Active Roll Stabilization, Function - GF32.34-P-0002GN
MODEL 166
as of model year 2016
with CODE 468 (ACTIVE CURVE SYSTEM)
MODEL 292
with CODE 468 (ACTIVE CURVE SYSTEM)
Block diagram
Function requirements, general
- Engine running
- Gear range "D" or "N"
System in general
Active roll stabilization increases and improves ride comfort and vehicle dynamics.
This is achieved by the following system features:
- Improved suspension comfort with transverse movement of the body
- Stabilization of the body in the case of transverse inclination, crosswind and uneven loading
- Increased driving safety due to forgiving lane-change behavior up to type-tested maximum speed
- Roll stabilization on bends and active wheel load distribution
- Increased responsiveness with significantly reduced steering angle requirements
- Improved torsional flexibility off-road (degree of axle articulation)
- Improved sensing of individual obstacles
On vehicles with active roll stabilization, the conventional (passive) stabilizer bars on the front and rear axle are replaced by 2 stabilizer bars which allow active intervention. Active intervention is made possible by the use of 2 hydraulic stabilizer bars.
The active roll stabilization control unit regulates the hydraulic pressure in the respective hydraulic stabilizer bar actuator based on a characteristics map.
The most important input variable is the lateral acceleration of the vehicle. This is measured, on the one hand, by the active roll stabilization lateral acceleration sensor and, on the other, is computed by the active roll stabilization control unit based on the following input factors:
- Steering wheel angle, via steering wheel angle sensor
- Steering wheel velocity, from steering column tube module control unit
- Vehicle speed, via left front axle rpm sensor, right front axle rpm sensor, left rear axle rpm sensor and right rear axle rpm sensor
The computed and measured lateral acceleration values are compared against each other. If the difference between the values remains below a specified limit value, the computed value is used to determine the roll moment. If the difference between the values exceeds the limit value, the measured lateral acceleration value is hidden. The roll moment acting proportionally on the vehicle body can be computed based on the lateral acceleration value.
The two active stabilizer bars must jointly build up an appropriate counteracting moment to reduce the roll angle. For this purpose, the active roll stabilization control unit computes the specified pressure in the hydraulic stabilizer bar actuators and determines the percentage of the counteracting moment which has to be provided by the front stabilizer bar and by the rear stabilizer bar.
The aim is for the vehicle handling characteristics to be stable and easy to control for the driver.
With active roll stabilization, the roll moment distribution can be dynamically adjusted between the front and rear stabilizer bar depending on the driving situation, e.g. depending on the vehicle speed.
If, for example, more roll moment is applied to the front stabilizer bar at a high vehicle speed, the handling characteristics of the vehicle tend toward understeer and stable straight-ahead travel. The vehicle demonstrates forgiving lane-change behavior.
In the medium vehicle speed range, the driver experiences a more responsive vehicle due to active roll control. The steering wheel angle requirements are reduced. With "Circuit 15 ON", an internal function test of the active roll stabilization control unit is performed at first.
Next, the electrical functions of the following sensors and actuators are tested:
- Active roll stabilization front axle pressure sensor 1
- Active Roll Stabilization front axle pressure sensor 2
- Active roll stabilization rear axle pressure sensor 1
- Active Roll Stabilization rear axle pressure sensor 2
- Active roll stabilization lateral acceleration sensor
- Active roll stabilization rear axle directional control valve
- Active roll stabilization rear axle pressure regulating valve
- Active roll stabilization front axle directional control valve
- Active roll stabilization front axle pressure regulating valve
- Active roll stabilization throttle valve
- Active roll stabilization safety valve
In addition, the hydraulic safety functions are tested after the vehicle is driven off for the first time. The system must have been active once.
For this purpose, a test pressure is built up between the active roll stabilization hydraulic pump and the active roll stabilization safety valve.
This tests whether the active roll stabilization safety valve is actually in the desired position (closed) when de-energized. At the same time, the function of the active roll stabilization front axle pressure regulating valve is tested.
This test is not noticeable inside the vehicle as no pressure is built up in the front axle hydraulic stabilizer bar actuator. Active roll stabilization is fully deactivated with the vehicle at a standstill or at a vehicle speed v < 5 km/h, all valves are deenergized. No active moments are adjusted with the vehicle at a standstill. A vehicle on a lean (e.g. single-sided load, vehicle on curb) thus does not cause any control intervention even though the active roll stabilization lateral acceleration sensor supplies a signal.
As of a vehicle speed v = 5 km/h, active roll stabilization is started and becomes fully active as of a vehicle speed of 30 km/h.
In the offroad transmission mode, the function is available when the vehicle is stationary in order to support the vehicle in the case of transverse inclination. The system remains active in the offroad transmission mode until the engine is switched off or the gear selection stage "P" is engaged.
The active roll stabilization function is broken down into the following conditions:
- Straight-ahead driving
- Cornering
- System failure
Straight-ahead driving
When the engine is running, the active roll stabilization hydraulic pump pumps hydraulic fluid into the system. The active roll stabilization safety valve is energized (open). The active roll stabilization throttle valve is not energized and thus restricts the oil feed to the active roll stabilization hydraulic pump. The pressure acting on one side of the hydraulic stabilizer bar actuator has no effect on the respective stabilizer bars because it is released via internal leakage.
The active roll stabilization front axle pressure regulating valve and active roll stabilization rear axle pressure regulating valve are not energized and are thus open. The hydraulic fluid can flow directly back into the active roll stabilization oil reservoir. The pressure in the system is low. It is determined only by the flow resistance of the lines and valves.
Cornering
When the vehicle drives into a curve, the active roll stabilization control unit actuates the following components directly using pulse width modulated signals:
- Active roll stabilization front axle directional control valve
- Active roll stabilization front axle pressure regulating valve
- Active roll stabilization rear axle directional control valve
- Active roll stabilization rear axle pressure regulating valve
- Active roll stabilization throttle valve
The active roll stabilization safety valve is still energized (open). The higher the lateral acceleration, the greater the hydraulic pressure that is built up in the hydraulic stabilizer bar actuators. The active roll stabilization control unit records the actual pressure and regulates it to the specified pressure via the following sensors:
- Active roll stabilization front axle pressure sensor 1
- Active Roll Stabilization front axle pressure sensor 2
- Active roll stabilization rear axle pressure sensor 1
- Active Roll Stabilization rear axle pressure sensor 2
The required rotation direction (left or right) of the respective hydraulic stabilizer bar actuator is defined in accordance with the path of the curve (right-handed or left-handed curve) by actuating the Active Roll Stabilization front axle directional control valve and the Active Roll Stabilization rear axle directional control valve. Due to the installation position of the stabilizer bars (in front of the front axle and behind the rear axle), the hydraulic stabilizer bar actuators have different rotation directions.
System failure
If a system fault is detected, an entry is made in the fault memory of the active roll stabilization control unit. At the same time, a message appears in the multifunction display of the instrument cluster. The active roll stabilization safety valve is no longer energized and is closed. The hydraulic fluid in the hydraulic stabilizer bar actuator of the front axle is confined, thus blocking the rotary actuator. The active stabilizer bar at the front axle then acts as a passive stabilizer bar. At the same time, the delivery flow of the hydraulic pump is routed directly and without pressure to the active roll stabilization oil reservoir. The rear axle hydraulic stabilizer bar actuator remains open. This produces safe vehicle handling characteristics tending towards understeer.
| Electrical function schematic for Active Roll Stabilization | PE32.34-P-2050-97NBA | ||
| Overview of system components of active roll stabilization | GF32.34-P-9998GN |