We'll save ourselves and our customers some significant money by using wireless sensors in places to connect a battery of machines across varying distances, many nearly 1000-ft distant. First issue: The sampling rates will vary from 10/sec to 1/sec, with some others transmitting on status change only. So it looks like we have options for powering the sensors. At these rates, can we economically use energy-harvesting methods? What's the general state of battery life for these conditions? There even are a few spots where photovoltaics (PV) is an option. Second issue: We heard that the level of encryption (say 128-bit) has a big effect on battery life, which no one seems to want to talk about. Any practical advice for these issues?
—from March '14 Control Design
Check Your Data Rate Needs
Technology can deliver low-power wireless sensing, but there are challenges such as range, rate, power and encryption to consider.
An excellent way to extend range is with wireless meshing technology. Mesh solutions based on 802.15.4e are far more robust and resilient than 802.15.4, and they'll provide the most flexibility and reliability. A 1,000-ft. range is workable, although environmental factors such as RF interference, frequency choice and the presence of physical objects in the line of sight will affect the range. To get the maximum range, you'll need an external antenna, rather than an on-board chip antenna. Gain and antenna type (directional or omni) will be determined by the local environment.
Ten samples per second is a high rate for a battery-powered sensor node. Do you require that much raw data for the application? You can preserve a lot of battery power if you push some event processing out to the sensor nodes and let them make decisions about how much data really needs to be transmitted. If high data rates are truly required, some wireless technologies will prove to be impractical.
Consider the total amount of data that will pass through the system. The total number of nodes, combined with their sampling rates, will determine the bandwidth that the system design must be able to handle.
Sampling rates heavily influence power budgets. As the sampling rate rises, you're looking at either larger batteries or shorter battery replacement intervals. Note that the industrial process world has gotten good results with WirelessHART. Many products boast multi-year battery life in applications that call for several samples per minute using just one or two D-cell lithium batteries.
Also Read: Color Sensors Displace Older Methods
Energy harvesting is becoming more viable all the time, but it does call for careful system choices. Photovoltaic power (PV) is still the most practical option, and it works well outdoors. Indoor PV provides much less power. It can work, but only in applications with lower sampling rates and lower range requirements.
To your second issue: encryption adds overhead. How much overhead will depend on data payload packet size. Always factor encryption overhead into the power and bandwidth budget specified by the vendor. And don't take chances with the "security by obscurity" rationale, where security depends on the secrecy of the system's implementation or its components. That's like thinking you can leave your back door unlocked because a burglar can't see it from the street.
Director of Product Management,
[The following response came as a result of posting the question to our own Control Design LinkedIn Group.]
Saving Money Isn't the Big Factor
From my personal experience with wireless instruments and sensors, the majority of these types of instruments have non-rechargeable batteries. Unless you have a huge amount in backup for them, you can't guarantee a continuous measurement of your process. On the other hand, it's the responsibility of a capable technical service group to ensure the minimum power consumption via device configuration.
Finally, saving money must not be a factor when you decide to use wireless instruments. The money that you save in installation and engineering might be spent in medium term by replacing batteries and paying for technical service.
I've seen that the only advantage about wireless instruments is if they can be locally powered by PV solutions or by ensuring continuous power from any power source.
Javier Ernesto Otálora Sánchez,
Control Design Engineer,
[These are responses received when we posted the question to LinkedIn's Industrial Automation & Process Controls group.]
Don't Worry About the Encryption Overhead
With the technology on the market today, you have a choice between proprietary wireless products or those conforming to a standard. Both ISA100 Wireless and WirelessHART conform to standards (different ones). Both use AES-128 encryption. Battery life for both depends on the frequency of transmission. ISA100 is capable of transmitting 10/sec, but not WirelessHART. Typically, battery life can be expected to be between one and five years. Either one can be powered by energy-harvesting devices, especially those using the ISA100.18 specification for energy-harvesting power connection. Reducing the length of the encryption key will have no effect on the power consumption, but you can't do it with current products anyway.
Dick Caro, Industrial Networking Consultant,
WirelessHART transmitters have demonstrated six-year battery life in the Arctic on actual installations. Update rates are generally one minute or longer on those applications. They should go for nine or 10 years. Naturally, as the update rate increases, the battery life will decrease. At one-second updates, battery life will be from about seven months to 1.5 years, depending on the transmitter type.
There are energy-harvesting products available today that can be interfaced with WirelessHART transmitters. Two primary energy sources are thermal and vibration. The normal method of interface is to use a purpose-built power module with provision for an external energy source. If the external energy source is providing enough power, it feeds the transmitter. If it drops out for any reason, then the power module picks up the slack and prevents loss of the transmitter from the network. With this supplemental methodology, the power module life on a one-second update application can be extended to four years or more.
Business Development Manager,
[These are from LinkedIn's Automation Engineers group.]
Just Do Monitoring
Have used wireless sensors for temperature transmission. The sampling rate was every 60 sec. The battery we used on them lasted from five to seven years. I did not see any significance of the encryption having an effect on the battery life. The more you transmit, waking the device up, the more power you would use. I am not confident with the equipment yet to use it for control at this time. For monitoring it is great.
HDL Process PC&IS engineer technician,
Procter & Gamble
Test Energy Consumption First
We have deployed hundreds of wireless sensor products.
There is a tradeoff on speed or sampling rate, power needed for the device, and radio and battery life. One of our suppliers will do testing on a previously unused device to determine minimum values for current, voltage and power uptime for stability. They then provide battery-life values, which drives your PM program.
Sales and marketing manager,
All About Frequency
I have dealt with solar-powered seismic sensors. Increasing the time between samples greatly decreased the power requirements, thus allowing the use of smaller and less expensive solar panels. The question is what is the minimum number of samples required per time period to meet the desired functionality.