Real-Time Tunnel Deformation Monitoring Technology Based on Laser and Machine Vision

applied sciences

Article Real-Time Tunnel Deformation Monitoring Technology Based on Laser and Machine Vision

Zurong Qiu * , Haopeng Li , Wenchuan Hu, Chenglin Wang, Jiachen Liu and Qianhui Sun

State Key Laboratory of Precision Measuring Technology and Instruments, School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China; [email protected] (H.L.); [email protected] (W.H.); [email protected] (C.W.); [email protected] (J.L.); [email protected] (Q.S.) * Correspondence: [email protected]

Received: 23 October 2018; Accepted: 7 December 2018; Published: 11 December 2018

Featured Application: The proposed tunnel deformation monitoring system was designed for tunnel during construction in dusty and dark environment with low visibility, and it also can be applied for the tunnel that has already been built.

Abstract: Structural health monitoring is a topic of great concern in the world, and tunnel deformation monitoring is one of the important tasks. With the rapid developments in tunnel traffic infrastructure construction, engineers need a portable and real-time system to obtain the tunnel deformation during construction. This paper reports a novel method based on laser and machine vision to automatically measure tunnel deformation of multiple interest points in real time and effectively compensate for the environment vibration, and moreover it can overcome the influence of a dusty and dark tunnel environment in low visibility. An automatic and wireless real-time tunnel deformation monitoring system, which is based on laser and machine vision and can give early warnings for tunnel collapse accidents, is proposed. The proposed system uses a fixed laser beam as a monitoring reference. The image acquisition modules mounted on the measured points receive the laser spots and measure the tunnel accumulative deformation and instantaneous deformation velocity. Compensation methods are proposed to reduce measurement errors caused by laser beam feasibility, temperature, air refraction index, and wireless antenna attitude. The feasibility of the system is verified through tunnel tests. The accuracy of the detection system is better than 0.12 mm, the repeatability is less than 0.11 mm, and the minimum resolution is 10 µm; therefore, the proposed system is very suitable for real-time and automatic detection of tunnel deformation in low visibility during construction.

Keywords: structural health monitoring; real-time monitoring; tunnel deformation measurement; machine vision; laser beam; wireless; low visibility

1. Introduction Structural health monitoring plays an indispensable role in diagnosing structure safety around the world [1], and tunnel deformation monitoring is one of the important tasks [2]. With the continuous expansion of the scale of tunnel trafﬁc infrastructure construction, tunnel collapse accidents have occurred frequently in recent years [3]; this indicates that the traditional methods for monitoring tunnel deformation are unable to meet the new safety monitoring requirements for large tunnel trafﬁc infrastructure [4], when real time becomes an important issue. In tunnel construction, the tunnel excavation surface is composed of tunnel face, side face, and roof face. Numerous workers are present on the excavation surface with dusty and dark environment in low visibility [5], and the tunnel

Appl. Sci. 2018, 8, 2579; doi:10.3390/app8122579 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2579 2 of 23 deformation is at a maximum; thus, monitoring the deformation of this fracture surface in real time is important. The current tunnel deformation monitoring technology, including non-machine vision measurement methods and machine vision measurement methods [6], however, is not effective enough. Most of the existing equipment is manually operated and susceptible to a dusty and dark environment in low visibility, and cannot meet real-time automatic monitoring and early warning requirements or provide timely warnings about dangerous deformation [7]. Non-machine vision measurement methods are widely used in tunnel deformation monitoring; however, they are susceptible to environmental and human factors, and take a long time. Tape extensometers and convergence gauge are unstable and operated manually; in addition, they are strongly influenced by environmental and human factors [8]. The leveling method mainly addresses the tunnel settlement value [9] and requires many monitoring points to be set; thus, the error accumulates with the increase in the tunnel length. The indoor global positioning system measurement method requires many reference signal transmitters; however, it is difficult to ensure the stability of the reference signal transmitters position. The traditional total station measurement method has a low degree of automation, exhibits strong subjectivity, and requires a fixed station; these features affect the tunnel construction period [10]. The new-generation total station measurement method allows stations to be freely monitored, compared with the traditional method [11,12], but it cannot be used to monitor all kinds of tunnels because of its high cost [13]. The fiber Bragg grating measurement method [14] can be used to monitor tunnel deformation constantly over a long period of time, but it is expensive, and the relevant equipment is easily damaged. Moreover, the equipment needs to be embedded into the structure or adhered onto the structure surface during monitoring, which results in a higher demand on the environment. A deformation monitoring system based on ultrasonic sensors [15] is automatic and in real time; however, it is only suitable for small- to middle-size tunnels, and it cannot be used to realize early warnings of dangerous deformation during the construction. Terrestrial laser scanning [16] achieves tunnel deformation monitoring; however, it takes a long time to obtain and process point cloud data, and cannot realize real-time monitoring. A machine vision measurement method for tunnel deformation develops a lot in the last few years [17]; however, existing machine vision measurement methods are not suitable for real-time tunnel deformation monitoring during construction with dusty and dark environments. A three-dimensional reconstruction method based on machine vision [18] needs to take lots of photos and the data processing algorithms are complex; this takes a long time and cannot meet real-time automatic monitoring and early warning requirements. A digital close-range photogrammetric method [19] is susceptible to tunnel dust and light intensity, heavy dust and a dark environment during construction reduces photo quality and detection accuracy. A photogrammetric method for tunnel detection based on a CCD camera [20] has a higher requirement for brightness and lighting equipment; this requires external power supply; when using batteries, the equipment is heavy and not easy to install; when using an electric generator, extra great vibration is produced and this affects the result. The infrared photogrammetric method [21] reduces dust impact at the price of decreasing photo brightness; as a result, the detection accuracy is low and cannot be used for real-time monitoring. Photogrammetric techniques for monitoring tunnel deformation [22] are only suitable for tunnels without dust after excavation, and it is also susceptible to dusty and dark environments. A tunnel detection method combining photogrammetry and laser scanning [23] is susceptible to the set position of the equipment; when the equipment is set deviating from the tunnel axis, the detection error increases obviously [24]; this indicates that the method is not suitable for a tunnel during construction, since the set position has a conflict with tunnel construction. This paper presents a real-time and automatic tunnel deformation monitoring system to realize real-time monitoring and early warning for dangerous tunnel deformation. Considering the dusty and dark tunneling environment, a fixed laser beam is used to transmit the monitoring reference from a stable point to a measured point through the dust and darkness in low visibility, and a shielded image acquisition module mounted on the measured points is used to acquire the laser spot image regardless Appl.Appl. Sci. Sci.2018 2018, 8, ,8 2579, x FOR PEER REVIEW 33 of of 23 23

the measurement result is affected by laser beam feasibility, temperature, air refraction index, and of the dust and darkness. However, because of the adverse tunnel environment, the measurement wireless antenna attitude. These factors are carefully examined in this study, and compensation result is affected by laser beam feasibility, temperature, air refraction index, and wireless antenna methods are proposed to reduce the measurement errors caused by them. The results show that the attitude. These factors are carefully examined in this study, and compensation methods are proposed proposed system not only achieves good detection accuracy, repeatability and resolution, but also to reduce the measurement errors caused by them. The results show that the proposed system not compensates for the shortcomings of the above methods, such as the low degree of automation, the only achieves good detection accuracy, repeatability and resolution, but also compensates for the lack of real-time measurement function and overcoming the influence of dusty and dark shortcomings of the above methods, such as the low degree of automation, the lack of real-time environments in low visibility. measurement function and overcoming the inﬂuence of dusty and dark environments in low visibility.

2.2. Methodology Methodology TheThe tunnel tunnel collapse collapse process process is is the the deformation deformation of of the the construction constructionfracture fracture surface surfacein inthe the tunnel,tunnel, whichwhich takestakes a fixed fixed point point of of the the earth earth as asa referenc a reference.e. At present, At present, the new the Austrian new Austrian tunneling tunneling method method[25], which [25], includes which includes designing designing an initial an support initial support and secondary and secondary lining to lining the construction to the construction fracture fracturesurface surface[26], is [mostly26], is mostlyadopted adopted in tunnel in tunnelconstruc construction.tion. The secondary The secondary lining liningis constructed is constructed on the onbasis the basisof the of initial the initial support support when when the thedeformation deformation of ofthe the initial initial support support becomes becomes stable [[27].27]. Therefore,Therefore, the the deformation deformation of of the the secondary secondary lining lining surface surface can can be be ignored; ignored; however, however, the the deformation deformation ofof the the excavation excavation surface surface and and the the edge edge part part of of the the initial initial support support is is significant. significant. The The proposed proposed system system takestakes a a fixed fixed point point of of the the earth earth as as monitoring monitoring reference reference and and measures measures the the deformation deformation of of excavation excavation surface.surface. Given Given the the good good directivity directivity and and energy energy concentration concentration of of a a laser, laser, as as well well as as the the dusty dusty and and dark dark tunnelingtunneling environmentenvironment inin lowlow visibility,visibility, a a laser laser beam beam is is used used to to transmit transmit the the monitoring monitoring reference reference fromfrom the the fixed fixed point point to theto the measured measured point point through thro theugh dust the anddust darkness, and darkness, thereby thereby forming forming an optical an projectionoptical projection of the fixed of the point fixed on point the intersecting on the intersec surfaceting nearsurface the near measured the measured point, which point, is which the optical is the equivalentoptical equivalent reference, reference, to monitor to themonitor tunnel the deformation tunnel deformation in the system. in the A system. sketch mapA sketch of the map system of the is shownsystem in is Figure shown1. in Figure 1.

Figure 1. System principle. Figure 1. System principle. A laser transmitter module, whose laser beam points to the intersecting surface near the measured pointsA and laser forms transmitter an optical module, projection whose of laser the fixed beam point points as to a laserthe intersecting spot, is firmly surface fixed near on thethe earth.measured To obtain points the and position forms relationshipan optical projection between of the the measured fixed point point as anda laser the spot, fixed is point, firmly an fixed image on acquisitionthe earth. To module obtain mounted the position on the relationship measured point between and locatedthe measured perpendicularly point and to the the fixed laser point, beam isan usedimage to acquisition acquire the lasermodule spot mounted image of on the the laser measured beam. This point module and located monitors perpendicularly the laser spot positionto the laser in thebeam image is used coordinate to acquire system the withlaser machine spot image vision of th technology.e laser beam. The This image module acquisition monitors mode the is shownlaser spot in Figureposition2. Thein the image image sensor coordinate acquires system the laser with spot machine image through vision technology. a laser receiving The image screen acquisition placed in frontmode of is the shown sensor. in Figure 2. The image sensor acquires the laser spot image through a laser receiving screen placed in front of the sensor. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 23 Appl. Sci. 2018, 8, 2579 4 of 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 23

Figure 2. Image acquisition module. Figure 2. Image acquisition module.module. The image acquisition module is fixed on the measured points. When the excavation surface sinksThe during imageimage acquisitionacquisitionthe deformation, modulemodule the isis ﬁxedfixedimage onon acquisition thethe measuredmeasured module points.points. will WhenWhen sink thethe along excavationexcavation with it, surfacesurface thereby sinkschanging duringduring the the deformation,laser deformation, spot position the the image inimage acquisitionthe imageacquisition modulecoordinate module will system. sink will along sinkThe with alongchange it, therebywith of theit, changing laserthereby spot thechangingposition laser spot the in the positionlaser image spot in coordinate theposition image insystem coordinate the imagereflects system. c oordinatethe deformation The changesystem. of of theThe the excavation change laser spot of surface, positionthe laser including in spot the imagepositionaccumulative coordinate in the image deformation system coordinate reﬂects and system theinstantaneous deformation reflects thdeformation ofe deformation the excavation velocity of the surface, [28]. excavation including surface, accumulative including deformationaccumulativeThe mathematical and deformation instantaneous model and instantaneous deformation of this principle velocity deformation is shown [28]. in velocity Figure [28].3. The geometric center of the laser is Thethe coordinate mathematical origin, modelmodel the ofof tunnel thisthis principleprinciple driving isis direction shown in is Figure the z3 -axis,3.. TheThe geometricthegeometric horizontal centercenter direction ofof thethe laserlaser is the isis thexthe-axis, coordinatecoordinate and the origin, verticalorigin, the theaxis tunnel tunnel is the driving drivingy-axis. direction Therefore,direction is the isa zthree-dimensionalthe-axis, z-axis, the horizontal the horizontal world direction coordinatedirection is the xis -axis,system the andx-axis,o-xyz the and is vertical established. the vertical axis is The theaxis ylaser -axis.is the beam Therefore,y-axis. points Therefore, ato three-dimensional the laser a three-dimensional receiving world screen coordinate andworld forms coordinate system a laser ospotsystem-xyz Pis on established.o-xyzthe isscreen. established. The laser The beam laser points beam to points the laser to receivingthe laser receiving screen and screen forms and a laser forms spot a Plaseron thespot screen. P on the screen.

FigureFigure 3. Mathematical3. Mathematical model. model.

Considering the horizontal and verticalFigure 3.position Mathematical information, model. the coordinates of the spot P in the Considering the horizontal and vertical position information, the coordinates of the spot P in world coordinate system o-xyz are (x, y). To examine the position information of laser spot P in the theConsidering world coordinate the horizontal system o-xyz and arevertical , position. To examine information, the position the coordinates information of of the laser spot spot P inP in image coordinate system, an image coordinate system on the laser receiving screen is established, as thethe world image coordinate coordinate system system, o-xyz arean image,. Tocoordina examinete thesystem position on informationthe laser receiving of laser spot screen P in is shown in Figure3. The o0x0 axis is parallel to the ox axis, and the o0y0 axis is parallel to the oy axis; the theestablished, image coordinate as shown system, in Figure an 3.image The ′′coordina axis teis systemparallel0 onto thethe laser axis, receiving and the screen′′ axis is is coordinates of laser spot P in the image coordinate system are (x , y0). The coordinate transformation established,parallel to asthe shown axis; in theFigure coordinates 3. The ′′of laser axis spotis parallel P in the to image the coordinate axis, and system the ′′ are axis ,′is . of spot P in the two coordinate systems is shown in Equation (1). The constants c and c are coordinate parallelThe coordinate to the transformationaxis; the coordinates of spot ofP inlaser the spottwo coordinateP in the image systems coordinate is shown0 system in1 Equation are (1).,′ The. conversion parameters, which are related to the relative position of the two coordinate systems: Theconstants coordinate transformationand are coordinate of spot conversionP in the two parameters, coordinate whichsystems are is relatedshown toin theEquation relative (1). position The constantsof the two coordinateand are systems: coordinate conversion( 0 parameters, which are related to the relative position x = x + c0 of the two coordinate systems: 0 =+. (1) y = y + c1 . (1) =+ =+ . (1) InIn the the three-dimensional three-dimensional world world coordinate coordinate =+ system system o -o-xyzxyz,, thethe laserlaser beambeam equationequation is shown shown as as Equation (2), and the equation of the intersecting surface near the measured point is shown as EquationIn the three-dimensional(2), and the equation world of coordinatethe intersecti systemng surface o-xyz, thenear laser the beammeasured equation point is isshown shown as as Equation (3), where A, B, C, and d are constant parameters and are related to the laser position: EquationEquation (2), (3), and where the Aequation, B, C, and of d the are intersecticonstant parametersng surface nearand arethe related measured to the point laser isposition: shown as Equation (3), where A, B, C, and d are constant parameters and are related to the laser position: A ×x×=×=×= B × y = C × z,, (2)(2) ×=×=×=. , (2)(3) z = d. (3) According to Equations (1)–(3), the coordinates=. of the laser spot in the image coordinate system(3) o′-xAccording′y′ can be obtainedto Equations as shown (1)–(3), in the Equation coordinates (4): of the laser spot in the image coordinate system o′-x′y′ can be obtained as shown in Equation (4): Appl. Sci. 2018, 8, 2579 5 of 23

According to Equations (1)–(3), the coordinates of the laser spot in the image coordinate system o0-x0y0 can be obtained as shown in Equation (4):

( 0 C×d x = A + c0 C×d . (4) y0 = B + c1

According to Equation (4), when the positions of laser and image acquisition module are invariant, that is, when the parameters A, B, C, and d are invariant, the coordinates of the laser spot in the image C×d C×d coordinate system A + c0, B + c1 remain fixed. Therefore, the monitoring reference can be transmitted from the fixed point of the earth to the measured points. When the image acquisition module sinks during tunnel deformation, the image coordinate system moves relative to the world coordinate system o-xyz. Supposing that the vertical and horizontal deformation amounts of the image acquisition module are ∆x and ∆y, respectively, the laser spot position in the image coordinate system also shifts ∆x and ∆y in the vertical and horizontal directions, respectively. Therefore, when the image acquisition module shifts in the vertical and horizontal directions along with the measured point, the change of the laser spot position in the image coordinate system reflects the deformation. In an actual situation, the laser beam is not absolutely perpendicular to the laser receiving screen. As a result, when the image acquisition module moves along the z-axis with the measured point, the laser spot will produce the displacement component in the vertical and horizontal directions. As shown in Figure3, α is the angle between the laser beam and the ox axis, β is the angle between the laser beam and the oy axis, and θ is the angle between the laser beam and the oz axis. Assuming that the displacement change of image acquisition module along the oz axis is ∆z, the errors of the horizontal and vertical deformation measurements are as shown in Equation (5):

( cosα ∆x = ∆z × cosθ cosβ . (5) ∆x = ∆z × cosθ

Assuming that the laser beam points obliquely to the receiving screen, the angles α = 73.3◦, β = 81.5◦, and θ = 18.9◦ (in fact, the tilt of the laser beam is not that particularly obvious). When the image acquisition module shifts 1 mm along the oz axis, ∆x = 0.3037 mm and ∆y = 0.1562 mm. Therefore, when the shifting displacement of the image acquisition module along the z-axis is not large (and in fact it is not large) [29], this error can be ignored. The laser transmitter module is mounted on the ﬁxed point of the earth distant from the construction site, and it should remain stable. However, in cases of emergency during construction, such as sudden vibrating, when the laser beam reference is shifted and rotated, the position of the laser spot in the image coordinate will change. Taking the x-axis coordinate as an example, if the displacements along the x-axis and the horizontal rotation angle of the laser beam reference are ∆x and ∆α, respectively, the resulting laser spot position error ∆x0 is as shown in Equation (6):

∆x0 = ∆x + L × tan∆α = ∆x + L × ∆α. (6)

In order to reduce this error, a data compensation method is proposed here. The data compensation method uses two backward lasers in the laser transmitter module and two ﬁxed distant reference monitoring image acquisition modules with higher stability to monitor the feasibility of the laser transmitter, as shown in Figure4, where Ls represents the distance between the laser transmitter module and the image acquisition modules, ∆α represents the horizontal rotation angle of the laser transmitter module, and ∆x represents the displacement along the x-axis of the laser transmitter module. Appl. Sci. 2018, 8, 2579 6 of 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 23

Figure 4. Sketch map. 1. reference monitoring image acquisition module A; 2. reference monitoring Figure 4. Sketch map. 1. reference monitoring image acquisition module A; 2. reference monitoring image acquisition module B; 3. laser transmitter module; 4. image acquisition module. image acquisition module B; 3. laser transmitter module; 4. image acquisition module. Given that Ls were dozens of meters, small displacement and rotation angle will have no effect Given that Ls were dozens of meters, small displacement and rotation angle will have no effect on these Ls; that is, the lengths of Ls were invariable before and after the position change of the laser on these Ls; that is, the lengths of Ls were invariable before and after the position change of the laser transmitter module. With the two reference monitoring image acquisition modules, two laser spot transmitter module.0 With the0 two reference monitoring image acquisition modules, two laser spot position errors ∆x1 and ∆x2 can be obtained. The image acquisition module detecting displacement position errors 0 ∆′ and ∆′ can be obtained. The image acquisition module detecting with error is ∆x3. The equations of two variables ∆x and ∆α were obtained, as shown in Equation (7): displacement with error is ∆′. The equations of two variables ∆ and ∆ were obtained, as shown in Equation (7): ( 0 ∆x1 = ∆x + L1 × ∆α 0 . (7) ∆′∆x =∆+= ∆x + L ×∆× ∆α 2 2 . (7) ∆′ =∆+ ×∆ According to Equation (10), ∆x and ∆α can be determined as shown in Equation (8): According to Equation (10), ∆x and ∆α can be determined as shown in Equation (8):

 ∆∆x0∆−∆ x0  ∆∆x = ∆∆x0 −−L ×× 1 2 1 1 L1−L2 0 0 . (8) ∆x1−∆x2 . (8)  ∆α=∆ ∆ = L 1 − L 2 Then,Then, the the detected detected displacement displacement can can be be compensated compensated in in Equation Equation (9): (9):

x∆0 −∆x0 ∆∆x0− x0 ∆ =0 ∆ −0 ∆ − ∆× 1 ∆ 2 + × ∆ 1 .∆ 2 (9) ∆x3 = ∆x3 − ∆x1 − L1 × + L3 × . (9) L1 − L2 L1 − L2 Therefore, the detected displacement error caused by sudden vibration is compensated successfully.Therefore, the detected displacement error caused by sudden vibration is compensated successfully.

3. System Composition 3. System Composition Figure5 shows the main composition of the real-time tunnel deformation monitoring system, Figure 5 shows the main composition of the real-time tunnel deformation monitoring system, and Figure6 shows the details. The working principle is brieﬂy described as follows. and Figure 6 shows the details. The working principle is briefly described as follows. Install the system in the tunnel and turn it on. After calibration, adjust the direction of laser to Install the system in the tunnel and turn it on. After calibration, adjust the direction of laser to the image acquisition module. The command to measure tunnel deformation is ﬁrst generated at the the image acquisition module. The command to measure tunnel deformation is first generated at the industrial computer and sent to the wireless adaptor by a USB port. The communication between industrial computer and sent to the wireless adaptor by a USB port. The communication between the the adapter and the image acquisition module is wireless. The wireless adaptor is shown in Figure7. adapter and the image acquisition module is wireless. The wireless adaptor is shown in Figure 7. The image acquisition module receives the command and begins to capture laser spot images and The image acquisition module receives the command and begins to capture laser spot images and sends them to the industrial computer through the corresponding wireless adaptor in real time. The sends them to the industrial computer through the corresponding wireless adaptor in real time. The software installed in the industrial computer receives the images and processes them, obtaining the software installed in the industrial computer receives the images and processes them, obtaining the tunnel accumulative deformation and instantaneous deformation velocity. If the deformation values tunnel accumulative deformation and instantaneous deformation velocity. If the deformation values exceed the safe threshold, another command is generated and sent to the sound and light alarm exceed the safe threshold, another command is generated and sent to the sound and light alarm module and the command center through a wireless adaptor, giving an early warning to workers module and the command center through a wireless adaptor, giving an early warning to workers and managers. and managers. Appl. Sci. 20182018,, 88,, x 2579 FOR PEER REVIEW 77 of of 23 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 23

Figure 5. Main composition of the real-time tunnel deformation monitoring system. Figure 5. Main composition of the real-time tunnel deformation monitoring system. Figure 5. MainMain composition composition of of the the real-time tunnel deformation monitoring system.

Figure 6. System composition: ( (aa)) laser laser transmitter transmitter module; module; ( (bb)) image image acquisition acquisition module; module; ( (cc)) sound sound Figure 6. System composition: (a) laser transmitter module; (b) image acquisition module; (c) sound andFigure light light 6. alarmSystem alarm module; module; composition: ( (dd)) hard hard (a )devices laser devices transmitter and and system system module; software. software. (b) 1.image 1.forward forward acquisition laser; laser; 2. module; 2.backward backward (c )laser; sound laser; 3. and light alarm module; (d) hard devices and system software. 1. forward laser; 2. backward laser; 3. electronicallyand3. electronically light alarm controlled module; controlled (two-dimensionald)two-dimensional hard devices and swivel swivelsystem ta table;ble; software. 4. 4.organic organic 1. forward glass glass wilaser; window;ndow; 2. ba 5. 5.ckward filter ﬁlter imaging laser; 3. electronically controlled two-dimensional swivel table; 4. organic glass window; 5. filter imaging screen;electronicallyscreen; 6. high-definition high-deﬁnition controlled two-dimensionalindustrial camera; swivel 7. da darkroom; rkroom;table; 4. 8. organic alarm alarm glasslight; wi9. ndow;alarm alarm 5.horn; filter 10. imaging power screen; 6. high-definition industrial camera; 7. darkroom; 8. alarm light; 9. alarm horn; 10. power switch;screen;switch; 11.6. high-definition reference reference monitoring industrial image camera; acquisition 7. da rkroom;module. 8. alarm light; 9. alarm horn; 10. power switch; 11. reference monitoring image acquisition module. switch; 11. reference monitoring image acquisition module.

FigureFigure 7. 7. WirelessWireless adaptor. Figure 7. Wireless adaptor. Figure 7. Wireless adaptor. 3.1. Laser Laser Transmitter Transmitter Module 3.1.3.1. LaserLaser TransmitterTransmitter ModuleModule All parts of this module, except the reference monitoring image acquisition module, module, are are rigidly connected,AllAll partsparts therebythereby ofof thisthis preventingpreventing module,module, exceptexcept the the deformation deformation thethe referencereference of of the monitoring monitoringthe module. module. Theimageimage The organic organicacquisitionacquisition glass glass window module,module, window and areare and therigidlyrigidly dirtthe dirtconnected,shroudconnected, shroud form therebytherebyform a sealed a sealed preventingpreventing space, space, which thethe which can deformationdeformation effectively can effectively preventofof thethe prevent module.module. the interference the TheThe interference organicorganic of tunnel glassglass of dusttunnel windowwindow to thedust andand module to thethe moduledirtanddirt shroudshroud guarantee and formform guarantee the aa sealed permeabilitysealed space,space,the permeability whichwhich of the can lasercan effectivelyeffectively beam.of the The laserpreventprevent electronically beam. thethe interferenceinterference The controlled electronically ofof tunneltunnel two-dimensional dustdustcontrolled toto thethe two-dimensionalmoduleswivelmodule table andand can guaranteeguarantee adjust swivel the table the horizontalthe can permeabilitypermeability adjust and the vertical horizontal ofof the anglesthe laserandlaser of vertical the beam.beam. laser angles beam,TheThe ofelectronicallyelectronically making the laser the beam, laser controlledcontrolled beammaking to thetwo-dimensionalpointtwo-dimensional laser to the beam intersecting to swivelswivel pointsurface tabletable to cancanthe near adjustadjustintersecting the measured thethe horizontalhorizontal surface points. andandnear The verticalvertical referencethe measured anglesangles monitoring ofof thethepoints. laserlaser image Thebeam,beam, acquisitionreference makingmaking monitoringthemodulethe laserlaser is beam behindbeam image toto and acquisitionpointpoint distant toto fromthethe module intersectingintersecting the other is behind parts susurfacerface ofand the nearneardistant laser the transmitterthe from measuredmeasured the module, other points.points. parts and TheThe itof is referencethe mountedreference laser transmittermonitoringonmonitoring a more stable module,imageimage ﬁxed acquisition acquisitionand point it is away mounted modulemodule from on the isis a construction behindbehindmore stable andand site,fixeddistantdistant monitoring point fromfrom away thethe the from otherother feasibility the partsparts construction of ofof the thethe forward laser lasersite, monitoringtransmitterlasertransmitter beam. module,themodule, feasibility andand it itof isis the mountedmounted forward onon laser aa moremore beam. stablestable fixedfixed pointpoint awayaway fromfrom thethe constructionconstruction site,site, monitoring the feasibility of the forward laser beam. monitoring the feasibility of the forward laser beam.

Appl. Sci. 2018, 8, 2579 8 of 23

3.2.Appl. Image Sci. 2018 Acquisition, 8, x FOR PEER Module REVIEW 8 of 23

3.2.A Image band-pass Acquisition filter Module film [ 30] matching with the laser frequency is coated on the exterior surface of the filter imaging screen [31]; this allows the laser beam to go through the screen. A thin layer of diffuse A band-pass filter film [30] matching with the laser frequency is coated on the exterior surface reflection coating is present on the interior surface of the filter imaging screen [32]. On this layer, the of the filter imaging screen [31]; this allows the laser beam to go through the screen. A thin layer of laserdiffuse beam reflection forms a coating laser spot is present with Lambertianon the interior reflection. surface of The the HDfilter industrial imaging screen camera [32]. constantly On this takes photoslayer, of the the laser laser beam spot, forms extracting a laser the spot position with La informationmbertian reflection. of the laser The spotHD inindustrial the image camera coordinate systemconstantly with takes machine photos vision. of the Thelaser band-pass spot, extracting filter the film position and darkroom information [33 of] the helps laser to spot reduce in the the stray lightimage interference. coordinate system with machine vision. The band-pass filter film and darkroom [33] helps to reduce the stray light interference. 3.3. Sound and Light Alarm Module 3.3. Sound and Light Alarm Module When the tunnel accumulative deformation and instantaneous deformation velocity exceeds the setWhen threshold, the tunnel the module accumulative is turned deformation on and and provides instantaneous an early deformation warning to velocity the command exceeds center throughthe set a threshold, sound and the light module alarm; is thisturned gives on aand timely provides and effectivean early warning early warning to the command against tunnel center collapse through a sound and light alarm; this gives a timely and effective early warning against tunnel accidents, thereby reducing casualties and property losses. collapse accidents, thereby reducing casualties and property losses. 3.4. Hard Devices and System Software 3.4. Hard Devices and System Software TheThe hard hard devices devices areare usedused to to realize realize the the commu communicationnication between between each eachmodule module and install and installthe the systemsystem software. software. TheThe mainmain functions of of the the software software for forreal-time real-time tunnel tunnel deformation deformation monitoring monitoring systemsystem include include measurement, measurement, control, control, communication, communication, and dataand storage.data storage. Considering Considering the multi-system the operation,multi-system cross operation, platform, cross and compatibilityplatform, and ofcomp theatibility system softwareof the system requirements, software requirements, LabVIEW [34 ] 2010 byLabVIEW NI (Austin, [34] TX,2010 USA)by NI is(Austin, adopted TX, asUSA) the is softwareadopted as development the software development platform. The platform. communication The betweencommunication the laser between transmitter the laser module, transmitter image module, acquisition image module,acquisition sound module, and sound light and alarm light module, andalarm hard module, devices and and hard system devices software and system is wireless. software The is flow wireless. chart ofThe the flow system chart softwareof the system is shown as software is shown as Figure 8. Figure8.

FigureFigure 8. 8. FlowchartFlowchart of the ofthe system system software. software. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 23

4. Test Results and Analysis

4.1. Principle Verification Test To verify the principle of the proposed tunnel deformation monitoring technology, a tunnel deformationAppl.Appl. Sci. Sci.2018 2018, 8 , process,8 2579, x FOR PEERwas designedREVIEW and simulated as shown in Figure 9. 99 of of 23 23 The image acquisition module was placed on a high-precision micro motion platform, which is a 4.one-dimensional Test Results and motion Analysis platform with a 1 μm accuracy to enable the image acquisition module to move4. Test with Results the andmoving Analysis platform and simulate the deformation process of the tunnel. The laser transmitter4.1. Principle module Verification was Testarranged on the other side, with the laser pointing to the filter imaging 4.1. Principle Veriﬁcation Test screen.To If verifythe change the principle in the coordinates of the proposed of the laser tunnel spot deformation center [35] in monitoring the image coordinatetechnology, system a tunnel is proportionaldeformationTo verify to process the the principle tunnel was designeddeformation, of the proposed and simulatedthen the tunnel correctness as deformationshown inof Figurethe monitoring tunnel 9. deformation technology, monitoring a tunnel principledeformationThe is image proved. process acquisition was designed module and was simulated placed on as a shown high-precision in Figure micro9. motion platform, which is a one-dimensional motion platform with a 1 μm accuracy to enable the image acquisition module to move with the moving platform and simulate the deformation process of the tunnel. The laser transmitter module was arranged on the other side, with the laser pointing to the filter imaging screen. If the change in the coordinates of the laser spot center [35] in the image coordinate system is proportional to the tunnel deformation, then the correctness of the tunnel deformation monitoring principle is proved.

Figure 9. Sketch map of the principleprinciple veriﬁcationverification experiment.experiment. 1. moving direction; 2. micro motion platform; 3. image acquisition module; 4. laser transmitter module.

The image acquisition module was placed on a high-precision micro motion platform, which is In the experiment, the micro motion platform was moved at an interval of 5 mm, and the a one-dimensional motion platform with a 1 µm accuracy to enable the image acquisition module displacement of the micro motion platform and the corresponding abscissa values of the laser spot to move with the moving platform and simulate the deformation process of the tunnel. The laser center in the image coordinate system in pixel units were recorded. transmitter module was arranged on the other side, with the laser pointing to the ﬁlter imaging The data relation curve is shown in Figure 10. In this curve, the x-axis is the displacement of the screen.Figure If the 9. change Sketch inmap the of coordinates the principle of verification the laser spotexperiment. center [1.35 moving] in the direction; image coordinate 2. micro motion system is micro motion platform, and the y-axis is the measured displacement, namely the abscissa value of proportionalplatform; to 3. the image tunnel acquisition deformation, module; then4. laser the transmitter correctness module. of the tunnel deformation monitoring the laser spot center. principle is proved. InIn the the experiment, experiment, thethe micromicro motionmotion platformplatform waswas movedmoved atat anan intervalinterval ofof 55 mm, mm, andand the the displacementdisplacement of of the the micro micro motion motion platform platform and and the the corresponding corresponding abscissa abscissa values values of of the the laser laser spot spot centercenter in in the the image image coordinate coordinate system system in in pixel pixel units units were were recorded. recorded. TheThe data data relation relation curve curve is is shown shown in in Figure Figure 10 10.. In In this this curve, curve, the thex x-axis-axis is is the the displacement displacement of of the the micromicro motion motion platform, platform, and and the they-axis y-axis is theis the measured measured displacement, displacement namely, namely the abscissathe abscissa value value of the of laserthe laser spot spot center. center.

Figure 10. Data relation curve.

According to the data relation curve, the displacement of the micro motion platform was linearly correlated to the measured displacement, and the linear correlation coefficient was close to 1, which demonstrates the correctness of the principle.

FigureFigure 10. 10.Data Data relation relation curve. curve.

AccordingAccording toto thethe datadata relationrelation curve,curve, thethe displacementdisplacement ofof thethe micromicro motionmotion platformplatform waswas linearlylinearly correlated correlated toto thethe measuredmeasured displacement, an andd the the linear linear correlation correlation coefficient coefﬁcient was was close close to to1, 1, which which demonstrates demonstrates the the correctness correctness of of the the principle. principle.

Appl. Sci. 2018, 8, 2579 10 of 23

Appl. Sci. 2018,, 8,, xx FORFOR PEERPEER REVIEWREVIEW 10 of 23 4.2. Image Acquisition Module Calibration Experiment 4.2. Image Acquisition Module Calibration Experiment The position information obtained by the image acquisition module is the plane two-dimensional The position information obtained by the image acquisition module is the plane data in the image coordinate system in pixels. Thus, the image coordinate system needs to be calibrated two-dimensional data in the image coordinate system in pixels. Thus, the image coordinate system to restore the true deformation value of the measured point [36]. needs to be calibrated to restore the true deformation value of the measured point [36]. 4.2.1. Horizontal Calibration Experiment 4.2.1. Horizontal Calibration Experiment Given the different positions of the measured points in the tunnel, the image acquisition module Given the different positions of the measured points in the tunnel, the image acquisition will oftenGiven roll the along different the z -axispositions after itof is the installed. measured As a points result, anin anglethe tunnel, will form the betweenimage acquisition the x0-axis module will often roll along the z-axis after it is installed. As a result, an angle will form between the moduleof the image will often coordinate roll along system the z-axis and theafterx -axisit is installed. of the world As a coordinateresult, an angle system. will form Given between this angle, the x′-axis of the image coordinate system and the x-axis of the world coordinate system. Given this xthe′-axis change of the in image the position coordinate information system and of the the laser x-axis spot of centerthe world obtained coordinate by the system. image acquisitionGiven this angle, the change in the position information of the laser spot center obtained by the image angle,module the cannot change directly in the represent position the information settlement and of thethe diameterlaser spot convergence center obtained of the measured by the image point. acquisition module cannot directly represent the settlement and the diameter convergence of the Therefore,acquisition the module horizontality cannot ofdirectly the image represent coordinate the settlement system needs and tothe be diameter calibrated. convergence of the measured point. Therefore, the horizontality of the image coordinate system needs to be calibrated. measuredA simpliﬁed point. Therefore, model for the the hori relationshipzontality of between the image the coordinate world coordinate system needs system to andbe calibrated. the image A simplified model for the relationship between the world coordinate system and the image coordinateA simplified system ismodel shown for in the Figure relationship 11[ 37]. Here, betweeno-xy representsthe world thecoordinate world coordinate system and system, the imageo0-x0y0 coordinate system is shown in Figure 11 [37]. Here, o-xy represents the world coordinate system, coordinaterepresents thesystem image is coordinateshown in Figure system, 11 and [37].θ represents Here, o-xy the represents rotation angle.the world The linecoordinatel is a horizontal system, o′-x′y′ represents the image coordinate system, and θ represents the rotation angle. The line l is a oline,′-x′y which′ represents is parallel the image to the oxcoordinateaxis and comessystem, from and a θ laser represents cast instrument. the rotation The angle. laser cast The instrument line l is a horizontal line, which is parallel to the ox axis and comes from a laser cast instrument. The laser cast producedhorizontal aline, horizontal which is laser parallel line to with the aox± axis10” accuracy.and comes Supposing from a laser that cast the instrument. slope of the The line laserl in cast the instrument produced a horizontal laser line with a ±10″ accuracy. Supposing that the slope of the line instrumentimage coordinate produced system a horizontal is k, the relationship laser line with between a ±10″ θaccuracy.and k is asSupposing shown in that Equation the slope (10): of the line l in the image coordinate system is k, the relationship between θ and k is as shown in Equation (10): ◦ − = − 1 θ = 180°180 −tan k.. (10) (10)

Figure 11. SimplifiedSimpliﬁed coordinate model.

After the rotation angle θθ was obtained, the horizontal calibrati calibrationon of the image coordinate system 0 could be completed by by rotating the the im imageage coordinate coordinate system system by by an an angle angle θθ around point o′,, as as the rotation center center in in the the reverse reverse direction. direction. The The hori horizontalzontal calibration calibration process process is isshown shown in in Figure Figure 12. 12 .

Figure 12. HorizontalHorizontal calibra calibrationtion experiment.

Before the horizontal calibration, when the image acquisition module horizontally moved along thethe micro motion platform, as as shown shown in in Figure Figure 99,, bothboth thethe x-axis and y-axis values of of the laser spot center in the image coordinate system changed with the movement. After the horizontal calibration was completed, only the x-axis value changed, thereby indicating that the horizontal calibration of Appl. Sci. 2018, 8, 2579 11 of 23

centerAppl. in the Sci. 2018 image, 8, x coordinateFOR PEER REVIEW system changed with the movement. After the horizontal calibration11 of 23 was completed, only the x-axis value changed, thereby indicating that the horizontal calibration of the Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 23 imageAppl.the coordinate imageSci. 2018 coordinate, 8, systemx FOR PEER wassystem REVIEW successful. was successful. The micro The mi motioncro motion platform platform moved moved at at an an interval interval11 of ofof 23 5 5 mm. mm. The experimental data curves are shown in Figure 13. The experimental data curves are shown in Figure 13. thethe image image coordinate coordinate system system was was successful. successful. The The mi microcro motion motion platform platform moved moved at at an an interval interval of of 5 5 mm.mm. The The experimental experimental data data curves curves are are shown shown in in Figure Figure 13. 13.

FigureFigure 13. Coordinate13. Coordinate change change before before and and afterafter horizontal calibration. calibration.

AccordingAccording to Figure toFigure Figure 13, 13. the13. 13, Coordinate Coordinate displacementthe displacement change change of before before theof th microand ande micro after after motion horizontal horizontalmotion platform platform calibration. calibration. was was from from 0 0 mm mm to to 70 70 mm, and themm, change and the in change the y-axis in the value y-axis after value calibration after calibration was as was follows: as follows: AccordingAccording to to Figure Figure 13, 13, the the displacement displacement of of th thee micro micro motion motion platform platform was was from from 0 0 mm mm to to 70 70 ∆Y = 318.52 − 317.01 = 1.51 (pixel). mm,mm, and and the the change change in in the the y y-axis-axis value value after after calibration calibration was was as as follows: follows: ∆Y = 318.52 − 317.01 = 1.51 (pixel). Therefore, within the permitted error, the y-axis values were basically unchanged, and the ∆∆YY = = 318.52 318.52 − − 317.01 317.01 = = 1.51 1.51 (pixel). (pixel). horizontal calibration of the image coordinate system was successful. Therefore, within the permitted error, the y-axis values were basically unchanged, and the Therefore,Therefore,In the tunnel withinwithin environment, thethe permittedpermitted a universal error,error, thearmthe y y-axiswas-axis us valuesvaluesed to hold werewere the basicallybasically laser cast unchanged,unchanged, instrument andand during thethe horizontalhorizontalhorizontalthe horizontal calibration calibration calibration calibration, of theof of the the image as image image shown coordinate coordinate coordinate in Figures systemsystem system 14 and was waswas 15, successful. successful.and the calibration method was the same. InEvery theInIn tunnel thetime the tunnel tunnelbefore environment, environment, environment,using the system, a universal a a universal universal the horizontal arm arm arm was was was calibration used us useded to to to hold hold holdwas the the requiredthe laserlaser laser castcast tocast be instrument instrumentdone first. during during the horizontalthethe horizontal horizontal calibration, calibration, calibration, as shown as as shown inshown Figures in in Figures Figures 14 and 14 14 15 and and, and 15, 15, theand and calibrationthe the calibration calibration method method method was was was the the the same. same. same. Every timeEvery beforeEvery time time using before before the using system,using the the thesystem, system, horizontal the the horizontal horizontal calibration calibration calibration was required was was required required to be to to done be be done done ﬁrst. first. first.

Figure 14. Universal arm.

FigureFigureFigure 14.14. 14. Universal Universal arm. arm.

Figure 15. Tunnel environment.

FigureFigureFigure 15.15. 15. Tunnel Tunnel environment. environment.environment. Appl. Sci. 2018, 8, 2579 12 of 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 12 of 23

4.2.2. Conversion Coefficient Coefficient Calibration Experiment The unitunit of of spot spot center center coordinates coordinates in the in imagethe image coordinate coordinate system system is pixel, is which pixel, cannot which represent cannot therepresent true displacement the true displacement directly [38 directly]. A sketch [38]. map A sk ofetch the map conversion of the conversion coefficient calibrationcoefficient experimentcalibration isexperiment shown in is Figure shown 16 in. The Figure laser 16. was The placed laser was on theplaced micro on motionthe micro platform motion andplatform pointed and to pointed the fixed to imagethe fixed acquisition image acquisition module, therebymodule, allowing thereby itallowing to move it with to move the micro with the motion micro platform. motion platform.

Figure 16. Conversion coefficientcoefficient calibration experiment. 1. image acquisition module; 2. movement direction; 3. micro motion platform; 4. laser. To restore the true displacement of the micro motion platform, the horizontal coordinate To restore the true displacement of the micro motion platform, the horizontal coordinate conversion coefficient α and the vertical coordinate conversion coefficient β needed to be conversion coefficient α and the vertical coordinate conversion coefficient β needed to be determined determined [39]. The calculation formulas are shown in Equation (11): [39]. The calculation formulas are shown in Equation (11): ( ∆ = × ∆ ∆x = α × ∆X ∆ = × ∆. .(11) (11) ∆y = β × ∆Y In Equation (11), (∆X, ∆Y) represents the displacement of the laser spot in the image coordinate systemIn Equationin pixels, (11),and ( (∆xX, ,∆∆yY) )represents represents the the real displacement displacement of the in laserthe world spot incoordinate the image system. coordinate The systemx-axis conversion in pixels, coefficient and (∆x, ∆ calibrationy) represents experimental the real displacement data are shown in in the Table world 1. coordinate system. The x-axis conversion coefficient calibration experimental data are shown in Table1. Table 1. x-axis conversion coefficient calibration experimental data. Table 1. x-axis conversion coefficient calibration experimental data. Displacement of Micro Abscissa Value of Laser ∆x Displacement of Micro Abscissa Value of Laser ∆X (pixel) Motion Platform (mm) Spot Center (pixel) ∆x (mm)(mm) ∆X (pixel) Motion Platform (mm) Spot Center (pixel) 0 636.39 — — 010 636.39567.28 —10 69.11 — 10 567.28 10 69.11 20 498.04 10 69.24 20 498.04 10 69.24 3030 428.44428.44 1010 69.6069.60 4040 360.53360.53 1010 67.9167.91 5050 291.98291.98 1010 68.5568.55 6060 223.58223.58 1010 68.4068.40 70 154.93 10 68.65 70 154.93 10 68.65

The x-axis conversion coefficient coefﬁcient can be obtained with Equation (11), and the arithmetic mean value formula is shown as Equation (12):

∆∆x ∑7 i ∑i=1 ∆∆X (12) α == i== 0.14540.1454.. (12) 7 The calibration process of the y-axis conversion coefficient β was the same as α; that is, β = The calibration process of the y-axis conversion coefficient β was the same as α; that is, β = 0.1454. 0.1454. The relationship between the positions of the filter imaging screen and the HD industrial The relationship between the positions of the filter imaging screen and the HD industrial camera is camera is invariant. Once the conversion coefficients α and β are determined, they are invariant and invariant. Once the conversion coefficients α and β are determined, they are invariant and only need only need to be calibrated once. to be calibrated once.

4.3. Data Processing Algorithm Comparison Test Because of of changes changes in in the the refractive refractive index index of of air air and and other other factors factors in a in long a long distance distance of 30 of m, 30 the m, thelaser laser beam beam will willslightly slightly deflect deﬂect during during the propagation, the propagation, and the and position the position of the of laser the laserspot on spot the on filter the imaging screen will vibrate. Given the isotropy of laser spot vibration, this experiment only addressed the x-axis coordinates of the spot, and the unit is pixels. Appl. Sci. 2018, 8, 2579 13 of 23

Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 23 Appl.ﬁlter Sci. imaging 2018, 8, x screenFOR PEER will REVIEW vibrate. Given the isotropy of laser spot vibration, this experiment13 onlyof 23 addressedAs shown the x in-axis Figure coordinates 17, the laser of the spot spot, position and the information unit is pixels. of 1000 spot images was processed, As shown in Figure 17, the laser spot position information of 1000 spot images was processed, whereinAs shownthe abscissa in Figure axis 17 was, the the laser acquisition spot position time. informationAs can be seen of 1000from spot the diagram, images was the processed, vibration wherein the abscissa axis was the acquisition time. As can be seen from the diagram, the vibration rangewherein of thelaser abscissa spot was axis big; was ther theefore, acquisition when the time. laser As canpropagation be seen fromdistance the diagram,is long, the the laser vibration spot range of laser spot was big; therefore, when the laser propagation distance is long, the laser spot centerrange ofdata laser at spot a single was big; time therefore, point could when not the laserbe used propagation to represent distance the ismonitoring long, the laser reference. spot center The center data at a single time point could not be used to represent the monitoring reference. The positiondata at a probability single time distribution point could of not the be 1000 used spot to representimages is theshown monitoring in Figure reference. 18. Although The the position laser position probability distribution of the 1000 spot images is shown in Figure 18. Although the laser spotprobability was in distributionvibration, the of position the 1000 spotprobability images distribution is shown in was Figure approximately 18. Although in the accordance laser spot with was spot was in vibration, the position probability distribution was approximately in accordance with Gaussianin vibration, distribution; the position therefore, probability stable distribution data was wasfound approximately to represent inthe accordance monitoring with reference Gaussian by Gaussian distribution; therefore, stable data was found to represent the monitoring reference by processingdistribution; a therefore,set of data stable over a data period was of found time. to represent the monitoring reference by processing a set processingof data over a aset period of data of over time. a period of time.

Figure 17. Laser spot position information of 1000 spot images. FigureFigure 17. 17. LaserLaser spot spot position position information information of of 1000 1000 spot spot images. images.

Figure 18. Position probability distribution. Figure 18. Position probability distribution. Given the the characteristics characteristics of of the the dataset, dataset, the the following following methods methods were were used used to process to process the thedata: data: (1) Given the characteristics of the dataset, the following methods were used to process the data: (1) calculation(1) calculation of the of theextremum extremum after after Gauss Gauss fitting, ﬁtting, (2) calculation (2) calculation of the of expectation the expectation from fromoriginal original data, calculation of the extremum after Gauss fitting, (2) calculation of the expectation from original data, (3)data, calculation (3) calculation of the of expectation the expectation after after low-pass low-pass filtering, ﬁltering, and and (4) (4) acquisition acquisition of of the the maximum (3) calculation of the expectation after low-pass filtering, and (4) acquisition of the maximum probability data from data probability distribution map. The results are shown in Table 2 2.. AsAs cancan bebe probability data from data probability distribution map. The results are shown in Table 2. As can be seen, thethe fourthfourth methodmethod has has the the highest highest accuracy; accuracy; therefore, therefore, this this method method was was used used to process to process the laser the seen, the fourth method has the highest accuracy; therefore, this method was used to process the laserspot positionspot position information. information. laser spot position information.

Appl. Sci. 2018, 8, x FOR PEER REVIEW 14 of 23

Appl. Sci. 2018, 8, 2579 14 of 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW Table 2. Results comparison. 14 of 23

MethodTable 2. Results comparison. Accuracy (mm) Table 2. Results comparison. 1. Calculation of the extremum after Gauss fitting 0.28 Method Accuracy (mm) 2. Calculation of the expectationMethod from original data Accuracy (mm) 0.42 1. Calculation of the extremum after Gauss fitting 0.28 3. Calculation1. Calculation of ofthe the expectation extremum afterafter Gauss low-pass ﬁtting filtering 0.28 0.21 2. Calculation of the expectation from original data 0.42 4. Acquisition2. Calculation of ofthe the maximum expectation probability from original data data 0.42 0.12 3. Calculation3. Calculation of the of theexpectation expectation after after low-pass low-pass ﬁlteringfiltering 0.21 0.21 4. Acquisition of the maximum probability data 0.12 4.4. Wireless Antenna4. Acquisition Attitude of the Test maximum probability data 0.12

4.4.4.4. Wireless WirelessAccording Antenna Antenna to the Attitude Attitude principle Test Test of electromagnetic wave emission, the antenna has a certain angle when transmitting and receiving electromagnetic waves, as shown in Figure 19, where the signal is According to the principle of electromagnetic wave emission, the antenna has a certain angle strongestAccording in the to main the principle lobe range. of electromagnetic When the antenna wave is emission,arranged, the if the antenna antenna has aattitude certain can angle be when transmitting and receiving electromagnetic waves, as shown in Figure 19, where the signal is whenadjusted transmitting to the right and position receiving, the electromagnetic intensity of the received waves, as signal shown will in be Figure enhanced. 19, where the signal is strongeststrongest inin thethe main main lobe lobe range. range. When When the the antenna antenna is arranged, is arranged, if the antennaif the antenna attitude attitude can be adjustedcan be adjustedto the right to the position, right position the intensity, the intensity of the received of the received signal will signal be enhanced. will be enhanced.

Figure 19. Schematic diagram of electromagnetic wave emission principle. Figure 19. Schematic diagram of electromagnetic wave emission principle. A spectrumFigure analyzer 19. Schematic was used diagram as a ofwireless electromag signalnetic detection wave emission tool principle.to analyze the size of the A spectrum analyzer was used as a wireless signal detection tool to analyze the size of the power power received under different poses. Because the attitudes of the transmitting antenna and the receivedA spectrum under different analyzer poses. was used Because as a the wireless attitudes signal of the detection transmitting tool to antenna analyze and the the size receiving of the receiving antenna were relative, the transmitting antenna was fixed horizontally during the powerantenna received were relative, under thedifferent transmitting poses. antenna Because was the ﬁxed attitudes horizontally of the duringtransmitting the experiment, antenna and and the the experiment, and the attitude of the receiving antenna was changed. receivingattitude of antenna the receiving were antennarelative, was the changed. transmitting antenna was fixed horizontally during the As shown in Figure 20a, when the receiving antenna was parallel to the transmitting antenna, experiment,As shown and in the Figure attitude 20a, of when the receiving the receiving antenna antenna was changed. was parallel to the transmitting antenna, the maximum signal intensity was −23.63 db. As shown in Figure 20b, when the receiving antenna the maximumAs shown signalin Figure intensity 20a, when was −the23.63 receiving db. As antenna shown inwas Figure parallel 20b, to when the transmitting the receiving antenna, antenna was vertical to the transmitting antenna, the signal strength was minimum, −35.68 db. Therefore, thewas maximum vertical to signal the transmitting intensity was antenna, −23.63 thedb. signalAs shown strength in Figure was minimum,20b, when the−35.68 receiving db. Therefore, antenna when using the instrument, the two antennas should be as parallel as possible to enhance the waswhen vertical using theto the instrument, transmitt theing two antenna, antennas the should signal bestrength as parallel was as minimum, possible to − enhance35.68 db. the Therefore, intensity intensity of the received signal. whenof the using received the signal. instrument, the two antennas should be as parallel as possible to enhance the intensity of the received signal.

(a) (b)

FigureFigure 20.20.Receiving Receivingantenna antenna(a) power powermap. map. ((aa)) Parallelparallel attitude;attitude;(b) ((bb)) verticalvertical attitude. attitude.

4.5.4.5. TemperatureTemperatureFigure Change Change 20. Receiving Test Test antenna power map. (a) parallel attitude; (b) vertical attitude. The system needs to meet the requirement of real-time measurement without interruption for 4.5. TemperatureThe system Change needs Testto meet the requirement of real-time measurement without interruption for a along long time; time; therefore, therefore, it itis isbound bound to tobe be affected affected by by the the change change of ofambient ambient temperature. temperature. However, However, the The system needs to meet the requirement of real-time measurement without interruption for a thelong long distance distance laser laser reference reference prop propagationagation has has a amagnifying magnifying effect effect on on the the laser’s laser’s own own angle change. long time; therefore, it is bound to be affected by the change of ambient temperature. However, the long distance laser reference propagation has a magnifying effect on the laser’s own angle change. Appl. Sci. 2018, 8, x 2579 FOR PEER REVIEW 15 of 23

Since the maximum temperature difference in the tunnel during day and night is only 4 °C, the ◦ linearSince thethermal maximum deformation temperature of the difference laser system inthe can tunnel be ignored, during but day the and angle night change is only will 4 haveC, the a linearlarge error.thermal As deformation shown in Figure of the 21, laser for system a temper canature be ignored, difference but 10 the °C angle during change 14 h, will the havex-axis a coordinate large error. ◦ Asdifference shown inof Figure the spot 21, forposition a temperature caused differenceby thermal 10 deformationC during 14 h,can the reachx-axis 38 coordinate pixels on difference a 50 m propagationof the spot position distance. caused For bythe thermal 4 °C temperature deformation di canfference reach 38in pixelsthe tunnel, on a 50the m thermal propagation deformation distance. ◦ willFor thecause 4 lessC temperature x-axis coordinate difference difference. in the tunnel, the thermal deformation will cause less x-axis coordinateTo reduce difference. the effect of thermal deformation on the precision of the system, the mechanical structureTo reduce was thedesigned effect of to thermal be symmetrical deformation to on theminimize precision the of theangle system, torsion the mechanicalcaused by structurethermal wasdeformation. designed to be symmetrical to minimize the angle torsion caused by thermal deformation.

Figure 21. Result of data change caused by thermal deformation.

4.6. System Accuracy Test Experiment 4.6.1. Detection Accuracy Test Experiment 4.6.1. Detection Accuracy Test Experiment After the above tests were performed, the real-time tunnel deformation monitoring system was After the above tests were performed, the real-time tunnel deformation monitoring system was ready to operate. A high-precision micro motion platform was used to simulate the tunnel deformation ready to operate. A high-precision micro motion platform was used to simulate the tunnel process, and it moved at an interval of 5 mm. The obtained detection displacement is shown in Table3. deformation process, and it moved at an interval of 5 mm. The obtained detection displacement is shown in Table 3. Table 3. Detection accuracy test experimental data.

Position of Micro LaserTable Spot3. Detection Center accuracyLaser Spottest experimental Center data. Motion Platform before Calibration after Calibration x (mm) |∆x−10| (mm) Position of Micro(mm) Motion Laser(pixel) Spot Center before (mm)Laser Spot Center after x |∆ − | Platform (mm) Calibration (pixel) Calibration (mm) (mm) (mm) (70, 0) (598.27, 317.10) (86.99, 46.11) — — (70, 0) (598.27, 317.10) (86.99, 46.11) — — (60, 0) (529.16, 317.29) (76.94, 46.13) 10.05 0.05 (60,(50, 0) 0) (459.92, (529.16, 317.45) 317.29) (66.87, 46.15) (76.94, 46.13) 10.07 10.05 0.07 0.05 (50,(40, 0) 0) (390.32, (459.92, 317.60) 317.45) (56.75, 46.18) (66.87, 46.15) 10.12 10.07 0.12 0.07 (40,(30, 0) 0) (322.41, (390.32, 317.84) 317.60) (46.87, 46.21) (56.75, 46.18) 9.88 10.12 0.12 0.12 (30,(20, 0) 0) (253.86, (322.41, 318.14) 317.84) (36.91, 46.25) (46.87, 46.21) 9.96 9.88 0.04 0.12 (20,(10, 0) 0) (185.46, (253.86, 318.42) 318.14) (26.97, 46.29) (36.91, 46.25) 9.94 9.96 0.06 0.04 (10, (0,0) 0) (116.81, (185.46, 318.61) 318.42) (16.88, 46.32) (26.97, 46.29) 10.09 9.94 0.09 0.06 (0, 0) (116.81, 318.61) (16.88, 46.32) 10.09 0.09 According to Table3, the monitoring system had a high detection accuracy better than 0.12 mm. According to Table 3, the monitoring system had a high detection accuracy better than 0.12 mm. 4.6.2. Repeatability Accuracy Test Experiment 4.6.2. Repeatability Accuracy Test Experiment The micro motion platform performed a linear reciprocating motion at an interval of 10 mm. ExperimentalThe micro data motion are shown platform in Table performed4. a linear reciprocating motion at an interval of 10 mm. Experimental data are shown in Table 4.

Appl. Sci. 2018, 8, x FOR PEER REVIEW 16 of 23

Appl. Sci. 2018, 8, 2579 Table 4. Repeatability accuracy test experimental data. 16 of 23

Position of System Detection Results Position of System Detection Results Micro Motion Table 4. Repeatability|∆| − accuracy Micro test experimental Motion data. |∆| − (x, y) (mm) (x, y) (mm) Platform (mm) (mm) Platform (mm) (mm) Position of System Detection Results Position of System Detection Results Micro(70, Motion 0) (86.95, 46.09) — Micro (60, Motion 0) (76.95, 46.13) 0.06 ||∆x|−10| ||∆x|−10| Platform(60, 0) (mm) (77.02,(x, y) (mm) 46.12) 0.07 Platform (70, (mm)0) (x (86.96,, y) (mm) 46.11) 0.01 (mm) (mm) (50, 0) (66.90, 46.15) 0.12 (60, 0) (76.89, 46.12) 0.07 (70, 0) (86.95, 46.09) — (60, 0) (76.95, 46.13) 0.06 (40,(60, 0) 0) (56.80,(77.02, 46.12) 46.19) 0.070.10 (70, (50, 0) 0) (86.96, (66.88, 46.11) 46.16) 0.01 0.01 (30,(50, 0) 0) (46.87,(66.90, 46.15) 46.21) 0.120.07 (60, (40, 0) 0) (76.89, (56.80, 46.12) 46.18) 0.07 0.08 (20,(40, 0) 0) (36.89,(56.80, 46.19) 46.23) 0.100.02 (50, (30, 0) 0) (66.88, (46.83, 46.16) 46.22) 0.01 0.03 (30, 0) (46.87, 46.21) 0.07 (40, 0) (56.80, 46.18) 0.08 (10, 0) (26.85, 46.27) 0.04 (20, 0) (36.91, 46.25) 0.08 (20, 0) (36.89, 46.23) 0.02 (30, 0) (46.83, 46.22) 0.03 (0,(10, 0) 0) (16.90,(26.85, 46.27) 46.32) 0.040.05 (20, (10, 0) 0) (36.91, (26.88, 46.25) 46.28) 0.08 0.03 (10,(0, 0) (26.88,(16.90, 46.32) 46.28) 0.050.02 (10, (0, 0) 0) (26.88, (16.89, 46.28) 46.33) 0.03 0.01 (20,(10, 0) 0) (36.91,(26.88, 46.28) 46.25) 0.020.03 (0, (10, 0) 0) (16.89, (26.96, 46.33) 46.29) 0.01 0.07 (20, 0) (36.91, 46.25) 0.03 (10, 0) (26.96, 46.29) 0.07 (30,(30, 0) 0) (46.88, 46.22) 46.22) 0.03 (20, (20, 0) 0) (36.89, (36.89, 46.26) 46.26) 0.07 0.07 (40,(40, 0) 0) (56.80, 46.20) 46.20) 0.08 (30, (30, 0) 0) (46.85, (46.85, 46.23) 46.23) 0.04 0.04 (50,(50, 0) 0) (66.89, 46.14) 46.14) 0.09 (40, (40, 0) 0) (56.78, (56.78, 46.18) 46.18) 0.07 0.07

According to Table 44,, thethe monitoringmonitoring systemsystem hadhad goodgood stabilitystability andand repeatability,repeatability, andand thethe repeatability accuracy was better than 0.11 mm.

4.6.3. System System Resolution Resolution The micromicro motionmotion platform platform moved moved slowly slowly with with the minimumthe minimum displacement, displacement, that is, that at an is, interval at an intervalof 1 µm. of In 1 μ them. systemIn the system accuracy accuracy test experiment, test experiment, when when the micro the micro motion motion platform platform moved moved 10 µ 10m, μthem, coordinates the coordinates of the of laser the laser spot fromspot thefrom system the system software software started started to change, to change, indicating indicating that the that system the systemresolution resolution was 10 µwasm. 10 μm.

4.7. Laser Laser Transmitter Transmitter Module Feasibility Test In orderorder toto simulate simulate the the dusty dusty and and dark dark tunneling tunneli environment,ng environment, this this test wastest conductedwas conducted on a foggyon a foggynight asnight shown as shown in Figure in Figure 22. 22.

FigureFigure 22. 22. TestTest environment in foggy and dark night.

The test sketch map is shown in Figure 2323.. Appl. Sci. 2018, 8, 2579 17 of 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 17 of 23

Figure 23. Test sketch map. 1. reference monitoring image acquisition module A; 2. reference Figure 23. Test sketch map. 1. reference monitoring image acquisition module A; 2. reference monitoring image acquisition module B; 3. image acquisition module I; 4. image acquisition module II. monitoring image acquisition module B; 3. image acquisition module I; 4. image acquisition module (L1 = 40 m, L2 = 46 m, L3 = 35 m, L4 = 38 m). II. ( =40 m, =46 m, =35 m, =38 m). As shown in Figure 23, the laser transmitter module was located on a precision swivel table that wasAs fixed shown on the in micro Figure motion 23, the platform. laser transmitter Two image modu acquisitionle was located modules on anda precision two reference swivel monitoring table that wasimage fixed acquisition on the modulesmicro motion were placedplatform. on bothTwo sidesimage of acquisition the laser transmitter modules moduleand two respectively, reference monitoringan Ls represents image the acquisition distance between modules the were laser placed transmitter on both module sides and of thethe imagelaser transmitter acquisition modules,module respectively, and Ls represents the distance between the laser transmitter module and the image ∆α represents the horizontal rotation angle of the laser transmitter module, and ∆x represents the acquisition modules, ∆ represents the horizontal rotation angle of the laser transmitter module, displacement along the x-axis of the laser transmitter module. and ∆ represents the displacement along the x-axis of the laser transmitter module. In this test, the two reference monitoring modules and the two image acquisition modules were In this test, the two reference monitoring modules and the two image acquisition modules were fixed, and the laser beam reference was shifted 12 mm and rotated 36”. According to Equation (9), fixed, and the laser beam reference was shifted 12 mm and rotated 36″. According to Equation 9, the the detected displacement can be compensated. The test data are shown in Table5. detected displacement can be compensated. The test data are shown in Table 5. Table 5. Laser transmitter module feasibility test data (unit: mm). Table 5. Laser transmitter module feasibility test data (unit: mm). Reference Monitoring Image Devices Image Acquisition Modules ReferenceAcquisition Monitoring Modules Image Devices Image Acquisition Modules NumberAcquisition 1 Modules 2 3 4 InitialNumber laser spot coordinates (92.02,1 48.13) (69.27,2 66.21) (80.42,3 60.30) (73.89,4 58.62) Final laser spot coordinates (97.18, 48.15) (73.36, 66.18) (62.22, 60.37) (55.12, 58.70) HorizontalInitial laser displacement spot coordinates before compensation (92.02, 48.13) 5.16 (69.27, 66.21) 4.09 (80.42, 18.20 60.30) (73.89, 18.71 58.62) HorizontalFinal laser displacement spot coordinates after compensation (97.18, 48.15) — (73.36, 66.18) — (62.22, 0.09 60.37) (55.12, 0.14 58.70) Horizontal displacement before 5.16 4.09 18.20 18.71 Accordingcompensation to Table5 , this compensation method can effectively reduce the error caused by small Horizontal displacement after sudden vibrations of the laser transmitter— module. — 0.09 0.14 compensation 4.8. Field Test AccordingTo verify theto Table stability 5, this and compensation accuracy of the method monitoring can effectively system, thereduce system the waserror taken caused to theby smallXingyuan sudden Town vibrations Tunnel inof Heilongjiangthe laser transmitter province module. for a field test. As shown in Figure 24, the tunneling environment was adverse and filled with dust, darkness 4.8. Field Test and sudden vibration during construction. Tunnel trolley, as shown in Figure 24a, ran between initial supportTo verify and secondary the stability lining; and this accuracy may block of the the lasermonitoring transmitting system, path, the sosystem that the was laser taken transmitter to the Xingyuanmodule was Town fixed Tunnel on the in earth Heilongjiang close to theprovince side face; for therefore,a field test. the proposed monitoring system had no conflictAs shown with in construction Figure 24, the as showntunneling in Figure environmen 24c. t was adverse and filled with dust, darkness and suddenAs shown vibration in Figure during 24b,d, construction. the dust is heavy, Tunnel this trolley, may decrease as shown the laserin Figure spot energy24a, ran received between by initialthe image support acquisition and secondary module, lining; so that this the lasermay transmitterblock the laser module transmitting used 500 path, mw highso that power the laserslaser transmitterwith an optical module lens was to achieve fixed on better the earth long-distance close to the transmission side face; therefore, effect. In addition, the proposed the distance monitoring from systemthe laser had transmitter no conflict module with construction to the measured as shown points in was Figure varying 24c. from 30 m to 80 m, and the laser spot energyAs shown decreased in Figure with increasing24b,d, the dust distance, is heavy, so that this a may laser decrease energy transmissionthe laser spot testenergy with received difference by thereceiving image distancesacquisition was module, conducted so that in thethe dustylaser transmitter tunnel to detect module the imageused 500 quality mw high acquired power by lasers image withacquisition an optical module. lens to The achieve test data better are long-distance shown as Table transmission6 effect. In addition, the distance from the laser transmitter module to the measured points was varying from 30 m to 80 m, and the laser spot energy decreased with increasing distance, so that a laser energy transmission test with difference receiving distances was conducted in the dusty tunnel to detect the image quality acquired by image acquisition module. The test data are shown as Table 6. Appl. Sci. 20182018,, 88,, x 2579 FOR PEER REVIEW 1818 of of 23 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 18 of 23

(a) (b)

Figure 24. Tunnel environment(c) during construction. (a) tunnel construction(d) trolley; (b) dusty and Figuredark tunneling 24. TunnelTunnel environment; environmentenvironment during( cdu) ringdeformation construction. construction. monitoring (a )( Tunnela) tunnel during construction construction construction; trolley; trolley; (b ()d dusty()b )tunnel dusty and darkfaceand darkenvironment.tunneling tunneling environment; environment; (c) deformation (c) deformation monitoring monitoring during construction; during construction; (d) tunnel face(d) environment.tunnel face environment. TableTable 6. Laser energy transmission test data.

ReceivingReceiving GrayGray Value Value ofTable of 6.Receiving LaserReceiving energy transmissionGrayGray ValueValue test of of data.Receiving Receiving GrayGray Value Value of of Distance (m) Laser Spot Distance (m) Laser Spot Distance (m) Laser Spot DistanceReceiving (m) GrayLaser Value Spot of DistanceReceiving (m) GrayLaser Value Spot of DistanceReceiving (m) GrayLaser Value Spot of Distance20 20 (m) Laser255 255Spot Distance45 45 (m) Laser 246246 Spot Distance 7070 (m) Laser 185185 Spot 25 255 50 233 75 171 2025 255 4550 246233 7075 185171 30 30255 255 55 55 221221 8080 169169 25 35255 255 50 60 210233 8575 155171 35 255 60 210 85 155 30 40255 255 55 65 196221 9080 139169 3540 255 6065 210196 8590 155139 40 255 65 196 90 139 According toto TableTable6 ,6, laser laser spot spot energy energy decreased decreas withed with increasing increasing receiving receiving distance, distance, but the but laser the laserspot According couldspot could still betostill Table obtained be obtained 6, laser by image spotby image energy acquisition acquisit decreas module,ioned module, with and increasing theand graythe grayreceiving value value of the distance, of laserthe laser spot but spot metthe lasermetmeasurement measurement spot could requirements; still requirements; be obtained this thisby indicates image indicates thatacquisit that theion proposedthe module,proposed system and system the isnot gray is susceptiblenot value susceptible of the to dustylaser to dusty spot and metanddark darkmeasurement environment. environment. requirements; The The laser laser spot thisspot image indicates image obtained obtained that in the 70 in proposed m 70 receiving m receiving system distance distance is not is shownsusceptible is shown in Figure in to Figure dusty 25a, and25a, darkand the highthe environment. high power power laser The laser with laser with optical spot optical lensimage lens is shownobtained is shown in in Figure in70 Figure m 25 receivingb. 25b. distance is shown in Figure 25a, and the high power laser with optical lens is shown in Figure 25b.

(a) (b)

Figure 25. (a) laser Laser spot spot image image( ain in) 70 70 m m receiving receiving distance; distance; ( (bb)) high high power power(b) laser laser with with optical optical lens. lens. Figure 25. (a) laser spot image in 70 m receiving distance; (b) high power laser with optical lens. InIn order order to verify verify the the stability stability and and accuracy accuracy of of the the monitoring monitoring system, system, a a field ﬁeld test test was was conducted conducted inin the theIn tunneling tunneling order to verifyenvironment. environment. the stability The The andimage image accuracy acquisition acquisition of the module module monitoring was was fixed ﬁxedsystem, on athe the field edge edge test of of was initial initial conducted support support inof the the tunnelingtunnel, as environment.shown in Figure The 2626a. imagea. acquisition module was fixed on the edge of initial support of the tunnel, as shown in Figure 26a. Appl. Sci. 2018, 8, 2579 19 of 23 Appl. Sci. 2018,, 8,, xx FORFOR PEERPEER REVIEWREVIEW 19 of 23

(a) (b)

FigureFigure 26. ((aa)) measured Measured points; points; ( (bb)) image image acquisition acquisition module module mounted mounted on on measured measured points.

As shown inin FigureFigure 27 27,, a a total total station station was was used used to to synchronously synchronously measure measure the the deformation deformation of the of thesame same measured measured point point in the in tunnel the tunnel every every 1 h, while 1 h, while the system the system conducted conducted real-time real-time monitoring monitoring during duringconstruction construction as shown as inshown Figure in 28Figure. 28.

FigureFigure 27. 27. TotalTotalTotal stationstation measurement.measurement.

Figure 28. SystemSystem measurement. measurement.

InIn thethe ﬁeldfield test, test, a a large large number number of dataof data were were obtained, obtained, and partand ofpart the of system the system measurement measurement results resultscorresponding corresponding with the with station the station measurement measurement results areresults presented are presented in Table in7. Table 7.

Appl. Sci. 2018, 8, 2579 20 of 23

Table 7. Partial ﬁeld test data.

Deformation from Deformation from No. Time Total Station (mm) Monitoring System (mm) 1 16:00 0 0 2 17:00 2.51 2.46 3 18:00 4.90 4.95 4 19:00 6.21 6.22 5 20:00 7.10 7.15 6 21:00 8.01 7.99

According to Table6, the monitoring system and total station measurement results are close, which indicates that the monitoring system has good accuracy, stability, and repeatability in a ﬁeld application.

5. Discussion

5.1. About Data Compensation Method for Environment Vibration The data compensation method for environment vibration is effective for small and sudden vibration. When the vibration of the laser transmitter module is large and continues for a while, the spot position in the reference monitoring image acquisition modules will change and reflect the vibration; then, the system will stop the measurement for the measured points until the vibration of the laser transmitter module stops, filtering out the influence of the vibration on the accuracy of the system.

5.2. Drawbacks and Improvements of Simulation Environment Because of the complex tunnel environment, the veriﬁcation test environment needs to be discussed for such a complicated engineering mechanics problem.

(1) For the principle verification test, image acquisition module calibration experiment and wireless antenna attitude test, a relative stable environment was needed; thus, these tests were conducted in the laboratory reasonably; (2) For the data processing test and temperature change test, a real tunnel environment was needed. However, because of the busy, dusty and vibrational construction sites, it was not only hard to study effects of single factor, but also inconvenient to conduct the tests in such a complex tunnel environment. Therefore, the tests were conducted in laboratory and studied without dust, and it would be better to simulate a dusty environment during the lab test; (3) For the system accuracy test, the deformation was simulated by a micro motion platform with a 1 µm accuracy. The micro motion platform was placed on a table and moved horizontally. However, for tunnel settlement deformation, it settled vertically. Given the isotropy of the image sensor for vertical and horizontal direction, the test was reliable, and the field test verified this adequately. In addition, it would be better to use a vertical micro motion platform for the deformation simulation in a vertical direction; (4) For the laser transmitter feasibility test, this test was conducted in a foggy night; thus, the simulation environment was similar to a tunnel, a dusty and dark environment in low visibility. (5) For the field test, the environment is dusty and even wet. In order to prevent the interference of tunnel dust and water mist, dirt shrouds and waterproof devices were used in laser transmitter module, image acquisition module and some electric parts. To improve the permeability of the laser beam, an automatic device needs to be added to clear the mud consisting of dust and water mist on the laser receiving screen and laser transmitter module glass window. Appl. Sci. 2018, 8, 2579 21 of 23

5.3. About Diffraction Rings of Laser Spots In order to improve a laser transmitting effect, a set of optical lens were used, and this produced additional diffraction rings, as shown in Figure 25a. Precisely because of the dusty environment, the diffraction effect decreased through the dust [40], and this had advantages in the image process.

5.4. About Application of the Proposed System The proposed system was designed for tunnel deformation monitoring in construction, but it is also suitable for the tunnel that has already been built. However, because of the heavy traffic or trains’ passage, an unavoidable noise is produced in operating tunnels, and the noise intensity is different and relates to traffic volume, vehicle speed and vehicle type. In order to solve this problem in the application of operating tunnels, a noise prediction model based on traffic volume, vehicle speed and vehicle type needs to be studied [41]. With this prediction model, a data compensation method should be proposed to compensate for the measurement error caused by unavoidable noise.

6. Conclusions This paper presents a novel method based on laser and machine vision to automatically measure tunnel deformation of multiple interest points in real time and effectively compensate for the environment vibration, and, moreover, it can overcome inﬂuence of dusty and dark environments in low visibility. An automatic and wireless real-time tunnel deformation monitoring system that is based on laser and machine vision, and can give early warnings for tunnel collapse accidents, is proposed. Compensation methods are proposed to reduce the measurement errors caused by laser beam feasibility, temperature, air refraction index, and wireless antenna attitude. Experimental results show that the proposed monitoring system has a high detection accuracy, which can compensate for the shortcomings of the traditional tunnel deformation monitoring method in highly automatic real-time monitoring. The proposed system can also avoid the conﬂict with tunnel construction, that is, it realizes real-time monitoring during construction. Moreover, the monitoring system has good stability and repeatability. The detection accuracy is better than 0.12 mm, the repeatability accuracy is less than 0.11 mm, and the minimum resolution is 10 µm.

Author Contributions: Z.Q. conceived the method and revised the paper; H.L. designed the experiments and wrote the paper; W.H. and C.W. designed mechanical and electrical structure; W.H. and J.L. designed and conducted the field test; W.H., J.L., and Q.S. developed system software and improved the experimental results. Funding: This research was financially supported by the National Natural Science Foundation of China (NSFC) (No: 51775378) and the National Key R&D Program of China (No.2017YFF0108102). Conflicts of Interest: The authors declare no conflict of interest.

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