Thursday, 18 August 2016

Types of active suspension (l). Hydroactive 3 for Citroën C5

In an ideal suspension system, the position of the wheels would not change with respect to the body. The purpose of the various active suspension systems is to control the stiffness of the damping.

In hydraulic and air systems, the height of the vehicle is also controlled depending on variations in weight and road conditions. Electronic and electro-mechanical systems are required for this. In this and subsequent posts, three examples of this type of suspension are going to be described: the Citroen C5 Hydroactive 3, the Audi A8 Air suspension and the VW Golf DCC Adaptive Chassis Control.

In this first post, we are going to look at the Hydroactive 3 suspension.

Hydroactive 3 from Citroën C5

This type of suspension allows the distance to the ground to be varied automatically depending on speed. There are two positions, sport and comfort, which automatically change the stiffness of the damping. These changes give greater stability due to the lowering of the centre of gravity by 15 mm at the front and 11 mm at the back, which reduces fuel consumption. On poor roads, the system can raise the height of the vehicle by up to 13 mm.

Hydroactive 3 facilitates choosing between the two suspension options, it allows switching, alternately and in real time, from a soft setting, which gives priority to comfort, to a hard one to improve stability, while taking into consideration the driving style and the road profile at all times.

The main elements involved in the system are:

1. Integrated hydroelectronic block.
2. Front bearing elements.
3. Front stiffness regulator with its sphere.
4. Front height sensor.
5. Rear hydropneumatic cylinders
6. Rear stiffness regulator with its sphere.
7. Rear height sensor.
8. Control unit.
9. Steering angle sensor.
10. Hydraulic fluid tank.
11. Accelerator pedal position sensor.
12. Braking pressure sensor.
-A simplified hydraulic system.

The operating principle of these systems is based on a variable height correction that depends on the amount of oil that enters the cylinders and on the absorption of the suspension oscillations through the compression and expansion of the gas inside the sphere.

These spheres are essentially hydraulic accumulators that have two chambers separated by a membrane, one of these is charged with nitrogen gas, while the other is linked to the hydraulic circuit. The nitrogen pressure is kept constant over time, which means its properties are fully maintained.

Each axle is equipped with a third sphere for flexibility, and with a stiffness regulator for the damping laws and the switching of the additional sphere. The principle consists of isolating these elements for sport mode and in activating them for comfort mode, this is based on information received from the height sensors, the steering wheel, the brake pressure and engine speed through the multiplexing network.

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