Development of a High Speed UWB Ground Penetrating Radar for Rebar Detection
Dryver Huston1, Tian Xia1, Anbu Venkatachalam1 and Xianlei Xu1,2
1University of Vermont, Burlington, VT2China University of Mining and Technology
This research was supported byUS Dept Commerce, NIST TIP Program Coop Agreement 70NANB9H901 with
Northeastern Univ., US Dept of Transportation Coop Agreement DTOS59-08-G00102
Acknowledgments
GPR Design Goals
• Operate at road speeds (60 mph)• Capture and process real-time data• Multi-channel
• Physical coverage across width of roadway
• Meet or exceed FCC mask requirements• FCC 02-48 compliant
• Compact, modular design
Technical challenge: High resolution sampling at highway speeds
• Issue FCC 02-48 Compliance• Limits radiated
emissions • Constrains pulse rate
and power
G o o d G P R p e n e t r a t i o n ,
b u t lo w r e s o lu t io n
I d e a l f o r G P R , b u t r e s t r ic t e d
H i g h a b s o p t io n , l i m i t s G P R p e n e t r a t io n
F C C 0 2 - 4 8 R a d ia t e d E m is s io n L im i t s o n U l t r a w i d e b a n d G r o u n d P e n e t r a t in g R a d a r
Design Strategies• Shaped Frequency
Low cost transmitter• Single Shot High
Speed ADC receiver• Low-profile, low-
weight, low emissions antenna
• Multichannel system
tStandard method of subsampling high speed radar signals requires sending and
receiving multiple signals.
tFull waveform sampling of high speed
radar signals allows sending and receiving fewer signals.
Signal
Amplitu
deSignal
Amplitu
de
Design Challenges• To operate accurately at normal travel speed (60mph), the sensing system needs to have fast sensingspeed and wide sensing area
• Moving the sensing antenna horizontally to scan awide area is slow process• Proposed solution: multiple sensing channels
Design Challenges• Operating at normal travel speed (60 mph), the pulse
repetition frequency (PRF) should be higher to achievegreater surface mapping resolution
• At 60 mph, to achieve mapping resolution of 10 cm at single shot dataacquisition, the pulse repetition frequency should be ~2.6 kHz• 60 miles in an hour -> 10 cm in 372 usec (travel speed)
Travel Speed – 60 mph
Pulse Repetition Frequency Mapping Scan Resolution
~3 kHz 10 cm
~30 kHz 1 cm
10 kHz 2.68 mm
100 kHz 0.268 mm
10 MHz 2.68 um
Ver. 3 System Configuration – Single Channel
Integrated Board with Pulser and LNA
FPGA
Antenna Transmit
Antenna Receive
12 V
10‐bit 8 GHz ADC w/ 0‐2 GHz bandwidth
Analog Data
Trigger
Trigger Confirm
PC 64 bit quad coreEncoder
QSBSSD Digital Data PCIe
Hardware Development(1)Radio Frequency Front End: Low Ringing High Amplitude UWB Pulse GeneratorThree functional units: Signal amplitude converter
current feedback OpAmp:THS3091 negative voltage converter:TL 7660
Gaussian pulse generator ;Step‐Recovary Diode.
Pulse shaping filter. a shunt resistor ; an inductor; a series connected capacitor.
UWB monocycle pulse generator circuit
Pulse measurement result
Mono-Cycle Pulse Generated
Monocycle Pulses at PRF 100 kHz
Monocycle Pulse
1/8/2014 10
Typical return trace and B-scan from rebar in concrete
Block Diagram of Two Channel SystemThe sensing circuit can be controlled to operate in different frequency bands to
facilitate the penetrating capability and measurement resolution according to thecharacteristics of the material under inspection.
PRF Square Wave
Pipeline Issues
Solutions
Digital Data Acquisition and Data Storage
Digitizer
Flow Chart of Digitizer
Digitizer-Agilent Acquiris
U1065A-004-FHZ DC282HZ
Sample rate: 8GHz, 10—bit
Pipeline Issues:• U1065A Digitizer
• Max 2.5MHz trigger frequency
• Each samples takes ~25ns to transfer to PC yet is acquired in only 125ps = 200x slower than required for full coverage
• Computer Storage / Operating System• Achieved ~920 Mb/s write
speed by memory-mapping existing buffer files
• Switching between files causes ~18,000us data gap every ~12 seconds
(2)Data Acquisition Module: High Speed Digitizer Configuration Sampling Rate: 8 GspsResolution: 10‐bitProblem: speed mismatchAcquisition : 125 ps/per sample;Transfer and save: 13 ns/per sample;
Data transfer speed is about 100 to 200 times slower than sampling data acquisition. Such large speed discrepancy could cause digitizer operation jam and data loss.
U1065A Acqiris real‐time AD converter
Techniques to solve pipeline speed mismatch:
(1) SAR mode (Simultaneous multibuffer Acquisition and Readout);
(2) Multi-thread data reading and writing;
(3) Solid state disk: Write speed is 140Mb/s, where the normal hard disk is 60Mb/s)
Third‐generation solid state disk
Digitizer configured in SAR mode
Multi‐threading configuration for data transfer and storage
(4)Digital Control
Two channel SynchronizationPRF variable microwave front end with two channels.
ADC synchronizationGenerate channel tag necessary to identify the data from each channel for post processing.
Block diagram of FPGA ADC Synchronization Control module
FPGA functional blocks for tunable synchronized clock generation
Field‐Programmable Gate Array (FPGA) from Xilinx
GPR Systems Integration
Data ProcessingChallenge: Data size is big. Assuming pulse repetition is 40KHz, then the data size is as follows:
sampling widow(ns) (MB)/ Per second (GB)/ Per Hour
10 3.8161 13.4160
40 15.2598 53.6475
The signal processing steps consists of four main parts, including Data preprocessing;
traces compression to eliminate accidental errors; background removal; band filter;
Target area detection; Morphological Filter, Normalized Energy Map
Hyperbola fitting. Depth evaluation
Apex coordinates extraction ; depth estimation;
Data Preprocessing(1)Vibration effect correctionTo eliminate vibration effect, level tracking and level correction operations are applied.
(2) Systematic noise reductionTo eliminate various systematic noises, including channel noise, antennas direct coupling and road surface reflections etc.
(3) Radio frequency interference (RFI) reduction To eliminate radio frequency interference in the test environment.
Image after vibration effect correction
Image after after systematic noise removal
Image after after RFI reduction
Conclusions
(1) Air-launched UWB GPR in development for reinforced concrete evaluation (bridge decks at highway speeds is primary goal).
(2) Full waveform digitization with buffered multithread data pipeline.
(3) Dual-band switchable source with FPGA control.(4) Customized signal processing methods, including
data pre-processing, target area detection and hyperbola fitting, have been developed.
(5) Future directions are to improve further GPR performance and to characterize rebar dimensional size and corrosion conditions.
Questions?