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大多數(shù)用戶只使用這種類型的信號，因此在許多情況下，現(xiàn)有設(shè)備已經(jīng)可以提供所需的任何附加數(shù)據(jù)。全球安裝了大約 4000 萬臺 Hart 設(shè)備，其中 90% 的用戶僅使用模擬 4…20 mA 信號。
因此不使用配置、校準或工廠管理功能。如果改進對未使用數(shù)據(jù)的訪問，估計有 20% 的設(shè)備可以提供有價值的信息。Hart 協(xié)議（高速可尋址遠程傳感器）在近 30 年前被標準化，現(xiàn)在是全球過程工業(yè)中常用的通信標準。
Hart 初的目標是通過將數(shù)字數(shù)據(jù)疊加在 4…20 mA 信號回路上來使模擬過程測量設(shè)備更智能。由于引入了其他傳輸方法（例如通過以太網(wǎng)或無線轉(zhuǎn)發(fā)數(shù)據(jù)），該標準多年來一直在發(fā)展。該技術(shù)的向后兼容性和來自不同制造商的設(shè)備的互操作性確保用戶不受供應(yīng)商特定或區(qū)域解決方案的限制。
Hart 協(xié)議提供了許多只有少數(shù)用戶知道的可能性。很多時候，DCS 或 PLC 就位或指定時沒有 Hart 功能。因此，控制系統(tǒng)不使用 Hart 設(shè)備提供的額外測量和診斷數(shù)據(jù)。只有維護人員使用 Hart 界面進行本地故障排除或使用手持設(shè)備進行校準。因此，有關(guān) Hart 設(shè)備功能的知識就會丟失。在這樣的背景下，它們的優(yōu)勢將再次凸顯出來。
配置：在簡單的情況下，可以通過 Hart 協(xié)議配置設(shè)備。預配置設(shè)置允許選擇操作參數(shù)，包括測量范圍、工程單位和傳感器類型。
校準：初始安裝和配置后，用戶可以通過設(shè)置零位和跨度、調(diào)整 4…20 mA 信號或考慮外部測量誤差源來校準設(shè)備。
診斷
Hart 設(shè)備具有豐富的診斷數(shù)據(jù)。他們中的許多人可以監(jiān)控傳感器或探頭的狀況，從而在早期指示不準確的測量。此外，可以檢測到可能導致將來出現(xiàn)問題的配置錯誤或操作模式。然而，重要的是，診斷數(shù)據(jù)用于確定大型設(shè)備的健康狀況或流程的質(zhì)量和效率。這有助于避免計劃外維護。
多變量設(shè)備：大多數(shù) Hart 設(shè)備都是多變量的，這意味著它們可以提供多個測量值。只有初級值由 4…20 mA 信號表示，而其他變量只能作為數(shù)字數(shù)據(jù)使用。例如，壓力變送器可以提供絕壓或表壓過程壓力以及環(huán)境溫度。
溫度變送器還可以為用戶提供過程和環(huán)境溫度以及冷端補償值。差壓變送器不僅提供差壓，還提供靜壓和環(huán)境溫度。科里奧利儀表提供質(zhì)量流量、密度和溫度等信息。不要忘記閥門定位器，它報告諸如閥桿的目標和實際位置以及執(zhí)行器壓力和向執(zhí)行器輸出信號等數(shù)據(jù)。
如果過去工廠操作員決定升級控制系統(tǒng)并想要訪問 Hart 數(shù)據(jù)，通常會安裝一個 RS485 Hart 多路復用器并將其連接到 PC。但是，大多數(shù) PC 沒有 RS485 接口，因此使用了 RS232 轉(zhuǎn)換器。目前，甚至連 RS232 接口都不再經(jīng)常集成在 PC 中。因此，用戶必須購買設(shè)備服務(wù)器才能將 RS485 協(xié)議轉(zhuǎn)換為以太網(wǎng)協(xié)議。事實證明，這是一個復雜的解決方案，需要配置和維護多個設(shè)備。
Hart 基金會近發(fā)布了 Hart IP 標準，本質(zhì)上是基于以太網(wǎng)的 Hart 協(xié)議。這為與 Hart 協(xié)議的通信提供了一個用戶友好且普遍可用的接口。它可以集成大多數(shù)工廠操作員已經(jīng)使用的工具：Pactware（或其他 FDT 容器）、Hart Server、Hart OPC Server、Simatic PDM 和 Honeywell FDM。因此，Hart IP 考慮了工業(yè) 4.0 和工業(yè)物聯(lián)網(wǎng) (IIoT) 的大趨勢，它們需要基于以太網(wǎng)的數(shù)據(jù)交換和高網(wǎng)絡(luò)連接性。
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Hart devices are rich in diagnostic data. Many of them can monitor the condition of a sensor or probe, indicating inaccurate measurements at an early stage. In addition, a configuration error or an operating mode can be detected that may lead to problems in the future. Most importantly, however, diagnostic data is used to determine the health of a large piece of equipment or the quality and efficiency of processes. This helps to avoid unplanned maintenance.
Multivariable Devices: Most Hart devices are multivariable, meaning that they can provide multiple measurement values. Only the primary value is represented by the 4…20 mA signal, while other variables are only available as digital data. Pressure transmitters can, for example, provide absolute or gauge process pressure as well as the ambient temperature.
Temperature transmitters may also provide the user with the process and ambient temperature as well as cold junction compensation values. Differential pressure transmitters provide not only differential pressure, but also static pressure and ambient temperature. Coriolis meters deliver information such as mass flow, density and temperature. Not to forget the valve positioners, which report data such as the target and actual position of the valve stem as well as the actuator pressure and the output signal to the actuator.
Missing serial interfaces
If in the past the plant operator decided to upgrade the control system and wanted to access the Hart data, an RS485 Hart multiplexer was usually installed and connected to a PC. However, most of the PCs do not have an RS485 interface, so an RS232 converter was used. Currently, not even an RS232 interface is often integrated in the PCs any more. Users must therefore purchase a device server for conversion from RS485 to Ethernet protocol. This proves to be a complicated solution with several devices to configure and maintain.
The Hart Foundation recently published a standard for Hart IP, essentially the Hart protocol over Ethernet. This provides a user-friendly and generally available interface for communication with the Hart protocol. It enables the integration of tools that most plant operators already use: Pactware (or other FDT containers), Hart Server, Hart OPC Server, Simatic PDM and Honeywell FDM. Hart IP thus takes into account the megatrends Industry 4.0 and Industrial Internet of Things (IIoT), which require Ethernet-based data exchange and high network connectivity.
