Design and function of acceleration skid control ASR II - RA4200140A800X(42-0800)
- General
Acceleration skid control (ASR) is an automatic system for improving the capacity for starting off and accelerating as well as driving stability. ASR is a logical extension of the anti-lock braking system (ABS).
Whilst with ABS the wheels are prevented from locking when braking, thus maintaining steerability and driving stability as the vehicle decelerates, ASR prevents the driving wheels skidding when starting off and accelerating. It thus provides optimum driving stability together with increased traction capacity.
For ASR the basic ABS components are supplemented by an additional hydraulic unit, a pressure supply unit, a pressure accumulator, a fourth wheel speed sensor, enhanced electronics and electronic accelerator pedal with control unit, nominal value sensor and actuator. Cruise control (Tempomat) is also contained in the electronic accelerator control unit.
Advantages
- Improves starting off and acceleration capacity due to increased traction; particularly advantageous on road surfaces with different levels of adhesion and on corners.
- Increases active safety in the dynamics of vehicle movement, since only a wheel which is not skidding can provide optimum propulsion without any loss of lateral control.
- Automatically adapts the engine torque to the ability of the wheels to transmit this to the road when the driver applies too much throttle.
- A function display provides information on the ASR control operation and provides a signal to the driver that he is driving the vehicle close to its limits.
- Driving with ASR II
Displays for ASR with various symbols are located in the instrument cluster.
Switching on the ignition
When the ignition is switched on (ignition/starter switch "position 2") the function indicator lamp and warning lamps in the instrument cluster (bulb check) come on and go out when the engine is running.
Driving mode
In driving mode the function indicator lamp comes on as soon as ASR is operating. The function indicator lamp comes on approximately 1 s after the ASR control mode is terminated. The function display does not come on if ASR control mode is in operation for a very brief period.
Control mode
The function indicator lamp provides information on the ASR control mode and provides a signal to the driver that he is driving the vehicle close to its limits.
He can thus better adapt his driving style to suit prevailing road conditions.
Indication
If the ASR warning lamp comes on when the engine is running, there is a fault in the ASR or in the electronic accelerator pedal. If ASR is inoperative, the ABS continues to operate. If there is a fault in the electronic accelerator pedal it is possible to continue driving with reduced engine performance (accelerator goes into emergency running mode). The driver notices this due to changes in the force required to operate the accelerator pedal (after a short travel, accelerator is difficult to operate).
If the ASR and ABS warning lamps come on when the engine is running, there is a fault in the ABS, which always causes the ASR to be switched off as well.
In the event of a fault in the ASR which also affects the ABS, both systems are once again switched off.
ASR snow chain switch (S76)
(Snow chain switch)
In deep snow and when snow chains are fitted, the sensitive, stability-oriented control of ASR can have a disadvantageous effect when starting off.
Therefore a special control logic has been programmed in for the starting off range in such situations, which can be selected by the driver via the snow chain switch.
The indicator lamp in the switch comes on when the snow chain switch is operated. After 50 ms continuous ASR control mode a higher drive slip is permitted at the wheels so that the vehicle can bite its way out in the above-mentioned situations.
Since this higher slip would have a negative effect on driving stability particularly at higher speeds, it is limited to the speed range < 40 km/h. The higher drive slip remains stored in memory up to 60 km/h and is automatically brought back into operation when the vehicle speed falls below 40 km/h. If 60 km/h is exceeded the memory is erased and the indicator lamp in the switch goes out.
- Overall function of ASR II
Location of components
Model 124
Model 126
Model 129
Model 201
Overall function ASR II
The speeds of all wheels are recorded and processed in the ASR control unit (N30/1). If the ASR control unit (N30/1) recognizes from the wheel speed signals that one or both driving wheels are tending to skid, the ASR control mode starts with the braking moment control circuit and the drive moment control circuit. The following processes are initiated:
- Brake moment control circuit
The skidding drive wheel is braked via the relevant wheel brake until it has achieved the most favorable acceleration skid range. In this way the other wheel can be driven to best effect (differential lock effect).
To brake the skidding wheel, part of the brake fluid under high pressure is led from the pressure accumulator into the brake caliper via the ASR hydraulic unit (pressure build-up).
A solenoid valve in the ASR hydraulic unit (A7/3) modulates the pressure characteristics in the brake caliper during brake moment control, in the pressure build-up phase, pressure holding stage, and pressure reduction stage.
For pressure reduction, the volume of brake fluid is returned from the brake caliper into the pressure accumulator via the return pump/ charge pump (A7/3m1). This control sequence is performed individually for each wheel brake of the drive wheels.
- Drive moment control circuit
In order to reduce drive moment which is too high (with wheels skidding) for optimum traction, there is an exchange of signals via the connection between the ASR control unit (N30/1) and the electronic accelerator control unit (N4/1).
The throttle valve position is reduced by the actuator relative to the accelerator pedal position set by the driver.
A check is constantly made in the ASR control unit as to whether the control functions can be cancelled, e.g. as a result of a sudden improvement in road surface adhesion, so that the drive moment applied by the driver via the accelerator pedal can come back into operation as soon as possible.
A distinction has to be made between three control ranges in ASR control mode:
- Control mode with one skidding drive wheel and at vehicle speeds <40 km/h.
Here the slip threshold of the drive moment control is higher than that of the brake moment control. In the event of control, the brake moment control is thus activated first. If the drive wheel nevertheless skids faster, the drive moment control is also activated. Thus priority is given to maximum traction.
- Control mode when both drive wheels skid or at a vehicle speed >40 km/h.
In this case, the slip threshold of the brake moment control is above that for the drive moment control.
Initially the throttle valve position is reduced in the event of a control. If the drive wheels nevertheless skid faster, the brake moment control is also activated. Thus priority is given to maximum driving stability.
- Control mode when cornering, at vehicle speeds between approx. 20 km/h and 120 km/h.
In this case, depending on the lateral acceleration, the drive moment control comes into operation even sooner than for control mode "B".
An ASR function indicator lamp (A1e21) provides information concerning the ASR control mode and signals to the driver that he is driving the vehicle close to its limits. The function of ABS is not impaired by ASR since the ASR control mode is interrupted when braking.
Brake moment control circuit
The brake moment control circuit builds on the electric/electronic and hydraulic components of the service brake with ABS.
Electrics/electronics
In addition to the ABS functions, all ASR functions are contained in a common ASR control unit (N30/1). Compared to the conventional ABS control unit, further input signals and 2 ASR microcomputers for ASR functions and additional output signals are provided.
Hydraulics
Building on the conventional ABS hydraulic unit a compact ASR hydraulic unit (A7/3) was developed for the ASR II.
Apart from the components familiar from the ABS, it contains a change-over valve for changing from braking mode to ASR control mode, pressure limiting valves as hydraulic safety equipment and the right rear axle solenoid valve (A7/3y4) for the right rear wheel brake.
The brake pressure necessary for ASR control mode comes from the pressure accumulator which is filled by the ASR pressurizing pump (M15) together with the return pump/charge pump (A7/3m1).
The speed signals of the wheel speed sensors (L6/1, L6/2, L6/3, L6/4) are continuously processed to the following parameters in the ABS/ASR control unit (N30/1).
- Acceleration skid
- Acceleration
- Vehicle speed (front wheel)
- Curve recognition
If the values determined exceed specified nominal values, output signals from the ASR control unit (N30/1) effect control of the 3 pressure phases (pressure build-up, pressure holding, pressure reduction) in the ASR hydraulic unit (A7/3) (brake intervention).
Depending on the vehicle speed, the brake moment control starts as soon as the differential speed between the front and the rear wheel (Δv) exceeds the slip threshold of the brake moment control.
Thus the braking of the relevant rear wheel is controlled by pressure build-up, pressure holding and pressure reduction in the appropriate rear wheel brake.
If the differential speed (Δv) falls below the slip threshold again, the system switches to pressure reduction for a pre-determined time interval.
Slip thresholds
Drive moment control circuit
The drive moment control circuit builds on the electric/electronic components of the ASR and is supplemented by the electronic accelerator pedal (EFP).
The electronic accelerator pedal comprises the following electrical/electronic and mechanical components:
- Electronic accelerator pedal nominal value sensor (R25)
- Electronic accelerator pedal actuator (M16/1)
- Electronic accelerator pedal control unit (N4/1)
- Control linkage and accelerator pedal
- Lost motion rod (redundancy rod)
The wheel speed signals from the wheel speed sensors (L6/1, L6/2, L6/3, L6/4) are constantly processed in the ASR control unit (N30/1) to the following parameters:
- Drive skid
- Acceleration
- Vehicle speed (front wheel)
- Curve recognition
- Control mode with one skidding drive wheel and at vehicle speeds <40 km/h.
If the values determined exceed specified nominal values, the ASR control unit (N30/1) causes the electronic accelerator pedal control unit (N4/1) to close the throttle valve.
Thus the electronic accelerator pedal (EFP)/Tempomat cruise control (TPM) actuator (M16/1) and thus the throttle valve are closed irrespective of the adjustment movements of the accelerator pedal.
Depending on the vehicle speed, the drive moment control starts as soon as the differential speed (Δv) exceeds the slip threshold of the drive moment control.
The drive moment is controlled by closing, holding and opening the throttle valve.
If the differential speed (Δv) falls below the slip threshold again, the throttle valve is opened in accordance with what the driver dictates.
- Brake moment control circuit
- Location of electrics/electronics components
Model 124
Model 126
Model 129
Model 201
- Functions of electrics/electronics
The corresponding functions in the ASR hydraulic units (A7/3) (brake moment control circuit) and the functions of the electronic accelerator pedal (drive moment control circuit) are controlled by the electric/electronic components.
Voltage is supplied to the ASR electronic control unit (N30/1) mainly via the overvoltage protection relay (K1/2).
The ASR control unit (N30/1) receives input signals from the following components:
- 4 wheel speed sensors
- Stop lamp switch
- Electronic accelerator pedal control unit (throttle valve actual value)
- Snow chain switch
- Pressure switch
The input signals are processed in the ASR control unit into output signals for the following components
- Solenoid valves in the hydraulic unit
- Relay (solenoid valves, return pump/charge pump)
- Pressurizing pump
- Electronic accelerator pedal control unit (throttle valve nominal value)
- ASR function indicator lamp
- Display lamp in snow chain switch
- ABS warning lamp
- ASR warning lamp
The ASR control unit (N30/1) decides which of the three following operating modes have to be engaged based on the input signals:
- Normal mode
- ABS control mode
- ASR control mode
Normal operation
In this case no speed differences exist (which require a control mode). All solenoid valves in the ASR hydraulic unit (A7/3) are in the basic position, thus de-energized. The system is ready for braking.
If a pressurizing signal from the pressure switch (A7/1s1) in the ASR hydraulic unit (A7/3) exists at the ASR control unit (N30/1), the ASR pressurizing pump (M15) and the return pump/charge pump (A7/3m1) is controlled.
The pressurizing process of the pressure accumulator continues until the pressure switch A7/3s1) gives the signal that the pressure accumulator is full.
The throttle valve actual value is continuously checked in the ASR control unit (N30/1) in order to check the function of the electronic accelerator pedal.
ABS control mode
From the available speed signals, the ASR control unit (N30/1), recognizes that the criteria for a ABS control exist.
The left or right front axle solenoid valves (A7/3y1 or A7/3y2) and left or right rear axle solenoid valves (A7/3y3 or A7/3y4) in the ASR hydraulic unit (A7/3) are controlled by the ABS circuits in the ASR control unit (N30/1) via the corresponding output stages. The left or right front axle solenoid valves (A7/3y1 or A7/3y2) in the ABS/ASR hydraulic control unit (A7/3) are controlled separately as in the conventional ABS. The left or right rear axle solenoid valves (A7/3y3 or A7/3y4) are controlled jointly for the ABS control process.
ASR control mode
The wheel speed signals are continuously processed in the ASR microcomputers in the ASR control unit (N30/1).
As soon as a drive wheel tends to skid, the ASR control unit (N30/1) controls the switchover solenoid valve (A7/3y5), the left and/or right rear axle solenoid valves (A7/3y3 and/or A7/3y4) in the ASR hydraulic unit (A7/3).
The left or right rear axle solenoid valve (A7/3y3 or A7/3y4) are controlled separately. In the pressure reduction switch position (refer to "hydraulics function") the return pump/charge pump (A7/3m1) is controlled.
During the ASR control mode the ASR function indicator lamp (A1e21) lights up.
If an excessive drive moment is established in the ASR microcomputers in the ASR control unit (N30/1), the throttle valve nominal value output signal to the electronic accelerator pedal control unit (N4/1) changes.
In this way the electronic accelerator pedal actuator (M16/1) is controlled with a particular signal from the electronic accelerator pedal control unit (N4/1) whereby the throttle valve is adjusted against the accelerator pedal movement. The pressure accumulator is not charged during ASR control mode.
The ASR control mode is interrupted by operating the brake pedal. Faults which influence ASR and ABS function are recognized in the ABS/ASR control unit (N30/1) and displayed by the ASR and ABS warning lamps (A1e22, A1e17). In the event of faults, which are related to ABS, both systems are always switched off.
Overvoltage protection relay (K1/1 or K1/2)
Voltage is supplied for the ASR electronic control unit via the overvoltage protection relay. Overload is prevented by the 10-A fuse.
NOTE: In addition to the designations on the plug listed below, all cables and plugs are designated with "L" and "E" (left) or "R" and "D" (right).Wheel speed sensors
Vehicles with ASR are equipped with 4 wheel speed sensors.
Models 124 and 201
The 2 wheel speed sensors on the front axle (L6/1, L6/2) can be recognized by the single edged tip (arrow) and by the long cable with coaxial plug (as on vehicles with ABS).
The left rear axle speed sensor (L6/3) is identified with "E" on the plug (arrow).
Right rear axle speed sensor (L6/4) is identified with "D" on the plug (arrow).
The plugs "D" and "E" of the wheel speed sensors are plugged into the plug connection, rear axle distributor (X62/8).
Model 126
The 2 wheel speed sensors on the front axle (L6/1, L6/2) can be recognized by the double edged tip (arrow) and by the long cable with coaxial plug (as on vehicles with ABS).
The left rear axle speed sensor (L6/3) is a straight design. The cable has a connector (L6/3x) as plug connection.
The right rear axle speed sensor (L6/4) is an angled design. The cable has a plug (L6/4x) as plug connection.
Model 129
The wheel speed sensors on the left or right of the front axle (L6/1 or L6/2) and left or right of the rear axle (L6/3 or L6/4) are connected to the ABS cable harness via the left (X62/7) and right (X62/6) plug connection, front axle distributor.
Location of the plug connection, left front axle distributor (X62/7) in left of the front axle component compartment.
The right rear speed sensor (L6/4) is an angled design. The plug (L6/4x1) is identified with the letter "D" (arrow).
The plugs (D, E) of the left or right rear axle speed sensors (L6/3 or L6/4) are plugged into the plug connection, rear axle distributor (X62/8) into the openings marked "D" and "E".
Models 124, 126, 129, 201
The speeds of the rear wheels are measured separately at the serrations on the rear axle shaft flange.
Model 126
ASR snow chain switch (S76)
Snow chain switch
This switch should be operated when driving in deep snow with snow chains fitted.
After the switch has been operated, an indicator lamp in the switch comes on which confirms that a higher slip threshold is preselected in the ABS/ASR control unit.
Operation: Press at the top -"ON"
Press at the bottom
- "OFF"
The slip thresholds in the lower speed range < 40 km/h are increased in stages after 50 ms control mode.
At vehicle speeds between 40 and 60 km/h the higher slip threshold is kept stored. If the speed exceeds 60 km/h the slip threshold changeover is erased again.
By increasing the slip thresholds, a higher differential speed between the front and rear wheels is possible. In this way, the vehicle can bite its way out of snow.
Thus maximum propulsion is achieved in deep snow when snow chains are fitted. For reasons of driving stability, this control is only allowed in the speed range up to 40 km/h.
Slip thresholds
ASR control unit (N30/1)
There are 4 versions of the ASR control unit (N30/1). The different characteristics for the electronic accelerator pedal are taken into account:
- for vehicles model 124 with 103 and 104 engine without speed signal output
- for vehicles model 129 with 103 and 104 engine with speed signal output
- for vehicles model 126 with 116/117 engine without speed signal output
- for vehicles model 129 with 119 engine with speed signal output
In order to avoid confusing ASR control units for the different engine variants, 2 lugs are cast on the side of the control unit connector. A recess at the respective different position is applied at the front claw.
2 grooves (arrows) are cast on the side of the ASR control unit at the guide for the connector. On the right of the locking device is a web at various positions (arrows).
The control unit comprises various circuit boards which are equipped with electronic components. Connection to the cable harness is via the 35-pin plug connection.
Apart from the ABS functions, the ASR control unit also contains all the ASR functions. It is divided up into functions as follows:
- Signal processing
- Input amplifier
- Logic sections
- Safety circuits
- Output stages
Signal processing
The input signals from the four wheel speed sensors are transformed into digital signals in the signal processing sections and transmitted to the ABS circuits and ASR microcomputer and processed.
Input amplifier
There is one input amplifier for each of the input signals from
- Stop lamp switch
- Throttle valve actual value
- Snow chain switch (S76)
- Pressure switch (A7/3s1)
Here the incoming digital signals are filtered and converted into a value which can be evaluated in the logic section and safety circuit.
Logic section
All input signals are processed in the logic sections of the ABS circuits and ASR microcomputer.
Wheel speed signals:
The processed wheel speed signals are continuously compared with each other and then with the specified slip thresholds (differential speeds between rear and front wheels, depending on the vehicle speed). The following values are determined by this comparative process:
- Drive slip
- Acceleration
- Front wheel driving speed
- Curve recognition
If the values determined exceed the specified slip thresholds then ASR control mode is initiated.
Adjustment of the throttle valve is controlled by the throttle valve nominal value output signal.
Stop lamp switch signal:
When the brake is operated, a signal is transmitted to the logic section. The solenoid valves are switched into the original position by the logic section via the respective output stages. In other words, ASR control mode is terminated immediately.
Throttle valve actual value signal:
In ASR control mode, the logic section controls the throttle valve closure of the electronic accelerator pedal via the throttle valve actual value according to the specified throttle valve nominal value.
Snow chain switch signal:
If there is a signal from the snow chain switch to the logic sections, an increased slip threshold is used in the logic and a signal issued to the indicator lamp via the corresponding output stage.
Pressure switch signal:
The pressurizing signal from the pressure switch causes the activation of the pressurizing pump and return pump/charge pump via the corresponding output stage.
Safety circuit
It is the task of the safety circuits to recognize faulty signals within the electronic control units and faults in the relays in the hydraulic unit as well as in the wheel speed sensors.
The ABS and ASR system is then switched off in the event of a fault.
The safety circuit also continuously monitors battery voltage. If the battery voltage is < 10 V, the system is switched off, until the voltage has again attained its nominal value. The following faults are recognized:
- Control unit defective
- Solenoid valve defective
- Wheel speed sensor defective or loose contact
- Relay defective
- Return pump/charge pump defective
- Accumulator leaking
- Pressure switch defective
Faults detected are displayed by the warning lamps coming on. When the ignition is switched off, all faults which have occurred briefly are erased again.
Output stages
The output signals from the ABS circuits and ASR microcomputers to the solenoid valves, relays, warning lamps etc. are amplified by appropriate output stages.