Servotronic
Servotronic
Servotronic (SVT)
Servotronic is a transverse dynamics system. Servotronic has the task of continuously adapting the steering servo depending on the driving speed and steering angle. For this purpose, Servotronic correspondingly controls the electrical current of the Servotronic valve so that the flow of hydraulic fluid assists steering to a greater or lesser extent. The current steering angle is additionally taken into account in order to better meter the steering assistance based on the steering angle. The steering servo is determined with the aid of characteristic maps and decreases with increasing driving speed, thus increasing the amount of steering torque required.
The Servotronic can be realized by different control units:
- Servotronic control unit (SVT)
- Integrated Chassis Management (ICM), first-generation only
Brief component description
The following components are described for the Servotronic:
- Hydraulic steering system overview
- Servotronic control unit (SVT)
- Servotronic valve
- ECO valve
- Integrated chassis management (ICM)
- DME (Digital Engine Electronics) or DDE (Digital Diesel Electronics)
Hydraulic steering system overview
Steering servo is realized by means of a conventional rack and pinion power steering gear. The size and direction of the hydraulic actuating forces acting on the rack depend on the rotational angle of the torsion bar spring. torsion bar spring is located between the steering shaft and pinion.
Servotronic control unit (SVT)
The Servotronic control unit is situated in the rear in the luggage compartment well.
The following signals are required for the Servotronic:
- Driving speed
- Status of the engine from the engine control
The Servotronic control unit is connected with the vehicle electrical system via the PT-CAN. But the Servotronic control unit has no connection for the wake-up line.
The Servotronic control unit does not require any steering angle. The current for the valves is specified by the Integrated Chassis Management (ICM) (M GmbH: specification through the DME).
The Servotronic control unit is supplied by the right rear power distribution box with terminal 15N.
Servotronic valve
The Servotronic controls the degree of assistance provided by the hydraulic steering depending on the vehicle's speed. The flow of hydraulic fluid is restricted to a greater or lesser extent depending on how the Servotronic valve is actuated. Restriction of the flow depends on the current actuating the Servotronic valve (pulse-width-modulated).
The Servotronic valve is only activated at Terminal 15N On and with the engine running. The Servotronic control loop controls the current for the connected Servotronic valve. A higher current results in greater steering assistance with the same steering torque.
The nominal current for the Servotronic valve is specified to the Servotronic control unit by the Integrated Chassis Management (ICM). Exception: For vehicles of the M GmbH, the DME specifies the nominal current. The nominal current varies between 0 mA (minimum) in 860 mA (maximum).
ECO valve
The ECO valve is secured to the power steering pump. ECO means: Electronically Controlled Orifice.
The power steering pump is controlled by the ECO valve as needed. The ECO valve regulates the volumetric flow of the hydraulic fluid, thus regulating the pump's hydraulic power. A tandem pump can be installed depending on the equipment (active steering, Dynamic Drive).
A modern rack and pinion power steering gear generates high angular wheel speed (wheel is turned faster). The hydraulic must have the corresponding high performance for the high angular wheel speeds. A sufficiently dimensioned conventional vane-type compressor would generate a high power loss. The fuel consumption would be increased unnecessarily. An improvement can be yielded with a controllable power steering pump. The ECO valve in the power steering pump regulates the volumetric flow depending on the requirement and lowers the ram pressure in the hydraulic system.
In this way, the ECO valve affects the power consumption of the power steering pump. Fuel consumption and carbon dioxide emissions (CO2) from the vehicle's engine are reduced accordingly.
The nominal current for the ECO valve is specified to the Servotronic control unit by the Integrated Chassis Management (ICM). Exception: For vehicles of the M GmbH, the DME specifies the nominal current. The nominal current varies between 0 mA (minimum) in 860 mA (maximum).
Integrated chassis management (ICM)
The Dynamic Stability Control (DSC) evaluates the individual wheel speed sensors. The DSC send the wheel speed signals to the Integrated Chassis Management (ICM) on the FlexRay.
The ICM control unit calculates a speed signal from this. The speed signal is made available to the Servotronic control unit (SVT) via the bus.
The central gateway module (ZGM) takes over the gateway function.
DME (Digital Engine Electronics) or DDE (Digital Diesel Electronics)
Digital Engine Electronics (DME) or Digital Diesel Electronics (DDE) undertakes the engine control with all components. DME (Digital Engine Electronics) or DDE (Digital Diesel Electronics) transfers the Engine On status. This status is a switch-on condition for the Servotronic.
The following graphic shows e.g. a DME near the engine.
The Digital Engine Electronics (DME) or Digital Diesel Electronics (DDE) is connected to the remaining bus system via the PT-CAN. The central gateway module (ZGM) takes over the gateway function.
System overview
System functions
The Servotronic determines most of the steering torque that driver has to apply. The Servotronic is definitively defined by the driving speed:
- Low driving speed range (urban driving, parking) = low steering torque
- High driving speed range (dual carriageway, motorway) = high steering torque
The Servotronic valve is only activated at Terminal 15N On and with the engine running. The Servotronic control loop controls the current for the connected Servotronic valve. A higher current results in greater steering assistance with the same steering torque.
The nominal current for the valves is specified to the Servotronic control unit by the Integrated Chassis Management (ICM). Exception: For vehicles of the M GmbH, the DME specifies the nominal current. The nominal current varies between 0 mA (minimum) in 860 mA (maximum).
The Servotronic determines the required steering servo as a function of following factors every 100 milliseconds:
- Current driving speed
The specified maximum or minimum values are used if the current value for driving speed is outside the defined range.
The following Servotronic system functions are described:
- Steering servo at low driving speeds
- Steering servo at high driving speeds
- Steering servo with Servotronic valve de-energized
Steering servo at low driving speeds
The steering torque is greatly assisted at low driving speeds. The steering assistance (steering servo) corresponds to the characteristic curve energized with 860 milliampere current supply. A low steering torque produces a high differential pressure between the hydraulic cylinders on the steering gear.
Steering servo at high driving speeds
The steering torque is less assisted at high driving speeds. The steering assistance (steering servo) corresponds to the characteristic curve energized with 0 milliampere current supply. A high steering torque produces a lower differential pressure between the hydraulic cylinders on the steering gear.
Steering assistance (steering servo) in the case of fault
When the Servotronic valve is de-energized the steering assistance (steering servo) corresponds to the characteristic curve energized with 0 milliampere current supply. A high steering torque produces a low differential pressure between the hydraulic cylinders on the steering gear.
In the event of a short circuit to B+, the entire vehicle voltage is applied to the Servotronic valve. This means that the Servotronic valve is fully actuated. The steering servo is at its maximum.
Notes for Service department
NOTICE: Observe repair instructions.
The service instructions do not replace the current information for service, nor the repair instructions. It is essential to refer to the repair instructions when carrying out repairs on components of the Servotronic system.
Failure of the component
If the Servotronic valve or ECO valve fails, the following behavior is to be expected:
- Fault entry in Servotronic control unit (SVT)
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.