Optimal Design of an IPM Motor for Electric Power Steering Application Using Simulated Annealing Method

Abstract

Electric power steering (EPS) in newer vehicles is becoming an alternative to the hydraulic power steering (HPS) because of the recent advances in electrical motors, power converters , sensors and digital control systems (Mir et al., 2003). In the EPS system, an electric motor is connected to the steering rack via a gear mechanism. Some sensors measure the torque on the steer and the angular position of the hand wheel. A control system receives these signals from the sensors, together with vehicle speed, turning rate, and gives operating commands to the electric motor drive, controlling steering direction and dynamics and driver effort. The control unit determines the amount of steering assist torque, which has to be also modified according to vehicle speed to maintain good steering feel. Electric power steering eliminates the need for a hydraulic power steering pump, hoses, hydraulic fluid, drive belt and pulley on the engine, therefore the total system is lighter than a comparable hydraulic system through the use of compact system units (Mir et al., 2003). Also, since EPS is an on-demand system that operates only when the steering wheel is turned, the fuel efficiency of vehicle equipped with such system is better than that of automotive equipped with an equivalent-output hydraulic system (Liao & Isaac, 2003). As a result, Electric power steering systems have many advantages over traditional hydraulic power steering systems in engine efficiency, space efficiency, and environmental compatibility. This motivates the great increase of EPS-equipped automotive recently (wilson, 2005). Electric power steering basically consists of a torque sensor and motor actuator couple. The sensor is attached to the steering column and measures the torque applied by the driver for moving the steering wheel. This torque signal is transmitted to a control power card that sends an amplified proportional power signal to the electric motor (in this paper we use interior permanent magnet (IPM) synchronous motor), which is engaged to the steering rack bar. An EPS system has the following two functions. First, it can reduce steering torque and present various steering feels. The steering torque (or driver torque) is defined as the one which a driver experiences (or a driver applies to the steering column) when turning the steering wheel. When an appropriate assist torque from an EPS system is applied in the 9