Suspension Control ( Automatic - Electronic ): Description and Operation
Porsche Active Suspension Management (PASM)
For the first time in the 911 series, actively adjustable dampers are available. The PASM is standard equipment on the 911 Carrera S (997). In comparison with the standard chassis, the vehicle with the PASM chassis is 10 mm lower.
The newly developed, variable damping system "Porsche Active Suspension Management" helps handle the demands of chassis. The aim was to maintain the high comfort level of the standard chassis while increasing performance at the same time.
The driver can choose between two programs: "Normal" and "Sport", both are selected via a button on the center console. The function light in the button comes on with the Sport setting. At the same time a damper icon appears in the instrument cluster display together with the text "PASM Sport". When the normal program is reactivated, the damper icon appears accompanied by the text "PASM Normal". The text and icon remain visible for 4 seconds and then disappear.
PASM combines two chassis rolled into one: one with equal measures of sportiness and comfort and one entirely dedicated to sportiness and the race circuit.
Normal mode (the basic setting when the vehicle starts) on the one hand provides the same excellent comfort as the standard chassis, on the other hand, the chassis approximates the sportiness of the sports chassis when controlled interventions are made with active PASM. In conjunction with specially developed software modules, the PASM ensures excellent performance and even greater driving safety in extreme situations.
Operating Principles of PASM
PASM selects the required damper firmness for each individual wheel from a precisely co-ordinated map in both the Normal and the Sport program. The possible damper settings range from comfortable to sporty. Both programs, which overlap slightly in some areas, are additionally superimposed with five special software modules to provide the optimum damper settings for every driving condition.
The system automatically selects the appropriate damper setting based on the PASM program selected and the driving condition identified. The Normal program offers comfortable settings with low damper forces.
Special control algorithms in the PASM software modules enable the chassis to offer greater active driving safety in extreme driving situations, even with the Normal program. To increase driving safety at higher speeds, the dampers are automatically switched to a firmer setting as speed increases.
The dampers switch to a firm characteristic when Sport mode is activated. This offers superior agility and excellent steering precision on uneven surfaces. If the system detects an uneven driving surface in Sport mode, it immediately switches to a softer characteristic to improve contact with the road surface. PASM selects the optimum damper setting for this softer characteristic from the Sport map.
Since extremely firm damping is not always the ideal solution in every driving situation (depending on the driving surface, the vehicle may start to bounce or shift), the intentional overlap between the Normal and Sport maps allows a noticeably soft setting to be selected if necessary. The driver gets an "active sports chassis" which automatically responds to the actual road surface and switches from a firm, sporty damping setting to a comfortable range as necessary. PASM switches back to the original characteristic as soon as the system detects a smooth road surface.
The following is a detailed description of the five software modules overlapping Normal and Sport mode.
Lane Change Module
The damper forces at both axles are immediately increased in response to rapid steering movements, for example sudden evasive manoeuvre. This reduces body tilt and instability, thereby significantly improving vehicle control even in extreme situations.
Vertical Control Module
In the Normal program, the damper force is increased as soon as the vertical movement of the body, for example when driving over uneven surfaces, rises over a specific threshold value. This prevents body instability.
In the Sport program, the damper setting is slightly reduced to improve contact between the road and the wheels as body movements increase. This results in a noticeable increase in riding comfort.
Lateral Acceleration Module
If specific, speed-dependent thresholds for lateral acceleration are exceeded when cornering in the Normal program, the damper force is increased by different, defined amounts for each side of the vehicle. This prevents vehicle instability and significantly increases driving precision.
In the event of large vertical movements and high lateral acceleration coinciding, the higher of the vertical-control and lateral-acceleration damping values is set. This happens if, for example, the damping in the Sport program was previously decreased by the vertical control module.
Brake Module
PASM switches to firm damping immediately when the system senses the brakes being applied in order to prevent vehicle nose-dive. This allows higher brake forces to be transmitted to the road faster. It switches back to a softer setting (this setting is different for the front and rear axle) after a specific amount of time. The result is improved road contact, and thus a shorter braking distance, particularly when braking on uneven surfaces.
Load Change Module
The damper characteristics for the front and rear axle are individually switched when accelerating heavily, releasing the throttle or changing lanes. In Normal mode, the dampers are briefly switched to a firmer damping setting. This avoids excessive lifting or diving at the front of the vehicle ("pitching"). In Sport mode, a softer damper characteristic is briefly selected (if necessary) to improve traction when accelerating, particularly on uneven surfaces.
The PASM system comprises the following components:
- Four dampers with continuously adjustable damping force (each with one bypass valve)
- PASM control module
- Two acceleration sensors for detecting vertical movement of the body (one at the damper dome at the front right and one at the rear left). Further signals such as lateral acceleration, steering angle, travel speed, brake pressure, engine torque, etc. are read in via the CAN bus.
- One button for selecting the program (Normal or Sport)
The system measures body movements via one acceleration sensor on each axle (front and rear). Values such as lateral acceleration, steering wheel angle, vehicle speed and information on possible braking operations are provided by the PSM via the CAN bus. Engine rpm and torque values are provided by the Motronic via the CAN bus.
Acceleration Sensor
The acceleration sensors are attached to the front right and rear left spring strut domes, respectively. The arrangement allows lifting, pitching and rolling movements of the body to be measured.
