Summary Lecture: Part 4 Wireless Sensor Networks Discovering Sensor Networks: Applications in Structural Health Monitoring

Distributed networks of wireless sensor nodes –gather critical information about the physical world –communicate the information to remotely located decision makers Example applications: –smart farming –health applications for the elderly –environmental monitoring –monitoring the structural health of a building or bridge

Our Original Motivation: Structural Health Monitoring for Disaster Prevention bridges public venues dams

We Learned about… Key concepts in Electrical and Computer Engineering, in particular: 1.Sensor read-out electronics and data conversion 2.Introduction to MEMS, microsystems and sensors 3.Radio-frequency (RF) wireless data communications 4.Wireless sensor networks Practical steps in the implementation of a wireless sensor network to monitor structural health –From a sensed parameter, through sensor readout and data conversion, to wireless transmission and forming a network

Wireless Sensor Systems SENSORS SIGNAL CONDITIONING (ELECTRONICS) MICROPROCESSOR ACTUATORS SIGNAL CONDITIONING (ELECTRONICS) SENSED PARAMETERS CONTROL PARAMETERS Sensor Readout Power & Control AnalogDigital RF/Wireless Transceiver Communications with Wireless Network Transducers RF

The Big Picture: Our Sensor Network Story Piezoresistive strain gages: metallic films whose electrical resistance varies with the strain We also observed that the performance of electronic devices and circuits can have undesired variation in their performance due to temperature, mechanical stress, light, etc. 1. A physical phenomenon (deflection, in our case) is converted into an electrical parameter (resistance).

The Big Picture: Our Sensor Network Story The Wheatstone bridge Assuming the bridge starts off in the balanced condition, if Rx varies due to an environmental condition (e.g. temperature, stress, etc.), a non-zero voltage will be detected across nodes D and B 2. The electrical parameter is converted to an analog signal via readout circuitry.

The Big Picture: Our Sensor Network Story Note that the output of the DAC is not a perfect representation of the original analog signal → we call this quantization error 3. Analog-to-digital conversion. ADC …

The Big Picture: Our Sensor Network Story The bridge output voltage data is converted to digital bits by a Freescale MC9S12C32 16-bit microcontroller unit (MCU) with on-board analog-to-digital converter (ADC) and transferred to students’ laptops via USB cable The project board DC supply rails are powered via the USB cable from a student’s laptop 4. Digital data stored in microprocessor or memory.

The Big Picture: Our Sensor Network Story Transmission uses Freescale AP13192USLK ZigBee transceivers ZigBee devices conform to the IEEE Low-Rate Wireless Personal Area Network (WPAN) standard This standard specifies operation in the unlicensed 2.4 GHz, 915 MHz and 868 MHz Industrial / Scientific / Medical (ISM) bands 5. Data formatted/encoded for transmission.

The Big Picture: Our Sensor Network Story 6. Formatted/coded data modulated on RF carrier (Zigbee uses something called QPSK, quadrature phase shift keying) and transmitted over the wireless channel. DAC … ×

The Big Picture: Our Sensor Network Story 7. Use of the channel by network nodes is determined by the Medium Access Control protocol. Aloha Simple random access If have data to send, just send it If collision occurs, try to resend again later Tends to be inefficient CSMA Random access, not as simple as Aloha If have data to send, listen first: channel free? send channel occupied? listen again later If collision occurs, try again later

The Big Picture: Our Sensor Network Story 8. Waveform received at the other node is demodulated and converted back to digital …

The Big Picture: Our Sensor Network Story Sensors may form a multi-hop wireless network –Or they may all report directly to a sink Information is synthesized and analyzed at the sink –Which, in turn, reports critical situations to some command- and-control center 9. Network of nodes allows for sensing along entire bridge span. Annotated map of sensor field: Average: sensor

Mission Accomplished: Disaster Averted During lab: –Sensor node underwent deflection: a digital voltage resulted –Voltage transmitted throughout network –Received by other sensor nodes (if in range and no collisions) Post-lab: –Convert received voltages to resistances, R –Using relationship between R, G and strain (  ), compute strain levels at each sensor node –Compare to strain threshold to determine unsafe strain levels –Generate warning and alert safety personnel, result

Think / Pair / Share Consider the wireless sensor system block diagram. What sources of error can you identify that will result in the output data at the receiving node deviating from the actual physical parameter being sensed?

Think / Pair / Share – Possible Answers What sources of error can you identify that will result in the output data at the receiving node deviating from the actual physical parameter being sensed? –Error in conversion of physical parameter to electrical parameter (readout circuit nonlinearity, calibration error…) –Quantization error in sensor ADC –Bit error in RF transmitter –Channel impairments (propagation loss, fading, noise, interference…) –Data packet collisions –Bit error in RF receiver –Quantization error in receiver ADC

Concepts, Trade-offs, and Tools Concepts DiscoveredSolution StrategiesTrade-offsTools  Perform analog-to- digital data conversion  Sense mechanical movement and convert it to electrical data  Transmit data efficiently through a wireless medium  Establish a wireless sensor network and transmit/receive data between the sensor nodes  Balanced Wheatstone bridge circuit for sensing resistance changes  Sensor to convert beam deflection to electrical resistance  Carrier sensing to improve data throughput  Self-organizing wireless network to relay and aggregate sensed data  Accuracy vs. quantization error  Sensor accuracy vs. speed  MAC scheme vs. efficiency  Sensor location vs. successful data reception throughout sensor network  Size/energy consumption of sensor node vs. performance  Freescale protoboard with readout circuit, microprocessor, and Zigbee transceiver  Strain gauge sensor with attached “beam”  CodeWarrior software interface

Other Applications lake water quality monitoring motion analysis precision agriculture

A Final Observation The development of sensors and wireless sensor networks is highly interdisciplinary, requiring team members with expertise spanning multiple areas of ECE and CS: –Sensor devices (materials science, microelectronics, MEMS) –Circuits and electronics –Digital signal processing –RF/wireless communications –Radiowave propagation (electromagnetics) –Wireless networking –Software (control, embedded computing, etc.)