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Alireza Haji-Valizadeh, technology development manager at ControlSoft, says it’s extremely useful to treat the control loop itself as the asset, rather than the devices that make up the control loop. Once you start thinking that way, he suggests, it becomes rather easy to extend the concept to the entire plant.
“Looking at loops as a cut on automation hierarchy and as a component in a plant-wide structure can help to devise an effective predictive maintenance strategy,” stated Haji-Valizadeh in the December 2005 issue of our sister magazine, Plant Services. “We propose to extend this view of a loop to incorporate all levels of the automation hierarchy. As an example, consider a reactor temperature control loop.” In Haji-Valizadeh’s view, all of the components from the reaction, the sensors, heaters, device networks, PID algorithm, PLC and its programs, all the way out to the HMI and human operators themselves, are functional loop components.
Thus, the same condition-based monitoring that works for the lowest device and final control element layer can be applied to optimizing the process itself, loop by loop.
Now Get Started!
If you’re still operating in the mode of driving your plant to failure and then fixing what’s broken, you might try to look at a simple asset management project. Keep it small, keep it simple, and pick one with the potential of decent payback. You might get hooked on the idea of keeping your plant running by fixing what’s going to break before it actually does.
MOST PEOPLE think of HART, when they think of it at all, as a means to calibrate and set up field devices and valve controllers [See “HART 6: The Very Model of a Modern Calibrator?” in March '05]. It’s far more than that. It was envisioned in the late 1980s as a way to communicate digitally with devices that were designed to operate on 4-20 mADC current loops. In 1993, the HART Communications Foundation was established to support HART (Highway Addressable Remote Transducer) technology. The HART communications protocol is open and available to foundation members (vendors of HART-enabled devices) and the HART User Group.
HART technology is a hybrid of analog and digital communications. The technology imposes a modulated two-way digital signal on the industry-standard current loop that carries the primary process signal information. This means that HART provides two separate control signals on the same wire: the fast and robust current loop for local control and remote monitoring, and the digital signal for diagnostics, digital process information, calibration data and configuration of the operating parameters of the device. Every HART device contains 35-40 standard items of information, including device identification, calibration data, process variables (both measured and calculated, so that inferred multiple variables can be reported from a single transmitter) and a wide set of diagnostic alerts.
HART, of course, originated the DD (device description) file, which is the basis of both DDL and EDDL technology. DDL (device description language) is used to create the DD file, an electronic data sheet describing all of the capabilities of the smart field device. This permits all DD-enabled host systems to communicate with all device features of a HART-enabled device. This is now an IEC international standard.
DD is also the basis for FDT/DTM (Field Device Type/Device Type Manager) technology, which can be used to produce detailed data files for use by open asset management systems.
Because of HART’s diagnostics codes, plants with hundreds or thousands of existing HART-enabled devices can almost immediately institute asset management projects without enormous expense.
Sometime in 2006, the HART Communications Foundation will release its HART Wireless protocol. As with all HART upgrades, this one should be backward compatible to even the earliest HART devices (there are over 50 vendors manufacturing HART-enabled devices). Emerson Process Management recently released its first product based on the draft HART Wireless Protocol.
HART Wireless will provide an even easier way to release the data trapped inside your HART devices. More than a half dozen vendors have already begun designing a variety of upgrade packages to upgrade your HART transmitters to wireless. These may be as expensive as new transmitters, or as inexpensive as a “slap-on” radio transmitter that can perch on the 4-20 mADC loop. Asset management data can therefore be collected wirelessly, without even a single change to the control system. The asset management system can sit on a completely different server, and be entirely independent of the control system.
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