Simulation Using GPSS
Modeling of chain conveyors and their equipment interfaces
WSC '96 Proceedings of the 28th conference on Winter simulation
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Most of the conveyors that appear in simulation models tend to be fairly simplistic. The conveyor only provides a means of moving an object from one place to another. It is driven by a single speed motor and very little, if any, control logic is applied during operation. A number of objects or products can be conveyed in this manner with no difficulty. For example, boxes, kitting tubs and pallets of bricks are all relatively sturdy and relatively insensitive to damage. In fact, the conveyor system may be designed such that the objects touch or accumulate at a given station. The flow of objects through the system is also important. If there is a momentary stoppage in the system, the input of objects to the system may be interrupted so that saturation is avoided.However, what if the product is very sensitive to damage and input to the system cannot be temporarily interrupted? Such a system requires a high degree of control over the operation of the individual conveyor sections. In order to develop a control philosophy and thereby shape the simulation logic, we must first understand the constraints of the product and how it is produced.We all know that glass is naturally fragile. Excluding breakage, the product may be rendered useless due to edge damage or surface damage. Edge damage occurs when adjacent pieces collide. Usually both are damaged beyond usefulness. Prevention of these collisions means that only one piece can occupy our smallest, definable control unit. In this case, the smallest, definable control unit is one conveyor section. Surface damage is due to abrasion. This is caused by relative motion between the piece and the conveyor. In practice, the constraint must be that adjacent conveyors must be running at the same surface speed.Using the previous two concepts as a foundation, we can develop control logic, and therefore simulation rules and constructs to deal with specific problems. Some of these problems are: right-angle transferring, equipment that necessarily runs at something other than line speed and increasing the separation between adjacent pieces. Sample simulation code will be presented in GPSS/H. The reader is invited to implement the constructs in other simulation languages.