DSP-based electric power assisted steering using BLDC

or steering column. This allows varying amounts of assistance to be applied depending on driving conditions. In the event of component failure, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems. Electric systems have an advantage in fuel efficiency because there is no hydraulic pump constantly running. Their other big advantage is the elimination of a belt-driven engine accessory, and several high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis. This greatly simplifies manufacturing.

The demand of electrically assisted power steering (EAS) has rapidly increased in past few years because of energy savings compared to Hydraulic Power Steering (HPS). Alternating current (ac) motors are designed to be highly efficient and easily controlled with modern power circuitry. Because of the developments in switching techniques, it is quite feasible to use ac motors with a battery supply as source. The traditional worm gear driven dc motor system is constrained by the limitations of the dc motor brushes and size of the motor for the same torque of BLDC. In this case BLDCmotor has been used as an actuator in the application for electric power steering. The BLDC motor provides high torque and easy control (Chan & Fang 2002; Chu et al 2001; Desai & Emadi 2005; Jun-Uk Chu et al 2004; Kevin Brown et al 1990; NamhunKim et al 2007). The basic mechanical properties of the vehicle are

essentially invariant among all of the available brands. The electrically assisted power steering system consists of BLDC motor mounted to the frame of the steering column and coupled to the wheels through a worm speed reducer. Electrically assisted power steering is shown in figure 1.

An electrically assisted power steering is composed of several parts such as torque sensor, engine speed sensor, vehicle speed sensor, steering column, torsion bar and electronic control unit.

Figure 1. Electrically assisted power steering.

DSP-based electric power assisted steering using BLDC motor 583

Torque sensor output gives the torque difference to be developed by the motor to reduce the effort required by the driver while he is steering. Engine speed signal is required to start the assistance only when the engine is ON in order to save the battery life. Vehicle speed signal is required to control the assistance developed by the motor (for the same level of torque signal) at various vehicle speed, as assistance requirement comes down as speed of the vehicle increases.

The control architecture consists of two layers of control, namely the assistance level control and the torque control. In the higher level of control architecture, vehicle speed signal works as a reference for controlling the assistance to be developed by the motor. In the inner layer torque sensor signal performs generation of torque. The torque output from motor is a function of torque sensor signal and it depends on the torque difference between the steering wheel and the wheel. The vehicle speed signal and engine speed signals are pulses with variable frequency. For system implementation, a DSP-based BLDC motor controller with three-phase