Parameter (Speed Sensitive) Steering

HYDRAULIC CONTROL, LEFT LOCK
(Parking)
Hydraulic servo assistance is introduced by the displacement of the servo valve spool (4) via the control-bar engaging in the servo valve spool.

In this way the pressure oil delivered from the power steering pump reaches the bearing surface area of the power piston (1a) via the open leading edges. The oil to be forced onto the ocher side of the power piston (1b) is led back to the oil reservoir via the leading edge of the spool valve

At the same time the pressure oil reaches the end faces of the spool valve and the P valve (Y10) via the throttling ports in the spool valve. Since the P valve (Y10) keeps the control orifice (Y10a) closed, the same pressure occurs on both end faces of the spool valve (no differential pressure) and the servo assistance is introduced after overcoming the basic load spring (12) of the spool valve (at low torque, approx. 3 Nm).





HYDRAULIC CONTROL, LEFT LOCK
(Driving Mode)
If hydraulic assistance is introduced during driving, the pressure oil is led into the return via the open control orifice (Y10a) of the Parameter Steering (PML) P valve (Y10) depending on vehicle speed. As a result of this process, a differential pressure is produced at the end faces of the spool valve (4) via the fixed orifice (arrow) in the governor slide valve (4a). The higher pressure on the end face of the spool valve selectively counteracts the torque on the steering wheel on left lock.

The torque to be applied at the steering wheel is adjusted between approx. 2-5 Nm - depending on speed.





HYDRAULIC CONTROL, RIGHT LOCK
(Parking)
The bearing surface area of the power piston (lb) subjected to pressure on right lock can be seen in the illustration

This pressure also occurs at the closed control orifice (Y10a) of the PML P valve (Y10). The end faces of the spool valve (4) are surrounded with virtually Repressurized oil (circulation pressure) determined by the fixed orifice (arrow) in the governor slide valve (4a).

This results in pressure equilibrium (no differential pressure) and servo assistance is introduced after overcoming the basic load spring (12) of the spool valve, with low torque at the steering wheel (approx. 2 Nm).





HYDRAULIC CONTROL, RIGHT LOCK
(Driving Mode)
If hydraulic assistance is introduced in driving mode, the control orifice (Y10a) of the PML P valve (Y10) opens and the pressure is introduced into the chamber of the spool valve depending on vehicle speed. This pressure is effective on the end face of the spool valve (4) which has been introduced.

Determined by the fixed orifice (arrow) in the governor slide valve (4a), a differential pressure occurs at the end faces of the spool valve. The torque to be applied at the steering wheel is adjusted between approx. 2-5 Nm - depending on vehicle speed.

FAILURE OF PARAMETER FUNCTION
If there is a fault in the electrical system the parameter function is switched off. The parameter steering then functions as a conventional power steering system. The P valve (Y10) is fully opened - basic position = producing the maximum possible reaction on the spool valve. The steering is therefore somewhat heavier to operate than in normal driving mode (up to 80 km/h).





FUNCTION OF MANUAL EFFORT LIMITATION
This is achieved with the aid of the governor slide valve (4a).

If the pressure differential is greater than the force of the preloaded governor slide valve spring (4b), the governor slide valve closes the restrictor orifices (arrows) and thus keeps the differential pressure and the steering wheel moment (manual moment) constant.