Control Strategies
Operating principle of the transmission.
In this transmission, the use of a dry double clutch in conjunction with an electromechanical control system means that two gears (transmission ratios) are engaged at the same time.
One of the clutches is engaged in driving mode, the other is already preselected when approaching the next gearshift with the clutch open.
Depending on the position of the accelerator pedal and the demand issued by the driver, the clutch of the previously activated gear is opened, while at the same time the clutch of the pre-selected gear is closed. As a result of this overlap in clutching, only minimal losses in tractive force are encountered during gearshifts.
Gearshift on a manual transmission
| Item | Description |
|---|---|
| 1 | Drive torque (Nm) |
| 2 | First gear |
| 3 | Second gear |
The illustration shows that in conventional manual transmissions, gearshifts result in a customary interruption of the propulsive force.
Gearshift process
| Item | Description |
|---|---|
| 1 | Drive torque (Nm) |
| 2 | First gear |
| 3 | Second gear |
With this gearshift, shown under load in the illustration, the power flow is only slightly restricted; there is constant propulsion perceptible.
Control
The TCM controls the clutch and gearshift system with the aid of the four brushless DC motors with integrated position sensors.
The information about the position of the DC motors is sent to the TCM. On the basis of this information, the TCM knows which gears are engaged and which clutch the power flow is routed through.
Gearshift Control
The gearshift control system is based on a software strategy for shift point determination which corresponds to the driving conditions and the driver input.
The TCM actuates the relevant DC motors in order to perform the automatic gearshift.
In order to be able to precisely determine the shift points on the basis of the selected driving program, the TCM receives the following information:
- Chosen transmission range
- Vehicle speed via the HS-CAN data bus
- Engine speed and torque as well as the throttle position via the HS-CAN data bus
- Engine temperature via the HS-CAN data bus
- Outside air temperature via the HS-CAN data bus for determining the viscosity of the transmission fluid when cold
- Steering angle from the steering wheel rotation sensor via the HS-CAN databus in order to avoid upshifts or downshifts during cornering.
- Information about braking interventions via the HS-CAN databus
- Input shaft speed for even and uneven gears from the respective speed sensor
Adaptive control
The TCM monitors each gearshift in order to enable smooth gearshifting under all driving conditions. To do this, the control unit controls the brushless DC motors for the clutch and gearshift system via an open-loop control system.
Adaptation of the following is performed:
- Clutch engagement points
- Clutch friction coefficient
- Individual positions of the synchronizer assembly
The adapted values are stored in the non-volatile RAM (read/write memory) of the control unit. This permits improved shifting smoothness and increases transmission service life.
Automatic mode, selector lever in the D position
The TCM adapts the shift points to match the driving conditions.
If special driving conditions are detected, the TCM switches to predefined characteristics.
Select-shift mode
The Select-Shift mode can only be activated if the selector lever is in the S position. Individual gears can be requested by operating the Select-Shift switch which is located on the side of the selector lever. Manual gearshifting will only be performed if the engine speeds do not exceed 6500 RPM or fall below predetermined values.
Moving the selector lever from N to R
The TCM only permits shifting to reverse gear if the vehicle speed is less than 14.5 km/h (10 mph).
At a vehicle speed above 14.5 km/h (10 mph), reverse gear is not engaged and the gearshift process is prevented.
Selector lever position N
In accordance with the calibration of the TCM, the following gears are engaged in the selector lever position N:
- During a shift from selector lever position R to N, 1st gear and reverse gear are engaged.
- During a shift from selector lever position D to N, 1st gear and N are engaged.
Selector lever position P
In accordance with the calibration of the TCM, 1st gear and reverse gear are engaged in the selector lever position P.
Altitude correction
Engine performance is reduced as air pressure decreases at higher altitudes. This situation is detected by the PCM.
In order to compensate for this operating situation, the TCM changes the shift points.
Speed control system
When the vehicle speed control system is switched on, a gear change can be executed by the TCM. This is done on the basis of the demand from the throttle plate position, which is controlled by the PCM.
Hill Start Assist
Hill start assist helps the driver when accelerating from a stop on an incline or side of a hill. Hill start assist supports the driver during drive-off situations on upward gradients. When the driver releases the vehicle brake, hill start assist prevents the vehicle from rolling back in the opposite direction.
Hot mode
The temperature of the clutch is calculated via a model in the TCM.
The following variables are incorporated in the calculation:
- Engine torque
- Rotational speed signals from the ISS A, ISS B, OSS and the engine speed
- Calculated torque transfer of the clutch
The Hot Mode function is used to prevent the clutch from being damaged as a result of excessively high temperatures. When hot mode is active, the clutch is engaged more quickly and the engine torque is reduced.
At a calculated clutch temperature at which the clutch lining would suffer heat damage, the following message is displayed on the message center:
- Transmission hot - stop the vehicle or accelerate
- Transmission hot - wait
- Transmission hot - wait 10 minutes.
Once the clutch has cooled down, the message "Transmission ready for operation" is displayed on the multi-function display of the instrument cluster.
At a calculated clutch temperature of more than 300 °C (572 °F) the clutches are disengaged.
Limp home mode
The TCM software contains functions which take control of the transmission if serious faults occur.
The fault characteristic decides which strategies are to be used.
The vehicle remains capable of restricted operation, unless there is a fault in the TCM itself or at the TR sensor.
Different measures are implemented depending on the current gear position and driving situation when the fault occurs:
- In the event of failure of an electric motor which actuates the lever actuator of the clutch, the TCM will only actuate the electric motor which is intact. If, for example, electric motor A has failed, then this path through the transmission is blocked (1st, 3rd and 5th gear). The TCM then only actuates electric motor B. This actuates the clutch of reverse, 2nd, 4th and 6th gear via the lever actuator.
- In the event of failure of the gearshift system or the rotational speed sensors, the fault reactions of the system range from the blocking of individual gears or the blocking of an entire path through the transmission (even/odd gears) to only permitting the vehicle to be driven in the currently engaged gear.
- In limp home mode, a text message is displayed in the instrument cluster and/or the MIL and/or the transmission warning light comes on (depending on the fault type).
When the engine is started again (ignition switched off for approximately 15 seconds), a self-test is performed in order to check whether there are any faults in the system. If the fault is still present, limp home mode is reactivated. If the fault is no longer present then it is no longer displayed on the instrument cluster and the MIL and/or the transmission control light will be off. However, the fault remains stored in the TCM.
In the event of a fault, it is recommended to continue driving so long as it is necessary and to look for the nearest workshop or to park the vehicle in a safe location.
Component Description
Double clutch with travel-controlled wear adjustment
As a result of wear to the clutch discs, the positions of the lever springs change, and this in turn alters the characteristic curves of the contact pressure and disengaging force. The consequence of this would be increased loads on the brushless DC motors of the electromechanical lever actuators.
In order to keep the position of the lever springs and therefore the contact pressure and disengaging force more or less constant, the double clutch has a travel-controlled wear adjustment mechanism.
It comprises the following main components:
- Clutch cover with adjusting and sensing ramps
- Adjustment ramp ring for each partial clutch
- Ramp ring for each partial clutch
- Clamping springs, adjustment tension springs and adjustment roller springs for each partial clutch.
Sectional view of clutch B of the double clutch
| Item | Description |
|---|---|
| 1 | Driving disc |
| 2 | Clutch disc B |
| 3 | Pressure plate B |
| 4 | Adjustment roller spring |
| 5 | Adjustment tension spring |
| 6 | Adjustment ramp ring |
| 7 | Clamping spring |
| 8 | Ramp ring |
| 9 | Clutch cover |
The adjustment of the clutch is triggered if - as a result of wear to the clutch lining - the lever spring for generating a specific contact pressure is pressed through further in the direction towards the engine. As a result of the additional travel, the clamping spring lifts off the ramp ring. As a result of the pre-loaded adjustment roller spring, the ramp ring is rotated on the ramp until the clearance between the clamping spring and the ramp ring has been compensated for. If the clutch is then fully opened (i.e. disengaged) again as a result of a gearshift process, the lever spring moves into a new position due to the rotation of the ramp ring, and this creates an air gap between the lever spring and the adjustment ramp ring. As a result of the adjustment ramp ring, which is also spring-loaded, it is then rotated until it abuts against the lever spring. The adjustment process is then complete.
Flow chart
| Item | Description |
|---|---|
| 1 | Instrument Cluster |
| 2 | BCM |
| 3 | Electric shift motors in the TCM Comments: actuate the shift shafts |
| 4 | DC clutch actuator motor A |
| 5 | DC clutch actuator motor B |
| 6 | TCM |
| 7 | ISS sensor A |
| 8 | ISS sensor B |
| 9 | OSS |
| 10 | The TR sensor |
| 11 | Hall sensors of the DC clutch actuator motor A and B |
| 12 | Hall sensors of the electric shift motors in the TCM |
| 13 | Select-shift switch |
| 14 | PCM |
| 15 | ABS |
| 16 | Steering wheel rotation sensor |
Function of the electronic components
ISS sensor A 1st gear, 3rd gear and 5th gear
| Item | Description |
|---|---|
| 1 | Spacer |
| 2 | ISS sensor A |
The ISS sensor A is attached to the transmission housing. The spacer washer provides a defined distance between the sensor and the transmitter wheel. The spacer washer has a thickness of 3.5 mm ± 0.05 mm for standard and single span transmissions and 2.0 mm ± 0.05 mm for super span transmissions.
The sensor detects the rotational speed of 3rd gear on output shaft B. It is a magneto-resistive sensor which measures the rotational speed and the direction of rotation.
ISS sensor B 2nd gear, 4th gear, 6th gear and reverse gear
| Item | Description |
|---|---|
| 1 | Spacer |
| 2 | ISS sensor B |
The ISS sensor B is attached to the transmission housing. The spacer washer provides a defined distance between the sensor and the transmitter wheel. The spacer washer has a thickness of 3.2 mm ± 0.05 mm for standard and single span transmission and 2.0 mm ± 0.05 mm for super span transmissions.
The sensor detects the rotational speed of 4th gear on output shaft B (hollow shaft). It is a magneto-resistive sensor which measures the rotational speed.
The OSS sensor
The OSS sensor is attached to the transmission housing.
The sensor detects the rotational speed via a transmitter wheel which is attached to the differential. It is a magneto-resistive sensor which measures the rotational speed.
The TR sensor
| Item | Description |
|---|---|
| 1 | The TR sensor |
| 2 | Cover plate |
| 3 | Actuating rod |
The TR sensor detects the position of the manual control lever shaft. The TR sensor is mounted on the transmission. The TR sensor allows the vehicle to start in PARK and NEUTRAL positions. The TR sensor output signal is linear over the measurement range and the TCM receives the selector lever position.
The signals of the TR sensor are used for the following functions:
- to recognize the selector lever position.
- to actuate the starter inhibitor relay.
- to actuate the reversing lamps.
No substitute signal is available for the TR sensor.
If the connection is cut, the vehicle cannot be started.
The TR sensor requires to be calibrated through the adaptive learning procedures.
Electric motors
| Item | Description |
|---|---|
| 1 | DC clutch actuator motor A Comments: Actuates the clutch for 1st, 3rd and 5th gear via the electromechanical lever actuator |
| 2 | Electric shift motor A in the TCM Comments: Controls the selector forks for 1st/5th gear and 3rd gear via the gear selector drum 1 |
| 3 | Electric shift motor B in the TCM Comments: Controls the selector forks for 2nd/6th gear and 4th/reverse gear via the gear selector drum 2 |
| 4 | DC clutch actuator motor B Comments: Actuates the clutch for 2nd, 4th, 6th and reverse gear via the electromechanical lever actuator |
All electric motors are designed as brushless DC motors.
| Item | Description |
|---|---|
| 1 | Motor control in the TCM |
| 2 | Stator coils |
| 3 | Rotor |
| 4 | Hall sensor |
The stator coils are actuated by the electronics in the TCM in such a way that a circumferential magnetic field is generated. The rotor runs behind the magnetic field. Via the Hall sensors, the TCM receives information on the position of the rotor and calculates by how many rotations the motor has been turned. This information is required by the TCM to actuate the shift forks in accordance with the specified torque angle and to actuate the clutches.