor and the database may be operable to check the process condition for each cell individually with the database being updated periodically. For example, the cell process condition may be updated at the commencement or termination of each shift. 
     The processor may include a causal framework for relating identified problems and cell process conditions to specific causes. The causal framework may form part of a learning algorithm of the processor which is improved and updated over time using data from the database, including feedback from staff about the validity of the causes identified and the effectiveness of corrective actions applied. This may also include conflicts which are observed and documented between the observational data and decisions and the numerical state information and automated decisions at level 1 of the control system. These updates may be subject to monthly review by management before becoming part of the knowledge base in the control system. Thus, the management level may employ causal trees containing the learning algorithm to provide a growing framework of decision support and, in the case of a smelting operation, cell diagnosis over time. 
     The database may have information associated with each cell and may contain process variable identifiers or “fingerprints” associated with specific problems, process events and/or cell process conditions. 
     The processor may further use a complexity measure to assess predictability of the process outcomes and the overall operation of the plant. 
     According to a second aspect of the invention, there is provided a method of controlling an industrial plant, the method comprising: 
     monitoring operation of the industrial plant by a plurality of sensors forming part of automatic control equipment; 
     transferring measured data from the sensors and observational (qualitative) data relating to operation of the industrial plant to a processor; 
     accessing a database containing operational data including data from the sensors and the observational data relating to operation of the industrial plant, as periodically updated by the processor; and 
     generating evolving process condition descriptions of each monitored component of the industrial plant and output information relating to operation of the industrial plant. 
     Thus, the method may include employing new formal control objectives based on the long term reduction in variability of the process, and on integrating human observation and decision making into the computational organisation of sensed information in traditional control systems. 
     The method may include forming three system levels, the automatic control equipment constituting a first system level, the processor and database constituting a second system level and a third system level being a management level. The method may include using the information output from the processor in the management level for effecting control of the industrial plant. The method may include configuring the levels to achieve an improvement in a number of operating variables of the plant. 
     The plant may be an aluminium smelting plant and the method may include configuring the levels to achieve improvements in a number of operational aspects of the plant. These operational aspects are generally not considered as part of the process condition of the industrial plant from a control viewpoint. More particularly, the operational aspects may include feed control to achieve desired alumina dissolution; feed control to reduce, and, if possible, eliminate, periods of sludge accumulation; compositional control to maintain the mass of aluminium fluoride at an approximately constant level in a bath in each cell and reduce compositional and temperature variation over time; energy balance control to maintain both sufficient superheat and actual bath temperature for alumina dissolution; energy balance control to inhibit periods of excessive superheat over time; statistical and causal analysis to continuously reduce variations across pot lines; and enterprise level management to assess actual pot line capabilities cell by cell to organise and prioritise improvement actions to improve capability over time and to optimise the production of metals with specifications matching sales orders. 
     The method may include operating the plant within a range for each variable as determined by variability within the process and which acts to reduce variation within each variable and other key process variables through identifying abnormal or systemic, damaging patterns of variation which can be related to a single dominant cause. Thus, the method may include correcting or minimising identified causes as appropriate, reducing the range of each process variable and improving process capability over time. 
     The method may include classifying variations of operating variables of the plant into one of a predetermined number of classes of variations. In particular, the method may include classifying variations in a process variable into one of three classes being: common cause or natural variation, special cause variation or structural variation. 
     The method may include taking into account information about the total process condition including process variable trajectory over a preceding period of time. The observational data may relate to the operational state of the individual cells, the method including formally monitoring and integrating the operational state into individual cell process conditions and the operational states including:—
         anode condition including red carbon, airburnt anodes, red stubs, spikes, cracked anodes;   bath condition including carbon dust, gap between bath and crust, bubble generation and location of evolution of the bubbles in the cell, bath level;   metal level and the projected metal tap history;   cover condition (remaining thickness and height on the anode connectors/stubs), crust damage, fume escape from superstructure;   alumina and bath spillage on electrical conductors (rods, beams, bus bars);   control action history over previous weeks including aluminium fluoride addition, alumina addition, extra voltage, excessive, unplanned anode beam movements, metals and bath transfers, etc;   cathode condition including cathode voltage drop (CVD) history, collector bar current density, instability history, anode changing observations, anode effect frequency, etc;   shell condition, including red plates, shell deformation and excessive heat rising to the catwalk from a certain shell location;   hooding condition—gaps, damage, fitment, door and quarter shield sealing;   bus bar and flexible damage, collector bars cut;   lack of duct gas suction as observed through fume escape into the pot room;   feeder operation, feeder chutes, feeder holes blocked, alumina not entering feeder holes;   side wall ledge condition, silicon carbide mass loss, history of silicon level in metal;   excessive liquid bath output from cells or from a pot room, indicating a change in heat balance causing melting of ledge, crust or dissolution of bottom sludge;   iron level in metal which is an indicator of bath level and anode condition;   trace elements in the metal which is indicative of trends in current efficiency over time;   flame colour, including blue flames, lazy yellow flames (sludge), bright yellow (sodium) shooting flames which may indicate some anode to metal direct contact in a cell; and   general housekeeping around each cell.       

     The method may further include monitoring each operational state automatically by the sensors, using regular cell observations or both by the sensors and by observation, information obtained from the monitoring process being integrated with state variable measurements to build a description of the cell process condition of each individual cell and its evolution over time. 
     The method may includes operating the processor and the database to check the process condition for each cell individually and updating the database periodically. For example, the cell process condition may be updated at the commencement or termination of each shift. 
     In addition, the method may include using a causal framework to relate identified problems and cell process conditions to specific causes. The method may include integrating the causal framework into a learning algorithm of the processor which is improved and updated over time using data from the database. Thus, the method may include employing causal trees containing the learning algorithm to provide a growing framework of decision support and, in the case of a smelting operation, cell diagnosis over time. 
     The database may have information associated with each cell and may contain process variable identifiers or “fingerprints” associated with specific problems, process events and/or cell process conditions. 
     The method may include using a complexity measure to assess predictability of the process outcomes and the overall operation of the plant. 
     According to a third aspect of the invention, there is provided a system for controlling an industrial plant, the system comprising: 
     automatic control equipment comprising a plurality of measurement sensors for sensing predetermined variables associated with components of the industrial plant; 
     a database containing operational data, including observational data, regarding the industrial plant; and 
     a processor in communication with the automatic control equipment and the database for receiving data from the sensors of the automatic control equipment and from the database, the processor using causal tree analysis comprising at least one continually updated learning algorithm to provide a framework of decision support and plant component diagnosis over time. 
     According to a fourth aspect of the invention, there is provided a method of controlling an industrial plant, the method comprising: 
     monitoring operation of the industrial plant by a plurality of sensors forming part of automatic control equipment; 
     transferring data from the sensors and observational data relating to operation of the industrial plant to a processor; 
     accessing a database containing operational data, including the data from the sensors and the observational data relating to operation of the industrial plant, as periodically updated by the processor; and 
     using causal tree analysis comprising at least one continually updated learning algorithm to provide a framework of decision support and plant component diagnosis over time. 
     According to a fifth aspect of the invention, there is provided automatic control equipment for a system for controlling an industrial plant, the system comprising: 
     a plurality of measurement sensors for sensing predetermined variables associated with components of the industrial plant; 
     a signal processing module responsive to the sensors and control input data; and 
     a classifier module in communication with the signal processing module for classifying variations of operating variables of the plant, as detected by the sensors, into one of a predetermined number of classes of variations. 
     According to a sixth aspect of the invention, there is provided a method of operating an industrial plant, the method comprising: 
     monitoring operation of the industrial plant by a plurality of sensors; 
     processing data from the sensors and other control inputs; and 
     classifying variations of operating variables of the plant, as detected by the sensors, into one of a predetermined number of classes of variations. 
     According to a seventh aspect of the invention, there is provided a method of operating an industrial plant, the method comprising 
     monitoring operation of the industrial plant by a plurality of sensors forming part of automatic control equipment; 
     transferring measured data from the sensors and observational data relating to operation of the industrial plant to a processor; 
     accessing a database containing operational data, including the data from the sensors and the observational data relating to operation of the industrial plant, as periodically updated by the processor, to provide mechanisms to assist in identification and removal of causes of variations in the measured data; and 
     combining automatic control as carried out by the automatic control equipment with said mechanisms to provide continuous improvement in the operation of the plant. 
     According to an eighth aspect of the invention, there is provided a system for controlling an industrial plant, the system comprising: 
     automatic control equipment comprising a plurality of measurement sensors for sensing predetermined variables associated with components of the industrial plant; 
     a database containing operational data, including observational data, regarding the industrial plant; and 
     a processor in communication with the automatic control equipment and the database for receiving data from the sensors of the automatic control equipment and from the database, the processor using a complexity measure to assess predictability of the plant. 
     The use of the complexity measure may provide an early warning of a trend to more chaotic or less reliable operation of the plant (and/or the people in the plant) over time which will not otherwise be detected by the more repetitive, regular operation of control inputs and process outputs. 
     According to a ninth aspect of the invention, there is provided a method of controlling an industrial plant, the method comprising: 
     monitoring operation of the industrial plant by a plurality of sensors forming part of automatic control equipment; 
     feeding data from the sensors and observational data relating to operation of the industrial plant to a processor; 
     accessing a database containing operational data, including the data from the sensors and the observational data relating to operation of the industrial plant, as periodically updated by the processor; and 
     using a complexity measure to assess predictability of the plant. 
     The complexity measure may further alarm deteriorating trends in the reliability of the plant or elements of it (for example a particular part of a potline or a whole potline may start to behave less reliably than others in the same smelter). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are now described by way of example only with reference to the accompanying drawings in which: 
         FIG. 1  shows a schematic block diagram of a system, in accordance with an embodiment of the invention, for controlling an industrial plant; 
         FIG. 2  shows a tabular representation of the system of  FIG. 1 ; 
         FIGS. 3-6  show a graphic representation of an example of the determination of state changes for a cell using a complexity measure; 
         FIG. 7  shows a tabular representation of a first control objective of the system; 
         FIG. 8  shows a tabular representation of a second control objective of the system; 
         FIG. 9  shows a tabular representation of a third control objective of the system; 
         FIG. 10  shows a tabular representation of a fourth and a fifth control objective of the system; and 
         FIG. 11  shows a tabular representation of a sixth control objective of the system. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In  FIG. 1  of the drawings reference numeral  10  generally designates a system, in accordance with an embodiment of the invention, for controlling an industrial plant. The plant is designated generally by the reference numeral  12 . The system  10  will be described with reference to its application to an aluminium smelting operation but it will be appreciated that the system  10  could be used in any industrial plant requiring control. The smelting operation or smelter  12  comprises a plurality of pot lines. Each pot line is constituted by a plurality of cells in which alumina is processed to form aluminium. Aluminium formed in the cells is tapped off periodically for casting or further processing downstream in the plant  12 . 
     The purpose of the system  10  is to minimize energy consumption and smelter emissions, maximize productivity and metal purity and promote a safer, healthier working environment over time by continuously reducing and removing variations in the process. The control objectives of the system  10  include:— 
     1. Feed control to achieve conditions for good alumina dissolution for a high proportion of the operating time through identifying early signs of poor dissolution conditions and acting automatically if heat or composition related or through operational decisions and intervention if not able to be corrected automatically.
 
2. Feed control to inhibit periods of sludge accumulation which are usually characterised by low alumina input during an underfeeding control mode and high alumina input during an overfeeding control mode and through this objective and objective 1 above jointly to minimize anode effects on all of the cells.
 
3. Compositional control which maintains the mass of aluminium fluoride at an approximately constant level in the whole cell but also provides signals and mechanisms to correct the causes of aluminium fluoride mass variation within cells and pot lines, thereby improving the stability of aluminium fluoride concentration in the bath over time.
 
4. Energy balance control which maintains both sufficient superheat and actual bath temperature for alumina dissolution and which inhibits large scale electrolyte freezing during normal cell operations such as anode setting, alumina feeding, metal level changes, or other additions to the cell.
 
5. Energy balance control which minimises periods of excessive superheat over time by signalling when causes of excessive superheat are present and by giving decision support to cause identification and elimination.
 
6. Continuous reduction in variation across the pot lines by categorising and finger printing sensor responses, integrating operational information with cell data histories and connecting the resulting cell process condition and identified process signals to proven causal trees. These causal trees contain a learning algorithm (as discussed below) and provide a growing framework of decision support and cell diagnosis over months and years of operation of the industrial process.
 
7. Enterprise level management which assesses actual pot line capability cell by cell and includes carbon plant and casthouse capabilities to organise and prioritise improvement actions over time. The system builds levels of planning information for future years based on cell and pot line constraints and tested solutions to result in maximized production, capture of higher purity metal and satisfying sales order metal specifications (through bath level, anode airburn and bath temperature variability reduction and linkage to the casthouse metal batch and furnace capture). It also matches anode capability and cell capability across their respective populations to reduce anode/cell adverse interactions over time.
 
     The above objectives are achieved through a three step control model of, firstly, observing the process, secondly, understanding the variation and, thirdly, controlling the outcome. Particular emphasis is placed on the multivariate nature and non-linearity of the system  10 . The system  10  is implemented within an architecture which systemizes observation of total process condition in the first control step, learning through a causal framework associated with the second control step and a human decision guidance module associated with the third control step. 
     The system  10  includes automatic control equipment  14 . The automatic control equipment  14  comprises a plurality of sensors  16  for monitoring operating variables associated with the smelter  12 . The automatic control equipment  14  and the sensors  16  constitute a first level of the system  10 . 
     A second level of the system  10  comprises a processor  18  which communicates with the automatic control equipment  14 . The processor  18  is further in communication with a database  20  in which operational data relating to the cells of the smelter  12  are stored. In addition to the sensor and routine numerical measurements, the database  20  contains data relating to problems and process variable identifiers or “fingerprints” associated with specific problems or events related to each cell of the smelter  12 . These data are derived from the sensors  16  as well as qualitative observational data as detected by operations staff, as illustrated schematically at  22 , which is input into the database via the processor  18 . 
     A third level of the system  10  is a management level  24  which uses data from the database  20  to control and improve operation of the smelter  12 , as will be described in greater detail below. 
     A tabular arrangement of the system  10  is shown in  FIG. 2  of the drawings. The automatic control equipment  14 , as indicated above, communicates with the sensors  16 . These sensors  16 , in turn, feed to a plurality of monitoring modules. Hence, the automatic control equipment  14  comprises an abnormal raw resistance monitor  26  for monitoring a rapidly acquired cell resistance signal and detecting patterns and frequencies indicative of abnormal operation. Together with alarming of cathode and anode abnormalities, the resistance monitor  26  is used in combination with a feed monitor module  28  which monitors the response of the individual cell resistances of the smelter  12  to the feeding of alumina at different rates corresponding to the different feed control modes. 
     A temperature/liquidus control module  30  also communicates with the feed monitor module  28  and with the raw resistance monitor  26 . This module  30  monitors changes in temperature, liquidus temperature and bath resistance per millimeter of anode beam movement (if these sensors are active) and also includes an alumina concentration dimension computed with reference to the feed monitor module  28 . 
     The automatic control equipment  14  further includes a signal processing module  32  which receives signals from all of the sensors  16  and other control inputs. It summarises the essential character (mean, range, trend, frequency) of these signals and controls the supply of the resulting information to the processor  18  of the system  10 . 
     The operating variables of the smelter  12  are classified in three classes, as will be described in greater detail below. To enable this to occur, the automatic control equipment  14  includes a classifier module  34 . This classifier module  34  classifies variations into one of the three classes. In addition, the classifier module  34  classifies the cells of the pot line. 
     In level two of the system  10 , the processor  18  includes a cell process condition module  36  which communicates with the database  20  for maintaining a history of state variables, operational observations and non-conformances of both as well as cell process complexity trends (which can show process state changes) for all cells. The cell process condition is tracked daily, weekly and monthly. Short and longer term aspects of the cell process condition are determined and updated periodically, for example, at the commencement or termination of each shift, weekly and/or monthly. 
     The cell process complexity trends are used to determine aspects of the cell state which are not evident from the normal physical measurements of process variables. Predictability of the system  10  is assessed using a complexity/information measure referred to as T-entropy. Briefly, T-entropy is an algorithmic technique which allows computation of the complexity of a finite string of characters which are produced by symbolically transforming information from visual and other analogue or digital signals. (A complete treatment of the derivation of T-entropy can be obtained from the following reference: Titchener, M. R., Gulliver, A., Nicolescu, R., Speidel, U. and Staiger, L. (2005)  Deterministic Complexity and Entropy . Fundamenta Informaticae, 64(1-4), 443-61.) 
     T-entropy is analogous to its thermodynamic equivalent which is most commonly referred to as the ‘level of disorder’ in a (chemical) system. Similarly, T-entropy evaluates the level of disorder in a finite, two-dimensional signal. T-entropy contains information which is not provided in traditional signal processing, where the repetitive, regular frequency characteristics of a signal are determined. The non-repetitive, chaotic or non-linear elements of signals in real world problems contain more information however. It is this complex, real world behaviour which is transduced through the T-entropy computation. 
     Using the example of a pseudo-resistance trace from a pre-bake operating cell with metal pad noise developing over a period of four hours,  FIG. 3  exemplifies the computation of T-entropy. In  FIG. 3 , an entropy surface  52  and its maximum entropy  54  for pseudo-resistance trace  56  is illustrated. 
     In  FIG. 4 , the maximum entropy  54 , an integral under the curve, or raster,  58  and a three-dimensional hybrid trace  60  are illustrated. Another “view” of this (in a z-y plane) is shown in  FIG. 5  of the drawings. This shows areas of density in the hybrid trace  60 . The two areas of density are shown at  62  and  64  with the transition between them shown at  66 . The three clusters  62 - 66  indicate three distinct states of the process with state changes associated with physical changes inside the cell which it is not possible to detect routinely by other sensors and methods. 
     The three clusters or states  62 - 66  identified in  FIG. 5  are plotted against time as a trace  68  together with the T-entropy output  52  and the pseudo-resistance output  56  as shown in  FIG. 6 . This state information  68  would not have been obtainable from the pseudo-resistance trace  56  alone and provides information concerning changes in the complexity of the cell behaviour. The information provided by the trace  68  is integrated into the cell process condition module  36  and is made available to the operational staff of the plant to enable analysis and remedial action, if necessary, to be undertaken. 
     The processor  18  makes use of a learning algorithm  38  for causal tree analysis. The learning algorithm comprises a causal framework  39  ( FIG. 11 ) for the pot line of the smelter  12  and relates identified problems and cell process conditions to specific causes. The processor  18  therefore communicates with the database  20  and under management authorisation the new causal/cell condition links and corrective actions are added to the framework so that the causal framework is improved and updated over time to render the learning algorithm  38  more applicable to prevailing circumstances. Operational staff at the plant provide feedback concerning the success of causal analysis and recommendations into the processor  18  to integrate the practical aspect of plant operation and to improve future control system actions. 
     The management level  24  includes an assessment module  40 . The assessment module  40  assesses pot line efficiency and production capability on a cell by cell basis, taking note of the incidence and severity of variations occurring in two of the classes, i.e. special cause variation and structural cause variation. 
     Additionally, the management level  24  comprises an analysis module  42  for effecting planning options analysis based on potential capability improvement and calculated risk over a period of time. 
     As indicated above, the classification module  34  classifies variations in each variable into one of three classes. These three classes are:— 
     common cause or natural variation, special cause variation and structural cause variation. 
     Common cause or natural variation is a variation where no dominant cause is acting and the mix of causes results in a basically random pattern of variation. This class of variation is not responded to automatically but may be the subject of process investigations if the magnitude of the variation is too high or has safety implications. 
     Special cause variation is one where a statistically significant, rarely encountered pattern of variation indicates that a dominant cause is influencing the process at that particular time and that this cause is not part of the way the process is normally run. This class of variation is detected by the sensors  16  of the automatic control equipment  14  and/or through the systematic observations of staff at the cells during routine daily operations and is investigated by the operations staff  22 . The staff  22  use the causal tree analysis of the processor  18  to determine and, if possible, eliminate the cause. 
     Structural variation occurs where non-random variation takes place often or routinely through the action of physical and chemical laws and the way the process is operated. Automatic corrective action is possible if undesirable structural variation is detected in the cell sensors  16 . This requires access to the database to determine established connections between the cell sensor responses, causes of variation and control objectives of the system  10 . This is achieved through finger printing the structural variations and observing corresponding changes in the process condition over time. 
     Insofar as the first step of observing the process is concerned, the control strategy of the system  10  does not rely solely on fixed or specified operating targets for the key process variables such as bath temperature, bath composition, and cell voltage. Rather, the control strategy operates over a range of process variables determined by variability within the process itself to produce a target cell process condition related to the desired process outcomes (for example energy efficiency, metal purity, anode effects, cell life, cost of production and safety). The control strategy acts to reduce variation in the range of these key process variables through identifying abnormal or systemic, damaging patterns of variations which can be related to a single dominant cause at a given point in time. The identified causes can then be corrected or eliminated as appropriate reducing the range of each process variable and improving the process capability over time. Thus, the system uses, in addition to the existing cell sensors  16 , new cell sensors such as bath superheat sensors, bath resistivity sensors, sensors for monitoring anode current variation at characteristic frequencies, cell off-gas temperature and flow rate sensors and other control inputs. 
     In addition, observational data as detected by the operation staff  22  include monitoring the operational state of the cells by monitoring of the following:
         anode condition including red carbon, airburnt anodes, red stubs, spikes, cracked anodes;   bath condition including carbon dust, gap between bath and crust, bubble generation and location of evolution of the bubbles in the cell, bath level;   metal level and the projected metal tap history;   cover condition (remaining thickness and height on the anode connectors/stubs), crust damage, fume escape from superstructure;   alumina and bath spillage on electrical conductors (rods, beams, bus bars);   control action history over previous weeks including aluminium fluoride addition, alumina addition, extra voltage, excessive, unplanned anode beam movements, metals and bath transfers, etc;   cathode condition including cathode voltage drop (CVD) history, collector bar current density, instability history, anode changing observations, anode effect frequency, etc;   shell condition, including red plates, shell deformation and excessive heat rising to the catwalk from a certain shell location;   hooding condition—gaps, damage, fitment, door and quarter shield sealing;   bus bar and flexible damage, collector bars cut;   lack of duct gas suction as observed through fume escape into the pot room;   feeder operation, feeder chutes, feeder holes blocked, alumina not entering feeder holes;   side wall ledge condition, silicon carbide mass loss, history of silicon level in metal;   excessive liquid bath output from cells or from a pot room, indicating a change in heat balance causing melting of ledge, crust or dissolution of bottom sludge;   iron level in metal which is an indicator of bath level and anode condition;   trace elements in the metal which is indicative of trends in current efficiency over time;   flame colour, including blue flames, lazy yellow flames (sludge), bright yellow (sodium) shooting flames which may indicate some anode to metal direct contact in a cell; and   general housekeeping around each cell.       

     The cell process condition elements monitored above can be monitored either by the operations staff  22  or by the sensors  16 . This information is integrated with state variable measurements to build a description of the total process condition of each individual cell and its evolution over time. 
     The cell process condition for each cell is tracked individually by the processor  18  and is updated periodically, for example, at the commencement or termination of each shift. The process condition description is also used in the automatic control equipment  14  and may be used for operational decisions during any shift for individual cells or for the pot line. The process condition is also used for process engineering investigations to correct individual cells manifesting long term problems and, finally, is used by the management level  24  which uses pot line condition for judging the capability of the smelter  12  to alter its operating settings, for example, production rate or energy usage. 
     As indicated above, understanding the variation in the operating variables comprises classifying these variations in one of the three classes. 
     The third step of the control system  10  is achieved by altering the traditional function of each level of the system  10  so that the new control objectives set out above are met. Insofar as the first level is concerned, the control system  10  seeks to achieve a systematic reduction in individual cell variations through corrective control of variables such as alumina feed, bath composition and energy input and relies on the integration of operator observations of cell condition and their subsequent well informed decisions and actions to remove causes of these variations. 
     The second level of the system  10  seeks to achieve pot line variation reduction through removal of causes of the variations. It further relies on pot line management which emphasises decision making using the database framework of variation and causes which is continually updated with evidence accumulating over time and is systematically linked to the operational observations and practical decision making so that contradictions between theoretical control decisions and direct observation are constantly being sought and resolved. It also facilitates individual cell process condition description and tracking using T-entropy trends to identify hidden state and state change information. This level also uses human decision guidance linking physical cell condition stimuli to detection and decision making. 
     The management level  24  of the system  10  is used to effect pot line capability assessment based on cell state, metal purity distribution data from the second level and quantified improvement potential. The cell distribution data is linked to improvement strategies such as reducing poorly performing cells or moving the entire cell distribution, as well as metal marketing and financial planning and control. 
       FIG. 7  shows a tabular implementation of the first control objective for achieving good alumina dissolution. The module letters correspond to the module labels in  FIG. 2  of the drawings. 
     Similarly,  FIG. 8  shows a tabular representation of the second control objective of elimination of periods of sludge accumulation without incurring anode effects. As is the case with the first control objective, the second control objective relies on operational observations triggered automatically by sensing and level 1 control logic to indicate specific observations (concerning the alumina feeders primarily) required to achieve the control objectives. 
       FIG. 9  shows a tabular representation of the third control objective of compositional control based on achieving near constant mass of aluminium fluoride in each cell and its improvement over time. In this control objective, there is, once again, a requirement for observational data and also operator input for adjustment of the cell or line, particularly in the case where the variation is identified as being special cause. The identification of adverse structural variation such as thermal and compositional cycling allows these to be related to the systemic causes embedded in the control system and the smelter  12  itself through the learning algorithm  38  at level 2 in the control system. 
       FIG. 10  is a combination of control objectives 4 and 5 to achieve energy balance control to maintain changes in temperature within a range which can be withstood by the cell without damage to the process. 
     In the case of control objective 6, this control objective is met substantially at level 2 of the system  10  and is shown diagrammatically in  FIG. 11 . Because the system design is now specifically aimed at improvement and not only control, the architecture of the level 2 system differs from previous supervisory systems. 
     Better understanding of what constitutes the cell process condition now enables a single screen view of the state of each cell of the smelter  12  and incorporates both state variable measurement and operational state attributes as well as the respective histories. One embodiment of this view is shown in the first part of  FIG. 11 . Each variable or attribute is described by a colour being red (R), orange (O), blue (B), or green (G) representing not only the last observation but also the stability of the observations over a specified operating period and within the stable, multi-dimensional control volume for selected groups of variables. Red and orange status indicates abnormal status conditions requiring attention and potential abnormality respectively. 
     Taking the example of “Alumina kg/d” the stability of the uni-variate measurement will be judged by the statistical stability of the custom of the “Alumina Daily addition. The capability of the cell with respect to “Alumina Feeding” will be judged by the flatness of the Cusum Chart. In other words: “Is the cell consumption of alumina matched to the metal production rate?” However, this variable is also combined into multivariate views of the whole cell process condition because of its interaction with the thermal and compositional balance. In this example of alumina feeding, kg/d of alumina fed during an underfeed mode and kg/d of alumina fed during an overfeed mode can be analysed as a bi-variate surface, leading to a state descriptor for feeding, as one element of the overall cell process condition. 
     The database  20  contains the normal comprehensive numerical information over time, but with new classes of discontinuous, cell specific information as shown in  FIG. 11 . This “event driven” data is stored in time stamped flat files and is used along with process variable fingerprints stored in the database to establish likely causes within the causal framework  39 . 
     The causal framework  39  is largely automated in its data queries and logic processing. It is designed to respond to management requirements in two ways by, firstly, providing causes and corrective actions for individual problems through request at any time. These requests can also be automated at a start of a shift through the cell process condition module  44 , if required. 
     The causal framework  39 , secondly, provides timed (daily, weekly, monthly) review reports to people within the organisation. These reports are configurable and summarise problems requested, those resolved and those with adverse consequences stemming from the advice provided, learning opportunities formulated (for authorisation) and conflicts between causal logic and observations (for resolution). This is provided on a human decision guidance module  46 . 
     The causal framework  39  drives improvements in control and in performance on the pot line by use of the enhanced database  20  and process condition descriptions to solve single cell and systemic pot line problems. 
     The presentation of summary data on the number of problems outstanding on the number of cells in various states of control is a stimulus for management attention and is facilitated by having a continuous tracker  48  of both cell process condition and identified cell problems. The tracker  48  aids in operational implementation of cell action plans as shown at  50 . 
     The tracker  48  plays an integral part of the management process embedded in level 2 of the system  10 . Decisions are based on the scientifically formulated and evidentially confirmed causal framework  39 , the diagnosed cell process condition and the computed trend in the complexity or chaotic nature of the cell condition using T-entropy. 
     The control objective 7 relies on achievement over time of the first six control objectives. It also requires that the measured and predicted future capability of the pot line is formally integrated into financial management and planning processes for the smelter  12 . This is achieved by the modules  40  and  42  of the management level  24 . The actual design of the modules  40  and  42  will depend on the enterprise level system which is in use at the smelter  12 . 
     It is therefore an advantage of the invention that an improved system  10  is provided which enables more accurate control of a smelter  12  to be achieved over a period of time by the use of observational data, a causal framework  39  and automatic control equipment  14  which is more integrated with the formal control objectives and with the observations of the staff. With the new system, reduction in variation in individual cells through integrated automatic and operational control decisions can be achieved over a period of time resulting, in the long run, in improved operating efficiencies of the smelter  12 . 
     A further advantage of the system  10  is that it achieves integration of energy, composition, alumina feed and operational controls with smelter improvement plans to minimise energy consumption and smelter emissions and to maximise production of metal of the highest possible purity/value over time. Still further, it facilitates a holistic assessment of the process condition of each individual cell, the process condition of each cell being maintained and updated over time. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. In particular, while the system and method have been described with reference to its application in an aluminium smelting plant, that has been done for ease of explanation only. The system and method are equally applicable in any industrial process where a set of equivalent observational data representative of the operational state of the process can be employed in improving the operation of the process.