Technology Update: Why WirelessHART?

With other industrial wireless communications options available, process industry engineers may ask, ‘What is WirelessHART, and what is it good for?’

02/28/2011


Greater network connectivity, especially wireless communications, can augment vast untapped potential of millions of process devices that are equipped with HART protocol, but not regularly sharing information with other systems about processes. WirelessHART protocol, linked with appropriate network hardware, is not limited to “rescuing” stranded devices. It can also be used to create or expand networks using no wires at all.

HART protocol, used by the process automation industry for more than 20 years, is used in 30 million HART devices installed in petrochemical, electrical power, food and beverage, oil and gas, water/wastewater, and other industries. With  about 10 percent of these are leveraged to their full potential, providing valuable data back to an assets management system or other data collection devices.

Since it was created, the HART protocol has gone through a number of revisions, spanning from HART5 to the newest standard, HART7. The HART Communication Foundation (the owner and standards-setting body of the HART protocol) ensures that each revision of the standard is backwards-compatible with the previous versions.

This means that a HART7 device can work with a HART5 device. While the feature set will be limited to the HART5 device, the two devices will be fully compatible. With the advancement to the new HART7 standard comes a wireless option known as WirelessHART. Because WirelessHART is a public standard, devices from multiple manufacturers can work together on the same network. By combining wired and wireless technology, WirelessHART increases the usability and extends the range of HART applications.

Most HART applications use the protocol to commission and calibrate instrumentation using a hand-held programmer in the field. Once the instrument is installed, the HART data does not come back to the control room, leaving the HART data in the device stranded.

WirelessHART provides a means to rescue these stranded devices, allowing the primary process variable to be presented back to a DCS (distributed control system) using the traditional 4-20 mA signal and providing all remaining HART data over a wireless network. The engineers can then use this additional data for predictive or scheduled maintenance, greater process resolution, or to clearly define instrument conditions or faults. 

 A native WirelessHART instrument can run on battery power, eliminating the need for a power wire, which eliminates the need to dig trenches and lay cabling, decreasing the deployment costs of traditional HART networks. It will allow measurements to be taken in areas that might have previously been unattainable because of the logistics, cost, or regulations of running wire.

Next questions are:

  1. How do you plan for a WirelessHART system?
  2. How difficult is it to set up?

A WirelessHART network can have three device types: WirelessHART instruments, adapters, and gateways.

WirelessHART instruments use battery power so are completely wireless. They transmit all information via the WirelessHART network. Adapters bolt onto a wired HART instrument, allowing the HART data to be available over the WirelessHART network. They leave the 4-20 mA signal intact for connection to a DCS system. Finally, the gateway allows all WirelessHART instruments and adapters to talk to one another. The gateway manages the network, defines routes, ensures security, and converts standard industrial protocols, such as MODBUS to HART, for communication between remote devices and the control room. 

To program WirelessHART instruments or adapters, the installer can generally use a standard HART handheld or modem, just as if installing a traditional HART instrument. WirelessHART gateways, on the other hand, are generally programmed via Web-Based Management, for easy browser-based configuration on  a PC. However, some gateways will allow programming via HART handhelds or modems, just like the instrumentation. This is ideal when a PC is not available or not allowed in specific areas.

As for WirelessHART network planning, the wireless network leverages mesh networking, meaning that every device can talk to every other device in the network, creating a wireless “web.” Using a mesh network system greatly simplifies design and deployment. “The general recommendation in deploying a WirelessHART network is that any installed device should have at least two other devices within 200 meters. In most cases, a WirelessHART device can communicate about 100 m (about 300 ft). As a best practice, plan for 100 m between devices, though it is possible to communicate greater distances with clear line of site.”

Every device should have a back-up path in case something should happen to its primary path. Which path is the primary and which is the secondary? The gateway will define this, so it does not need to be defined when designing the WirelessHART network.

In general, the more devices in a WirelessHART network, the better it is, as a higher number of devices increases distance and redundancy. However, increasing network size does have some drawbacks. As network size increases, so does chatter from all of the wireless devices. This can lower response times from the network. It can also lower battery life because devices that are not directly being asked for data may have to route information from a device deeper in the network. There is no firm line to determine when this chatter is too great. It really depends on the application.

Breaking a large network into smaller clusters can increase system performance. By using the cluster method, more time slots will be available for remote devices. This allows faster update times (if needed). With less node routing required, battery life will increase. To divide a network into clusters, add more gateways to the system.

Traditional gateways are wired back to the control room using an Ethernet connection. If a gateway is remotely located in the plant, this additional wiring can reduce the cost savings associated with a WirelessHART network.

Some gateways, such as the Phoenix Contact WirelessHART gateway (RAD-WHG/WLAN-XD), integrate a WLAN (802.11) network in addition to the wired Ethernet connection. This WLAN network connects concentrated HART data to the control room without wires. Using WLAN and a wired connection for backhaul, the user can implement redundancy in the network. Or, if a wired backhaul is used, the mobile operator can use the WLAN network to interface to the gateway, which helps network maintenance.

Because this wireless protocol is designed for the industrial environment, it can overcome interference and maintain security. It uses 128-bit advanced encryption standard (AES) and sophisticated authentication processes to ensure the integrity of the information being communicated.

Overall, WirelessHART offers a cost-effective means to maximize the efficiency of an existing HART system. It also simplifies networking of a new HART system. The WirelessHART protocol is designed to be simple, providing 300-feet connections to any neighboring WirelessHART device in the same network. Like traditional HART devices, it is programmed using HART handheld or modems.

- Ira Sharp is product marketing lead specialist-wireless for Phoenix Contact, www.phoenixcontact.com/wireless.

www.hartcomm.org