GF42.40-P-2100PP ASC Drive Torque Control Circuit, Function

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

Function of ASR
Critical driving conditions cannot only occur when braking, but also when starting off and accelerating (particularly on a smooth road on a mountain) or when cornering if the drive wheels spin. In this situation ASR adapts the engine torque to the drive torque which can be transmitted to the road in each case. The ASR has 2 independent closed loops, the brake torque controller and drive torque controller. Both closed loops are matched to each other and are effective depending on the vehicle situation.

Brake torque controller
Its task is to build up the brake pressure in one or both brake cylinders of the drive wheels. 3 switch positions of the intake valves and exhaust valves are possible for each hydraulic channel. In the acceleration skid control [ASR] case in addition 2 solenoid valves and the self-priming high pressure/return flow pump are actuated. With the actuation the intake valve opens and the switchover valve closes. Apart from the valve actuation the self-priming high pressure/return flow pump is actuated via a relay by the controller logic with the first valve actuation signal.

The brake fluid can then be drawn out of the brake fluid reservoir via the master brake cylinder. When the intake valve is open the brake pressure produced by the high pressure/return flow pump reaches the wheel brake cylinder in question.

The control concept is based on the fact that basically the side stability and traction of a wheel is monitored and regulated via wheel slip. With the exceeding of the so-called instability limit a control cycle is started, brake pressure modulation taking place with pressure buildup, pressure holding and pressure reduction phases, which is ended again when the stability limit is reached.

Pressure modulation is proportional to brake torque and is specially matched to the brake in each vehicle model series. Changes to the brake design must be taken into account in the ASR [acceleration skid control] so that the ratio of torque to pressure match. The instability criterion is met, i.e. the stability limit is exceeded, if the wheel slip and wheel acceleration have exceeded a threshold value.

These threshold values must have been exceeded for a certain time for the start of control. As a result faulty controls due to road bumps are prevented. In the case of instability a distinction is made between "select low" and "select high" control. In the event of a single-sided instability below a vehicle speed of approx. v = 40 km/h and driving straight ahead the "select high" control mode applies, i.e. to attain an optimum traction the brake is used as a locking differential function. The drive torque control is switched to insensitive compared with the brake torque control, in order to have available plenty of engine torque available for traction. The "select low" control applies in all other cases. The brake torque control and drive torque control then intervene almost at the same time, in order to quickly stabilize the drive wheels so that these can transmit lateral stability forces and the vehicle is prevented from breaking away.

Drive torque controller
The drive torque control has as actuator the engine throttle valve via an electronic engine control and the ignition timing adjustment. If the drive torque control instability criterion is met, the interventions are activated after a certain time. The short-term additional ignition timing adjustment intervention takes place at the start of control and in the event of large changes in specified torque As with the brake torque control the control concept is based on keeping the wheel drive slip under a threshold value. If it is exceeded, a drive torque is reduced until the wheel slip is again in the permissible range. This is followed by the careful addition of drive torque up to the next instability.