Electric power-assisted steering has well established itself in the automotive vehicle market. In principle there are two types: electro-hydraulic power steering (EHPS), where an electric motor drives a hydraulic pump (a similar pump is used in classical power steering) and electric power steering (EPS), where the electric motor directly assists the steering motion.

While current market research sees penetration of EHPS at around 8% and staying constant over the next few years, EPS is seen as growing very strongly. Currently, the market share for EPS is 25% and is expected to be equipped in every second vehicle sold in the next 10 years.

EHPS is the system with more components compared to EPS, since next to the electric motor, usually a brushless DC (BLDC), and the necessary electronic control unit (ECU) it still retains the hydraulic pump. The advantage of EHPS over conventional power steering is that the drive of the hydraulic pump can be controlled and becomes more energy efficient. The pump does not need to build up pressure if there is only a little or no steering required. This cannot be achieved with a conventional power steering system (or only with a lot of effort and cost), because the pump is driven directly via the combustion engine.

For an EPS there is no hydraulic system anymore. The electric motor is assisting the steering motion directly. The absence of the hydraulic system means there is less cost. In case of failure there is, however, an increase in risk by the electric motor acting on the steering column directly. This has led to a delay in introduction of these systems and has opened a market opportunity for EHPS, which has less risk associated with it in case of failure. However, through redundancies for critical sub-components (similar to the aircraft industry) and extensive validation testing, this risk has been reduced.

The next evolution for power steering will be steer-by-wire systems, where there is no mechanical link between steering wheel and steering axle anymore. The electric motor steers directly and the movement of the steering wheel by the driver is only detected by sensors and communicated to the motor.

An ECU unit, as shown in Figure 1, is needed to drive the electric motor. It consist of three principle functional blocks:

1. The control module.
2. The power module.
3. The control software.

The control module drives the power module by turning the power switches on and off and performing certain diagnostic functions. The power module consists of three half-bridges. Each half-bridge powers one phase of the three-phase electric motor. The power switches are field-effect transistors (FETs). And the control software consists of algorithms to control speed and torque of the electric motor.

This module consists of a microprocessor that is mounted on a PCB along with peripheral components. The module requires several inputs e.g., signals from the angle sensors that determine the exact angular position of the electric motor's rotor (there is a sensorless drive also, where the angular position is inferred from the response to pulses superimposed on the phase signals). It monitors the temperature of the power module, protects against overvoltages and currents and carries out emergency strategies in case of failure.

The control module drives the power module via its output signals. The turn-on/turn-off sequences are dependent on the speed and torque requirements of the steering system.