外文翻译---基于petri网的plc开发程序

外文翻译---基于petri网的plc开发程序内容摘要：

notation for PLC programs in that is assumes the internal memory is set by external events. Transitions A, D and G are not necessary for modelling of the ladder logic in that they model the environment of the door, not the door and mats themselves. Observation of the demonstrates the obvious problems with the logic. Firstly the door may open or close when a person is stood on mat B, the so called ~hospital case39。 . Secondly the allows for the door to be left open. The ladder logic, however would always detect a person on mat B and close the door. It is relatively easy to stop the door opening or closing when a person is on mat B. Rungs 1 and 4 should be amended as shown below. This appears simple but would not work in practice. Reexamine the new ladder logic and recall the operation of a PLC, scan internal memory, establish logical connections, action outputs. The initial scan will detect on A, not on B, and set internal memor y, odf, open door flag. A second scan will detect odfand the door will begin to open. However as soon as the door opens a fraction the person may step offmat A and start to pass through the door. A scan at this instant will detect neither A nor B, neither odf or cdf will be set and the door will halt part way open. Secondly, assume that the person is patient and waits for the door to be fully open before stepping through onto mat B. A scan at this stage will set cdf, close door flag. Nothing further will be effected until the person leaves mat B. Subsequently at scans cdf will not be set, no one on B, rung 4 will never evaluate to true and the door will never close. The programmer overes this problem by latching,2 reexamine figures 1 and 2 where an output is reused as an input to latch the motor until the stop button is depressed, a Note that some PLCs have self latching relays. Returning to the door. It is necessary to latch the open door flag until the 4 door is fully open. A door open sensor, dos, detects when the door is fully open, it resets when the door starts to close. Figure A5 Figure A6 Figure A7 shows that the open door flag is set when on A, not on B, door not fully open。 and reset when the door reaches the fully open point. This corresponds to the rung shown in figure A8. Figure A7 Figure A8 These basic s。