DIRAVI Power Steering
The DIRAVI system was developed by the French motor company Citroen in the early 1970s. The direct translation of the acronym means "steering with controlled return." Americans might better know it as the Speedfeel system. This is a fully hydraulic system that has no electrical components whatsoever. The basic design follows the unassisted rack and pinion design that's been used since motor cars were first invented. There is a hydraulic ram placed at the centre of the rack with a dividing plate between two fluid powered pistons. One of the pistons is half the mass of the other, though the two pistons remain balanced because the smaller of the two is under constant hydraulic pressure while the pressure of the other varies. At this point, the rack will remain neutral and keep the centring pinion to which the wheels are attached level, meaning the wheels will point straight ahead.
The steering wheel is connected to the hydraulic control unit, which contains a centring piston and slide valve. When the wheel is turned, the slide valve proportionally decreases or increases the hydraulic pressure behind the larger of the two pistons on the rack, causing the pinion to shift proportionally to one side or the other and turn the wheels. When the steering wheel is released, the weight of the centring piston will bring it back to the neutral position and straighten the car's tires.
Servotronic Power Steering
A servotronic system is part hydraulic and part electric. A hydraulic servo, or piston-like device is attached to the vehicle's pinion. If the servo's piston retracts, it turns the piston and the wheels in one direction; if it extends, it turns them in the other direction.
A direct electrical linkage runs from the steering wheel to the Power Steering Control System (PSCS). The PSCS judges how much force the servo will need to exert to turn the wheels as directed by the steering wheel based on the vehicle's speed. Less force is needed to turn a car's tires at low speeds than high speeds. Having determined this, the PSCS sends an electrical impulse to the servo, engaging the hydraulics and turning the wheels.
Electric Power Steering
The Electrical Power Steering, or EPS, uses only electrically powered machinery to steer the car's wheels in the same manner as the servotronic system. Instead of a hydraulic servo, an electrical motor is attached to the pinion, doing the exact same job. As before, the wheel is attached to a PSCS, which relays the amount of electrical energy required to turn the car's tires the specified angle and direction to the motor. The entire process runs off of the car battery, and as a result can cause the batteries to be replaced more often than vehicles with the other two systems.