INTRODUCTION - ELECTRICALLY CONTROLLED STEERING SYSTEM

SPECIFICATIONS

The EPS (Electrical Power Steering) produced by DELPHI has a power assisted steering device which is designed to lighten the effort required from the driver on the steering wheel especially during steering manoeuvres at low speed without however making it too light in normal driving conditions.

1. EPS Electrical Power Steering

2. Mechanical steering box

3. Battery

4. Engine compartment junction unit

5. Body Computer

Electric power steering offers the following advantages compared with hydraulic power steering:

- The system has fewer components and therefore weighs less and is less complex.

- The installation and/or servicing times are reduced and simpler.

- The electric power steering system only absorbs energy from the engine when power assistance is requested, improving the performance of the vehicle and reducing consumption and emissions.

- A reduction in operating noise improves driving comfort.

- Reduction in pollution because the electrical energy used is clean.

- Variation in power assistance depending on the speed of the vehicle

- Steering return to the centre (active return)

- Steering return oscillation damping

- Possibility of selecting power assistance (Normal / City)

ACTIVATION STRATEGIES

The system operating strategy is illustrated in the diagram below.

BASIC OPERATION

Depending on the driver''s requirements (torque at the steering wheel) and the speed of the vehicle, the electric steering node management control unit (NGE) controls the electric servo motor that assists the steering column to rotate.

The motor applies a torque to the actual steering column by means of a worm screw mechanism, thereby saving the driver the effort of steering.

Variable servo assistance depending on the speed of the vehicle

As the speed of the vehicle increases the user proportionally increases the effort applied to the steering wheel because the resistant force at the wheels decreases as the vehicle speed increases.

As a result, making use of the vehicle speed signal, the NGE implements a lesser degree of steering assistance.

Active return

The return stage refers to the realignment function normally produced by the geometry of the vehicle''s front section when the steering is released after steering.

This function is designed to make the realignment quicker causing the servo motor to intervene to help the normal geometric effect.

The active return correction varies depending on the speed of the vehicle:

- It is greatest at low speeds

- It is lowest at high speeds.

The servo motor carries out the active return of the steering wheel depending on the steering angle in relation to the centre. The greater the steering angle, the greater the effort of the motor to realign the wheels.

Steering return oscillation damping

After the steering wheel is released, following steering, the vehicle chassis produces oscillations (A) which, if they persist for a while, can become tiresome.

The servo motor reduces the extent of the oscillations (B) during the return to the straight line driving route and intervenes more at high speeds.

Power assistance that can be selected

The user can choose between two driving modes using the button in the control panel:

- Normal for normal power assistance at medium and high speeds,

- City for easier driving whilst parking and at low speeds thanks to greater assistance.

Nge calibrations

The system manages 5 different driving calibrations. The NGE receives vehicle information (engine type and steering box type) from the Body Computer via the CAN and selects the correct calibrations from inside its memory.

The two NGE configurations are illustrated in the diagram below:

Axially and lengthwise adjustable column

Fixed Column

COMPOSITION

The composition of the assembly is illustrated in the diagram below.

1. Microprocessor

2. CAN interface

3. Supply circuits

4. Engine phase operating circuit (EBMD)

5. Power electronics (FET)

6. Analogue signals interface

7. Position and torque sensor

8. Servo mechanism

9. Electric motor (with engine position sensor)

Reduction motor

The reduction motor unit comprises an aluminium casting secured to the vehicle chassis.

Located at the side of the reduction motor casting, the servo motor provides the power assistance gear with a torque with a ratio of 22:1 by means of a worm screw.

The reduction unit gear, coaxial and in one piece with the steering column, is made of steel, whilst the outer ring gear is made of pressed plastic. The worm screw and the gear have been designed so that the angles ensure the reversibility of the coupling.

The metal part of the gear is fitted on the output shaft which transmits the total steering forces (in other words the torques for the servo motor and the driver).

The input and output shafts are connected to one another by a "calibration torsion bar" which allows an angular movement of between +8° degrees and -8° (mechanical ends of travel present the torsion from being increased further).

When there is resistance at the wheels, the input shaft twists the torsion bar therefore the input shat and the output shaft are offset at an angle proportional to the torque applied to the steering wheel.

A torque sensor, fitted inside the reduction unit measures the difference of the angle between the input shaft and the output shaft and supplies an electrical signal to the control unit that is proportional to the difference.

The reduction unit casing also has the task of retaining the outer part of the "torque and position sensor" and lastly the input shaft support cover is secured to the casing where the steering wheel is fitted and houses both the ignition switch and the steering column switch unit.

On the version with an adjustable column, the angle and the axial position of the steering wheel can be adjusted on the vehicle.

The reduction unit casing must not, under any circumstances, be dismantled. It would also NOT be possible to reassemble this complex system like in the factory.

Control unit

The control unit processes the input signals received from the sensors and operates the electric motor, providing a suitable current to achieve the desired assitance torque. It also manages the communication on the CAN and carries out the continuous autodiagnosis of the system to ensure correct operation. The communication on the CAN is managed using the diganostic equipment.

The values relating to the speed of the vehicle, the alternator, the City/Normal mode and the type of tuning are read on the CAN line.

The position and torque signals coming from the sensors represent the basic values with which the microprocessor processes the output data in terms of current supplied to the engine.

The steering power assistance control unit is fastened to the actual electric steering casing and interfaces with the wiring by means of two separate connectors: one 10 pin and one 2 pin.

CONTROL UNIT PIN OUT

The diagram below shows the control unit pin out.

Connector A:

A - Battery +

B - Battery -

Connector B:

1 - Ignition

2 - CAN HI 2

3 - CAN LO 2

4 - N.C.

5 - N.C.

6 - N.C.

7 - CAN HI

8 - CAN LO

9 - N.C.

10 - N.C.

Connector C:

1 - Position 3 (ESP only)

2 - Position 1

3 - Sensor power supply

4 - T2

5 - N.C.

6 - Position 2

7 - Earth

Description of the signals

Normal / City driving mode selection signal

The Normal / City function has the task of varying the power assistance torque depending on the speed of the vehicle. It is possible to switch from a basic (normal) configuration to a City one using the button in the control panel.

The NGE system receives the Normal / City signal from the CAN and implements the requested strategy. The body computer control unit is prepared to receive the analogue signal that is altered when the button is released switching on the signal when the torque applied by the driver is more than 1 Nm.

During the ignition OFF/ON cycle, the Body Computer control unit maintains the status requested previously by the user.

The steering is always assisted (with the CITY function on the power assistance is greater) and decreases as the speed increases.

CAN serial line

The control unit is capable of receiving/sending information on the CAN with this network interface operational from when the key is turned on until it is turned off.

Signals received / sent via the CAN

The following signals are received from the NGE via the CAN:

- Vehicle speed

- Failure light status

- Engine running signal (D+)

- Fault diagnosis

- Vehicle speed signal error

- Normal/City mode

- Type of tuning (calibration)

The following signals are sent by the NGE control unit via the CAN:

- System status (failure)

- Power assistance activated signal (EPS Active)

- Fault diagnosis

- Steering wheel absolute position (only for NGE version with versions with ESP)

OPERATION

The diagram showing the operation of the system is illustrated below.

1. city driving selector

2. ignition key

3. alternator voltage

4. vehicle speed

5. steering load

6. failure warning light status sending

7. failure warning light status return

8. failure and/or City warning light on

9. steering angle position signal (only with VDC)

10. city / normal signal from NBC

11. tuning type signal (calibration) from NBC

Operational behaviour

The operational behaviour of the system is illustrated in the table below.

ELECTRIC MOTOR

Specifications

The electric motor is the three-phase self-switching synchronous type (without blades) with a permanent magnet rotor.

The distribution of the power and the control of the phases are regulated by the NGE management control unit.

Composition

The composition of the electric motor is illustrated in the diagram below.

The motor rotor is constructed through the application of permanent magnetic material.

There is a disc with magnets (2) fitted to it secured to the motor rotor (1), whilst there are three Hall effect semiconductors (3) on the fixed part on the crankshaft output side with the task of notifying the control unit of the angular position of the crankshaft through 360°. Therefore the position of the rotor is measured by the position sensors incorporated in the casing (Hall effect sensors) allowing the control unit to supply the suitable phases.

To reduce noise and pulsations at the steering column the motor is fitted with three insulators for the stator and one for the rotor.

Operation

The engine absorption current ranges from 1 A to 80 A; the maximum absorption condition exists at equivocal manoeuvres at high steering wheel rotation speeds.

The motor has been designed to provide the torque required, in other words it has been designed to help the user depending on the effort required on the steering wheel.

The wheels offer different resistance to the steering column depending on whether they are resting on ice or asphalt, for example.

As a result the motor provides the necessary torque.

The control unit controls every single (three phase) winding. As the rotor has permanent magnets it tends to pursue and reach the centre of the magnetic field.

The control unit controls the position of the rotor through the signals sent by the Hall effect semiconductors.

The exact position of the rotor helps the control unit to move the current in the coils concerned and therefore ensure that the motor is always working

The activation of the individual coil is carried out using the FET bridge illustrated above with a working frequency of 18 KHz using the PWM (Pulse Width Modular) method for each individual coil.

The diagram illustrates how when the current is supplied to one winding, it is requested in another, whilst in the third winding no current flows through. On switching the current flows passing through the coils change.

The synchronized, self-switching motor always operates at the torque requested by the force exerted by the user on the steering wheel and as the torque requested increases, the value of the current passing through the windings increases proportionally: this value will be around zero when no torque is requested by the engine.

The sensor

The sensor that measures torque and position is fitted on a single casing secured to the reduction unit casting whilst the input and output shafts are free to rotate driving the moving measurement parts.

As stated previously, the sensor has two functions: it measures the torque and the position of the output shaft.

In measuring the torque it can be compared with a potentiometer where the resistive support is secured to the output shaft and the cursor is securd to the input shaft. The twisting moment on the torsion bar between the two shafts determines the value of the torque applied between the steering wheel and the steering wheels.

Through the signal from this potentiometer the control unit is capable of understanding the effort that the driver is exerting on the steering wheel and the direction of the torque. Through the same principle (potentiometer), it measures the angular position of the output shaft in relation to the centre (wheels straight). The control unit is capable of understanding by how many degrees the steering wheel has moved in relation to the centre via the signal.

On versions with ESP the sensor in question also has the function of a steering angle sensor and is designed to calculate the number of revolutions for the steering wheel and how many degrees per turn. The information supplied by the torque/position sensor is sent via the C CAN from the NGE node for the optimum management of the electronic stability control managed by the NFR.

It is prohibited to carry out tests on the sensors using any instruments: any fault diagnosis MUST be carried out using the electric steering control unit.

Lower Shaft

The connection between the steering column output shaft and the steering box pinion is made through a telescopic type intermediate shaft that is attached to the pinion by means of a splined fork with 26 teeth and a bolt.

Electric steering system protective fuses

The 70A power fuse protecting the direct supply system from the battery is located in the engine compartment on the battery. The ignition-controlled power supply (+15) is protected by a 7.5A fuse located on the Body Computer.