Close integration of intelligent field devices provides many benefits, and a recent field test gives the proof BY john wheelis

Intelligent field devices have been used in process plants for over 10 years. As the use of these devices has increased, and the discussion on fieldbus devices and networks has continued, it is apparent that many additional benefits can be obtained through tighter integration of the intelligent devices with the control system.

Numerous proposed benefits from the use of intelligent field devices range from remote calibration, remote diagnostic capabilities (including self-monitoring, device diagnostics, and process data validation), greater measurement accuracy and stability, improved calibration capabilities, reduced maintenance costs and improved process performance.

Additional benefits that can be obtained from a tighter integration of field instruments with the controls include increased process visibility, automation of desired response to field device malfunction, on-line re-ranging, reduction in field wiring and I/O and integration of field data with plant maintenance.

Field tests prove positive

A field test was conducted in an agricultural products plant to determine the practical effects of this tighter integration.

Firstly, intelligent devices can now communicate additional information on the 4-20 mA signal about the process or the device itself, via multiple variables from a transmitter or digital valve controller.

Typically, a mass flowmeter is able to communicate the mass flow rate, mass flow total, standard volumetric flow rate and total, density and process temperature. Further, a digital valve controller can communicate its actual valve position, actuator pressure, input current from the control system, internal temperature, and have travel alerts set instead of installing limit switches, in addition to many other user-configured maintenance alerts.

For some process indications and control loops (where high-speed is not required) the HART digital signal can be used directly, eliminating the A to D conversion, which introduces one of the largest errors in the control loop. The control of a non-critical flow loop and several process indications was demonstrated successfully using the HART signal.

Several important benefits are provided by the use of digital communications, including concurrent operation of the master on the communications (the control system interface), with devices acting as secondary masters.

The capability to have the handheld or the PC configuration system (secondary master) operate and communicate concurrently with the field device and the control system interface (primary master) was demonstrated successfully.

During the field test, the maintenance software was used to interrogate the digital valve controller for configuration and on-line diagnostic information.

The field test successfully demonstrated the capability to re-range an indirect measurement either in terms of the transmitter units (pressure) or in terms of the process units (flow rate).

An example would be a pressure measurement used to measure the rate of flow to a vessel. The pressure transmitter was ranged from 0 to 48 inches H2O, inferring a flow of 0-1 gallon per minute.

The alerts, the actual valve position, and the actuator pressure from the digital valve controller provided information to the operator that was either not available, or only inferred from other information. Integration advantages shown during the test were:

{{* automation of desired response to field device malfunctions;* on-line re-ranging for changes;* reduction in field wiring and control system I/O; and* provision of a platform for integration of field data to PC- based applications.}}

The integration of the intelligent devices into the process control system allows for the status data to be used not only for the operational aspects which have been discussed, but also for additional maintenance needs.

The solution to these additional needs can be included in software applications written for personal computers that are designed to help manage the information from and on the intelligent devices, as part of the plant’s assets.