Static and Dynamic Monitoring of Civil Engineering Structures by Microwave Interferometry Garry Spencer and Mark Bell 1 PRODUCTS IBIS range APPLICATIONS IBIS - FL LANDSLIDE & DAM MONITORING IBIS - FM SLOPE STABILITY IN MINING IBIS - FS STRUCTURE MOVEMENTS 2
3 IBIS FS Hardware 4
Techniques behind the IBIS FS system The IBIS FS product utilises two radar techniques: 1. Frequency Modulated - Continuous Wave (FM-CW) technique for obtaining the 1-Dimensional Range Profile with Range Resolution. 17.1 17.3 GHz Single Transmit & Receive Burst 2. Interferometric technique 5 Interferometric Technique The interferometric analysis provides data on object displacement by comparing phase information, collected in different time periods, of reflected waves from the object, providing a measure of the displacement with an accuracy of less than 0.01mm (intrinsic radar accuracy in the order of 0.001 mm.) First acquisition TX RX ϕ 1 d d = λ 4π ( ϕ ϕ ) 2 1 Second acquisition TX RX ϕ 2 6
1-D Interferometric technique The displacement is measured in the direction of the line of sight of the system. To calculate the real displacement is needed to know the acquisition geometry h R d p α d d = d p sin(α) sin(α ) = h R d = d p R h The distance R is measured by IBIS-S 7 IBIS-FS measured scenario is determined by Antenna beamwidth (depending on the model, from 10 to 50 ). User s defined maximum range (up to 1 Km) IBIS-FS Acquisition Mode IBIS-FS Range resolution n-th range bin The measured scenario is divided into range bins, whose number depends on the range resolution (0.75 m minimum, constant with distance). Two targets at the same distance from the radar fall on the same range bins. Range profile Distance (m) 8
IBIS-FS: 1-dimensional range profiles Range Profile: one dimensional image with 0.5m range resolution Welding lines are good reflective points IBIS-FS installed at the turbine pillar base (height 60m) 9 Measurement accuracy: IBIS vs. Total Station Test objective: comparison between IBIS-S results and a high-performance Total Station in measuring a target displacement Target distance: 33m Forced displacement: 3 x 1mm step towards IBIS-S and -3mm back 2 x 0.5mm step towards IBIS-S and -1mm back 5 x 0.1mm step towards IBIS-S and -0.5mm back 10
Measurement accuracy : IBIS vs. Total Station IBIS-S results Total station results 1mm displacements 0.5 1 mm displacement 1mm steps Radial Displacement [mm] 0-0.5-1 -1.5-2 -2.5-3 -3.5 0 500 1000 1500 2000 time [sec] Displacement (m) 0,0005 0,0000-0,0005-0,0010-0,0015-0,0020-0,0025-0,0030-0,0035 0 20 40 60 80 100 120 Measure Number 0.5mm steps Radial Displacement [mm] 0.4 0.2 0-0.2-0.4-0.6-0.8 0.5 mm displacement 0,5 mm displacement 0,0005 Displacement (m) 0,0000-0,0005-0,0010-1 -1.2 0 200 400 600 800 1000 time [sec] -0,0015 60 80 100 120 140 160 180 Measure Number 11 Measurement accuracy : IBIS vs. Total Station IBIS-S results 0.1 mm displacement Total station results 0,1 mm displacement 0.10 0,0003 0.1mm steps Radial Displacement [mm] 0.00-0.10-0.20-0.30-0.40-0.50 Displacement (m) 0,0000-0,0003-0,0005 0 500 1000 1500 2000 2500 3000 3500 4000 4500 time [sec] -0,0008 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Measure Number 12
Static Monitoring Corners Load position IBIS-FS 13 IBIS-FS corner reflectors From the structure plan is possible to determine whether reflectors are needed or not. BEAMS CORNER REFLECTORS 14
Bridge testing: static live load test : Linear Variable Differential Transformer to measure displacement SNR (db) 90 80 70 60 50 40 A B C D E F G H IBIS-FS installation Range profile of P2-S P3-S span 30 0 10 20 30 40 50 60 70 Ground-Range (m) 15 Bridge testing: static live load test : Linear Variable Differential Transformer displacement (mm) 10 0-10 -20-30 LC1 B D F H LVDT 0 200 400 600 800 1000 1200 1400 time (s) 16
Bridge testing: static live load test : Linear Variable Differential Transformer displacement (mm) 10 0-10 -20 LC1-8.84-15.62 LVDT -20.50 IBIS-S LVDT -30 0 200 400 600 800 1000 1200 1400 time (s) 17 Static settlements of bridge s pier and beam P1 P2 P3 P4 P4 P3 P2 P1 0,2 0,1 Vertical movements of the 4 points 6,5 m 5 m 0 spostamento [mm] -0,1-0,2-0,3-0,4-0,5-0,6 0 500 1000 1500 2000 2500 3000 3500 4000 tempo [s] P1 P2 P3 P4 18
Dynamic Monitoring Corners IBIS-FS 19 Dynamic Monitoring: Capriate bridge Measurement objective: comparison with accelerometers, resonance frequencies and modal shape retrieval Central arch length (m): 62.5 20
Dynamic Monitoring: Capriate bridge To make a comparison between the results of IBIS-S system and accelerometers system 6 corner reflector were installed at the same position of accelerometers 21 Dynamic Monitoring: Capriate bridge Bridge photograph and range profile 22
Dynamic Monitoring: Capriate bridge Velocity comparison for Test Point 22 Velocity [mm/sec] IBIS-S acc Zoom on the first 15 sec direct comparison IBIS-S Time [sec] Velocity [mm/sec] accelerometer Time [sec] 23 Dynamic Monitoring: Capriate bridge acc Frequency analysis comparison on 3000sec acquisition duration IBIS- FS Acc detected frequency IBIS-S detected frequency Percentage error Hz Hz % 2,617 2,595 0,84 3,164 3,182-0,57 6,641 6,608 0,50 8,086 8,077 0,11 24
Dynamic Monitoring: Capriate bridge f = 2.617 Hz f = 3.164 Hz f = 6.641 Hz f = 8.086 Hz Modal shape obtained by accelerometer data 25 Dynamic Monitoring: Capriate bridge Normalized Mode Shape Normalized Mode Shape 1.2 0.6 0.0-0.6 1.2 0.6 0.0-0.6 Accelerometer IBIS-S sensor Accelerometer IBIS-S sensor Modal shapes comparison -1.2 0 23 46 69 92 115 Distance along deck (m) -1.2 0 23 46 69 92 115 Distance along deck (m) f=2.617hz f=3.164hz 26
Normalized Mode Shape Dynamic Monitoring: Capriate bridge 1.2 0.6 0.0-0.6 Accelerometer IBIS-S sensor -1.2 0 23 46 69 92 115 Distance along deck (m) f=6.641hz Modal shapes comparison Normalized Mode Shape 1.2 0.6 0.0-0.6 Accelerometer IBIS-S sensor -1.2 0 23 46 69 92 115 Distance along deck (m) f=8.086hz 27 In Summary: Interferometry deflection measurements were measured and compared with accelerometer and Linear Variable Differential Transformer fully validating the instrument results. The IBIS-FS can be rapidly deployed for short-term displacement and vibration monitoring with complete measurement and set up time being less than one hour A great deal of information can be captured from a number of points on a variety of structures very quickly making the unit an excellent alternative for economical static and dynamic surveys 28
Questions? Thanks for your attention 29