Abstract:
Modelling of industrial processes is simplified with the use of Mixed Logical Dynamic (MLD) framework. Optimal control problems can be generated for application to industrial processes. For example, two arbitrarily connected MLD blocks are automatically merged to obtain one composite MLD block. Via a repeated use of the procedure, any arbitrarily complex system containing the complete description of an industrial process can be generated from the simplest MLD building blocks. The optimal control problem is generated via adding an MLD block whose unique output becomes the cost functional of the problem. In a graphical environment, any specific industrial process may be reproduced by instantiating blocks from a library of basic MLD elements or atomic MLD blocks and by properly connecting them. In case an appropriate library is available, this process will not require any expert knowledge from the end user apart from the ability to build the graphical interconnections mentioned.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to EP Application 04405350 2 filed in the EPO on 7 Jun. 2004, and as a continuation application under 35 U.S.C. §120 to PCT/CH2005/000317 filed as an International Application on 6 Jun. 2005 designating the U.S., the entire contents of which are hereby incorporated by reference in their entireties. 
    
    
     FIELD 
     Industrial process modelling, optimal control and advanced scheduling are disclosed. It departs from a method of merging two arbitrarily interconnected Mixed Logical Dynamic (MLD) subsystems into a combined MLD system and a method of designing a model based controller or scheduler for real-time control or scheduling of an industrial process. 
     BACKGROUND INFORMATION 
     Mixed logical dynamical (MLD) systems, as introduced e.g. in the article “Control of Systems Integrating Logic, Dynamics, and Constraints”, A. Bemporad and M. Morari, Automatica 35(3), 1999, pp 407-427, represent a mathematical framework for modeling systems described by interacting physical laws, logical rules, and operating constraints, generally called “hybrid systems”. MLD systems are determined or described by linear dynamic equations subject to linear mixed-integer inequalities involving both continuous, i.e. real-valued, and binary, i.e. boolean-valued, variables. The variables include continuous and binary states x, inputs u and outputs y, as well as continuous auxiliary variables z and binary auxiliary variables δ, as described in the following equations:
 
 x ( t+ 1)= Ax ( t )+ B   1    u ( t )+ B   2 δ( t )+ B   3   z ( t )  (Eq. 1a)
 
 y ( t )= Cx ( t )+ D   1   u ( t )+ D   2 δ( t )+ D   3   z ( t )  (Eq. 1b)
 
 E   2 δ( t )+ E   3   z ( t )&lt;= E   1   u ( t )+ E   4   x ( t )+ E   5   (Eq. 1c)
 
In general, the variables mentioned are vectors and A, B i , C, D i , E i  are matrices of suitable dimensions.
 
     In order to be well posed, the MLD system (Eqs. 1) must be such that for any given x(t) and u(t) the values of δ(t) and z(t) are defined uniquely. Formulations or relationships in the form of Eqs. 1 appear naturally when logical statements are written as propositional calculus expressions, or when bounds on the states are set explicitly. Among the advantages of the MLD framework are the possibility to automatically generate the matrices of MLD systems from a high-level description. MLD systems generalize a wide set of models, among which there are linear hybrid systems and even nonlinear systems whose nonlinearities can be expressed or at least suitably approximated by piecewise linear functions. 
     The drawback of the MLD systems approach is the relative complexity of the theory, which in turn makes the modelling and maintenance of a complex industrial system difficult for people without a background in mixed integer optimization. 
     Model predictive control (MPC) is a procedure of solving an optimal-control problem, which includes system dynamics and constraints on the system output, state and input variables. The main idea of model predictive control is to use a model of the plant or process, valid at least around a certain operating point, to predict the future evolution of the system. Based on this prediction, at each time step t the controller selects a sequence of future command inputs or control signals through an on-line optimization procedure, which aims at optimizing a performance, cost or objective function, and enforces fulfillment of the constraints. Only the first sample of the optimal sequence of future command inputs is actually applied to the system at time t. At time t+1, a new sequence is evaluated to replace the previous one. This on-line re-planning provides the desired feedback control feature. 
     Model Predictive Control can be applied for stabilizing a MLD system to an equilibrium state or to track a desired reference trajectory via feedback control. Standard routines can then be used to recast the previous control problem into a Mixed Integer Linear Programming (MILP) of the following form: 
                         min     v   _       ⁢     f   ⁡     (     v   _     )         =         min     v   _       ⁢         g   _     T     ⁢     v   _     ⁢           ⁢     s   .   t   .           ⁢   A     ⁢     v   _         ≤     b   _         ,           (     Eq   ⁢   .2     )               
where the optimization vector  v  includes the aforementioned MLD input and auxiliary variables u, δ, z via  v   T =[ u   T  δ T    z   T ], the vector  g  denotes a cost vector, the matrix A is a constraint matrix and the vector  b  is termed a boundary vector. Standard procedures can be adapted to handle time varying parameters such as cost and price coefficients. The solution of the MILP problem can be obtained by reverting to the commercial optimisation problem solver for solving linear, mixed-integer and quadratic programming problems called CPLEX (http://www.ilog.com/products/cplex/).
 
     Model predictive control (MPC) in combination with Mixed Logical Dynamical (MLD) systems descriptions has been used for modeling and control of processes in the utility automation and process industry. By way of example, a method of scheduling a cement production is described in the article “Using Model Predictive Control and Hybrid Systems for Optimal Scheduling of Industrial Processes”, by E. Gallestey et al., AT Automatisierungstechnik, Vol. 51, no. 6, 2003, pp. 285-293. 
     SUMMARY 
     The establishment of Mixed Logical Dynamic (MLD) systems is disclosed in order to model and control complex industrial processes, based on MLD subsystems representing sub-processes of said industrial processes and in view of an automated control, scheduling or simulation of said industrial processes. Exemplary methods and a computer program for merging two arbitrarily interconnected MLD subsystems into one combined MLD system are disclosed, as well as a method of designing a model based controller or scheduler for real-time control or scheduling of an industrial process. Further exemplary embodiments are evident from the disclosure herein. 
     In one aspect, the original MLD subsystems are related or connected via pairs of dependent input and output variables, which pairs are transformed into additional auxiliary variables of the combined MLD system. The equations describing the original MLD subsystems and involving said dependent input and output variables in turn are converted into constraints of the combined MLD system involving the additional auxiliary variables. Hence, one aspect of the disclosure concerns the automatic merging of two arbitrarily connected MLD subsystems into one composite MLD system. 
     In an exemplary embodiment, a repeated use of the merging procedure allows to generate any arbitrarily complex MLD system containing the complete description of an industrial process. Provided that the industrial process can be decomposed in several sub-processes, each MLD subsystem representing a sub-process is established independently from the others in the MLD framework, and the industrial process is reproduced as a sequence of interconnected MLD blocks. 
     In one aspect of the exemplary embodiment, an optimal control problem is generated via adding an ultimate MLD block whose unique output becomes the objective function of the problem. In combination with Model Predictive Control (MPC) techniques, both on-line optimal control problems and scheduling applications can be generated and solved. 
     Such an exemplary embodiment can simplify the modelling of complex industrial processes in the Mixed Logical Dynamic (MLD) framework and allows to automatically generate and solve optimal control problems and scheduling applications for said industrial processes. For example, this can be done by graphically representing any MLD subsystem by a block whose input/output ports match the input/output variables of the MLD formulation. In a graphical environment, any specific industrial process may then be reproduced by instantiating blocks from a library of basic MLD elements or atomic MLD blocks and by properly connecting them in order to obtain a graphical representation of the process. In case an appropriate library is available, this procedure should not require undue expert knowledge from the end user, but a mere ability to build graphical interconnections. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional features and advantages will be apparent to those skilled in the art upon reading the following detailed description of exemplary embodiments which are illustrated in the attached drawings, wherein: 
         FIG. 1  shows two exemplary MLD subsystems being merged into a combined MLD system, and 
         FIG. 2  depicts exemplary MLD subsystems representing the sub-processes of the grinding section of a cement plant. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows two exemplary MLD blocks or subsystems S 1  and S 2 , defined respectively by the matrices A 1 , B 1   1 . . . 3 , C 1 , D 1   1, . . . , 3 , E 1   1, . . . , 5  and A 2 , B 2   1 . . . . 3 , C 2 , D 2   1, . . . , 3 , E 2   1, . . . , 5 , as well as by the vectors of states  x   1 ,  x   2 , the vectors of inputs  u   1 ,  u   2 , the vectors of outputs  y   1 ,  y   2 , the vectors of real auxiliary variables  z   1 ,  z   2 , and the vectors of Boolean auxiliary variables  δ   1 ,  δ   2 . The MLD subsystem S 1 , S 2  are interconnected as indicated by the arrows joining the two blocks. The goal is to merge S 1  and S 2  in order to obtain one single or combined MLD system S described by the matrices A, B 1 . . . 3 , C, D 1, . . . , 3 , E 1, . . . , 5  and the corresponding vectors of states  x , inputs  u , outputs  y , real auxiliary variables  z  and Boolean auxiliary variables  δ . 
     The matrices of the combined system S are obtained by creating matrices comprising the corresponding matrices of the subsystems S 1 , S 2  along their diagonal as a starting point, e.g. by setting 
               A   =     [           A   1         0           0           A   2     ⁢                   ]       ,         
and by subsequently eliminating or exchanging rows and/or columns as explained below. Likewise, the vectors of the combined system S are created by augmenting the vectors of S 1  with the corresponding vectors of S 2  as a starting point, e.g. by setting  x   T =[ x   1   T    x   2   T ], and by subsequently eliminating or rearranging their elements if necessary. It is to be noted that the matrices and vectors of the combined system S are uniquely designated and maintain their designation throughout the subsequent operations performed on their rows, columns or elements.
 
     The vectors of states, inputs and outputs can contain both real (denoted with the index r) and Boolean (index b) variables. If this is the case, all the real variables will have to be grouped and placed in the corresponding vector before the Boolean ones. Among the inputs and outputs certain variables are independent of the other MLD block (denoted with the prefix i) and some are dependent (prefix d). Pairs of the dependent variables are represented in  FIG. 1  by a common arrow pointing from one MLD block (S 1 , S 2 ) to the other MLD block (S 2 , S 1 ). 
     The exemplary embodiment according to  FIG. 1  will be detailed in the following, yet less general configurations with e.g. only one, two or three pairs of dependent variables are also conceivable and can be treated in the framework of the present invention. 
     The resulting state vector  x  of the combined MLD block S consists of a composition of all the states of S 1  and S 2  and is defined as  x =[x r1 ,  x   r2 ,  x   b1 ,  x   b2 ]. Due to the real variables of both systems ( x   r1 ,  x   r2 ) being placed before the Boolean variables ( x   b1 ,  x   b2 ), the matrix A of the combined MLD system S, based on the matrices A 1 , A 2  of the subsystems S 1 , S 2  as shown above, has to have the rows and columns corresponding to  x   b1  swapped with the rows and columns of  x   r2 . Likewise, the other matrices B 1 . . . 3  involved in the state dynamic of the system S, based on the corresponding matrices (B 1   1 . . . 3 , B 2   1 . . . 3 ) of the subsystems S 1 , S 2 , undergo, in a first step, a corresponding swap of rows. Equally, in a first step a swap of columns is executed on the matrices C, E 4 ; of the system S multiplying the resulting state vector  x  and being based on the corresponding matrices (C 1 , E 1   4 ; C 2 , E 2   4 ) of the subsystems S 1 , S 2 . 
     The resulting output vector  y  of the combined MLD block S consists of a composition of the independent outputs of S 1  and S 2  and, taking into account the “real” before “Boolean” principle, is defined by y=[i y   r1 , i y   r2 , i y   b1 , i y   b2 ]. Again, the matrices involved in the output dynamic (C, D 1, . . . , 3 ) of the system S are based on the corresponding matrices (C 1 , D 1   1, . . . , 3 ; C 2 , D 2   1, . . . , 3 ) of the subsystems S 1 , S 2 . Along the same idea, the rows of these matrices corresponding to i y   b1  will have to be swapped in a first step with the rows corresponding to i y   r2 . Additionally, as the dependent outputs are lost, the corresponding rows of the C, D 1, . . . , 3  matrices have to be deleted. 
     The resulting input vector  u  of the combined MLD block S consists of a composition of the independent inputs of S 1  and S 2  and is defined as u=[i u   r1 , i u   r2 , i u   b1 , i u   b2 ]. Again, the real variables of both systems are being placed before the Boolean variables. The dependent inputs have been dropped. This implies that the input matrix B 1 , in a second step, will undergo a swap between the columns corresponding to i u   b1  and i u   r2  and, in a third step, will have to have the columns corresponding to the dependent inputs (d u   r1 , d u   b1  and d u   r2 , d u   b2 ) deleted. Likewise, the other input matrices D 1 , E 1  of the system S, being based on the matrices (D 1   1 , E 1   1 ; D 2   1 , E 2   1 ) of the subsystems S 1 , S 2 , will undergo a swap of columns between i u   b1  and i u   r2  and, in a second step, will have the columns corresponding to the dependent inputs (d u   r1 , d u   b1 ; d u   r2 , d u   b2 ) deleted. 
     The eliminated real dependent inputs d u   r1 , d u   r2  will then be substituted by additional real auxiliary variables  z   12 ,  z   21  added to the real auxiliary variables  z  of the combined system S. Thus the real auxiliary variables of the MLD system S,  z =[ z   1 ,  z   2 ,  z   12 ,  z   21 ], include the original auxiliary variables ( z   1 ,  z   12 ) of S 1  and S 2  plus the ones introduced by the S 1 -S 2  and S 2 -S 1  real interconnections. Similarly, the Boolean dependent inputs d u   b1 , d u   b2  will be replaced by additional Boolean auxiliary variables  δ   12 ,  δ   21  added to the Boolean auxiliary variables  δ  of the combined system S. This transformation is obtained or executed by appending the columns deleted from the matrices B 1 , D 1  and E 1  (observing a change of sign for the latter) to the matrices B 3 , D 3 , and E 3  if real or to the matrices B 2 , D 2 , and E 2  if Boolean. 
     The equations d y   1 =d u   2  and d y   2 =d u   1  between the dependent inputs and outputs of the two subsystems S 2  and S 1  represent the connections between the subsystems. The information contained therein has to be retained and included in the formulation of the combined MLD system S. However, as the dependent variables (d y , d u ) do not appear in the final formulation, this is not a straightforward task. In the case of the S 1 -S 2  connection (short straight arrows in  FIG. 1 ), the original expression for the dependent outputs d y   1  of subsystem S 1  is comprised in the set of equations
 
   y     1   =C   1     x     1   +D   1   1     u     1   +D   1   2   δ   1   +D   1   3     z     1 .  (Eq. 1b)
 
As a result of the row-elimination procedure above, these expressions for d y   1  are absent from the resulting equations for the output variable y of the combined MLD block S.
 
     Eq. 1b comprises terms of the type D 1   1 d u   1  wherein the dependent inputs d u   1  have to be replaced by the additional auxiliary real and/or Boolean variables ( z   12 ,  δ   12 ) introduced above. Likewise, the dependent inputs d u   2  have to be replaced by the additional auxiliary variables ( z   21 ,  δ   21 ) introduced above. Thus, the equations d u   2 =d y   1  translate into
 
[   z     i   21 / δ   i   21   ]=d y     r1   /d y     b1   ={tilde over (C)}   i   1     x     1   +{tilde over (D)}   i   1   1   i u     1   +{tilde over (D)}   i   1   2 [ δ   1   T   δ   12   T ] T   +{tilde over (D)}   i   1   3   [ z     1   T     z     12   T ] T   (Eq. 1d)
 
where the tilde and the subscript matrix index i({tilde over (C)} i   1 , {tilde over (D)} i   1   1 . . . 3 ) signify that only the i-th rows and the relevant columns of the matrices C 1  and D 1   1 . . . 3  are exploited. This equality constraints involving only state variables ( x   1 ), independent input variables (i u   1 ) and auxiliary variables [ z   1    z   21    z   12 ;  δ   1    δ   12    δ   21 ] obtained with the previous manipulations or substitutions can therefore be added as additional rows to the matrices E i  of the combined MLD system S. Additional constraints in terms of MLD inequalities are formed by turning every single equality or line in the above vector equation Eq. 1d into two inequalities or rows by noting the former both in form of &lt;= and in the form of &gt;=. The case of the S 2 -S 1  connection (long returning arrows in  FIG. 1 ) is similar and results in exchanged indices.
 
     The combined or merged MLD system S resulting from the procedure outlined above in turn can be considered as a subsystem itself. Via iterated application of the procedure, a complex MLD system S tot  representing all relevant technical aspects of an arbitrarily complex industrial installation or process can be generated. Thereby order of merging of the “basic” MLD subsystems or “atomic” MLD blocks does not have any impact on the final result and may be chosen at will. 
     In order to use the MLD system for a model based control procedure an objective function or cost functional ƒ has to be defined. In the context of the present invention, this is most conveniently done via adding a functional MLD subsystem S cost  comprising exactly one independent scalar output variable y cost  representing the objective to be optimized. This functional MLD subsystem S cost  is ultimately merged to the complex MLD system S tot  representing all relevant technical aspects of the physical process. Thereby, all the outputs of the complex MLD system S tot  that are related to costs are regarded as dependent outputs with corresponding dependent inputs of the functional MLD subsystem S cost . Obviously, there is no restriction on the number of independent inputs u serving as control variables or command inputs of the single-output MLD system S that results from the aforementioned ultimate merger. 
     The exemplary method is illustrative and not restrictive. Such exemplary methods can be applied to many industrial processes with a limited effort. In the following, a particular implementation of the exemplary method in a cement plant is presented in more detail with reference to  FIG. 2 . 
     Cement mill scheduling implies deciding when to produce a certain cement grade on which mills. Due to the number of mills, grades, silos, conveyor belts and the various operating constraints the problem is quite complex. In a hybrid system framework, the mills are treated as finite state or on/off machines that associate different energy consumptions and throughputs depending on the cement grade produced. Generally, a library of basic MLD blocks or subsystems is provided for each basic element or sub-process of the complex industrial process being modeled. In a cement mills scheduling example, these blocks typically consist of the MLD formulation for a “mill”, “silo” and “electricity”. In particular, the representation of the grinding section of a cement plant according to  FIG. 2  comprises one MLD block associated to the Cement Mill (one input, three outputs), one MLD block associated to the Cement Silo (one input) and one associated to the Electricity section (one input, two outputs). The optimal control problem is generated via adding a functional or cost MLD block, of which the unique output variable y cost  becomes the objective function of the problem. The next step is to iteratively merge two interconnected MLD blocks to obtain a combined block, until only one block is left. The final MLD block, resulting from the merging process, will have as inputs all the independent inputs of the various MLD subsystems, i.e. the input “production” in the exemplary embodiment of  FIG. 2 , and as output the unique output of the cost block, representing the objective function of the optimal control problem. The arrows in  FIG. 2  represent interconnections between the individual blocks or subsystems just as in  FIG. 1 . 
     It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.