Antilock Brakes / Traction Control Systems: Description and Operation
System descriptionPSM 0101
Information and warning lights
The driver is informed of the PSM control activities by means of the display lights in the instrument cluster. All display lights illuminate for a lamp check when the ignition Is switched on. The information and warning lights are triggered via the CAN drive in Cayenne.
1 - ABS warning light: illuminates if the ABS is faulty.
2 - Brake warning light: illuminates if brake system is faulty.
3 - ASR/PSM warning light: illuminates if PSM is faulty or if the 'PSM OFF switch' is off. The warning light flashes if ASR or PSM control is on.
PSM OFF switch
The ASR/PSM function can be switched on/off using the PSM OFF switch.
- Switch on ignition, internal PSM control module function test complete, control lamps are off, PSM is switched on.
- Actuate PSM OFF switch, PSM is switched off and the control lamp comes on.
- Actuate PSM OFF switch again, PSM is switched on and the control lamp is off. For safety reasons, the driving dynamics control is reactivated temporarily for the duration of one brake pedal actuation.
Steering-angle sensor
Steering-angle sensor 'A' integrated into steering column switch
The steering-angle sensor supplies information about the slip angle of the front wheels to the PSM control module (intention of driver in relation to direction of travel). Vehicle behavior with respect to its transverse dynamics is calculated in the PSM control module based on the information received from the steering-angle sensor and the vehicle speed. The steering-angle sensor is integrated into the steering column switch and sends messages via the CAN drive. Without the information from the steering-angle sensor, the PSM control module cannot produce a picture of the desired direction of travel.
INITIALISING
The steering-angle sensor in Cayenne must be initialised after the following work is carried out:
- Whenever the power supply to terminal 30 is interrupted (PSM indicator light on).
- The steering-angle sensor must be calibrated after all work on the running gear.
Combination sensor
1 - Plug connection
x - Vehicle transverse axle 'transverse acceleration'
y - Vehicle direction of travel 'axial acceleration'
z - Vehicle high axle 'rate of turn/yaw speed'
NOTE:
- After installing/replacing the combination sensor, the integrated sensors for axial and transverse acceleration must be calibrated using the menu with the Porsche System Tester II No. 9588.
- Calibration is required in order to enter the PSM system and must only be performed when 'vehicle is positioned horizontally and brake is not actuated'.
The combination sensor is fastened to the centre tunnel near the centre console. The functions of the sensors listed below are integrated into the combination sensor:
- Rate-of-turn sensor
- Axial acceleration sensor
- Transverse acceleration sensor
The combination sensor communicates (signal transfer) with the PSM control module via the separate CAN data bus 'internal CAN'.
The functions that are integrated into the combination sensor are described below:
- Rate-of-turn sensor:
The rate of turn is recorded using a micromechanic system (Coriolis force). Without measurement of the rate of turn, it is not possible for the PSM control module to detect whether the vehicle is developing a tendency for spinning. 1)
1) Coriolis force: named after the French mathematician Gustave Gaspard Coriolis (1792-1843). Coriolis force occurs in rotating systems.
- Axial acceleration sensor:
Axial acceleration is measured in order to support the theoretically determined vehicle speed. In unfavourable conditions, the actual vehicle speed of four-wheel drive vehicles could not be determined reliably without the axial acceleration measurement.
- Transverse acceleration sensor:
The transverse acceleration sensor supplies the PSM control module with information about the transverse accelerations occurring in the vehicle. Together with the information from the steering-angle sensor and the rate-of-turn sensor, the current vehicle handling behavior with respect to its transverse dynamics is calculated. The sensor element is a damped spring-and-mass system. Without measurement of the transverse acceleration, it is not possible for the PSM control module to detect which vehicle movements must be managed in a stable way in given road conditions.
Active speed sensor
CAUTION: Resistance measurement destroys the active speed sensor.
The wheel speed sensors supply the PSM control module with information about the current wheel speed.
The speed sensors installed in Cayenne are called 'active speed sensors'.
Active speed sensors require an external power supply. Two-wire cable is used to connect to the control module. The control module supplies the active speed sensor with power and ground. The function of the sensor element integrated into the active speed sensor is based on the effect of thin metal layers of 'permalloy', which change their electrical resistance as a function of the strength and direction of a magnetic field (anisotropic magnetoresistive effect).
The pulse sender for the active speed sensor is integrated into the wheel bearing 'wheel bearing with multiple-pole ring'. 48 magnetic plates (north/south poles), distributed at equal distances, are integrated into the sealing ring of the wheel bearing. Contact to the active speed sensor is established via an air gap. The rotary motion of the wheel bearing/sealing ring changes the resistance of the metal layers in the sensor element of the active speed sensor.
An electronic circuit in the sensor element converts the alternating multiple or reduced power consumption, which is determined by the changing resistance, into a digital speed signal and sends this to the PSM control module. The speed signal that is received is processed further in the PSM control module. The amplitudes of the signal remain constant, do not depend on the speed of the wheel, and are sent until the vehicle is stationary, 1)2)
1. Permalloy: soft magnet (Ni-Fe alloy), exceptionally suitable as base material for sensors/weak magnetic fields.
2. Anisotropic magnetoresistive effect: change in electrical resistance in accordance with the strength and direction of a magnetic field. Frequently used for recording weak magnetic fields.
Hydraulic unit
The hydraulic unit is screwed to the PSM control module and installed in the radiator tank. The hydraulic unit contains a dual-circuit pump, which is controlled via a plug connection located on the underside of the 'PSM control module'. The required pressure is built up here and distributed to the affected wheels.
NOTE: In the event of damage, the hydraulic unit can only be replaced together with the PSM control module.
Active brake booster
Brake booster with components (tandem brake booster)
1 - Brake booster (tandem brake booster)
2 - Brake pedal pulse sender actuated (with release switch)
3 - Pressure sensor
4 - Brake master cylinder
5 - Distance sensor
6 - Brake fluid expansion tank
Brake booster
The brake booster is separated by two membranes into two chambers - a vacuum chamber and a pressure chamber (atmospheric pressure). It also contains a magnetic coil, metal piston and ventilation valves for vacuum control. The magnetic coil is connected electrically to the PSM control module.
Brake pedal pulse sender actuated
The 'release switch' (designed as a changeover switch) is integrated into the 'brake pedal pulse sender actuated' and is used for brake detection. When the driver actuates the brake pedal, the magnetic coil in the 'active brake booster' moves towards the brake master cylinder and the release switch changes the contact position, thereby changing the signal output from 1 to 2. 'Signal 2' informs the PSM control module that the driver is braking. It the release switch is in position 1 (driver is not braking) and the PSM control module detects on the basis of the values of the incoming signal that driving dynamics control via the brakes is required, the PSM control module activates the magnetic coil on the voltage side. A magnetic field forms in the magnetic coil through which current flows and this magnetic field attracts a metal piston, which in turn releases ventilation valves. The vacuum that builds up in the brake booster presses through the membranes on to the brake master cylinder piston and the brake pressure is built up. The PSM control module controls the vacuum that builds up via the voltage supply to the magnetic coil. Magnetic coil activation is set when the driver actuates the brake pedal.
Active pressure sensor
The brake pressure sensor records the brake pressure (desired deceleration), which is used by the PSM control module to calculate the wheel brake forces (longitudinal forces). If driving dynamics control is necessary during the braking procedure, the existing wheel brake forces are included to calculate the lateral traction.
Distance sensor
The membranes in the brake booster are connected to a potentiometer. The potentiometer changes its resistance to reflect the movement of the membranes. The distance of the membranes in the brake booster is calculated using the measured resistance.