Evaluate your application's energy-harvesting vibrational profile with a Slam Stick

Margery Conner, Technical Editor - 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. Evaluate your application’s energy-harvesting vibrational profile with a Slam Stick image 2

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.