Nutz and Boltz of Wireless HART™

The mesh

The main secret to the success of WirelessHART™ installations is the mesh. Using the global standard IEEE 802.15.4 2.4 GHz radio and channel-hopping technology, WirelessHART™ installations are flexible, secure, reliable, and simple.

All field devices have the same wireless capabilities, and as the number of devices in the mesh grows, the more secure and reliable it becomes. The more devices in the mesh, the greater the number of pathways exist from any single device to an access point, from an access point to any single device, or from a device to another device.

At the same time, the user experience mimics using wired HART, with the same tools and practices users have become familiar with. The same command structure is also used in WirelessHART™, and WirelessHART™ is compatible with HART-enabled control systems and EDDL.

“End users get a state-of-the-art standard wireless infrastructure,” says Emerson‘s Zornio, “supported by a full set of vendors they know and trust, so they can invest with confidence. At the same, time,” Zornio goes on, “WirelessHART™ builds on the experience, knowledge, work practices, and investments end users have made in HART over the last 15+ years.”

Every device is a router

The WirelessHART™ network consists of field devices, adapters, access points, and gateways. Somewhere on the network, either built into a gateway or separately, are the Network Manager and the Security Manager. No devices are network end points, or reduced function devices to be orphaned by changing signal conditions.

The Network Manager maintains and updates routes. It establishes links between neighboring devices, and it allocates bandwidth. There is a single Network Manager (can be redundant) in a WirelessHART™ network that can manage multiple gateways and access points. The number of access points needed is determined by the number of devices and average update period, using a simple formula (see “HART 7: Installing and Making Systems Work” in this issue). Geographic location may also be a factor. The more devices you add, the more access points. Multiple access points per gateway mean redundancy and faster throughput, and even better “path diversity.”

All communication paths in the mesh network are continuously exercised (primary and redundant).  All WirelessHART™ devices are routing-capable to maximize the number of possible redundant pathways, increase the reliability, and simplify network design.

WirelessHART™ technology’s priority-based routing enhances reliability. Its first priority is to keep the network operational. Second is to deliver the process monitoring and control data. Third, enable management and troubleshooting of the devices. Fourth, deliver the alarms. All alarms are time-stamped and latched. That way alarms are never lost, and the alarm log details in what order the problems occurred.

User deployment and operation is simple. WirelessHART™ devices are configured using the same tools as wired HART devices. Only the Join Key and Network ID need to be loaded into the device for it to join the network. These items can be loaded by the user or preloaded at the factory, and the device does the rest.  The device joins the network and, working with the Network Manager, it embeds itself in the mesh. The Network Manager then continuously grooms the mesh to ensure reliable, low-latency communications as plant conditions change and evolve.

Because every WirelessHART™ device is also a router, any device can direct and re-transmit data from any other device. Users do not have to maintain, stock, and learn how to operate two kinds of field devices.

Every device reports battery life in days and as a PV

Some of the most important process variables are those that today are not measured because the cost of wiring them into the control system is simply unmanageable. These include remote level measurements and asset-management information. In such applications, low-power sensors, many operating on batteries, are critical to making these measurements economically viable. Battery-operated WirelessHART™ devices make these measurements practical.

In keeping with the spirit and tradition of HART to “keep it simple,” the HART 7 standard requires that battery life be reported as a PV (process variable) and reported in “days remaining.” This key performance indicator enables operators and maintenance personnel to see simply and clearly when a battery in a given WirelessHART™ device must be changed.

Interoperability and Interchangeability

WirelessHART™, like all the rest of the HART 7 specification, is built on backward-compatibility and Electrical Interchangeability. HCF’s definition is rigorous: the freedom to use any manufacturer of HART devices, wired or wireless, and to mix manufacturers and products in the same wireless network.

With WirelessHART™, users may mix any number of manufacturers’ devices with any vendor’s Registered HART wireless adapter and any other vendor’s Registered HART wireless access point or gateway. The same HART Universal Commands are required in all WirelessHART™ devices, just as they are in HART wired devices. The same Universal diagnostics exist in every HART device, wired or wireless, and may be used in exactly the same way. The same tools work with all HART devices regardless of manufacturer, and the EDD for that HART device (wired or wireless) works on all EDD-compliant hosts.

And any of those devices may be replaced in kind by another HART-enabled device from any manufacturer with instantaneous interoperability.

This incredible flexibility is provided on top of the largest installed user base of devices in the world—more than 26 million HART-enabled devices as of mid-2008.

As Sean Ireland, wireless product manager for Siemens Energy and Automation notes, “WirelessHART™ gives us the opportunity to provide flexibility to the customer for installation, a lower cost for installation, the ability to enable new applications previously not accessible due to technology or cost limitations, and improved information availability, which should result in process improvements for our customers.”

Proposed global standard

WirelessHART™ was built on proven industry standards. The radio is based on the IEEE 802.15.4-2006 standard. WirelessHART™ is also built on the IEC HART Standard, IEC 61158, and on the IEC 61804-3 standard for EDDL. The wireless portion of HART 7 also has been submitted for ballot to become a global IEC standard.

The 2.4 GHz frequency band used in WirelessHART™ radios was selected because of its international availability. It is available, usually without licensing, in almost every country, and is reserved for industrial, scientific and medical uses, and other commercial communications needs. Many cordless telephones also operate in this band, as do other IEEE radio standards, such as 802.11(a, b, g and n), the WiFi standard.

WirelessHART™ bandwidth usage

Many wireless applications and protocols are in use already in the modern industrial plant: cellular and VoIP telephony, WiFi, WiMAX, Bluetooth, and other wireless sensor networks.

WirelessHART™ communication is designed to result in low bandwidth usage. The communication protocol is a channel-hopping TDMA protocol.  This protocol has 100 10-ms “communication slots” per second over 15 channels, allowing multiple devices to communicate in the same slot, but at different frequencies. This avoids interference and reduces multi-path fading. WirelessHART™ also provides priority-based message routing so the network stays up, and alarms and critical data packets get the appropriate priority. The Network Manager continuously assesses network health and retries and may “blacklist” any of the 15 channels to reduce latency and prevent interference from existing networks that might be in use.

“An upper bound on bandwidth consumption can be estimated, assuming a large number of devices being in a single geographic region, so that any device in the region can hear all other devices in the region,” says Wally Pratt, chief engineer of the HART Communication Foundation. “The HART standard provides estimates on communication range that would define the boundary size (diameter) of the spherical region containing that large number of devices or access points in the network. The number of devices in the region does not limit the number of devices or access points in the network.  The network can have many devices with multiple hops and cover an area much larger than the ‘region’ we are considering here.”

For example, if one were to have 10,000 WirelessHART™ devices located in a spherical region, meeting the above criteria with an average update period of 60 seconds, communicating through 15 access points, the bandwidth consumed would be approximately 4.3% The diameter of the spherical ‘region” would depend on whether the installation is indoors or out and the configured transmit power setting for the devices. Consequently, the region can vary from 200 meters in diameter to less than 20 meters in diameter.

Extrapolating to a more general basis for industrial plant applications, the bandwidth consumption by a large WirelessHART™ network will be no more than 10% of the available bandwidth in the 2.4 GHz band in any industrial plant.

A WirelessHART™  network with any conceivable number of devices cannot consume all of the 2.4 GHz bandwidth in any installation.(For a bandwidth usage calculator for WirelessHART™ networks, see “HART 7: Installing and Making Systems Work” in this issue.)

Coexistence with other networks

“Coexistence is the ability of one system to perform a task in a given shared environment in which other systems perform their tasks and may or may not be using the same set of rules.” This is the IEEE’s definition of “coexistence” and serves to illustrate the difficulties faced by the HART 7 design team when they faced the creation of a wireless version of HART.

There are a very large number of wireless networks and devices that WirelessHART™ must coexist with, including cordless telephones, microwave ovens, WiFi, WiMAX and other wireless sensor networks, such as Zigbee and Bluetooth.

WirelessHART™ is designed using standardized IEEE radios, clear-channel assessment, blacklisting, channel-hopping, mesh-to-the-edge networks, and redundant path routing, to coexist with any other network in the 2.4 GHz band, and not only to not interfere with other networks, but also to not allow other networks to interfere with WirelessHART™.

Interfacing with control systems

A WirelessHART™ gateway is a remote I/O system for connecting WirelessHART™ devices to plant control and/or asset mangaement systems (new or existing). Since WirelessHART™ is backward compatible with wired HART, the WirelessHART™ gateway allows integration to plant systems in the same way I/O systems do for traditional wired HART devices. Like existing remote I/O, gateways will offer a variety of host-side interfaces to standard industrial communication networks like Modbus, Profibus, Profinet, Foundation Fieldbus, OPC XML, RS-485, Ethernet, and others.

Every major asset management software vendor produces HART-enabled products that will interface directly with the digital data provided by the WirelessHART™ gateway. So all users have to do is connect either a native WirelessHART™ product or a WirelessHART™ adapter to an existing HART-enabled device, achieve communications at the gateway, and connect the gateway to their existing HART-enabled software, whether it is a control, asset-management, or CMMS system. This releases the “stranded” HART diagnostic information and makes it possible to implement state-of-the-art predictive maintenance where it was not cost-effective before.

WirelessHART™ security

Industrial control systems have traditionally been built with little regard for security. Denial-of-
service attacks, spoofing of identities, authentication issues, industrial espionage, and terrorist attacks, along with invasion by organized crime and damage caused by disgruntled employees are among the threats faced by all control systems.

HART 7 permits a single system of secure communications. Critical measurements should be wired using HART-enabled instruments. Less critical measurements and control parameters can be wireless. But all HART 7 communications are secure.

WirelessHART™ is secure. The data is encrypted with 128-bit AES encryption, a strong cipher. Provisioning (hooking up a new device to the network) is accomplished by secure “join keys” which are entered into the device via a secure wired connection. All messages are encrypted and individual session keys ensure that only valid devices can pass information in the network. End-to-end data encryption is employed to prevent sensitive data from being intercepted. Message Integrity Codes (MICs) are generated after the data is encrypted to sign the data end-to-end. Denial-of-service attacks are mitigated with channel-hopping and redundant paths provided by the mesh infrastructure. Password protection (secure keys), white lists, and black lists prevent unauthorized devices from joining the network and communicating on the network. The use of individual encryption codes and passwords on a point-to-point basis provides secure authentication for WirelessHART™.

Registration program for devices

Like all HART devices, all WirelessHART™ devices must undergo detailed testing before receiving the right to display the “HART Registered” mark.

WirelessHART™ security diagnostics

WirelessHART™ devices have all the same universal set of diagnostics as wired HART devices, plus additional standardized diagnostics specific to the wireless connection.  Among the diagnostics available to all WirelessHART™ devices are

• Device malfunction—Simply stated, the number one alarm to monitor (check device immediately)
• Configuration change—System verification of configuration changes (confirm device reconfiguration was correct)
• Cold start or reset—Power supply problems (monitor for continued problems and repair as needed)
• More status available—More information exists to troubleshoot device (a DD-enabled host application can retrieve additional device specific diagnostics)
• Loop Current fixed—The analog output should not be used for control (may or may not be an error)
• Loop Current saturated—The analog range has been exceeded (the digital value should be used for the measurement)
• Non PV out of limits—Varies by device type: for example, a multivariable device with a temperature measurement may have exceeded operating range (verify device accuracy)
• PV out of limits—Device measurement range has been exceeded (verify device accuracy)
• Maintenance Required
• Device Variable Alert
• Low Battery
• Critical Power Failure

All devices build and maintain a list of neighbors. All devices report their neighbor list and network health. Some of the network health parameters include “average latency measurement” per device, “path down” alarms, “number of lost packets” (should be zero), and “devices-per-access-point” loading.

The WirelessHART™ specifications were built on proven industry standards with support from nearly all major vendors of automation systems and devices. The HART Protocol continues to be enhanced to meet the changing needs of the process industry and to adapt to new instrumentation technology, such as wireless. The backward-
compatibility of HART Protocol revisions ensures that an investment made in a HART device or system is protected into the future.

The wide adoption of WirelessHART™ by automation product manufacturers ensures that WirelessHART™ devices have the same security as investments that HART devices have proven to have for over 18 years.