For more than 20 years, HART Communication has been the fieldbus protocol that has been used the most. There are over 30 million installed HART-enabled devices, worldwide. That's more than double the number of devices installed by the next most common fieldbus. Why is this true? HART Communication has always been built on open standards. The IEC recognizes HART as part of its Industrial Communication Networks and Device Description Language (EDDL) standards. WirelessHART is built upon the IEEE 802.15.4 standard for low-power mesh radio networks. And WirelessHART itself is now an IEC standard, IEC62591. Another reason is that HART maintains the conventional 4-20mA signal, another international standard.
Typically, most users are familiar with the use of handheld communicators, such as the Rosemount 375 or 475 handheld communicators, or of PC-based communications software, such as ProComSol's Devcon2000 Smart Device Communicator. These devices are for manual programming or diagnostic checking of a plant's HART-enabled field devices.
And since WirelessHART is HART, anything you can do with a handheld or PC-based communicator, you can do wirelessly through WirelessHART.
There is so much more that you can do with your already existing HART-enabled devices, so let's look at what some end users have done.
How Wired HART Works
"HART" is an acronym for Highway Addressable Remote Transducer. The HART Protocol superimposes digital communication signals at a low level on top of the 4-20mA. This enables two-way field communication and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument, and there is no interference with the 4-20mA signal.
HART technology is a master/slave protocol, which means that a smart field device (slave) only speaks when spoken to by a master. The HART Protocol can be used in various modes such as point-to-point or multidrop for communicating information to/from smart field instruments to control systems. HART Communication occurs typically between a smart field device and a control or monitoring system. Best of all, HART Communication occurs using standard instrumentation-grade wire and using standard wiring and termination practices.
The HART protocol provides two simultaneous communication channels: the 4-20mA analog signal and a digital signal. The 4-20mA signal communicates the primary measured value (in the case of a field instrument) using the 4-20mA current loop—the fastest and most reliable industry standard. Additional device information is communicated using a digital signal that is superimposed on the analog signal.
The digital signal contains information from the device, including device status, diagnostics, additional measured or calculated values, etc. Together, the two communication channels provide a low-cost and very robust complete field communication solution that is easy to use and configure.
What You Can Do With Wired HART
Lower Your Maintenance Costs by 10%. Mitsubishi Chemical's HART Plant of the Year for 2009 is using the HART Communication capability of over 800 interoperable field devices integrated with its DCS and asset management systems through multiplexers and HART-enabled I/Os to access real-time continuous process variables and diagnostics. By accessing this real-time intelligent data Mitsubishi Chemical is able to diagnose abnormal process conditions and track equipment health 24 hours a day. As a result, peak production performance has improved, with an estimated operational savings for the plant of $20,000 to $30,000 USD per day.
"Diagnostic parameters that help detect signs of an abnormal situation or degrading performance are difficult to obtain with simple handheld devices because they require a manual, time-consuming, step-by-step approach," says Takayuki Aoyama, team leader, instrumentation group, Mitsubishi Chemical. "HART technology made it possible to access this data without manual operation. This made it much easier for us to gather data and detect abnormal situations from field devices and has reduced maintenance costs by 10 percent."
Manage Your Inventory. Tank level and inventory management is an ideal application for a HART multidrop network. The HART network digital update rate of two process variables per second is sufficient for many tank-level applications. A multidrop network provides significant installation savings by reducing the amount of wiring from the field to the control room, as well as the number of I/O channels required. In addition, many inexpensive process-monitoring applications are commercially available to further cut costs.
HART multiplexers are available from multiple vendors, such as Pepperl+Fuchs and Phoenix Contact.
In fact, as Gerrit Lohmann, Pepperl+Fuchs' wireless product manager, points out, "When a HART 5 device is already installed and wired in the field, it does not initially make much sense to upgrade this device with a WirelessHART Adapter just to access the diagnostic data. Normally, a HART Multiplexer is the superior solution in terms of investment and operational costs. It has been shown that the hardware costs alone are significantly higher when implementing a WirelessHART solution verses a standard HART Multiplexer installation. Furthermore, the performance of a wired HART Multiplexer could be considered more stable when considering site constraints, distance limitations and network-related issues."
Lohmann says, "With a HART Multiplexer, a short process shutdown will probably be required during the installation."
He goes on to note that there are advantages to using WirelessHART. "A plant shutdown is normally not required since the devices can be upgraded individually; however, the instrument may be offline for a short time during the instrument commissioning process. The existing wiring in the marshalling area is not affected, so during and after installation of the WirelessHART Adapters, the plant should continue to operate. There is normally enough space to mount the WirelessHART adapter on or near the field instrument, so installation is relatively easy. A HART Multiplexer requires control room cabinet space. This premium area is normally occupied with other existing hardware. As a result, the costs of an additional cabinet or junction box must be considered."