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Discovering Sensor Networks: Applications in Structural Health Monitoring 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 dams public venues 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 Transducers (ELECTRONICS) Sensor Readout ACTUATORS Analog CONTROL PARAMETERS Communications with Wireless Network SIGNAL CONDITIONING Power & Control SIGNAL CONDITIONING (ELECTRONICS) Digital MICROPROCESSOR SENSORS SENSED PARAMETERS Wireless Sensor Systems RF RF/Wireless Transceiver The Big Picture: Our Sensor Network Story 1. A physical phenomenon (deflection, in our case) is converted into an electrical parameter (resistance). • 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. The Big Picture: Our Sensor Network Story 2. The electrical parameter is converted to an analog signal via readout circuitry. • 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 The Big Picture: Our Sensor Network Story 3. Analog-to-digital conversion. ADC 101 110 100 011 100 110 111… • Note that the output of the DAC is not a perfect representation of the original analog signal → we call this quantization error The Big Picture: Our Sensor Network Story 4. Digital data stored in microprocessor or memory. • 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 The Big Picture: Our Sensor Network Story 5. Data formatted/encoded for transmission. • Transmission uses Freescale AP13192USLK ZigBee transceivers • ZigBee devices conform to the IEEE 802.15.4-2003 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 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 101 110 100 011 100 110 111… × 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. 101 110 100 011 100 110 111… The Big Picture: Our Sensor Network Story 9. Network of nodes allows for sensing along entire bridge span. • 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 commandand-control center Annotated map of sensor field: Average: sensor Mission Accomplished: Disaster Averted • During lab: • Post-lab: – Sensor node underwent – Convert received voltages deflection: a digital to resistances, R voltage resulted – Using relationship between – Voltage transmitted R, G and strain (e), throughout network compute strain levels at each sensor node – Received by other sensor nodes (if in range and no – Compare to strain collisions) 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 Discovered Perform analog-todigital 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 Solution Strategies 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 Trade-offs 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 Tools Freescale protoboard with readout circuit, microprocessor, and Zigbee transceiver Strain gauge sensor with attached “beam” CodeWarrior software interface Other Applications lake water quality monitoring precision agriculture motion analysis 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.)