INTRODUCTION In the last 30 years, advances in electronics have revolutionized many aspects of the automobile industry. Areas like engine management and safety systems, such as anti-lock braking systems (ABS), traction control systems (TCS), and electronic stability control (ESC) systems, have received particular attention. These safety systems involve the use of electronic control units to modulate the brake and accelerator pedal inputs provided by the driver in order to control the slip of individual tires during emergency braking (ABS) or accelerating (TCS), or to control the stability of the vehicle by braking individual wheels (ESC) [ Zan00, Alb96, Ack99]. ABS is by no means a new innovation, and its development and acceptance has occurred over a number of decades. The first ABS system was developed by Dunlop Maxaret in 1952, and was used on aircraft landing systems [Vel01]. In 1978, Robert Bosch GmbH introduced the modern anti-lock braking system for passenger vehicles [Mar02-a, Mar02-b]. By the 1990s, ABS was a common option on many vehicles, and is now a standard feature, or atleast an optional feature, on nearly all new vehicles. In 1971, the Buick division of GM introduced MaxTrac as the first TCS, which was used to detect rear wheel spin and modulate the engine power delivered to those wheels in order to provide the most traction possible. Since then, more sophisticated TCS systems have been developed by different companies, such as Cadillac and Robert Bosch GmbH, and involve an engine management controller that cooperates with the brake system in order to prevent the driven wheels from spinning out. A comprehensive overview of the history, operation, and types of slip control systems can be found in [Bur93]. The primary task of a slip control system, such as ABS or TCS, is to influence the longitudinal dynamics of a vehicle by preventing the tires from locking up when braking or spinning out when accelerating, thereby enhancing the directional stability of the vehicle. According to a study conducted by the Monash University Accident Research Centre, ABS has reduced the risk of multiple vehicle crashes by 18% and the risk of run-off-road crashes by 35% [ Bur04]. Another study conducted by the National Highway Traffic 2012-01-0248 Published 04/16/2012 Copyright © 2012 SAE International doi:10.4271/2012-01-0248 saealtpow.saejournals.org Development of a Fuzzy Slip Control System for Electric Vehicles with In-wheel Motors Kiumars Jalali, Thomas Uchida, John McPhee and Steve Lambert Univ of Waterloo ABSTRACT A two-passenger all-wheel drive urban electric vehicle (AUTO21EV) with four direct-drive in-wheel motors and an active steering system has been designed and developed at the University of Waterloo. A novel fuzzy slip control system is developed for this vehicle using the advantage of four in-wheel motors. A conventional slip control system uses the hydraulic brake system in order to control the tire slip ratio, which is the difference between the wheel center velocity and the velocity of the tire contact patch along the wheel plane, thereby influencing the longitudinal dynamics of a vehicle. The advantage of the proposed fuzzy slip controller is that it acts as an ABS system by preventing the tires from locking up when braking, as a TCS by preventing the tires from spinning out when accelerating. More importantly, the proposed slip controller is also capable of replacing the entire hydraulic brake system of the vehicle by automatically distributing the braking force between the wheels using the available braking torque of the in-wheel motors. In this regard, the proposed fuzzy slip controller guarantees the highest traction or braking force on each wheel on every road condition by individually controlling the slip ratio of each tire with a much faster response time. The performance of the proposed fuzzy slip controller is confirmed by driving the AUTO21EV through several test maneuvers using a driver model in th