CITROEN DS series Technical Manual page 26

U
The Citroën Guide
strument panel informs about the setting selected (it does
not indicate the mode the suspension is currently in).
The second input comes from a vehicle speed sensor.
This inductive magnet tachogenerator generates 4 pulses
per rotation, that is approximately 5 pulses per meter trav-
eled (although this depends somewhat on tire size). It is lo-
cated on the gearbox where the speedometer cable atta-
ches, or in some versions, on the cable itself. The ECU deter-
mines the acceleration of the car by evaluating changes in
vehicle speed for the duration of one second.
Another input arrives from the steering wheel angle
and speed sensor, an optoelectronic device consisting of
two infrared light beams, interrupted by a rotating disc
with 28 holes. The ECU senses the quadrature signal
changes of both sensors to effectively increase the resolu-
tion of the sensor (28 pulses per steering wheel revolution)
by a factor of four. This produces one edge change every
3.214 degrees of steering wheel rotation. The direction of
turning can be determined by the sequence of the edge
changes.
To make decisions, the computer needs to know the
straight ahead position of the steering wheel. The sensor
does not have a built-in zero position (as it would not al-
ways work, due to misalignment and wear in the mechani-
cal components). The computer uses heuristics instead:
First, the straight line position is assumed if the vehicle
speed is above 30 km/h and the steering wheel position
was not changed (an error margin of up to 4 pulses is al-
lowed) for the last 90 seconds. Second, we know the maxi-
mum number of pulses in both directions from the center
(lock to lock angle divided by two). If the steering wheel is
found to turn more than this value (an error of up to 4
pulses is accepted here, too), this is a clear indication of an
incorrect center reference: in this case the center position
will be adjusted by the surplus.
The rotational speed of the steering wheel is determined
by measuring the time elapsed between the individual
pulse edges coming from the sensors.
A similar sensor informs the computer about the move-
ment of the car body. Two infrared beams, the disc hav-
ing 45 notches, similarly quadrupled by the ECU. Exces-
sively long intervals are considered coming from slow
height changes resulting from the driver selecting a differ-
ent height setting, and are consequently discarded.
The sensor is connected to the front anti-roll bar, to the
right of the height corrector linkage. Due to its location, it is
capable of detecting both squat and dive, and to some ex-
tent, body roll. But as the sensor is mounted off-center, its
sensitivity to roll is about three times less than the sensitivity
to squat and dive. In all directions, it can measure both
movement amplitude and speed of movement, using the
same process as the steering wheel sensor does.
The throttle pedal position sensor is located below
the dashboard, right next to the pedal mechanism, where
the pedal can operate its sprung lever as it moves. The sen-
sor is a potentiometer with an integrated serial resistor in
the wiper's circuit.
The entire travel of the potentiometer is quantized into
256 steps by the analog-digital converter inside the ECU.
The 5 V reference is supplied by the ECU itself. Due to the
gas pedal initial position and maximum travel, about 160
to 220 steps out of 256 are being actually used.
Suspension: Hydractive I
The brake pressure sensor is a simple pressure acti-
vated switch located on a hydraulic conduit connector
block, right next to the ABS block, at the bottom of the left
front wing, in front of the wheelarch, under the battery.
The switch makes contact at 35 bars of braking pressure.
The door/tailgate open switches are located on the
door frame and in the boot latch. The door switches are all
wired together in parallel and connected to one input line
(and routed to the interior light dimmer and timer as well).
The tailgate switch is connected to the other input line (and
routed to the boot light and the tailgate opened detection
input for the status display on the dashboard, too; the door
open and bonnet open signals for the status display are gen-
erated by a separate set of switches, independent of the
ones used for the suspension).
The usual ignition switch provides a power-on signal,
triggering and internal reset and self diagnostic run in the
ECU. Turning the ignition on and off also triggers internal
events that guarantee proper pressure equalization be-
tween the center and corner spheres.
The brain behind the suspension
The ECU is a small microcomputer sensing the input signals
coming from the various sensors. A very interesting and im-
portant aspect of the system is that it uses the driver of the
car as a major part of its intelligence, making the operation
very simple but effective. To achieve this, most of the sen-
sors read the controls the driver operates.
The software contains the description of various condi-
tions (status of the input lines and internal timers) govern-
ing when to activate-deactivate the electrovalve switching
the suspension to either hard or soft mode. These condi-
tions can be formulated as rules.
Every main input sensor has an associated rule: when the
value collected from the sensor exceeds a specific thresh-
old, the suspension is put into hard mode and the com-
puter starts a timeout counter. For the suspension to return
to soft mode at the end of the timeout period, the thresh-
old must not be exceeded again during this time. If it was ex-
ceeded, the suspension stays in hard mode and the timeout
starts all over again.
There are four additional rules overriding the normal op-
eration—even if the sensor inputs call for a generic rule to
be applied, these four conditions are checked first:
U the computer puts the suspension into soft mode when
the ignition is turned on or off. This setting prevails until
30 seconds elapse or the vehicle speed exceeds 30 km/h,
whichever comes first;
U
if the computer determines any problem with its own op-
eration or any of the input or output devices (including
inconsistent values like no body movement but a vehicle
speed above 30 km/h), the suspension will be switched
to hard mode and stay there until the ignition is turned
off or the doors are opened with the vehicle speed below
30 km/h. The ECU does run a self-diagnostic routine
when the ignition is turned on but some sensors cannot
be tested at this time, only during normal use;
U whenever the suspension stays in hard mode for more
than one minute, the computer switches to soft mode
momentarily to assure the equalization of pressures in
the corner and center spheres. If the circumstances still
26