Example showing suspension strut with air spring and active damper |
1 | Air bellows |
2 | Front axle shock-absorber strut (hydropneumatic system) |
3 | Piston chamber |
4 | Rear axle shock absorber |
P32.50-2426-75
Example showing electrohydraulic suspension on front axle |
1 | Air bellows |
2 | Front axle shock-absorber strut (hydropneumatic system) |
5 | Hydraulic line |
6 | Air suspension control unit, compressor and valves |
7 | Electrohydraulic controller unit |
P32.50-A028-75
Example showing electrohydraulic suspension on rear axle |
1 | Air bellows |
4 | Rear axle shock absorber |
7 | Electrohydraulic controller unit |
P32.50-A027-75
Overview
This document contains information on:
General
Function requirements
Function
General
The electrohydraulic suspension is a fully active suspension system based on the 48 V on-board electrical system technology. It combines an air suspension system with electrohydraulic dampers. The electrohydraulic suspension adapts both the springing and the damping at each individual wheel to suit the ambient conditions.
Function requirements
Ignition ON (circuit 15)
Function
The electrohydraulic suspension supplements the air suspension with a semi-supporting hydropneumatic system. In this, the air suspension bears the basic load of the vehicle body and regulates the level. The hydropneumatic system produces dynamic forces which are superimposed on the forces of the air suspension. The vehicle body is thus actively supported and damped, e.g. under longitudinal and lateral acceleration or when driving on uneven roads.
At each of the four wheels, a hydraulic damper is installed inside the axle, which contains an adjustable throttle valve at both working chambers and a hydraulic accumulator. The hydraulic damper is connected through hydraulic lines to an electrohydraulic controller unit. The air suspension control unit actuates the electrohydraulic controller units at all four wheels. This control unit also actuates the valves and the compressor of the air suspension system.
When an electrohydraulic controller unit is actuated, the integrated hydraulic pump moves the hydraulic fluid between the annular space and the piston space in the connected hydraulic damper. A pressure difference can be set here inside the hydraulic damper. This pressure difference generates a force at the piston of the hydraulic damper, that is set to ensure that the required dampening effect is achieved on the vehicle body.
The system primarily uses information from the following sensors:
Pressure sensors
Body acceleration sensors
Lateral acceleration sensor
Level sensors at each wheel
With this sensor information, the air suspension control unit mainly actuates the following actuators:
Damping valve units at each wheel
Electrohydraulic controller units at each wheel
Influencing variables
The electrohydraulic suspension detects the driving situation and the vehicle load, and adapts the control accordingly. The adaptation is dependent on the following factors:
Driving style, e.g. dynamic
Road surface, e.g. surface undulations
Vehicle load
Drive program, e.g. Comfort
Active lean-over in bends
The active lean-over in bends also has the following function requirements:
Vehicle speed between 30 km/h and 160 km/h
If the Curve drive program is activated, the vehicle actively leans into the curve. This improves ride comfort when cornering. Active lean-over in bends is only available in a certain vehicle speed range. Outside this vehicle speed range, the Curve transmission mode is equivalent to the Comfort drive program.
When the vehicle drives into a curve, the air suspension control unit primarily actuates the following components:
Left front electrohydraulic controller unit
Right front electrohydraulic controller unit
Left rear electrohydraulic controller unit
Right rear electrohydraulic controller unit
Recovery mode
The recovery mode also has the following function requirements:
Off-road program activated
Vehicle speed below 15 km/h
Recovery mode is a function of the electrohydraulic suspension which can help the driver to free a vehicle which has become stuck on loose ground (e.g. sand or snow).
When recovery mode is activated, the body is moved in slow vertical oscillations. In this way, the wheels are intermittently more heavily loaded so that they have more grip, enabling the vehicle to be freed.
In the following cases, recovery mode is deactivated automatically:
A hard impact on the underbody of the vehicle is detected.
After a period of 30 seconds.
Vehicle speed above 15 km/h.
A door is opened.
Individual wheel control
The individual wheel control also has the following function requirements:
Vehicle speed below 15 km/h
Individual wheel control is a function of the electrohydraulic suspension with which the vehicle level can be adjusted at each wheel individually. This can help to improve the alignment of the body when driving off-road.
In the following cases, individual wheel control is deactivated automatically:
A hard impact on the underbody of the vehicle is detected.
Vehicle speed above 15 km/h.
A door is opened.
Schematic diagram of visual road mapping |
8 | Visual road mapping |
P32.50-2422-74
Visual road mapping
The visual road mapping also has the following function requirements:
Engine in operation
Vehicle speed between 7 km/h and 130 km/h
If the multifunction camera is installed, visual road mapping is available to the electrohydraulic suspension. With this, the road in front of the vehicle is captured by the camera. This information about the road surface enables the suspension struts to be actuated prior to surface undulations is such a way as to significantly reduce movements of the body when the undulations are driven over.
Visual road mapping may be impaired or inoperative in the following cases:
With inadequate illumination of the road, e.g. in darkness.
With snow, rain, fog, heavy slush, dazzle, direct sunlight or alternating light conditions.
If the windshield in the area of the multifunction camera is soiled, fogged, damaged or covered.
If the road surface has no visible structure or is reflective.
If the distance to the preceding vehicle is too short.
On stretches of road with very small curve radii.
Active roll stabilization
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.
The passive torsion bars on the front and rear axle are replaced by two torsion bars that provide active intervention. Active intervention is made possible by the use of hydraulic stabilizers.
The relevant 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 the rotational speed sensors of the four wheels
The active roll stabilization control unit regulates:
Hydraulic fluid flow of hydraulic pump
Oil pressure in hydraulic system of active roll stabilization
Rotation direction of hydraulic stabilizer bar actuator on front and rear axle
The aim is for the vehicle handling characteristics to be stable and easy to control for the driver.
Function schematics | |||
Function schematic for ADS | Model 257, 290 with code 489 (AIR BODY CONTROL) | ||
Model 167 with code 490 (Electrohydraulic suspension) | |||
Model 213, 238 with code 489 (AIR BODY CONTROL) | |||
Additional base function | |||
Level control system, basic function | Model all (CAR) | ||
Control units | |||
Air suspension control unit, basic function | Model all (CAR) | ||
Components | |||
Acceleration sensor, basic function | Model all (CAR) | ||
Active roll stabilization lateral acceleration sensor, basic function | Model all (CAR) | ||
Electrohydraulic controller unit, basic function | Model 167 |