97 Rack and pinion electric power assisted steering

The traditional hydraulic actuated power assisted steering requires weighty high pressure equipment, which incorporates an engine driven high pressure pump, fluid reservoir and filter, reaction valve, high pressure hoses, servo cylinder, piston, ram and a suitable fluid. There is a tendency for fluid to leak due to severe overloading of the steering linkage when driving against and over stone kerbs and when manoeuvring the car during parking in confined spaces. The electric power assisted steering unit is relatively light, compact, reliable and requires a maximum current supply of between 40 and 80 amperes when parking (depending on the weight imposed on the front road wheels) and does not consume engine power as is the case of a hydraulic power assisted steering system which does apply a relatively heavy load on the engine.

9.7.1 Description and construction (Fig. 9.41) The essentials of a rack and pinion electric power assisted steering comprises an input shaft attached to the steering wheel via an intermediate shaft and universal joint and a integral output shaft and pinion which meshes directly with the steering rack, see Fig. 9.41. A torsion bar mounted in the centre of the hollow input shaft joins the input and output shafts together and transfers the driver's manual effort at the steering wheel to the pinion output shaft. Electrical servo assistance is provided by an electric motor which supplies the majority of the steering torque to the output pinion shaft when the car's steering is being manoeuvred. Torque is transferred from the electric motor to the output pinion shaft through a ball bearing supported worm gear and a worm wheel mounted and attached to the output pinion shaft.

Antilock brake

Diagnostic socket

Ignition switch

Input shaft

Battery

Torsion bar

Potentiometer V sensor

Lever arm & pin

Slide sleeve

Ball & diagonal groove

Diagnostic socket

Ignition switch

Input shaft

Battery

Torsion bar sensor

Alignment adjustment nut

Electric Power Steering System

Fig. 9.41 Rack and pinion electric power assisted steering system

Alignment adjustment nut sensor

Fig. 9.41 Rack and pinion electric power assisted steering system

Relative angular misalignment between the input and output shafts is measured by transforming this angular movement into an axial linear movement along the input shaft by means of a slide sleeve, control ball, internal diagonal groove and a peg and slot. The slide sleeve which fits over the input shaft can move axially relative to the input shaft and rotates with the output shaft due to the peg and slot. Proportionate axial movement of the slide sleeve to the misalignment of the input to the output shafts is achieved by the internal diagonally formed groove in the slide sleeve and the control ball held in the shoulder part of the input shaft. Any axial slide-sleeve movement is registered by the rotary potentiometer (variable resistor) through the potentiometer arm and pin which is located in the slide sleeve's external groove.

When the steering is initially turned against the tyre to road surface grip resistance, the input torque applied to the steering is transferred to the pinion output shaft through the central torsion bar. The torsional twist of the torsion bar, that is, the angular misalignment of the input and output shafts, is proportional to the input effort at the steering wheel before the servo electric motor responds and supplies the extra input torque to the pinion output shaft to produce the desired amount of steering turn by the front road wheels. Should the electric servo assistance fail for any reason, then the steering input effort will be entirely provided by the driver though the torsion bar; under these conditions however the driver will experience a much heavier steering. A limit to the maximum torsion bar twist is provided when protruding ridges formed on the input and output shafts butt with each other.

An electronic control unit which is a microprocessor takes in information from various electrical sensors and then translates this from a programmed map into the required steering assistance to be delivered by the servo electric motor. Mechanical power is supplied by a servo electric motor which is able to change its polarity so that it can rotate either in a clockwise or anticlockwise direction as commanded by the direction of steering turn, the drive being transferred from the output pinion shaft via a warm gear and warm wheel. The large gear reduction ratio provided with this type of drive gearing enables the warm wheel to rotate at a much reduced speed to that of the warm gear and enables a relatively large torque to be applied to the output pinion shaft with a moderately small electric motor.

Steering wheel torque is monitored in terms of relative angular misalignment of the input and output shafts by the slide-sleeve movement, this is then converted into an electrical signal via the interlinked rotary potentiometer sensor. Engine and road speed sensors enable the electronic control unit to provide speed-sensitive assistance by providing 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 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 output shafts are aligned as when the steering wheel is in a neutral no turning effort position, the control ball will be in the central position of the diagonal control groove. Correspondingly the potentiometer lever arm will be in the horizontal position, with zero signal feed current to the electronic control unit and the power supply from the electronic control unit to the servo electric motor switched off. Note the potentiometer is calibrated with the wiper arm in its mid-track position to signal a zero feed current.

Clockwise right hand turn (Fig. 9.42(a)) When the steering wheel is turned clockwise to give a right hand turn, the input torque applied by the steering wheel causes a relative angular misalignment between the input and output shaft, this being proportional to the degree of effort the driver applies. As a result the control ball rotates clockwise with the input shaft relative to the output shaft, and since the slide sleeve cannot rotate independently to the output pinion shaft due to the peg and slot, the flanks of the diagonal groove are compelled to slide past the stationary control ball, thus constraining the slide sleeve to an axial upward movement only.

Accordingly the rotary potentiometer lever arm will twist anticlockwise thereby causing the wiper arm to brush over the wire or ceramic resistive track. The change in resistance and current flow signals to the electronic control unit that servo assistance is required, being in proportion to the amount the slide sleeve and rotary potentiometer moves. Once the initial effort at the steering wheel has been applied the torsional twist of the torsion bar relaxes; this reduces the relative misalignment of the input and output shafts so that the rotary potentiometer lever arm moves to a reduced feed

Basics Rack And Pinion Steering

Sectioned (a) Clockwise (b) Neutral (c) Anticlockwise view right hand position left hand turn turn

Fig. 9.42(a-c) Operating principles for a rack and pinion electric power assisted steering current position or even to zero feed current position. At this point the electronic control unit switched 'off the electrical supply to the servo electric motor so that servo assistance via the warm gear and warm wheel to output pinion shaft comes to an abrupt end.

Anticlockwise left hand turn (Fig. 9.42(c)) When the steering wheel is turned anticlockwise to negotiate a left hand turn, the input effect applied by the driver to the steering wheel causes a relative angular misalignment between the input and output shafts, the relative twist of the torsion bar being proportional to the driver's input effort on the steering wheel. Due to the rotary movement of the input shaft, and control ball relative to the pinion output shaft, the diagonal groove in the sleeve will be forced to move over the stationary control ball in a downward axial direction since the peg and slot only permits the slide sleeve to move axially. The vertical downward displacement of the sleeve is relayed to the rotary potentiometer lever arm which will now partially rotate in a clockwise direction; its wiper arm will therefore brush over the resistive track, and an appropriate signal current will then be fed to the electronic control unit. The servo electric motor is then switched on, and thereby rotates the worm gear and in turn the worm wheel but at much reduced speed (due to the very large gear reduction ratio provided by a worm gear and worm wheel) in an anticlockwise direction. As the input torque effort by the driver on the steering wheel is reduced almost to nil, the relative misalignment of the input and output shaft will likewise be reduced; correspondingly the rotary potentiometer wiper arm will move to its mid-resistance position signalling zero current feed to the electronic control unit; it therefore switches off and stops the servo electric motor.

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