Emerson’s Craig Abbott provides some technical considerations for your IEC 62591 WirelessHART field device network.
A frequently asked question about the WirelessHART 2.4GHz radio network is “How far can these devices transmit?” Depending on local units of measurement, the rule of thumb distance for the standard antenna is 225 meters or 750 feet. Interestingly, in an industrial setting, distance is only a part of the story and is not a determining factor for the suitability of WirelessHART for the majority of installations.
Let’s look at radio signal strength in theory, and then a practical example.
For a simple wireless path, the transmitting radio generates a signal at its nominal power level, which is then amplified by its antenna. The radio waves (electromagnetic) then travel outwards in all directions. Even directional antennae transmit in all directions, just much stronger in one direction than others. As the radio waves travel, they will hit objects that absorb or reflect some of the signal, weakening it. Eventually, the signal reaches a receiving antenna that amplifies the signal (and any noise) before the signal reaches the receiving radio. For a reliable radio link, the received signal needs to be stronger that the minimum strength that the receiving radio can detect, and stronger than any noise.
Emerson instruments and the wireless gateway measure and report the incoming signal strength as a “Received Signal Strength Indicator” or RSSI. We recommend a signal strength of -75dB to maintain a highly reliable communications link. This is a very conservative figure that leaves headroom to overcome temporary changes to the environment that reduce signal strength – obstructions, noise or even heavy rain. A signal strength less than -75dB will still work, but not at the 99.995% reliability that Emerson aspires to.
The role of distance in weakening a radio signal is analogous to the effect of distance on visible light. If you turn on a small light in a room and place a book one meter away, you may find that the book is well lit and legible. At two meters the book is less well lit and may be difficult to read and at three meters, too dark to read. If you think of a cone between the light and the book at one meter, and then extend that cone out to two and three meters, you would notice that the base of the cone becomes larger and larger whilst the book remains the same size. The total amount of light does not change, but book becomes only a fraction of the area that the light covers. The actual relationship is an inverse square, so that compared to the light at one meter; the light level is only 1/4 at two meters and 1/9 at three meters.
Radio signals decay in exactly the same manner, weakening at the inverse square of the distance. In 1945, the engineer Harald T. Friis, developed an equation to describe the relationship between the radio power at the receiving radio (Pr) and the transmitter radio power (Pt), based upon the amplification (gain) of the transmitting and receiving antennae (Gt and Gr), the radio signal wavelength (λ) and the distance between the antennae (R). Notice that the inverse square relationship (divide by R2) in the equation below:
If the power units are in dBm and the antenna gains in dB then this can be rearranged into:
As stated, the rule of thumb distances commonly quoted for standard Emerson WirelessHART transmitters are 225m or 750ft. If we substitute these for “R” in the Friis equation, along with the radio strength and antennae gain from the instrument specifications, we get an RSSI of -75dB. It should not be a surprise that the maximum quoted distance is at the recommended lower limit of received signal strength.
One can easily claim greater distances if a signal strength threshold lower than -75dB, and the reduced reliability is accepted. Additionally, where a point-to-point topology is used rather than a mesh network, omni-directional antennae can be replaced by directional antennae, which will boost the signal strength and allow the signal to be detected at much greater distances.
A transmitter with the exact same radio as a WirelessHART transmitter, but with a high gain (14dB), directional transmitting antenna, broadcasting to a high gain (8dBi) omni-directional receiving antenna would have an RSSI of -75.2dB at 1800m. This is an adequate signal strength at eight times the quoted range of a WirelessHART device. This distance seems impressive, however, how does it compare to a meshed network in practice, in a process plant?
An industrial facility on a one kilometre by one kilometre plot, with a WirelessHART gateway in the middle, would place each corner of the plant 707m from the gateway. This is well within the 1800m range of a point-to-point configuration. WirelessHART, with a nominal 225m range may seem limited, however, the ability to form a mesh network with up to seven hops means that only three, 235m hops would cover the distance, with the RSSI expected to be a reasonable -75.5dB. WirelessHART takes advantage of the fact that every measurement point is a repeater, so any location in a plant can be easily reached by relaying messages via neighbouring transmitters.
An even greater demonstration of the advantage of WirelessHART is to consider the impact of any obstacles – of which there are many in a process plant. A typical office wall with plasterboard (drywall) on each side would reduce signal strength by 6dB. Consider a building in the path of a radio signal where the signal would have pass though one wall to enter the building and another wall to exit the building, the two walls causing a 12dB signal loss. If we include a 12dB signal loss in the Friis equation, with a target RSSI of -75dB, it shows that a WirelessHART transmitter would be reduced to a 56m range and high gain directional transmitter to 469m. Note that the recommended distances between wireless instruments in a process plant are 150m for light density infrastructure, 75m for medium density, and only 30m for high-density plants.
A point-to-point network that is limited to 469m is just not going to reach a gateway that is over 700m away, at least not reliably.
The beauty of WirelessHART is that interference is avoided by relaying the signal through a repeater that is not in line with any obstacles. The full, end-to-end pathway from the originating transmitter to the gateway does not need to be a straight line and can go around, over or under any object.
An additional benefit is that every repeater is also a signal booster. Whilst one radio hop through an obstacle may be severely impacted, the repeaters after the obstruction are free to broadcast at full signal strength. Depending on the number of obstacles, additional hops may be required, however, WirelessHART allows for up to seven such hops. Where a direct pathway fails, WirelessHART is far more likely to get the message across.
Distance is a de facto measurement for comparing radios. It is a useful guide, however, needs to be understood within the context of the radio network. In an industrial facility with buildings, pipe work and tanks, short hops through a meshed topology is far more appropriate than powerful point-to-point links. Over time, process plants often change and new infrastructure is added. WirelessHART, that automatically responds, and re-routes radio pathways around new obstacles will provide a more reliable instrumentation network for years to come.
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