D. Van Oudheusden

Developing railway timetables which guarantee a better service

Abstract In order to improve passenger service, a waiting cost function, weighting different types of waiting times and late arrivals, is designed and minimised. The approach is applied to a small part of the Belgian railway network. In the first phase of the approach, ideal buffer times are calculated to safeguard connections when the arriving train is late. These buffer times are based on the delay distributions of the arriving trains and on the weighting of different types of waiting times. In a second phase, standard linear programming is used to construct an improved timetable with well-scheduled connections and, whenever possible, with ideal buffer times. Simulation compares different timetables and optimises the LP timetable. For the case of the Belgian railway network, the final result is a timetable with well-scheduled connections and a waiting cost that is 40% lower than the current timetable. Since only LP modelling is applied, the proposed technique is very promising for developing better timetables—even for very extensive railway networks.

Districting for salt spreading operations

Abstract The districting problem presented in this paper involves the partitioning of the road network in a region into sub-networks (or districts), to facilitate the organization of the operations to be performed within the region. Typically, each district contains one local center (depot) whose location is given, while the operations involve different types of routing, with routes starting and ending at the depot. For public sector applications like salt spreading and road maintenance, this partitioning is a real distinct stage in the organization and planning of services, and as is the case with location, districting is of a non-operational nature. Relevant characteristics of well-designed districts are: ability to support good routing, balance in workload, compactness of the sub-areas, centrality of the depot, etc. We present a heuristic procedure for our districting problem. First we partition the road network into small cycles, then we aggregate them into districts in two phases. Phase 1 uses an approach based on bin packing principles, while in Phase 2 a multi criteria approach is used. We illustrate the procedure and discuss its merits for the salt spreading operations in the province of Antwerp.

District design for arc-routing applications

In this paper we address the problem of district design for the organisation of arc-routing activities. In particular, the focus is on operations like winter gritting and road maintenance. The problem involves how to allocate the road network edges to a set of depots with given locations. The collection of edges assigned to a facility forms a district in which routes have to be designed that start and end at the facility. Apart from the ability to support good arc routing, well-designed districts for road-maintenance operations should have the road network to be serviced connected and should define clear geographical boundaries. We present three districting heuristics and evaluate the quality of the partitions by solving capacitated arc routing problems in the districts, and by comparing the solution values with a multi-depot CARP cutting plane lower bound. Our experiments reveal that based on global information about the distribution system (ie the number of facilities or districts, the average edge demand and the vehicle capacity) and by using simple guidelines, an adequate districting policy may be selected.

Infield logistics planning for crop-harvesting operations

Crop-harvesting operations are typically carried out with combine harvesters. The harvested product is transferred to one or more tractors every time the combine harvesters storage capacity is reached. The efficiency of the process can be significantly improved by computing optimal routes and interactions for the harvest vehicles in the field. Furthermore, an automated method for generating itineraries for the harvest vehicles facilitates the planning for autonomous agricultural vehicles. The infield logistics problem is formulated as an integer linear programming vehicle routing problem with additional turn penalty constraints, but, because of the high number of decision variables, it is not possible to solve cases of realistic field size. The solution time of the infield logistics problem is considerably reduced by reformulating it as a modified minimum-cost network flow problem. This specific structure allows the exact solution of intermediate-size planning problems in a much shorter time period. The result of solving the infield logistics problem with the proposed modelling approaches is a set of itineraries (‘tours’), covering the entire field. Each ‘tour’ is characterized by the combine harvesters start and end points and the positions where the combine harvester needs to be unloaded. The planning models minimize non-productivity (i.e. the time when a combine harvester travels in a field without harvesting). The results indicate that coordination between combine harvesters and tractors is also improved.

Automatic production planning of press brakes for sheet metal bending

This paper discusses the planning module of system that has been developed in a research project investigating the possibilities for automatic planning of press brakes. Only limited attention has been given to production-planning problems for bending processes in the sheet-metal industry. A well-considered production-planning module provides an important opportunity for cost saving and flow-time reduction. The module will support and may replace an experienced planner. Under certain assumptions, the planning problem can be modelled both ways as a Travelling Purchaser Problem (TPP) and as a Generalized Travelling Salesperson Problem (GTSP). A hierarchical decomposition approach is presented to solve the TPP, while the GTSP is solved using guided local search (GLS). These methods are compared based on a set of problems from a real production environment and perform well.

Algorithms for the Design Verification and Automatic Process Planning for Bent Sheet Metal Parts

Abstract Sheet metal bending processes require detailed process planning in order to eliminate infeasible set-ups or avoidable economically demanding manipulation requirements. In this paper it is demonstrated how the complexity of a bend sequencing task can be handled by means of an efficient reduction of the search field through well-chosen representation schemes and the identification of geometric constraints. The expertise established by experienced process planners was integrated in search algorithms based on precedence constraint solving and dynamic branch-and-bound techniques. A case study is used to illustrate these procedures. Additionally, the characteristics of a dedicated collision detection algorithm, developed for fast sequence verification during search procedures, are outlined.

Branch and Bound Algorithms for Single Machine Scheduling with Batching to Minimize the Number of Late Jobs

This paper considers the problem of scheduling a single machine to minimize the number of late jobs in the presence of sequence-independent family set-up times. The jobs are partitioned into families, and a set-up time is required at the start of each batch, where a batch is a maximal set of jobs in the same family that are processed consecutively. We design branch and bound algorithms that have several alternative features. Lower bounds can be derived by relaxing either the set-up times or the due dates. A first branching scheme uses a forward branching rule with a depth-first search strategy. Dominance criteria, which determine the order of the early jobs within each family and the order of the batches containing early jobs, can be fully exploited in this scheme. A second scheme uses a ternary branching rule in which the next job is fixed to be early and starting a batch, to be early and not starting a batch, or to be late. The different features are compared on a large set of test problems, where the number of jobs ranges from 30 to 50 and the number of families ranges from 4 to 10.

Tooling layout on a press brake for sheet metal air bending

Air bending is a forming process by which very complicated three-dimensional parts are produced on a press brake. A bending operation is performed on a station consisting of a punch and a die. A part is produced by performing a sequence of bending operations, often requiring different stations. The stations that are necessary to produce the part are mounted on a press brake. This paper discusses several approaches for the optimal placement of the stations on a press brake. Process constraints and objectives regarding efficiency are taken into account. Both exhaustive enumeration and neighbourhood search are used for solving the formulated problems. The reported computational experiments show that all problems can be solved satisfactorily, and that, in a dynamic industrial environment, neighbourhood search is more appropriate because of the limited available computation time.

A minimum cost network flow model for the maximum covering and patrol routing problem

This paper shows how the maximum covering and patrol routing problem (MCPRP) can be modeled as a minimum cost network flow problem (MCNFP). Based on the MCNFP model, all available benchmark instances of the MCPRP can be solved to optimality in less than 0.4s per instance. It is furthermore shown that several practical additions to the MCPRP, such as different start and end locations of patrol cars and overlapping shift durations can be modeled by a multi-commodity minimum cost network flow model and solved to optimality in acceptable computational times given the sizes of practical instances.

The development and application of an integrated production planning methodology for sheet metal working SMEs

Sheet metal working remains an important industry. It is argued that, to be competitive, the complete production chain needs to be optimised. Therefore, this article focuses on the optimisation of the production plan. The sheet metal shop is configured as a two-stage flow shop with laser cutting and air bending. In the literature, theoretical production planning models can be found for both individual processes. Unfortunately, those planning models have a low applicability in sheet metal working small and medium sized enterprises (SMEs) due to the fact that they only focus on one single production step while in reality the planning decisions taken at the cutting stage affect the production plan for the air bending stage. An integrated production planning methodology is proposed to overcome this problem of individual optimisation by taking into account relevant bending information already at the cutting stage. An integer program is presented, incorporating some important practical requirements: (1) the planning model is suited for normal workpieces, rush orders, special workpieces and multiple-machine sheet metal shops and (2) the computational times to generate the production plan are limited. Ten real-life industrial test cases are used to evaluate the proposed methodology. The integrated production planning methodology results in schedules with a reduced makespan and a reduced set-up time at the press brake compared to the current way of planning. All planning models are developed in close cooperation with sheet metal working SMEs to facilitate the implementation process.