Development of a High Speed UWB Ground Penetrating Radar ...edg.uchicago.edu/~eric/uvm_gpr_system.pdf · Development of a High Speed UWB Ground Penetrating Radar for Rebar Detection

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?