Advanced Vehicle Technology Episode 2 Part 9 pdf

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 352 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 0 480 180 120 60 30 0 30 60 120 180 480 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 PP R r Movement ratio Fig. 9.37 (a±d) Variable ratio rack and pinion steering suitable for power assisted steering 353 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 Ðlowspeed)(Figs 9.38(b) and 9.39(b)) Rotating 354 Rack 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) (2) (3) (a) Neutral position (4) (1) 6 5 7 6 Fig. 9.38 (a±d) Speed sensitive rack and pinion power assisted steering with rotary reaction control valve 355 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 (4) (1) Inner check valve Outer check valve RP 6 5 7 Speedo ECU (3)(2) EHT CO-V Right hand side (b) Turning left anticlockwise (low speed) 6 Fig. 9.38 contd 356 Left hand side R P (4) (1) Inner check valve Outer check valve RP 6 5 7 6 CO-V (2) (3) EHT ECU Speedo Right hand side Ball guide grooves Ball thread grooves Reaction piston (c) Turning left anticlockwise (high speed) Fig. 9.38 contd 357 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 side (4) (1) P R Right hand side ECU Speedo (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 358 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 359 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) (1) (3) (4) R P Sleeve Rotor Torsion bar Supply short slot Return long slot Fig. 9.39 contd 360 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) (1) (3) (4) R P Fig. 9.39 contd 361 [...]... 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 3 62 100 80 Km/h 160 Km/h 94 /3 /h Km 60 20 0 Km/h Fluid pressure (bar) 80 40 /2 40/6 40 30/3 14 /2 20 18/3 17/6 10 /2 0 8 6 /2 6 4 2 0 2 8.7/3 4 6 8 Steering wheel torque (Nm) Fig 9. 40 speeds Speed sensitive power... more assistance at low vehicle speed when manoeuvring in a restricted space and to reduce this assistance progressively with rising speed so that the driver experiences a positive feel to the steering wheel Note the engine and vehicle speeds are monitored by the tachometer and antilock brake sensors respectively 9. 7 .2 Operating principle (Figs 9. 42( a±c)) Neutral position (Fig 9. 42( b)) When the input... 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... 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 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... reaches 30 bar With a higher vehicle speed of 80 km/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 160 km/h the rise in servo pressure assistance for an input torque rise ranging from 2 to 6 Nm only increases from... fluid pressure on the spring side of the check valve ball is much lower, the ball valve is forced to open thus 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... 10.8(a)) Under these conditions the steered wheels become unstable as they tend to twitch from side to side when the vehicle travels along a straight path A rear wheel drive vehicle has the front wheel steer pivot axis inclined backward to produce positive castor (Fig 10 .9( a)) As the vehicle is propelled from the rear (the front wheels are pushed by the driving thrust transmitted by the rear drive wheels),... 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 Clockwise rotation of the steering wheel (turning right Ð low speed) (Fig 9. 39( c)) Rotation of... upper and lower wishbone arms or axle beam which supports the vehicle body are slightly raised This unstable state produces a downward vehicle weight component which tends to return both steered wheel assemblies to a more stable straight ahead position In other words, the pivot inclination produces a self-centring action which is independent of vehicle speed or traction but is dependent upon the weight... axis is pulled forwards The swivel balls or pin mounting swing to the rear of the contact patch centre, due to the vehicle rolling resistance acting through the rear wheels, opposing any forward motion The effects of castor angle can be seen in Fig 10 .9( a and b), when the steering is partially turned on one lock The trail or lead distance between the pivot centre and contact patch centre rotates as . shaft. 100 80 60 40 20 0 86 42 02 46 Steering wheel torque (Nm) Fluid pressure (bar) 94 /3 40 /2 0 Km/h 20 /hKm 80 Km/h 160 Km/h 30/3 18/3 14 /2 10 /2 6 /2 17/6 40/6 8 8.7/3 Fig. 9. 40 Speed sensitive. Note the engine and vehicle speeds are monitored by the tachometer and anti- lock brake sensors respectively. 9. 7 .2 Operating principle (Figs 9. 42( a±c)) Neutral position (Fig. 9. 42( b)) When the input and. 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 3 62 about 14 bar