Electric power steering, in which an electric motor provides assistance in steering effort, has the problem that the driver's feel can be made worse by the inertia and friction of the motor.
To solve this problem, the use of a small motor with large output is necessary. Either by increasing the supply voltage or by the use of rare-earth -metal magnets, EPS offering sufficient assistance for stationary vehicles and satisfactory driver's feel could be developped for vehicles up to 1.5 liter.
For 0.55 liter vehicles with relatively small steering effort, it was designed for the motor to provide assistance during low-speed driving only and to be disconnected from the steering system with an electromagnetic clutch at higher speeds. In this manner, satisfactory driver's feel could be secured for this class of vehicles.
Incorporation of the differential elements into the control system contributes to improving the response and eliminating self-exited vibration of the system. Such elements also serves to reduce the effect of inertia, but an excessively strong differential element causes adverse effects, such as poor stability when hands are released from the steering wheel.