A Concept for an Electrohydraulic Brake System with Adaptive Brake Pedal Feedback

Abstract

This paper describes a concept for an electrohydraulic brake system for electric vehicles. The concept offers the possibility to generate about any brake pedal feedback for the driver, so that the perception of the brake pedal can be influenced and adapted to actual driving conditions. The brake system 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. Two examples are presented to show how different brake pedal feedback characteristics can be implemented. The developed control concept is analyzed in a simulation with a detailed nonlinear model of the brake system in an electric vehicle environment. The simulation results show a very good performance for recuperative together with friction-based braking without negatively affecting the pedal feedback.

1 Introduction

The demand for reducing the CO2 emission in spite of increasing individual mobility requires new drive technologies as an alternative to combustion engines. One approach which has been an actual researched topic in the last years is the development of electric vehicles. For these vehicles in addition to the electric drive and battery technology the brake system is of great importance. That offers the possibility to develop new brake systems which fulfill high technical requirements. Due to increasing demands for energy efficiency, driving comfort and safety, these systems have to be controlled with high speed.
This paper describes a concept for an electrohydraulic brake system for electric vehicles where the brake pedal feedback force to the driver is controlled by an electric power brake booster. The concept offers the possibility to generate about any brake pedal feedback for the driver, so that the perception of the brake pedal can be influenced and adapted to actual driving conditions.
The concept and results shown in this paper have been developed in the context of the research project “Simulationsgestützter Entwurf für Elektrofahrzeuge” (Simulation-Based Development for Electric Vehicles) co-funded by the “Ministerium für Wirtschaft, Mittelstand und Energie des Landes Nordrhein-Westfalen”. Partners in the work performed are dSPACE GmbH, the LEA institute and C-Lab from the University of Paderborn and DMecS GmbH & Co. KG.

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 y_bra and the control variable T_mot,req .
The desired brake pedal feedback for the driver is given by the reference force F_fs,ref
which can be derived from variables for the vehicle dynamics in the vector
y_veh and variables of the brake system in the vector y_obs also provided by the observer. The
functionality of the reference generator is the topic of the following chapter.
3 Reference Generator
The reference generator, which is part of the EHBS controller, generates the desired
brake pedal feedback with a specific brake pedal feel for the driver and the actual

3 Reference Generator

The reference generator, which is part of the EHBS controller, generates the desired
brake pedal feedback with a specific brake pedal feel for the driver and the actual