Patent Publication Number: US-9888620-B2

Title: Allocation of printed circuit boards on fitting lines

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent document is a § 371 nationalization of PCT Application Serial Number PCT/EP2013/058523, filed Apr. 24, 2013, designating the United States, which is hereby incorporated by reference, and this patent document also claims the benefit of DE 10 2012 211 814.4, filed on Jul. 6, 2012, which is also hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present embodiments relate to a method for allocating printed circuit boards on fitting lines for fitting the printed circuit boards or other subassemblies with components. The embodiments also relate to a control device for a production or assembly line for fitting printed circuit boards with components. The embodiments also relate to a computer program product and to a computer-readable medium. 
     BACKGROUND 
     In the field of electronics production, in particular, printed circuit boards or subassemblies to be produced are produced on surface mounted technology (SMT) fitting lines by surface mounting. On account of technical restrictions, however, not every printed circuit board may be produced on every fitting line. The printed circuit boards also may have different production times on the fitting lines. In addition, the maximum production time capacities of the fitting lines is not exceeded. 
     DE 10 2009 013 353 B3 shows a method for equipping such a fitting line. 
     Printed circuit boards on fitting lines of a fitting system may be allocated manually or semi-automatically on the basis of empirical values or heuristics. In this case, it has been shown in practice that unbalanced allocations are being made again and again, which cause high utilization of one component of a fitting line with low utilization of another component, with the result that the fitting system may not be used in an optimal manner. 
     SUMMARY AND DESCRIPTION 
     The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary. The present embodiments may obviate one or more of the drawbacks or limitations in the related art. 
     The object of the present embodiments is to provide an improved technique for allocating printed circuit boards on fitting lines. 
     A fitting system includes a plurality of fitting lines for fitting printed circuit boards with electronic components. A method for allocating printed circuit boards to the fitting lines includes acts of detecting requirements for fitting a plurality of printed circuit boards with respectively allocated components and allocating the printed circuit boards to fitting lines under predetermined specifications by integer linear programming. Equipment families for the fitting lines are determined on the basis of the allocation and the allocation is repeated until a criterion has reached a predetermined threshold. The criterion is formed on the basis of the numbers of equipment families of the fitting lines. 
     In this case, an equipment family is determined as a set of printed circuit boards that may be fitted with components on a fitting line without changing the set of component types that are kept ready at the fitting line for fitting. The set of component types kept ready at the fitting line is also called equipment. A sufficiently large number of components of each component type may be kept ready at the fitting line. 
     More printed circuit boards than may be included in an equipment family may be allocated to a fitting line since an arbitrary number of component types may not be kept ready at the fitting line. The fitting line is therefore occasionally subjected to an equipment change in which the equipment for a first equipment family is exchanged for the equipment for a second equipment family. The rarer these equipment changes are and the fewer component types that have to be exchanged during an equipment change, the more cost-effectively the fitting system may be operated. 
     It has been shown that the total number of equipment families used in the fitting system may be a more realistic quality indicator than component variance of a fitting line, for example. In this case, the component variance of a fitting line is given by the number of different components that are fitted to one of the printed circuit boards allocated to the fitting line. The method may therefore determine allocations of printed circuit boards to fitting lines that allow improved utilization of the entire fitting system. 
     In addition, the method may be flexibly configured to actual conditions or desires of an operator of the fitting system. A global optimization approach may be achieved by using integer linear programming. It is possible to extend the method. Runtime environments for carrying out an optimization by integer linear programming are available in the form of commercial products, for example, Ilog or Xpress. Such so-called standard solvers are continuously being improved, with the result that it may be expected that the allocation using the described method will be able to be carried out even more quickly and/or with further optimization in future. 
     The described method may allow a very good allocation of printed circuit boards to fitting lines within a relatively short runtime. The method may provide improved results with respect to the quality of the results or the required runtime in comparison with other optimization approaches. In addition, the method may be used to find allocations of printed circuit boards to fitting lines that allow balanced numbers of equipment families and the formation of an increased number of constant shuttle tables. 
     In order to determine the equipment families, a method that is separate from the integer linear programming may be called. The separation of integer linear programming and determination of the assessment criterion makes it possible to avoid the situation in which the integer linear programming has to process an excessively complex allocation problem and therefore either needs a long time in order to create an allocation or the created solution is less effective. 
     If the solver is sufficiently powerful, the allocation may also be carried out, in another embodiment, directly with respect to minimizing the number of equipment families for the fitting lines by integer linear programming. 
     In one embodiment, the criterion includes the condition that the number of equipment families of each fitting line is greater than a predetermined threshold. Convergence of the optimization by integer linear programming may be accelerated thereby. 
     In another embodiment, the criterion includes the condition that the number of equipment families of each fitting line is less than a predetermined threshold. An excessively high utilization of one of the fitting lines may be prevented thereby. 
     In different variants, one or both of the above-mentioned predetermined thresholds may be individually predetermined for one or more of the fitting lines. This makes it possible to take into account an empirical value per se as a threshold that has proved to be useful. 
     In one embodiment, the criterion includes the condition that differences between the numbers of equipment families of each fitting line are below a predetermined threshold. An allocation of printed circuit boards to fitting lines may therefore be optimized to the effect that the fitting lines have the same numbers of equipment families as far as possible. A compromise between high allocation quality and the method runtime may be chosen in an improved manner by increasing or decreasing the predetermined threshold. In one embodiment, the threshold may also be zero. In another embodiment, the criterion may also include a quantified difference between the numbers of equipment families of the individual fitting lines. In this case, the allocation of printed circuit boards to fitting lines may also be optimized with respect to the smallest possible difference in repeated iterations of the method. 
     In another embodiment, the allocation of printed circuit boards in an equipment family to a fitting line is prevented if the equipment family is allocated a small number of printed circuit boards or a small track consumption. For the number of printed circuit boards and the track consumption, it is possible to respectively state a threshold below that the respective value is recognized as “low”. Devices that are intended to stockpile different components at a fitting line in this case include a predetermined number of tracks that may be each 8 mm wide. This makes it possible to force a reallocation of printed circuit boards in the relevant equipment family to another fitting line. 
     In a similar manner, an allocation of a printed circuit board to a fitting line may also be fixed by virtue of a change in an allocation that has already been made being prevented if the corresponding equipment family is allocated a large number of printed circuit boards or a high track consumption. In this case too, it is possible to state threshold values above which the respective value is recognized as “high”. The optimization of the allocation of printed circuit boards to fitting lines may be accelerated or improved thereby. 
     A computer program product includes a program code for carrying out the described method if the computer program product runs on an execution device or is stored on a computer-readable medium. The computer program product may be created in a conventional programming language (for example, C++, Java). The processing device may include a commercially available computer or server with corresponding inputs, outputs, and storage. 
     A control device for allocating printed circuit boards to fitting lines of a fitting system is set up to carry out the method described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts one embodiment of a fitting system. 
         FIG. 2  depicts an embodiment of equipment families in the fitting system from  FIG. 1 . 
         FIG. 3  depicts an embodiment of a flowchart of an optimization method using integer linear programming. 
     
    
    
     DETAILED DESCRIPTION 
     Linear optimization is one of the main methods in the field of mathematical optimization and deals with the optimization of linear target functions over a set that is restricted by linear equations and inequalities. Linear optimization is the basis of the solution methods of (e.g., mixed) integer linear optimization. 
     Advantages of linear optimization include the following features: (1) global optimization approach; (2) easily expandable, (3) good commercial standard solvers (e.g., SCIP, CPLEX, Ilog, Xpress), which are widespread and proven in practice; and (4) for a determined solution, it is known how far away it is at most from the optimal solution (e.g., gap). 
       FIG. 1  depicts a fitting system  100 . The fitting system  100  includes a plurality of fitting lines  110  and a control device  115  for allocating printed circuit boards  120  to the fitting lines  110 . Each fitting line  110  may include a transport system  125  and one or more fitting machines  130 . Each fitting machine  130  includes one or more fitting heads  135  that are each set up to receive components from a constant table  140  or a variable table  145  and to position them at a predetermined position on the printed circuit board  120  that is on the transport system  125 . 
     During the fitting process, the printed circuit board  120  may be still with respect to the fitting machine  130 . The tables  140 ,  145  each include a multiplicity of feeding devices  150 , only one of which is illustrated by way of example. Each feeding device  150  keeps a supply of components  155  of a predetermined type ready. Although each feeding device  150  may be configured to keep different components  155  ready and different feeding devices  150  are fitted to a table  140 ,  145 , the tables  140 ,  145  may be completely exchanged for reasons of speed if a fitting machine  130  is supplied with components  155  that are not kept on one of the fitted tables  140 ,  145 . 
     Since such a change may be associated with a production standstill, the aim is to keep the number of tables  140 ,  145  to be changed low. If a table is not exchanged during a conversion process, it is referred to as a constant table  140 , otherwise it is referred to as a variable table  145 . Otherwise, there are no functional differences between a constant table  140  and a variable table  145 . 
     The printed circuit board  120  is to be fitted with a number of different components  155 . In order to minimize frequent changing of variable tables  145  and ideally to maximize a number of constant tables  140 , the control device  115  is set up to optimize the allocation of a printed circuit board  120  to one of the fitting lines  110 . In this case, specific properties of each fitting line  110  and of each fitting machine  130  may be taken into account just like properties of the printed circuit boards  120  and of the components  155  to be fitted on the latter. 
       FIG. 2  depicts an illustration  200  of equipment families, which are also called “clusters”. A first printed circuit board  205 , a second printed circuit board  210 , and a third printed circuit board  215 , which each correspond to one of the printed circuit boards  120  in the fitting system  100  from  FIG. 1 , are considered. The first printed circuit board  205  is allocated a first set  220  of components  155 , the second printed circuit board  210  is allocated a second set  225  of components  155 , and the third printed circuit board  215  is allocated a third set  230  of components  155 . In an exemplary manner, the sets  220  to  230  each include five different component types, of which components  155  are used in different numbers. Components  155  of one component type may not be distinguished from one another within the scope of the method  200  being considered. The sets  220  to  230  of components  155  are therefore allocated sets  235  to  245  of component types. Each of the components  155  in the allocated set  220  to  230  occurs only once in the sets  235  to  245 . 
     An equipment family includes those printed circuit boards  120  that are allocated for being fitted with components on a fitting line  100 . In the example from  FIG. 2 , the printed circuit boards  205  and  210  are allocated to the same fitting line  110  and form a first equipment family  250 . The first equipment family  250  therefore requires first equipment  260  with component types of the combination set of the sets  235  and  240  that are allocated to the printed circuit boards  205  and  210 . In the example illustrated, the first equipment  260  includes six component types. With the provision of the component types of the first equipment  260 , the printed circuit boards  205  and  210  in the first equipment family  250  may be fitted on the fitting line  110  without conversion. 
     The third printed circuit board  215  is allocated to a different fitting line  110  and alone forms a second equipment family  255 . Second equipment  265  allocated to the second equipment family  255  includes five component types. 
     If the sets  220  and  225  include many components  155  of the same component type, the first equipment  260  only includes a small number of different component types. An attempt may be made to allocate the printed circuit boards  120  to equipment families  250 ,  255  in such a manner that as many different printed circuit boards  120  as possible may be processed with the fitting line  110 . 
     The third printed circuit board  215  is allocated to a different fitting line  110  and alone forms a second equipment family  255 . Second equipment  265  allocated to the second equipment family  255  includes five component types. 
     If the third printed circuit board  215  was allocated to the same equipment family as the printed circuit boards  205  and  210 , the equipment allocated to the resulting equipment family would contain the component types of the combination set of the sets  235  to  245 , that is to say seven component types in the present example. If the fitting line  110  has only receiving capacities for six different components  155 , for example, the equipment family formed in this manner may not be allocated to the fitting line  110  and the third printed circuit board  215  no longer fits in the equipment family allocated to the fitting line  110 . 
     An equipment change is required in order to fit printed circuit boards  120  in another equipment family with components on the fitting line  110 . In this case, one or more feeding elements, (in particular, a variable table  145  or a conveyor  150 ), may be changed on a fitting machine  130  of the fitting line  110 . 
     The number of equipment changes may be reduced and a number of constant tables  140  may be increased by reducing the number of equipment families  250 ,  255  of the fitting line  110 . This may be achieved by skillful allocation of the printed circuit boards  120  to the equipment families  250 ,  255 . The optimization of the allocation of printed circuit boards  120  to the equipment families  250 ,  255  is explained in more detail below with reference to  FIG. 3 . 
     A method  300  for allocating printed circuit boards  120  to fitting lines  110  is illustrated in the form of a flowchart in  FIG. 3 . 
     At act  305 , a starting allocation of printed circuit boards  120  on fitting lines  110  is determined and a current allocation is equated with the starting allocation. Different heuristics that may also include manual specifications or restrictions are possible for determining the starting allocation. 
     At act  310 , a subset of printed circuit boards  120  is selected from the printed circuit boards  120  to be allocated. This selection may be made in such a manner that different subsets are selected during successive runs through the method  300 . 
     At act  315 , one or more alternative allocations of printed circuit boards  120  in the subset to the fitting lines  110  are formed, (e.g., using integer linear programming). These allocations are made with respect to  FIG. 2  in such a manner that the sum of the numbers of equipment families  250 ,  255  is minimized across all fitting lines  110 . A weighted criterion of a plurality of individual criteria may also be used for the allocation, with the result that yet further parameters may also be minimized or maximized as far as possible in order to achieve an optimized allocation. 
     At act  320 , in this case, the quality of the determined allocations is determined by forming an individual or combined criterion. The criterion is formed on the basis of a number of equipment families for the fitting lines  110 . In one embodiment, a method that is separate from the integer linear programming is called for this purpose, which method first of all forms the equipment families for the fitting lines on the basis of the determined allocation and then forms the sum of the determined equipment families over all fitting lines  110 . 
     In yet another embodiment, the criterion may include the compliance with a predetermined condition. For example, the condition may include the fact that the number of equipment families of each fitting line  110  is greater or less than a predetermined threshold, the threshold respectively being able to be individually allocated to a fitting line  110 . If the condition has been met, the criterion may be set to a first predetermined value, otherwise it may be set to a second value. It is therefore possible to prevent an allocation from being accepted if the condition has not been met. 
     In further embodiments, it may also be required as a condition that differences between the numbers of equipment families of each fitting line  110  are below a predetermined threshold. This threshold may be zero, in particular. In one variant, the differences may also be expressed in a quantified manner as a criterion, with the result that the method  300  may carry out optimization with respect to the criterion. 
     In further embodiments, the criterion may be composed of a plurality of individual criteria that may be added together or multiplied by one another using weighting factors to form an overall criterion. For example, individual criteria may relate to one or more of the above-mentioned criteria, a degree of utilization of a fitting machine  130 , the ratio of constant tables  140  to variable tables  145  or a degree of utilization of a fitting machine  130 . 
     At act  325 , a check is then carried out in order to determine whether a predetermined abort criterion has been reached. The abort criterion has been satisfied if the criterion or the overall criterion from act  320  has reached a predetermined threshold value. The allocation may also be aborted if a predetermined determination time for the method  300  has expired. This makes it possible to prevent endless optimization of the allocation. 
     At act  330 , the determined allocations are output if the abort criterion has been satisfied. 
     At act  335 , otherwise, those allocations from the determined allocations that are intended to be optimized further may be selected. For example, the allocation determined last may be dropped in favor of an allocation determined in a previous run if the older allocation has a better criterion or overall criterion. 
     At act  340 , in this case, the current allocation is set to one of the allocations to be optimized and the method  300  may run through again from act  310 . If a plurality of allocations that may be optimized have been selected, the method  300  may also be accordingly branched repeatedly in a parallel manner. 
     Mathematical Background 
     The use of exact mathematical methods makes it possible to achieve considerably better solutions than with heuristics previously used in practice. In further contrast thereto, good production times may also be achieved with these methods. 
     When allocating printed circuit boards  120  or subassemblies to a fitting line  110 , not every printed circuit board  120  may be produced on every fitting line  110  on account of technical restrictions. The printed circuit boards may also have different production times on the fitting lines  110 . In addition, the maximum production time capacities of the fitting lines  110  is not exceeded. 
     When allocating printed circuit boards  120  on fitting lines  110 , the following aims may be pursued: (1) as many printed circuit boards  120  as possible may be able to be produced using constant tables  140  in order to reduce the conversion effort; (2) the number of equipment families (“clusters”) on the fitting lines  110  may be as small as possible in order to reduce the amount of conversion time; (3) as little equipment as possible (for example conveyor  150 ) may be required; and (4) the total production time for the printed circuit boards  120  may be as minimal as possible. 
     An attempt may be made to achieve these aims by striving for the best possible component coverage of the printed circuit boards  120  of a fitting line  110  or minimizing the sum of component variances of the fitting lines  110 . 
     IP models (IP stands for integer programming or integer program or integer optimization model) are used to determine an optimized allocation of printed circuit boards  120  to fitting lines  110 . This determination may be carried out using a known standard solver. 
     Indices 
     
         
         L Set of SMT fitting lines  110  of the system  100   
         R Set of printed circuit boards  120   
         C Set of component types  155   
         R c  Set of printed circuit boards with component type c 
         R l  Set of printed circuit boards that may be fitted with components on line l
 
Parameters
 
Time r,l  Total production time for printed circuit board r on line l
 
TimeLimit l  Production time limit on line l
 
Binary Variables
 
Assign r,l  Allocation of printed circuit board r to line l
 
Setup c,l  Use of a component c on a line l
 
IP Formulation
 
       
    
     
       
         
           
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     In order to improve the allocation of printed circuit boards  120  to fitting lines  110 , the solver may be provided with conditions. The conditions imposed are aimed at optimizing the allocation of printed circuit boards  120  on the fitting lines  110  in such a manner that the sum of the numbers of equipment families of all fitting lines  110  of the fitting system  100  is minimized. 
     Using this specification makes it possible to achieve an improved distribution of the printed circuit boards  120  across the fitting lines  110 , in which case balanced sizes of equipment families may be achieved. The fitting system  100  may therefore be operated with improved efficiency or utilization. In addition, the allocations may be determined more quickly than previously known with the described approach using an IP solver. 
     Although the invention was illustrated and described more specifically in detail by the exemplary embodiments, the invention is not restricted by the disclosed examples and other variations may be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention. 
     It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification. 
     While the present invention has been described above by reference to various embodiments, it may be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.