Suspension Control ( Automatic - Electronic ): Description and Operation
Porsche Active Suspension Management (PASM)
The new Boxster (987) and Boxster S (987) are the first Boxster series to offer a chassis with actively adjustable dampers as optional equipment.
In comparison with the basic chassis, the vehicle with the PASM chassis is 10 mm lower. The aim has been to maintain the high comfort level of the standard chassis while at the same time increasing performance.
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 activated, 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 in one: One with equal measures of sportiness and comfort (N) and one entirely dedicated to sportiness (S) for 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 regulative 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.
Mode of Functioning of PASM
PASM selects the required damper hardness for each individual wheel from a precisely coordinated 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 routines to provide the optimum damper settings for every driving condition.
The system automatically selects the appropriate damper hardness 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 routines 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 harder damper setting as speed increases.
The dampers switch to a hard 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 hard 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 hard, sporty damping setting to a comfortable range as necessary. PASM switches back to the original characteristic as soon as the road surface is smooth enough.
The following is a detailed description of the five software routines overlapping Normal and Sport mode.
Lane Change Routine
The damper forces at both axles are immediately increased in response to rapid steering movements, for example sudden evasive maneuver. This reduces body lean and instability, thereby significantly improving vehicle control even in extreme situations.
Vertical Control Routine
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 damping is slightly reduced automatically to improve contact between the road and the wheels as body movements increase. This also results in a noticeable increase in comfort.
Lateral Acceleration Routine
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 routine.
Brake Routine
PASM switches to harder damping at the start of a braking operation to reduce vehicle nose dive when braking. This way, higher brake forces can 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 therefore a shorter braking distance, particularly when braking on uneven surfaces.
Load Change Routine
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 harder damping setting in these driving conditions. 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.
Components
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 in the area of the front right and rear left spring struts respectively. The arrangement allows lifting, pitching and rolling movements of the body to be measured.
PASM Control Module
The PASM control module is installed on the front wall (footwell passenger's side) at the right.
