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大多數(shù)用戶只使用這種類(lèi)型的信號(hào)，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(wàn)臺(tái) Hart 設(shè)備，其中 90% 的用戶僅使用模擬 4…20 mA 信號(hào)。
因此不使用配置、校準(zhǔn)或工廠管理功能。如果改進(jìn)對(duì)未使用數(shù)據(jù)的訪問(wèn)，估計(jì)有 20% 的設(shè)備可以提供有價(jià)值的信息。Hart 協(xié)議（高速可尋址遠(yuǎn)程傳感器）在近 30 年前被標(biāo)準(zhǔn)化，現(xiàn)在是全球過(guò)程工業(yè)中常用的通信標(biāo)準(zhǔn)。
Hart 初的目標(biāo)是通過(guò)將數(shù)字?jǐn)?shù)據(jù)疊加在 4…20 mA 信號(hào)回路上來(lái)使模擬過(guò)程測(cè)量設(shè)備更智能。由于引入了其他傳輸方法（例如通過(guò)以太網(wǎng)或無(wú)線轉(zhuǎn)發(fā)數(shù)據(jù)），該標(biāo)準(zhǔn)多年來(lái)一直在發(fā)展。該技術(shù)的向后兼容性和來(lái)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(shí)候，DCS 或 PLC 就位或指定時(shí)沒(méi)有 Hart 功能。因此，控制系統(tǒng)不使用 Hart 設(shè)備提供的額外測(cè)量和診斷數(shù)據(jù)。只有維護(hù)人員使用 Hart 界面進(jìn)行本地故障排除或使用手持設(shè)備進(jìn)行校準(zhǔn)。因此，有關(guān) Hart 設(shè)備功能的知識(shí)就會(huì)丟失。在這樣的背景下，它們的優(yōu)勢(shì)將再次凸顯出來(lái)。
配置：在簡(jiǎn)單的情況下，可以通過(guò) Hart 協(xié)議配置設(shè)備。預(yù)配置設(shè)置允許選擇操作參數(shù)，包括測(cè)量范圍、工程單位和傳感器類(lèi)型。
校準(zhǔn)：初始安裝和配置后，用戶可以通過(guò)設(shè)置零位和跨度、調(diào)整 4…20 mA 信號(hào)或考慮外部測(cè)量誤差源來(lái)校準(zhǔn)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(zhǔn)確的測(cè)量。此外，可以檢測(cè)到可能導(dǎo)致將來(lái)出現(xiàn)問(wèn)題的配置錯(cuò)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(jì)劃外維護(hù)。
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(gè)測(cè)量值。只有初級(jí)值由 4…20 mA 信號(hào)表示，而其他變量只能作為數(shù)字?jǐn)?shù)據(jù)使用。例如，壓力變送器可以提供絕壓或表壓過(guò)程壓力以及環(huán)境溫度。
溫度變送器還可以為用戶提供過(guò)程和環(huán)境溫度以及冷端補(bǔ)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(mén)定位器，它報(bào)告諸如閥桿的目標(biāo)和實(shí)際位置以及執(zhí)行器壓力和向執(zhí)行器輸出信號(hào)等數(shù)據(jù)。
如果過(guò)去工廠操作員決定升級(jí)控制系統(tǒng)并想要訪問(wèn) Hart 數(shù)據(jù)，通常會(huì)安裝一個(gè) RS485 Hart 多路復(fù)用器并將其連接到 PC。但是，大多數(shù) PC 沒(méi)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(gòu)買(mǎi)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(shí)證明，這是一個(gè)復(fù)雜的解決方案，需要配置和維護(hù)多個(gè)設(shè)備。
Hart 基金會(huì)近發(fā)布了 Hart IP 標(biāo)準(zhǔn)，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(gè)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(shì)，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.
