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【GE IC697MDL750】
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PLC程序設計的功能表圖方法
概 述
功能表圖 ( Function Chart ) 亦稱順序功能表圖 ( Sequence Function Chart ),簡稱SFC。
在前面我們已經系統(tǒng)地介紹了梯形圖設計方法,這種方法絕大多數是采用經驗設計方法,是從傳統(tǒng)的繼電器邏輯設計方法繼承而來,它的基本設計思想是:被控制過程由若干個狀態(tài)所組成,每個狀態(tài)都由輸入的某些命令信號建立,輔助繼電器用于區(qū)分狀態(tài)且構成執(zhí)行元件的輸入變量,而輔助繼電器的狀態(tài)由輸入的命令信號控制,正確找出輔助繼電器、命令信號及執(zhí)行元件之間的邏輯關系,也就基本完成了程序設計任務。
經驗法僅適用于簡單的單一順序問題的程序設計,且設計無一定的規(guī)律可循,對稍復雜的程序設計起來顯得較為困難,而對具有并發(fā)順序選擇順序的問題就更顯得無能為力,故有必要尋求一種能解決更廣泛順序類型問題的程序設計方法。
功能表圖是一種能很好解決上述問題的程序設計方法,它是描述控制系統(tǒng)的控制過程、功能、特性的一種圖形,它zui初很象一種工藝性的流程圖,它并不涉及所描述的控制功能之具體技術,是一種通用的技術語言。這種設計方法很容易被初學者接受,對有一定經驗的技術人員而言也會提高設計效率,有資料稱這種設計方法可減少2/3的設計時間,且用此法設計出的程序調試、修改、閱讀也很容易。
這種設計方法是在80年代初由法國科技人員zui先提出的,因為它有許多*性,很快得到了推廣,法、德等國并對此推出了相關的國家標準,IEC了于88年公布了類似的標準,我國也已在86年頒布了功能表圖的國標。
功能表圖法在PLC程設中有兩種用法:
直接根據功能表圖的原理研制PLC,即將功能表圖作為一種編程語言直接使用,目前已有此類產品,多數應用在大、中型PLC上,其編程主要通過CRT終端,直接使用功能表圖輸入控制要求。
用功能表圖說明PLC所要完成的控制功能,然后再據此找出邏輯關系并畫出梯形圖。這種應用法較多,本節(jié)主要討論這種方法。
功能表圖的基本概念
功能表圖是一種描述順序控制系統(tǒng)過程、功能和特性的圖形表示方法。主要由步、轉移,有向線等元素組成。
步
步是控制系統(tǒng)中一相對不變的狀態(tài),在功能表圖中,步通常表示某個或某些執(zhí)行元件的狀態(tài),其符號見圖5-1。步又分成起始步、動步、靜步。
1.起始步:
起始步對應于控制系統(tǒng)的初始狀態(tài),是系統(tǒng)運行的起點。一個控制系統(tǒng)至少要有1個起始步,起始步的符號見圖5-2
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