Conceptual model framework

To use a structured step-by-step approach of conceptual modeling we use the framework presented by Robinson (2008b) which consists of the following five steps (see Figure 6-1):

 Understanding the problem situation,

 Determining the modelling and general project objectives,

 Identifying the model outputs (responses),

 Identifying the model inputs (experimental factors),

 Determining the model content (scope and level of detail), identifying any assumptions and simplifications.

6.2.1 The problem situation

The key requirements of a conceptual model are validity, credibility, feasibility and usefulness (Stewart Robinson, 2008a). Understanding the problem situation, which is input for the conceptual model, is thus an important first step. We elaborated on the problem situation in previous chapters, but it remains to link the problem situation to the conceptual model requirements. In the Rotra case the model should:

 Provide sufficiently accurate insight on the number of AGVs required in the system to handle a certain number of semi-trailers within predefined time intervals (validity).

 Rotra and other consortium partners must have confidence in the model (credibility).

 Be feasible to build within given time and data constraints.

 Be useful, that is, flexible, applicable to a multitude of scenarios, visually attractive and modularly build (the ability to extend the complexity of the model by other researchers).

To summarize, the problem situation of the case study: Rotra wants to research the possibility of using autonomous vehicles to shunt semi-trailers at a new to-be-build multimodal cross-dock. Within this research an important factor is the number of AGVs required to process all semi- trailer movements during the day, given certain arrival- and departure times.

6.2.2 Modelling and general project objectives

The objective of Rotra is to minimize the Total Cost of Ownership (TCO) of the AGVs. The model should therefore provide insight in the number of AGVs required, given a certain amount of semi- trailers arriving and departing during the day. This also involves assessing the impact of the number of AGVs on waiting times and the throughput and responsiveness of the system. We will use a time-scale of 6 days a week, which is current practice at Rotra, and try to provide the greatest amount of flexibility possible in the model such that is can be used to do many different experiments. We also need many runs per experiment to be able to do good statistical analysis of the impact of the experimental factors used in the different experiments. Furthermore we use a simple 2D visualization and the model should be understood by a moderately experienced modeler such that he can further extent the system or do more experiments.

6.2.3 The model outputs

 Time-series of daily throughput and cycle times of arriving and departing semi-trailers

 Bar chart of daily AGV occupancy (working, idle, stopped, waiting, charging).

 Mean, standard deviation, minimum and maximum daily throughput of all docks

 Mean, standard deviation, minimum and maximum occupancy of the Arrival/Departure Parking and the Empty Trailer Parking

 Mean, standard deviation, minimum and maximum daily; number of jobs assigned, utilization, response time, travel time, waiting time, charging time and idle time of all AGVs.

 Mean, standard deviation, minimum and maximum daily conflicts resolved by Conflict Manager.

6.2.4 The model inputs

 Number of docks

 Number of arriving and departing transports

 Number of AGVs deployed

 Number of parking slots at Arrival/Departure parking

 Dimensions of guide-path design (distance between docks, distance between parking slots, length dock area, length crossroads, length parking slots)

 Number of loads per arriving semi-trailer (uniform distribution with a minimum 10 and maximum value)

 Look ahead length to determine how far the Conflict Manager should check for the presence of other AGVs.

 Maximum number of semi-trailers allowed in system

 (De)coupling time of semi-trailer with a truck

 (De)coupling time of semi-trailer with an AGV

 Forward and rearward driving speed of AGV

 Charging alternative of AGVs (continuous, battery swap, conventional charging).

 Parking Manager policy (available slot closest by)

 Vehicle Routing policy (shortest path)

 Vehicle Scheduling policy (auction mechanism and bid evaluation as discussed in Section 5.5)

 Conflict Manager priority ruling (as discussed in Section 5.9)

 Battery Manager policy (continuous, battery swaps or conventional charging) 6.2.5 Model scope and level of detail

The simulation model includes all operations required to pick-up and drop-off semi-trailers at various locations around the cross-dock at pilot location Velp using AGVs. We focus on the shunting of semi-trailers only and assume that the heterogeneous fleet can be viewed as homogenous when it comes to the maneuvering-, coupling- and driving times. We also assume that the inter-arrival time of semi-trailers follow a certain probability distribution. We further reviewed the worst-case-scenario (3-axle non-steered semi-trailer) for the guide-path design and use the distances associated within this guide-path design as if they were applicable for all semi- trailers. The impact of this simplification is assumed to be very small compared to the non-type specific handling and driving times (e.g. average speed of AGV does not depend on the length or load of a semi-trailer). We further ignore the inner operations of the cross-dock such as

loading/unloading and assume that this requires a predetermined amount of time and it never raises any exceptions or delays. We thus also make estimates on the consolidation (which arriving trailers contain how much cargo for which departing trailers). We also assume that AGVs never break down and maintenance is carried out during system down-time. The AGVs in the system are modeled as a black-box and driving from an origin to a destination takes a certain amount of time (based on the maximum forward speed) as well as rearward docking (based on the maximum rearward speed). Next to that, the coupling and uncoupling of semi-trailer with the AGVs is assumed to never fail and requires a fixed amount of time. A detailed overview of the model scope and assumptions can be found in Appendix 5 and the corresponding level of detail in Appendix 6.