SpecMate: Where the HART Is

HART 's 16 Diagnostic Status Bits Can Provide Early Warning of Device Problems

 

This time of the year, after big trade shows such as the annual ISA Expo and the biennial Chem Show are done making news, comes a reminder that there's a lot of analog process data handling being dealt with within the process equipment builder community.

So, it's a good time for a quick review of the HART fieldbus protocol, the most popular device data network protocol in the process industries.

Many users know HART provides communication of multiple data items between remote devices and a host controller or computer. What most HART users don't know is there is a large amount of data that can be communicated between their existing HART devices and control systems. Typical user estimates of available data items range from 3-10 values in addition to the process variable. The actual number is 35-40 values depending on the type of HART device.

The most important data items available from HART devices are the process variables. The primary process variable is continuously transmitted in two formats: 4-20 mA analog and digital as part of the HART protocol. The primary process variable is also transmitted as a percent of range. Finally, the loop current in milliamps can also be accessed to validate the signal being received by the controller.

The primary process variable digital value is expressed as an IEEE floating point number with up to 32-bit precision. This far exceeds the standard 12-bit precision offered with most PLC analog input modules.

Many existing, and most new HART devices have multivariable measurement capabilities. This is true of many pressure and temperature devices; most flow, level, and analytical devices; and all valve positioners and valve controllers

Up to three secondary process variables, in addition to the primary process variable (total of four), can be simultaneously transmitted from a HART device to a host in a single message. This multivariable capability can be exploited in a number of ways. One of the better ways for your customers to contribute to the improvement of operational profitability is to use the on-board temperature sensor located in virtually all HART devices. This sensor measures the internal temperature inside the device, not the ambient or process temperature. The manufacturer uses internal device temperature to characterize the output during wide temperature fluctuations, thereby eliminating most of the temperature-related error of the device. A user can digitally transmit the on-board device temperature to the control system using the HART protocol.

HART devices provide eight diagnostic status bits (16 bits with the new HART 6 protocol). These status bits can be used to provide early warning of device problems. Handheld communicators can access this information when connected to the instrument loop, but many applications would benefit from continuous monitoring of these status bits.

This monitoring function is provided with some HMI software packages, but not all control and monitoring systems have this functionality. When a HART status bit changes, a HART loop monitor can provide both LED indication and a relay output.

The relay output can be connected to an existing control and monitoring system, and the loop monitor can also provide a 4-20 mA signal based on one of the HART process variables. This allows a HART instrument to be interfaced to a control system simply and quickly.

HART-compatible temperature transmitters can be used to avoid a process shutdown. A HART loop monitor can be configured to use the status bits to provide a relay output indicating sensor failure. This differentiates between a sensor problem and potentially dangerous operating condition.

Most HART users know that HART instruments can be configured remotely from any point on the HART data highway. This calibration can be performed with handheld calibrators or through a host computer.

HART transmitters feature programming options that go beyond more common universal input capabilities. The arrival of simple and functional Windows-based calibration and configuration software allows faster setup and more precise settings than with a handheld communicator. Setup is simplified and time is saved by using a host computer for calibration. Configuration via a computer is especially advantageous for multiple instruments with the same calibration parameters. A PC configuration can be stored to disk and downloaded to multiple transmitters.

Periodic instrument calibration is a necessity for all processes, but continuous calibration can be a requirement for certain applications. If the composition of a fluid or gas that is being measured changes continuously and if this change affects calibration parameters, then continuous calibration will maintain the accuracy of process measurements.

Asset management software programs make extensive use of the device identification information available through HART. This information is read by the asset management program and stored in a database, allowing the program to automatically populate the database with pertinent information related to each instrument's tag number, manufacturer, device type, final assembly number, and serial number.

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