With the DSC3, BMW will introduce in 09/96 the most modern dynamic control system currently available in the automotive sector.
This control system is designed to avoid instabilities, e.g. vehicle skidding, even more effectively.
In order to effectively meet these control objectives, it is necessary to precisely record both longitudinal-dynamic as well as transverse-dynamic drive statuses. In addition to the four wheel speed sensors and the steering angle sensor a further set of sensors is required for this purpose:
The rotation rate sensor is capable of signalling the slightest deviations from the set course determined by the steering angle sensor.
The transverse acceleration sensor senses lateral drift of the vehicle which does not spin but nevertheless is still off course.
The pressure sensor measures the brake pressure applied by the driver.
With the aid of this information and the four wheel speeds, the vehicle speed and various engine and transmission data, the DSC3 control unit can assess critical driving situations which do not correspond to the driver's requirements (steering lock) and can draw conclusions with regard to road conditions.
From these data, the DSC3 control unit calculates within milliseconds the oversteer and understeer characteristics of the vehicle as well as the location, duration and intensity of the necessary brake intervention.
The DSC3 control unit is powered via four lines:
Pin No. |
Signal |
|---|---|
1 |
12 V supply (terminal 87) |
51 |
12 V supply (terminal 30) |
28, 29 |
Ground (terminal 31) |
Note
At "ignition ON", the positive supply at pin No. 1 is switched on immediately via the DME main relay but it is switched off only after approx. 5 second delay at "ignition OFF" (switch-off delay by DME control unit).
The DSC3 control unit is linked via the CAN bus to the DME, EML and AGS control units. The steering angle sensor is also connected to this bus.
These control units permanently exchange the most diverse data in the form of telegrams via the CAN bus:
The AGS control unit signals the currently engaged gear (gear ratio) to the DSC3 control unit for the purpose of calculating the drive torques.
In this way, the DSC3 detects low frictional values and prevents downshifts in order to avoid annoying reciprocating gearshifts.
When driving off with low frictional values, the DSC3 control unit instructs the transmission management system via the CAN bus to engage 2nd gear at an earlier stage.
The DSC3 control unit has self-diagnosis capabilities. Any faults which occur are stored in a non-volatile memory and can be read out for the purpose of troubleshooting.
The following diagnostic functions have been realized:
Important!
The DSC3 control unit features a non-volatile fault code memory. In warranty cases, print out test code and do not delete defect code memory!
The DSC3 has two indicator lamps. These are
These two indicator lamps serve the purpose of indicating system statuses and system faults of the DSC3 in the instrument cluster.
When the ignition is switched on (terminal 15) the two indicator lamps are activated and their function checked. The two indicator lamps go out after approx. two seconds if no faults are found in the control unit self-test.
The safety lamp is off when the ABS is in standby mode. If the control unit detects a fault in the ABS system which leads to deactivation of the ABS, the ABS safety lamp lights up thus signalling failure of the system.
Activation of the safety lamp has been changed compared to the previous ABS systems:
The ABS safety lamp is connected to the 12 Volt supply via terminal 15. If the ABS is fully operable, the control unit applies high level at pin 32. This status is recognized by the electronic circuitry in the instrument cluster and the safety lamp is not switched to ground.
If there is a fault in the ABS system which results in its deactivation, the control unit switches the high level to low level.
This status initiates the instrument cluster electronics to switch ground through to the safety lamp so that it lights.
The same status as for an ABS fault is assumed automatically if the control unit is not connected.
The advantage of this new method of activation is that it renders the spring contacts in the plug connector or the intricate jumper circuit via the valve relay unnecessary.
The lamp is off when the DSC3 is fully operable, it flashes during a DSC or ASC control procedure.
The lamp lights continuously if a fault is found in the DSC3 system or if the system is temporarily deactivated by means of the DSC passive button.
The multifunction lamp receives its 12 V supply via terminal 15 and, depending on the relevant functions, receives clocked or continuous ground from the control unit.
The multifunction lamp receives ground via a spring contact in the plug connector in order to draw the driver's attention to failure of the DSC3 even when the control unit is not connected. This contact is open when the control unit is connected.
Note
During diagnosis (= communication with DIS) both indicator lamps light irrespective of whether there is a system fault or not.
The brake light switch informs the DSC3 control unit as to when the brake pedal is operated.
During ASC control, brake intervention is terminated immediately, the intake valve closed and the changeover valve opened.
The ABS control procedure is also terminated immediately if during ABS controlled braking the brake is released thus opening the brake light switch.
In the case of the DSC3 the entire system (ABS, ASC and DSC) is switched off if the brake light switch is defective.
The handbrake switch informs the DSC3 control unit that the handbrake is applied. This results in an increase in the slip threshold for the engine drag torque control (MSR) by 20 km/h.
As of ignition lock position 2 (terminal 15) and after successful control unit self-test, the ASC/DSC scope of functions can be deactivated with the passive button. The DSC multifunction lamp lights in the instrument cluster when the ASC/DSC functions are switched off. The ABS control function, however, is still operable.
The ASC/DSC function is reactivated by pressing the button again as long as the difference between the maximum rear wheel speed and minimum front wheel speed is less than 20 km/h.
Active speed sensors are generally fitted on the E38 and E39 model series.
These active speed sensors operate in accordance with the Hall principle, in which changes in the magnetic field strength produce alternating voltages with the frequency used as a measure for the wheel speed. In contrast to the passive speed sensors, the actual sensor element and electronics for signal conditioning are integrated in the active speed sensors.
Advantages of active sensors compared to the previous passive sensors:
The speed signals are square-wave signals with constant amplitude (low level = 0.75 V, high level = 2.5 V), frequencies within the range from 0 ... approx. 1700 Hz are obtained for the speed range from 0 ... 250 km/h dependent on the tyre circumference.
The DSC3 control unit provides the voltage supply to the active sensors (7.6 ... 8.4 V direct voltage).
The wheel speeds are also required by other control units, for instance, the instrument cluster electronics (IKE) or electronic damper control (EDC). The speed signals are correspondingly processed in the DSC3 control unit for this purpose.
The speed output signals are square-wave signals with constant amplitude (low level < 1 V, high level > 11 V). The frequency is dependent on the wheel speeds and is within the range from 0 too 1700 Hz (0 to 250 km/h).
The steering angle sensor is installed at the bottom end of the steering spindle in front of the steering flexible coupling.
Connector assignments:
Pin No. |
Signal |
|---|---|
1 |
12 V supply (terminal 30) |
2 |
12 V supply (terminal 87) |
3 |
CAN (+) |
4 |
CAN (-) |
5 |
Ground (terminal 31) |
The sensor supplies a total steering angle signal with a resolution of approx. 0.7 o on the CAN bus.
A status variable is also sent on the CAN bus which provides information on the validity of the steering angle signal.
At the end of the assembly line or after replacing sensors, initialization must take place as part of DSC3 diagnosis with the diagnostic tester with the steering or the front wheels exactly in the straight-ahead position. During this procedure, the electrical offset of the sensor is calibrated and stored permanently in the sensor (EEPROM).
An identification number which is sent by the steering angle sensor along the CAN bus is additionally stored in the sensor and in the DSC3 control unit. If the IDs differ it will not be possible to enter the DSC3 system and steering angle initialization will be requested in DSC3 diagnosis.
This is intended to avoid introducing a faulty offset value when using a brand-new steering angle sensor or when installing a sensor which has already been used in another vehicle.
In addition to the electrical offset, while driving, the DSC3 logic continuously determines an offset for the steering centre, e.g. as occurs when adjusting the toe. The DSC3 logic checks the plausibility of the steering angle signal (by comparing it with other sensor signals).
In the same way as the EDC steering angle sensor, this sensor consists of two precision potentiometer wipers which are offset by 90 o and which supply signal voltages of 0 to approx. 4.5 V.
The processor logic (microcontroller with CAN module) additionally incorporated in the sensor calculates the current steering wheel rotation from the individual voltages and monitors the plausibility of the total steering angle (90 o monitoring).
The sensor logic looses the current steering wheel rotation value if the voltage supply is interrupted (e.g. as the result of disconnecting the battery). In this special case, the current steering wheel rotation is re-calculated by statistical evaluation of the front wheel speeds (sent via CAN).
On the one hand, the total steering angle signal is used in the DSC3 logic to precisely determine the vehicle speed (when cornering).
On the other hand, it acts as an input signal for the higher-ranking vehicle controller:
The DSC3 logic calculates the "vehicle rotation rate" required by the driver from the steering angle signal.
Definition:
The rotation rate signal corresponds to the vehicle rotation speed about the vertical axis.
The rotation rate sensor is mounted under the driver's seat.
Connector assignments:
Pin No. |
Signal |
|---|---|
1 |
Negative supply (0 V) |
2 |
Positive supply (12 V) |
3 |
Sensor signal |
4 |
Reference signal (2.5 V) |
5 |
Test signal |
The sensor supplies a signal voltage of 0.7 - 4.3 V. The difference between the signal voltage and reference voltage ranges from -1.8 to +1.8 V and corresponds to a rotation rate of -50 to +50 o /s.
The sensor is monitored with regard to its permissible offset values as of "ignition ON".
In addition, the plausibility of the values is constantly checked while driving by comparison with the following signal information:
The effective signal is measured continuously every 20 ms as of "ignition ON".
In order to check the electrical function of the sensor, the effective signal is overlaid by an electrical offset during every second cycle. This offset overlay is controlled by the test line. The constancy of this offset is checked in the DSC3 control unit.
Oscillating cylinder which is excited by various piezo-elements to an oscillatory frequency of 14 kHz.
The cylinder is deflected by the effect of a transverse force triggered by the transverse acceleration while cornering.
The cylinder deflection is set back to zero by an electronic control circuit; this value which represents a measure for the relevant rotation rate is then converted into an analog voltage i.e. linear characteristic.
The measured rotary speed is compared with the rotary speed required by the driver (from the steering angle information) and the driveable limit rotary speed (derived from the transverse acceleration information).
The vehicle controller corrects the rotary vehicle speed as required by specific brake interventions at the individual wheels.
This is intended to ensure a stable driving status under all driving conditions (braking, propulsion and free rolling).
The transverse acceleration sensor is installed horizontally next to the driver's seat on the door sill approx. 5 cm in front of the B-pillar.
Connector assignments:
Pin No. |
Signal |
|---|---|
1 |
Sensor signal |
2 |
Negative supply (0 V) |
3 |
Positive supply (5 V) |
The sensor supplies an analog voltage of 0.5 - 4.5 V which corresponds to a measuring range of -1.5 - to +3.5 g.
The offset value is 1.7 V (vehicle stationary on horizontal surface).
After initialization, the DSC3 control unit calibrates the transverse acceleration signal both while the vehicle is stationary as well as while driving and continuously monitors its plausibility by comparing it with other sensor signals.
The transverse acceleration sensor is a capacitive sensor.
Under the effects of acceleration, a moving capacitor plate in the sensor is raised from a fixed capacitor plate. The signal voltage which occurs corresponds to the initiated acceleration.
The transverse acceleration signal is used in the DSC3 logic as an input signal for the higher-ranking vehicle controller. A nominal rotation rate is calculated on the basis of the measured transverse acceleration. On reaching the cornering limit speed this, nominal rotation rate corresponds to a cornering rate which is still stable under the given driving conditions.
The pressure sensor is mounted on the boost piston unit in the output to the front axle circuit.
Connector assignments:
Pin No. |
Signal |
|---|---|
1 |
Negative supply (0 V) |
2 |
Positive supply (5 V) |
3 |
Sensor signal |
The sensor supplies an analog signal with a measuring range from 0 to 250 bar.
The zero point is calibrated constantly after initialization of the system with "ignition ON" and is concluded after 100 ms provided the brake light switch is not operated.
Steel diaphragm with piezo-resistive measuring principle based on the distortion of the diaphragm with subsequent conversion into a linear characteristic from 0 to 5 V.
Monitoring with regard to plausible signal values by the DSC3 control unit in a time frame of 100 ms under following conditions:
The following statuses are monitored by the electronic control:
When calibrated (zero calibration), the system pressure must not exceed 5 bar for a certain period of time provided the brake light switch is not operated.
The signals from the brake light switch (BLS) and pressure sensor are combined in the system and their sum forms a redundant BLS signal which in turn is monitored by the electronic control with regard to logic statuses.
The hydraulic unit is located between the boost piston unit and wheel brake cylinder.
It modulates the brake pressure during ABS controlled braking and controls or regulates the brake pressure during an ASC or DSC control procedure.
Essentially, the design and function of the hydraulic unit correspond to that of ABS/ASC5.
A changeover valve and a prebooster valve must be additionally provided due to possible brake intervention at the front axle during a DSC control procedure.
The following are therefore integrated in the housing:
When terminal 15 is switched on, all valves are applied to 12 V and are switched to ground by the control unit during a control procedure.
Power is not applied to the 4 inlet solenoid valves and the 2 changeover valves are open, the 4 outlet valves and the 2 admission valves are closed.
The return pump is driven by an electric motor which, when active, is powered via the ABS motor relay.
During ABS control, the pump returns the brake fluid drained off during the pressure reduction phase to the master brake cylinder.
It produces the necessary brake pressure during ASC or DSC control with brake intervention.
The return pump has only one direction of rotation, i.e. both during ABS as well as during ASC/DSC control.
The changeover valve must be closed and the prebooster valve open if the pump is to generate brake pressure during an ASC/DSC control phase.
The pump draws in the brake fluid via the open prebooster valve and then delivers it to the wheel brake cylinder, in which the necessary brake pressure is to be built up. The system pressure build-up is secured by the closed changeover valve.
A hydraulically operated pressure relief valve integrated in the two changeover valves prevents the delivery pressure of the pump increasing above 161 (+/-25) bar during an ASC/DSC control phase.
The prebooster pump of the unit is activated with positive and negative supply directly by the control unit during an ASC/DSC control phase.
When activated, the pump delivers brake fluid from the reservoir of the tandem master brake cylinder into a chamber between two pistons of the piston unit.
A second line leads from this chamber back to the reservoir. The throttle nozzle installed in the return line facilitates the necessary pressure build-up (backpressure principle).
A pressure relief valve in the pump prevents the pump pressure increasing to more than 15 bar. If the pressure set at the pressure relief valve is exceeded, the valve opens hydraulically and a short-circuit occurs in the pump. This effectively prevents further increase of the inlet pressure.
The ABS motor relay is activated by the DSC3 control unit during certain phases of the ABS, ASC or DSC control procedure.
The ABS motor relay switches the return pump on and off.
The return pump returns the brake fluid which has collected in the accumulator back into the brake circuit between the closed inlet valve and master brake cylinder (pressure reduction phase during ABS controlled braking) or conveys the brake fluid under pressure through the opened inlet valve into the relevant wheel brake cylinder (pressure build-up phase during ASC and DSC controlled braking).
The ABS valve relay is activated by the DSC3 control unit as of terminal 15.
The ABS valve relay switches the 12 V supply voltage for all solenoid valves in the hydraulic unit on and off.