DIRAVI
DIRAVI is the name given by Citroën to its proprietary power steering system, first seen in 1970. DIRAVI is an acronym for ""Direction à rappel asservi"" literally meaning "steering with controlled return" more accurately described in English as "power steering with power assisted return". In the UK, it was marketed as "VariPower" and in the U.S. as "SpeedFeel." This was the first commercially available variable assist power steering arrangement, allowing the motorist power assist when parking, but recognizing that less steering assistance was needed at high speed. This feature is now spreading to mainstream vehicles, using electrical motors rather  than hydraulic actuation. Citroën engineer Paul Magès invented the system as part of its effort to engineer a practical high horsepower, front wheel drive car - a new type of vehicle at the time. This DIRAVI system is an addition to the integrated Citroën hydropneumatic suspension and braking system. These all drew power from an engine driven pump and hydraulic accumulator. This unique Citroën power operated self centring steering system is fitted to Citroën SM, Citroën CX (most), Citroën XM (early Left Hand drive V6), Maserati Quattroporte II and Maserati Khamsin. How It Works. The steering wheel is connected to the  hydraulic control unit, which contains a slide valve, the control gears and linkage, and the centring piston and cam. The steering rack pinion is connected to the hydraulic control unit through an adjuster, to allow setting of the centre point of the steering. This then drives one of the control gears, the other being driven by the steering wheel. The control linkage consists of two gears coupled with rods through ball-joints. When the gears are turned relative to each other, the rods move the slide valve, allowing fluid in or out of the rack. This moves the rack, which  in turn moves its pinion, turning the control gears back to their centre position. Compare this with a conventional power steering system, which relies on the flexing of a strong spring to control the valve. The steering rack is almost conventional. As with a normal power-assisted steering setup, there is a rack-and-pinion, and a stepped hydraulic ram with a dividing plate (the piston) in the middle. On one side of the plate, the piston area is half the area of the other, giving twice the area for the fluid to work on. Thus, with full system pressure  on the small area side of the ram, and half system pressure on the large area side, the ram remains perfectly balanced and centered. This is because the lower pressure is working on a larger surface area. The smaller side of the piston is at constant hydraulic pressure, the other side only varies in pressure. NOTE: The author seems to be describing the DS power steering piston and rod, on which the piston is in the middle of the rod. The SM's steering cylinder has the piston on the end of the rod, which rod's cross-sectional area  is half the area of the other side of the piston. Full hydraulic pressure is admitted to the rod side at all times while the pressure on the other side of the piston (its "head") is regulated. The steering actually had the same "assist" at all speeds — the steering was hydraulically locked against steering movement of the wheels from the road ("feedback") up to the capacity of the unit. The reduction in 'assist' was achieved by a piston/roller pushing on a heart shaped cam geared to the steering shaft (hence the one turn to full lock), which was fed  with system pressure so that as its pressure rose with increasing road speed, the steering assistance seemingly reduced and the steering centering effort rose. However, full steering wheel turning was available at all speeds, though considerable force was necessary to turn the steering wheel at high road speed. Enough pressure was admitted to the centering unit to return the wheels to the straight ahead position when the car was not moving. The centering pressure was regulated by a flyweight centrifugal governor driven by the pinion (secondary) shaft of the manual gearbox and by a proportioning valve connected to the fluid  pressure in the automatic gearbox, which pressure was proportional to the speed of the output shaft. The pressure increased all the way to , and a subsidiary function of this feed was to turn off the air conditioning fans above . When there is no pressure available to operate the ram, the steering wheel will mechanically move the rack directly but with significant play through a split shaft. One side from the steering wheel drives a pin which mates with a slot on the output shaft connected to the rack. The free play in this emergency mechanical system is necessary for the  normal play free pressure operated Diravi system to operate its feedback control loop. In practice this heavy & inaccurate manual steering character of Diravi is only required when the hydraulic system has failed, whence emergency system prioritisation firstly sacrifices the steering system. Added NOTE: The mechanical unpowered steering effort is very heavy. Driving the car without power to the steering should be used only to guide the car to a safe place to stop. Attempting to turn the steering wheel without power when the car is not moving may break the steering wheel. Because the DIRAVI system is much more sensitive  than conventional systems, something must be done to prevent the driver over-controlling at high speeds. This is the job of the heart shaped centring cam inside the unit. A pressure loaded piston with a roller on the end runs against the edge of this cam. This pressure comes from a centrifugal governor proportioning valve driven from the gearbox (on manual gearboxes - on automatic gearboxes the gearbox's internal governor pressure controls the centring pressure). At low speeds, the centring piston pressure is 290 psig, to provide a light degree of self centring when parking etc. Proportionally the self centring  pressure rises to a maximum of about 800 psig. at 80 km/h, at which self centring forces become a maximum, stiffening the steering but not excessively so. However, the zero feedback/kickback is because of the perfectly neutral geometry of the steering (just as in DS, ID, or GS), Disadvantages. and destabilising forces (potholes, blowouts, ruts, etc) simply do not affect a steering of neutral geometry, and the driver feels nothing at the wheel.