An investigation of abs strategies for articulated vehicles (2004)

This paper investigates anti-lock braking control strategies for heavy goods vehicles. A mathematical model of the braking of an experimental articulated vehicle is developed. The model includes the details of vehicle dynamics, the ABS control algorithm as well as the mechanics of the foundation brakes and the pneumatic and mechanical systems. Parameters for the model are determined from tests on an instrumented experimental tractor semi-trailer. The model is validated by comparing its performance to experimental results for the instrumented vehicle undergoing straight-line braking tests on several test track surfaces. The simulation and experimental results show good agreement. The model is used to evaluate the benefits of wheel-slip control. The results indicate that wheel-slip control has the potential to reduce braking distances by approximately 25%, however some hardware modifications may be needed to achieve this. INTRODUCTION The braking system is a critical safety component of a vehicle. ABS systems are mandatory on European heavy vehicles. This paper considers the performance of ABS system and whether improvements could be made to stopping performance. In early computer simulations, Fancher showed that the steering axles play an important part in the braking of articulated vehicles (Fancher 1976). He later compared a worst-wheel ABS strategy to an independent wheel strategy (Fancher 1985) and showed that lightly loaded vehicles can jack-knife in braking-in-a-turn manoeuvres nearing the limits of tyre-road adhesion, when independent wheel control is used on a tractors rear wheels. These simulations were later extended to include driver models in a closed loop system (MacAdam 1985). The conclusions were that the rear axle of a tractor should be equipped with a worst-wheel strategy to assist drivers to maintain path control, and an independent wheel control strategy should be implemented on steering axles so as to improve the deceleration performance. Van Zanten (Zanten 2002) discussed the extension of ABS which controls individual wheel behaviour to ESP (Electronic Stability Program)  which controls the whole vehicle. A combination of sensor measurements and estimation algorithms, using model-matching control can be used to prove feedback signals. The adaptation of these intelligent systems from passenger cars to commercial vehicles, as well as systems designed originally for commercial vehicles, was discussed by Palkovics and Fries (Palkovics 2001). Commercially available systems, incorporating anti-jack-knifing and stability enhancement systems, include the Haldex EB+ and the Knorr-Bremse ROP (Roll Over Protection). Conventional air-actuated ABS systems for heavy vehicles use a bang-bang control approach to controlling wheel rotation under braking. There appears to be some scope for alternative strategies. McLoughlin (McLoughlin 1985) analysed wheel-slip control as an alternative to conventional ABS strategies, and concluded that wheel-slip control offers advantages for a range of tyre friction characteristics. Wheel-slip control seeks to continually optimise the slip during braking to generate maximum deceleration. Hardy and Cebon (Hardy 1995) used a validated ABS simulation model to show that wheel-slip control can improve the stopping distance of heavy trucks fitted with either steel or air suspension on rough roads. The benefits are particularly pronounced when the road surface roughness increases. Implementation of wheel-slip control however is not straightforward due to limitations caused by pneumatic time delays and the difficulty in measuring absolute vehicle speed under braking. These problems were discussed by Gissinger et al (Gissinger 2003), who concluded that a complete redesign of brake actuation and hardware was required to implement wheel-slip control. A number of researchers have investigated wheel-slip control for passenger cars as well as heavy commercial vehicles. Kimbrough (Kimbrough 1994; Kimbrough 1999) combined a simple bilinear tyre model with a Lyapunov Stability Criterion so as to develop a method to account for tyre saturation. The inherent robustness of Fuzzy Logic control has made the strategy a frequent choice in developing advanced wheelslip control algorithms (Akey 1995; Akey 1996; Chen 2000; Will 2000; Yu 2002). Jun (Jun 1998) has presented a comparison of conventional threshold ABS, fuzzy logic, sliding mode and PID control. The fuzzy control was the easiest to implement and calibrate but in its implemented form was poor in performance. This paper presents an early stage investigation into ABS systems for heavy vehicles. Chapter 2 presents a programme of field tests on an instrumented vehicle, chapter 3 develops a mathematical model of the vehicle and chapter 5 compares the measurements with simulation results. Finally chapter 6 presents an early investigation of wheel-slip control for heavy vehicles. FIELD TESTS Brake system modifications Braking tests were performed on the tractor / semi-trailer combination shown in Figure 1. In order to investigate the performance of the trailer ABS system, the brakes on the tractor unit were disabled during the tests. Figure 1. Tractor and semi-trailer combination used for testing. A simple pneumatic circuit that could apply a constant brake pressure to the trailer brakes was developed (Figure 2) and inserted on the truck just upstream of the tractor/trailer coupling. This circuit enabled the brake-pressure signal to be adjusted using the regulator prior to the test run. This pre-selected pressure was then applied down the service line of the trailer by the solenoid valve, using a switching signal generated by the operator in the tractor cab. The two-way valve is a safety measure that ensures that the driver an override the test-brake pressure by use of the normal foot-brake pedal or the handbrake, if required. Supply line to tractor Service line to tractor Solenoid valve Regulator Supply line from compressor Service demand from foot-pedal 2-way valve Figure 2. Modifications to brake pneumatics Instrumentation and data logging The instrumentation used on the test vehicle is illustrated in figure 3. Two ICONs (industrial computers from Mektronika Systems, specifically designed for data logging and control) were used for logging the test data. One was located on the tractor and the other on the trailer. The ICONs logged at 250 Hz and low pass filters were set at 35 Hz. The ICONs communicated via CAN (Control Area Network) with a Global Controller computer, which was located in the tractor. ICON ICON Brake demand Longitudinal Tractor Acceleration Brake Switch Tap into wheel speed sensors Tractor Speed REV Pressure Chamber Pressures Global Controller