International Journal of Automotive Technology, Vol. 5, No. 2, pp. 77-88 (2004) Copyright © 2004 KSAE 1229-9138/2004/016-02 77 ACTIVE DIRECT TILT CONTROL FOR STABILITY ENHANCEMENT OF A NARROW COMMUTER VEHICLE D. PIYABONGKARN, T. KEVICZKY and R. RAJAMANI * Department of Mechanical Engineering, University of Minnesota, 111 Church Street S.E., Minneapolis, MN 55455 (Received 5 September 2003; Revised 27 January 2004) ABSTRACT-Narrow commuter vehicles can address many congestion, parking and pollution issues associated with urban transportation. In making narrow vehicles safe, comfortable and acceptable to the public, active tilt control systems are likely to play a crucial role. This paper focuses on the development of an active direct tilt control system for a narrow vehicle that utilizes an actuator in the vehicle suspension. A simple PD controller can stabilize the tilt dynamics of the vehicle to any desired tilt angle. However, the challenges in the tilt control system design arise in determining the desired lean angle in real-time and in minimizing tilt actuator torque requirements. Minimizing torque requirements requires the tilting and turning of the vehicle to be synchronized as closely as possible. This paper explores two different control design approaches to meet these challenges. A Receding Horizon Controller (RHC) is first developed so as to systematically incorporate preview on road curvature and synchronize tilting with driver initiated turning. Second, a nonlinear control system that utilizes feedback linearization is developed and found to be effective in reducing torque. A close analysis of the complex feedback linearization controller provides insight into which terms are important for reducing actuator effort. This is used to reduce controller complexity and obtain a simple nonlinear controller that provides good performance. KEY WORDS : Receding horizon control, Direct tilt control, Narrow tilting vehicle, Feedback linearization 1. INTRODUCTION The development of narrow vehicles is a promising alternative that is being proposed to address increasing traffic congestion and limited highway capacity in metro- politan areas. In order to provide an acceptable replace- ment for today's average passenger car, these vehicles should retain the perceived safety and the ease and comfort of driving a regular four-wheeled vehicle. Narrow vehicles currently used in urban transportation (e.g. motorcycles) require the driver to balance the vehicle while it is turning, meaning that the vehicle must be tilted into the curve to compensate for the tilting moment of the centripetal force generated by the tires. These vehicles also lack the level of safety that the majority of commuters would prefer. This could be addressed by increasing the dimensions of such a vehicle, preferably in terms of height, not to compromise the benefits gained from the narrower lane track. Building narrow vehicles taller tends to increase their tilt and the chances of a rollover during tight cornering. Active tilt control systems that assist the driver in balancing the vehicle (and perform automatic tilting while cornering) have to be essential parts in any narrow vehicle system design that intends to provide a reason- able alternative to the current mainstream in personal transportation. The F300 Life Jet (mercedes-benz.com) for example, was developed by Daimler-Chrysler as an experimental prototype that demonstrates the feasibility of the narrow-tilting vehicle paradigm. There are two basic types of control systems that could be used for tilting (Hibbard and Karnopp, 1993; 1996): (1) Direct tilt control, in which an actuator is used in the vehicle suspension to control tilt. (2) Steering tilt control (Gohl, 2003), in which the steer- ing actuator is used to achieve the required tilt angle. These approaches provide automatic tilting of the vehicle so the driver needs to perform only lateral control to keep the vehicle in the lane. This paper will focus on the development of an auto- matic direct tilt control system for a generic narrow vehicle model that describes a prototype narrow commuter vehicle built at the Mechanical Engineering Department of the University of Minnesota. Photographs of the proto- type vehicle are shown in Figure 1. A direct tilt control (DTC) system uses an actuator in the suspension to apply a leaning torque, M t on the vehicle. This allows the controller to tilt the vehicle to a desired angle. There are two issues that need to be addressed in a DTC system: (1) The first issue is deter- *Corresponding author. e-mail: rajamani@me.umn.edu