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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.
by Steve Grady, Cymbet
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 control. 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) is the term used to power the devices. 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 electromechanical 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.
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 to 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 Table1. 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.
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, batteries 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 Intra Ocular 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.
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 4 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 on-board 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.
Millions of Wireless Sensor Networks will be deployed due to the rising installation costs of hardwired 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.
Cymbet, Elk River, MN. (763) 633-1780. [www.cymbet.com]