Development of ABS using HILs

An efficient simulation platform for the development of anti-lock brake system (ABS) is presented. To reduce computational burden due to the rigidity of hydraulic unit comprising brake system, the actual hydraulic brake assembly is taken .from the target vehicle and integrated to a virtual 18-DOF-vehicle dynamics model. The latter is used to describe dynamics of the target vehicle under extreme maneuvers. Thus HILS allows the investigation of not only the dynamic behavior, but also hydraulic response of the vehicle with a complete assess to all system parameters and state variables. Introduction Tuning the control algorithm of ABS, most manufacturers of ABS have relied on road tests, especially in winter. The technique has some limitations: cost, the period of development, safety and so on. Also, it can not define control parameters quantitatively. In many cases of the road tests, the tuning of parameters depends on a driver's experiences or interjection. Also the road tests could not be performed under uniform condition; for example, pressure of brake pedal, friction coefficient and so on. More complicate the structure of modulator is, more difficult the modeling of hydraulic brake system is. To solve those problems, HILS (Hardware in the Loop System) is suggested. The simulation platform consists of a virtual vehicle dynamic model and a real brake system. The real brake system is used to reduce computational burden due to the rigidity of a hydraulic unit. Consequently, it is shown that the results of the simulation almost coincides with those of the road tests. Construction of the HILS The vehicle under consideration is modeled on 18 DOF system from AUTOSIM. It employes Macperson strut front suspension and a typical steering system. The steering system is modeled on 6-bar linkage with Ackermann geometry. [21[31 The front suspension model is simplified as a spring and damper system with closed kinematics chains. To reduce the computational burden, the tension bar is removed, and the joint has unique rotation movement. r41 The HILS is made up with three modules, namely, hydraulic module, DSP module and ABS ECU. Hydraulic module: Hydraulic module is a modified brake system to maintain same conditions in every test. An air cylinder is applied instead of brake pedal because it can give same brake pressure every test. And the hydraulic modulator, which is adapted in HILS, is a commercial solenoidsolenoid type one. To pick up the variation of brake pressure at wheel, a strain gauge type sensors is set up at each wheel. DSP module: This module is used to generate wheel speed signals, which is calculated from pressure signal and to operate the virtual vehicle dynamic model. It consists of four parts, DSP 8 0-7803-5729-9/99/$10.00 O 1999 IEEE KORUS’99 220 Information Systems and Technologies board, I/O card, LAN card and host PC. The DSP board contains vehicle dynamic model. The I/O card receives 4 signals from pressure sensors and generates 4 wheel speed. LAN card is in charge of interfacing between host PC and DSP board and booting on DSP board. And host PC is used to initialize state variables and parameters, and to save data for analysis of control results. ECU: ABS ECU determine the road surface conditions using the emulated speed signals, and provide appropriate control action to prevent brakes from locking. Data transfer between modules Hydraulic module to DSP module: Once ABS ECU controls the hydraulic module, pressure signal at each wheel is changed and .@e changed signal is picked up and transferred to DSP module. Then DSP module calculates the speed of each wheel using vehicle dynamic model and generates wheel speed data. DSP module to ABS ECU: The wheel speed signals calculated from DSP module are transferred to ABS ECU. And ABS ECU determines whether the situation is to start control or not. ABS module to hydraulic module: In comparison with wheel speed signals from DSP module, ABS ECU determines control state. From the decision, ABS ECU controls hydraulic pressure in the real system. System validation The off-line simulations have been conducted to verify the reliability of 18-DOF vehicle model. With existing road-test results at hand, the brake pressure signals are taken as the input for the system of nonlinear equations of motion and the resulting wheel speed signals are compared with measured wheel speed signals. Then, the system parameters including the road-surface conditions are tuned to minimize the difference between the simulation and road-test results as illustrated by Fig 1 and 2