- Email: [email protected]
Review on Global Chassis Control*
5th IFAC Symposium on System Structure and Control Part of 2013 IFAC Joint Conference SSSC, FDA, TDS Grenoble, France, February 4-6, 2013
Review on Global Chassis Control ⋆ Carlos A. Vivas-L´ opez ∗ Diana Hern´ andez-Alcantara ∗ ∗ Juan C. Tud´ on-Mart´ınez Ruben Morales-Menendez ∗ ∗
Tecnol´ ogico de Monterrey, Av. Garza Sada # 2501, 64849, Monterrey M´exico {ca.vivas.phd.mty, A00469139, jc.tudon.phd.mty, rmm}@itesm.mx Abstract: Control of the vehicle dynamics has been the objective of research in the resent years by the Original Equipment Manufacturers , suppliers, and Research and Development institutes, this interest relies on the statement that by controlling the performance of the vehicle dynamics the safety and comfort of the passengers can be improved. In response to this issue have arisen various chassis control systems which focus on a particular objective, but are not designed to interact jointly with other systems. Because this situation that the concept of Global Chassis Control (GCC ) its been proposed, GCC states that it is possible the interaction of different chassis control systems in order to reach a common objective, either safety or comfort, depending on the vehicle situation. A review of the different approaches the GCC objective is proposed. Keywords: Vehicle Dynamics, Chassis Control, Vehicles, Active Vehicle Suspension, Active Control 1. INTRODUCTION The vehicle dynamics control has been the object of research in the recent years by Original Equipment Manufacturers (OEM), suppliers, and research and development institutes. Due to state regulations calling for safer and vehicles, and the world trends seeking for intelligent cars, TechCast-LLC [2012], GCC, also known as Integrated Vehicle Dynamics Control, rises as a suitable solution. GCC proposes the integration of different vehicle control systems in order to meet the objectives of safety/comfort for the passengers, rather than just put them altogether and let them pursuit its own objectives. This integration while keeping the same, or less, costs of sensors and actuators. He [2005] remarks this advantage of the GCC, which can share sensors information and actuators functions in the integration, avoiding duplicated sensors or actuators. The vehicle dynamics theory defines different effects in the vehicle that can be classified in three directions: Vertical, Lateral and Longitudinal, Fig. 1 presents the vehicle axis system. Depending on what dynamic direction we are looking some variables become relevant. Vertical dynamics refers to the movements that affects mainly the comfort of the passenger: pitch(φ) (around Y axis), roll(θ) (around X axis), and vertical acceleration(¨ z ) (vertical acceleration). The longitudinal dynamics refers to the stability of the vehicle: longitudinal velocity (v), wheel rotational velocity (ω), and tire slip ratio (λ). Lateral dynamics also refers to stability: lateral displacement (y), side slip angle (β), and yaw(ψ) (around Z axis).
⋆ Authors thank Autotronics and Development of Products for Emerging Markets research chairs at Tecnol´ ogico de Monterrey.
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Fig. 1. Vehicle dynamics Three vehicle subsystems have been studied: steering, suspension, and brake/driveline, Fig.2. Each can be controlled with an electronic control system, for example, for controlling the steering system the controller can modify the driver’s steer angle command, Active Steering (AS ). For braking, each wheel can be braked individually, the Antilock Brake System (ABS ) or Electronic Stability Control (ESC ). The driveline can regulate the power transfer for each wheel, the Anti-Slip Regulation (ASR), and for suspension, the damper can change its damping coefficient, Continuos Damping Control (CDC ). Individually they can counteract the effects of some of those dynamics, the AFS alone can control the tire slip ratio, vertical velocity and helps in the control of the yaw and lateral displacement. But most of these control systems were designed to operate reaching its own goal, but when they are put to operate simultaneously the result can be a degradation of the general performance, because of their interactions. The main objective of the GCC is to integrate those chassis control systems avoiding conflicts on their actions by coordinating them. Sato et al. [1992] evaluate different configurations of vehicle control systems like Active Suspension (A-SUS ), Active 4 Wheel Steering (A-4WS ), Traction Control and ABS. His results demonstrate the advantage of controlled vehicle systems over passive ones, and how a coordinated integration improves the overall vehicle performance. 10.3182/20130204-3-FR-2033.00040
2013 IFAC SSSC Grenoble, France, February 4-6, 2013
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Fig. 2. Vehicle systems: (a) Brake, (b) Steering, and (c) Suspension. (b)
Obviously the sub-system integration is not the only issue that comes with the GCC. The complexity of the resulting integrated control system, because of the wide number of signals that need to be managed and processed by the control units, this lead to a more powerful Electronic Control Units (ECU ). Another issue is the stability during switching between control laws. Tavasoli et al. [2012] overcomes this stability problem by defining a switching gain. This gain is defined as X = n ∗ Xc1 + (n − 1)Xc2 , where X is the overall objective, Xc1 and Xc2 are the individual control laws objectives and n is the switching gain.
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Also with the increasing sensor incorporation to the vehicle, the reliability becomes an issue in GCC that in Hwang and Lee [2006] is taken in consideration, by creating redundancy in instrumentation, whereas virtual or physical sensors, and a discrimination algorithm to diagnose the fault situation and correcting it using the redundancy. General fault tolerant methods will be discussed further. According to the literature review made by Shibahata [2005] the evolution of the chassis control systems can be separated in three generations. The first one starts in the middle 80’s with the introduction of Four Wheel Steering (4WS) by Sano et al. [1986] and a Direct Yaw Control (DYC ), Shibahata et al. [1992], an example of DYC is the one presented in Canale and Fagiano [2008], they use an Nonlinear Model Predictive Control approach to control the yaw moment of the vehicle by a Rear Active Differential. The systems in this generation were intended to be a secondary control system, additional systems which support the primary control system. The second generation became the control system itself, instead of support the control system, the driver interacts directly with the control system, this generation was represented by systems like Variable Gear Ratio Steering system and the X-by-wire control systems. The current generation according to Shibahata is the integration generation, where various systems are intended to interact in order to achieve performance in the vehicle beyond the level reached by the systems alone. Also the driver assistance systems become important in this generation, they can be consider as another vehicle control system that supports the drivers intentions, and needs to interact with the other control systems. The control systems which represents this generation correspond to GCC systems and here is where this paper is situated. The integration of different vehicle control systems can be seen in the literature with OEM ’s own concepts of integration, like GM ’s project “Trilby”, Schilke 876
Fig. 3. Control system topologies: (a) Centralized,(b) Decentralized, and (c) Multi-layer Control Structure. et al. [1988], Nissan,Imaseki and Kobari [Jul 10 1990], or Toyota, Yokoya et al. [1990]. The outline of this article continues as follows: The section 2 discuses the different control topologies, the section 4 presents the different approaches to Integrated Chassis Control from a single objective integration to a full Dynamics Integration structure, the section 5 discuses the integration from the fault tolerant point of view. Finally, section 6 concludes this review.
2. CONTROL TOPOLOGY For better understanding about the GCC approach it is necessary to describe the basic topologies found in literature. To achieve integration proposed by GCC concept, the global control system needs to interact with the existing controllers of the vehicle. Through the time many control structures have been detected in the literature, according to Yu et al. [2008] three topologies can be found: decentralized, centralized, and multi-layer control structure. Each one presents its benefits and limitations to achieve the GCC goal. The Multi-layer is the more suitable for this purpose.
2013 IFAC SSSC Grenoble, France, February 4-6, 2013
2.1 Decentralized Control In a decentralized control structure, Fig. 3b, every subsystem has its own independent controller and control goals which commands its particular actuator. The interaction among different control loops is limited to shared information obtained from a communication bus. This type of control architecture was used in the early chassis control integration. In this scheme the integration lies on the OEM ’s, while the supplier provides its systems interconnection options. An example of decentralized control architecture is a vehicle equipped with ABS and semiactive suspension, the control systems seek for their own goal ignoring the action of the other system. 2.2 Centralized Control In a centralized control structure, Fig. 3a, the main goal consists in a single main controller that send the control signals directly to the subsystems actuators. This kind of information flow increases the computational load in the central ECU demanding more powerful units. In the other hand the centralized structures are rigid, which limited its reconfiguration capabilities for integrating new elements in the control systems. If there is a new sub-system for the control loop the only solution is to redesign the full control system. Also, this type of topology forces the OEM ’s to open its architecture. 2.3 Multi-Layer Control In the multi-layer scheme three layers can be identified, the coordination layer, the controller layer and the actuation layer, Fig. 3c. Acording to Gordon et al. [2003] the coordination controller layer has two main functions, first based on the dynamic state of the vehicle it determines desired set-points for the controller layer, the second, is to set a specific mode of operation into the actuators. The second layer receives the control signals from the coordination one and it selects and control its own control subsystem in the indicated mode by the upper layer. For instance, depending on the overall vehicle situation the suspension system can operate in comfort mode or in road-holding mode. 3. INTEGRATED CHASSIS CONTROL APPROACHES Research and Development institutes and OEM ’s have turn the sight to the GCC, their interest rely on the intention of achieving safer and more comfortable commercial vehicles. The scope of their researches is to develop integrated strategies that improve the overall performance of the vehicle in different driving conditions. This section is divided in three different approaches, (1) to integrate the available chassis systems to improve one direction (lateral, longitudinal or vertical) of dynamical performance, (2) to make control in more than one direction, and (3) the control strategies that try to reach the full direction vehicle dynamics control.
in this subsection are presented some approaches that by integrating different chassis control systems they improve a single characteristic or vehicle property. In Valasek et al. [2004] the objective is to reduce the braking distance in an emergency situation by integrating the semi-active suspension system with an ABS algorithm in a multi-layer topology, in this structure the suspension is used to reduce the fluctuations of the vertical forces between the road and the tire, when the upper layer switch its mode to emergency mode. Wei et al. [2006] propose and multi-layer integration of AFS and a torque based DYC to control the lateral vehicle dynamics using fuzzy logic, the control variable is the yaw moment which is divided into 3 states. The first state correspond to a low yaw moment and only the AFS acts, in the second level both the AFS and DYC gets in action, an in the last level only the DYC should be controlling the yaw moment. G´asp´ar et al. [2007a] and G´asp´ar et al. [2007b] proposes the multi-layer integration of suspension and braking to prevent a rollover situation in a vehicle during cornering, the controller uses a weight function that relates its magnitude with the level of danger of the vehicle state, this function is sectioned by two thresholds the first one is used to indicate a possible rollover situation and the suspension system only tries to revert the situation, then if the situation continues the function reaches the critical level and the braking system starts its action to prevent the rollover. Poussot-Vassal et al. [2008] and Poussot-Vassal et al. [2011b] propose the multi-layer integration of steering and braking systems to improve the lateral dynamics but they approach the issue from the LPV /H∞ framework, where a GCC controller determines the needed brake torque and steer angle to meet the desire yaw rate. Hwang et al. [2008] proposes a method that integrates AFS and ESC to improve the lateral dynamics of the vehicle, this is made by two methods, the first one uses a multilayer architecture where a supervisory layer coordinates the actions of the two systems separately, the other uses a centralized control who integrates the two systems in one ECU, the results demonstrate that the unified controller overcomes the performance of the supervisory one, but this controller presents the disadvantage that needs to modify the system architecture to integrate the two systems. Kangwon et al. [2009] integrate ESC and CDC, with a multi-layer scheme, to control the lateral dynamics during emergency situations, like in a single line change maneuver. The supervisory layer determines the mode of operation of each individual control system based on the deviation from the desire yaw rate.
3.1 Single Purpose Integration
In Wu et al. [2010] they propose the centralized integration of braking force control, active rear wheel steering control and Active Yaw moment Control to improve vehicle handling and line keeping performance, the controller is optimized by LMI - H∞ also using a driver model to give the steering commands in line kipping procedures.
Some approaches of integration of vehicle control systems are intended to achieve only a single objective of control,
Seongjin and Kyongsu [2011] integrates ESC and AFS to create a multi-layer Integrated Chassis Control (ICC )
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system and an Active Anti-Roll Bar to create an Active Roll Control System (ARCS ). The main objective of this integration is to improve the ARCS capabilities for control vertical dynamics, specially the Roll angle. Integrating the ARCS with other chassis control systems, the results prove that the performance of an stand-alone ARCS can be dramatically improved with the integration. In Seongjin et al. [2012] they use the same ICC system, formed by ESC and AFS, to design a DYC to improve lateral stability of the vehicle using the Weighted Pseudo-inverse based Control Allocation (WPCA) approach. 3.2 Multi-Purpose Integration A more advance integration of chassis control systems are when the integration aims to control two or multiple objectives but without actually fully control the vehicle dynamics, these approaches get closer to the GCC concept. In Andreasson and B¨ unte [2006] they present a centralized approach to GCC using an inverse dynamics model, which main objective is to control de horizontal (lateral and longitudinal) dynamics, although they ensure that it can be extended to full dynamics control. The method is capable to be reconfigured at any time and it is tested using different vehicles configurations. Mokhiamar and Abe [2006] propose the integration of the four wheels in the control of the horizontal dynamics of the vehicle, the proposed control system consist on the integration of DYC, Front Wheel Steering (FWS), and Rear Wheel Steering (RWS), the results show that this control system overcomes the performance of the one obtained of a FWS + ABS vehicle in severe cornering conditions (large amplitude sine input). Falcone et al. [2007] they integrate the braking and steering systems to improve longitudinal and lateral dynamics. The idea is to make the vehicle capable of perfectly follow a trajectory by centralized Model Predictive Control (MPC) algorithms, the results were satisfactory compared with a single steering controlled vehicle. Poussot-Vassal [2008] in its dissertation proposes a multilayer GCC approach using the LPV /H∞ framework, integrating braking, suspension, and steering systems, he divides the vehicle dynamics in two, vertical and horizontal (lateral, longitudinal), the vertical dynamics are controlled by means of the suspension system, using semi-active dampers, the objective is to achieve compromise between road holding and comfort. The horizontal dynamics are controlled by the steering and braking systems, which control the yaw rate and lateral acceleration. In Miura et al. [2008] they design a multi-layer control system that integrates traction and braking systems by means of a called Active Center Differential (ACD) system and ABS, the aim of this control system called Super All Wheel Control (S-AWC) from Mitsubishi Motors Co. is to improve the longitudinal and lateral dynamics with those systems and its application results in a reduction of steering effort of the driver. Xiao et al. [2009] propose an integration scheme implemented by a multi-layer architecture with an ESC + 878
Active Suspension System (ASS) configuration. It consists in two layers, the upper layer monitors the driver’s intentions and vehicle dynamic state. With this information it computes a corrective yaw moment Mz , and generated the distributed torques MESP and MASS to the controller layer. The lower layer, execute the control commands of the upper layer, the ASS system is controlled by a LQG technique and the ESC by Adaptive Fuzzi Logic (AFL). The integration proposed by Xiao controls the vertical and the lateral dynamics, the emphasis in the lateral stability. In Wanki et al. [2011] and Cho et al. [2012] they integrate ESC and AFS to improve agility, maneuverability and, vehicle lateral stability, the controller uses a multi-layer topology with a supervisor and control algorithm layers, the strategy depends on a threshold of the yaw rate to determine wether to use maneuverability or agile mode. Also this control system uses the yaw rate to maintain the side slip angle in a low level. In the same way Jangyeol et al. [2010] use the same chassis systems but its goal is to improve vehicle lateral stability and maneuverability while preventing rollover, it is noteworthy that their control scheme was implemented with human in the loop configuration, this to demonstrate the effectiveness of the algorithm when interacts whit the drivers inputs. 3.3 Complete Dynamics Integration The approaches that completely fulfill the GCC concept are the ones that control the vehicle dynamics in the three directions, vertical, lateral, and longitudinal. In Lu et al. [2010] and in Lu et al. [2011] a complete GCC integration is presented, using braking, suspension, and steering to control all the vehicle dynamics, the algorithm uses a multi-layer topology controller, with a supervisory layer that detects and classify the vehicle state in seven predefined conditions and coordinates the roll of the integrated chassis systems, then a controller layer, which includes each system controller, follows the decisions of activation and mode of operation of the supervisory layer, in order to control each state with a particular strategy. In Poussot-Vassal et al. [2011a] a multi-layer control system was designed considering the vertical and horizontal dynamics, the vehicle chassis systems involved were: suspension using active dampers, and brakes. The design were made in two different controllers, one for the vertical dynamics and other for the lateral, the link between the two controllers it the “monitor”. The monitor is an upper layer that, based in the wheel slip ratio, coordinates the tuning parameters of the chassis systems to overcome conflicts and guaranty a well coordinated action. 4. GLOBAL CHASSIS FAULT-TOLERANT CONTROL During last decade, advanced control systems in the automotive industry have been proposed to consider features of safety and fault tolerance in order to maintain the desired performances: comfort, handling, vehicle stability, etc. When the controller is designed to be robust to vehicle model uncertainties or instrument failures, the Fault-Tolerant Controller (FTC ) is passive; while, if the
2013 IFAC SSSC Grenoble, France, February 4-6, 2013
controller uses on-line information for the fault accommodation is considered active, i.e. a Fault Detection and Isolation (FDI ) module is used. The major effort of FDI systems and FTC in vehicle dynamics is applied in only one system (suspension, steering or braking without any chassis integration) and some of them belongs to the single purpose integration approach. For adding fault-tolerance in the active suspension control system, Chamseddine and Noura [2008] proposes an active FTC based on sliding mode theory, in this case the vertical, lateral and longitudinal dynamics are evolved. On the other hand, an experimental validation of a methodology for detecting faults in semi-active or active dampers based on parity equations is shown in Fischer and Isermann [2004], a sedan car is used as test-bed, i.e. inherently the chassis motion includes the global rotational and translational dynamics. Related to the lateral stability control, in Bosche et al. [2009] is proposed a passive FTC when the steering actuators (front or rear) fail, the LPV controller is designed by using a parameter-dependent Lyapunov matrix of the lateral and longitudinal vehicle dynamics. For monitoring the vehicle chassis performance, a design methodology of an FDI module for the suspension, steering and braking is developed as a Multi-layer framework in Pisu et al. [2003]; the proposed approach was illustrated in details in the braking system. A GCFTC with multi-purpose integration or complete dynamics integration is a hard task that recently has been studied, e.g. in G´asp´ ar et al. [2010] a Multi-layer FTC is proposed to guarantee road holding and roll stability when sensor and actuator faults appear (analysis in the vertical and lateral chassis dynamics), the reconfiguration of the proposed active FTC is based on LPV control theory. 5. IDEAL INTEGRATED CHASSIS CONTROL SYSTEM According to the literature review and the actual state of the technology, an ideal Integrated Chassis Control System approach is proposed. The propose must meet the following aspects: • Full dynamics integration. • Multi-layer control stricture. In this scheme the computational load is divided into several ECU s, also this architecture gives the system the capability of been reconfigured easily. • Safety-comfont oriented. In the list of priorities the main purpose of the system integration must be safety, followed by comfort. • Multi-mode operation. It is necessary for the system to be capable of operate in different control modes, because there is no a single mode of operation that can achieve optimal performance in all conditions. • Fault tolerant. In order to be able to counteract against undesired situations. 6. CONCLUSIONS This literature review has shown the benefits of vehicle control systems integration in terms of vehicle dynamic performance. Also based on the contributions cited above, the multi-layer control structure, nowadays, seems to be 879
the suitable solution for integrated vehicle control systems, because the computational effort is divided among all the individual subsystem controllers and the supervisory controller. The X-by-wire technology trend now is an everyday topic of research and whit it comes the issues of fault tolerant and reliability. Although there is much research on these issues, these increase the dimension of the complete system making it more expensive and complex. The challenges that future research will be focus on are: 1. Optimization of control algorithms to make them realtime implemented in the actual available hardware. 2. Development of more reliable and fault tolerant control systems to avoid over redundancy of sensors and actuators that increase the complexity of the complete vehicle system. 3. Development of more powerful ECU that can support real-time centralized control schemes. REFERENCES J. Andreasson and T. B¨ unte. Global Chassis Control based on Inverse Vehicle Dynamics Models. Vehicle System Dynamics, 44(sup1):321–328, 2006. J. Bosche, A.E. Hajjaji, and A. Rabhi. Actuator FaultTolerant Control for Vehicle Dynamics. In 7th Safeprocess, pages 1103–1108, 2009. M. Canale and L. Fagiano. Vehicle Yaw Control using a fast NMPC Approach. In 47th IEEE Conf on Decision and Control, pages 5360–5365, dec. 2008. A. Chamseddine and H. Noura. Control and Sensor Fault Tolerance of Vehicle Active Suspension. IEEE Trans. on Control Systems Tech., 16(3):416–433, 2008. Wanki Cho, Jaewoong Choi, Chongkap Kim, Seibum Choi, and Kyongsu Yi. Unified chassis control for the improvement of agility, maneuverability, and lateral stability. Vehicular Technology, IEEE Trans on, 61(3):1008–1020, march 2012. P. Falcone, F. Borrelli, J. Asgari, H.E. Tseng, and D. Hrovat. A Model Predictive Control Approach for Combined Braking and Steering in Autonomous Vehicles. In Mediterranean Conf on Control Automation, pages 1–6, Jun 2007. D. Fischer and R. Isermann. Mechatronic Semi-active and Active Vehicle Suspensions. Control Eng. Practice, 12: 1353–1367, 2004. P. G´asp´ar, Z. Szab´o, J. Bokor, C. Poussot-Vassal, O. Sename, and L. Dugard. Global Chassis Control using Braking and Suspension Systems. In IAVSD, editor, 20th Symp of the Int Association for Vehicle System Dynamics, 13-17, 2007a. P. G´asp´ar, Z. Szab´o, J. Bokor, C. Poussot-Vassal, O. Sename, and L. Dugard. Towards Global Chassis Control by Integrating the Brake and Suspension Systems. In 5th IFAC Symp on Advances in Automotive Control, 2007b. P. G´asp´ar, Z. Szab´o, and J. Bokor. LPV Design of Faulttolerant Control for Road Vehicles. In Conf. on Control and Fault Tolerant Systems, France, pages 807–812, 2010. T. Gordon, M. Howell, and F. Branda. Integrated Control Methodologies for Road Vehicles. Vehicle System Dynamics, 40(1-3):157–190, 2003.
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Review on Global Chassis Control ⋆ Carlos A. Vivas-L´ opez ∗ Diana Hern´ andez-Alcantara ∗ ∗ Juan C. Tud´ on-Mart´ınez Ruben Morales-Menendez ∗ ∗
Tecnol´ ogico de Monterrey, Av. Garza Sada # 2501, 64849, Monterrey M´exico {ca.vivas.phd.mty, A00469139, jc.tudon.phd.mty, rmm}@itesm.mx Abstract: Control of the vehicle dynamics has been the objective of research in the resent years by the Original Equipment Manufacturers , suppliers, and Research and Development institutes, this interest relies on the statement that by controlling the performance of the vehicle dynamics the safety and comfort of the passengers can be improved. In response to this issue have arisen various chassis control systems which focus on a particular objective, but are not designed to interact jointly with other systems. Because this situation that the concept of Global Chassis Control (GCC ) its been proposed, GCC states that it is possible the interaction of different chassis control systems in order to reach a common objective, either safety or comfort, depending on the vehicle situation. A review of the different approaches the GCC objective is proposed. Keywords: Vehicle Dynamics, Chassis Control, Vehicles, Active Vehicle Suspension, Active Control 1. INTRODUCTION The vehicle dynamics control has been the object of research in the recent years by Original Equipment Manufacturers (OEM), suppliers, and research and development institutes. Due to state regulations calling for safer and vehicles, and the world trends seeking for intelligent cars, TechCast-LLC [2012], GCC, also known as Integrated Vehicle Dynamics Control, rises as a suitable solution. GCC proposes the integration of different vehicle control systems in order to meet the objectives of safety/comfort for the passengers, rather than just put them altogether and let them pursuit its own objectives. This integration while keeping the same, or less, costs of sensors and actuators. He [2005] remarks this advantage of the GCC, which can share sensors information and actuators functions in the integration, avoiding duplicated sensors or actuators. The vehicle dynamics theory defines different effects in the vehicle that can be classified in three directions: Vertical, Lateral and Longitudinal, Fig. 1 presents the vehicle axis system. Depending on what dynamic direction we are looking some variables become relevant. Vertical dynamics refers to the movements that affects mainly the comfort of the passenger: pitch(φ) (around Y axis), roll(θ) (around X axis), and vertical acceleration(¨ z ) (vertical acceleration). The longitudinal dynamics refers to the stability of the vehicle: longitudinal velocity (v), wheel rotational velocity (ω), and tire slip ratio (λ). Lateral dynamics also refers to stability: lateral displacement (y), side slip angle (β), and yaw(ψ) (around Z axis).
⋆ Authors thank Autotronics and Development of Products for Emerging Markets research chairs at Tecnol´ ogico de Monterrey.
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Fig. 1. Vehicle dynamics Three vehicle subsystems have been studied: steering, suspension, and brake/driveline, Fig.2. Each can be controlled with an electronic control system, for example, for controlling the steering system the controller can modify the driver’s steer angle command, Active Steering (AS ). For braking, each wheel can be braked individually, the Antilock Brake System (ABS ) or Electronic Stability Control (ESC ). The driveline can regulate the power transfer for each wheel, the Anti-Slip Regulation (ASR), and for suspension, the damper can change its damping coefficient, Continuos Damping Control (CDC ). Individually they can counteract the effects of some of those dynamics, the AFS alone can control the tire slip ratio, vertical velocity and helps in the control of the yaw and lateral displacement. But most of these control systems were designed to operate reaching its own goal, but when they are put to operate simultaneously the result can be a degradation of the general performance, because of their interactions. The main objective of the GCC is to integrate those chassis control systems avoiding conflicts on their actions by coordinating them. Sato et al. [1992] evaluate different configurations of vehicle control systems like Active Suspension (A-SUS ), Active 4 Wheel Steering (A-4WS ), Traction Control and ABS. His results demonstrate the advantage of controlled vehicle systems over passive ones, and how a coordinated integration improves the overall vehicle performance. 10.3182/20130204-3-FR-2033.00040
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Fig. 2. Vehicle systems: (a) Brake, (b) Steering, and (c) Suspension. (b)
Obviously the sub-system integration is not the only issue that comes with the GCC. The complexity of the resulting integrated control system, because of the wide number of signals that need to be managed and processed by the control units, this lead to a more powerful Electronic Control Units (ECU ). Another issue is the stability during switching between control laws. Tavasoli et al. [2012] overcomes this stability problem by defining a switching gain. This gain is defined as X = n ∗ Xc1 + (n − 1)Xc2 , where X is the overall objective, Xc1 and Xc2 are the individual control laws objectives and n is the switching gain.
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Also with the increasing sensor incorporation to the vehicle, the reliability becomes an issue in GCC that in Hwang and Lee [2006] is taken in consideration, by creating redundancy in instrumentation, whereas virtual or physical sensors, and a discrimination algorithm to diagnose the fault situation and correcting it using the redundancy. General fault tolerant methods will be discussed further. According to the literature review made by Shibahata [2005] the evolution of the chassis control systems can be separated in three generations. The first one starts in the middle 80’s with the introduction of Four Wheel Steering (4WS) by Sano et al. [1986] and a Direct Yaw Control (DYC ), Shibahata et al. [1992], an example of DYC is the one presented in Canale and Fagiano [2008], they use an Nonlinear Model Predictive Control approach to control the yaw moment of the vehicle by a Rear Active Differential. The systems in this generation were intended to be a secondary control system, additional systems which support the primary control system. The second generation became the control system itself, instead of support the control system, the driver interacts directly with the control system, this generation was represented by systems like Variable Gear Ratio Steering system and the X-by-wire control systems. The current generation according to Shibahata is the integration generation, where various systems are intended to interact in order to achieve performance in the vehicle beyond the level reached by the systems alone. Also the driver assistance systems become important in this generation, they can be consider as another vehicle control system that supports the drivers intentions, and needs to interact with the other control systems. The control systems which represents this generation correspond to GCC systems and here is where this paper is situated. The integration of different vehicle control systems can be seen in the literature with OEM ’s own concepts of integration, like GM ’s project “Trilby”, Schilke 876
Fig. 3. Control system topologies: (a) Centralized,(b) Decentralized, and (c) Multi-layer Control Structure. et al. [1988], Nissan,Imaseki and Kobari [Jul 10 1990], or Toyota, Yokoya et al. [1990]. The outline of this article continues as follows: The section 2 discuses the different control topologies, the section 4 presents the different approaches to Integrated Chassis Control from a single objective integration to a full Dynamics Integration structure, the section 5 discuses the integration from the fault tolerant point of view. Finally, section 6 concludes this review.
2. CONTROL TOPOLOGY For better understanding about the GCC approach it is necessary to describe the basic topologies found in literature. To achieve integration proposed by GCC concept, the global control system needs to interact with the existing controllers of the vehicle. Through the time many control structures have been detected in the literature, according to Yu et al. [2008] three topologies can be found: decentralized, centralized, and multi-layer control structure. Each one presents its benefits and limitations to achieve the GCC goal. The Multi-layer is the more suitable for this purpose.
2013 IFAC SSSC Grenoble, France, February 4-6, 2013
2.1 Decentralized Control In a decentralized control structure, Fig. 3b, every subsystem has its own independent controller and control goals which commands its particular actuator. The interaction among different control loops is limited to shared information obtained from a communication bus. This type of control architecture was used in the early chassis control integration. In this scheme the integration lies on the OEM ’s, while the supplier provides its systems interconnection options. An example of decentralized control architecture is a vehicle equipped with ABS and semiactive suspension, the control systems seek for their own goal ignoring the action of the other system. 2.2 Centralized Control In a centralized control structure, Fig. 3a, the main goal consists in a single main controller that send the control signals directly to the subsystems actuators. This kind of information flow increases the computational load in the central ECU demanding more powerful units. In the other hand the centralized structures are rigid, which limited its reconfiguration capabilities for integrating new elements in the control systems. If there is a new sub-system for the control loop the only solution is to redesign the full control system. Also, this type of topology forces the OEM ’s to open its architecture. 2.3 Multi-Layer Control In the multi-layer scheme three layers can be identified, the coordination layer, the controller layer and the actuation layer, Fig. 3c. Acording to Gordon et al. [2003] the coordination controller layer has two main functions, first based on the dynamic state of the vehicle it determines desired set-points for the controller layer, the second, is to set a specific mode of operation into the actuators. The second layer receives the control signals from the coordination one and it selects and control its own control subsystem in the indicated mode by the upper layer. For instance, depending on the overall vehicle situation the suspension system can operate in comfort mode or in road-holding mode. 3. INTEGRATED CHASSIS CONTROL APPROACHES Research and Development institutes and OEM ’s have turn the sight to the GCC, their interest rely on the intention of achieving safer and more comfortable commercial vehicles. The scope of their researches is to develop integrated strategies that improve the overall performance of the vehicle in different driving conditions. This section is divided in three different approaches, (1) to integrate the available chassis systems to improve one direction (lateral, longitudinal or vertical) of dynamical performance, (2) to make control in more than one direction, and (3) the control strategies that try to reach the full direction vehicle dynamics control.
in this subsection are presented some approaches that by integrating different chassis control systems they improve a single characteristic or vehicle property. In Valasek et al. [2004] the objective is to reduce the braking distance in an emergency situation by integrating the semi-active suspension system with an ABS algorithm in a multi-layer topology, in this structure the suspension is used to reduce the fluctuations of the vertical forces between the road and the tire, when the upper layer switch its mode to emergency mode. Wei et al. [2006] propose and multi-layer integration of AFS and a torque based DYC to control the lateral vehicle dynamics using fuzzy logic, the control variable is the yaw moment which is divided into 3 states. The first state correspond to a low yaw moment and only the AFS acts, in the second level both the AFS and DYC gets in action, an in the last level only the DYC should be controlling the yaw moment. G´asp´ar et al. [2007a] and G´asp´ar et al. [2007b] proposes the multi-layer integration of suspension and braking to prevent a rollover situation in a vehicle during cornering, the controller uses a weight function that relates its magnitude with the level of danger of the vehicle state, this function is sectioned by two thresholds the first one is used to indicate a possible rollover situation and the suspension system only tries to revert the situation, then if the situation continues the function reaches the critical level and the braking system starts its action to prevent the rollover. Poussot-Vassal et al. [2008] and Poussot-Vassal et al. [2011b] propose the multi-layer integration of steering and braking systems to improve the lateral dynamics but they approach the issue from the LPV /H∞ framework, where a GCC controller determines the needed brake torque and steer angle to meet the desire yaw rate. Hwang et al. [2008] proposes a method that integrates AFS and ESC to improve the lateral dynamics of the vehicle, this is made by two methods, the first one uses a multilayer architecture where a supervisory layer coordinates the actions of the two systems separately, the other uses a centralized control who integrates the two systems in one ECU, the results demonstrate that the unified controller overcomes the performance of the supervisory one, but this controller presents the disadvantage that needs to modify the system architecture to integrate the two systems. Kangwon et al. [2009] integrate ESC and CDC, with a multi-layer scheme, to control the lateral dynamics during emergency situations, like in a single line change maneuver. The supervisory layer determines the mode of operation of each individual control system based on the deviation from the desire yaw rate.
3.1 Single Purpose Integration
In Wu et al. [2010] they propose the centralized integration of braking force control, active rear wheel steering control and Active Yaw moment Control to improve vehicle handling and line keeping performance, the controller is optimized by LMI - H∞ also using a driver model to give the steering commands in line kipping procedures.
Some approaches of integration of vehicle control systems are intended to achieve only a single objective of control,
Seongjin and Kyongsu [2011] integrates ESC and AFS to create a multi-layer Integrated Chassis Control (ICC )
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system and an Active Anti-Roll Bar to create an Active Roll Control System (ARCS ). The main objective of this integration is to improve the ARCS capabilities for control vertical dynamics, specially the Roll angle. Integrating the ARCS with other chassis control systems, the results prove that the performance of an stand-alone ARCS can be dramatically improved with the integration. In Seongjin et al. [2012] they use the same ICC system, formed by ESC and AFS, to design a DYC to improve lateral stability of the vehicle using the Weighted Pseudo-inverse based Control Allocation (WPCA) approach. 3.2 Multi-Purpose Integration A more advance integration of chassis control systems are when the integration aims to control two or multiple objectives but without actually fully control the vehicle dynamics, these approaches get closer to the GCC concept. In Andreasson and B¨ unte [2006] they present a centralized approach to GCC using an inverse dynamics model, which main objective is to control de horizontal (lateral and longitudinal) dynamics, although they ensure that it can be extended to full dynamics control. The method is capable to be reconfigured at any time and it is tested using different vehicles configurations. Mokhiamar and Abe [2006] propose the integration of the four wheels in the control of the horizontal dynamics of the vehicle, the proposed control system consist on the integration of DYC, Front Wheel Steering (FWS), and Rear Wheel Steering (RWS), the results show that this control system overcomes the performance of the one obtained of a FWS + ABS vehicle in severe cornering conditions (large amplitude sine input). Falcone et al. [2007] they integrate the braking and steering systems to improve longitudinal and lateral dynamics. The idea is to make the vehicle capable of perfectly follow a trajectory by centralized Model Predictive Control (MPC) algorithms, the results were satisfactory compared with a single steering controlled vehicle. Poussot-Vassal [2008] in its dissertation proposes a multilayer GCC approach using the LPV /H∞ framework, integrating braking, suspension, and steering systems, he divides the vehicle dynamics in two, vertical and horizontal (lateral, longitudinal), the vertical dynamics are controlled by means of the suspension system, using semi-active dampers, the objective is to achieve compromise between road holding and comfort. The horizontal dynamics are controlled by the steering and braking systems, which control the yaw rate and lateral acceleration. In Miura et al. [2008] they design a multi-layer control system that integrates traction and braking systems by means of a called Active Center Differential (ACD) system and ABS, the aim of this control system called Super All Wheel Control (S-AWC) from Mitsubishi Motors Co. is to improve the longitudinal and lateral dynamics with those systems and its application results in a reduction of steering effort of the driver. Xiao et al. [2009] propose an integration scheme implemented by a multi-layer architecture with an ESC + 878
Active Suspension System (ASS) configuration. It consists in two layers, the upper layer monitors the driver’s intentions and vehicle dynamic state. With this information it computes a corrective yaw moment Mz , and generated the distributed torques MESP and MASS to the controller layer. The lower layer, execute the control commands of the upper layer, the ASS system is controlled by a LQG technique and the ESC by Adaptive Fuzzi Logic (AFL). The integration proposed by Xiao controls the vertical and the lateral dynamics, the emphasis in the lateral stability. In Wanki et al. [2011] and Cho et al. [2012] they integrate ESC and AFS to improve agility, maneuverability and, vehicle lateral stability, the controller uses a multi-layer topology with a supervisor and control algorithm layers, the strategy depends on a threshold of the yaw rate to determine wether to use maneuverability or agile mode. Also this control system uses the yaw rate to maintain the side slip angle in a low level. In the same way Jangyeol et al. [2010] use the same chassis systems but its goal is to improve vehicle lateral stability and maneuverability while preventing rollover, it is noteworthy that their control scheme was implemented with human in the loop configuration, this to demonstrate the effectiveness of the algorithm when interacts whit the drivers inputs. 3.3 Complete Dynamics Integration The approaches that completely fulfill the GCC concept are the ones that control the vehicle dynamics in the three directions, vertical, lateral, and longitudinal. In Lu et al. [2010] and in Lu et al. [2011] a complete GCC integration is presented, using braking, suspension, and steering to control all the vehicle dynamics, the algorithm uses a multi-layer topology controller, with a supervisory layer that detects and classify the vehicle state in seven predefined conditions and coordinates the roll of the integrated chassis systems, then a controller layer, which includes each system controller, follows the decisions of activation and mode of operation of the supervisory layer, in order to control each state with a particular strategy. In Poussot-Vassal et al. [2011a] a multi-layer control system was designed considering the vertical and horizontal dynamics, the vehicle chassis systems involved were: suspension using active dampers, and brakes. The design were made in two different controllers, one for the vertical dynamics and other for the lateral, the link between the two controllers it the “monitor”. The monitor is an upper layer that, based in the wheel slip ratio, coordinates the tuning parameters of the chassis systems to overcome conflicts and guaranty a well coordinated action. 4. GLOBAL CHASSIS FAULT-TOLERANT CONTROL During last decade, advanced control systems in the automotive industry have been proposed to consider features of safety and fault tolerance in order to maintain the desired performances: comfort, handling, vehicle stability, etc. When the controller is designed to be robust to vehicle model uncertainties or instrument failures, the Fault-Tolerant Controller (FTC ) is passive; while, if the
2013 IFAC SSSC Grenoble, France, February 4-6, 2013
controller uses on-line information for the fault accommodation is considered active, i.e. a Fault Detection and Isolation (FDI ) module is used. The major effort of FDI systems and FTC in vehicle dynamics is applied in only one system (suspension, steering or braking without any chassis integration) and some of them belongs to the single purpose integration approach. For adding fault-tolerance in the active suspension control system, Chamseddine and Noura [2008] proposes an active FTC based on sliding mode theory, in this case the vertical, lateral and longitudinal dynamics are evolved. On the other hand, an experimental validation of a methodology for detecting faults in semi-active or active dampers based on parity equations is shown in Fischer and Isermann [2004], a sedan car is used as test-bed, i.e. inherently the chassis motion includes the global rotational and translational dynamics. Related to the lateral stability control, in Bosche et al. [2009] is proposed a passive FTC when the steering actuators (front or rear) fail, the LPV controller is designed by using a parameter-dependent Lyapunov matrix of the lateral and longitudinal vehicle dynamics. For monitoring the vehicle chassis performance, a design methodology of an FDI module for the suspension, steering and braking is developed as a Multi-layer framework in Pisu et al. [2003]; the proposed approach was illustrated in details in the braking system. A GCFTC with multi-purpose integration or complete dynamics integration is a hard task that recently has been studied, e.g. in G´asp´ ar et al. [2010] a Multi-layer FTC is proposed to guarantee road holding and roll stability when sensor and actuator faults appear (analysis in the vertical and lateral chassis dynamics), the reconfiguration of the proposed active FTC is based on LPV control theory. 5. IDEAL INTEGRATED CHASSIS CONTROL SYSTEM According to the literature review and the actual state of the technology, an ideal Integrated Chassis Control System approach is proposed. The propose must meet the following aspects: • Full dynamics integration. • Multi-layer control stricture. In this scheme the computational load is divided into several ECU s, also this architecture gives the system the capability of been reconfigured easily. • Safety-comfont oriented. In the list of priorities the main purpose of the system integration must be safety, followed by comfort. • Multi-mode operation. It is necessary for the system to be capable of operate in different control modes, because there is no a single mode of operation that can achieve optimal performance in all conditions. • Fault tolerant. In order to be able to counteract against undesired situations. 6. CONCLUSIONS This literature review has shown the benefits of vehicle control systems integration in terms of vehicle dynamic performance. Also based on the contributions cited above, the multi-layer control structure, nowadays, seems to be 879
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