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projects:fram

Smart City Batteryless, Wireless and Serviceless Sensor Node

Design assumptions

The aim of this project is to build batteryless and serviceless device for measuring passing cars count and speed.

The project is not finished due to lack of essential parts (radio quartz and inductor for boost converter).

Still I think it is cool therefore I am posting it.

Applications

  • Intelligent traffic control
  • Industrial sensor networks
  • Security systems
  • Intelligent lighting
  • Crowd management

System does not have to be limited to the cars. Piezo sensors can detect any moving mass including both vehicles and humans.

Location

image/svg+xml

Device can be located under asphalt. The piezo generators will be located under track of car wheels. The wireless link with local wired node allows for minimum invasion into road structure.

Block diagram

image/svg+xml MSP430FRAM BoostConverter RadioTransmitter INT Piezos+Rectifiers Otherenergysources Energystorage

The mechanical energy from the passing cars is transformed using piezoelectric transducers into electric power. The current is rectified and boosted in high-efficiency, low input voltage boost converter. The energy is stored in capacitor. Other sources of energy can be connected as well.

MSP430 is in deep sleep until external interrupt wakes it up. Radio is completely cut off from the power unless MCU opens MOSFET.

Experiments

Alongside hardware development I am experimenting with the piezoelectric power generation.

I am using 35 [mm] Piezo plates: lh3.googleusercontent.com_-5d7hnvwaete_usd7oeq3vbi_aaaaaaaaes8_qcuaohb52-q_s902_img_20130222_150722.jpg

lh6.googleusercontent.com_-xcjbsjsfne4_usd5swk-q2i_aaaaaaaaesk_x4wxpneshom_s1059_img_20130222_111941.jpg Test setup consist of piezo, diode rectifier, 100 [μF] capactor, diode and 2 [kΩ] resistor. Few gentle bends charge the capacitor allowing diode to light after button press.

lh5.googleusercontent.com_-o2wvy7cn_v8_usd5wch8jgi_aaaaaaaaes4_y1lgp-svbgs_s1059_img_20130222_141554.jpg In this setup 4 piezos were connected in parallel to increase power. Rigid bar applies force to the centers of piezos. Piezos are mounted on soft base allowing them to bend slightly. This setup produces much higher voltage on capacitor. lh6.googleusercontent.com_-uevrcaln_cw_usd5wj91q8i_aaaaaaaaes4_mhqsphodh9q_s1059_img_20130222_141525.jpg I was able to charge capacitor easily up to 6V.

Piezos are made of solid material and will break if applied force is too high. Broken piezo will produce much less power if any at all.

lh5.googleusercontent.com_-aryn30ysb9g_usd5wahsk-i_aaaaaaaaes4_tof0w-wysdy_s1059_img_20130222_141740.jpg Close-up of the setup. The LED is bright.

Maybe this simple method of energy generation could be used for powering “active” cat's eyes on roads?

Hardware

Hardware design considerations:

  • Low leakage current capacitors
  • Maximum radio power - folded dipole antenna
  • Low cost - minimal board size, components on one side

Schematics

Layout

Software

Application is simple and can be represented by following flow diagram:

image/svg+xml Go to deepsleep Configure GPIO & Timer Start timer Stop timer store data Check data count Set radio send data GPIO INT 1 GPIO INT 1 RESET Not enough Enough

After power-up MCU initializes all peripherals and goes into deep sleep. Two external interrupts are responsible for measuring the vehicle speed. First interrupt starts timer, the second one stops it and stores time in FRAM. If timer overflow occur measurement is discarded. After several measurements MCU activates radio, sets it and burst data stored in FRAM.

Summary

  • GHz radio might be attenuated by asphalt. More experiments are required to estimate the best radio settings for minimal power and usability.

Prototype

ASCII

The device is connected to MSP-EXP430FR5739 for debugging and control signals emulation.

References

Datasheet

Application note

Other

Tools

projects/fram.txt · Last modified: 2013/04/01 22:38 by mkucia