RACK & PINION & SERVOTRONIC STEERING
Conventional rack and pinion power steering is used in the X3 (E83) and Servotronic is available as an option. The design and functioning principle of the Servotronic option are nothing new but have been modified as described in this chapter. The servotronic steering system controls the amount of power assistance based on the current road speed. The hydraulic pressure is electronically adapted to the current road speed, with greater power assistance available at lower road speeds and less power assistance available at greater road speeds.
The E83 is equipped with hydraulic assisted rack and pinion steering. The total ratio of the steering is: 18.9:1. The total rack stroke is 81.0 mm (1.7 turns of the steering wheel). The hydraulic ports on the steering gear have quick release couplings. The hydraulic pump maximum pressure is limited to 127 bar. E83 vehicles are equipped with "W" shaped cooling loops (hoses) to assist in cooling. The hydraulic pump does not have a pump end shutdown feature. The hydraulic pump could be damaged after approximately 1 minute if the steering is kept on full lock (end stop) for a long period.
Scheme 1
SERVOTRONIC INTRODUCTION
Servotronic does not regulate the steering force support in a conventional way by using the engine speed, rather it is dependent upon the car's road speed. The Servotronic regulates the hydraulic pressure in the power-steering pump. Adaptation of the hydraulic pressure to the car's road speed has the effect of
- High steering force support at low road speeds.
- Minimum steering force support at high road speeds.
This means that minimum force is required for steering force at slow road speeds. As the road speed increases the steering force support is continuously reduced. Contact to the road surface therefore becomes more direct and steering is more precise. Consequently, steering is very comfortable and maneuverability is maximized, for instance when parking and maneuvering at slow speeds.
COMPONENTS
Note. The following actuators are involved in the Servotronic.
Servotronic Control Unit (SVT)
The SVT control unit specifies the nominal values for controlling the Servotronic valve (hydraulic pressure). The SVT control unit is connected to the PT-CAN (powertrain CAN) and K-Bus (body bus).
DSC Control Unit (Dynamic Stability Control)
The DSC control unit delivers the car's road speed signal. The DSC control unit and SVT control unit are connected by the PT-CAN.
DME or DDE (Digital Engine Electronics or Digital Diesel Electronics DDE)
The Servotronic receives the signal of whether the engine is running from the DME or DDE over the PT-CAN. The Servotronic only activates the Servotronic valve when the engine is running.
Instrument Cluster (KOMBI)
The instrument cluster delivers the terminal status (e.g. terminal 15 ON) and the kilometer reading.
Servotronic Valve
The Servotronic valve is an electro-hydraulic converter located in the power-assisted steering oil circuit. The Servotronic valve restricts the hydraulic current and consequently reduces the hydraulic pressure. The Servotronic valve is activated by a characteristic curve delivered by the SVT control unit.
SYSTEM FUNCTIONS
The Servotronic is based upon the following system functions
- Initialization of the Servotronic.
- Recording of the speed.
- Supplying the Servotronic valve with power.
- Undervoltage detection and overvoltage detection.
Initialization of the Servotronic
The Servotronic is active from terminal 15 ON. A short initialization phase for the Servotronic (under 1 second) is necessary when the engine starts in order to reach the characteristic curve for a low road speed as fast as possible. A plausibility check for supplying the Servotronic valve is performed during the initialization phase.
Recording of the Speed
The road-speed signal is calculated by the DSC and supplied to the PT-CAN. The Servotronic specifies an acceleration value for plausibility of the road-speed signal. A road speed is calculated from the acceleration value. At a low acceleration the road-speed signal supplied by the DSC is used. At a higher acceleration the Servotronic control unit calculates the road speed internally. The road speed is stored in the Servotronic control unit.
Supplying the Servotronic Valve with Power
The Servotronic valve is supplied with power in response to the car's road speed. The power-assisted steering characteristic is memorized in a characteristic curve. The characteristic curve specifies the power based on the car's road speed. The characteristic curve is written using 16 reference points. The Servotronic valve is no longer supplied with power when the engine is OFF (over the PT-CAN). Minimum steering force support is set (high road speed) when the Servotronic valve is no longer supplied with power. Equally, the Servotronic valve is also no longer supplied with power if there is a fault-memory entry.
Undervoltage detection and overvoltage detection
The Servotronic valve is no longer supplied with power under the following conditions
- Supply Voltage Less Than 9 Volts
- Supply Voltage Higher Than 17 Volts
Voltage dips during ignition faults are detected.
SWITCH-ON CONDITIONS
Switch-on conditions for the Servotronic are
- Terminal 15 ON
- Engine Must Be Running
The Servotronic only activates the Servotronic valve when the engine is running. When the engine has started and a road-speed signal is not supplied, the Servotronic sets the maximum steering force support. If a plausible road-speed signal is detected, the Servotronic valve is activated by the characteristic curve. If there is still not a (or not a plausible) road-speed signal 5 seconds after the engine has started, the minimum steering force support is set.