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 harmonize 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.
Steering servo is realized by means of a conventional rack and pinion power steering gear. The size and direction of the hydraulic 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.
The control unit responsible for the Servotronic function contains the following components:
- Servotronic output stage
- Servotronic software
The Servotronic function can be realized by different control units:
- E70 without active steering built after 04/2010 = Servotronic control unit (SVT)
- E70 with active steering = active steering (AL)
- E71, E71M and E70M = Integrated Chassis Management (ICM)
Servotronic is optionally available for the E70 without active steering (AL) as from 04/21010. Vehicles without active steering do not have active steering (AL). The Servotronic control unit (SVT) is additionally installed in these vehicles.
NOTICE: Vehicles without active steering.
This functional description describes the Servotronic function on the E70 built after 04/2010 without active steering.
Brief component description
The following components are described for the Servotronic:
- Servotronic control unit (SVT)
- Dynamic stability control (DSC)
- Steering column switch centre (SZL)
- DME (Digital Engine Electronics) or DDE (Digital Diesel Electronics)
- Junction Box Electronics (JBE)
- Car access system (CAS)
- Servotronic valve
Servotronic control unit (SVT)
The Servotronic control unit (SVT) is responsible for the functions that can also be fully integrated in the following control units:
- Integrated chassis management (ICM)
- Active Steering (AL)
The following vehicles are equipped with the Servotronic control unit (SVT):
- E70 with optional equipment Servotronic without active steering
The Servotronic control unit (SVT) is installed behind the Car Access System (CAS).
The following signals are required for the Servotronic:
- Steering angle from steering column switch cluster (SZL)
- Driving speed from Dynamic Stability Control (DSC)
- Engine status from Digital Engine Electronics (DME) or Digital Diesel Electronics (DDE)
- Terminal status from Car Access System (CAS)
The Servotronic valve is only activated at Terminal 15 On and with the engine running.
Dynamic Stability Control (DSC)
Dynamic Stability control (DSC) evaluates the individual wheel speed sensors and calculates the driving speed. The speed signal is made available to the Servotronic control unit (SVT).
Dynamic Stability control (DSC) supplies the speed signal. The speed signal is one of the signals required to activate the Servotronic control unit (SVT).
Steering column switch centre (SZL)
The optical sensor for measuring the steering angle is integrated on the printed circuit board of the steering column switch cluster (SZL). The steering angle sensor is designed as a contactless, optical measuring system. The system consists of an encoded disc and an optical sensor. The encoded disc is connected by a driver directly to the steering wheel. The encoded disc turns within the optical measuring system as the steering wheel is moved.
The steering column switch cluster (SZL) evaluates the steering angle and sends this information via K-CAN2.
The steering angle sensor is installed in the steering column switch cluster (SZL). The steering angle sensor measures the steering angle optically and without contact. The steering angle sensor is fitted on the printed circuit board with the evaluation electronics. The steering angle sensor consists of the following components:
- Encoding disc
- Optical sensor
The encoded disc is connected to the steering wheel via the coil spring cassette. When the steering wheel is moved, the encoded disc moves within the optical sensor. Various line patterns for evaluation are arranged on the encoded disc.
The steering column switch cluster (SZL) is connected to the Dynamic Stability Control (DSC) via the F-CAN. The steering angle is sent via the F-CAN.
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 one of the activation conditions for the Servotronic.
The following graphic shows the Digital Engine Electronics (DME).
DME (Digital Engine Electronics) or DDE (Digital Diesel Electronics) is connected via the engine wiring harness.
The Digital Engine Electronics (DME) or Digital Diesel Electronics (DDE) is connected to the remaining bus system via the PT-CAN. The junction box electronics (JBE) are responsible for the gateway function.
DME (Digital Engine Electronics) or DDE (Digital Diesel Electronics) supplies the engine status. The engine status is one of the signals required to activate the Servotronic control unit (SVT).
Junction Box Electronics (JBE)
The junction box electronics (JBE) are mechanically and electrically connected to the front power distribution box.
The junction box electronics (JBE) combine numerous functions in a single control unit. The junction box electronics (JBE) are connected via a number of plug connections.
The junction box electronics (JBE) are the data interface between the PT-CAN and K-CAN.
The following graphic shows the installation location of the junction box electronics (JBE) in the E70.
Car access system (CAS)
The Car Access System (CAS) is the control unit responsible for terminal monitoring and is woken by the junction box electronics (JBE).
The Car Access System (CAS) supplies the terminal status. The engine status is one of the signals required to activate the Servotronic control unit (SVT).
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. The Servotronic valve is a solenoid valve.
The Servotronic valve is connected to the Servotronic control unit (SVT) by means of a 2-pin plug connection. The Servotronic control unit (SVT) provides voltage and ground for the Servotronic valve.
System overview
System functions
The Servotronic determines most of the steering torque that driver has to apply. Servotronic is largely defined by the driving speed and steering angle:
- 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 15 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 is determined from a characteristic map with driving speed and steering angle as the parameters. The characteristic map is stored in the coding data and has 10 data points for the driving speed and 6 data points for the steering angle. Intermediate values of the nominal current are determined by linear interpolation of adjacent data points. A corresponding function limits the value from the characteristic curve to 0 and 860 milliampere.
The Servotronic determines the required steering servo as a function of following factors every 100 milliseconds:
- Current driving speed
- Current steering angle value
The corresponding value in the characteristic map is activated depending on the situation. The specified maximum or minimum values are used if the current value for driving speed or steering angle is outside the defined range. It is possible to switch over between two characteristic maps depending on the chassis and suspension set-up.
The Servotronic evaluates the CAN message in order to switch over between the normal characteristic map and the sport characteristic map. The signal is compared to the coding value sport mode. If the values are equal the Servotronic switches over to the sport characteristic map. If the message fails or in the event of an invalid value, the characteristic map last selected remains active. After a reset Servotronic uses the comfort status and therefore the normal characteristic map. However, the status is not stored in the Servotronic control unit (SVT).
As an example, the following graphic shows the characteristic map for driving speed with steering angle 0 degrees on the E70 with Dynamic Drive and Electronic Damper Control.
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 working cylinders.
Steering servo at high driving speeds
The steering torque is barely 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 working cylinders.
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 working cylinders.
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
The Servotronic makes available a diagnostic interface via the CAN-bus.
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
The Servotronic control unit (SVT) monitors the following areas with self-diagnosis:
- Voltage
- Activation of Servotronic valve
- Communication
- Check Control message
- Fault code memory
- Fault regeneration
- Coding
If the Servotronic 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.