Design andevaluationofaunifiedchassiscontrolsystemforrollover.pdf

isRollover mitigation controlUnified chassis controlvelopmenrabiliedperfocontroller, and the human driver are investigated through a full-scale driving simulator on the VTTwhich consists of a real-time vehicle simulator, a visual animation engine, a visual display, and suitablebya smalla disproportionatelythe vehicle control system. Accordingly, in 2002, NHTSAtime-andmethod for rollover prevention that employs an optimal tire forceARTICLE IN PRESSContents lists available at ScienceDirectControl EngineeringControl Engineering Practice 18 (2010) 585597(Yoon, Kim, & Yi, 2007). Since the lateral acceleration is theE-mail address: kyisnu.ac.kr (K. Yi).distribution (Schofield & Hagglund, 2008). Yoon and Yi proposed arollover index that indicates the danger of vehicle rollover as wellas an index-based rollover mitigation control system to reducethe rollover index through Electronic Stability Control (ESC)0967-0661/$-see front matter & 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.conengprac.2010.02.012nCorresponding author. Tel.: +82 2 880 1941; fax: +82 2 882 0561.automotive industry as it does not consider the effects ofsuspension deflection, tire traction aspects, or the dynamics ofLiu proposed a robust active suspension for rollover prevention(Yang & Liu, 2003) and Schofield and Hagglund proposed aunderstand a vehicles likelihood of rollover, the rolloverresistance rating program was proposed by NHTSA which usesthe static stability factor (SSF), which is the ratio of half the trackwidth to the height of the center of gravity (CG), to determine therollover resistance rating. The SSF has been questioned by theand Peng proposed an anti-rollover algorithm based on theto-rollover (TTR) metric (Chen & Peng, 2001). In this research,differential braking is selected as the actuation methodology.Ungoren and Peng evaluated a vehicle dynamics control (VDC)system for rollover prevention (Ungoren & Peng, 2004). Yangportion of severe and fatal injuries. Almost 11 million passengercars, SUVs, pickups, and vans crashed in 2002, yet only 2.6% ofthese involved a rollover. However, the percentage of fatal crashesthat involved the occurrence of rollover was about 21.1%, whichis significantly higher than the corresponding percentages forother types of crashes (NHTSA, 2003). In order to help consumersmotions through differential braking and active front steering(Wielenga & Chace, 2000). Several studies have been undertakenon rollover detection and its prevention and Hac et al. haveproposed an algorithm that detects impending rollover and anestimator-based roll index (Hac, Brown, & Martens, 2004). Chen1. IntroductionVehicle rollover is a serious problemtransportation and a report publishedTraffic Safety Administration (NHTSA)though rollover constitutes onlyaccidents, it does, however constitutehumanvehicle interfaces. The VTT has been developed and used for the evaluation of the UCC undervarious realistic conditions in the laboratory making it possible to evaluate the UCC controller in thelaboratory without risk of injury prior to field testing, and promises to significantly reduce the cost ofdevelopment as well as the overall cycle development time.& 2010 Elsevier Ltd. All rights reserved.in the area of groundthe National Highwayhas found that, evenpercentage of alllargepublished another announcement with regard to a tentativedynamical rollover test procedure (NHTSA, 2001).Most existing rollover prevention technologies can be classi-fied into two types, namely, (1) the type which directly controlsthe vehicle roll motion through an active suspension, an activeanti-roll bar, or an active stabilizer (Chen & Hsu, 2008) which canprevent rollover by raising the rollover threshold; and (2) the typewhich indirectly influences roll motions by controlling the yawVehicle stabilityVirtual test trackDesign and evaluation of a unified chassprevention and vehicle stability improvementJangyeol Yoona, Wanki Choa, Juyong Kanga, BongyeongaSchool of Mechanical and Aerospace Engineering, Seoul National University, 599 Gwanangno,bMando Corporation Central R&D Center, 413-5 Gomae-Ri, Giheung-Eub, Yongin-Si, Kyonggi-Doarticle infoArticle history:Received 6 May 2009Accepted 23 February 2010Available online 23 March 2010Keywords:Full-scale driving simulatorHuman-in-the-loop evaluationabstractThis paper describes the dethe control scheme on a virtuato improve vehicle maneuve(ESC) and active front steeringvehicle stability is improvdriver, the overall vehicleits interactions with the humjournal homepage: /locontrol system for rolloveron a virtual test trackKoob, Kyongsu Yia,nGwanak-Gu, Seoul 151-742, Republic of Korea449-901, Republic of Koreat of a unified chassis control (UCC) scheme and the evaluation ofl test track (VTT). The UCC scheme aims to prevent vehicle rollover, andty and its lateral stability by integrating electronic stability control(AFS). The rollover prevention is achieved through speed control, and thevia yaw rate control. Since the UCC controller always works with thermance depends not only on how well the controller works but also onan driver. Vehicle behavior and the interactions between the vehicle, thecate/conengpracPracticeARTICLE IN PRESSand evaluations prior to field testing. For this reason, a full-scaledriving simulator on a virtual test track (VTT) has been developedand used in a human-in-the-loop evaluation of the UCC where theJ. Yoon et al. / Control Engineering Practice 18 (2010) 585597586dominant factor in vehicle rollover, much research into rolloverprevention has proposed the use yaw motion control to reducethe lateral acceleration. However, since these rollover preventionschemes only focus on reducing the lateral acceleration, vehiclemaneuverability and lateral stability cannot be guaranteed (Yoon,Cho, Koo, & Yi, 2009). For instance, when the rollover preventioncontroller works to reduce the lateral acceleration, this tends to bein the opposite direction to the intentions of the driver which maycause the vehicle to deviate from the road, thereby resulting in anaccident. Studies have been conducted to prevent rollover whilemaintaining good lateral stability. Jo et al. proposed a VDC systemfor rollover prevention and ensuring lateral stability (Jo, You,Jeong, Lee, & Yi, 2008). In this research, a VDC is designed andactivated in descending order of priority rollover prevention,excessive side-slip angle, and under-steering/over-steering of thevehicle. However, this method leads to reduction of themaneuverability or rollover prevention.Nomenclaturea distance from the center of gravity (CG) to the frontaxleaylateral acceleration of the vehicleay,desdesired lateral accelerationay,ccritical lateral accelerationay,msensor measurement of the lateral accelerationb distance from CG to the rear axlem vehicle masst tread (track width)vxlongitudinal velocity of the vehiclevx,desdesired longitudinal velocity of the vehiclevylateral velocity of the vehicleCfcornering stiffness of the front tireCrcornering stiffness of the rear tireFxlongitudinal tire forceFx,1longitudinal tire force of the front-left wheelFxflongitudinal tire force of the front sideFxylongitudinal tire force of the rear sideFor this reason, the unified chassis control (UCC) algorithm hasbeen designed to prevent vehicle rollover while, at the same time,ensuring good maneuverability and lateral stability by integratingindividual chassis control modules, such as ESC and active frontsteering (AFS). A vehicle speed control algorithm has beendesigned to prevent rollover and an algorithm for controllingthe yaw motion has been designed to improve the maneuver-ability and the lateral stability. The proposed UCC works toenhance the maneuverability and the lateral stability in normaldriving situations without danger of rollover. When the risk ofrollover increases, the proposed UCC works to prevent vehiclerollover and at the same time ensures the vehicle can continu-ously move in the path intended by the driver. In order to detectan impending vehicle rollover, the rollover index (RI), as proposedin a prior study (Yoon et al., 2007), is employed.Since the UCC controller always works with the driver, theoverall vehicle performance will depend not only on how well thecontroller works but also on its interactions with the humandriver. Therefore, a closed human-in-the-loop evaluation wouldbe a more effective way of designing the UCC controller thanperforming open-loop simulations that use the prescribed steer-ing and velocity profiles (Chung & Yi, 2006). Moreover, theevaluation of active safety systems, such as UCC, active cruisecontrol, collision warning, collision avoidance, etc., rely heavily onfield testing that entails time-consuming and expensive trials, andVTT, based on the concept of rapid control prototyping (RCP), hasbeen described in Lee (2004).In this paper, the control performance of the proposed UCCalgorithm has been investigated by a real-time human-in-the-loop simulation, using a vehicle simulator on a VTT. The tests,based on the VTT, are conducted by thirteen drivers and theresults have been analyzed in detail and summarized here.2. Unified chassis controller designoften significant danger (Han & Yi, 2006a). A model-basedsimulation makes it possible to perform exhaustive design trialsFyflateral tire force of the front sideFyrlateral tire force of the rear sideFy,1lateral tire force of the front-left wheelFzfvertical tire force of the front sideFzrvertical tire force of the rear sideFz,1vertical tire force of the front-left wheelFz,2vertical tire force of the front-right wheelFz,3vertical tire force of the rear-left wheelFz,4vertical tire force of the rear-right wheelIzmoment of inertia about the yaw axisMzdirect yaw momentb side slip angle of the vehicledftire steer anglef vehicle roll anglefthroll angle threshold_f vehicle roll rate_fthroll rate thresholdg yaw rategddesired yaw rateIn this study, the UCC system is designed to prevent a vehiclerollover and to improve both the maneuverability and the lateralstability of the vehicle by integrating the individual chassiscontrol modules such as the ESC and AFS. There are three controlmodes, namely, ROM, ESC-c, and ESC-b, which stand for rolloverprevention, maneuverability and lateral stability, respectively.The proposed UCC works to enhance the maneuverability and thelateral stability in normal situations without danger of rollover.The improvement in maneuverability and lateral stability isachieved by reducing the yaw rate error between the actualyaw rate and the desired yaw rate, based on the drivers steeringinput and the vehicles side slip angle. When the risk of rollover ishigh, the proposed UCC works to reduce vehicle rollover and, atthe same time, improves the maneuverability and the lateralstability. As mentioned in the previous section, since priorresearch concerning rollover mitigation (ROM) control, i.e., anRI-based ROM control (Yoon et al., 2007), is only focused on theprevention of vehicle rollover, then vehicle maneuverability andlateral stability cannot be guaranteed. For instance, since vehiclerollover generally occurs at large lateral accelerations, prior RI-based ROM controllers operate to reduce the lateral acceleration.This control strategy tends to control the vehicle in the oppositedirection intended by the driver which may cause the vehicle todeviate from the road resulting in accidents. For this reason, an RI/vehicle stability (VS)-based UCC controller is designed to preventARTICLE IN PRESSvehicle rollover and at the same time ensuring that the vehiclecan continuously move in the intended path of the driver.Fig. 1 shows a schematic diagram of the RI/VS-based UCCstrategy where the proposed UCC system consists of upper andlower-level controllers where the upper-level controllerdetermines the control mode, such as rollover prevention,maneuverability level, and lateral stability; it also calculates thedesired braking force and the desired yaw moment for itsobjectives. Each control mode generates a control yaw momentand a longitudinal tire force in line with its coherent objective.The lower-level controller calculates the longitudinal and lateraltire forces as inputs of the control modules, such as the ESC andthe AFS.2.1. The upper-level controller: decision, desired braking force, anddesired yaw momentThe upper-level controller consists of three control modes anda switching logic. A control yaw moment and the longitudinal tireforce are determined in line with its coherent control mode sothat the switching across control modes is performed on the basisof the threshold. Based on the drivers input and sensor signals,the upper-level controller determines which control mode is to beselected, as shown in Fig. 2.In this study, RI is used to detect an impending vehicle rolloverwhere the RI is a dimensionless number that can indicate the riskof vehicle rollover and it is calculated through: the measuredlateral acceleration, ay, the estimated roll angle,f, the estimatedroll rate,_f, and their critical values which depend on the vehiclegeometry in the following manner (Yoon et al., 2007):In (1), C1, C2, and k1are positive constants (0oC1o1,0oC2o1), C1and C2are weighting factors, which are related tothe roll states and the lateral acceleration of the vehicle, and k1isa design parameter which is determined by the roll angle-ratephase plane analysis. These parameters in (1) are determinedthrough a simulation study undertaken under various drivingsituations and tuned such that an RI of 1 indicates wheel-lift-off. Adetailed description for the determination of the RI is provided inprevious research (Yoon et al., 2007). The lateral acceleration caneasily be measured from sensors that already exist on a vehicleequipped with an ESC system. However, additional sensors areneeded to measure the roll angle and the roll rate, although it isdifficult and costly to directly measure these (Schubert, Nichols,Fig. 1. RI/VS-based UCC strategy.RIC1ftC12C12C12C12_fth_ftC12C12C12C12C12C12fthfth_fth01AC2ayC12C12C12C12ay,cC18C191C0C1C0C2ftC12C12C12C12ft2_ftC16C172r0BB1CCA, f_fC0k1fC16C1740RI0, f_fC0k1fC16C17r08:1J. Yoon et al. / Control Engineering Practice 18 (2010) 585597 587Fig. 2. Control modes for the proposed UCC system.ARTICLE IN PRESS012345678Time secNo control 43.2mphControl 45.6mphRoll angle 012345678Time secNo control 43.2mphContro l45.6mphLateral acceleration No control 43.2mphControl 45.6mph-15-10-5051015-15-10-505101511.5Roll angle deg/secay m/sJ. Yoon et al. / Control Engineering Practice 18 (2010) 585597588Wallner, Kong, & Schiffmann, 2004). For this reason, the roll angleand the roll rate are estimated by a model-based roll stateestimator (Park, Yoon, Yi, & Kim, 2008).The proposed RI is evaluated using vehicle test data obtainedfrom the MANDO Corporation. Note that the test data used in thisevaluation are not the outcome from the proposed UCC system. Inother words, the control algorithm of MANDO is different fromthe one described in this paper so that the test results show littledifference compared with the desired results. Fig. 4 shows thevehicle test data and the rollover index for the fishhook test whichhas been developed by NHTSA, as a dynamical test for theprediction of dynamic rollover propensity and the test results areused for vehicle evaluation. The fishhook test maneuver isdescribed in Fig. 3.Fig. 4(a) shows the time histories of the steering angle of twotest cases where the entrance speeds are 43.2 and 45.6 mph,respectively, but the vehicle stability control input is applied onlyfor the 45.6 mph case. In both cases, either one or two wheels arelifted off at about 4.2 s, and the rollover indices increase overFig. 3. Fishhook maneuver developed by NHTSA (adopted from Corrsys-Datron).unity. However, once the control input is selected, the roll angleand the lateral acceleration are decreased, and the rollover indexalso decreases below unity, as shown in Fig. 4(b)(d). In contrastwith the control case, the roll angle, the lateral acceleration, andthe rollover index increase over unity in the non-control case.Consequently, the vehicle is rolled over at about 6 s.If the RI exceeds a particular threshold, then the rolloverprevention mode, ROM, is activated, otherwise, the controller is ineither the maneuverability mode or in the lateral stabilitymode. Under a small side slip angle, the controller is in themaneuverability mode, ESC-c, if the error between the actual yawrate and the desired yaw rate exceeds a particular threshold.The condition of activation of the lateral stability mode isdetermined by the vehicle side slip angle. If the side slip angleexceeds the threshold value, the controller is in the lateralstability mode, ESC-b and the side slip angle can be successfullyestimated in real time from already existing vehicle sensors(You, Hahn, & Lee, 2009).The maneuverability and the lateral stability are ensured bythe yaw moment control method and rollover prevention isachieved by the yaw moment/speed control. The upper-levelcontroller calculates the desired braking force, DFx, for rolloverprevention and the desired yaw moment, Mz, for maneuverabilityand lateral stability. The state-transition diagram for the requiredSteering wheel angle 012345678-200-1000100200Time secSWA degNo control 43.2mphControl 45.6mphcontrol mode switching in the upper-level controller is given inFig. 5.The signals used for the state transitions are the yaw rate error,ge, the side slip angle, b, and the RI so that each event in Fig. 5represent a switching condition, and the conditions of itsactivation are described in Table 1. When the vehicle state iseither ESC-c or ESC-b, as shown in Fig. 5, the yaw moment controlis applied and generates the desired yaw moment to track a targetyaw rate. In ESC-c, a target yaw rate is generated on the basis ofthe drivers steer