Anticlockwise rotation of the steering wheel turning left low speed Figs 938b and 939b Rotating

Reservoir Pump

Valve sleeve Inner check valve Outer check valve Inner reaction chamber Reaction piston (RP)

Outer reaction chamber Torsion bar

Reservoir Pump

Right hand side

Outer orifice Inner orifice

Teflon ring seal

Left hand side

Electronic speedometer

Right hand side

Outer orifice Inner orifice

Teflon ring seal

Left hand side

Electronic speedometer

Electronic control unit (ECU)

(a) Neutral position

Fig. 9.38(a-d) Speed sensitive rack and pinion power assisted steering with rotary reaction control valve

Fig. 9.38 contd

the steering wheel in an anticlockwise direction twists the control valve rotor against the resistance of the torsion bar until the corresponding leading edges of the elongated groove in the valve rotor and sleeve align. At this point the return path to the exit port '4' is blocked by control edges '2' while fluid from the pump enters port '1'; it then passes in between the enlarged control-edge gaps to come out of port '3', and finally it flows into the right-hand power cylinder chamber.

yuiuc iiiicau . .

(c) Turning left grooves grooves p anticlockwise (high speed)

yuiuc iiiicau . .

(c) Turning left grooves grooves p anticlockwise (high speed)

Fig. 9.38 contd

Fig. 9.38 contd

Conversely fluid from the left hand side power cylinder chamber is pushed towards port '2' where it is expelled via the enlarged trailing control-edge gap to the exit port '4', then is returned to the reservoir. The greater the effort by the driver to turn the steering wheel, the larger will be the control-edge gap made between the valve sleeve and rotor and greater will be the pressure imposed on the right hand side of the power piston.

Return long slot Sleeve Rotor

Torsion bar

Supply short slot

Left hand

Power cylinder and piston

(a) Neutral position

Fig. 9.39(a-c) Rack and pinion power assisted steering sectional end views of rotary reaction control valve

Left hand

Power cylinder and piston

(a) Neutral position

Fig. 9.39(a-c) Rack and pinion power assisted steering sectional end views of rotary reaction control valve

When the vehicle is stationary or moving very slowly and the steering wheel is turned to manoeuvre it into a parking space or to pull out from a kerb, the electronic speedometer sends out its minimal frequency signal to the electronic control unit. This signal is processed and a corresponding control current is transmitted to the electro-hydraulic transducer. With very little vehicle movement, the control current will be at its maximum; this closes the transducer valve thus preventing fluid pressure from the pump reaching the reaction valve piston device and for fluid flowing to and through the cut-off valve. In effect, the speed sensitive rotary control valve under these conditions now acts similarly to the conventional power assisted steering; using only the basic rotary control valve, it therefore is able to exert relatively more servo assistance.

Anticlockwise rotation of the steering wheel (turning left — high speed) (Figs 9.38(c) and 9.39(b)) With increasing vehicle speed the frequency of the electronic speedometer signal is received by the electronic control unit; it is then processed and converted to a control current and relayed to the electro-hydraulic transducer. The magnitude of this control current decreases with rising vehicle speed,

Sleeve Rotor

Torsion bar

Supply short slot Return long slot

Left hand

(b) Turning left - anticlockwise rotation of the steering wheel

Sleeve Rotor

Torsion bar

Supply short slot Return long slot

Left hand

(b) Turning left - anticlockwise rotation of the steering wheel

Fig. 9.39 contd correspondingly the electro-hydraulic transducer valve progressively opens thus permitting fluid to reach the reaction piston at a pressure determined by the transducer-valve orifice opening. If the steering wheel is turned anticlockwise to the left (Fig 3.38(c)), the fluid from the pump enters radial groove '5', passes along the upper longitudinal groove to radial groove '7', where it circulates and comes out at port '3' to supply the right hand side of the power cylinder chamber with fluid.

Conversely, to allow the right hand side cylinder chamber to expand, fluid will be pushed out from the left hand side cylinder chamber; it then enters port '2' and radial groove '6', passing through the lower longitudinal groove and hollow core of the rotor valve, finally returning to the reservoir via port '4'. Fluid under pressure also flows from radial groove '7' to the outer chamber check valve to hold the ball valve firmly on its seat. With the electro-hydraulic transducer open fluid under pump pressure will now flow from radial grooves '5' to the inner and outer reaction-piston device orifices. Fluid passing though the inner orifice circulates around the reaction piston and then passes to the inner reaction chamber check valve where it pushes the ball off its seat. Fluid then escapes through this open check valve back to the reservoir by way of the radial groove '6' through the centre of the valve rotor and out via port '4'. At the same time fluid flows to the outer piston

(c) Turning right - clockwise rotation of the steering wheel

Fig. 9.39 contd reaction chamber and to the right hand side of the outer check valve via the outer orifice, but slightly higher fluid pressure from port '7' acting on the opposite side of the outer check valve prevents the valve opening. However, the fluid pressure build-up in the outer piston reaction chamber will tend to push the reaction piston to the left hand side, consequently due to the pitch of the ball-groove helix, there will be a clockwise opposing twist of the reaction piston which will be transmitted to the valve rotor shaft. Accordingly this reaction counter twist will tend to reduce the fluid gap made between the valve sleeve and rotor longitudinal control edges; it therefore brings about a corresponding reaction in terms of fluid pressure reaching the left hand side of the power piston and likewise the amount of servo assistance.

In the high speed driving range the electro-hydraulic transducer control current will be very small or even nil; it therefore causes the transducer valve to be fully open so that maximum fluid pressure will be applied to the outer reaction piston. The resulting axial movement of the reaction piston will cause fluid to be displaced from the inner reaction chamber through the open inner reaction chamber check valve, to the reservoir via the radial groove '6', lower longitudinal groove, hollow rotor and finally the exit port '4'.

As a precaution to overloading the power steering, when the reaction piston fluid pressure reaches its pre-determined upper limit, the cut-off valve opens to relieve the pressure and to return surplus fluid to the reservoir.

Clockwise rotation of the steering wheel (turning right — low speed) (Fig. 9.39(c)) Rotation of the steering wheel clockwise twists the control valve against the resistance of the torsion bar until the corresponding leading control edges of the elongated grooves in the valve rotor and sleeve are aligned. When the leading groove control edges align, the return path to the exit port '3' is blocked while fluid from the pump enters port '1'; it then passes inbetween the enlarged control-edge gap to come out of port '2' and finally flows into the left hand power cylinder chamber.

Conversely, fluid from the right hand side power cylinder chamber is displaced towards port '3'where it is expelled via the enlarged gap made between the trailing control edges to the exit port '4'; the fluid then returns to the reservoir. The greater the misalignment between the valve sleeve and rotor control edges the greater will be the power assistance.

Clockwise rotation of the steering wheel (turning right — high speed) (Figs 9.38(d) and 9.39(c)) With increased vehicle speed the electro-hydraulic transducer valve commences to open thereby exposing the reaction piston to fluid supply pressure.

If the steering wheel is turned clockwise to the right (Fig. 9.38 (d)), the fluid from the pump enters the radial groove '5', passes along the upper longitudinal grooves to radial groove '6' where it circulates and comes out at port '2' to supply the power cylinder's left hand side chamber with fluid.

Correspondingly fluid will be displaced from the power cylinder's right hand chamber back to the reservoir via port '3' and groove '7', passing through to the lower longitudinal groove and hollow core of the rotor valve to come out at port '4'; from here it is returned to the reservoir.

Fluid under pressure will also flow from radial groove '6' to the reaction piston's outer chamber check valve thereby keeping the ball valve in the closed position. Simultaneously, with the electro-hydraulic transducer open, fluid will flow from radial groove '5' to the inner and outer reaction-piston orifices. Fluid under pressure will also pass though the outer orifice, and circulates around the reaction piston before passing to the reaction piston's outer chamber check valve; since the fluid pressure on the spring side of the check valve ball is much lower, the ball valve is forced to open thus causing fluid to be returned to the reservoir via the radial groove '7', lower elongated rotor groove, hollow rotor core and out via port '4'. At the same time fluid flows to the inner chamber of the reaction piston via its entrance orifice. Therefore, the pressure on the spring side of its respective ball check valve remains higher thus preventing the ball valve opening. Subsequently pressure builds up in the inner chamber of the reaction piston, and therefore causes the reaction piston to shift to the right hand side; this results in an anticlockwise opposing twist to the reaction piston due to the ball-groove helices. Accordingly the reaction counter twist will reduce the flow gap between corresponding longitudinal grooves' control edges so that a reduced flow will be imposed on the left hand side of the power cylinder. Correspondingly an equal quantity of fluid will be displaced from the reaction piston outer chamber which is then returned to the reservoir via the now open outer check valve. Thus as the electro-hydraulic transducer valve progressively opens with respect to vehicle speed, greater will be the fluid pressure transmitted to the reaction piston inner chamber and greater will be the tendency to reduce the flow gap between the aligned sleeve and rotor valve control edges, hence the corresponding reduction in hydro-servo assistance to the steering.

9.6.4 Characteristics of a speed sensitive power steering system (Fig. 9.40)

Steering input effort characteristics relative to vehicle speed and servo pressure assistance are shown in Fig. 9.40. These characteristics are derived from the microprocessor electronic control unit which receives signals from the electronic speedometer and transmits a corresponding converted electric current to the electro-hydraulic transducer valve attached to the rotary control valve casing. Accordingly, the amount the electro-hydraulic transducer valve opens controls the degree of fluid pressure reaction on the modified rotary control valve (Fig. 9.38(c)). As a result the amount of power assistance given to the steering system at different vehicle speeds can be made to match more closely the driver's input to the vehicle's resistance to steer under varying driving conditions.

Referring to Fig. 9.40 at zero vehicle speed when turning the steering, for as little an input steering wheel torque of 2 Nm, the servo fluid pressure rises to 40 bar and for only a further 1 Nm input rise (3 Nm in total) the actuating pressure can reach 94 bar. For a vehicle speed of 20 km/h the rise in servo pressure is less steep, thus for an input effort torque of 2 Nm the actuating pressure has only risen to

Steering wheel torque (Nm)

Fig. 9.40 Speed sensitive power steering steering wheel torque to servo fluid pressure characteristics for various road speeds

Steering wheel torque (Nm)

Fig. 9.40 Speed sensitive power steering steering wheel torque to servo fluid pressure characteristics for various road speeds about 14 bar and for an input of 3 Nm the pressure just reaches 30 bar. With a higher vehicle speed of 80km/h the servo pressure assistance is even less, only reaching 10, 18 and 40 bar for an input torque of 2, 3 and 6 Nm respectively; however, beyond an input torque of 6 Nm the servo pressure rises very steeply. Similarly for a vehicle speed of 160km/h the rise in servo pressure assistance for an input torque rise ranging from 2 to 6 Nm only increases from 6 to 17 bar respectively, again beyond this input torque the servo pressure rises extremely rapidly. These characteristics demonstrate that there is considerable servo pressure assistance when manoeuvring the vehicle at a standstill or only moving slowly; conversely there is very little assistance in the medium to upper speed range of a vehicle, in fact the steering is almost operated without assistance unless a very high input torque is applied to the steering wheel in an emergency.

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