Murphy’s 7 laws of industrial wireless communications

Applying wireless technologies for industrial communications doesn’t have to be as tough as Murphy’s Law (if anything can go wrong, it will), if you remember these Murphy’s 7 laws of industrial wireless communications. Wireless troubleshooting tips follow, including the number-one cause of wireless woes.

10/18/2010


Applying wireless technologies for industrial communications doesn’t have to be as tough as Murphy’s Law (if anything can go wrong, it will), as long as you remember these Murphy’s 7 laws of industrial wireless communications. Do you know the number-one cause of wireless woes? That’s included below.

1. You can’t trump the laws of physics. Stick with the lowest frequency possible.

Even Murphy knows better than to argue with physics. Industrial applications typically operate in “license free” ISM frequency bands that vary from country to country. The most common frequencies are:

  • 2.4 GHz—nearly worldwide;
  • 915 MHz band—North America, South America, some other countries; and
  • 868 MHz band—Europe.

Radio frequency (RF) power is measured in milli Watts (milliwatts; mW) or in a logarithmic scale of decibels (dB), or decibels referenced to 1 mW of power (dBm). Since RF power attenuates as a logarithmic function, the dBm scale is most useful. These scales relate as follows:

               1 mW = 0 dBm

A 2-fold increase in power yields 3 dB of signal.

               2 mW = 3 dBm

A 10-fold increase in power yields 10 dB of signal.

               4 mW = 6 dBm

A 100-fold increase in power yields 20 dB of signal.

               10 mW = 10 dBm

               100 mW = 20 dBm

               1 W = 30 dBm

As frequency rises, available bandwidth typically rises, but then distance and ability to overcome obstacles is reduced. For any given distance, a 2.4 GHz installation will have roughly 8.5 dB of additional path loss when compared to 900 MHz. However, lower frequencies require larger antennas to achieve the same gain.

2. Protocols and software can be downright unfriendly to wireless. Make sure your application software won’t choke.

Before you lift a finger toward the perfect wireless installation, think about the impact of wireless communications on your application. Acceptable bit error rates are orders of magnitude higher than wired communications. Most radios quietly handle error detection and retries for you at the expense of throughput and variable latencies.

Software must be well-designed and communication protocols must be tolerant of variable latencies. Not every protocol can tolerate simply replacing wires with radios. Protocols sensitive to inter-byte delays may require special attention or specific protocol support from the radio. Do your homework up front to confirm that your software won’t choke, that the intended radio is friendly toward your protocol, and that your application software can handle it as well.

3. Transmit power does not necessarily equal long-range performance. You must know the receive sensitivity.

The more sensitive the radio, the lower the power signal it can successfully receive, stretching right down to the noise floor. There is such variety in “specsmanship” for radio sensitivity that it can be difficult to compare products meaningfully. Unless you’re in a high RF noise environment, the odds are good that the noise floor will be well below the receive sensitivity, so the manufacturer’s rated receive sensitivity will be a key factor in your wireless system and range estimates.

You can often improve your receive sensitivity, and therefore your range, by reducing data rates over the air. Many radios give the user the ability to reduce the communication rate to maximize range. Also, receive sensitivity improves at lower frequencies, giving 900 MHz radios a significant range advantage.

Contrary to popular opinion, no black art is required to make a reasonable prediction of the range of a given radio signal.

The mathematical expression for successful radio reception is:

        TX power + TX antenna gain – Path loss – Cabling loss + RX antenna gain

         – 10dB fade margin > RX radio sensitivity or (less commonly) RF noise floor

Most parameters above are easily gleaned from the wireless device manufacturer’s data. That leaves only path loss and, in cases of heavy RF interference, RF noise floor as the two parameters you must establish for your installation.

4. Unexpected background radio noise can tank your installation; find it first.

RF background noise comes from many sources, ranging from solar activity to high-frequency digital products to all forms of other radio communications. That background noise establishes a noise floor, the point where the desired signals are lost in the background ruckus.

If your environment has high degrees of RF noise in your frequency band, then use the noise floor figures instead of radio receive sensitivity in your calculations. When in doubt, look around. Antennas are everywhere nowadays—on the sides of buildings, water towers, billboards, chimneys, and even disguised as trees. Interference may not be obvious.

5. Mother Nature can wreak havoc on signal strength. Always give yourself a safety margin.

Fade margin is a term critical to wireless success. Fade margin describes how many dB a received signal may be reduced without causing system performance to fall below an acceptable value. Walking away from a newly commissioned wireless installation without understanding how much fade margin exists is the number-one cause of wireless woes.

Establishing a fade margin of no less than 10dB in good weather conditions virtually ensures that the system will continue to operate effectively in a variety of weather, solar, and RF interference conditions. Outside conditions also require a cabinet for protection or select IP67 outdoor-rated wireless units (such as B&B Electronics’ Zlinx Xtreme radios and I/O modules; see photos) to handle Mother Nature. (See the next law about wireless signal obstacles; metal is not your friend.)

6. Only an eternal optimist would ever attempt a system at the manufacturer’s maximum advertised distance. Remember the real world: Stay clear of obstacles and maintain line of sight. 

In a clear path through the air, radio signals attenuate with the square of distance. Doubling range requires a four-fold increase in power, therefore:

  • Halving the distance decreases path loss by 6dB.
  • Doubling the distance increases path loss by 6dB.

When indoors, paths tend to be more complex, so use a more aggressive rule of thumb, as follows:

  • Halving the distance decreases path loss by 9dB.
  • Doubling the distance increases path loss by 9dB.

Radio manufacturers advertise “line of sight” range figures. Line of sight means that, from antenna A, you can see antenna B.  For every obstacle in the path, de-rate the “line of sight” figure specified for each obstacle in the path. The type, location, and number of obstacles will all impact path loss. Obstructions located close to the antennas will cause the most dramatic loss.

Don’t underestimate the distance between antennas. If it’s a short-range application you can pace it off.  If it’s a long-range application, increase distance accuracy quickly by using a global positioning system (GPS) or Google Maps.

The most effective way to reduce path loss is to elevate the antennas.  At approximately 6 feet high (2 m), line of sight due to the Earth’s curvature is about 3 miles (5 km), so anything taller than a well-manicured lawn becomes an obstacle.

Industrial installations often include many reflective obstacles leading to numerous paths between the antennas. The received signal is the vector sum of each of these paths. In multiple-path environments, simply moving the antenna slightly can significantly change the signal strength.

Some obstacles are mobile. More than one wireless application has been stymied by temporary obstacles such as a stack of containers, a parked truck or material handling equipment. Remember, metal is not your friend. An antenna will not transmit out from inside a metal box or through a storage tank.

Path loss rules of thumb:

  • To ensure basic fade margin in a perfect line of sight application, never exceed 50% of the manufacturer’s rated line of sight distance. This in itself yields a theoretical 6dB fade margin, still short of the required 10dB.
  • De-rate more aggressively if you have obstacles between the two antennas, but not near the antennas.
  • De-rate to 10% of the manufacturer’s line of sight ratings if you have multiple obstacles, obstacles located near the antennas, or the antennas are located indoors.

7. It’s not all about the radio; use the wrong antenna or cable, and you’re toast.

Antennas increase the effective power by focusing the radiated energy in the desired direction. Using the correct antenna not only focuses power into the desired area but also reduces the amount of power broadcast into areas where it is not needed.

If your job site is already bristling with other antennas, try to separate yours as much as possible. Most antennas broadcast in a horizontal pattern, so vertical separation is more meaningful than horizontal. Try to separate antennas by a minimum of two wavelengths, 26 inches (0.66 m) at 900 MHz, or 10 inches (0.25 m) at 2.4 GHz.

Use high-quality RF cable between the antenna connector and your antenna, and ensure that all connectors are high-quality and carefully installed to help signal propagation. Factor in a 0.2 dB loss per coaxial connector in addition to the cable attenuation itself. Typical attenuation figures for two popular cable types are listed below.

               Loss per 10 feet (~3 meters) of coaxial cable length

               Frequency

RG-58U

LMR-400

               900 MHz

1.6 dB

0.4 dB

               2.4 GHz

2.8 dB

0.7 dB

While long cable runs to an antenna create signal loss, the act of elevating the antenna another 25 feet (7.6 m) can compensate for those lost dB.

Following these 7 Murphy’s Laws of industrial wireless should help make your next wireless installation painless. 


- Mike Fahrion is the chief engineer at B&B Electronics. He oversees the company’s development of next-generation industrial wireless products, including the Zlinx Xtreme IP67-rated family of radios and I/O modules. Fahrion writes the politically incorrect newsletter, eConnections.


- Also read:

www.bb-elec.com/wireless

 

www.bb-elec.com/WirelessRangeEstimator - B&B Electronics provides an online Wireless Range Estimator

Outdoor-rated wireless I/O modules, radio modems

www.controleng.com/wireless - Control Engineering industrial wireless articles, tips, advice



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
The true cost of lubrication: Three keys to consider when evaluating oils; Plant Engineering Lubrication Guide; 11 ways to protect bearing assets; Is lubrication part of your KPIs?
Contract maintenance: 5 ways to keep things humming while keeping an eye on costs; Pneumatic systems; Energy monitoring; The sixth 'S' is safety
Transport your data: Supply chain information critical to operational excellence; High-voltage faults; Portable cooling; Safety automation isn't automatic
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Maintaining low data center PUE; Using eco mode in UPS systems; Commissioning electrical and power systems; Exploring dc power distribution alternatives
Synchronizing industrial Ethernet networks; Selecting protocol conversion gateways; Integrating HMIs with PLCs and PACs
Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.