ROLL RATE BASED STABILITY CONTROL - THE ROLL STABILITY CONTROL ™ SYSTEM Jianbo Lu Dave Messih Albert Salib Ford Motor Company United States Paper Number 07-136 ABSTRACT precise detection of potential rollover conditions and driving conditions such as road bank and vehicle This paper presents the Roll Stability Control ™ loading, the aforementioned approaches need to system developed at Ford Motor Company. It is an conduct necessary trade-offs between control active safety system for passenger vehicles. It uses a sensitivity and robustness. roll rate sensor together with the information from the conventional electronic stability control hardware to In this paper, a system referred to as Roll Stability detect a vehicle's roll condition associated with a Control™ (RSC), is presented. Such a system is potential rollover and executes proper brake control designed specifically to mitigate vehicular rollovers. and engine torque reduction in response to the The idea of RSC, first documented in [10], was detected roll condition so as to mitigate a vehicular developed at Ford Motor Company and has been rollover. implemented on various vehicles within Ford Motor Company since its debut on the 2003 Volvo XC90. INTRODUCTION The RSC system adds a roll rate sensor and necessary control algorithms to an existing ESC system. The The traditional electronic stability control (ESC) roll rate sensor, together with the information from systems aim to control the yaw and sideslip angle of a the ESC system, help to effectively identify the moving vehicle through individual wheel braking and critical roll conditions which could lead to a potential engine torque reduction such that the desired path of vehicular rollover. Such critical roll conditions need a vehicle determined through the driver’s inputs (e.g., to be discriminated from those due to road bank steering input) can be maintained. That is, ESC variations and to be characterized with respect to systems help the vehicle to follow the driver’s intent vehicle loading variations. RSC then applies pressure such that the driver maintains good control of the to the brake(s) on the wheel(s) of the outside of the vehicle regardless of the variation of road conditions. turn. This reduces lateral force and helps keep the inside wheels firmly on the ground, thus reducing the Beyond yaw and sideslip control, brake controls in likelihood of a rollover event. ESC systems have been pursued to mitigate vehicular rollovers in recent years. For example, [1] describes Although a complete RSC system includes many an enhanced system over Driver Stability Control algorithm modules such as sensor off-set systems for commercial trucks. [2] proposes a stand- compensation, sensor signal filtering and processing, alone function called Anti-Rollover Braking (ARB) sensor plausibility, active wheel lift detection, when an impending rollover of a vehicle is sensed. In software enhancement of brake hydraulics, [3], engineers from Bosch describe a rollover longitudinal velocity computation, etc., this paper mitigation function over its ESP system. Continental focuses on vehicle roll dynamics and state estimation Teves has developed an Active Rollover Prevention as well as the RSC control strategy. Interested readers (ARP) system. [4] proposes a Rollover Control may find more details on those topics from various Function (RCF). Note that the aforementioned patents granted to Ford Motor Company such as (but systems use only ESC hardware. In addition to ESC- not limited to) [11],[12],[13],[14],[15] and [16]. based brake controls, other chassis control systems have been pursued to mitigate rollovers, see [5], [6], This paper is organized as follows. The vehicle roll [7], [8] and [9] for more details. stability and state estimation are discussed in the next section. The sequential section provides a brief In order to achieve smooth rollover control without description of vehicle loading estimation. Wheel lift sacrificing other vehicle dynamics performance detection is discussed in the next section. The last two attributes with respect to road and driving condition sections focus on various RSC control strategies and variations, precise detection or prediction of a the conclusions. potential rollover event is critical. Due to the lack of Ford RSC 1 VEHICLE ROLL DYNAMICS SENSING AND The critical roll angle defining a potential rollover STATE ESTIMATION event is the relative roll angle θxbm between the vehicle body and the moving road, which is defined Vehicular roll instability (rollover) is the condition as where a vehicle has divergent roll motion along its roll axis. θ = θ +θ (1) b xbm xbw xwm 2 or θxbm = θxb −θxm (2) s 2 b3 If the magnitude of is greater than a threshold s3 θxbm for a certain duration, the vehicle is likely to be roll- instable. θxb θxbw The relative roll angle θxbm may be determined through laser height sensors which measure the distances of the vehicle body at the sensor mounting locations from the road surface along the direction of θxwm m3 m2 the laser beams. However using them in mass production for rollover detection purpose is generally θxm cost prohibitive with the current technology. Therefore using the other sensors equipped with the vehicle is desired. Figure 1. The roll angle definitions for a vehicle driven on a banked road. RSC ESC Consider a vehicle driven on a general road surface. Traction Control ABS Figure 1 shows a rear view of the vehicle. Its roll 1988 1998 2001 2003 2005 - instability can be identified and characterized by Key Key Key Key Sensor Sensor Sensor Sensor using the vertical travel of the wheel centers with Inputs: Inputs: Inputs: Inputs: respect to the smooth road surface. That is, it is said Wheel Same as Steering Roll Rate to be roll instable if it has sustainable two wheel lift Speeds ABS Wheel from the road surface (both wheels are on the inside Angle, Yaw Rate, of a turn). Lateral The RSC AdvanceTrac® Acceleration, System was with RSC is The roll instability can also be determined by using MC Pressure, first available standard on Longitudinal on the 2003 Explorer, various roll information. In order to define the Acceleration Volvo XC90 Mountaineer, various roll angles, we define two coordinate systems: Aviator and Navigator a body-fixed coordinate system _ with axes _. , _/ since the 2005 MY, E-350 Econoline _ and 0 (called the body frame) and a road coordinate Extended Passenger system j with axes j , j and j which is vans since the 2006 . / 0 MY, and attached to the road surface but moves and yaws with Expedition, Escape, the vehicle body (called a moving road frame). The Edge, roll angle of the vehicle body with respect to the sea Explorer SportTrac, o Lincoln MKX, level is denoted as u_ , the road bank angle with Mazda CX – 9, Land Rover LR2, respect to the sea level is denoted as ouj , the roll Mazda Tribute and Mercury angle between the wheel axle and the road surface is Mariner for 2007 MY denoted as outj (which is called a wheel departure angle), and the roll angle between the body and the o axle of the wheels is denoted as u_t(which is called Figure 2. The evolution of the sensors used in a chassis roll angle). vehicle stability control systems. Ford RSC 2 The sensor set used in the RSC system evolved from to the vehicle reference velocity calculated based on the initial sensors equipped on an anti-lock brake the wheel speed sensor signals. system (ABS), see Figure 2. It includes a centralized motion sensor cluster called the RSC sensor cluster, a Based on (3), it is not hard to find the following: steering wheel angle sensor, four wheel speed sensors, a master cylinder pressure sensor, etc. The (i) the roll rate sensor only provides global RSC sensor cluster adds a roll rate senor to the ESC information of the sensor frame with respect motion sensor cluster, i.e., it is composed of a roll " to the sea level, θxb as in Figure 1, which rate sensor, a yaw rate sensor, a lateral accelerometer cannot be directly used as a control variable and a longitudinal accelerometer, which are packaged to drive the RSC system; together along the three orthogonal directions. (ii) the global roll and pitch angles can be Since the measuring directions of the RSC sensor determined from the accelerometers if the cluster do not always coincide with the directions of lateral velocity v is known, however in the body frame _ , it is necessary to define a sensor ys frame p . The angular differences between frame _ reality, it is unknown; and frame p are called the sensor misalignments, which are usually generated due to the mounting (iii) the lateral velocity or sideslip angle can be errors when the RSC sensor cluster is attached to the determined from the lateral acceleration vehicle body. Although the sensor misalignments are sensor signal if the global roll angle can be relatively small, they may need to be corrected in determined; order to avoid potential signal contamination. In addition to the sensor misalignments, oftentimes the (iv) the roll rate sensor will have non-zero misalignment between the vehicle body and the road output even if there is no roll attitude surface due to unevenly distributed loading inside the change when there is yaw rate on a pitched vehicle may also need to be corrected. For example, a road. vehicle with heavy loading near the rear axle might cause the RSC sensor cluster to be tilted with a pitch Since there are uncertainties in the roll rate sensor angle relative to the road surface. These signal and in the computation of the pitch angle, misalignments can be conditionally determined based direct integration of the first equation in (3) is not on the sensor and the calculated signals and the practical due to the potential of integration drift.