ASC drive torque control circuit, function - GF42.40-P-2100PP

MODEL 163 as of 1.9.02, 203, 209 with CODE (472a) Electronic Stability Program (ESP) 

ASR function 

Critical driving conditions can occur not only under braking but also if the drive wheels spin when driving off and accelerating (especially on smooth uphill roads) or when cornering. In this situation the ASR adapts the engine torque to produce the optimum drive torque that can be transmitted to the road. The ASR has 2 independent control loops, the braking torque controller and the drive torque controller. The two control loops are matched to each other and become active depending on the vehicle situation.

Braking torque controller 

This has the task of increasing the brake pressure in one or both brake cylinders of the drive wheels. There are 3 possible switch positions for the inlet and outlet valves in each hydraulic channel. In the case of ASR control, 2 solenoid valves and the self-priming high-pressure and return pump are also actuated. On actuation, the intake valve opens and the changeover valve closes. In addition to valve control, the self-priming high pressure and return flow pump is started by the control logic with the first valve control signal via a relay.

The brake fluid can then be drawn in from the brake fluid reservoir via the master brake cylinder. If the inlet valve is open, the brake pressure produced by the high-pressure and return pump then travels to the relevant wheel brake cylinder.

The control concept is based on the fact that the lateral stability and traction of a wheel are monitored and controlled primarily by means of the wheel slip. If the so-called instability limit is exceeded, a control cycle is started in which brake pressure modulation with pressure increase, hold and reduction phases takes place, which is terminated when the stability limit is reached.

The pressure modulation is proportional to the braking torque and is specially matched to the brakes in each vehicle model series. Alterations to the brake configuration must be taken into account in the ASR so that the ratio of moment to pressure is correct. The system interprets the instability criterion as having been met (the vehicle has reached the stability limit) once wheel slip and acceleration rates exceed a defined threshold.

For control to start, these thresholds must have been exceeded for a certain length of time. This prevents disruptive control sequences from occurring due to irregularities in the road surface. When instability occurs, the system distinguishes between "select low" and "select high" control. The system responds to instability on one side of the vehicle during straight-line operation below speeds of approximately v = 40 kph with closed-loop control in its "select high" mode, in which the brakes are employed to simulate the functionality of a limited-slip differential and ensure maximum traction. The drive torque control is switched to insensitive with respect to the braking torque control to have sufficient engine torque available for traction. In all other cases, "select low" control applies. The braking torque control and drive torque control then act practically simultaneously to stabilize the drive wheels quickly so they can transmit lateral force and avoid the vehicle breaking away.

Drive torque control circuit 

The drive torque control circuit uses as its actuator the engine throttle valve via an electronic engine control system and ignition timing. If the drive torque control instability criterion is fulfilled, intervention is activated after a predefined time. At the start of control and when large changes in nominal torque occur, the additional short-term ignition timing intervention is performed. As with brake moment control, the control concept is based on keeping the wheel slip below a threshold level. If it is exceeded, reduction of the drive torque takes place until the wheelslip is again within the permissible range. The drive torque is then carefully increased until the next instability occurs.