New Ways to Power Instrumentation

01 June 2007

With the increased use of wireless networks and remote instrumentation, engineers are looking for non-traditional ways to supply electricity to their devices.

In a wired world where mains power is available and devices communicate via direct I/O wiring or fieldbus,
instrumentation can report continuously. Whatever flow, pressure, or other variable, the data supply never stops.

But the standard 4-20 mA signal consumes a large amount of current from a battery, discharging most
batteries in a matter of days. However, if a measurement can be taken at some interval, the device can operate intermittently and save power.

Some process variables are critical enough to merit continuous monitoring, and those will likely be hardwired. Other points may only need checking once every few seconds, every minute, every hour, or longer. If a device can ‘wake’ itself up, take a single reading, transmit the number, and go back to ‘sleep’ until it repeats the process in another hour, the power consumption can be cut by 99% compared to continuous operation. Such a device, using current designs and an appropriate battery, could operate for
20 years without maintenance.

‘Wireless communication has made power the critical issue,’ says Rob Conant, of Dust Networks. His company is collaborating with Emerson Process Management to supply wireless networks for its instruments.

Dust has made significant advances in creating efficient communications technology for industrial instrumentation. Its embedded radios are capable of turning on, uploading a process variable, transmitting the variable, and shutting off all within 10ms. Receivers are synchronised to turn on at the same time to capture the signal. In a normal duty cycle where the device updates every 10 seconds, the equipment is in sleep mode 99.9% of the time. This type of power utilisation makes self-contained supplies very practical.

Exotic batteries
Alternative power approaches also have to accommodate the desire to eliminate any need for maintenance. An instrument that has to have its batteries changed once a month or even once a year simply won’t get installed unless its need is very critical. Plant operators want devices that require virtually no attention, with battery life measured in years.

Standard alkaline batteries (alkalinemanganese dioxide) are adequate for most consumer applications but they are not suitable in the demanding world of industrial instrumentation.

Lithium formulations largely have taken over due to their high power density and long life spans. There are many types, and capabilities vary considerably. The widely available lithium-manganese dioxide batteries will certainly outperform their alkaline competitors in a digital camera, but they pale in comparison to more advanced technologies. The type commonly used in today’s wireless offerings is lithium thionyl chloride. While alkaline batteries put out a maximum of 1.5 V, the powerful lithium variety put out 3.6 V and maintain this consistently until they reach the exhaustion point. They can operate at temperatures from - 55º to 150ºC and have a self-discharge rate of less than 1% per year, giving a possible life of 20 or more years.

Can a battery really last 20 years? ‘Yes,’ says Sol Jacobs of Tadiran Batteries. ‘We know of water meter-reading devices that have been in use for 23 years with the original batteries, and the cells that we made 20 years ago aren’t as good as the ones we make today.’

Solar collectors
Photovoltaic panels have been around for decades and are often the first choice for remote equipment of all types when used in the right location.

The panels are made in all shapes and sizes, use a variety of technologies, and come from a large number of manufacturers. Efficiencies range from 4-24%, with a correlation of price to efficiency. Most cost competitive technologies have around 15% efficiency. Panels can be linked together to provide any required voltage and amperage. Under good sun conditions they can put out about 100 W per square metre, but
since the availability of sun determines actual output, it’s difficult to give rules of thumb on how big a set has to be for a given application.

For remote instrumentation, a typical installation includes panels feeding a controller, which regulates charging a group of batteries. The instrumentation and transmitters are powered by the batteries and not the cells directly. The combination of panel output and battery size should be able to provide constant power, even during extended bad weather or snow, so the same installation in a sunny area can have a smaller panel and fewer batteries than in a northern, rainy area.

Power scavenging
The idea of scavenging environmental energy sources such as vibration and heat has been around for some time. Applications are becoming more practical. An instrument near a compressor, for example, could have its supply fed by a small generator that produces current by harnessing vibrations from the machinery.

Perpetuum Ltd. is currently producing units for sale while it develops the technology. Its PMG7 vibration
microgenerator is available and newer models are in the pipeline. These units don’t produce a huge amount of energy; the output can be as high as 1.2 mA, which typically feeds a capacitor to provide additional surge capacity for the instrument.

Microgenerators operate best at specific frequencies, so they are tuned to respond to vibrations typically created by 50 Hz motors commonly used in industrial applications. When installed, the tuning can be adjusted to maximise output for the specific installation. Units are very small (roughly the size of half a
C-cell battery), can be installed in any position, and require no maintenance. They do depend on the equipment running, so if the host compressor or other equipment shuts down, the power stops.

Acceptance of this technology will depend on reliability and cost effectiveness. With present technology, the power output of a microgenerator lithium thionyl chloride battery. With a 0.5 mA continuous load, within the
comfort level of either device, the battery could last 30,000 hours, so power harvesters will have to compete with this type of performance.

With experiments continuing on wireless transmission of power, the trend toward self-contained and wireless instrumentation will continue, which will pressure equipment designers to improve efficiency and create new ways to collect and communicate data with minimal power use. The process has just begun and there is much to gain with new developments.

—Peter Welander, Control Engineering

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