Functional Description
Automatic Stability Control plus Traction (ASC +T):
Functional Description:
General
ASC+T Interconnected System:
General Block Diagram:
The task of the system is to control the slip of the drive wheels, irrespective of the condition of the road surface and corresponding to the coefficient of friction with the aim of achieving optimum driving stability at propulsion.
During a control phase, the engine torque is reduced by way of the DME (ignition intervention) and EML (throttle control system). If this measure is not sufficient, the drive wheel which is spinning too fast is additionally braked by means of the wheel brake in conjunction with a hydraulic actuator (plunger).
Display and Switching Logic:
The ASC multi-function lamp in the instrument cluster lights constantly when the ignition is switched on (lamp check). The system is switched on after engine start and can be switched off as required with the passive button. The ASC lamp is off when the system is switched on.
The ASC lamp flashing in the instrument cluster indicates to the driver that the ASC+T control is active.
If the ASC lamp lights permanently with the engine running, this indicates that the system has switched off automatically due to a defect or it was switched off by hand.
Safety Information:
The ASC+T system must be switched off before testing the vehicle on a brake or performance test stand.
The ignition must be switched off before towing a vehicle with the front axle raised.
Throttle Control via EML:
The throttle is controlled by means of the electronic throttle control system (EML).
If impermissibly high wheel spin occurs in a certain driving situation, the ASC+T determines the maximum drive torque which can be transmitted and constantly signals the necessary throttle position to the EML for the duration of the control phase.
The EML constantly signals the current throttle angle (set by driver) to the ASC+T via the DKV line. If the throttle angle is to be reduced, the EML receives this information via the DKR line. If the ASC+T requires that the throttle angle is increased, engine (engine drag torque control), this information is signaled to the EML via the DKE line.
Ignition Timing Control via DME:
A further measure to reduce the engine torque is to retard the ignition timing.
In ASC+T mode (control) the digital motor electronics (DME) receives a digital signal from the ASC+T and consequently retards the ignition timing.
Transmission Intervention via EGS:
When the ASC+T is active, a digital signal is sent to the electronic transmission control. As a result, the gearshift characteristic is changed within the EGS in order to facilitate smooth shifting or to temporarily suppress pending gearshifts.
Brake Intervention via the Plunger Hydraulic Unit:
General:
The fastest way of reducing the slip of a drive wheel is direct braking of the spinning wheel with the relevant wheel brake.
This brake intervention increases the stability and steerability of the vehicle and supplements the measures: Throttle control and ignition angle adjustment. In addition, individual wheel-controlled braking can achieve a differential lock effect on the power axle on road surfaces with different grip conditions.
Individual braking of the drive wheels occurs only within a speed range of 0 to 40 km/h. When.starting off on a regular road surface and generally at speeds above 40 km/h, the brake is used only temporarily to reduce the drive torque.
Operating Principle:
The brake pressure necessary for braking the wheel is applied via the two-plunger hydraulic unit. This hydraulic component is installed between the ABS hydraulic unit and the rear wheel brakes.
Via two solenoid valves, compressed oil (mineral oil) is applied from the pressure accumulator to the primary side of the plungers, thus displacing them. Due to this displacement, brake pressure is built up in the relevant wheel brake on the secondary side (brake fluid). If the brake pressure is to be reduced once again, the pressure in the primary circuit is reduced by way of the solenoid valves and the plunger moves back to its initial position.
Block Diagram of Hydraulic Plunger System:
Block Diagram of Hydraulic Plunger System:
Image Legend:
1,2 Rear brake line
3,4 Front brake line
5 Plunger
6 Solenoid valves
7 Pressure accumulator
8 Pressure switch
9 Accumulator charge valve
10,11 Rear wheel brakes
12,13 Front wheel brakes
14 Hydraulic tandem pump
15 Level control
16 Oil reservoir (mineral oil)
Safety Circuit:
The control unit itself, as well as all components of the ASC+T system are constantly monitored for defects irrespective of the switching status of the ASC passive button. In the same way, the connections (interfaces) to other systems (DME, EML, EGS) are constantly checked or checked during a control procedure. If a fault occurs in the overall system, the ASC lamp in the instrument cluster is activated and the system switched off.. If a defect occurs during a control procedure, the control function can be completed via the intact control circuit depending on the severity of the defect. The ABS/ASC+T control unit concept allows for separate deactivation of both individual systems, whereby any ABS defect results in deactivation of the ASC+T system.