Abstract:
In a check schedule decision method, after simulating the production line to produce a time-varying number of in-process workpieces in a designated process unit, a check-available time period for the designated process unit is detected based on a comparison of the time-varying number of in-process workpieces to a predetermined level. Further, the production line is simulated under a constraint that the designated process unit is stopped operating during the check-available time period to produce a simulated output of the production line being simulated. When the simulated output is not smaller than a predetermined target figure, a check schedule of the designated process unit is decided within the check-available time period.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to management of a production line including a plurality of process units, and in particular to a method for deciding an inspection schedule of the process units. 
     2. Description of the Related Art 
     A production line having a plurality of process units arranged in series must be periodically checked for proper and stable operation. Since a process unit must stop operating in order to be checked, it is very important to decide upon an inspection schedule of the process units so that an output of the production line is not decreased by a large amount. 
     Conventionally, a simulation technique is used to decide such upon an on schedule. More specifically, a model of the production line is created by adding many constraints and then a managing operator explores the model&#39;s behavior displayed on screen, that is, a change of the number of in-process workpieces in the simulated production line. Based on the time-varying number of in-process workpieces, the managing operator decides the inspection schedule of the process units. 
     However, such a model is only as good as its input constraint data. If the input constraint data is not correct, the model does not accurately mimic the behavior of the real-world production line being simulated. In the case where the simulation is performed without consideration of the speed of workpieces conveyed, for example, the time-varying number of in-process workpieces substantially departs from that of the real-world production line, resulting in reduced reliability in line control. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method which decides a checking schedule so as to provide reliable and stable operation of a production line even while some process units are being checked. 
     Another object of the present invention is to provide a method which determine a checking schedule so as to provide an acceptable output while maintaining an acceptable number of in-process workpieces in a production line even while some process units are being checked. 
     According to the present invention, after simulating the production line to produce a time-varying number of in-process workpieces in a designated process unit, an available checking time period for the designated process unit is detected based on a comparison of the time-varying number of in-process workpieces versus a predetermined level. Further, the production line is simulated under a constraint that the designated process unit is stopped operating during the available checking time period to produce a simulated output of the production line being simulated. When the simulated output is not smaller than a predetermined target figure, a check schedule of the designated process unit is decided within the available checking time period. 
     In the case where a plurality of available checking time periods are detected for the designated process unit, it is preferable that the production line is simulated under a constraint that the designated process unit is stopped operating during a selected available checking time period while sequentially selecting the available checking time periods until the simulated output obtained by this step is not smaller than the predetermined target figure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing an embodiment of an inspection scheduler according to the present invention; 
     FIG. 2 is a flowchart showing an embodiment of an inspection schedule decision method according to the present invention; 
     FIG. 3 is a waveform diagram showing an example of the time-varying number of in-process workpieces in the embodiment; and 
     FIG. 4 is a schematic diagram showing an example of simulation result to explain an operation of the embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a periodic-inspection scheduler which uses a simulation technique to decide on an inspection schedule of process units arranged in series forming a production line. In the periodic-inspection scheduler, a processor 101 such as a CPU runs a periodic-inspection schedule decision program stored in a memory 102 while using a production line simulator 103, as will be described later. Needless to say, a simulation program of the production line simulator 103 may be incorporated into the periodic-inspection schedule decision program stored in the memory 102. 
     An input device 104 is used to input production line data required for the simulation and a monitor 105 displays the result of a periodic-inspection schedule decided by the program on screen. The production line data includes a maximum capacity and a processing time of each process unit, the processing sequence of the process units, the number of workpieces to be processed, the number of in-process workpieces in each process unit, and a target output of the production line. 
     The descriptions of first and second embodiments of the inspection-schedule decision method according to the present invention will be described hereinafter referring to FIGS. 2-4. 
     FIRST EMBODIMENT 
     Referring to FIG. 2, the above production line data together with a simulation period Ts and designation data for designating at least one process unit to be checked are input by the managing operator through the input device 104 (step S201). When receiving the above data, the processor 101 starts the production line simulator 103 performing the simulation using the production line data in the designated simulation period Ts (step S202). 
     The simulator 103 returns the time-varying number Np of in-process workpieces in each process unit and an output N RESULT  of the simulated production line to the processor 101. Based on the time-varying number Np of in-process workpieces in each process unit, the processor 101 extracts a time period Tc during which the time-varying number Np is not greater than a predetermined value Nb which is X% of an average of number of workpieces which would be processed by each process unit to be checked (step S203). For example, the value Nb is set to 10% of an average of number of processed workpieces a day in each process unit. 
     In the first embodiment, a single process unit is designated as a process unit to be checked. As a matter of practicality, it is useful to decide the checking period of a single process unit in the production line. More specifically, in the case where the number Np of in-process workpieces in a single process unit varies with time as shown in FIG. 3, the processor 101 compares Np with Nb and, in this case, extracts two time periods Tc 1  and Tc 2  within the simulation period Ts. There may be cases where such a time period is not found. These time periods Tc 1  and Tc 2  are used as available checking periods or candidate periods in the following steps. 
     If there is at lease one check-available period within the simulation period Ts (YES in step S204), one available checking period Tc is selected and then it is determined whether the available checking period Tc is equal to or longer than the checking time T CHECK  required to check that process unit (step S205). When Tc≧T CHECK  (YES in step S205), the checking period of the process unit is set to the available checking period Tc (step S206) and then the simulation is performed under that constraint (step S207). In other words, under the constraint that the process unit is stopped operating during the check time T CHECK  within the available checking period Tc, the simulation of the production line is performed again by the simulator 103. 
     The simulator 103 returns a simulated output N RESULT  of the production line to the processor 101. If the simulated output N RESULT  is equal to or greater than the target output N TARGET  (YES in step S208), the check time T CHECK  of the process unit is determined within the available checking period Tc and is displayed on the monitor 105 (step S209). 
     On the other hand, when Tc&lt;T CHECK  (NO in step S205) or when N RESULT  &lt;N TARGET  (NO in step S208), a subsequent available checking period is selected (step S210) and then control goes back to the step S204. Further, if there is no available checking period within the simulation period Ts (NO in step S204), the simulation period Ts is extended (step S211) and the simulation is performed again for the extended simulation period Ts. 
     In FIG. 3, for example, if the time period Tc 1  is shorter than T CHECK  or if N RESULT  &lt;N TARGET  for the time period Tc 1 , another time period Tc 2  is selected and the above steps S204-S209 are performed for the time period Tc 2 . Further, since the available checking period Tc is selected so that the time-varying number Np is smaller than the predetermined value Nb which is 10% of an average number of workpieces which would be processed by the process unit (step S203), the effect of the inspection on an output of the production line would be small. Especially, since the time period TC 2  provides a lower Np than the time period Tc 1 , a better balance of the number of in-process workpieces and a better output would be obtained, resulting in reliable and stable operation of the production line. 
     SECOND EMBODIMENT 
     As shown in FIG. 4, an available checking period Tc may be obtained for each of a plurality of process units to be checked. It is assumed for simplicity that two process units P i  and P j  of the process units P 1  -P N  are designated as a process unit to be checked in simulation. 
     In the second embodiment, when receiving the respective time-varying numbers N i  and N j  of in-process workpieces in the process units P i  and P j  from the simulator 103, the processor 101 compares Np i  with Nb i  and extracts time periods Tc i1  and Tc i2  as described before. Similarly, the processor 101 compares Np j  with Nb j  and extracts a time period Tc j1 . Therefore, if these time periods satisfy the conditions of steps S204 and S205 of FIG. 2, two checking schedule candidates are obtained, that is, a first checking schedule candidate of Tc i1  and Tc j1  and a second checking schedule candidate of Tc i2  and Tc j1 . 
     Subsequently, the processor 101 performs the simulation under the constraint of the first checking schedule candidate of Tc i1  and Tc j1 . In other words, under the constraint that the process units P i  and P j  are stopped operating during the check time T CHECKi  within the check-available period Tc i1  and the check time T CHECKj  within the check-available period Tc j1 , respectively, the simulation of the production line is performed again by the simulator 103. If the simulated output N RESULT  is equal to or greater than the target output N TARGET , the first checking schedule of T CHECKi  and T CHECKj  is determined. 
     On the other hand, if the simulated output N RESULT  is smaller than the target output N TARGET , the second checking schedule candidate of Tc i2  and Tc j1  is selected, and then the simulation under that constraint is performed. When there is no checking schedule candidate, the simulation period Ts is extended and the above steps will be repeated until the simulated output N RESULT  is equal to or greater than the target output N TARGET .