Evaluate your application's energy-harvesting vibrational profile with a Slam Stick
To know whether your application is suitable for vibrational energy harvesting, you must determine its vibrational profile. Mide Technology's Slam Stick data logger and open-source software can ease your profiling task.
Margery Conner, Technical Editor -- EDN, October 20, 2011
Midé Technology’s energy-harvesting
devices rely on resonant harvesting.
To extract the maximum
available vibrational energy from
their environment, the harvesters
must be “tuned” to match the vibration. To
know whether your application is suitable for
this type of harvesting, you must determine
the vibrational profile by using an accelerometer.
To ease the task of profiling,
Midé designed the Slam Stick—a data
logger that measures acceleration in
all three axes—with the form factor
of the familiar USB (Universal
Serial Bus) stick.1. The Slam Stick uses an 8-bit
Microchip PIC18F25J50
microcontroller, which
integrates a full-speed
USB 2.0 transceiver and
a 10-bit, 10-channel ADC.
The Slam Stick also uses
Analog Devices’ three-axis
ADXL345 accelerometer. According to Tim Gipson,
a design engineer at Midé,
using a high-Q resonating
piezoelectronic beam
to harvest energy requires
accurate knowledge of the
vibration frequency, ideally
within 1 to 2 Hz. 
The
ADXL345 internally generates
its own sampling
clock; however, this clock
frequency can vary from part
to part. In response, Midé
runs an accurate, 32-kHz
oscillator along with the
accelerometer to determine
its actual sampling rate and
stores a correction factor
in the recording file, giving
fractional-hertz accuracy.
The
ADXL345 internally generates
its own sampling
clock; however, this clock
frequency can vary from part
to part. In response, Midé
runs an accurate, 32-kHz
oscillator along with the
accelerometer to determine
its actual sampling rate and
stores a correction factor
in the recording file, giving
fractional-hertz accuracy.
2. The reverse side of the Slam
Stick shows its lithium-polymer
battery. The device charges in
one to two hours after you plug
it into a USB port. A green LED
indicates when it’s ready to go.
The company evaluated some
thin-form-factor supercapacitors
early in the design, but the
lower self-discharge and flatter
discharge curve of the lithium-polymer
battery keep the
design small and simple.
3. The Slam Stick works with its open-source
analysis software. The screen on the top shows
the recorded g force versus time over the Stick’s
default 10-sec recording period. The middle
screen shows an expanded view of an FFT (fast
Fourier transform), which converts the amplitude
versus time of the data to frequency versus
time. The bottom screen shows a spectrogram
of the shoe strike.
The software doesn’t currently tell you how much energy is being generated because the amount of energy available for harvesting depends on several factors beyond the frequency and amplitude. For example, the same piezoelectric beam with a small proof mass at the end versus a large proof mass in the center might tune to the same frequency but have different power outputs in the same environment.
Talkback
-
An FFT doesn't produce frequency versus time; it produces frequency versus amplitude!
Steve Hebrock - 2012-10-4 13:45:44 PDT
