The idea of wireless sensors that report measurements over communications protocols, such as 802.15.4, including ZigBee and other similar standards, is gaining ground. A key part of the concept is that such sensors—with their low power demand and duty cycle—should derive their power from batteries or should extract their power requirements from energy they "harvest," or "scavenge," from the environment. Start-up company Perpetuum has designed its PMG7 generator (picture) to glean sufficient energy from the vibration of rotating equipment—pumps, fans, and other mechanical equipment—that ac-induction motors drive to power a sensor and wireless-transceiver node.

Perpetuum's generator is electromechanical rather than piezoelectric, a principle that some researchers have used to scavenge vibration energy. Its fundamental structure is a sprung beam that carries a pair of permanent magnets. Vibration sets the magnets in motion, and a coil comprising many turns of fine wire lies between the magnets. Circuitry in the generator rectifies the ac and charges a capacitor or supercapacitor, which in turn powers the sensor node. The design can gather useful amounts of energy because the sprung beam is a resonant structure, tuned to the 50- or 60-Hz vibrations that all ac motors produce. The essence of the Perpetuum product is the design of the resonant beam, which has a wide mechanical bandwidth; a range of vibration frequencies set it in motion, and users need not tune each generator to a specific motor. The generator offers a mechanical-tuning feature, but Perpetuum's Chief Executive Officer, Roy Freeland, asserts that the PMG7 works immediately when you attach it to almost any ac motor driving mechanical equipment. Freeland believes that such a power source is essential for the deployment of 802.15.4 networks: "When you present the wireless-sensor idea to, for example, process engineers, they are enthusiastic until you mention the word 'battery,'" he says. "We know of an oil company that will not accept any form of battery-powered equipment on its sites."

The PMG7 generator outputs at much as 5 mW at 3.3V from a vibration level of 100 mg. This power is sufficient to drive a wireless node to periodically report sensor data, such as temperature, pressure, or the spectrum of the same vibrations that power the generator. US manufacturer RLW Inc has employed this principle to build its accelerometer-based condition-monitoring sensor node. It transmits raw-accelerometer spectrum for remote analysis, but Perpetuum says enough energy is available to perform an FFT within the sensor and to report processed data. Users can bolt a sensor node to the motor, pump, or fan housing, or they can simply and temporarily attach a magnetic clamp for true "instant-sensor-network" operation.

Perpetuum's target price for high-volume production is less than £100. Its price at introduction is higher, but Freeland says: "The product is already competitive with a battery-powered alternative when you take into account the lifetime cost of using batteries and include the cost of intervention by service personnel to replace them."