Journal of Advanced Concrete Technology
Research and development of infrastructure diagnostic robot system (ALP) for detailed
inspection of concrete structures at elevated heights--Manuscript Draft-Manuscript Number:
Article Type:

Scientific Paper

Full Title:

Research and development of infrastructure diagnostic robot system (ALP) for detailed
inspection of concrete structures at elevated heights

Abstract:

This paper describes R&D of an inspection diagnostic system that applies robot
technology to the detailed inspection of infrastructure such as concrete walls in highplaces. ALP uses a vacuum suction pad improved by testing on real structures as amoving mechanism. ALP is also fitted with a measurement system that consists of ahigh-resolution camera, an electromagnetic wave radar, and a hammering sounddiagnostic device. The result is a highly capable self-propelling inspection system thatcan carry out detailed inspections of infrastructure in difficult-to-access locations suchas high places.

First Author:

Junichiro Noima, Ph.D.

Order of Authors:

Junichiro Noima, Ph.D.

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Research and development of infrastructure diagnostic robot system (ALP) for detailed
inspection of concrete structures at elevated heights----------------------------------------------------------------------------------------Junichiro Nojima (Manager of civil Engineering Dept., PhD, J-POWER DESIGN (JPD),
Kanagawa, Japan), Toshiaki Mizobuchi (Professor, PhD, Hosei University, Tokyo, Japan),Kenji Hayashi (President of robot production company Stella Rtec Inc., Japan)----------------------------------------------------------------------------------------Abstract
This paper describes R&D of an inspection diagnostic system that applies robot technology to
the detailed inspection of infrastructure such as concrete walls in high places. ALP uses a
vacuum suction pad improved by testing on real structures as a moving mechanism. ALP is
also fitted with a measurement system that consists of a high-resolution camera, anelectromagnetic wave radar, and a hammering sound diagnostic device. The result is a highly
capable self-propelling inspection system that can carry out detailed inspections ofinfrastructure in difficult-to-access locations such as high places.
----------------------------------------------------------------------------------------1. Introduction
Infrastructure maintenance and management in Japan is a challenge owing to the rapid
aging of the infrastructure that was mass produced during Japan' period of high economic
growth and the shortage of skilled personnel. On the other hand, wholesale restoration ofJapan's enormous social capital stock would not necessarily be most efficient and a morecalibrated approach would work better. Hence, the assessment of the existing infrastructure isrequired to design optimal measures, yet superficial inspection based on simple visualinspection would fall short of providing the needed data. Accurate judgment of the causes ofdeterioration and whether it is of a progressive nature is required to determine the necessity ofmeasures.
Use of non-destructive inspection for the detailed inspection of concrete structures has
merits such as eliminating human error arising from differences among inspectors andrevealing conditions inside concrete not detectable from external visual inspection alone. Useof the electromagnetic wave inspection method among the various nondestructive inspection
methods allows measurement of cover concrete and thereby accurate judgment of whether
repair during the service period is called for to remedy progressive deterioration. Further, byapplying the chloride ion content determination technique using electromagnetic wave radar*developed by the authors, it is possible to determine the chloride ion content at the location ofrebar, allowing prediction of latent stage deterioration from chloride attack.
However, in the case of structures in high places, scaffolds or the like are required for1human access, making detailed inspection more difficult. In recent years, the use of UAVs forinfrastructure inspection has been increasing. However, detailed inspection such as describedabove necessitates bringing measuring equipment of a certain weight in stable contact withthe wall surface, and considering the counterforce that must be applied to the wall surface toestablish sufficiently stable contact with the structure, the use of UAVs is problematic.
Therefore, we focused on vacuum suction pads and decided to develop an infrastructure
diagnostic robot system called ALP. ALP is an abbreviation of the word "alpinist," and assuch it carries the implied meaning of achieving the goal of executing detailed inspections
even in difficult conditions such as high elevations (Nojima et al. 2018). Current
wall-climbing robots from around the world are listed in Table 1. ALP was developed withthe goal of achieving an apparatus capable of self-propelling up and down and left and rightwhile stably adsorbed on a wall while equipped with various measuring devices.
2. Moving mechanism
2.1
Suction units
2.1.1 Suction unit mechanism (Figures 1, 2)
ALP has five suction units, three on the upper side and two on the lower side of ALP.
All suction units consist of the same suction/travel mechanism, and only the control
firmware in each suction unit’s microprocessor differs. Besides a microprocessor, eachsuction unit has also an adsorption unit equipped with a vacuum pump and a vacuum seal
for secure adsorption on walls. The placement of the suction units on ALP is flexible as
they can be mounted where desired on the ALP frame.
Each suction unit has an absorption unit mechanism that can move left and right, up anddown, and extend and retract perpendicularly to the wall surface so as to achieve a good fitof the adsorption unit and its vacuum seal against the surface. Each suction unit iscontrolled independently with its own suction unit microprocessor, and the suction unit
microprocessors are controlled by a main microprocessor via CAN interface.
2.1.2 Adsorption Power (Figure 3)
ALP can adsorb to rough and irregular wall surfaces such as weathered concrete and tiled
surfaces with shallow joints with little air leakage.
The adsorption power of the adsorption unit is very strong. The adsorption power iscalculated simply by π × R × R × Vcur / Vabs,
where π = Pi; R = radius of the pad seal (cm), Vcur = current vacuum value in the pad’s
vacuum chamber (kPa), and Vabs = absolute vacuum value (kPa).
For example, when the pad seal diameter is 16 cm and the vacuum value is -75 kPa in the
pad’s vacuum chamber, the calculated adsorption power is about 150 kg.
This value is the adsorption force in the direction perpendicular to a smooth wall surface.
In actual practice, the adsorption power changes dynamically according to conditions suchas vacuum leakage due to suboptimal contact surface conditions, rotational moment in the2

downward direction of the suction pad under gravity, shifting loads on the suction pads due
to slipping, movement of ALP, and movement of the center of gravity caused by movementof the measuring devices mounted on ALP.
2.1.3 Structure of ALP’s adsorption unit seal
To maintain adsorption power, the adsorption unit seal is sophisticatedly designed to adapt
to wall surfaces. The adsorption unit seal that contacts the wall surface is a cylindricalshape with a trumpet shaped edge made of silicone. The adsorption unit seal is over 10 mm
high, which allows it cope with irregularities up to about 5 mm high and weatheredconcrete walls.
However, the surface layer of existing concrete structures tends to deteriorate under the
influence of leaching and drying in places where it is exposed to rain, developing higher air
permeability compared with parts that are shielded from rain (Torii et al. 1994).
Experiments carried out on concrete walls with high air permeability in the surface layer
revealed that it is not possible to achieve a stable vacuum state only with an adsorption unitseal.
On the other hand, the torrent method (Torrent 1994) is available as a surface layer quality
test method. To eliminate the influence of air permeability of the surface layer of concrete,
the torrent method employs a two-chamber vacuum cell. Taking inspiration from thismethod, the vacuum suction pads used by ALP were designed to achieve a stable vacuum
state even on sub-optimum surfaces such as existing concrete whose surface quality has
deteriorated by placing cushioning material around the sealing material so as to extend theair permeation path. (Figure 4)
The seal patent application number is Japanese Patent Application No. 2013-216032, theanti-gravity mechanism patent application number is Japanese Patent Application Nos.
ongu 2017-002965 and 2018-002441, and the unit style robot configuration patentapplication number is Japanese Patent Application No. 2015-234253.
2.1.4 Related product (Figure 5)
As a related product, we also developed the Universal Gondola Stabilizer (UGS) based on
ALP’s suction pad architecture.
Gondolas are used for traveling up and down high-rise buildings. Since they are suspended
from the rooftop by cables only, they are subject to shaking by work reaction force, strong
winds, and the like. Thus gondola work crews must contend with the risk of falling andreduced work efficiency caused by gondola shaking.
In remedy these problems, we developed the UGS system, which constantly adsorbs to thewalls of buildings even while ascending and descending, thereby eliminating gondola
shaking.
The UGS system can easily be mounted to and removed from existing gondolas.
2.2

Moving mechanism of ALP adsorption unit3

2.2.1 Left/right movement mechanism of adsorption unit (Figure 6)
The adsorption unit can move left and right to the desired position inside the suction unit
frame. This movement is made possible by two belts provided above and below the suction
unit and fixed to the adsorption unit, and that can be rotated clockwise andcounterclockwise. The rotation of the belts is controlled by a microprocessor on theadsorption unit that controls a system consisting of belts, pulleys, a servomotor, and an
encoder. The microprocessor on the adsorption unit can be communicated with andcontrolled by ALP’s main microprocessor via the CAN interface.
2.2.2 U/down movement mechanism (Figure 6)
Similarly to the left/right movement mechanism described above, the adsorption unit canalso move up and down to the desired position inside the suction unit frame. Movement ismade possible by rotating clockwise or counterclockwise two belts provided on the left andright sides of the suction unit and fixed to the adsorption unit. The rotation of these belts is
controlled by a microprocessor mounted on the adsorption unit and a mechanism that
includes belts, pulleys, a servomotor, and an encoder. The microprocessor on theadsorption unit is communicated with and controlled by ALP’s main microprocessor via
the CAN interface.
2.2.3 Adsorption unit extension/retraction mechanism (Figure 7)
The adsorption unit can also extend toward and retract from the wall surface. This is doneby extending or retracting the suction seal part of the adsorption unit facing the wall by
turning the servomotor ball screw provided on the adsorption unit. Extension is done byextending the suction seal at the extremity of the adsorption unit that faces the wall in the
direction of the target wall surface. When the suction seal contacts the surface, the suction
seal part stops extending and vacuum suction is performed, making the suction seal part
adhere to the wall surface and fixing the adsorption unit to the target wall surface.
2.2.4 ALP climbing motion on wall surface (Figures 6, 7)
As described above, the adsorption unit can move right/left and up/down in the suction unit.
Further, the suction seal part of the adsorption unit can perpendicularly extend toward andretract from the wall surface.
If the suction unit moves downward while the suction seal part is adsorbed to the wall
surface, the suction unit moves upward on the wall surface.
If the adsorption unit moves downward while the suction seal part is not adsorbed to thewall surface, the suction unit does not move and only the adsorption unit moves downward.
In this way, depending on whether or not the suction seal part is adsorbed to the wall
surface, the suction unit will either ascend the wall surface or not.
As described above, as the various suction units repeatedly perform this operation, ALP isable to climb up the wall while securely clinging to its surface. The operation of the suction
units and the adsorption units is made possible by controlling the suction units and the main
microprocessor.
4

2.3
ALP control method, power supply and safety
2.3.1 ALP remote control (Figures 8, 9)
ALP can be operated by remote control via wireless or via a composite cable combining an
optical fiber cable, a power cable, and a safety suspension cable. The main microprocessor
controlling the entire ALP system can also be controlled from a remote PC via wireless,and it is also possible to connect a PC mounted on ALP to a remote PC by optical fiber
cable for communication and control purposes.
In the case of control via optical fiber cable, very fast communication speed is achieved,
allowing not only control of ALP, but also control of the measurement tools and acquisitionby the remote PC of the enormous amounts of measurement data they generate, including
photographic data.
Use of a long-distance composite cable allows not only the acquisition of huge amounts ofmeasurement data, but also power supply and even safe suspension.
2.3.2 ALP power consumption (Figure 9)
ALP uses 100 VAC and consumes less than 3 A.
In the case of wireless operation, a separate power supply cable must be provided, and thecase of operation by composite cable, power supply is provided by the power supply cable
incorporated in the composite cable.
2.3.3 Safety (Figure 10)
ALP normally climbs walls on its own.
ALP is designed to use commercial power supplies.
If the commercial power supply used by ALP is momentarily interrupted, the UPS (Figure9) will supply electric power to ALP automatically until commercial power supply
resumes.
While ALP is designed to climb walls on its own, if it were to drop due to failure, ALP’s
fall will be arrested by safety suspension cables or the composite cable, which incorporates
a safety suspension cable (Figure 10).
3. Measurement mechanism
The measuring mechanism can move the measuring devices by 1 m in the horizontal direction
and stably press them against the wall surface for precise measurement.
As shown in Figure 11, the measurement mechanism is equipped with a high-resolution
camera, a hammering device, and an electromagnetic wave radar. The high-resolution camera
has a resolution of 51.5 million pixels that allows it to discern cracks just 0.2 mm in width. The
hammering device uses a solenoid magnet to strike the wall surface, picking up the reflectedsound with a microphone to detect scaling of concrete. The electromagnetic wave radar canestimate the position of the rebar and the depth of cover concrete. Moreover, it allows

5estimation of the chloride content in the cover concrete by using the attenuation rate of theelectromagnetic waves reflected from rebar.
3.1 Image capture system using high-resolution camera
Close visual inspection is an inspection action that consists in approaching the object to
be inspected and visually checking for deterioration such as cracks and spalling, recording
observations in the form of sketches. In the case of ALP, a robot approaches the surveyed
object on behalf of a person, so a camera must be mounted to perform close visualinspection. For cracks, auxiliary instruments such as a crack scale are usually required formeasurement and recording of the width and length of cracks.
For the purpose of supporting close visual inspection by ALP, experiments using imaging
with a high-resolution digital camera and 3D modeling were carried out. The camera used
was a 51.5 million pixel camera, shown in Figure 12, and a test shooting platform was also
employed, with the aim of capturing cracks with a width as small as 0.2 mm, equivalent to a
4-pixel span. Further, since the 3D modeling is done using a precision photogrammetry
technique, the camera position was moved using a slider mechanism so that to obtain an
overlap ratio between captured images of 60% or more, and a total of seventy pictures
covering a surface area of 1 m2 were taken.
The acquired image data were combined to create a 3D model using Structure from
Motion (SfM) (Snavely et al. 2006,2007) (see Figure 13), and the accuracy of theconstructed 3D model was verified by distance measurement with a TIN mesh (see Figure14).Beforehand, a qualified concrete diagnostician confirmed with a crack scale crack
widths of 0.1 mm and 0.2 mm, and the crack widths were measured at the same locations
on the 3D model. As a result, the measured value was found to be 0.131 mm in the 0.1 mm
crack range, and 0.210 mm in the 0.2 mm crack range. From the above, it was confirmedthat a 3D model allowing detection of cracks of width as small as 0.1 mm can beconstructed by using a high-resolution digital camera from the shooting position of ALP.
3.2 Electromagnetic wave radar
To detect the position of rebar in the concrete and the thickness of the cover concrete, anelectromagnetic wave radar was mounted on ALP. Elsewhere, the authors have reported on
the possibility of determining the chloride ion content of concrete through the use ofelectromagnetic waves (Mizobuchi et al. 2003-2008). Furthermore, studies by Kurumisawa et
al. have shown the possibility of verifying differences in chloride ion content using the rebar
reflected waveform of electromagnetic waves in specimen experiments using the finite
difference time domain method (FDTD). This technology is used to determine chloride ioncontent by non-destructive inspection of structures near the coast and roads wheresnow-melting agents are used. Compared with the conventional method combining coresampling and potentiometric titration, this technology excels both in terms of the amount of6

acquired data and implementation cost.
Given that the electromagnetic wave radar is mounted on ALP, it should be lightweight andradio controllable, and thus an electromagnetic wave radar of the specifications listed in Table
2 was adopted instead of the equipment that was used for determination of chloride ioncontent by electromagnetic wave radar.
3.2.1 Principle of technology for determination of chloride ion content in concrete byelectromagnetic waves (SAE) (Nojima et al. 2011-2016)
When the electromagnetic wave is sinusoidally oscillating at a single angular frequency𝜔,assuming it as a one-dimensional wave equation, equations (1) and (2) are derived using thevector wave equation.
𝛻2𝐸 + 𝑘2𝐸 = 0
2

(1)

2

𝛻 𝐻+𝑘 𝐻 =0
(𝑘 2 = 𝜔2 𝜀𝜇 − 𝑗𝜔𝜇𝜎)(2)where E is the electric field (V/m) and H is the magnetic field (A/m). where E is the electricfield (V/m) and H is the magnetic field (A/m), k is the propagation constant, and σ is the
conductivity (S/m).
Usually, when propagation constant k is a complex number, it can be divided into a realpart and an imaginary part,𝑘 = √𝜔 2 𝜀𝜇 − 𝑗𝜔𝜇𝜎 = 𝛽 − 𝑗𝛼 (𝛼 > 0, 𝛽 > 0) (3)with α and β as follows.
𝜀𝜇

𝜎2

1⁄2

𝛼 = 𝜔√ 2 {√1 + 𝜔2𝜀2 − 1}𝜀𝜇𝜎2

𝛽 = 𝜔√ 2 {√1 + 𝜔2 𝜀2 + 1}

(4)
1⁄2

(5)

When a plane wave with polarization in the x direction propagates in the z direction, thefollowing equation is obtained when expressed using terms α and β.
𝐸 = 𝑥̂𝐸0 exp(−𝑗𝑘𝑧) = 𝑥̂𝐸0 exp(−𝛼𝑧 − 𝑗𝛽𝑧) (6)The real part β of transmission constant k in equation (3) is the phase constant, and theimaginary part α is the attenuation constant, indicating that as the electromagnetic wave
propagates, its
gradually attenuates. In other words, the magnitude of the electric field |𝐸| is expressed by7

the attenuation function shown in equation (7), and as the electromagnetic wave travels in thez direction, it attenuates. Thus, the higher the value of attenuation constant α, the shorter the
propagation distance at which the electromagnetic wave attenuates.
|𝐸| = |𝐸0 | exp(−𝛼𝑧)(7)Chloride attack occurs when chloride ions from airborne salt and snow-melting agents
penetrate concrete and diffuse in the concrete pore solution, speeding up the progress ofcorrosion of rebar in concrete. The phenomenon of the conductivity of sea water being higher
than that of ultra pure water and tap water is due to the fact that strong electrolytes such as
NaCl dissolve in water, forming ions that play the same role as free electrons in metals,
thereby increasing the electrical conductivity of the aqueous solution. The same phenomenon
occurs when chloride ions are present in the pore solution of concrete, causing conductivity σ
to rise and resulting in a higher attenuation constant of electromagnetic waves, per equation(7).
In view of the above, the authors focused on the following in earlier studies.
1) Development of a theory of electromagnetic wave attenuation in the presence of chloride
ions in concrete2) Determination of the degree of influence of the major electromagnetic wave attenuation
factors of "chloride ion content," "electromagnetic wave propagation distance," "moisture
content," and "temperature" by varying these factors in concrete specimens.
3) Development of electromagnetic wave attenuation simulation from the attenuation
characteristics of electromagnetic waves in concrete obtained from electromagnetic wave
attenuation theory and specimen experiments (hereafter, SAE (Simulation of Attenuation
Electromagnetic waves), Equation (8))4) Determination of electromagnetic wave attenuation and chloride ion content in concrete
specimens and actual structures under various conditions with SAE
The amplitude value (Figure 16) as used here indicates the ratio of the reflected energy
from rebar to the input wave, which is similar to electric field magnitude |𝐸| in Equation (7).
Figure 17 shows the method of amplitude value acquisition by electromagnetic wave radar in
SAE.
3.2.2 Study toward assessment of actual structures
With a view to applying SAE to the assessment of actual structures with ALP, thefollowing items were investigated.
[1] Investigation of stability of amplitude value of electromagnetic wave radar[2] Influence of surface roughness on irregular surface reflection of electromagnetic
waves

8

[1] Investigation of stability of amplitude value of electromagnetic wave radar
The amplitude value applied in the present study was confirmed in past research to change
as the temperature rises when the antenna circuit inside the equipment is energized, and tostabilize as the temperature becomes constant. Since the electromagnetic wave radar to be
mounted on ALP differed from the electromagnetic wave radar used in past research, theoptimum stabilization wait time was determined using specimens.
For the chloride ion content of 0 kg/m3 and cover thickness of 50 mm, the measured value
was found to come within ±5% of the value measured at 1 hour from measurement start after
the lapse of 19 minutes. For the chloride ion content of 2.4 kg/m3 and cover thickness of 100
mm, the value came within ±5% after the lapse of 17 minutes. Based on the above results, it
was determined that the measured value can be considered to become stable at about 20
minutes from the start of measurement, since the cover thickness and chloride ion content of
concrete are not known when performing measurement of actual structures. (Figure 18)[2] Investigation of the influence of surface roughness on irregular surface reflection ofelectromagnetic waves
Indoor test specimens with varying surface roughness were fabricated and used to
examine the influence of surface roughness on irregular surface reflection ofelectromagnetic waves. In addition, the concrete surface of an actual structure wasmeasured with a 3D scanner and the obtained data was processed for data analysis.
(Figure 19)
As a result, it was confirmed that when the surface roughness is large, incident waves
are scattered by the concrete surface and the reflection amplitude value from rebars issmaller compared with concrete that has a smoother surface. (Figure 20)
Figure 21 shows the results of chloride ion content estimation carried out on a concrete
structure by the sea. The salinity maps shown on the middle and bottom level in thisfigure indicate differences in estimated values based on the magnitude of correction done
in view of the representative surface roughness of each block indicated by a red frame as
measured by 3D scan. This figure confirms that the amount of penetration by chloride
ions in the two rightmost red frames is small as the result of surface roughness correction,and that the concrete structure is sound. Further, the sea side of the two rightmost red
frames is a ship anchoring location, and as such it is protected from the influence of
airborne salt.
The result of collecting cores from each red frame and calculating the chloride ioncontent by potentiometric titration, and the chloride ion content determined byelectromagnetic wave SAE are shown in Figure 22. This figure confirms that throughappropriate assessment of the influence of surface roughness, values that closely
approximate actual measurement values can be obtained.
9

Since ALP is not equipped with a 3D scan device, the influence of surface roughness
will likely be assessed through the use of precision 3D models obtained from SfM.
3.3 Hammering device
The hammering device mounted on ALP uses a compact solenoid magnet designed
specifically for automatic striking to achieve hammering action, and the resulting reflectedsound is recorded with a condenser microphone and used to detect scaling and other flaws.
Generally, bending vibration theory is applied to the reflected sound produced by scaling
parts. As shown in Figure 23 (Utagawa et al. 2013), bending vibration can be used to detectflaws due to the fact that the thinner a member is, the larger the bending vibration it produces.
Somewhat similarly for the hammering device of ALP, the characteristic of scaling parts that,compared with the initial wave of the impact sound (P1), the reflected sound (P2) is of greater
amplitude, whereas the opposite holds true for the sound parts of concrete, is used to detectscaling. (Figure 24)
3.3.1 Improvement of exploration depth of hammering system
In the conventional type created during the early development period, sufficient
hammering energy was not achieved due to the short stroke of the hammer part, the small
hammering energy, and voltage drop of the connecting cable. Therefore, a new hammering
device improving all these points was created. As a specific countermeasure against thevoltage drop, the diameter of the cable was changed to about twice the previous thickness toincrease the voltage applied to the solenoid and to prevent the voltage drop of theconnecting cable. The old and the improved hammering devices are shown in Figure 25 andFigure 26.
Shape: φ 45 mm ×106 mm (excluding protrusions)Sound recording: Waterproof condenser microphone
Hammer: φ 10 mm cylindrical stainless steel hammer
Hammer stroke of 9 mm, hammering cycle fixed to 2 Hz

3.3.2 Specimen test
To verify the striking element improvement effect, specimens with a simulated void 400
mm wide, 100 mm long, and 100 mm thick were prepared, and the performance comparisonof the conventional type and the improved type was conducted.
The area under each of the numbers lined up from the left side of the specimen shown in
Figure 27 corresponds to the hammering points. The area under [2] has a simulated void 10
mm below the concrete surface that measures 50 mm × 50 mm × 10 mm (width × length ×
thickness), the areas under [4] has a simulated void 30 mm below the concrete surface that10

measures 50 mm × 50 mm × 10 mm, and the area under [2] has a simulated void 50 mm
below the concrete surface that measures 50 mm × 50 mm × 10 mm. The other areas under[1], [3], [5], and [7] are sound areas free of flaws.
The test conditions in this evaluation were the following four cases. (Figures 28, 29)
 Case 1: 9 V driven conventional type
 Case 2: 12 V driven conventional type
 Case 3: 9 V driven improved type
 Case 4: 12 V driven improved type
The test results of each case were as follows.
[Case 1: 9 V driven conventional type]The measurement results show reliable detection of the 10 mm deep void, but
unreliable detection of the 30 mm deep and 50 mm deep cavities.
[Case 2: 12 V driven conventional type]The measurement results show reliable detection of the 10 mm deep void, but
unreliable detection of the 30 mm deep and 50 mm deep cavities. This is presumably due
to the voltage drop of the cable and still insufficient striking force despite the increased
drive voltage.
[Case 3: 9 V driven improved type]The measurement results show reliable detection of the 10 mm deep void, as well as
the possibility of detecting cavities even at depths of 30 mm and 50 mm.
[Case 4: 12 V driven improved type]The measurement results show reliable detection of the 10 mm and 30 mm deep
cavities. The 50 mm deep void too gave a discernible waveform, showing the possibilityof detecting cavities even at this depth.
3.3.2 Performance verification on actual structures
Performance verification of the hammering device was carried out at the N2U-BRIDGE(New Bridge) facility (Figure 30) for bridge inspection technology study and research at
Nagoya University. Performance verification was carried out at the following four places.
[1] Simulated void specimen depth: 30 mm, size: 500 mm × 500 mm[2] Simulated void specimen depth: 80 mm, size: 600 mm × 350 mm[3] Cracked test specimen[4] Former Shibue River―cracked part on back of removed deck slab
The results of the performance verification are shown below.
[1] Simulated void specimen depth: 30 mm, size: 500 mm × 500 mm (Figures 31, 32)
As the result of the performance verification, a clear difference between the hollow
part and the sound part was confirmed. In the hollow part, the peak values of P1 and P2
are both large, clearly indicating a void detection waveform.
11[2] Simulated void specimen depth: 80 mm, size: 600 mm × 350 mm (Figures 33, 34)
As the result of the performance verification, a clear difference between the hollow
part and the sound part was confirmed. Regarding the detected waveform for the void atthe depth of 80 mm, while in some locations the void could not be detected as such, at thecenter of the hollow portion, the waveform clearly shows the characteristics of a void,and clearly differs from the waveform obtained from the sound parts.
[3] Cracked specimen depth: 80 mm, size: 300 mm × 300 mm (Figures 35, 36)
The performance verification showed that the cracked part returned a waveform similar
to that of the sound part, and that the wavelengths of both P1 and P2 both being long
makes it difficult to distinguish between the sound part and the cracked part.
[4] Former Shibue River―cracked part on back of removed deck slab (Figures 37, 38)
The performance verification showed that delamination due to cracks can be detected
quite clearly. While detection is possible also to some extent from P1 and P2, in scaling
parts, P2 is clearly larger than P1. A clear difference could be seen between the
waveforms obtained from sound parts surrounded by cracks or parts without severedelamination, and parts with severe delamination near cracks. Figure 38 shows acolor-coded representation of the numerical values measured at the marked location
showing delamination.
From the above, taking into consideration the results of the specimen test and theperformance verification on actual structures, it was confirmed that the maximum void
detection depth of the hammering device mounted on ALP is 50 mm or less.
4. Experimental results on actual structure
The ALP demonstration experiment was carried out on the side wall of the intake gate concretepier of an arch type dam. Because the ALP is heavy, it was moved from the transport vehicle to
the wall of the target using a lifter. Thereafter, ALP self-propelled along the wall and acquired
accurate data by remote control. The power supplied to ALP was supplied by power cable froma small 0.9 kVA gas-cylinder-type generator. The acquired data was analysed over five hours
to generate a three-dimensional model by SfM. Figure 39 shows the high-resolution
three-dimensional model obtained as the result of fine inspection by ALP, and the dataacquired by the hammering device and electromagnetic wave radar. The data shown in Figure
39, which covers an area of approximately 1 m2, took approximately 2 hours to acquire,
including ALP operation and measuring time.
4.1 Comparison with measurement by humans
12Alongside measurement by ALP, measurement by humans was also conducted at thesame locations, and the respective results were compared. The purpose of this comparison
was to find out whether ALP can produce results comparable to those achieved by humans
at inspection locations that humans are unable to access.
The results are summarized in Figure 40. As seen from Figure 40, in terms of precise 3Dmodel by SfM and the results of the electromagnetic wave survey, there were slightdifferences between the data acquired by ALP and by humans, but only to a limited extent,
and the data produced by ALP was confirmed to be suitable for practical use without any
problem. This is due to the fact that the vacuum suction pads used by ALP provided stable
adsorption even on concrete walls with unevenness. On the other hand, the hammering
device was unable to achieve stable hammering against unevenness on walls. Goingforward, it will be necessary to improve ALP's pressing mechanism or the solenoid driven
striking element.
4.2 Utilization of acquired data as detailed inspection data
ALP is capable of close-up overlapping photographic capture with a high-resolution
camera in high places with good stability. The acquired images can then be used to create aprecise 3D model with SfM. Figures 41 and 42 show 3D models created from the image
data acquired by ALP. ALP makes it possible to simultaneously obtain non-destructive
inspection results from electromagnetic wave radar and hammering device, allowing
judgment of factors and progress of deterioration even without engineers near the target
structure. Further, ALP surveys are non-destructive inspection, allowing simultaneous
inspection of the same location by two or more times. Moreover, for local inspection
records using crack scales or the like, ALP's ability to create 3D models with local
coordinates makes it possible to precisely judge the progress of deterioration. Based on theabove, ALP can be said to produce valuable data that can be used to verify the deteriorationof concrete with considerable precision.
On the other hand, ALP moves slowly compared with UAVs. Considering the time andcosts involved in inspection, it would be desirable to first confirm deterioration locations
from general imaging results obtained by UAV and then carry out detailed inspection by
ALP in a specific range.
5. Conclusions
The results obtained by research and development of the ALP infrastructure diagnostic
robot system capable of operating under difficult conditions such as high places are
summarized below.
(1) A system to acquire data required for detailed inspection of concrete without directaccess by people was created and tested.
(2) Vacuum suction pads that can stably adsorb onto uneven concrete walls were created.
13

(3) To further improve the judgment accuracy of engineers regarding flaws in concrete,multiple types of non-destructive inspection and precise 3D modeling allow detailed
inspection.
On the other hand, as the whole ALP system has considerable weight and its moving speed
is slow, it can take a lot of time to access target locations, and in some cases, obstacles may
well prevent access altogether. Going forward, we expect various innovations and beneficial
effects from synergies produced by combining the vacuum suction pads developed this timeand other technologies.
Finally, This ALP practical development project was obtained by the subsidy of New Energy
and Industrial Technology Development Organization (NEDO).

14

References----------------------------------------------------------------------------------------・Nojima, J., Mizobuchi, T., (2018). “Research and development of infrastructure diagnostic robot
system (ALP) by concrete wall moving mechanism using vacuum suction pad.” Syner Crete’18,
13, 135-146.
・Torii, K., Kawamura, M. and Ueda, S., (1994) “Over several 80 years deterioration of concrete
situation”, Cement Science and concrete Technology, No.48, pp.518-523. (in Japanese)
・R.J.Torrent, (1992), “A two-chamber vacuum cell for measuring the coefficient of permeability to
air of the concrete cover on site”, Material and Structures, 25, pp.358-365.
・Snavely, N., Seitz,S.M., and Szeliski,R., (2006), “Photo Tourism : Exploring image collections in
3D”, ACM Transactions on Graphics (Proceedings of SIGGRAPH 2006).
・Snavely, N., Seitz,S.M., and Szeliski,R., (2007) , “Modeling the World from Internet Photo
Collections”. Int’l. J. Computer Vision.
・Mizobuchi, T., Arai, J. and Suda, K., (2003), “Experimental Study on Measurement of Chloride
Content using Electromagnetic Wave in Reinforced Concrete Structures”, Structural Faults andRepair-2003, 241-248.
・Arai, J., Mizobuchi, T. and Suda, K., (2003), “Study on Measurement of Chloride Content using
Electromagnetic Wave in Reinforced Concrete Structures”, Non-Destructive Testing in Civil
Engineering.
・Mizobuchi, T., Suda, K., Hayashi, D. and Yokozeki, K., (2005)，“Experimental Study on
Applicability of Measuring Method of Chloride Content using Electromagnetic Wave in
Reinforced Concrete Structures”，The 11th International Conference on Fracture.
・Hayashi, D., Mizobuchi, T., Suda, K. and Yokozeki, K., (2005)，“Experimental Study on
Applicability of Measuring Method of Chloride Content using Electromagnetic Wave in
Reinforced Concrete Structures”, The Third US-Japan Symposium on Advancing Applications
and Capabilities in NDE.
・Mizobuchi, T., Yokozeki, K. and Ashizawa, R., (2006), “Applicability of Estimation of Chloride
Content using Electromagnetic Wave in Reinforced Concrete Structures”, Structural Faults andRepair-2006
・Mizobuchi, T., Yokozeki, K., Watanabe, K. , Hiraishi, M. and Ashizawa, R., (2007),“Monitoring
System of Chloride Content in Cover concrete using Electromagnetic Wave and Impedance
Method”, Proceedings of the 6th International Conference on Fracture Mechanics of Concrete
and Concrete Structures,1865-1876.
・Mizobuchi, T., Yokozeki, K., Watanabe, K. and Ashizawa, R., (2007), ” Applicability of Estimationof Chloride Content using Electromagnetic Wave in coastal Reinforced Concrete Structures” ,Proceeding of the 5th International Conference on Concrete under Severe Conditions ofEnvironment and Loading, 499-512.
・Mizobuchi, T., Yokozeki, K., Watanabe, K. and Ashizawa, R., (2008), “Applicability of Estimationof Chloride Content in Cover concrete using Electromagnetic Wave and Impedance Method”,
on Site Assessment of Concrete, Masonry and Timber.

15

・Mizobuchi, T., Yokozeki, K., Watanabe, K. and Ashizawa, R., (2008), “Applicability of Estimation
for Distribution of Chloride Content in Cover concrete using Electromagnetic Wave”, Structural
Faults and Repair-2008.
・Nojima, J. and Mizobuchi, T., (2011), “Applicability of Electromagnetic Wave Method for
Estimation of Chloride Content of Coastal Structures”, Concrete Solutioms-2011.
・Nojima, J. and Mizobuchi, T., (2011), “Application of an Electromagnetic Method for Evaluation of
Chlorine Distribution in Concrete”, NDTMS-2011.
・Mizobuchi, T. and Nojima, J., (2011), “Applicability of Estimation of Chloride Content in Coverconcrete of Coastal Reinforced Concrete Structures using Electromagnetic Wave”,
NDTMS-2011.
・Nojima, J., Ikeda, H.,Mizobuchi, T., (2012) “Possibility of Estimation of Chloride Content inConcrete to Existing Structures Using Electromagnetic Wave”, Structural Faults andRepair-2012.
・Nojima, J., Ikeda, H., Uchida, M and Mizobuchi, T., (2013). ”Theoretical And Experimental StudyOn Estimation Of Chloride Content In Concrete Using Electromagnetic Wave”, SPIE Smart
Structures NDE 2013
・Nojima, J., Uchida, M. and Mizobuchi, T., (2015). “Study on possibility of estimation of chloride
content in coastal reinforced concrete structures using electromagnetic waves.” ICCRRR 2015.
・Nojima, J., Uchida, M., Arai, J. and Mizobuchi, T., (2015). “A Study on Estimation Method of
Chloride Content in Concrete Using Electromagnetic Waveform.” Proceedings of the Japan
Concrete Institute, Vol.37, No.1, 1675-1680. (in Japanese)・Nojima, J., (2015). “Study on Accuracy Improvement and Practical Application for Estimation of
Chloride Content in Concrete Structures Using Electromagnetic Wave Method.” PhD Thesis,
Hosei University. (in Japanese)・Nojima, J., Uchida, M., Arai, J and Mizobuchi, T., (2016). “Fundamental study on improvement of
estimation accuracy of content of chloride ions in concrete using electromagnetic waves
method.” Journal of Japan Society of Civil Engineers E-2, vol.72, No.2, 109-125. (in Japanese)
・Utagawa, N. and Misaki, N., (2013) “Diagnosis of tunnel lining concrete”, Acoustical Society ofJapan, vol.69, No.3, 127-132. (in Japanese)

16Figure & Table----------------------------------------------------------------------------------------Table 1 Wall-climbing robot
ALP(Figure 1)
Developer
Stella Rtec
URL
http://www.stellartec.co.jp/

Vortex VRAMICM Climber

Vortex HC LLC
https://www.cnet.
com/news/wall-cl
imbing-robot-suc
ks-it-up

ICM
http://www.icm.cc/Adsorption
methodAdsorption
wall
StatusVacuum pump

BlowerBlowerIrregular surface<5-7 mmCommercialized

Irregular surface<2-4 mmCommercialized

Irregular surface<2-4 mmCommercialized

PayloadClimbing
speed

Heavy
Slow

Extremely light
Fast

Light
Fast

Fig. 1 ALP

suction unit

adsorption unit

suction unit

Fig. 2 ALP(suction side)

17

Wall
Climbing
Robot
SRI
https://www.sri.com/sites/default/fi
les/brochures/sri_wallclimbingrobo
ts.pdfElectrostatic

Mecho-Gecko

Irregular surface

Irregular surface

Academic
purposeExtremely light
Extremely slow

Academic
purposeExtremely light
Extremely slow

UCLA
http://www.ijmer.
com/papers/Vol2_Issue6/CP26434
74351.pdf
Van der Waals

slide stopper
friction material

adsorption seal

adsorption unit
Fig.3 Adsorption unit
Steel painting
Efflorescence

Unevenness of manholes

Surface quality has deteriorated

Unevenness of joint

Fig.4 Adsorption target of vacuum suction pads

18

Fig.5 Universal Gondola Stabilizer

Anti-rotation

Fig.6 Suction/travel mechanism (1)shortenextendFig.7 Suction/travel mechanism (2)19cameraControl PC

main microprocesser

complexcable

Fig.8 ALP remote control

control
PC

UPS

complex cable

Fig.9 ALP power consumption

20

safetywinch

Fig.10 Safety

Hight-defintion camera

Hammering device

Fig. 11 ALP mounted measuring instruments

21

Hammering device

Shooting camera

PENTAX 645Z

Using lens

PENTAX 645Z-35mm

Shooting distance

48.4cmAngle of view (a cut)

Approximately 40×30cm

Total pixels

8156×6192

ISO

200(Default)

Data capacity

JPG:1.94GB/70 pictures

Fig. 12 High-resolution digital camera

22

Calculated value on analysis soft ware
Measured value: 0.102mm

Measured value: 0.319mm
Measured value: 0.272mm

Measured value: 0.115mm
Measured value: 0.231mm

Measured value: 0.177mm

Measured value: 0.393mm

Fig. 13 Concrete wall surface (3D model)

Wa: width of crack

Wc=0.089mm
Wb=0.052mm
Wa=0.039mm

Fig. 14 Concrete wall (3D model + TIN mesh)23

Table 2 Electromagnetic wave radar parameters
item

content

Frequency

1GHz

Survey mode

Time measurement,
distance measurement

Thickness of rebar
detection

5～300mmCover resolution
Horizontal
direction
resolution
weightRange shallow：Approximately 1mm
deep ：Approximately 2mm
Depth under 75mm ：over 75mm
Depth over 75mm ：depth over
Approximately 1kg（including battery）

μ𝑟 μ0 𝑧|𝐸w| = ζ ∙ |𝐸w0 | ∙ exp [−{ξt ∙ κ𝑤 ∙ 𝜎c+𝑎𝑛𝑖𝑜𝑛 } ∙ √∙ ]ε𝑟 ε0 2(8)

where|𝐸w| ：Output amplitude value|𝐸w0 |：Input amplitude valueζ：Synthetic factor of penetration and scatteringξt：Conductivity correction factor from concrete temperature

κw ：Conductivity correction factor from concrete relative humidity
𝜎c+𝑎𝑛𝑖𝑜𝑛 ：Concrete conductivity considering chloride ion content (S/m)μ𝑟：Relative permeability of concrete ( = 1)μ0：Permeability in the air (H/m：4π×10-7)ε𝑟：Relative permeability of concrete

ε0：permittivity in the air (F/m：8.85418782×10-12)

Fig. 15 Basic equation 𝑧of SAE(Simulation of Attenuation Electromagnetic waves)：損失媒質中の伝搬距離(m)
図-17

SAE (Simulation of Attenuation Electromagnetic waves) 基本方程式

24

Amplitude
value

Fig. 16 Amplitude value of electromagnetic wave radar

-

-

Cl

Cl
-

Cl
The reflected energy ofthe electromagnetic
wave becomes smaller
due to Cl -Re-barFig. 17 Method of amplitude value acquisition with SAE

25000

Amplitude value

20000
50mm
70mm

15000

100mm

10000

5000

0

0

10

20

30

40

50

60

Time（minute）
Fig. 18 Comparison of variations in power supply measurement

25

Case ②

Case ①Case ③

Surface condition of specimens

Results of measurement by 3D scannerFig. 19 Concrete surface roughness and 3D scanning measurement results
650

Amplitude value A

600
550
500

A = -34.18R + 618.5
R = 0.995

450
400
350
300
0.0

0.5

1.0

1.5

2.0

2.5

3.0

Average roughnessAverage roughness
R
Fig. 20 Results of indoor test (surface roughness)

26

Concrete slab

Before surface roughness correction

kg/m3
After surface roughness correction

kg/m3
Fig. 21 Comparison of chloride ion content maps before and after surface roughness correction

Estimated chloride ion（kg/m3）4

32
Before surface
表面粗度補正roughness前
correction

1

After
surface
表面粗度補正roughness後
correction

0
0

1

2

3

4

Chloride ion in concrete（kg/m3）
Fig. 22 Difference between values estimated by surface roughness correction and measured
values27

Bending vibration

Thin member: easy to bending vibration

Thick member: hard to bending vibration

Fig.23 Bending vibration theory

P1
P1

P2

P2

Sound part

Void partFig. 24 Reflected sound waveform of ALP hammering device (X axis: Time (μs), Y axis:
Recorded sound (mV))

28

Height=
10.6cm

Improved

Old

Height=9.1cm

Old

Improved

Fig. 25 Comparison of old and improved hammering devices

Fig. 26 Overview of improved striking element

①

②

③

④

Fig. 27 Test specimen for hammering device evaluation

29

⑤

⑥

⑦

P1
P1

P2

P2

Sound partSound part

P2

P1

P2

P1

10 mm deep void

10 mm deep void

P1
P1

P2

30 mm deep void

30 mm deep void

P1
P1

P2

P2

P2

50 mm deep void

50 mm deep void

Fig. 28 Reflected sound waveform of case 1 (left) and case 4 (right) (X axis: Time (μs), Y axis:
Recorded sound (mV))

30

500
9V old type450

12V old type
9V improved type400

12V improved type

P2 (mV)

350

300
Hollow part

250
200
150

100

Sound part50
0
0

50 100 150 200 250 300 350 400 450 500
P1 (mv)Fig. 29 Relationship between measurement results and void judgment

Fig. 30 N2U-BRIDGE (New Bridge) facility

31

Fig. 31 Measurement situation (specimen [1])Hollow partSound partFig. 32 Verification results of sound part and hollow part (specimen [1])Fig. 33 Measurement situation (specimen [2])Hollow partSound partFig. 34 Verification results of sound part and hollow part (specimen [2])32Fig. 35 Measurement situation (specimen [3])Sound part

Cracked partFig. 36 Verification results of sound part and cracked part (specimen [3])33Fig. 37 Measurement situation (specimen [4])数値の単位は（mV）
1‘

Delamination due to cracks

1“

16

-43

1

2

2’

3

4

2”

5

6

7

91

473

473

473

168

219

472

204

38

3’

3”

87

472

4‘

4”

150

473

5’

5”

148

217

150 green,
以下：緑151-250:
151～250：黄緑
250over
以上：赤Under 150:
Yellow green,
250: redFig. 38 Numerical values indicating state of delamination

34

Fig. 39 ALP mounted measuring instruments35

【ALP】【humans】

ALP is easy for shadows to appear

ALP has a slight difference in distance

ALP varies greatly

Fig. 40 Comparison of data acquired by ALP and data acquired by humans

36

Fig. 41 3D model created from images obtained by close inspection by ALP (approx. 30 minutes)

Fig. 42 Expanded view of 3D model from bird's-eye perspective

37