Projects:2014S1-38 Semi-Passive Wearable Sensors

Project Introduction

Motivation

Older people may have the risk of falling when they move from one position to another. People tend to stay away from video monitoring because studies have shown that people do not like to be videoed and images are seen as an invasion of privacy, especially with older people.

Motivation of this project is to provide smart living spaces of tomorrow for older people through non-invasive activity monitoring using wearable sensors and environmental sensors.

Aim and goals

Our aim is to prototype a new generation of battery-less sensors for monitoring human motion. We build our prototype based on the WISP4.1 DL version.

Our design goals are:

To develop our WISP tag with accelerometer and barometer.

To test the performance of the WISP tag.

To implement RFID communication between reader and WISP tag.

Introduction

The WISP is a passive Computational RFID that harvests operating energy from, and communicates with, UHF RFID readers. It can also process data from its onboard sensors. We developed the new WISP which has extended functionality with its advanced accelerometer and barometer, SPI and I2C bus, and copper wire antenna.

The new WISP provides a RFID-scale, fully programmable, battery-less sensing platform that executes programs on its MSP430 microcontroller. The hardware and software are both derived from the open source WISP 4.1DL wiki.

          
                                                  Figure 1. System Structure

Hardware Development

Analog Front End

-Dipole antenna

-Schottky diode HSMS-285x

-5 voltage-doubling stages

-M4 modulation, backscatter

-70 Ω impedance matching, 915MHz

                     
                                                Figure 2. Analog Front End

Digital section

-Supervisory circuitry for waking up WISP tag

-Voltage level translator

-Microcontroller (MSP430F2132)

·Ultra-low power consumption

·Standby Mode: 0.7uA

·SPI and I2C peripherals

·Small size, flash RAM

Figure 3. MSP430 microcontroller

WISP with MSP430F2132

PCB design:

Characteristics:

-4 Layers board in FR4 material

-GND layer between Analog (Top layer, 915MHz) and Digital part (Bottom layer)

-Pin connection between MCU and sensors for both software and hardware communication

-Drilled hole for antenna soldered using copper wire and SMA connector soldered for impedance matching

-Extra copper space for snap on button connected to antenna

                 
                  Figure 4. PCB design of WISP with MSP430F2132

Fabricated and assembled result:

                                 
                        Figure 5. WISP with MSP43F2132

Sensors

-Accelerometer ADXL362

·Ultra-low power consumption: 1.8uA @ 100 Hz ODR, 2V

·12-bit output resolution

·SPI bus interface

Figure 6. Accelerometer ADXL362

-Barometer BMP180

·25cm minimum detection

·16-bit output resolution

·19-bit for ultra-high pressure resolution

·I2C bus interface

Figure 7. Barometer BMP180

Sotware Development

SPI implementation in CCS 6.0 software

The flow chart of ADXL362 spi implementation:

                         Figure 8. Flow chart for ADXL362 SPI implementation

I2C implementation in IAR 5.4 software

The flow chart of BMP180 I2C implementation:

                 Figure 9. Flow chart for ADXL362 SPI implementation

I2C implementation in IAR 5.4 software

The flow chart of power cycle in WISP

                 Figure 10. Flow chart for power cycle of WISP

Results

-Successfully developed the WISP tag.

-Successfully tested the accelerometer ADXL362 and barometer BMP180 using SPI and I2C bus.

-Successfully matched the impedance and make the WISP communication with RFID reader.

                         
                  Figure 11. WISP with MSP430F2132 and dipole antenna using copper wire

                             Figure 12. Communication testing between reader and WISP tag

Conclusion

This project designs and develops a WISP board integration by using barometer and accelerometer. One of the distinctive property of this project is that the whole system is passive. Power received from RFID Reader and accumulated by power harvester. Power is very limited. And because of this, components used in this project like microcontroller, barometer and accelerometer should have lower power consumption. Also PCB designed in this project should be very small, which brought many unexpected problems during the process.

Future Work

-To develop WISP tag with microcontroller MSP430FR5969

-To improve power sensitivity

-To test the data collision among several WISP tags

-To perform encryption in RFID data transfer

Group Members

Heranudin

Qiushi Li

Zhigeng Qian

Supervisor

Said Al-Sarawi

Damith Ranasinghe