Tuesday, March 8, 2011

Develop Design Approach of PLC Programs

The developed design approach is illustrated by the means of a program segment of a transport system. The example shows that the entire semantic could be described in a concise and coherent way with better understandability and fewer elements compared to the original programming in relay ladder logic.

The task- independent behavior of the robot is derived from the MSD-design process. The dynamic components are the feeder, horizontal drive (x-axis), vertical drive (z- axis), rotary actuator, and the gripper. These components are represented with a module-chart and state chart description to describe the I/O ports and the behavior. The material-handling robot might be controlled with a decentralized controller module. Therefore the next step implies the restructuring of the controller node and the modeled components to be controlled (Figure 6, left-hand side). Since the robot is a shared resource we may select a common resource state pattern from the database. The specific behavior is always wrapped up with a requested state in order to avoid accessing the resource at the same time. The task- oriented behavior is encapsulated within the active state. The controller module should provide two modes denoted with a dashed line since they are not task-neutral. In the supply mode material bins are transferred from the external transport system (AGV) to the internal transport system. The collect mode accomplishes the reverse operation. Each of these modes consists of 11 elementary control actions in a determined sequence for the first control actions of the supply mode.

This approach aims at reduced development time and higher software quality. Testing and debugging of the PLC program is integrated in the manufacturing system design process. Visualized formal modeling techniques increase the understandability of the PLC programs, particularly, for non-experts with different perspective and expertise. Combined with the simple but powerful axiomatic design principles, this model-based design approach accelerates the generation of correct PLC program code.

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