2 Electrohydraulic Brake System and Control
Figure 1 shows the setup of the electrohydraulic brake system (EHBS). It consists of a brake pedal mechanism, an elastic push rod, a tandem master brake cylinder, an EHBS actuator, a complete valve control unit and wheel brakes. The EHBS actuator consists of a current controlled electric motor and a gear which converts the motor
torque to an assisting force, therewith serving as an electric power brake booster.
Figure 1 Electrohydraulic brake system
A brake pedal displacement applied by the driver is propagated through the pedal gear ratio and the elastic push rod to the primary piston of the master brake cylinder.
This displacement is assisted by the EHBS actuator by inducing an additional force on the piston. The displacement of the piston in the master brake cylinder generates a
pressure build-up and therewith the braking torques at the wheel brakes. While in (1) the pressure in the master brake cylinder is used as a controlled variable for a superimposed control of the EHBS, in this paper the push rod force will be controlled. With the underlying control concept, the pressure build-up is an automatic by-product. This EHBS control is particularly suitable for vehicles with recuperative braking, as combined recuperative and friction-based braking can be accomplished with minimal influence on the brake pedal feedback. The control concept is comparable to the control concept for electric power steering systems described in (2).
Figure 2 shows the EHBS control in the vehicle and driver environment.
Figure 2 EHBS control in vehicle and driver environment
The inputs to the EHBS are the driver foot force Fdr, the recuperation torque Trec of the electric vehicle motor, the torque request Tmot,req for the EHBS actuator and the velocities of the wheels in the vector Ωwl. The vector-valued output Tbra contains the braking torques at the wheels to decelerate the vehicle. In an outer loop the driver controls the vehicle velocity vveh. The driver foot force Fdr as the control variable of this loop is generated in interaction with the brake system. Another loop is built by the EHBS controller that generates the torque request Tmot,req to provide the braking assistance. The controlled variable of this loop is the push rod force. The aim of this EHBS control is to ensure a fast and well-damped transient response to a reference force for the push rod, steady-state accuracy and to reject disturbances. Since in the steady-state the push rod force is proportional to the driver foot force the control generates the brake pedal feedback.
The EHBS controller, again, consists of an observer-based state-feedback controller (3, 4) with a nonlinear reference generator, as shown in figure 3.
Figure 3 EHBS controller
Feedforward of the reference force Ffs,ref and of disturbance variables in the vector xd yields the steady-state accuracy of the control for considered classes of reference and disturbance excitations. Feedback of the brake system state variables in the vector xp provides the fast and well-damped transient response to the reference force. Since not all variables can be measured, an observer is required to estimate these variables by using the measurement variables in the vector ybra and the control variable Tmot,req . The desired brake pedal feedback for the driver is given by the reference force Ffs,ref which can be derived from variables for the vehicle dynamics in the vector yveh and variables of the brake system in the vector yobs also provided by the observer. The functionality of the reference generator is the topic of the following chapter.