DIRAVI
In
the DIRAVI system invented by Citroën, the force steering the wheels comes from
the car's high pressure hydraulic system and is always the same no matter what
the road speed is. Turning the steering wheel moves the wheels simultaneously
to a corresponding angle via a hydraulic cylinder. In order to give some
artificial steering feel, there is a separate hydraulically operated system
that tries to turn the steering wheel back to centre position. The amount of
pressure applied is proportional to road speed, so that at low speeds the
steering is very light, and at high speeds it is very difficult to move more than
a small amount off centre. Citroën engineer Paul Magès invented the system as
part of its effort to engineer a practical high horsepower, front wheel drive
car.
As
long as there is pressure in the car's hydraulic system, there is no mechanical
connection between the steering wheel and the road wheels. This system was
first introduced in the Citroën SM in 1970, and was known as 'VariPower' in the
UK and 'SpeedFeel' in the U.S.
While
DIRAVI is not the mechanical template for all modern power steering arrangements,
it did innovate the now common benefit of speed sensitive steering. In the late
1960s, General Motors offered a variable-ratio power steering system as an
option on Pontiac and other vehicles.
Citroen DIRAVI type
closed-center arrangement.
The SM DIRAVI is based on the DS Dirass with a closed-center
arrangement, speed powered steering wheel and forced return. The CX DIRAVI
differ structurally with respect to the SM DIRAVI by another control command.
Herein, the hydraulic arrangements with a different kind of
differentiaalregelaar are grouped together next to the axis. XM DIRAVI equals
the CX DIRAVI with the difference that the eccentric disk has no recess in the
straight position. Furthermore, the straight-ahead position with a rotary knob to
adjust under para vane. This is done because it DIRAVI wheelhouse is accessible
from the ZPJ (4) V6 engine compartment.
Functioning
Neutral position
1. Principle of control when cornering
A very small revolution of the driving shaft makes piston T1 slides, chamber B is then related either to incoming pressure or to outgoing pressure: chamber B develops hydraulic volume variations.
The size of the hydraulic section to chamber B and the onset of the pressure depends on how fast the driver turns his steering wheel.
A very small revolution of the driving shaft makes piston T1 slides, chamber B is then related either to incoming pressure or to outgoing pressure: chamber B develops hydraulic volume variations.
The size of the hydraulic section to chamber B and the onset of the pressure depends on how fast the driver turns his steering wheel.
The
piston of the rack hydraulic command deals with two opposite forces of the same
values:
The
balance is triggered by valve T1.
fig.1
Left turn - Fig. 2
The volume variation in chamber B (increase) pushes on S of the piston a
dominating force pushing the rack to the left
This efficient control lets the driver a very small effort to give to the
steering wheel. This gives a vehicle that is very easy to drive in a city and
enhance very low speed manoeuvres
Fig.2
Because
of the cam design, the effort increases with the cornering angle ; this allows
the driver an even higher precision for path modifications.
2 - Principle of the self-centering steering gear on straight lines
The very little effort to apply to the steering wheel when cornering requires a perfect centralizing system to keep the car with an efficient stability.
To preserve this stability whatever the speed, the return effort to set back the steering wheel is applied on the cam by the roller increases with the speed through the centrifugal regulator.
This centralizing effort increase enhances the precision when rapid course changes.
The regulator speed connected to the front of the gearbox depends on the speed of the vehicle.
The very little effort to apply to the steering wheel when cornering requires a perfect centralizing system to keep the car with an efficient stability.
To preserve this stability whatever the speed, the return effort to set back the steering wheel is applied on the cam by the roller increases with the speed through the centrifugal regulator.
This centralizing effort increase enhances the precision when rapid course changes.
The regulator speed connected to the front of the gearbox depends on the speed of the vehicle.
Neutral position 50 km/h - Fig. 3
The
pressure given by the centrifugal regulator pushes the return piston and puts
the system at its center position.
Neutral position 200 km/h - Fig. 4
Because
of the centrifugal force, the leads move apart, piston T2 moves and increases
the pressure on the return piston increasing the effects of the centralizing
system.
Feature
- Fully hydraulic (no direct
mechanical connection between the steering wheel shaft and the steering
pinion during normal operation).
- Power operated unlike conventional
power assisted steering systems.
- Rapid self centering to straight
ahead position - whenever the engine is running the steering wheel will
return to center, even when parked
- Artificial feel inbuilt - centering force
varies in proportion to vehicle speed and/or steering wheel deflection.
- Power for the system from a
regulated high pressure hydraulic pump which also operates the brakes and
suspension system.
- The steering is operated by a rack
and pinion system which normally only works as a feedback loop.
- The rack takes the form of a double
acting hydraulic ram, but is capable of taking over full steering function
in the event of a hydraulic failure. During normal operation the rack and
pinion merely provides a position indication to the steering control valve
through the pinion shaft. The rack and pinion do the actual steering only
in the absence of system hydraulic pressure.
- A hydraulic power safety
prioritisation valve sets hydraulic fluid availability to each circuit
in the Citroën system.
- In a hydraulic system failure, order
of loss is first steering, then suspension, then brakes.
Advantages
- No steering kickback - blowouts, potholes,
ruts, etc. cannot affect the steering wheel or the direction of the
steered wheels which can only be moved by steering wheel input, since
apparent feedback is entirely artificial and bears no relation to the
actual forces on the front wheels from the vehicle's inertia and the
roadway
- Requires minimal physical exertion -
In the SM the steering wheel can be turned lock-to-lock with one finger
when the car is standing
- Very fast (minor steering inputs
equal large front wheel movements)
- Can be set up permitting vehicle to
travel in a straight line without driver input on a constant camber road
in still conditions
Disadvantaged
- No feedback to the driver - apparent
feedback is entirely artificial and bears no relation to the actual forces
on the front wheels from the vehicle's inertia and the roadway
- Driver must survey visual cues to
determine approach of road holding limit
- Requires familiarization - novices
find DIRAVI too fast and sensitive
- Cannot allow both hands to leave
steering wheel when navigating curves - because of rapid self centering
REFERENCE
