TECHNOLOGY IN CONTEXT
Energy Harvesting for Low-Power Nerworks
Zero Power Wireless Sensors Using Energy Harvesting
Eliminating wires has given us wireless sensor networks. But maintaining and changing batteries has been a big chore and expense. The use of energy harvesting technologies can finally offer zero power wireless sensors and open huge new application areas.
STEVE GRADY, CYMBET
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Sensor networks are gaining widespread use in factories, industrial complexes, commercial and residential buildings, agricultural settings and urban areas, serving to improve energy efficiency, safety, reliability, automation and security. These networks perform a variety of useful functions including industrial process control, and monitoring. These include lighting, heating and cooling controls in residential and commercial buildings; structural health monitoring of bridges, buildings, aircraft and machinery; tire pressure monitoring systems (TPMS); tank level monitoring and patient monitoring in hospitals and nursing homes to name a few.
To date, most sensor networks have used wired connections for data communications and power. The cost of installing a sensor network using copper wire and conduit, along with their support infrastructure has become extremely cost-prohibitive. There are new solutions using various wireless solutions such as Wireless Hart, ISA100, ZigBee, Bluetooth Smart and IP-based 6LowPAN to network sensor devices to eliminate the data communications wiring. However, the wireless sensors still need to be powered, and using disposable primary batteries such as alkaline or lithium coin cell batteries has been the solution. But these batteries wear out and changing them out is often an expensive proposition. OnWorld Research has estimated that this battery change-out cost will approach $1 Billion in 2013. What is needed is a solution that harvests the ambient energy around the wireless sensor device and we can cut the power cord forever.
Zero Power Wireless Sensors Are the Solution
Wireless Sensor Networks (WSNs)are based on devices that need to be powered by a source other than main line power or batteries. 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.
One drawback to moving toward a WSN installation has been the poor reliability and limited useful life of batteries needed to power the sensor, radio, processor and other electronic elements of the system. This limitation has to some extent curtailed the proliferation of wireless networks. The legacy batteries can be eliminated through the use of ambient energy harvesting (EH) 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.
Components of an EH-Powered Wireless Sensor
A zero power wireless sensor as shown in Figure 1 consists of five basic elements. An energy harvesting transducer converts some form of ambient energy to electricity while an energy processing stage collects, stores and delivers electrical energy to the electronic or electro-mechanical devices resident at the sensor node using MPPT power conversion and solid state battery storage. A microcontroller or variant thereof, receives the signal from the sensor, converts it into a useful form for analysis, and communicates with the radio link.
Zero power wireless sensor using energy harvesting diagram.
A sensor detects and quantifies any number of environmental parameters such as motion, proximity, temperature, pressure, pH, light, strain, vibration and many others. And, of course, a radio link at the sensor node transmits and receives the processor information on a continuous, periodic, or event-driven basis between a host receiver and data collection point.
Design Steps for EH-Powered Systems
In order to successfully design and deploy energy harvesting powered wireless sensors, it is important to analyze and implement four key aspects. First you must identify what sources of ambient energy are available surrounding the wireless sensor such as light, thermal gradients, vibration/motion, etc. After choosing the energy harvesting transducer appropriate to the available source, calculate the amount of energy to be produced by the transducer under all ambient conditions.
In micropower energy harvesting systems, it is important to design for high-efficiency energy conversion, energy storage and power management. Implementing electronics for maximum peak power point tracking (MPPT) energy conversion is important.
The first step is to calculate the application power requirements and minimize them to fit available input EH power. Every sensor use case and power mode must be identified and characterized for power consumption. Legacy sensor designs that were line-powered will need to be redesigned to save power. The sleep mode of all components is to be used as often as possible. Microcontroller firmware must be written to be “energy aware” so as to include no polling loops, check input power and battery charge levels, and be able to change the wireless transmission duty cycle depending on energy status, etc.
Finally, pick the right size for energy storage. All energy harvesting-based sensors require an energy storage device such as solid state batteries. These rechargeable batteries need to be selected for the correct capacity to power the device when EH power input is absent, but must also charge quickly when EH power is restored. Right sizing is important and bigger batteries are not always better.
Energy Harvesting Electronics
Energy harvesting transducers are a source of power that is regularly or constantly available. This power source could come in the form of a temperature differential, a vibrational source such as an AC motor, a radiating or propagating electromagnetic wave, or a light source, as examples. Any of these power sources can be converted to useful electrical energy using transducers designed to convert one of those forms of power to electrical power. The most common transducers are shown in Table 1. The efficiency and power output of each transducer varies according to transducer design, construction, material and operating temperature, as well as the input power available and the impedance matching at the transducer output.
Energy harvesting transducer comparisons.
Zero power wireless sensors require high-efficiency, 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, none of the transducers producing power can be relied on under all circumstances to continuously supply power to the load. While each transducer delivers power within an output range and with some regularity, they do not store energy. Consequently, when that source of power is not present, there is 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 make 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. New rechargeable solid state batteries (SSBs), such as the Cymbet EnerChip, can overcome the limitations of legacy batteries and supercaps.
The output of the sensor is typically connected to a microcontroller that processes the signal created from measuring the parameter of interest, such as temperature, pressure or acceleration, and converts it to a form that is useful for data transmission, collection and analysis. Additionally, the microcontroller usually feeds this information to the radio and controls its activation at some prescribed time interval or on the occurrence of a particular event. It is important that the microcontroller and radio operate in low power modes whenever possible in order to maximize the power source lifetime. MCU manufacturers such as NXP, Microchip, Texas Instruments and Energy Micro have paid particular attention to reducing power consumption in all operating modes.
New innovative small wireless sensors can now be created that use EH power. A photo and diagram of a highly integrated EH-powered intraocular pressure sensor is shown in Figure 2.This device was created by the University of Michigan and is used by glaucoma patients to measure the pressure in the eyeball. The total volume of the device as shown on the penny is one cubic millimeter. All the wireless sensor components reviewed in earlier sections are represented in this device: A MEMS pressure sensor is connected to a low power MCU with an A/D converter and power management; a solar cell powers the device and charges the integrated solid state battery. A wireless antenna broadcasts the pressure in the eye to a receiver wand placed a few inches away.
Intra ocular pressure sensor - Courtesy of University of Michigan.
Design engineers can easily experiment with energy harvesting-based wireless sensors using evaluation kits, which are available from global distributors such as Digi-Key, Mouser, Avnet and Farnell. An example is shown in Figure 3 of the Cymbet EnerChip CC Solar Energy Harvesting EVAL-10 kit combined with the Texas Instruments eZ430-RF2500 wireless evaluation kit to create a solar-based EH wireless temperature sensor. In this case, the onboard EnerChip CC CBC3150 connects directly to the solar cell and provides the energy processing functions and solid state battery energy storage for the TI wireless end device.
Cymbet EVAL-10 Solar EH Kit with TI eZ430-RF2500 Wireless Kit.
Millions of Wireless Sensor Networks will be deployed due to the rising installation costs of hard-wired sensor systems, the 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. Energy harvesting enables the reality of long-life, maintenance-free zero power wireless sensor networks.
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