Wireless Factory Sensors Get Kinetic

Dec. 1, 2006
The factory floor is changing radically. Usually powered by batteries, wireless sensors have been available for transmitting data for quite some time. The greater use of wireless communications standards like ZigBee has accelerated these applications. Bu

The factory floor is changing radically. Usually powered by batteries, wireless sensors have been available for transmitting data for quite some time. The greater use of wireless communications standards like ZigBee has accelerated these applications. But sensor batteries have limited lifetimes and require frequent replacement, limiting their use.

The PMG7 microgenerator module from Perpetuum Ltd. lifts these restrictions. This small, 190-gm module converts the kinetic energy of rotating equipment powered by ac induction motors to electrical energy to power wireless sensors. It won't ever need a battery or maintenance (see the figure).

The micromachined generator is based on a design developed at the University of Southampton in the U.K. Unlike other energy " scavengers" or "harvesters" that rely on the piezoelectric principle, it employs the Farady principle using an electromechanical approach.

The PMG7 can be installed easily on rotating equipment like fans, blowers, motors, pumps, and other factory floor devices. It can be bolted on, screwed in place, or held in place by magnets.

Perpetuum's CEO Roy Freeland says it will work when it's immediately attached to "well over 90% of all the ac motors in a plant worldwide."

A resonant strung beam in the PMG7 carries a pair of permanent-magnets comprising many turns of fine wire between the magnets. Vibration sets the magnets in motion, creating an ac potential in the coil. This generated voltage is rectified and charges a capacitor (or supercapacitor), which in turn powers the sensor. The resonant beam is tuned to 50- or 60-Hz vibrations that ac motors produce.

The mechanical approach used in the PMG7 provides a wide mechanical bandwidth. Operating half-power bandwidth ratings are typically ±0.2 Hz for the PMG7-50 and PMG7-60 and ±0.4 Hz for the PMG7-100 and PMG7-200. These models differ in the maximum center-frequency tuning range of 51, 61, 101, and 121 Hz, respectively. This center frequency can be manually tuned during installation by a screw, with a sensitivity of 1.0 Hz per turn, to maximize power generation.

And that power is more than adequate. An output range of 0.1 mW to several milliwatts, depending on the level of vibration, is possible. The module produces up to 5 mW at 3.3 V from a vibration level of 100 mg. It also can supply up to 0.4 mW at 3.3 V from vibration levels of 25 mg. That's enough power for the sensor's transmitter to send up to 6 kbytes of data every few minutes. Or, smaller amounts of data can be sent from, say, a simple temperature-sensor reading several times a second.

Perpetuum is targeting a price for the PMG7 of less than 100 Euros, though the initial introductory price might be somewhat higher. According to the company, this price point should make it very competitive with other battery-powered wireless-sensor methods, considering the lifetime battery replacement costs and the cost of the personnel needed to make those replacements.

RLW Inc., which builds accelerometer-based signalconditioning modules for wireless sensor nodes, already uses the PMG7. The company has performed successful field trials on its products, using the PMG7 in its S5NAP wireless sensor nodes, with the U.S. Navy and Yorkshire Water Co. in the U.K. A major international oil company also has successfully used the S5NAP wireless sensor nodes for a trial installation on a test pumping circuit.

The microgenerator has other uses in addition to industrial settings. The PMG27 is designed for use on military and civilian helicopters and aircraft that carry health and human monitoring systems (HUMS). Widely used in civil and military aircraft, HUMS technology provides significant safety and cost benefits by improving maintenance scheduling and providing accurate usage recording.

Perpetuum is now investigating much smaller silicon MEMS devices to enable the development of even smaller complete system packages. It predicts that someday, a 5-by 5- by 1.5-mm silicon package could be made to produce a few hundred microwatts of output under suitable conditions.

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