INTRODUCTION The growing demand for reduction in fuel consumption and CO 2 emissions with a view to protection of the global environment is accompanied by a rise in the attention being given every year to vehicles equipped with electric power trains, including electric vehicles (EV) and hybrid electric vehicles (HEV). These EVs and HEVs activate regenerative brakes when they decelerate, recovering kinetic energy in the form of regenerative electric power, and so are able to reduce fuel consumption. Various braking systems that enable regenerative coordinate braking that coordinate the distribution of regenerative braking force and friction braking force have been proposed for that purpose( 1),(2),(3),(4),(5). One of these is the electric servo brake system( 6). In order to decelerate according to the driver's intention when regenerative-friction brake coordination is in operation, the sum of the regenerative braking force and the friction braking force must always be matched to the demand from the driver (7),(8). In its application to the plug-in hybrid vehicle (PHEV) (9), the electric servo brake system has highly accurate brake pressure control that functions cooperatively with regenerative brakes, and this has enabled the system to provide deceleration characteristics that do not fluctuate regardless of the distribution of regenerative braking force and friction braking force.On the other hand, when a vehicle stalls on a sloping road or other such location and the stalled condition continues while the vehicle drive motor outputs driving force, the electric power train will generate heat. Reducing the driving force in order to prevent overheating results in the vehicle rolling back down the slope. Also, during deceleration when adaptive cruise control is in operation, there are cases when energy loss occurs due to the application of friction braking. That is to say, there is latitude for enhancement of operability and reduction of fuel consumption. Therefore a stall cooperative control that operates with the electric servo brake system to prevent overheating and rolling back down a slope during a stalled condition was developed, together with regenerative adaptive cruise control and hill-start assist, and applied to the PHEV. This paper describes these technical elements involved in the electric servo brake system control that has been applied in the PHEV. SYSTEM OVERVIEW The electric servo brake system is shown in Figure 1. This system is made up of a pedal feel simulator and pedal stroke sensor in a pedal operating unit, a tandem motor cylinder in a braking pressure generating unit, and an electronic control unit (ECU). The pedal feel simulator generates the pedal 2013-01-0697 Published 04/08/2013 Copyright © 2013 SAE International doi:10.4271/2013-01-0697 saepcelec.saejournals.org Application of Electric Servo Brake System to Plug-In Hybrid Vehicle Naoto Ohkubo, Satoshi Matsushita, Masayuki Ueno, Kohei Akamine and Kunimichi Hatano Honda R&D Co., Ltd. ABSTRACT An electric servo brake system applied for use on electric vehicles was applied for use on plug-in hybrid vehicles in order to achieve fuel-savings together with good brake feel and enhanced operability for plug-in hybrid vehicles. The electric servo brake system is made up of highly accurate braking pressure control that functions cooperatively with regenerative brakes together with a structure in which pedal force is not influenced by braking pressure control. The configuration of these components enabled good braking feel even when the power train was being switched from one drive mode to another. Automated pressurization functions that are intended for plug-in hybrid vehicles and that operate with electric servo brake systems were also developed. These developed functions include stall cooperative control that functions cooperatively with the power train, regenerative coordinate adaptive cruise control, and hill-start assist. The application of these automated