WirlessHART Networks in the Process Industry

In the process industries, misinformation about WirelessHART networks is prevalent among many instrument Engineers. This article sets the record straight by debunking seven common myths related to the WirelessHART networks.

1. WirelessHART is Unsafe

False. WirelessHART is safe. But why? A wide range of tools make this so.

Encryption—Communications are always encrypted by a WirelessHART network, which employs a 128-bit AES encryption system (Advanced Encryption Standard)—an industry standard in several fields of wired communication. However, it is not possible to disable the encryption.

The security manager in the WirelessHART gateway administers the following three parameters:

• Network ID
• Join key
• Session key

A network ID and join key is required to integrate a WirelessHART transmitter into a network. Once these are entered, the transmitter initially looks for the network with the right ID and when it detects such a network, sends a "Join Request" message with the key configured. The join key of the transmitter is then checked by the WirelessHART gateway. If the join key is correct, the transmitter is accepted by the network. The communication is encrypted by a session key. A separate session key is provided to every network subscriber which means they can only be accepted into a network with the join key. However, the encrypted communication of the other subscribers is not decrypted by this.

Access list—Once commissioning is completed, the acceptance of new network subscribers can always be disabled. This approach ensures that new network subscribers are not integrated within the network, even when the network ID and the join key are correct. In order to integrate a new subscriber into the network, this function can be disabled, or alternatively the UID (Unique Identifier = unique device serial number) of the network subscriber can be manually entered into the gateway. A network subscriber that no longer appears in the subscriber list of the gateway is also disregarded by other network subscribers when messages are forwarded.

Join counter—When a network is integrated with a WirelessHART transmitter, the WirelessHART records this data in the so-called join counter. The join counter of the device is increased if it joins the same network or if it is restarted. A join counter is included in both the gateway and the network subscriber and these cannot be read out. When a device attempts to integrate into a network with a join counter that does not correspond with the gateway, the gateway will decline it. Therefore, one device cannot be substituted with another device without this being noticed, even if both possess the same UID.

Nonce counter—A nonce counter exists in each transmitted message. This contains, among others, the UID and the number of messages sent by the transmitter to date. This mechanism is used to mark each message in a unique way. When a message is intercepted to resend it again later, it will be identified as obsolete and hence rejected. This method prevents any manipulation in the communication.

Modifying the network parameters—The network ID, network parameters and join key can only be modified by the gateway itself, or at a WirelessHART transmitter locally through the display or a service interface. This information can never be modified by a network subscriber or hacker in the network.

2. WirelessHART Networks are Too Expensive

It is true that WirelessHART devices are more expensive when compared to wired HART devices. But, more importantly, how do costs for the overall communication investment compare?

WirelessHART devices are more expensive because:

• They use high frequency components
• They contain ultra low power electronics to get prolonged battery life
• They need measures to achieve explosion protection

However, it is important to consider the entire solution and not just the devices alone. This solution involves material, engineering hours and labor hours.

Infrastructure for wired devices—Generally, the measurement signal of a new wired device has to be linked to a DCS or PLC to use the data. This is either achieved by the system's local I/O, a fieldbus connection or a remote I/O system. Although this is shown to be easy during a new installation (greenfield), this may rise to a challenge for a present installation (brownfield). There must be a spare capacity (terminals, channels, free slots) so as to add the new component. Another major concern is bringing the wires from the measurement to the I/O, needing routing and protection of the junction boxes, device cabling, cable trays and glands and all their accessories. It is essential to arrange, prepare and install all these infrastructures. An accessible location also has to be identified, or else this access has to be gained by other ways, such as by setting up a scaffold tower.

Engineering and labor costs—Before starting all this, Engineers have to develop a plan to identify which I/O makes sense, where cables can run and how this work can be carried out. The documentation should be constantly updated to track the location of wires.

Hazardous areas— Hazardous areas further increase the efforts and difficulty compared with general purpose areas. Technical issues and local conditions have to be considered by Engineers. An Explosion Protection Expert must verify the planned installation, including zone separation and a secure power supply.

Wireless device break-even points—Obviously, some amount of planning and installation are also required for a WirelessHART network. The key difference involves the effort because only the WirelessHART gateway needs a powered installation. Affordability will be determined by local conditions. The WirelessHART devices can be installed in any way that optimizes the measurement; separation of explosion zones occurs by default because there is no physical connection between the zones aside from the mechanics (e.g. a thermowell).

However, how much could be saved? The wireless solution obtains a breakeven point for the first installation of three or four WirelessHART devices, in addition to a single gateway. For instance, consider a popular device, a monitored heat-exchanger with two outputs and two inputs.

The heat exchanger will require four temperature transmitters. So assume:

• Four temperature transmitters
• A distance of 100 m between the scheduled junction box and control room
• 10 m of cables between each transmitter and the junction box

Realizing this solution will cost roughly USD 20,000, where only 20% indicates the cost of the temperature transmitters.

In the case of wireless, assume:

• Four temperature transmitters
• A distance of 10 m between the WirelessHART gateway and control room

Realizing this solution will account about USD 15,000, where 80% indicates the cost of the gateway and the WirelessHART devices.

Therefore, the wireless solution saves as much as 25% when compared to the wired one, and it will save even more in time. In effect, this solution could be available in a quarter of the time. And the next heat exchanger? If it is wired, it will cost an extra USD 20,000. If it is wireless, it will only add the cost of the new WirelessHART devices as the gateway is already available.

While three wireless solutions can be obtained for the price of two wired solutions, four wireless solutions can be obtained in the same time as a single wired solution.

3. WirelessHART Networks are Unreliable

A communication link for process control as well as for monitoring should be reliable and available as required. Examples of communication failures just when needed are a well-known fact. So, a wireless communication can ever be reliable? Interestingly it can be more reliable than cable. A time-synchronized, frequency hopping, meshed network is used to accomplish this.

Meshed network—As described before, all networks have a gateway that converts the wireless data into wired data ready for a PLC or DCS. The majority of wireless communications have a star architecture, which means all network participants connect only to the head or star center. Prime examples for a star topology are WLAN and mobile phone communication. WirelessHART has a mesh architecture, not a star one. The participants within a meshed network are communicating with the gateway and also among one another. In addition, the wireless devices inform the gateway which other participants they can communicate with.

Other wireless participants in range are known as neighbors. After analyzing information about the neighbors, the gateway generates a routing table that includes information on which network participant has which neighbors. Since participants can reach each other, they can route the data packets from and to their neighbors. In this manner, redundant communication paths for each network participant can be created by the gateway. If one communication path fails, the sender will automatically switch to a redundant path. As each transmitted packet has to be acknowledged by its receiver, a broken link can be easily detected.

RSSI and path stability—The radio signal strength indicator, also known as RSSI, indicates the quality of a communication link to the gateway. By knowing this, the gateway can find out whether the signal level is already too low or whether sufficient reserve strength is available. As the gateway receives the RSSI of each single communication link, it can readily differentiate between low and high level signals. The gateway also counts the data packets that are lost during transmission for each communication link. The gateway can detect paths with high losses and retransmissions by comparing the overall number of transmitted packets within a network. Both kinds of information are used by the gateway to detect good or bad paths within a network. Hence, the gateway can now pick the good paths that should be used by the network participants to communicate.

FHSS and DSSS—In order to ensure reliability, WirelessHART uses two techniques: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). WirelessHART is a frequency hopper in its 2.4 GHz band. The radio channel changes following each transmission between two network participants. Hopping across multiple frequencies has been shown to be a proven method to sidestep interference and resolve RF difficulties. When a transmission is blocked, the subsequent transmission will be to an alternate participant on a different frequency. The outcome is simple but very resilient in the face of common RF interference.

DSSS can transmit more than the required information, and sends eight bits for each single information bit. Each bit is encrypted such that the main bit is restored even when less than half of the eight bits are received. This makes the communication more powerful against short disturbances and data no longer has to be re-transmitted, which saves energy, time, and bandwidth.

Redundancy—As each WirelessHART device can route data for other devices, a network topology can be set up with redundant paths for each network participant. A reliable communication with the gateway can be ensured by having at least three independent and good communication paths. The gateway can establish all the information concerning network traffic, topology and quality of the communication paths.

4. The Range of WirelessHART Networks is Too Short

A common question concerns the maximum distance that can be covered by WirelessHART. Sometimes the issue is confused by answers relating to surroundings and obstructions. What range does a WirelessHART device really need to achieve? The sensible answer revolves around repeaters, the network setup and bandwidth.

Network setup—Getting the wireless data to a gateway that converts it into wired data ready for a PLC or DCS is the main aim of the network. A WirelessHART network that is properly setup has a minimum of three devices within range of each other, including the gateway. This guarantees a reliable connection to the gateway. Further, it must be ensured that the gateway is located towards the center of the network, or else devices next to the gateway become pinch points that reduce battery life and may lead to network failure.

If these recommendations for network setup are followed, a coverage of almost 200 feet can be obtained even in a highly obstructed area. In fact, coverage will usually expand to 300 feet. In large installations, more measuring points will be installed. This will automatically expand the network coverage because new WirelessHART devices will route the communication for other devices.

Frequency spectrum and bandwidth—In order to reduce power consumption, the number of device transmissions should be reduced to whatever is necessary to serve an application. The number of re-transmissions should also be kept as low as possible. WirelessHART uses time-division multiple access to avoid collisions. This means all links have their own unique time slot to communicate. Should this link fail for some reason or other, transmission will pass to another link.

License-free 2.4GHz ISM band is used by WirelessHART. This band can also be used by other applications (Scientific, Industrial, and Medical Band). Hence, it is important that WirelessHART share its bandwidth with all other technologies operating in the same band. This will result in collisions and re-transmissions for each device within the network as these various networks are not synchronized to each other (Bluetooth, WLAN and so on).

To maintain a stable and reliable network, time slots for re-transmissions should be reserved even if seldom required. More time slots are required for faster update rates of a device and the bandwidth of the total available network reduces. In reality, having an update rate of 1 second can easily lead to a maximum amount of 12 devices within a single gateway. As an alternative, two WirelessHART networks can be operated in parallel but this will also lead to collisions and reduce the bandwidth of both networks. Instead of having one long range network, two short-range networks that cover numerous areas with just small overlapping areas will improve their stability and device battery lifetime.

Repeater or routing device—At times, a measuring point is too far away from a network to connect, but this can be resolved by installing extra routing devices. While any WirelessHART device can be used for this purpose, the best fit is a device that is compact, can be easily installed and offers an easily replaceable battery.

5. WirelessHART Devices Constantly Need New Batteries

What would a wireless device be that needs a power cord—not completely wireless ofcourse. Therefore, it is mandatory to have a reliable and independent power supply. This requirement can be met by batteries, but with the disadvantage of their limited energy. It goes without saying that dead batteries should be replaced to get a battery-powered device running again. But, how big is this disadvantage really?

The WirelessHART devices from ABB employ an industrial-standard D-size primary cell. This cell was especially designed for extended operating life over a wide temperature range of -55 °C to +85 °C to meet the needs of process industries. However, how much lifetime is achievable? It depends. It is a hard fact that battery life cannot be predicted but rather it acts like the fuel consumption of a car. Some need less, some need more, based on speed and acceleration, traffic and vehicle weight.

In order to increase battery life, ABB electronics has an ultra-low power design- less by a factor of 20 compared to a traditional 4-20 mA HART device. All components have been carefully selected by their functionality and their power consumption. The aim of the design is to consume as little energy as possible, including software. For instance, sub-circuits power down if not required. Therefore, the sensor itself powers down between two measurements and the display. In case the update rate is slow enough, the device will fall into a "deep-sleep mode" between two measurements as frequently as possible.

The update rate is the user-defined interval at which point a wireless device begins a measurement and transmits the data to the gateway. Also, the update rate has the greatest effect on battery life— when the update rate is the faster, the battery life will be lower. This means the update rate has to be as slow as possible, but at the same time should meet the application needs. Based on the time constant of the process variable, the update rate has to be three to four times faster for monitoring open loop control applications. It also has to be 4 to 10 times faster for regulatory closed loop control and certain types of supervisory control.

Special attention should be paid for update rates that are faster than four seconds. With these faster rates, the device will be prevented from falling into the deep-sleep mode. Faster rates will also consume relatively more power, and thus affect the overall number of devices that can be handled by a single gateway.

Burst command setup—All WirelessHART devices are able to burst up to three separate HART commands. Obviously, the update rate of each command can be setup separately. However, the device attempts to go into deep-sleep mode as much as possible, as described before. By default, the update rates are set up as multiples of each other. This provides the device the best conditions to save as much energy as possible.

Network topology—Battery life can also be affected by mesh-functionality because each device has routing capability. If a single device behaves as a parent for another device and both devices are setup with the same burst configuration, then the parent should transmit data twice as often as its child. Also, the most power saving network topology is one that has all devices within an effective range of the gateway. While this is not possible all the time, this should be considered prior to placing the gateway. The gateway should be placed more or less in the center of a planned network so as to prolong battery life. In this fashion, the devices that behave as parents would be distributed uniformly — not depending on only a few devices to route data.

Knowing all these facts about battery life, what can be expected? Considering all these energy-saving recommendations and assuming the following:

• Bursting one command
• Using the device at 21 °C
• Having a direct communication path to the gateway
• Having three child devices with the same update rate

Under these conditions, the battery life may last up to:

• 5 years with an update rate of 8 seconds
• 8 years with an update rate of 16 seconds
• 10 years with an update rate of 32 seconds

If the device has a key position for routing within the network or if a faster update rate is preferred, ABB's Energy Harvester option can reliably relieve the battery. Most importantly, standard batteries are used by ABB's WirelessHART transmitters, making them easy to procure. While this will not save battery life, it will definitely save money.

6. WirelessHART Networks Require Specialists to Set Up

Most Engineers believe that it is laborious and annoying to set up a wireless network. It takes time when ensuring safe communication, getting everything running and including all preferred network participants. But is this true? What do we actually need to do to get a WirelessHART network running?

The wireless elements of a WirelessHART network comprise of:

• Field devices linked to the process or plant equipment. Obviously, they will all be WirelessHART capable
• A set of network parameters: Network ID and Join Key
• A gateway that allows communication between host applications and the field devices in the WirelessHART network

That’s it. Now, users can set up their network in a few simple steps:

Input of network parameter—To get the gateway into proper operation, users should input the network parameter. This can be achieved easily through the integrated web browser of the WirelessHART gateway. Most gateways provide this comfortable way of configuration. Now, the network participants can join the network, they also will require the network parameters. Here is the easiest way: users can order them with the preferred network parameters, or alternatively they must manually input the parameters.

Since a maintenance port is provided by all WirelessHART devices, users can use the tools that are already available for wired HART devices. This eliminates the need for more equipment. WirelessHART devices can be operated just like the wired HART devices. Moreover, ABB WirelessHART devices can be brought into operation by simply using their HMI. Again, users do not have to worry about security because it is all built-in.

Update rate —All WirelessHART devices are capable of bursting their measurement values and by default all ABB WirelessHART devices can burst HART command 9 at an interval of 16 seconds. This includes the dynamic variables TV, SV, PV, QV (for devices with multiple outputs) with the status of each and the remaining battery lifetime. WirelessHART devices burst HART command 48 every 32 seconds—the additional device status information. Therefore, users do not have to deal with the burst configuration. However, the update rates or the commands can be changed as required.

Placement of field devices and gateway— Users should first begin with the gateway installation. They must find an appropriate place for it and power it up. Since the gateway serves as the connection between the WirelessHART network and host application, it will require a power supply and wired connection to the DCS. Once the WirelessHART devices are prepared, they can now be installed in the field.

Installation can be carried out in the same way as wired HART devices. However, WirelessHART devices do not have wires and hence require less effort. This is particularly true in dangerous areas where nothing will cross the zones and output devices do not have to be checked with its ex parameters against an input device. Once the devices are powered up, they will automatically appear in the network. The gateway handles everything and users no longer have to worry about meshing the net or which device communicates with which.

7. WirelessHART is Too Slow

When users are asked for the desired speed to cover an application, they will usually answer: as fast as possible. However, the update rate for WirelessHART devices in a network can be separately configured between once per second and once per hour. Is that fast enough for everything? Before answering this question too quickly, it is important to look at a few considerations.

Usage—First, users should examine the applications for which a WirelessHART network is meant for: condition monitoring and process supervision. Note, the wireless sample/update rate should be:

• 4 to 10 times faster for regulatory closed loop control
• 3 to 4 times faster than the process time constant for open loop control and condition monitoring applications

For measurements in present process industries, over 60% simply monitor conditions- not for control applications. Therefore, a WirelelessHART update rate that is equal or greater than one second may fit most of these applications. Apparently, other factors may apply as well.

Timing—In the case of wired devices, update rates and timing are not usually taken into account. Operators and Engineers assume that the values in the DCS are actually real time values from the process, obtained by oversampling. In reality, signals are usually changed and scaled from the initial sensor element prior to reaching the DCS. Therefore, in a standard wired installation, latencies also exist in the measurement values. These facts are seldom known to instrument Engineers, and they simply assume that these values are timely enough. In the realm of WirelessHART, time stamps in the data packets spell out how old a measurement value is. This indicator allows Engineers to evaluate latencies and suitably react to them.

Thinking differently—Instrument Engineers in the process industries should know how fast a process value can change for condition monitoring as well as control applications. No extra knowledge is required for WirelessHART. With regard to wired installations, this knowledge impacts a PLC or DCS. For WirelessHART, it impacts the planning of the network itself. Since the bandwidth is a limited resource, instrument Engineers should consider how fast the update rate has to be rather than how fast it could be.

Comparing speeds—A speed of 1200 bits per second is provided by the traditional FSK-HART loop. In fact, HART on RS-485 cable is restricted to 38,400 bits per second. A speed of 250,000 bits per second is provided by WirelessHART, which means WirelessHART is over 200 times faster than wired HART and also six times faster than HART over RS-485 cable. By assigning the "Fast Pipe" to a network participant, the wireless gateway offers a high-bandwidth connection that is four times faster than a normal one. This is suitable for transmitting large amounts of data, such as uploading and downloading a complete configuration.

Terminology

This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.

For more information on this source, please visit ABB Measurement & Analytics.