Micro Datalogging Transceiver Networks for Dynamic Activity & Structural Performance Monitoring

Transcription

1 Micro Datalogging Transceiver Networks for Dynamic Activity & Structural Performance Monitoring C.P. Townsend & S.W. Arms MicroStrain,, Inc. 294 N. Winooski Ave., Burlington, VT microstrain.com web: tel fax

2 Introduction Telemetry systems capable of data transmission from multiple sensors have been developed; they use battery power (1), or inductively coupled energy from an external coil to an internal power reception coil (2). We have produced improved microtransmitters which include a programmable microprocessor and non-volatile memory; the digital data format allows for error checking at the receiver. The transmitter gain, filter settings, and sensor input types (differential vs. single ended) may be changed in software (3,4).

3 Introduction (continued) Our existing microtransmitter designs (Figures 1-3) are tiny and can be used to transmit multichannel data from various transducers (such as micromachined accelerometers), but these are not capable of high speed sampling. Datalogging allows for very fast data collection, but our dataloggers required a direct (hard wired) connection to a host computer in order to allow the data to be downloaded to the host computer for short term viewing and for long term analysis.

4 Fig 1: StrainLink microtransmitters Smallest commercially available wireless sensor node

5 Fig 2: Multichannel, addressable sensing modules (ASM's) RF Tx Power coil InAmp, A/D, mux microprocessor

6 Fig. 3: Remotely Powered & Interrogated Total Knee Replacement Johnson & Johnson, Scripps Inst., NK Biotek, MicroStrain,, Inc.

7 Objective To design and build a complete system for fast data collection and bi-directional wireless data communications from an implanted or worn sensor array to a remote data acquisition system (DAS) hosted by a personal computer.

8 Methodology A tiny digital transceiver, datalogger, analog signal conditioner, and triaxial accelerometer package (figures 4 & 5) were combined to. provide remote, high speed sampling and PC based wireless download of three orthogonal linear accelerations.

9 Methodology (continued) The subject is allowed to perform the activity to be monitored (running, skiing, boxing, football, tennis, etc.) The host transceiver antenna is placed within 95 feet of the subject. The host triggers the datalogger to begin logging by sending a "request to begin sampling" byte to the remote unit. This request contains a (PC based) time stamp in order to allow data from multiple nodes to be synchronized (by the host PC/receiver). The time stamp is updated to allow multiple requests to be sent to remote units, which insures reliable triggering.

10 Methodology (continued) The duration of logging is 5 to 20 seconds typically. For three channels, 5 seconds of sampling occupied approx. 0.5 megabits of memory. The subject is brought within range of the host transceiver. The host sends a "request to download data" byte to the datalogger.. At 9600 bits per second (baud) rate, wireless download is completed in 60 seconds. To collect more data from the same subject, the host sends a "request to begin sampling" byte again. This overwrites the datalogger's memory new data. The host PC is also capable of saving and displaying data downloaded from the remote datalogger.

11 Fig. 4: Micro Datalogging Transceiver System Diagram Li-ion battery, micropower voltage regulator triaxial, piezoresistive accelerometer s multiplexer, instrumentation amplifier successive approximation A/D converter (12 bit resolution) Narrowband (916 MHz) SAW transceiver MicroChip 8-bit, low power, microcontroller Flash EEPROM

12 Fig 5:Wireless Datalogging Transceiver 2 khz sweep rate of 8 channels Unique 16 bit address Remotely or internally triggered Vibration & impact monitoring Gold

13 Multidrop Wireless Sensing Networks Direct, multi-channel sensor input Transceiver nodes may be added or subtracted as needed Bi-directional (polled) or unidirectional (TDMA) network architectures 2-55 year battery life Web-enabled enabled receiver provides XML data stream Gold

14 How versatile? Software programming allows one hardware design to operate with many sensor types Linear & angular accelerometers, strain gauges, magnetometers, capacitive & inductive sensors, thermocouples, temperature & humidity sensors

15 Results The system was validated using a vibro-acoustic electrodynamic actuator. Accelerometers were calibrated against a precision accelerometer reference by rotating the accelerometer cluster on each of three axis, and using the datalogger, transceiver, and PC based data acquisition software (DAS) to record waveforms at the frequencies of interest. Using these systems, the University of Pennsylvania (New Bolton Center Large Animal Clinic, Kennett Square, PA) evaluated the effect of track surfaces and shoe design on the impact levels experienced by thoroughbred race horses.

16 Specifications Remote Datalogger/Transceiver PCB size: 2" by 2" by.25" Data Storage Capacity: 2 Mbytes (approx. 1 million data points) Data Sample Rate: programmable, from 32 to 2048 sweeps/second Data Sample Duration: programmable from 1 to 65,535 data sweeps; for 2048 sweeps per second, sample duration is 32 seconds Triggering Modes: 1) autotrigger,, user specified sensor voltage from a specific channel, or 2) on command from RS-232 or RF link Number Input Channels: programmable from 1 to 8 chs: : 4 full diff. inputs to InAmp (90 db CMRR min., volts common mode max), and 4 SE inputs (0-3 3 volts max) Remote Battery Lifespan: 95 hours est. w/ LiIon AA size 950 milliamp- hr capacity, 3 active MEMs accelerometers, RF transmit inactive, RF receive active Bridge Excitation: +3 volts DC at 50 milliamps max, OR pulsed DC w/ programmable sampling delay for energy conservation

17 Pulsed Bridge Power for Inductive Position Sensor (DVRT) Current pulse into bridge: 10 nanoamperes For 100 samples/sec, 1 microamp required 60 pts required to achieve 12 bit resolution, so average power required was 60 microamps Typical LVDT/DVRT circuits require 5000 to microamps Stainless packaged, hermetically sealed subminiature DVRT s

18 Datalogger sampling time after pulse precisely controlled to allow inductive component of impedance to be detected Pulsed MicroPower DVRT Response Device time = 0s and pulse turned time = 35uS Device sampled here 2 Output (Volts) mm 6 mm E E E E E E E E E-05 Time (Seconds)

19 Specifications (continued) Programmable Gain: for differential input channels 1-4, 1 user programmable from 5 to 2000 Programmable Offset: user programmable through Windows software, maximum allowable input offset is 600 millivolts Radio Frequency (RF) Transceiver Carrier: 916 MHz, narrowband, FCC compliant for license free use in the United States RF Programming & Downloading: baud, wireless RF, pulse code modulated, ASK RS-232 Programming & Downloading: baud direct serial RS-232 cable w/ miniature connector Data Download Time: 1000 data points per second Range for Remote RF Trigger: 100 feet (30 meters) typical LOS Range for Bi-Directional RF link: 100 feet typical, LOS

20 Discussion RF trigger with time stamp allows multiple wireless dataloggers to be synchronized with each other and with other motion analysis systems PC based host can address one or all datalogging transceivers 16 bit addressing allows thousands of sensing clusters to be employed in a scalable network

21 Discussion (continued) Sensing nodes do not communicate at the same time, so increasing remote sensing nodes does not increase the complexity of the base station High costs associated with installation and maintenance of long cable runs and the inherent reliability problems of outdoor electrical connections are eliminated

22 Discussion (continued) For structural monitoring, wireless sensor clusters may be permanently installed or deployed on a temporary basis to facilitate an instrumented inspection Scalable system architecture allows multiple sensor clusters of strain, vibration, acoustic emission, and temperature to be interrogated over a relatively large expanse such as a bridge

23 Biomedical applications Measure human activity and athletic performance more easily with small, wireless dataloggers Document musculoskeletal and neurological system functions & track progress over time after surgery/rehabiliation rehabiliation (balance, posture, tremor, gait, range of motion) Measure exposure to repetitive workplace tasks Report on the shock (g levels) experienced during work, sports

24 Industrial & commercial applications Rotating & vibrating machinery (drive shafts, helicopter rotors, turbines, power generators) chemical processes, pipelines, storage tanks heating ventilation and air conditioning (HVAC) trucks, railcars and other mobile assets food processing, storage, transport and retail agriculture, environmental (irrigation, hazards) security networks (border patrol, intruder detection smart structures (civil, military)

25 Reference s 1) J. Berilla et al., Design & Fab.. of multich.. strain gauge telemetry syst., Implantable Telemetry in Orthopaedics, Forsch. der FU, Berlin ) G. Bergmann,, et al, Multich.. strain gauge telemetry for orthopedic implants. J. Biomech. Vol. 21 No. 2, pp , 178, ) D. D Lima,, et al, An implantable telemetry system to measure intra-articular articular tibial forces,, ORS, Anaheim, CA ) Townsend et al: "Remotely powered, multichannel, microprocessor based telemetry system for a total knee implant", SPIE's 6 th Int'l Symp.. on Smart Structures and Materials, Newport Beach, CA, March 1999

26 Acknowledgments NSF SBIR Phase II & IIB, no NSF SBIR Phase II, no VT EPSCoR Phase O SBIR, July 1999 New Bolton Center, Large Animal Research Center, Kennett Square, PA