什么是 PLC？可編程邏輯控制器簡介


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什么是 PLC？本文將對可編程邏輯控制器進行簡潔的定義并解釋其基本組件。

可編程邏輯控制器（PLC）在當今各種工藝和制造業中無處不在。PLC 最初設計用于取代機電繼電器系統，以便為修改控制系統的操作提供更簡單的解決方案。無需重新接線大量繼電器，只需從 PC 或編程設備快速下載即可在幾秒鐘內更改控制邏輯。

在本文中，我們將介紹 PLC 的概念并解釋其基礎知識。

支持信息

  • 控制教科書 ~ 第 IV 卷 – 控制：可編程邏輯控制器 (PLC)
  • 控制教科書 ~ 第 IV 卷 – 控制：梯形圖

 

什么是 PLC？

PLC 是一種工業級數字計算機，專門用于執行控制功能 - 特別適用于工業應用。

如今，大多數 PLC 都是模塊化的，允許用戶添加各種功能，包括離散和模擬輸入和輸出、PID 控制、位置控制、電機控制、串行通信和高速網絡。與繼電器組等舊技術相比，PLC 更容易排除故障和維護，更可靠、更經濟高效，而且用途更廣泛。

 

圖 1.一套現代工業 PLC。圖片由西門子提供

 

Modicon 是“模塊化數字控制器”的縮寫，它是1968 年發明的第一個 PLC 產品的名稱，也是發明它的品牌，盡管現在歸施耐德電氣所有。

雖然它看起來不像典型的家用計算機，但 PLC 的核心與大多數人在日常生活中使用的計算機和智能設備中所見的技術相同。

 

基本組件

PLC 由幾個基本部件組成。不同制造商的產品可能外觀略有不同，但每個部件的用途和范圍相同。這些部件包括電源、中央處理器 (CPU)、輸入/輸出卡以及放置輸入/輸出 (I/O) 卡的背板或機架。

如圖 2 所示，背板在所有獨立組件之間建立電氣連接，使 PLC 具有模塊化設計。該電氣連接包括電源和通信信號。許多 PLC 制造商在背板上使用專有通信協議，以便 I/O 可以安全地與 CPU 通信。

 

 

圖 2. Rockwell ControlLogix 背板。圖片由Rockwell提供

 

電源

根據應用和安裝情況，電源可接受 120VAC 或 24VDC?。如上所述，該電壓通過背板總線傳輸，為 CPU 和 I/O 模塊供電，這些模塊以“卡”的形式提供。這些卡可以快速添加到托架中的插槽中或從中移除。

需要注意的是，CPU 的電源不為現場設備（如傳感器和線圈）供電。該電源連接必須單獨與卡建立。

中央處理器 (CPU)

CPU 是 PLC 的實際“大腦”，使其成為一臺計算機。即使是小型、非模塊化 PLC 也包含 CPU。輸入信號來自 I/O 卡，邏輯程序根據信號做出決策。如果需要，CPU 會根據信號和條件的變化命令輸出打開和關閉。

程序可能包括數學運算、計時、計數和通過現代網絡協議共享信息等高級功能。對于較舊的中繼系統來說，許多這些操作都極其困難，甚至不可能。

輸入/輸出卡

傳感器、按鈕、開關、繼電器、螺線管，甚至聯網設備都與連接到 I/O 卡上的螺釘端子的 I/O 信號共享信息。可以根據每臺機器和設??施的需求選擇這些卡。在一些小型 PLC中，I/O 端子不是模塊化卡，而是方便地安裝在 PLC 的側面。

由于 I/O 設備構造種類繁多，因此分立模塊和模擬模塊的種類也多種多樣，模擬模型有“源”與“匯”，“電壓”與“電流”等術語。

 

 

編程裝置和人機界面

PLC 本身之外還有兩個非常重要的組件：編程設備和人機界面 (HMI)。編程設備可以是同一制造商生產的臺式計算機、筆記本電腦或手持設備。一些小型 PLC 甚至在前面有按鈕，允許編程非常基本的邏輯操作，而無需使用計算機。

雖然編程設備允許用戶查看和修改 PLC 上運行的代碼，但 HMI 提供了一種顯示信息和獲取輸入的方法，對整個控制系統進行建模。HMI 通常不提供任何修改邏輯程序的方法。

圖 3 顯示了HMI 觸摸屏，可用于控制室或靠近過程的“現場”。這些類型的交互式顯示器非常常見，通常會直接安裝在 PLC 外殼上或附近供操作員使用。

 

 

圖 3.一些觸摸面板可以與過程交互或僅監控控制系統。圖片由Phoenix Contact提供

 

在現代 HMI 出現之前，工廠操作員依靠墻上的模擬儀表和燈泡指示器來了解其流程的狀態。打開閥門以釋放管道中的壓力或降低溫度控制回路無法通過控制室中的計算機完成，而是必須通過無線電傳送給外部操作員，由其手動執行操作。將這些舊控制室與配備 HMI 和高級圖形的現代控制室進行對比。

 

概括

自動化和控制工程師設計 PLC 的這種高級視圖。這些工程師了解控制系統、控制算法和配置。他們為流程建模并確定所需系統的大小和規模。然后購買 PLC 及其所有組件。這可能是一個帶有八個 I/O 卡的機架，也可能是二十個機架，它們都通過工業網絡進行通信，具有數千個 I/O 點 - 所有控制都由單個 CPU 完成。