Zero Power Wireless Sensors
Wireless Sensor Networks (WSNs) is the term that is used for wireless sensor and control networks that use batteries or Energy Harvesting techniques to power the device. With the availability of low cost integrated circuits to perform the sensing, signal processing, communication, and data collection functions, coupled with the versatility that wireless networks afford, we can move away from fixed, hard-wired network installations in both new construction as well as retrofits of existing installations.
A major drawback to moving toward large scale wireless sensor network installations is the poor reliability and limited useful life of batteries needed to supply the energy to the sensor, radio, processor, and other electronic elements of the system. This limitation has to some extent curtailed the proliferation of wireless sensor networks. Legacy batteries can be eliminated through the use of Energy Harvesting techniques which use an energy conversion transducer tied to an integrated rechargeable power storage device. This mini “power plant” lasts the life of the wireless sensor. Click here for the ISSCC paper describing the millimeter scale sensor shown above.
Energy Processors and Solid State Batteries Enable Zero Power Wireless Sensors
Zero Power Wireless Sensors require energy processing low power management circuitry to condition the transducer output power, store energy and deliver power to the rest of the wireless sensor. In most environments, any of transducers producing power cannot be relied on under all circumstances to continuously supply power to the load. While each transducer delivers power at some amplitude and with some regularity, they do not store energy. Consequently, when that source of power is not present, there would be no power to supply the load in the absence of an energy storage device. Moreover, the transducers typically do not deliver power at the proper voltage to operate the electronic system; therefore, conditioning of transducer power is essential to making the power useful in operating the sensor, processor, and transmitter. In particular, without an energy storage device, it would be difficult or impossible to deliver the pulse current necessary to drive the wireless transmitter. Traditional rechargeable energy storage devices such as supercaps and coin cell batteries have severe limitations with respect to charge/discharge cycle life, self-discharge, and charge current and voltage requirements.
Energy Processors use Maximum Peak Power Tracking
An Energy Processor, such as Cymbet’s EnerChip™ EP CBC915, provides all of the energy conversion, energy storage, and load power management for the Zero Power Wireless Sensor. In order to produce high efficiency transducer energy conversion the Energy Processor performs maximum peak power tracking by emulating the impedance of the transducer. The Energy Processor also coordinates all the power-up sequencing even from a dead start with no charge in the system. The Energy Processor also provides power and energy status information to the Microcontroller so the system can be made “Energy Aware”. Figure 3 shows the EnerChip EP Energy Processor and EnerChip solid state batteries on the EVAL-09 Universal Energy Harvesting kit. This kit interfaces to any type of EH transducer, converts and stores the harvested energy and provides a regulated output voltage to a target system.
Energy Harvesting vs. Nonrechargeable Batteries
Wireless sensor systems are becoming more prevalent due to the rising installation costs of hard-wired sensor systems, availability of low cost sensor nodes, and advances in sensor technology. Energy Harvesting-based autonomous wireless sensor nodes are a cost-effective and convenient solution. The use of Energy Harvesting removes one of the key factors limiting the proliferation of wireless nodes - the scarcity of power sources having the characteristics necessary to deliver the energy and power to the sensor node for years without battery replacement. Significant economic advantages are realized when Zero Power Wireless Sensors are deployed vs. hard-wired solutions. Additional savings are realized by removing the significant costs of battery replacement. Combining Energy Harvesting transducers, an Energy Processing Power Module, low power sensor, an energy aware Microcontroller, and an optimized RF Radio link delivers the reality of long life, maintenance-free Zero Power Wireless Sensor Networks.