The software program Tecnomatix Plant Simulation makes a distinction between two types of objects: “material flow objects” and “moving unit objects”. A material flow object is used to model a component that generates, destroys and routes moving unit objects. In general a material flow object is fixed to some place in the simulation environment. The following four components, which are discussed in Subsection 5.2.1 till 5.2.4, are modelled as material flow objects in our simulation model:
• Zone
• Crossing
• Depot
• Yard and vessel
Moving unit objects are used to model the flow of materials through a simulation model.
Subsection 5.2.5 till 5.2.6 will describe the two components that are modelled as material flow objects in the simulation model, namely: AGVs and containers. Material flow objects as well as moving unit objects can activate so-called “methods”. These methods, which use the programming language SimTalk, enable the programmer of the simulation model to control the behaviour of certain objects in the simulation environment.
5.2. Modelling of the main elements of the zone control traffic approach 43
5.2.1 Zones
The Tecnomatix component “track” is used to model a zone. This track component is unidirectional and has a certain length and width. Vehicles will travel along the direction a track, after they have entered it. Furthermore the capacity of a track component depends on its size and the size of the vehicles that are on the track. The following two control sequences (methods) are activated when a sensor, which is placed at a predefined position on the track, is triggered by an AGV:
• Method: Arriving the Zone
The method Arriving the Zone will be activated by an AGV when the total body of the AGV is in the zone. This method updates the status of the previous zone.
This method will also update the status of the previous crossing and will set the desired speed of the AGV to the maximum speed in case it concerns the first zone of a lane. Furthermore the method will call the routing algorithm if the zone is an EZ of a lane (see also Section 5.4). Finally the last zone and the next zone of the vehicle will be updated. The programming code of this method is described in Appendix E.1.
• Method: Leaving the Zone
The programming code of this method is described in Appendix E.2. The method Leaving the Zone will be activated by an AGV when the front of the AGV passes a modelled sensor on the track. The position of this sensor is constrained because of the following two reasons:
– The method Leaving the Zone should not be activated before the method Arriving the Zone is activated.
– The AGV needs enough space to brake and stop within the zone whenever the method Leaving the Zone will be activated.
The method Leaving the Zone checks if an AGV can leave the zone. The decision to leave a certain zone is based on the traffic rules that are explained in Paragraph 3.3. The method Leaving the Zone will call the method CDP Algorithm if the AGV is located in an EZ. This method checks if there will be formed a cyclic deadlock in the case the vehicle would leave the zone. This method CDP Algorithm is given in Appendix E.7. The vehicle will start braking if the conditions of the traffic rules are not satisfied (see Section 3.3). The AGV will start accelerating at the moment the conditions of the traffic rules are fulfilled. The method Leaving the Zone will change the status of the current zone to orange and the status of the next zone will be changed to red. In the case the current zone is an EZ, the method Leaving the Zone will set the status of the crossing to occupied and adjust the length of the track of the next crossing. This adjustment is based on the position of the current zone and the position of the next zone of the vehicle. The length of the
44Chapter 5. Implementation of the zone control model in a simulation model of the ACT
track of a crossing changes to simulate the extra/less distance that a vehicle will travel when it has to make a corner on a crossing.
Note that a lane is not modelled as one object, but the lane consists of multiple objects (zones) which are connected with each other.
5.2.2 Crossing
The crossing is modelled with the Tecnomatix component “track”. It is possible that the crossing consists of multiple track objects. Multiple “track” objects are needed in order to model the movement of multiple vehicles which are passing the same crossing simultaneously. Furthermore each crossing has a property that states which parts of the crossing are occupied by a vehicle that is passing the crossing. This information is used by the Leaving the Zone method for making a “go ahead” or “stop” decision.
The method Arriving the Crossing is activated when the body of a vehicle is completely inside a crossing. This method adjusts the speed of a vehicle in the case the vehicle should reverse his direction or make a turn. The method Arriving the Crossing is displayed in Appendix E.3.
5.2.3 Depot
The Tecnomatix object “buffer” is used to model a depot. The capacity of the buffer is infinite and has a processing time of 0 seconds. The method Arriving the Zone will be called by an AGV when the total body of the AGV is in the depot in order to update the status of the previous zone of the vehicle.
5.2.4 Yard and vessel
The Yard/vessel is modelled by the Tecnomatix object “store”. Containers are placed in, or dispatched from this store in order to simulate the loading and dispatching of a vessel/Yard. Initially each vessel/Yard is filled with a predefined number of containers.
5.2.5 AGV
The Tecnomatix object “transporter” is used to model an AGV. This transporter is able to transport containers from the loading point (Yard/Quay) to their destination (Yard/Quay). A certain destination object can be assigned to the vehicle in order to control the routing of the vehicle. Several parameters are assigned initially to each ve-hicle. The most important initial parameters are the size, maximum speed, acceleration and deceleration of the vehicle.