This article will review practical applications for energy
harvesting technology and discuss the best way to incorporate low
power RF radio technology to power wireless sensors in a simple
point to point, star or device mesh network.
Numerous industrial and commercial markets - including building
automation, transportation infrastructure and medical device alarm
monitoring - have already developed energy harvesting products that
are available today.
Building automation is a key application area. Commercial
building occupancy sensors, thermostats and light switches can be
installed with mechanical or solar energy harvesting devices that
can eliminate the control wire from existing installations.
A wireless network with energy harvesting technology can tie all
the sensors together and reduce lighting and HVAC costs by
switching off power to non essential functions when the building is
not occupied.
Another industrial application is transportation infrastructure
like road, rail and bridge strain gauges. By pairing strain gauges
with a wireless radio and relying on the continuous energy of
surface vibration, wind, or solar energy to power the strain
sensors, the sensors are able to transmit exception based data when
a key component of the infrastructure starts to deflect (hopefully)
before it breaks.
Energy harvesting is also being used in commercial applications.
For example medical devices designed for the elderly in home or
outpatient market are equipped with accelerometers and medical
sensors running on heat or body movement energy harvesting
technology.
A sudden fall or change in heart rate will trigger a request for
help to advise emergency providers when they have fallen or are not
okay.
Now let’s consider the best way to add energy scavenging technology
with a radio and take advantage of existing features to develop
product in which it’s not necessary to keep changing batteries once
the product is deployed.
When discussing low power radios, we will focus on radios that have
the following capabilities or features: they can go to sleep mode
and be off 95% of the time; they can go from deep sleep to ready to
transmit in 300 milliseconds or less; they consume less than
500micro amps of current in the transition phase and typically
consume less than 16 ma when transmitting and 20 ma when receiving
data.
The following best practices will accelerate the development
process of products featuring energy harvesting and RF
technology
- Use the lowest possible duty cycle. Send your data only when
needed and do not send more data than necessary. A large amount of
sensor activity is exception based messages. Only send the message
when the sensor value has gone out of limits set up for the
application.
- Minimize the receive window length of your radio in
applications testing. This window length can be adjusted
adaptively. The default is to use a narrow window and increases the
window when packets are missed as a result of receive timeout.
- Use the built in receive signal strength indicator (rssi)
feature of the radio to reduce the transmit power or receive
sensitivity in your point to point product so that the radio can
dynamically adjust based upon the other devices operating in the
vicinity of your radio. There is no need to increase your transmit
power and shout across the room when you can talk or whisper and
get your message through the first time. The difference in transmit
mode current can be as much as 10 ma between using output levels
of-12dBm and 0dbm.
- Use the lowest possible voltage setting. RF devices have
reduced current draw at low voltages and use the on chip regulator
with low quiescent current to maximize battery lifetime.
- When you do have data to send, use the FIFO buffer function of
the RF-IC to store the data. Use the buffer read/write mode to
minimize SPI operations. SPI communications consumes
power.
- When you receive incoming messages, discard false packets that
are not intended for your product possibly coming from another
unwanted radio system. Minimize the time in RX processing false
packets by checking the carrier sense, valid preamble, valid sync
word valid byte length and valid address feature options built
into today’s radio technology. Once you have done that, then and
only then wake up the MCU and use the automatic CRC check and
discard the packet if the CRC fails. Interrupt the MCU if the CRC
is okay.
- Spend time selecting a crystal that takes into consideration
clock drift between the two devices. Increased drift means
increased time in the receiver to ensure packet reception. The
receiver must be turned early especially in polling mode
architectures. The problem with turning on early means an increase
probability of receiving false packets that require additional
receiver time where we are driving to maximum sleep time for
battery life.
- Spend time squeezing every possible electron from transition
states of the radio. For example spend time drilling into the state
diagrams of the data sheet. Develop a power budget spreadsheet
which includes the typical values for power down, crystal
oscillator start up, PLL startup, full TX_RX time and power down
for your application. For two way protocols, go as quickly as
possible from transmit to receive mode or vice versa.
There is a tremendous amount of effort in the low power radio
alliances and users groups like the ZigBee Alliance, EnOcean
Alliance, and IPSO to develop best use practices.
For further
information on development platforms for your
product development like the Texas Instruments EZ430/RF2500- SHE
with solar power harvesting technology.
Author is Mark Grazier, who works for Texas
Instruments