英文参考文献原文复印件及译文.doc

英文参考文献原文复印件及译文专业: 电气工程及其自动化姓名: 曹丽倩学号: 100062630 指导教师: 高敬格完成日期 2014 年 6月第22卷第1期昆明冶金高等专科学校学报 200 6年1月 Jou mal of Kunming Metallurgy College - 1- Relays The Programmable Logic Controller Early machines were controlled by mechanical means using cams, gears, levers and other basic mechanical devices. As plexity grew, so did the need for a more sophisticated control system. This system contained wired relay and switch control elements. These elements were wired as required to provide the control logic necessary for the particular type of machine operation. This was acceptable for a machine that never needed tobe changed or modified, but as manufacturing techniques improved and plant changeover to new products became more desirable and necessary, a more versatile means of controlling this equipment had tobe developed. Hardwired relay and switch logic was cumbersome and time consuming to modify. Wiring had tobe removed and replaced to provide for the new control scheme required. This modification was difficult and time consuming to design and install and any small "bug" in the design could bea major problem to correct since that also required rewiring of the system. A new means to modify control circuitry was needed. The development and testing ground for this new means was the . auto industry. The time period was the late 1960's and early 1970's and the result w