01 October 2006

Want power for a motor condition monitoring sensor? Forget batteries. Try these

The vision of wireless sensor networks is warmly embraced by many control vendors and condition-based maintenance personnel in industrial plants. What they’re not so happy about is the prospect of deploying thousands of batteries and then changing them, every few months, to make the communications work.

Roy Freeland, chief executive at U.K. startup company Perpetuum Ltd., (www.perpetuum.co.uk) thinks he has the answer. He calls it ‘vibration energy harvesting.’

A Cambridge graduate engineer who has run several technology-based companies (Meggitt Electronics,
Bowthorpe (Spirent), and United Industries), Mr. Freeland has taken charge of the company, originally spun out of Southampton University. In the past year the microgenerator was vastly improved, important trial contracts were signed (U.S. Navy, Yorkshire Water in the U.K., and an ‘international oil company’), and early this year an additional $4 million in new investment was secured. Things are looking up.

The idea of generating electricity from vibrations has been around for at least a decade, but most of the early devices, based on the piezoelectric effect, produced only a few microwatts. Perpetuum’s simple electromechanical device generates power in the low milliwatt range. The microgenerator uses a coil positioned between four permanent magnets (shown in red in the diagram), which are at the end of a
cantilever beam. Vibration causes the magnets to bounce up and down and generates electricity according to the Faraday effect. The ac current from the coil is rectified through a power conditioning circuit and stored in capacitors, which are discharged when needed to power something, like a sensor or a wireless transmitter.

The key patent in the device, the PMG7, is the sprung beam that supports the magnets. It is designed to resonate at the motor’s 50 or 60 Hz and produce the greatest amounts of electricity at these points. Users can ‘tune’ the microgenerator with a screwdriver to achieve maximum current output. The next generation PMG17 is designed to be much less dependent on the 50/60 Hz motor frequencies.

As surprising as it seems, even the smoothest running motor produces enough kinetic energy through vibration to power the microgenerator. The minimum is 25 mg (where g is the gravitational force, not grams), to generate electricity sufficient to power a sensor and transmitter. At 100 mg Perpetuum says it can produce 5 mW at 3.3 volts. For temperature sensing there is enough power to transmit a value several times a second; a 6,000 byte vibration spectrum might be transmitted every 20 to 30 minutes. By comparison, the life of one (3.5Whr) AA battery at 1mW output is about 4 months.

Technology convergence favours the microgenerator concept. Much lowpower research has been done to
produce mobile telephones, notebook computers and loop-powered instruments, and the benefit of this work is that sensors and microprocessors today need only a fraction of the current they required a generation ago.

And industrial applications of wireless technology are beginning to take off. In semiconductor manufacturing
equipment, an engineer at Intel said ‘The conventional cost to equip a machine with six sensors is $8,000. A
wireless system is about $1,000.’

With the installation cost of a single wired sensor in a refinery or petrochemical plant running into tens of thousands of dollars, it is easy to see why companies are interested in wireless devices. But they don’t want
batteries, either.

The first product
The microgenerator by itself is interesting, but not useful unless it’s coupled with the sensor, and so
Perpetuum has teamed up with RLW Inc. (www.rlwinc.com), a company in State College, Penn., U.S.A. that specialises in Condition Based Maintenance applications. Together the two companies have produced the S5NAP, which packages the PMG7 microgenerator with RLW’s accelerometer and temperature sensor and an IEEE 802.15.4 (the basis of the ZigBee standard) compatible radio transmitter.

Several S5NAP units are currently being tested at an incinerator operated by Yorkshire Water in the U.K. (photo). The sensors are held onto the motors and pumps by magnets so they can be moved about the plant. The 802.15.4 radio has been working perfectly in the plant, in spite of all the metal obstacles.

Perpetuum’s next development is the PMG17. Unlike the current PMG7, which works with a narrow range of
frequencies around 50 or 60 Hz, and needs to be fine tuned for each motor to get maximum output, the PMG17 will not be so frequency sensitive, and will work with 90% of all motors with no adjustment (see figure at left).

Beyond that, Perpetuum is working on the next generation: a 5mm x 5mm silicon MEMS microgenerator chip, for which they have secured EU development funds.

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