Download RF Harvesting for remote Enviromental Sensing

RF HARVESTING FOR
REMOTE
ENVIROMENTAL
SENSING
Abstract: Battery-free wireless sensing can be utilized for solving
problems concerning, CO2 emissions, climate change and wildlife
migration patterns, to smart grid technologies, building automation and
structural health monitoring. Due to the scope of its application, RF
harvesting for remote sensing has become a grabbing topic for research
and development with the USDA, the Oil & Gas industry, and the DOT.
The goal of this project was to design, build and test a battery-free
wireless RF harvesting system capable of collecting and transmitting two
forms of data, and to harvest from a common RF bandwidth. A team of
five engineers were divided into communications and power
minimization, RF harvesting and power management sub-teams.
Temperature and power harvested were chosen to be the test sensor
data, with a data collection rate of 1 sample/30 minutes and
transmission rate of 1 transmission/hour as conservative objectives.
During the end of the build phase, the prototype was used for testing
and characterization of the system, the data collection rate had been
tuned to 1 sample/2.5 minutes with a transmission rate of 1
transmission/5 minutes within an area of radiated energy near 5.6 dBm.
Evan Andrews, EE
Ethan Heilicher, EE
Devin Madden, EE
Matt Ryan, EE
Ian Tomeo, EE
RF HARVESTING FOR REMOTE ENVIROMENTAL SENSING
Introduction
Battery-free wireless sensing has applications in an extraordinary number of different fields, providing
smart, efficient solutions to contemporary technological problems. By being wireless, sensing can be performed
in remote locations such as wildlife habitats, the top of skyscrapers, or inside CO2 producing machinery. By
harvesting power itself and not needing a battery, the device can appeal to an increasingly sustainability
conscious market, and can even label itself a ‘green’ device. Research on similar devices is being carried out by
the USDA, the DOT and even Oil and Gas industry giants, demonstrating the relevance of such technology.
This project aims to harvest ambient RF energy, rectify and store it, then use that stored energy to power a
microcontroller that takes data and transmits it to a home base.
Background/Theory
Antenna Theory:
For the purposes of this project, a dipole antenna was used to harvest the transmitted ambient RF energy. This
was chosen due to its simple construction and the ease of rectification of the collected RF transmission. Given a
desired transmission frequency to be harvested, equation (1) can be used to calculate the inductance of the
antenna. This equation can be simply put as:
2𝑙
3
(1) 𝐿 = .002𝑙 [ln ( 𝑎 ) − 4], where ‘l’ and ‘a’ are the length and radius in centimeters. The inductance is in µH.
1
(2) 𝑓 = 2𝜋√𝐿𝐶
One can then use Friis equation in order to find the ratio of transmitted power to received power, given to be:
𝜆
(2) 𝑃𝑅 = 𝑃𝑇 𝐺𝑇 𝐺𝑅 [4𝜋𝑅]
2
The rectification process can be accomplished by placing a diode between the two poles. As an antenna
transmits and receives electromagnetic radiation, the rectification process takes the electrical component of this
radiation and allows for conversion into usable DC power.
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MSP430:
For this project the greatest concern is by far the amount of current and voltage available at any given
point. Due to this, a device that is able to operate under these strict conditions is necessary. The CC430F137
microcontroller has a wide supply voltage range of 1.8 V - 3.6 V. In addition, it has an active mode current
draw of 160 uA/MHz and a standby mode current draw of 2.0 uA. This, along with the fact that it has a built in
radio transceiver make it an optimum choice for this experiment.
The EM4305137 development kit provides the CC430F137 device on a printed circuit board with built
in output ports, LEDs, and antenna. Utilizing two such kits allows more time to be spent on more critical
portions of the project, mainly power management. The main goal on the cc430 side of the project is to utilize
as little power as possible. This means operating in standby (low power) mode as often as possible and only
entering active mode to take a data measurement or transmit data. More details regarding how this is
accomplished will be presented in a later section of the paper.
P2110
The Powercast P2110 is a system-on-a-chip solution for low power RF applications. The P2110 provides RF
energy harvesting and power management for battery-free, micro-power devices. The chip converts RF energy
to DC voltage and stores the charge on an internal capacitor. When a charge threshold on the capacitor is
reached, the internal boost-converter boosts the voltage to the desired output voltage level and enables the
voltage output, Vout. When the charge on the internal capacitor declines to the lower threshold, the voltage
output is turned off. The output voltage can be regulated from 1.8V to 5.25V, with a maximum output current of
50mA.
Connected to the Vout pin of the chip is a larger external capacitor. This capacitor is slowly charged while
simultaneously providing enough current to maintain the microcontroller in low power mode. Once the external
capacitor reaches its nominal voltage of 3.2V, the microcontroller comes out of low power mode and transmits
the data samples it collected. After transmission, the microcontroller returns to low power mode, allowing the
external capacitor to charge.
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The efficiency of the RF-DC conversion process is a function of the power content of the input signal.
Figure 1: Efficiency of RF-DC Conversion vs. RF input power
At the frequency of 915MHz, with an input power of 5.6dBm, the RF to DC conversion efficiency is 55%.
Considering the low cost of the chip and the ease of implementation, this efficiency is quite high. The trade-off
of having such high efficiency is the input power requirement of the P2110. The chip requires a minimum input
power of -11dBm. This means the device will not be able to operate without a dedicated transmitter.
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Methods
RF Sources
Two sources of RF energy were implemented in the design. The first source is the Alien ALR-9650 RFID
Base Station. In a typical application, the base station transmits pulsed signals between 902.75 and 927.25 MHz
(centered at 915MHz) that are received by RFID tags. The tags use the energy to send an acknowledge signal
back to the base station. In this RF harvesting application, the pulses of energy are received by the P2110,
rectified, and stored in a capacitor. The base station contains an internal circular-polarized transmit/receive
antenna. The output power from the base station was set to its maximum of 4W EIRP (effective isotropic
radiated power). The antenna used to measure the received power was a 0dBi gain, omni-directional dipole
whip antenna.
Transmit Power [dBm] Distance [m] Received Power [dBm] Omni Antenna Polarization
36
0.3048
5.606
Vertical
17
0.3048
-22
Vertical
Table 1: Available power at a distance of 1 foot from the base station
The second RF source used in the design was the Agilent MXG Analog Signal Generator (M/N: N5181A).
Unlike the base station, the signal generated by the MXG is constant. The limiting factor of the MXG is the
power content of the generated signal. The P2110 requires a minimum RF input power of -10dBm. The MXG
can output a maximum of 17dBm at 915 MHz. At a distance of one foot from the transmitter, the measured
power available was -22dBm, 12dBm less than what the P2110 requires for operation. Due to this limitation, the
MXG was unable to be used as the dedicated transmitter for testing and demonstration.
Another advantage of using the base station is the practical aspect of RFID. As RFID becomes more widely
used, more base stations will be placed around buildings, campuses, warehouses, etc. This leads to an increased
opportunity to take this otherwise wasted ambient RF energy and use it for a multitude of very-low-power
applications.
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Current Limiter:
The LM134 3-terminal adjustable current source was implemented to reduce the amount of current capable of
flowing from the output of the P2110 into the external 1000uf storage capacitor. This was done due to the
uncharged cap acting as a drain to the internal voltage doubler within the P2110; this causes a lose of charge
before the voltage can be boosted, preventing the desired voltage from being reached. By implementing the
current limiter, the voltage doubler is capable of reaching a higher, desired voltage before storing on the
external capacitor.
Low-Power Mode:
Low-Power Mode is used by the microcontroller to enter a state in which the CC430 requires less
current to remain in operation without entering a state of BOR (Brown Out Reset). When any of the four low
power modes are entered the CPU (Central Processing Unit) shuts off, however the CPU will immediately
power up when any interrupt is recognized by the microcontroller’s interrupt vector. When LPM3 (Low Power
Mode Three) is entered in the C code by setting the corresponding bits in the status register (see user’s
guide) the CPU, SMCLK, and MCLK are disabled however ACLK is still active.
When the on board VLO (Very-Low-Power Low-Frequency Oscillator) is set to source ACLK which in
turn is used to source TIMERA, TIMERA1 is used to generate an interrupt after the counter reaches a
predetermined value loaded into TACCR0 (Timer ‘A’ Capture and Control Register 0) by being set in up mode.
Since the VLO has a known frequency of about 10[kHz] TACCR0 is set with a value that when reached by the
counter a few times will correspond to a 2.5[min] delay.
After the time interval of 2.5[min] has been reached the microcontroller will stay in active mode whilst
it utilizes the ADC12 (12 bit Analog to Digital Converter) to take a voltage reading of the capacitor providing
power to the microcontroller itself as well as the on board Temperature Sensor. The ADC12 is set up for Single
Sample Single Channel Conversion of 8 bits. The microcontroller only samples 8 bits at a time in order to cut
down the amount of information which will have to be sent to the receiving microcontroller. When a
Temperature sample is taken the C code will enter an ADC12 ISR that loads the sampled data into an array
called TxBuffer that holds the information to be transmitted. The second voltage sample is taken from the
capacitor the code once again enters the ADC12 ISR and loads the sampled data into a different place in
TxBuffer. The micro controller will then once again enter LPM3 and wait for a timer interrupt to wake the CPU
after 2.5[min]. After another round of sampling the CC430 is ready to transmit the data it has acquired.
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The power loss due to sampling is negligible due to the very short amount of time that the ADC12 is
actually sampling in the C code in comparison to the power gained by the RF and power portion of this system.
After 5[min] waiting in LPM3 the stored data will be transmitted with transmission and receiver code provided
by Texas Instruments. For this code to allow for the transmission of acquired data at 868[MHz] to occur
properly the PMM (Power Management Module) of the device must be set to 2. The PMM controls the core
voltage level of the CPU and thus must be set to this value to allow for enough power to reach the radio for it to
even enter a region of operation as stipulated by the device specific datasheet of the CC430f5137. An LED is
turn on to indicate transmission. The 4 bytes of data are then transmitted by the radio through the air and to the
receiving antenna of another CC430 microcontroller connected to a computer with Labview. The transmitting
CC430 then resets the radio core, turns off the LED, and enters low power mode to repeat the 5[min] cycle.
The code loaded onto the receiving CC430 is only modified to read the contents of RxBuff after a
successful transmission of data and uses a Printf statement to write the 4 bytes into a text file to be saved into a
directory on the computer to which it is connected. This data file is read into Labview, manipulated to represent
physical measurements and plotted on their respected graphs over time.
Results
Implementing the above techniques results in a standby current draw of 3.65 uA at 3.1 V. This value is only
1.65 uA larger than the idealized standby current listed in the CC430 data sheet (without powering the other
elements of the development board). In addition, the device successfully exits low power mode and draws only
20.6 mA during a transmit operation. The goal of this project is (with a dedicated transmitter) to collect data
every 15 min and transmit once every 30 min. However, due to the impressive performance the data collecting
unit and the leanness of the CC430 code design this goal was surpassed by a factor of 12 with no losses in data
reliability. This results in a data collection rate of every 2.5 min and a transmission rate of once every 5 min.
The Labview VI (Virtual Instrument) on the receiving end has several different functions. It collects data
from the CC430 and plots both temperature and supply voltage versus time. In addition, it processes the data
and performs the necessary calculations to modify the voltages given by the ADC into recognizable temperature
and voltage values. The Labview VI is also responsible for keeping track of the last 12 samples taken, which
allows it to produce meaningful graphs that provide a clear indication of the values of temperature and supply
voltage over time.
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Figure 2 - Microcontroller Current Draw During Collection and Transmission
Figure 3 - Storage Capacitor Discharge During Transmission
The current draw of the microcontroller was observed as seen in figure 2. The graph represents a time period
where the microcontroller samples and transmits. The first plateau corresponds to taking a data point, the
second plateau is from transmission, and the peak is due to the LED on the development kit blinking. Figure 3
demonstrates the charge loss on the external cap during the same operation, data collection and transmission.
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Discussion
- Limitations on charge time due to LM134
Due to the required voltage drop across the LM134 limiter, a reduction in charge and charge timing is
produced. The shut off voltage varies as the external storage capacitor voltage increase in relation to regulated
output voltage and the designed limited current flow. An effect of this shortened charge time and magnitude
causes a diminishing of charging period for every charge cycle; this can be observed as a false capacitor
charging pattern over time on the output. Using the pseudo-charging pattern, one can characterize the new
charging system similarly. Increasing duty cycle, which is dependent on the capacitance of internal capacitor
and storage capacitor, one can increase optimum storage time for the particular application at hand.
- Code Difficulties
Mixing C and Assembly was a big problem in this project the ret function in assembly would not push
the correct value off of the stack onto the program counter. we were never able to debug this problem and thus
could not pass global variables into and out of an assembly function call. When returning from an interrupt from
taking samples in assembly the stack would also push the wrong return address onto the program counter. Due
to the fact that most of the time the CPU is shut down we determined the power loss from writing the code
entirely in C would be negligible.
The Low Power modes were difficult to master at first because of the radio operations however after
making a decision to keep the PMM core voltage level constant the problems which arose from entering and
exiting LPM stopped. The CPU would not wake up or the program counter would jump to seemingly random
places in the microcontroller’s memory when using switch the PMM core voltage level from 1 during the LPM
portion of the code to 2 for radio operation. Increasing the PMM to 2 during LPM3 increased the amount of
current the microcontroller used in LPM3 if the radio was not reset but when it was discovered resetting the
radio after transmitting turned the radio off the current which was being drawn decreased by 96[uA].
A BOR would cause the microcontroller to use much more power than it would in active mode and thus
an initial charge time is required before even turning the microcontroller on.
The SPI mode of the original RF rectification chip did not operate properly and we could not discover a
definite reason why however we can speculate that it may have been due to incorrect solder connections when
pinning out the TMS37157.
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Conclusion
In industry today, sustainability is king. With a plethora of potential energy crises on the horizon, as well
as the constant search for lower power (and thus lower cost) devices, sustainable devices and technologies are
being highly sought after. College campuses are being powered by wind turbines, more and more homes are
covered in solar cells, and hybrid cars are become the norm.
RF ID technology is simple and pervasive, but its potential for larger scale applications has yet to be
fully explored. Traditionally an RF ID contains a small rectenna that broadcasts a tiny piece of data stored on it
when an RF ID reader is present. It can broadcast that small data piece because the reader transmits power that
is received by the rectenna. For our implementation of this technology, we wanted to scale up the RF ID so that
it would run off of ambient RF signals and transmit environmental data back to a home base.
Ambient RF signals are abundant today, from wi-fi signals to cell phone signals, so implementing an RF
harvester is relatively practical. By tuning an antenna to the desired frequency range and building a rectifying
boost circuit and temporary storage manager, a low-power source can be constructed. By combining this with a
microcontroller with built-in temperature and voltage sensors, and transmitter, the full system was assembled.
The antenna receives power, the circuit manages the power, and the microcontroller takes data and transmits.
Acceptable performance was achieved, taking data every 2.5 minutes and transmitting every 5 minutes when
using the RF ID reader as a source (5.6dBm). For a remote application, data would only be required every 30
minutes and transmitting every hour, resulting in only -5.2dBm required from a source.
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