Wheel Alignment of a Suspension Module Unit Using a Laser Module

sensors

Article Wheel Alignment of a Suspension Module Unit Using a Laser Module

Seong Han Kim 1 and Kang In Lee 2,*

1 School of Intelligent Mechatronics Engineering, Sejong University, 209 Neungdong-ro, Gunja-dong, Gwangjin-gu, Seoul 05006, Korea; [email protected] 2 R&H Research Lab, Hyundai Motor Company, 150, Hyundaiyeonguso-ro, Namyang-eup, Hwaseong-si, Gyeonggi-do 18280, Korea * Correspondence: [email protected]; Tel.: +82-31-5172-5162; Fax: +82-31-368-5748

Received: 19 January 2020; Accepted: 11 March 2020; Published: 16 March 2020

Abstract: Vehicle wheel alignment inspection is generally carried out using a computer vision-based system. Due to its inspection mechanism using four wheel centers, the computer vision-based system cannot be applied to the wheel alignment inspection of suspension module units. However, when a vehicle suspension module is being developed, there is no complete car ready for wheel alignment inspection even though it is a very important procedure for suspension property tests. This study proposes a novel and efficient way to inspect vehicle wheel alignment for suspension modules. Two laser modules and several mechanical jigs were employed for wheel alignment inspection, allowing the toe and camber angles of the suspension module to be measured. For accurate wheel alignment results, calibration of the laser modules was performed prior to the inspection. This calibration procedure adjusts the yaw and pitch angles of the laser module so that they can be orthogonal to the mounting jig. For the calibration, a novel method of using laser straightness was adopted and, consequently, 0.02 degrees of orthogonality was achieved. The wheel alignment inspection results were determined then verified using a vision system with two cameras. In order to use this vision system, two cameras were used and a new method of modifying the measurement mechanism was developed. According to the verification results, the proposed wheel alignment inspection provided very high measurement accuracy. The wheel alignment inspection mechanism proposed in this study can not only give very reliable results but also provide a cost-efficient method of inspecting the wheel alignment of suspension modules to automakers.

Keywords: Wheel alignment inspection; Laser module; Vision system for wheel alignment inspection

1. Introduction Vehicle wheel alignment inspection is generally conducted in automobile maintenance shops. It includes the measurement of the steering axis inclination (SAI), caster angle, camber angle, and toe angle. SAI (also known as king pin inclination (KPI)) and caster angle are the inward and backward tilt, respectively, of the suspension toward the center of the vehicle. They are determined by the positions of the upper and lower ball joint pivots, which are geometrically unchanged and thus not adjustable once the vehicle has been designed. Camber angle is the inward or outward tilt of the front tires as viewed from the front, and toe angle is the side-to-side diﬀerence in distance between the front and rear of the front tires. These camber and toe angles are adjustable and should be continuously managed by the vehicle owner, because if a vehicle has incorrect camber and toe angles, the vehicle can experience several issues such as uneven tire wear, steering wheel pulling, steering wheel shimmy, and sometimes serious vehicle vibration [1,2].

Sensors 2020, 20, 1648; doi:10.3390/s20061648 www.mdpi.com/journal/sensors Sensors 2020, 20, 1648 2 of 13

Wheel alignment inspection in automobile maintenance shops is generally conducted by a computer vison-based system [3,4] comprising specially designed target boards and image acquisition modules, including charged coupled device (CCD) video cameras and infrared light emitting diodes (IR LEDs) as illuminants [5,6]. Wheel alignment inspection using this computer vision-based system needs a full car, which means it needs four wheels for the inspection. The detailed inspection method is described in Section2. When a new vehicle goes into production, its mechanical and electronic modules are manufactured and tested before they are installed in the vehicle. For instance, a suspension module, which is composed of a sub-frame, suspension springs, shock absorbers, lower and upper arms, knuckles, rubber bushes, stabilizer links, and a stabilizer bar, is produced and tested before it is installed into the vehicle. Figure1 shows a suspension module that is undergoing preliminary tests. The preliminary tests generally include dynamic performance tests, kinematic and compliance (K&C) tests, and durability tests [7,8]. These tests are mainly affected by the geometrical dimensions and mechanical strength of the module parts, and the shape and stiffness of the rubber bushes. In particular, the initial shape deformation of the rubber bushes affects the test results due to preloads and wheel alignment and, accordingly, the preloads on the rubber bushes and the wheel alignment should be thoroughly managed before the tests. Preloads on rubber bushes are usually caused by two factors—suspension module assembly procedures and incorrect wheel alignment [9]. Whereas preloads using assembly procedures can be removed by run-in procedures, preloads by wrong wheel alignment are difficult to remove. Furthermore, when a full vehicle is not yet available, the computer vision-based system widely used in automobile maintenance shops cannot be applied to the wheel alignment inspection because it needs four wheels for the inspection.

Figure 1. Suspension module under preliminary tests.

2. OverviewThere have of Typical been several Wheel studies Alignment that can Inspection be applied tofor the a wheelFull Car alignment inspection of suspension modules. Dongyoub Baek has proposed a simple and inexpensive method using a consumer-grade The typical wheel alignment inspection for a full car uses a computer vision-based system depth-sensing camera such as Kinect to replace the computer vision-based system [5]. The method [14,15]. Figure 2 shows the overall structure of the typical wheel alignment inspection system, which proposed in his study generates point clouds within a region of interest (ROI) that contain the consists of one vehicle lift with four turnplates, four target boards, image acquisition modules geometrical information of the wheel. He verified the developed method by comparing the wheel including four pairs of CCD cameras and IR LEDs illuminants, and a computer console with software alignment results with the results from the existing method. Jieh-Shian Young has proposed a micro including various wheel setting specifications. For the wheel alignment inspection of a vehicle, the control unit (MCU)-based camber inspection system with a 3-axis accelerometer [10]. He emphasizes vehicle is placed on the lift. Each wheel should be positioned on each turnplate of the lift to have four the fact that the axes of the accelerometer can be misaligned to the axes of the camber inspection system wheels steered freely. Afterward, one target board is attached onto each wheel. The target boards and, therefore, in his study he proposes the calibration method to amend these axis misalignments. have materials that reflect the illumination from the IR LEDs so the wheel angles (i.e., toe and camber Rocco Furferi has proposed a 3D machine vision-based system for the contactless reconstruction of angles) can be measured by the CCD cameras. All wheel alignment values are transmitted to the vehicle wheel geometry, with a particular reference to characteristic planes [11]. The effectiveness computer console, which displays the measured values. of the proposed method in his study was verified by comparing the method with the measurement results using a commercial 3D scanner.

Figure 2. Overall structure of a typical wheel alignment inspection system for a full car [3].

When all target boards are mounted on the wheels, the technician turns the steering wheel to the left and right ends to determine the neutral position of the front wheels. Then, the steering wheel is positioned at the neutral position and the technician moves the vehicle back and forth. By moving the vehicle back and forth, each plane of four wheels (Planes A1, A2, A3, and A4) and their normal vectors (nA1, nA2, nA3, and nA4) are obtained, as shown in Figure 3a. Along with the normal vectors, the centers of Planes A1–A4 are also determined, and these centers create another plane (Plane B) and its normal vector (nB), as shown in Figure 3b. The typical wheel alignment method using a computer vision-based system uses Plane A and B and their normal vectors to measure the wheel alignment. Figure 4 describes how to measure the camber and toe angles of a vehicle. When there is an angle, α, between nA and nB, 90-α is the camber angle. A total of four camber angles can be obtained from each plane of the wheels. On the other hand, when nA is projected onto Plane B, it creates a vector (nA'), and the angle between nA' and the centerline of two front wheels is the toe angle of the front wheels. A total of four toe angles can be obtained. Sensors 2020, 20, 1648 3 of 13

This study proposes a new and cost-efficient method to inspect vehicle wheel alignments for suspension modules. The proposed method employs two laser modules that are mounted into the wheel knuckles. With the laser modules, toe and camber angles can be measured by measuring the distance of the laser mark projected on the opposite side of the laser mounting jig. For accurate wheel alignment results, the calibration procedure of the laser module—which makes the laser module perfectly orthogonal to the laser mounting jig—was performed before wheel alignment inspection. After the wheel alignment inspection, the results were verified by a vision system. For the verification, a new method using complementary metal-oxide semiconductor (CMOS) cameras was also developed. The wheel alignment inspection method proposed in this study can provide very accurate results and a cost-efficient way for automakers to inspect the wheel alignment of suspension modules. In addition, it can be also applied in the wheel alignment inspection of motorcycles and unconventional automobiles [12,13]. Figure 1. Suspension module under preliminary tests.

2. Overview Overview of of Typical Typical Wheel Alignment Inspection for a Full Car The typicaltypical wheelwheel alignment alignment inspection inspection for for a full a fu carll usescar uses a computer a computer vision-based vision-based system system [14,15]. [14,15].Figure2 Figureshows 2 the shows overall the structure overall structure of the typical of th wheele typical alignment wheel alignment inspection inspection system, which system, consists which of consistsone vehicle of liftone with vehicle four turnplates,lift with four four turnplates, target boards, four image target acquisition boards, modulesimage acquisition including fourmodules pairs includingof CCD cameras four pairs and of IR CCD LEDs cameras illuminants, and IRand LEDs a computerilluminants, console and a withcomputer software console including with software various includingwheel setting various speciﬁcations. wheel setting For specifications. the wheel alignment For the inspection wheel alignment of a vehicle, inspec thetion vehicle of a isvehicle, placed the on vehiclethe lift. is Each placed wheel on shouldthe lift. beEach positioned wheel should on each be turnplate positioned of theon each lift to turnplate have four of wheels the lift steered to have freely. four wheelsAfterward, steered one targetfreely. boardAfterward, is attached one target onto each board wheel. is attached The target onto boards each wheel. have materials The target that boards reﬂect havethe illumination materials that from reflect the IR the LEDs illumination so the wheel from angles the IR (i.e., LEDs toe so and the camber wheel angles angles) (i.e., can toe be measuredand camber by angles)the CCD can cameras. be measured All wheel by alignmentthe CCD valuescameras. are All transmitted wheel alignment to the computer values console,are transmitted which displays to the computerthe measured console, values. which displays the measured values.

FigureFigure 2. 2. OverallOverall structure structure of of a a typical typical wheel wheel alignmen alignmentt inspection inspection system for a full car [3]. [3].

When all target boards are mounted on the wheels, the technician turns the steering wheel to the When all target boards are mounted on the wheels, the technician turns the steering wheel to left and right ends to determine the neutral position of the front wheels. Then, the steering wheel is the left and right ends to determine the neutral position of the front wheels. Then, the steering wheel positioned at the neutral position and the technician moves the vehicle back and forth. By moving is positioned at the neutral position and the technician moves the vehicle back and forth. By moving the vehicle back and forth, each plane of four wheels (Planes A1, A2, A3, and A4) and their normal the vehicle back and forth, each plane of four wheels (Planes A1, A2, A3, and A4) and their normal vectors (nA1, nA2, nA3, and nA4) are obtained, as shown in Figure3a. Along with the normal vectors, vectors (nA1, nA2, nA3, and nA4) are obtained, as shown in Figure 3a. Along with the normal vectors, the the centers of Planes A1–A4 are also determined, and these centers create another plane (Plane B) and centers of Planes A1–A4 are also determined, and these centers create another plane (Plane B) and its its normal vector (nB), as shown in Figure3b. The typical wheel alignment method using a computer normal vector (nB), as shown in Figure 3b. The typical wheel alignment method using a computer vision-based system uses Plane A and B and their normal vectors to measure the wheel alignment. vision-based system uses Plane A and B and their normal vectors to measure the wheel alignment. Figure4 describes how to measure the camber and toe angles of a vehicle. When there is an angle, α, Figure 4 describes how to measure the camber and toe angles of a vehicle. When there is an angle, α, between nA and nB, 90-α is the camber angle. A total of four camber angles can be obtained from each between nA and nB, 90-α is the camber angle. A total of four camber angles can be obtained from each plane of the wheels. On the other hand, when nA is projected onto Plane B, it creates a vector (nA’), plane of the wheels. On the other hand, when nA is projected onto Plane B, it creates a vector (nA'), and the angle between nA’ and the centerline of two front wheels is the toe angle of the front wheels. and the angle between nA' and the centerline of two front wheels is the toe angle of the front wheels. A total of four toe angles can be obtained. TheThe camber camber and and toe toe angles angles of of a avehicle vehicle are are syst systematicallyematically managed managed by by the the vehicle’s vehicle’s automaker automaker becausebecause they they can can cause cause uneven uneven tire tire wear, wear, steer steeringing wheel wheel pulling, pulling, steering steering wheel wheel shimmy, shimmy, and and sometimessometimes serious serious vehicle vehicle vibrations. vibrations. Camber Camber angl angleses are are generally generally within within ±2 ±2 degrees, degrees, but but most most modernmodern passenger passenger cars cars generally generally have have negative negative camber camber angles, angles, whereas whereas toe toe angles angles relatively relatively have have a a Sensors 2020, 20, 1648 4 of 13 smallsmall angle angle range range of of ± ±0.2 0.2 degrees degrees [2,16]. [2,16].

FigureFigureFigure 3. 3. Creation 3. CreationCreation of of wheel wheel planes planesplanes and and normalnormal normal vectors. vectors. vectors. (a ()a Plane()a Plane) Plane A. A. (b A.) ( Planeb ()b Plane) Plane B. B. B.

Figure 4. Camber and toe angles from Planes A and B. (a) Camber angle. (b) Toe angle. FigureFigure 4. 4. Camber Camber and and toe toe angles angles from from Planes Planes A A and and B. B. (a ()a Camber) Camber angle. angle. (b ()b Toe) Toe angle. angle. The camber and toe angles of a vehicle are systematically managed by the vehicle’s automaker 3.3. Wheel Wheelbecause Alignment Alignment they can cause Inspection Inspection uneven tire for for wear, Suspension Suspension steering wheel Module Module pulling, steering wheel shimmy, and sometimes serious vehicle vibrations. Camber angles are generally within 2 degrees, but most modern passenger The typical wheel alignment inspection in Section 2 requires± four wheels, because the centers of carsThe generallytypical wheel have negative alignment camber inspection angles, whereasin Section toe 2 angles requires relatively four wheels, have a small because angle the range centers of of four wheels form a plane and the camber and toe angles are measured with respect to this plane. four wheels0.2 degrees form [2 ,a16 plane]. and the camber and toe angles are measured with respect to this plane. However,However,± when when a asuspension suspension module module is is in in develop developmentment for for a anew new vehicle vehicle and and its its K&C K&C characteristics characteristics needneed3. to Wheelto be be tested tested Alignment for for performance performance Inspection evaluation forevaluation Suspension purpos purpos Modulees,es, the the typical typical wheel wheel alignment alignment inspection inspection cannot cannot bebe applied appliedThe because typicalbecause wheel the the suspension alignmentsuspension inspection module module inhas has Section only only2 tworequires two wheels. wheels. four wheels,Theref Therefore, becauseore, a anew thenew centersmethod method of of inspectinginspectingof four the wheels the wheel wheel form alignment alignment a plane and is is ne the needed cambereded for for andsingle single toe suspension angles suspension are measured modules. modules. with respect to this plane. However, when a suspension module is in development for a new vehicle and its K&C characteristics 3.1.3.1. Wheelneed Wheel to Alignment beAlignment tested forInspection Inspection performance Mechanism Mechanism evaluation purposes, the typical wheel alignment inspection cannot be applied because the suspension module has only two wheels. Therefore, a new method of inspecting TheThe proposedproposed wheelwheel alignmentalignment inspectioninspection emplemploysoys laserlaser modules.modules. AA pairpair ofof preciselyprecisely the wheel alignment is needed for single suspension modules. manufacturedmanufactured jigs jigs are are mounted mounted into into both both knuckles, knuckles, as as shown shown in in Figure Figure 5, 5, and and two two laser laser modules modules are are orthogonallyorthogonally3.1. Wheel mounted Alignment mounted in Inspection in the the jigs. jigs. Mechanism The The laser laser modules modules face face each each other other and, and, if if the the laser laser beams beams from from two two sources coincide with each other, it means that the camber and toe angles of both wheels are zero, as sources coincideThe proposed with each wheel other, alignment it means inspection that the camber employs and laser toe modules. angles of both A pair wheels of precisely are zero, as shown in Figure 6. shownmanufactured in Figure 6. jigs are mounted into both knuckles, as shown in Figure5, and two laser modules are orthogonally mounted in the jigs. The laser modules face each other and, if the laser beams from two sources coincide with each other, it means that the camber and toe angles of both wheels are zero, as shown in Figure6. Sensors 2020, 20, 1648 5 of 13

FigureFigure 5. Wheel5. Wheel knuckle knuckle and and mounted mounted jig. jig.

FigureFigure 6. Zero6. Zero camber camber and and toe toe angles. angles.

FigureFigure7 7describes 7describes describes a toe a angleatoe toe angle measurementangle measurement measurement according according according to the proposed to tothe the proposed wheel proposed alignment wheel wheel inspection.alignment alignment Wheninspection.inspection. a toe angleWhen Whenoccurs a toe a toe angle at angle the occurs left occurs wheel, at atthe the the left left laser wheel, wheel, module the the laserin laser the module module left wheel in inthe isthe left horizontally left wheel wheel is ishorizontally steered horizontally and, accordingly,steeredsteered and, and, itsaccordingly, laseraccordingly, mark onits its thelaser laser right mark mark knuckle on on the the jig ri horizontallyghtright knuckle knuckle movesjig jig horizontally horizontally away from moves themoves right away away laser from from source. the the Byrightright measuring laser laser source. source. the horizontalBy By measuring measuring distance the the horizontal between horizontal the distance distance source between and between the laserthe the source mark, source and the and toethe the anglelaser laser ofmark, mark, the leftthe the wheeltoetoe angle angle can of be ofthe measured. the left left wheel wheel The can can toe be anglebe measured. measured. can be The calculated The toe toe angle angle using can can Equationbe be calculated calculated (1). using using Equation Equation (1). (1). 𝑑 1 dt 𝑑 θθθtan=tantan− (1)(1)(1) L𝐿 𝐿 wherewherewhere d𝑑t is𝑑 is the isthe the horizontal horizontal horizontal distance distance distance between between between a a laser lasera laser sourceso sourceurce andand and thethe the markmark mark fromfrom from the the other other laser laser source, source, L𝐿 is𝐿 is wheel iswheel wheel tracktrack track width,width, width,and and andθ θis θ is right isright right or or left or left left toe toe angle.toe angle. angle. In In thethe the samesame same manner,manner, manner, whenwhen when aa cambercambera camber angleangle angle occursoccurs occurs atat at thethe the leftleft left wheel,wheel, wheel, thethe the laserlaser laser modulemodule module in in thethe the leftleft left wheelwheelwheel is verticallyis vertically steered steered and, and, accordingly,accordingly, accordingly, thethe the laserlaser laser markmark mark verticallyvertically vertically movesmoves moves away away from from the the right right laser laser source.source. By By measuringmeasuring measuring thethe the verticalvertical vertical distance distance distance between between between the the the source source source and and and laser laser laser mark, mark, mark, the the camber the camber camber angle angle angle can can be can obtained.bebe obtained. obtained. The The only The only dionlyﬀ erencedifference difference between between between toe and toe toe camberand and ca camber anglesmber angles isangles the is direction isthe the direct direct of laserionion ofmarks, oflaser laser marks, as marks, shown as as inshownshown Figure in 8 inFigure. TheFigure camber 8. 8.The The camber angle camber can angle angle be calculated can can be be calculated calculated using Equation using using Equation (2).Equation (2). (2).

𝑑𝑑 dc (2) φtanϕφtan= tan 1 (2)(2) − L𝐿 𝐿 wherewhere 𝑑 𝑑 is isthe the vertical vertical distance distance between between a lasera laser source source and and the the mark mark from from the the other other laser laser source, source, where dc is the vertical distance between a laser source and the mark from the other laser source, and and φ is left or right toe angle. ϕ andis left φ or is right left or toe right angle. toe angle. Sensors 2020, 20, 1648 6 of 13

Figure 7. Toe angle measurement. (a) Left toe. (b) Right toe. Figure 7. Toe angle measurement. (a) Left toe. (b) Right toe. FigureFigure 7. 7. Toe Toe angle angle measurement. measurement. ( a(a) )Left Left toe. toe. ( b(b) )Right Right toe. toe.

Figure 8. Direction of laser mark by toe and camber angles. Figure 8. Direction of laser mark by toe and camber angles. 3.2. Laser Module Setup andFigureFigure Calibration 8. 8. Direction Direction of of laser laser mark mark by by toe toe and and camber camber angles. angles. 3.2. Laser Module Setup and Calibration 3.2.3.2. Laser ForLaser accurate Module Module Setup wheelSetup and and alignment Calibration Calibration inspection, each laser module should be orthogonally mounted into eachForaccurate jig. If a wheellaser module alignment is not inspection, perfectly each orthogonal laser module to the should jig, then be orthogonally measurement mounted errors can into ForFor accurate accurate wheel wheel alignment alignment inspection, inspection, each each laser laser module module should should be be orthogonally orthogonally mounted mounted result.each jig. Especially If a laser modulefor toe angles is not perfectlywhose setting orthogonal range to is the usually jig, then from measurement −0.2 to +0.2 errors degrees, can result. the intointo each each jig. jig. If If a a laser laser module module is is not not perfectly perfectly orthogonal orthogonal to to the the jig, jig, then then measurement measurement errors errors can can orthogonalityEspecially for toeof the angles laser whose modules setting is very range crucial is usually to the from measurement0.2 to +0.2 results. degrees, In the this orthogonality study, a laser of result.result. EspeciallyEspecially forfor toetoe anglesangles whosewhose settingsetting rangerange isis usually−usually fromfrom −−0.20.2 toto +0.2+0.2 degrees,degrees, thethe modulethe laser with modules an adjustable is very crucial beam size to the was measurement chosen in order results. to prevent In this the study, measurement a laser module inaccuracies with an orthogonalityorthogonality of of the the laser laser modules modules is is very very crucial crucial to to the the measurement measurement results. results. In In this this study, study, a a laser laser thatadjustable can be beam caused size by was a chosenlarge beam in order size. to The prevent minimum the measurement diameter of inaccuracies the beam thatis 1 mm can bewhich caused is modulemodule with with an an adjustable adjustable beam beam size size was was chosen chosen in in order order to to prevent prevent the the measurement measurement inaccuracies inaccuracies adjustedby a large by beam a screw size. head. The The minimum specifications diameter of ofthe the laser beam module is 1 mm usedwhich in this is study adjusted are byshown a screw in Table head. thatthat cancan bebe causedcaused byby a a large large beambeam size.size. TheThe minimumminimum diameterdiameter ofof thethe beambeam isis 1 1 mm mm whichwhich is is 1.The speciﬁcations of the laser module used in this study are shown in Table1. adjustedadjusted by by a a screw screw head. head. The The specifications specifications of of the the la laserser module module used used in in this this study study are are shown shown in in Table Table AfterAfter the the laser laser module module is is mounted mounted in in the the jig, jig, it itss yaw yaw and and pitch pitch angles angles should should be be adjusted adjusted for for 1.1. completecomplete orthogonality. orthogonality. This This study study used used a a kinematic kinematic adapter adapter that that was was installed installed between between the the laser laser AfterAfter the the laser laser module module is is mounted mounted in in the the jig, jig, it itss yaw yaw and and pitch pitch angles angles should should be be adjusted adjusted for for modulemodule and and jig. jig. This This kinematic kinematic adapter adapter is is designed designed no nott only only to to facilitate facilitate the the integration integration of of the the laser laser completecomplete orthogonality. orthogonality. This This study study used used a a kinematic kinematic adapter adapter that that was was installed installed between between the the laser laser modulemodule in in the the jig, jig, but but also also to to provide provide pitch pitch and and yaw yaw adjustments. adjustments. By By tightening tightening or or loosening loosening the the pitch pitch modulemodule and and jig. jig. This This kinematic kinematic adapter adapter is is designed designed no not tonly only to to facilitate facilitate the the integration integration of of the the laser laser andand yaw yaw screws screws (as (as shown shown in in Figure Figure 9a),9a), the the laser laser module module can can be be completely completely orthogonal orthogonal to tothe the jig. jig.In modulemodule in in the the jig, jig, but but also also to to provide provide pitch pitch and and yaw yaw adjustments. adjustments. By By tightening tightening or or loosening loosening the the pitch pitch additionIn addition to the to thekinematic kinematic adapter, adapter, a pair a pair of radial of radial ball ball bearings bearings were were also also used used for forthe theintegration integration of andand yaw yaw screws screws (as (as shown shown in in Figure Figure 9a), 9a), the the laser laser module module can can be be completely completely orthogonal orthogonal to to the the jig. jig. In In theof thelaser laser module, module, because because the the procedure procedure ofof calibrat calibratinging the the laser laser module module requires requires rotating rotating the the laser laser additionaddition to to the the kinematic kinematic adapter, adapter, a a pair pair of of radial radial ball ball bearings bearings were were also also used used for for the the integration integration of of modulemodule with with respect respect to to the the jig. jig. By By using using two two radial radial ball ball bearings bearings in series in series (as shown (as shown in Figure in Figure 9b), 9theb), thethe laser laser module, module, because because the the procedure procedure of of calibrat calibratinging the the laser laser module module requires requires rotating rotating the the laser laser bearingthe bearing rotational rotational error error caused caused by th bye bearing the bearing tolerance tolerance can canbe prevented. be prevented. modulemodule with with respect respect to to the the jig. jig. By By using using two two radial radial ball ball bearings bearings in in series series (as (as shown shown in in Figure Figure 9b), 9b), the the bearingbearing rotational rotational error error caused caused by by th thee bearing bearing tolerance tolerance can can be be prevented. prevented.

FigureFigure 9. 9. LaserLaser module module for for calibration. calibration. ( (aa) )Pitch Pitch and and yaw yaw adjustment adjustment via via screws. screws. (b (b) )Two Two radial radial ball ball bearingsbearings for for rotating. rotating. FigureFigure 9. 9. Laser Laser module module for for calibration. calibration. ( a(a) )Pitch Pitch and and yaw yaw adjustment adjustment via via screws. screws. ( b(b) )Two Two radial radial ball ball bearingsbearings for for rotating. rotating. Sensors 2020, 20, 1648 7 of 13

Figure 10 shows the laser module calibration procedure. A laser module is mounted in the jig and switched on. The laser beam is projected to the board at a far end and the laser mark creates a circle on the board as the laser module is rotated. The larger the orthogonal error is to the jig, the larger the circle is. By tightening or loosening the pitch and yaw screws, the circle can be decreased to its smallest possible size. The orthogonal error can be calculated by Equation (3). r γ = tan 1 (3) − l where l is the distance from the laser module and a board, r is the diameter of the circle, and γ is the angle between the circle center and the laser module.

Figure 10. Orthogonal calibration of the laser module.

InFigure this study,10 shows a laser the laser beam module was projected calibration onto pr aocedure. board located A laser 8000 module mm is away mounted from in the the laser jig module,and switched achieving on. The a 0.02 laser degree beam orthogonal is projected error. to the board at a far end and the laser mark creates a circle on the board as the laser module is rotated. The larger the orthogonal error is to the jig, the larger the circle is. By tighteningTable or loosening 1. Specifications the pitch of the and laser yaw module. screws, the circle can be decreased to its smallest possible size. The orthogonal error can be calculated by Equation (3). Beam Shape (Collimated) Elliptical Beam Size 𝑟ф1.0 mm~5.0 mm γtan (3) Focused Spot Diameter 𝑙 30 µm Operating Voltage 4.9 V~5.2 V where 𝑙is the distance from the laserWavelength module and a board,630 r nm~645 is the diameter nm of the circle, and γ is the angle between the circle center andPower the laser module. 4.0 mW~5.0 mW In this study, a laser beam was projected onto a board located 8000 mm away from the laser 3.3.module, Experimental achieving Setup a 0.02 degree orthogonal error. Figure 11 shows an experimental setup for the wheel alignment inspection using laser modules. Table 1. Specifications of the laser module. A suspension module for the wheel alignment inspection is mounted onto jigs. The jigs are also screwed to a U-beam that isBeam placed Shape on two(Collimated) posts that are fixedElliptical on a surface table. The knuckle is linked to suspension components such as the lower arm, shock absorber, and trailing arm, and its Beam Size ф 1.0 mm ~ 5.0 mm kinematic motion is geometrically determined by the suspension components. Accordingly, the toe and camber angles of the suspensionFocused module Spot varyDiameter with the vertical30 position μm of the knuckle. In this study, the knuckle was lifted to the vertical position predetermined by the automaker for accurate wheel Operating Voltage 4.9 V ~ 5.2 V alignment inspection, and two hydraulic suspension jacks were used to lift the knuckles. Two wheel alignment measurement jigsWavelength with the calibrated laser modules630 nm were ~ 645 mounted nm onto the knuckles. Proper reflective surfacesPower have been employed in many4.0 mW studies ~ 5.0 using mW laser devices to perform measurements [17,18]. In this study, two laser-detecting cards were attached to each surface where the laser light was projected. These cards were photosensitive and enabled the easy location of ultra-violet (UV)3.3. Experimental and visible lightSetup beams and focal points. Figure 11 shows an experimental setup for the wheel alignment inspection using laser modules. A suspension module for the wheel alignment inspection is mounted onto jigs. The jigs are also screwed to a U-beam that is placed on two posts that are fixed on a surface table. The knuckle is linked to suspension components such as the lower arm, shock absorber, and trailing arm, and its kinematic motion is geometrically determined by the suspension components. Accordingly, the toe and camber angles of the suspension module vary with the vertical position of the knuckle. In this study, the knuckle was lifted to the vertical position predetermined by the automaker for accurate wheel alignment inspection, and two hydraulic suspension jacks were used to lift the knuckles. Two wheel alignment measurement jigs with the calibrated laser modules were mounted onto the knuckles. Proper reflective surfaces have been employed in many studies using laser devices to perform measurements [17,18]. In this study, two laser-detecting cards were attached to each surface where the laser light was projected. These cards were photosensitive and enabled the easy location of ultra- violet (UV) and visible light beams and focal points. Sensors 2020, 20, 1648 8 of 13

Figure 11. Experimental setup for wheel alignment inspection.

4. Results & Verification Verification In order to verifyverify the wheel alignment inspection proposed in this study, a newnew wayway ofof usingusing a vision-based system with two cameras was also developed.developed. As previously mentioned, the computer vision-based inspection system with four CCD cameras that is widely used in mostmost automotiveautomotive maintenance shopsshops needs needs four four wheel wheel centers centers to measureto measure the the toe andtoe and camber camber angles. angles. Accordingly, Accordingly, these cannotthese cannot be applied be applied during during verification. verification. In this In study, this study, two CMOS two CMOS cameras cameras and their and accessoriestheir accessories were adoptedwere adopted for verification. for verification. A suspension module was designated as a targettarget forfor verification.verification. The target’s camber and toe angles werewere measured measured by by the the laser laser and cameraand camera modules, modules, and the and results the wereresults compared. were compared. The proposed The wheelproposed alignment wheel methodalignment using method laser modulesusing laser measures modules camber measures and toecamber angles and by measuringtoe angles theby horizontalmeasuring andthe verticalhorizontal distances and vertical of the laser distances marks. of Accordingly, the laser marks. its sensitivity Accordingly, depends its on sensitivity the wheel trackdepends width on ofthe the wheel suspension track width module. of Inthe the suspension case of the module. target suspension In the case module, of the thetarget laser suspension module’s sensitivitymodule, the was laser 36.53 module’s mm/degree, sensitivity which was means 36.53 that mm/degree, if one degree which shifts means in thethat toe if one or camber degree angleshifts (shownin the toe in or Figure camber8), then angle the (shown laser mark in Figure moves 8), 36.53 then mmthe laser horizontally mark moves or vertically. 36.53 mm horizontally or vertically. 4.1. Verification System Configuration and Inspection Mechanism 4.1. VerificationThe two-camera System optical Configur systemation usedand Inspection for verification Mechanism consists of two CMOS cameras, two reference plates,The two two-camera wheel target optical plates, system an image used data for acquisitionverification device, consists a powerof two source, CMOS and cameras, jigs. It wastwo originallyreference plates, designed two for wheel measuring target vehicleplates, wheelan image movements, data acquisition whereby device, each wheel a power was source, monitored and byjigs. a cameraIt was capturingoriginally thedesigned wheel asfor well measuring as its reference vehicle plate wheel [15 ].movements, The specifications whereby of theeach optical wheel system was aremonitored described by ina Tablecamera2. Thecapturing two wheel the wheel target platesas well are as mountedits reference into plate each knuckle[15]. The of specifications the suspension of module,the optical whereas system the are two described reference inplates Table are2. The fixed two by jigs.wheel Each target reference plates plateare mounted is positioned into closeeach toknuckle each wheelof the targetsuspension plate, module, but usually whereas in the the front two of re theference wheel plates target are plate, fixed asby shown jigs. Each in Figurereference 12. Theplate cameras is positioned are fixed close at to a distanteach wh positioneel target from plate, the but suspension usually in module the front so thatof the they wheel can target capture plate, the wheelas shown target in Figure and reference 12. The cameras plates in are one fixed picture at a di frame.stant position These high from resolution the suspension CMOS module cameras so that can measurethey can thecapture toe andthe wheel camber target angles and of reference the wheel plat targetes in plates one picture by detecting frame. theirThese relative high resolution motions CMOS cameras can measure the toe and camber angles of the wheel target plates by detecting their relative motions with respect to the reference plates. For this measurement mechanism, some preparatorySensors 2020, 20 ,procedures 1648 are required. 9 of 13 The preparatory procedures consist of three steps: 3D space creation, absolute coordinate creation,with respect and to local the coordinate reference plates. creation For and this wheel measurement alignment mechanism, inspection. some preparatory procedures are required. Table 2. Specifications of camera system. The preparatory procedures consist of three steps: 3D space creation, absolute coordinate creation, and localType coordinate of camera creation and wheel alignmentcomplementary inspection. metal-oxide semiconductor (CMOS)

Acquisition frequency Table 2. Speciﬁcations of camera system.500 Hz

MeasurementType range of camera complementary metal-oxide± 45 semiconductor° (CMOS) DistanceAcquisition camera/wheel frequency adapter 5000.5 Hz m Measurement range 45 ± ◦ PositionDistance accuracy camera /wheel adapter ± 0.50.1 mmm Position accuracy 0.1 mm ± Angular accuracyAngular accuracy ± 0.0150.015 ° ± ◦

FigureFigure 12. VerificationVerification system for wheel alignment inspection. i)(i) 3D 3D Space Space Creation Creation There are many ways to create a 3D space using ca camerasmeras [19–21]. [19–21]. In In this this study, study, 3D 3D space space creation creation with IR cameras and retro-reflectiveretro-reflective markers was employed. Each camera was equipped with an IR pass filter filter and a ring of IR LEDs aroundaround thethe lens,lens, whichwhich periodicallyperiodically illuminated the measurement space with IR light.light. Retro-reflectiveRetro-reflective markers,markers, on on the the other other hand, hand, reflected reflected the the incoming incoming IR IR light light back back to tothe the cameras. cameras. The The IR reflectionsIR reflections were were detected detected by the by cameras, the cameras, then internallythen internally processed processed by the opticalby the opticalsoftware. software. This system This calculatessystem calc theulates 2D markerthe 2D positionsmarker positions in image in coordinates image coordinates with high with precision. high precision.In an optoelectronic In an optoelectronic system, a system, minimum a minimum of three markersof three markers are basically are basically required required to measure to measure a rigid abody rigid [ 22body], but [22], for but the for system the system employed employed in this in study, this study, multiple multiple markers markers were were randomly randomly placed placed onto ontothe object the object in order in order to create to create the 3D the space 3D space of the of object. the object. Figure Figure 13a shows13a shows the markersthe markers placed placed on theon thesuspension suspension module module and and jigs jigs for thefor 3Dthe space3D space creation. creation. After After sticking sticking hundreds hundreds of markers of markers onto onto the theobject, object, hundreds hundreds of photos of photos were were taken taken by a by camera. a came Thesera. These photos photos were were sent sent to the tohost the host computer, computer, then thenprocessed processed by the by optical the optical software software to create to the create 3D spacethe 3D of thespace object. of the Figure object. 13 bFigure shows 13b the createdshows the 3D createdspace of 3D the space suspension of the suspension module with module retro-reflective with retro-reflective markers and markers IR LED and cameras. IR LED cameras. Sensors 2020, 20, 1648 10 of 13

FigureFigure 13. 3D13. 3D space space creation. creation. ( a()a) Markers Markers on the the suspension suspension module module and and jigs. jigs. (b) Created (b) Created 3D space 3D space using markers. using markers. Figure 13. 3D space creation. (a) Markers on the suspension module and jigs. (b) Created 3D space (ii) Absoluteii) Absoluteusing Coordinate markers. Coordinate Creation Creation Retro-reflective markers were also placed on the target wheel plates. In addition to the markers, Retro-reflective markers were also placed on the target wheel plates. In addition to the markers, thereii) Absolute were single-dot-type Coordinate Creation markers along the rim of the target wheel plates, as shown in Figure 13a. there were single-dot-type markers along the rim of the target wheel plates, as shown in Figure 13a. By calculatingRetro-reflective the center markers of the were single-dot-type also placed on ma therkers target on wheelthe target plates. wheel In addition plate, the to thepositions markers, of By calculating the center of the single-dot-type markers on the target wheel plate, the positions of each eachthere wheel were centersingle-dot-type can be defined markers in thealong 3D the space. rim Figureof the target14 shows wheel the plates, procedures as shown that inwere Figure used 13a. to wheelcreateBy center calculating the canabsolute be the defined coordinatecenter inof the of thesingle-dot-type 3D suspension space. Figure modu ma rkers14le. shows First, on the an the targetabsolute procedures wheel plane plate, thatis required the were positions usedto create to of create the absolutetheeach absolute wheel coordinate centercoordinate, can of be and the defined the suspension ground in the surface3D module. space. is thFiguree First, absolute 14 an shows absoluteplane. the Once procedures plane the centers is required that of were each toused target create to the absolutewheelcreate coordinate, platethe absolute are defined, and coordinate the the groundline of thatthe suspension surfaceconnects isthese modu the centers absolutele. First, is alsoan plane. absolute defined, Once plane as the shown is centers required in Figure of to each create 14a. target wheelThisthe plate absolute line areis defined defined,coordinate, as thethe and lineCenterline. the that ground connects This surface Centerline these is the centersabsolute also represents is plane. also Once defined, the thecenter centers as between shown of each inthe Figuretarget two 14a. Thiswheel linewheel is centers.plate defined are The asdefined, the normal Centerline. the linevector that Thisfrom connects Centerline the absoluthese alsocenterste plane represents is (thealso defined,surface the center table)as shown between to thein Figure center the two 14a. of wheel centers.CenterlineThis Theline normalis is defineddefined vector as the from ZCenterline. axis the (Figure absolute This 14b). Centerline plane Ideally, (the the also surfaceZ representsaxis and table) Centerline the to center the are center between supposed of Centerlinethe to two be is orthogonal each other. However, they are not actually orthogonal due to the machining and assembly definedwheel as thecenters. Z axis The (Figure normal 14 vectorb). Ideally, from thethe Zabsolu axiste and plane Centerline (the surface are supposedtable) to the to becenter orthogonal of tolerance of the jigs. This is the reason that the line from the two wheel centers is defined as the eachCenterline other. However, is defined they as the are Z not axis actually (Figure 14b). orthogonal Ideally, duethe Z to axis the and machining Centerline and are assemblysupposed to tolerance be Centerlineorthogonal and each not other. the YHowever, axis. The they Centerline are not actuand allythe orthogonalZ axis create due one to plane,the machining and the Yand axis assembly can be of the jigs. This is the reason that the line from the two wheel centers is defined as the Centerline definedtolerance on of that the plane. jigs. This This isY theaxis reason is not onlythat theon theline plane from created the two by wheel the Centerline centers is and defined the Z as axis, the andbut notCenterline is the also Y perpendicular axis.and not The the Centerline Y to axis. the TheZ axis and Centerline (Figure the Z axis14c). and createOncethe Z the axis one Z createand plane, Y oneaxes and plane, are the defined, Yand axis the then can Y axis the be Xcan defined axis be on thatis plane.defined defined Thison by that the Y axisplane. right-hand is This not only Yrule, axis onas is shown thenot only plane in onFi createdgure the plane14d by[23]. created the The Centerline XYZby the coordinate Centerline and the created and Z axis, the by Z butthese axis, is also perpendicularproceduresbut is also perpendicular tois the basic Z axis coordinate (Figureto the Z axis14 neededc). (Figure Once to create 14c). the Z Oncetwo and local the Y axes Z coordinates and are Y axes defined, atare each defined, then wheel. the then X axisthe X is axis defined by theis right-handdefined by the rule, right-hand as shown rule, inFigure as shown 14d in [ 23Figure]. The 14d XYZ [23]. coordinate The XYZ coordinate created by created these proceduresby these is the basicprocedures coordinate is the neededbasic coordinate to create needed two local to create coordinates two local at coordinates each wheel. at each wheel.

Figure 14. Absolute coordinate creation. (a) Centerline. (b) Z axis. (c) Y axis. (d) X axis.

FigureFigure 14. Absolute14. Absolute coordinate coordinate creation. creation. ((a) Centerline. (b ()b Z) Zaxis. axis. (c) (Yc )axis. Y axis. (d) X ( daxis.) X axis.

(iii) Local Coordinate Creation and Wheel Alignment Inspection Two local coordinates are created at each wheel by transferring the absolute coordinate to each wheel, and the toe and camber angles are obtained by the angular differences between the local iii) Local Coordinate Creation and Wheel Alignment Inspection Sensors 2020, 20, 1648 11 of 13 Two local coordinates are created at each wheel by transferring the absolute coordinate to each wheel, and the toe and camber angles are obtained by the angular differences between the local coordinatecoordinate and and the the target target wheel wheel plane, plane, asas shownshown in Figure 15a.15a. Camber Camber angles angles are arethe angular the angular differencesdifferences of the of the Y axis Y axis with with respect respect to to the the XX axis,axis, and toe toe angles angles are are the the angular angular differences differences of the of Y the Y axis withaxis with respect respect to the to the Z axis,Z axis, as as shown shown in in Figure Figure 1515b.b.

FigureFigure 15. Local15. Local coordinate coordinate creation creation and and wheelwheel alignment inspection. inspection. (a) (Locala) Local coordinate coordinate creation. creation. (b) Camber(b) Camber and and toe toe angle angle measurement. measurement.

4.2. Verification4.2. Verification Results Results and and Discussion Discussion VerificationVerification tests tests were were performed performed three three timestimes for for each each wheel, wheel, and and the the results results are shown are shown in Table in Table3. 3. As mentionedAs mentioned earlier, earlier, toe toe angles angles ofof generalgeneral passenger passenger vehicles vehicles are usually are usually set between set between −0.2 and 0.2 and degrees, so the toe angles in the verification tests were also controlled in the same range. In the same− 0.2 degrees, so the toe angles in the verification tests were also controlled in the same range. In the manner, the camber angle was controlled between −2 and 2 degrees. For the entire verification tests, same manner, the camber angle was controlled between 2 and 2 degrees. For the entire verification the maximum difference of the measurement values between− the laser and camera modules was 0.071 tests,degrees the maximum in the left dicamberfference angle, of theand measurementthe minimum difference values between was 0.007 the degre laseres in and the cameraright camber modules was 0.071angle. degreesThe mean in value the leftof the camber absolute angle, difference and the was minimum 0.027. Considering difference the was cost 0.007and preparatory degrees in the righttime camber difference angle. between The mean the two value measurement of the absolute methods, diff theerence proposed was wheel 0.027. alignment Considering inspection the cost in and preparatorythis study time seems diff toerence offer very between meaningful the two results. measurement methods, the proposed wheel alignment inspectionThe in proposed this study wheel seems alignment to offer inspection very meaningful method uses results. two laser modules that are mounted into the wheel knuckles, and the toe and camber angles are determined by measuring the distance of the laser mark projected onto the oppositeTable side 3. Measurement of the laser mounting and verification jig. Because results. of this measurement approach, when both wheels have non-null toe and camber angles, it can affect the measurement results. For Difference instance, if the left wheel exhibits one degreeLaser of (deg) camber Camera angle, (deg)then it basically has 36.53 mm of the laser (Laser-Camera) (deg) mark distance on the right side of the knuckle plate. However, if the right wheel exhibits one degree of Left 0.15 0.127 0.023 1 − − − camber angle at the same time, thenRight the laser mark0.15 distance0.158 of the left wheel becomes 0.008 36.534 mm, which − − is 1.00011 degrees. Therefore, thisLeft error can 0.01be negligible.0.008 Furthermore, the 0.018measurement results can Toe 2 − also be affected by the manufacturingRight tolerances 0.01 of the0.013 jigs. Especially 0.023for the knuckle jigs, the − parallelism and perpendicularity Lefttolerances 0.16may be critical 0.187 to the measurement0.027 results. In this study, 3 − Right 0.15 0.176 0.026 in order to prevent measurement errors, the parallelism and perpendiculari− ty tolerances of the Left 1.45 1.379 0.071 knuckle jigs were 0.01 mm1 and the surface −roughness was− 0.1 μm. − Right 1.51 1.482 0.028 − − − Left 0.10 0.073 0.027 Camber 2 Table 3. Measurement− and verification− results. − Right 0.05 0.043 0.007 LeftLaser 1.50 Camera 1.461 Difference (Laser-Camera) 0.039 3 Right 1.45 1.482 0.032 (deg) (deg) (deg)−

The proposed wheel alignmentLeft inspection −0.15 method –0.127 uses two laser modules –0.023 that are mounted into 1 the wheel knuckles, and the toe andRight camber −0.15 angles are–0.158 determined by measuring 0.008 the distance of the laser mark projectedToe onto the opposite side of the laser mounting jig. Because of this measurement Left 0.01 –0.008 0.018 approach, when both wheels2 have non-null toe and camber angles, it can aﬀect the measurement results. For instance, if the left wheelRight exhibits0.01 one –0.013 degree of camber 0.023 angle, then it basically has 36.53 mm of the laser mark distance on the right side of the knuckle plate. However, if the right wheel exhibits one degree of camber angle at the same time, then the laser mark distance of the left wheel becomes 36.534 mm, which is 1.00011 degrees. Therefore, this error can be negligible. Furthermore, the measurement results can also be aﬀected by the manufacturing tolerances of the jigs. Especially for the knuckle jigs, the parallelism and perpendicularity tolerances may be critical to the measurement results. In this study, in order to prevent measurement errors, the parallelism and perpendicularity tolerances of the knuckle jigs were 0.01 mm and the surface roughness was 0.1 µm. Sensors 2020, 20, 1648 12 of 13

5. Conclusions This study proposes a novel method for vehicle wheel alignment inspection. Unlike the wheel alignment inspection for full cars, which makes use of four wheel centers to measure toe and camber angles, wheel alignment inspection of a suspension module in development requires a new approach, since one suspension module has only two wheel centers. This study employed two laser modules for inspecting the wheel alignment of suspension modules in development. Two laser modules were orthogonally mounted onto the jigs of the left and right wheel knuckles, and each beam from the two laser sources was projected onto the other jig plane. By measuring the horizontal and vertical distances of the laser marks from the laser sources, the toe and camber angles of each wheel plane were obtained. The measurement accuracy of the proposed wheel alignment inspection can be guaranteed only when perfect orthogonality between the jig and the laser module is achieved. For perfect orthogonality, the angle between the jig and the laser module was precisely adjusted by rotating the laser module. Using a laser module mounted on the jig and a plane board placed at the far end of the laser module, the laser beam mark drew a circle on the board as the laser module rotates. By measuring and minimizing the diameter of the circle, the orthogonality between the jig and laser module was adjusted. By means of adjustment, 0.02 degrees of orthogonality was achieved. After setting up the wheel alignment conditions for a suspension module using several experimental parts such as a U-beam, surface table, pillars, and jigs, the wheel alignment inspection for the suspension module was performed. In this study, a vision system with two CMOS cameras was employed to verify the proposed wheel alignment inspection method. For the verification, two ultra-high resolution CMOS cameras were used and a 3D space were created by the cameras. In this 3D space, one absolute coordinate system from two wheel centers was created and two local coordinate systems at each wheel center were also created. By measuring the rotation angles between the absolute and local coordinate systems, the toe and camber angles of each wheel plane were obtained. From the verification results, it can be seen that the proposed wheel alignment inspection using laser modules can provide very accurate measurement results. The wheel alignment inspection proposed in this study can not only give very reliable results but was also very quick and cost-efficient. Therefore, it can save time and cost for automakers.

Author Contributions: S.H.K. conceived and designed the wheel alignment inspection system, and wrote the paper. K.I.L. performed the experiments and supervised the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

α Angle between normal vectors of Plane A and B β Angle between centerline and normal vector of Plane A θ Toe angle (deg.) Horizontal distance between laser source and the mark from d_t the other laser source (m) L Wheel track width (m) ϕ Toe angle (deg.) Vertical distance between laser source and the mark from the d_c other laser source (m) γ Angle between circle center and laser module (deg.) r Diameter of circle drawn by laser mark (m) l Distance from laser module and board (m) Sensors 2020, 20, 1648 13 of 13

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