Advanced Vehicle Technology Episode 2 Part 9 pdf

9.6.3 Operation of the rotary control valve and power cylinder Neutral position Figs 9.38a and 9.39a With the steering wheel in its central free position, pres-surized fluid from the pum

the epicyclic gear set does not operate in the fourth

quadrant even under full steering lock conditions

9.5 Variable-ratio rack and pinion

(Fig 9.37(a±d))

Variable-ratio rack and pinion can be made to

improve both manual and power assisted steering

operating characteristics For a manual rack and

pinion steering system it is desirable to have a

moderately high steering ratio to provide an almost

direct steering response while the steering wheel is

in the normally `central position' for straight ahead

driving and for very small steering wheel angular

correction movement Conversely for parking

manoeuvres requiring a greater force to turn the

steering wheel on either lock, a more indirect lower

steering ratio is called for to reduce the steering

wheel turning effort However, with power assisted

steering the situation is different; the steering wheel

response in the straight ahead driving position still

needs to be very slightly indirect with a relatively

high steering ratio, but with the power assistance

provided the off-centre steering response for

manoeuvring the vehicle can be made more direct compared with a manual steering with a slightly higher steering ratio The use of a more direct low steering ratio when the road wheels are being turned on either lock is made possible by the servo action of the hydraulic operated power cylinder and piston which can easily overcome the extra tyre scrub and swivel-pin inclination resisting force The variable-ratio rack is achieved by having tooth profiles of different inclination along the length of the rack, accordingly the pitch of the teeth will also vary over the tooth span

With racks designed for manual steering the centre region of the rack has wide pitched teeth with a 40 flank inclination, whereas the teeth on either side of the centre region of the rack have

a closer pitch with a 20 flank inclination Con-versely, power assisted steering with variable-ratio rack and pinion (see Fig 9.37(c)) has narrow pitch teeth with 20 flank inclination in the cen-tral region; the tooth profile then changes to a wider pitch with 40 flank inclination away from the central region of the rack for both steering locks

Fig 9.36 (a±d) Principle of rear steering box mechanism

Pressure angle

20°

Pressure angle

40°

(a) Central rack teeth (b) Off-centre rack teeth

Wide pitch (P)

Narrow pitch (p) Wide pitch (P)

(c) Variable-ratio tooth rack

Large p.c.d.

more direct

Transition

Small p.c.d Transition Large p.c.d 30

25

20

15

5

0

Turning steering wheel to left Turning steering wheel to right

Steering wheel and pinion rotation (deg) (d) Rack and pinion movement ratio from lock to lock of the steering wheel

Fig 9.37 (a±d) Variable ratio rack and pinion steering suitable for power assisted steering

With variable-ratio rack and pinion involute

teeth the rack has straight sided teeth The sides

of the teeth are normal to the line of action,

therefore, they are inclined to the vertical at the

pressure angle If the rack has narrow pitch `p'

20 pressure-angle teeth, the pitch circle diameter

(2R) of the pinion will be small, that is, the point

of contact of the meshing teeth will be close to the

tip of the rack teeth (Fig 9.37(a)), whereas with

wide pitched `P' 40 pressure-angle tooth contact

between teeth will be near the root of the rack

teeth (Fig 9.37(b)) so its pitch circle diameter (2R)

will be larger

The ratio of steering wheel radius to pinion pitch

circle radius (tooth contact radius) determines the

movement ratio Thus the smaller the pitch circle

radius of the pinion for a given steering wheel size,

the greater will be the movement ratio (see Fig

9.37(d)), that is, a smaller input effort will be

needed to steer the vehicle, but inversely, greater

will be the steering wheel movement relative to the

vehicle road wheel steer angle

This design of rack and pinion tooth profile can

provide a movement-ratio variation of up to 35%

with the number of steering wheel turns limited to

2.8 from lock to lock

9.6 Speed sensitive rack and pinion power

assisted steering

9.6.1 Steering desirability

To meet all the steering requirements the rack and

pinion steering must be precise and direct under

normal driving conditions, to provide a sense of

feel at the steering wheel and for the steering wheel

to freely return to the straight ahead position after

the steering has been turned to one lock or the

other The conventional power assisted steering

does not take into account the effort needed to

perform a steering function relative to the vehicle

speed, particularly it does not allow for the extra

effort needed to turn the road wheels when

man-oeuvring the vehicle for parking

The `ZF Servotronic' power assisted steering is

designed to respond to vehicle speed requirements,

`not engine speed', thus it provides more steering

assistance when the vehicle is at a standstill or

moving very slowly than when travelling at speed;

at high speed the amount of steering assistance may

be tuned to be minimal, so that the steering

becomes almost direct as with a conventional

man-ual steering system

9.6.2 Design and construction (Fig 9.38(a±d)) The `ZF Servotronic' speed-sensitive power assisted steering uses a conventional rotary control valve, with the addition of a reaction-piston device which modi-fies the servo assistance to match the driving mode The piston and rotary control valve assembly comprises a pinion shaft, valve rotor shaft with six external longitudinal groove slots, valve sleeve with six matching internal longitudinal groove slots, torsion bar, reaction-piston device and an electro-hydraulic transducer The reaction-piston device is supported between the rotary valve rotor and valve sleeve, and guided internally by the valve rotor via three axially arranged ball grooves and externally guided by the valve sleeve through a multi-ball helix thread

The function of the reaction-piston device is to modify the fluid flow gap formed between the valve rotor and sleeve longitudinal groove control edges for different vehicle driving conditions

An electronic control unit microprocessor takes

in speed frequency signals from the electronic speedometer, this information is then continuously evaluated, computed and converted to an output signal which is then transmitted to the hydraulic transducer mounted on the rotary control valve casing The purpose of this transducer is to control the amount of hydraulic pressure reaching the reaction-piston device based on the information supplied to the electronic control unit

9.6.3 Operation of the rotary control valve and power cylinder

Neutral position (Figs 9.38(a) and 9.39(a)) With the steering wheel in its central free position, pres-surized fluid from the pump enters the valve sleeve, passes though the gaps formed between the long-itudinal groove control edges of both sleeve and rotor, then passes to both sides of the power cylin-der At the same time fluid will be expelled via corresponding exit `sleeve/rotor groove' control-edge gaps to return to the reservoir The circulation

of the majority of fluid from the pump to the reservoir via the control valve prevents any

build-up of fluid pressure in the divided power cylinder, and the equalization of the existing pressure on both sides of the power piston neutralizes any

`servo' action

Anticlockwise rotation of the steering wheel (turning left

Ð low sp eed) (Figs 9.38(b) and 9.39(b)) Rotating

Pinion

shaft

Reservoir

Pump

Valve sleeve Inner check valve Outer check valve Inner reaction chamber

Outer reaction chamber Torsion bar

Reaction piston (RP)

Valve rotor shaft Outer orifice Inner orifice

Teflon ring seal

Electronic speedometer

Electronic control unit (ECU) Power piston

Power cylinder

Electro-hydraulic transducer (EHT)

Left

hand

side

Right

hand

side

Cut-off valve (CO-V)

(a) Neutral position

6

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

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

Left hand side

R

P

Inner check valve Outer check valve

RP

Speedo ECU

(3) (2)

EHT CO-V

Right hand side

(b) Turning left anticlockwise (low speed) 6

Fig 9.38 contd

Left hand side

R

P

Inner check valve Outer check valve

RP

7

6

Speedo

Right hand side

Ball guide grooves

Ball thread grooves

Reaction piston (c) Turning left

anticlockwise

(high speed)

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

con-trol-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

Left hand

P R

Right hand side

(3)

EHT

(2) co-v

6 5

Inner check valve Outer check valve

RP

(d) Turning right clockwise (high speed)

7

6

Fig 9.38 contd

When the vehicle is stationary or moving very

slowly and the steering wheel is turned to

man-oeuvre 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

move-ment, 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

sen-sitive rotary control valve under these conditions

now acts similarly to the conventional power assisted steering; using only the basic rotary con-trol valve, it therefore is able to exert relatively more servo assistance

Anticlockwise rotation of the steering wheel (turning left Ð high speed) (Figs 9.38(c) and 9.39(b)) With increasing vehicle speed the frequency of the elec-tronic speedometer signal is received by the electro-nic control unit; it is then processed and converted

to a control current and relayed to the electro-hydraulic transducer The magnitude of this con-trol current decreases with rising vehicle speed,

Return long slot

Sleeve

Rotor

Torsion bar

Supply short

slot

Reservoir

Pump

Right hand

Left hand

Power cylinder and piston

(a) Neutral position

(4)

(2) (1)

(3)

(4)

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

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

steer-ing 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 cir-culates 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

Left hand

(b) Turning left – anticlockwise

rotation of the steering

wheel

(4)

(2)

(4)

R

P

Sleeve Rotor

Torsion bar

Supply short slot

Return long slot

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

pre-vents the valve opening However, the fluid

pres-sure 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

trans-mitted to the valve rotor shaft Accordingly this

reaction counter twist will tend to reduce the fluid

gap made between the valve sleeve and rotor

long-itudinal 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 pres-sure will be applied to the outer reaction piston The resulting axial movement of the reaction pis-ton 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 steer-ing, when the reaction piston fluid pressure reaches

(c) Turning right – clockwise

rotation of the steering

wheel

Right hand

(4)

(2)

(4)

R

P

Fig 9.39 contd

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 Ð 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

elon-gated 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

mis-alignment between the valve sleeve and rotor control

edges the greater will be the power assistance

Clockwise rotation of the steering wheel (turning

right Ð high speed) (Figs 9.38(d) and 9.39(c))

With increased vehicle speed the electro-hydraulic

transducervalve commences to open therebyexposing

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

longi-tudinal grooves to radial groove `6' where it

circu-lates 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

pis-ton'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, hol-low 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 pres-sure 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 cylin-der 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-hydrau-lic 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 con-trol 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 vehi-cle 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 con-trol 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 resist-ance 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