# New scheduling rules for a dynamic flexible flow line problem with sequence-dependent setup times

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## Abstract

In the literature, the application of multi-objective dynamic scheduling problem and simple priority rules are widely studied. Although these rules are not efficient enough due to simplicity and lack of general insight, composite dispatching rules have a very suitable performance because they result from experiments. In this paper, a dynamic flexible flow line problem with sequence-dependent setup times is studied. The objective of the problem is minimization of mean flow time and mean tardiness. A 0–1 mixed integer model of the problem is formulated. Since the problem is NP-hard, four new composite dispatching rules are proposed to solve it by applying genetic programming framework and choosing proper operators. Furthermore, a discrete-event simulation model is made to examine the performances of scheduling rules considering four new heuristic rules and the six adapted heuristic rules from the literature. It is clear from the experimental results that composite dispatching rules that are formed from genetic programming have a better performance in minimization of mean flow time and mean tardiness than others.

### Keywords

Scheduling Dynamic flexible flow line Simulation Heuristics Genetic programming## Introduction

Scheduling involves the allocation of resources over a period of time to perform a collection of tasks (Baker 1974). It is a decision-making process that plays an important role in most manufacturing and service industries (Pinedo 1995). The hybrid flow line (HFL) scheduling problem is defined in the literature, e.g., Kianfar et al. (2012) and Gómez-Gasquet et al. (2012). In HFL, there are *g* stages and there is at least one stage with more than one machine where the jobs arrive continuously during time and pass the stages sequentially from stage one through *g* with the same order. If the jobs skip some stages, it is called flexible flow line (FFL) scheduling problem, e.g., Kurz and Askin (2004), Quadt and Kuhn (2005) and Kia et al. (2010).

Salvador (1973) proposed for the first time a definition of the HFL problem for minimizing makespan. He presented a branch and bound method to solve the problem. A double-stage hybrid flow shop problem, with one machine in stage two, is examined for minimizing makespan by Gupta (1988). He proved that the problem was NP-hard and developed a heuristic rule for it. Therefore, FFL is NP-hard. Sawik (1993) proposed a heuristic rule to minimize makespan for a limited buffer FFL problem and later proposed a new rule for the same problem with no in-process buffer (Sawik 1995). Kia et al. (2010) proposed two new scheduling rules with sequence-dependent setup times (SDST) considering non-zero job arrival times for a dynamic FFL problem. Although several papers have been written on the extent of hybrid flow shop and hybrid flow line, most of them are limited to a special case of a double stage (e.g., Gupta 1988; Guinet et al. 1996) or to a particular framework of machines in each stage (e.g., Kochhar and Morris 1987; Sawik 1993, Sawik 2002; Mirabi et al. 2014; Maleki-Darounkolaei et al. 2012). The papers that were in FFL with non-zero job arrival times focused on the heuristic rules that were rarely seen, except Kia et al. (2010). Jolai et al. (2012) proposed a novel hybrid meta-heuristic rule in FFL with SDST. A new dispatching rule for two-stage FFL is proposed by Li et al. (2013). Finally, a comprehensive survey in scheduling problems is presented by Allahverdi (2015) and also Neufeld et al. (2016).

In this paper, scheduling rules are studied to solve a dynamic flexible flow line scheduling problem with SDST using simulation. We used genetic programming to create new composite dispatching rules and a discrete-event model that examined the performance of scheduling rules in terms of mean flow time and mean tardiness.

The rest of the paper is organized as follows. The definition of the problem is given in “Problem definition”. “Problem mathematical modeling” introduces a mathematical model for considering the problem. The next section “Scheduling rules” presents those rules adapted from the literature. The genetic programming framework that is illustrated for generating composite dispatching rules is represented in “Genetic programming”. The next section is the “Simulation model”, and in “Experiment design” the details of experiments are designed and presented for scheduling rules. “Experiment results” provide the results and analyses, and the final section is the “Conclusion”.

## Problem definition

The number of stages is

*g*and also there is at least one stage with more than one machine.All jobs visit all stages through stage g, while skipping some stages is possible.

Machines that are placed in each stage are identical.

All machines are always available.

Preemption is not allowed.

There is no buffer constraint.

A machine can process at most one job at a time and a job can be processed by at most one machine at a time.

No job can be processed in one stage, except when the processing had been completed on the previous stage.

All jobs are not in the system from the beginning, but they can continuously enter over time.

The sequence-dependent setup time is assumed for every job on each machine.

The setup time of every job on each machine is sequence dependent.

When a job comes into the system, its characteristics, i.e., processing times in each stage, setup times and due dates, are identified.

There is no priority between jobs.

There is no machine breakdown.

## Problem mathematical modeling

In this section, the problem mathematical model is presented. The problem model is 0–1 mixed integer linear programming.

### Notation

*n*:Number of jobs.

- \(i, j\):
Index of the number of job, \(i,\, j = 1, 2, \ldots , n\).

*g*:Number of stages in the shop.

*l*:Index of the stage, \(l = 1, 2, \ldots , g\).

*m*^{l}:Number of parallel machines at stage

*l.**k*:Index of the machine, \(k = 1, 2, \ldots , m^{l}\).

*r*_{i}:Arrival time of job

*i.**d*_{i}:Due date of job

*i.**p*_{i}^{l}:Processing time of job

*i*at stage*l.**s*_{ij}^{l}:Setup time from job

*i*to job*j*at stage*l.**C*_{i}^{l}:Completion time of job

*i*at stage*l.**T*_{i}:Tardiness of job

*i.**F*_{i}:Flow time of job

*i.**X*_{ijkl}:1 if job

*j*is processed immediately after job*i*on machine*k*at stage*l*, otherwise 0.

### Mathematical model

Eq. (1) shows the linear convex combination of the dual criteria problem. The objective function is minimization of mean flow time and mean tardiness. The constraints (2) and (3) calculate the mean flow time and mean tardiness value, respectively. The constraints (4) and (5) guarantee assigning only one job to each sequence position at each stage. Furthermore, the constraints (6) and (7) guarantee assigning one job to the first and last sequence position on each machine at each stage, respectively. It is clear that job 0 and job (*n* + 1) are not real jobs and just stated for formularization. The constraint (8) ensures that each job at each stage is processed once. The constraint (9) forces consistent sequence at each stage. The constraint (10) ensures that job processing cannot be started before release time of the job at the first stage. The constraint (11) forces that just at the first stage, the completion time for each job cannot be less than the sum of the release, processing time and setup time of that job. The constraint (12) states that at each stage on the particular machine, starting the processing of the next job before completing the previous job is not possible. The constraint (13) states that for each job, its processing at the next stage cannot be started before completing it at the previous stage. The constraint (14) states that completion time for each job at each stage is non-negative. The constraint (15) determines the tardiness value for each job and constraint (16) states that the tardiness is non-negative. The constraint (17) determines the flow time value for each job and, finally, the constraint (18) shows that the problem variables are binary.

## Scheduling rules

*Simple priority rules*(*SPR*): These rules usually consist of just one parameter and are suitable for single-objective problems such as process time and due date.*Composite dispatching rules*(*CDR*): These rules consist of the application of a combination of several SPRs, and when the machine becomes free then this CDR evaluates the queue and then chooses a job with the most priority level for processing on the machine.

If the CDR is made well, then it is proper for solving real multi-objective problems. In the literature, e.g., Barman (1997), it is clear that CDRs have a better performance than SPRs. Furthermore, Jayamohan and Rajendran (2000) stated that there were no rules with a good performance considering flow time and due date. So, we intend to indicate the efficiency of the proposed new CDRs and also compare them with the six scheduling rules in Kia et al. (2010) that are presented by considering two objectives of mean flow time and mean tardiness for the DFFL environment. The six scheduling rules in Kia et al. (2010) that are adapted for this study are as follows:

*Earliest modified due date (EMDD)*: In this rule at each stage whenever a machine becomes free, a job is to be chosen that has the highest priority considering the earliest modified due date among all jobs waiting in the queue for processing. The modified due date of job

*i*on the stage of

*q*at the time of

*t*is calculated as follows:

*Wilkerson and Irvin’s rule (W&I)*: According to this scheduling rule at each stage whenever a machine is free, a job with the highest priority is chosen between two jobs

*i*and

*j*waiting in the queue for processing. This priority is stated by Eq. (20). If Eq. (20) is true, then the job with a shorter processing time is selected; otherwise, the job with an earlier due date will be selected. In fact, W&I rule uses both SPT and EDD according to the system status:

*Hybrid shortest processing time and cyclic heuristics (HSPTCH):* In this rule at each stage whenever a machine is free at the first step, jobs waiting in the queue for processing on the machine are arranged according to SPT. In the second step, a job that has minimum completion time on the machine relative to the other jobs waiting in the queue, according to the sequence-dependent setup time of that job, is allocated to the machine.

*Hybrid least work remaining and cyclic heuristics (HLWKRCH):* In this rule at each stage whenever a machine is free at the first step, jobs waiting in the queue for processing on the machine are arranged according to the least total remaining process time. In the second step, jobs possessing minimum completion time on the machine, in relation to the other jobs waiting in the queue, is allocated to the machine according to the sequence-dependent setup time of that job.

*Hybrid earliest modified due date and cyclic heuristics (HEMDDCH):* In this rule at each stage whenever a machine becomes free at the first step, jobs waiting in the queue for processing in the machine are arranged according to EMDD. In the second step, a job having the least completion time on the machine, in relation to the other jobs waiting in the queue, is allocated to the machine, according to the sequence-dependent setup time of that job.

*Hybrid Wilkerson & Irvin and cyclic heuristic (HW&ICH):* In this rule at each stage whenever a machine is free at the first step, jobs waiting in the queue for processing in the machine are arranged according to *W&I*. In the second step, a job having the least completion time on the machine in relation to the other jobs waiting in the queue is allocated to the machine according to the sequence-dependent setup time of that job.

In this paper, we focus on a computational method to make an effective CDR to solve a DFFL problem by a suitable algebraic combination of SPRs, but due to the width of the operator and parameter space, CDR’s efficiency evaluation is very difficult in comparison with applying SPRs manually. So, we used genetic programming to evaluate it.

## Genetic programming

Genetic programming (GP) is one of the evaluation computing methods based on the survival and reproduction principle (Koza 1992). Each individual, i.e., computer program, is a syntax tree in the random initial population produced in GP including a set of function and terminals; thus, it is essential to select the function and terminal set accurately to create proper CDRs for solving DFFL problems. The function and terminal set and GP parameter setting are stated in the two following subsections.

### Function and terminal set

There are various function and terminal sets that can affect the results’ quality and efficiency. Each of these terminals includes a dispatching rule that only a few of them are used due to the reduction of the search space.

Terminal set

Terminal | Terminal meaning |
---|---|

ReleaseDate | Release date of a job (RD) |

DueDate | Due date of a job (DD) |

ProcessingTime | Processing time of a job for each operation (PT) |

CurrentTime | Current time (CT) |

RemainingTime | Remaining processing time of each job (RT) |

avgTotalProcTime | Average total processing time of each job (aTPT) |

Function set

Function | Function meaning |
---|---|

+ | Addition |

− | Subtraction |

× | Multiplication |

/ | Division |

ADF(x1,x2) | Automatically defined function |

avgTotalProcTime | Average total processing time |

### GP parameter setting

Choice of parameter values for GP

Parameter | Parameter values |
---|---|

Population size | 100 |

Number of generations | 200 |

Creation type | Ramped half and half |

Maximum depth for creation | 5 |

Maximum depth for crossover | 15 |

Crossover probability | 90% |

Swap mutation probability | 3% |

Shrink mutation probability | 3% |

Number of best rules copy to new generation | 4 |

It generates the first half randomly with a maximum depth of 5 and the second half with a variable depth between 1 to maximum depth. The population (rules) size is 100 and we generate it 200 times. We maintain variation via crossover, mutation, and the creation type ramped half and half. At each time, we arrange the generated population based on the performance measurement and then copy the four best rules in the following population to be preserved and not to be deleted in the next generation. The information of parameter values is summarized in Table 3.

## Simulation model

Conditions like random arrival times, machine breakdowns and due date changes state a dynamic scheduling environment. The simulation is one of the ways to analyze the dynamic environment. In this paper, the arrival time of jobs is random, so a DFFL environment is present. A developed discrete-event simulation model is presented to evaluate the four best CDRs and the six heuristic rules. We use C++ programming language on the PC with 2.2 MHz CPU and 512 MB RAM.

*g*) is fixed and equals 8. The probability of job skip in each stage is defined in three separate levels of (0.00, 0.05 and 0.40) and the occurrence probability of these three levels is equal. The number of machines in the

*l*th stage (

*m*

^{l}) also follows uniform distribution [3–5]. The due date is also calculated according to Naderi et al. (2009) as follows:

*p*

_{i}is the total process time of the job

*i*,

*s*

_{i}the total mean setup times of job

*i*at all stages,

*r*

_{i}the arrival time of job

*i*and random a random number with uniform distribution of (0,1). The mean interval time parameter follows the Poisson distribution and is calculated as follows:

*μ*

_{p}is the mean process time of every job at each stage,

*μ*

_{g}the mean number of non-skipped stages of each job,

*U*the percentage of workshop utilization and

*M*the mean of the total number of machines in the shop. Since

*μ*

_{g}equals

*g*× (1 −

*μ*

_{skip}) and

*M*equals

*g*×

*μ*

_{m}, we have

*μ*

_{skip}is the mean skip probability and

*μ*

_{m}the mean machine number parameter at each stage. In this study, the parameter values are

*μ*

_{p}= 40,

*μ*

_{skip}= 0.15,

*μ*

_{m}= 4,

*α*= 9 and 10.

Simulation parameters

Parameter | Value |
---|---|

Number of jobs ( | 1450 |

Number of stages in the shop ( | 8 |

Number of parallel machines at stage | Uniform [3–5] |

Processing time of job | Uniform [20–60] |

Setup time of job | Uniform between [4–12] and [8–24] |

Skipping probability of each job at each stage | 0.00, 0.05 and 0.40 |

Mean of inter-arrival time ( | 9–10 |

- (I)
Initialization.

- (II)
Job data generation.

- (III)
Timing.

- (IV)
Events.

- (V)
Scheduling.

- (VI)
Status.

- (VII)
Report.

- Step 1:
Run module (I) and set type of scheduling rule.

- Step 2:
Run module (II).

- Step 3:
Run module (III) and set

*t*=*0*. - Step 4:
Run module (IV) to determine the next event and advanced simulation time to the next event time.

- Step 5:
Run module (V) [according to the selected scheduling rule in step (I)] to schedule/reschedule the system.

- Step 6:
Run module (VI) to analyze the status of the model.

If the termination condition is not met, go to step 3.

- Step 7:
Run module (VII) to report the computational results.

## Experiment design

Experimental settings for the scenarios

Scenario | Experimental setting | Purpose of investigation | |
---|---|---|---|

Shop utilization percentage (U) | Setup time ratio (%) (s) | ||

DFFL-I | 95 | 20 | Base case—analyze the performance of scheduling rules |

DFFL-II | 95 | 20, 40 | Analyze the effect of changing setup time ratio |

DFFL-III | 95, 85 | 20 | Analyze the effect of changing the mean inter-arrival time |

## Experiment results

*H*

_{0}), all of the means are the same, while, in the alternative hypothesis (

*H*

_{1}), at least two of the means are significantly different. The four elite scheduling rules are obtained from GP at each iteration. Table 6 shows that the best four evolved scheduling rules after ten iterations of GP are simplified by algebraic operations. For example, the release date of a job (RD) plus five times the processing time of a job for each operation (PT) plus two times the average total processing time of each job (aTPT) are defined as the first proposed rule (GP-1) that was achieved from genetic programing.

GP-generated dispatching rules

Rule | Rule expression |
---|---|

GP-1 | RD + 5PT + 2aTPT |

GP-2 | 7aTPT + 11PT + 12RD |

GP-3 | 3RD + 2DD + 3aTPT + PT-2RD |

GP-4 | 2DD + 8RD + 2aTPT−5PT |

As mentioned above, all experiments are conducted in ten iterations and the results of the scenarios are stated in the following three subsections considering the performance measurements.

### Scenario results for the overall objective function

Overall objective function values for different scenarios

EMDD | W&I | HSPTCH | HLWKRCH | HEMDDCH | HW&ICH | GP-1 | GP-2 | GP-3 | GP-4 | |
---|---|---|---|---|---|---|---|---|---|---|

DFFL-I | 700.9 | 701.0 | 717.4 | 698.2 | 691.3 | 693.2 | 647.5 | 646.7 | 647.2 | 647.1 |

DFFL-II | 854.3 | 852.8 | 860.1 | 861.3 | 833.9 | 834.1 | 832.9 | 832.7 | 833.3 | 833.2 |

DFFL-III | 638.0 | 637.4 | 657.0 | 633.3 | 628.3 | 629.4 | 627.0 | 626.2 | 626.9 | 626.5 |

### Scenarios’ results for the mean flow time

Mean flow time values for different scenarios

EMDD | W&I | HSPTCH | HLWKRCH | HEMDDCH | HW&ICH | GP-1 | GP-2 | GP-3 | GP-4 | |
---|---|---|---|---|---|---|---|---|---|---|

DFFL-I | 3502.7 | 3502.9 | 2966.8 | 2947.8 | 3454.7 | 3464.2 | 3235.5 | 3231.3 | 3234.2 | 3233.3 |

DFFL-II | 4269.3 | 4261.8 | 3500.7 | 3567.1 | 4167.2 | 4168.1 | 4162.2 | 4161.5 | 4164.3 | 4164.0 |

DFFL-III | 3187.9 | 3185.1 | 2732.4 | 2683.6 | 3139.5 | 3145.2 | 3132.9 | 3129.1 | 3132.7 | 3130.3 |

### Scenario’s results for mean tardiness

Mean tardiness values for different scenarios

EMDD | W&I | HSPTCH | HLWKRCH | HEMDDCH | HW&ICH | GP-1 | GP-2 | GP-3 | GP-4 | |
---|---|---|---|---|---|---|---|---|---|---|

DFFL-I | 0.518 | 0.525 | 155.0 | 135.8 | 0.503 | 0.505 | 0.502 | 0.501 | 0.503 | 0.503 |

DFFL-II | 0.585 | 0.611 | 200.0 | 184.9 | 0.574 | 0.590 | 0.570 | 0.561 | 0.568 | 0.569 |

DFFL-III | 0.495 | 0.505 | 138.1 | 120.7 | 0.499 | 0.506 | 0.500 | 0.495 | 0.498 | 0.496 |

## Conclusion

In this paper, we used the GP framework to obtain proper and effective CDRs for solving a DFFL problem. Finally, we created four evolved CDRs and proposed them for solving the DFFL problem. Experimental results indicated that the four proposed CDRs with 95% CI obtained a better solution in comparison with selected scheduling rules from the literature.

For future research, it is possible to consider the variation of the terminal set and ADF to develop the GP framework. It is also possible to use GP to find proper and effective CDRs for different objectives. Furthermore, it is useful to apply GP for other scheduling environments such as job shop or open shop and dynamic assumptions like machine breakdowns.

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