Electric Power Steering (EPS) or Motor Driven Power Steering (MDPS) is fitted to the majority of small and medium sized cars.
In the first of two articles, Blue Print looks at the column mounted type fitted to the Hyundai i30, a system developed by TRW and is similar in operation to other systems.
Like any electromechanical component, the MDPS will eventually fail and the Hyundai unit is no exception.
A motor drives a worm shaft, which turns the worm wheel (connected to the steering column) giving the motor speed reduction and torque increase necessary to assist the driver in turning the steering column.
A computer receives information about how much steering torque the driver is applying and delivers a motor torque to suit.
Modern units use brushless motor technology and contactless opto-electronics are used to provide information on the steering wheel torque and movement.
The motor uses permanent magnets in the armature; modern rare earth magnets (neodymium) have very strong magnetic fields and this means that there is no need for armature windings and mechanical commutator which makes it more energy efficient and gets rid of the weakest part of any DC motor.
The EPS system uses a brushless motor, where the rotor (armature) turns inside the stator.
Three separate stator windings produce electromagnets, the polarity of the magnet depends on the direction of current flow so the current flowing through winding A from A1 to A2 will produce a south pole at A1 and north pole at A2.
Reversing the flow from A2 to A1 will produce the opposite effect.
By switching the current flow to winding B, the rotor magnet is attracted to it (unlike poles attract) and the rotor turns in a clockwise direction.
So by consecutively energising the three windings in order, the rotor will turn.
The control system uses MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) which have the ability to switch large currents at great speed and generate less heat than ordinary transistors.
Switching on transistors 3 and 4 will also energise stator field coils 1 and 2 but the current will flow in the opposite direction and so the polarity of the stator magnets will be reversed.
Sequencing the transistor switching will control the direction and by using Pulse Width Modulation (PWM) of the supply current, the speed and torque of the motor is controlled.
Speed and position
Switching of the field has to be synchronised to the position of the rotor and this is done by rotor position sensors.
Hall sensors, as on the Hyundai, are polarity sensitive and switch on and off as the poles of the sensor rotor move past it.
There are three Hall sensors which mean that the controller can work out the position of the rotor even when it is stationary.
Steering torque and angle position sensor
The MDPS unit fitted to the Hyundai uses a combined Steering torque and angle sensor and has no electrical contacts.
A torsion bar separates the upper and lower parts of the column, allowing a small angle of displacement between the two shafts, relative to the torque applied and is limited by stops.
Slotted discs with different width slots are connected to the upper and lower parts of the column, sitting between a light source and a photodiode array that picks up the shadow created by the slotted discs.
When the steering is turned the image is captured, processing the displacement of the slots and the rate at of movement in the steering discs to give steering torque, direction, steering angle position and rate of turn.
Information from two sensors is compared for reliability and if one sensor should fail the system can still operate and a DTC will be set.
When turned, the torque applied is measured and a suitable torque is applied to the steering column by the motor.
When the steering wheel is stopped, the motor continues to drive the lower part of the column until there is no load on the torsion bar and assistance stops.
Absolute Steering Position (ASP)
This is the datum used to identify the straight ahead position and is set in the factory but may need to be reset after installation or if wheel alignment has taken place.
You will need a suitable scantool such as the G-Scan or G-Scan2 to perform the task.
Blue Print catalogues all eight variants of Hyundai i30 from 2007 onwards, supplying everything from filtration, braking, clutch and steering and suspension, to engine management, cooling and transmission components.
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