8/12/2019 65 File 35402

1/12

8/12/2019 65 File 35402

2/12

122

Steering-column drive shaft

Steering box

Drop arm

Drag link

Axle beam

Spring seat

King pin

Stub axleTrack rod arm

Track rod end joint

Track rod

Drag link ball joints

Drag link arm

Forward

direction

Steering wheel

Figure 4-1: Axle Beam Steering System

is, suspension in the vertical deflection and in the horizontal steering direction. In the steering systems shownin Figure 4-1, Figure 4-2 and Figure 4-3, the connection between the steering wheel and the steering linkageis pure mechanical connection. There is no any hydraulic or electric power assist in the steering process.A rack and pinion or a gearbox is used to reduce the driver steering effort. However, this multiplicationis achieved at the expense of having to multiply the distance the input effort has to move (steering wheel

rotation) to bring about the desired angular rotation to the steering road wheels about their swivel pins.Due to this issue, these types of steering systems are limited to small steering applications.

4.3 Power Assisted Steering (PAS) System

To reduce the physical steering effort the driver has to apply to the steering wheel, a steering gear boxratio is used so that for a given input force on the steering wheel the output torque is amplified in rotatingthe steering stub axle. However, this multiplication of output torque is achieved at the expense of havingto multiply the distance the input effort has to move (steering wheel rotation) to bring about the desiredangular rotation to the steering road wheels about their swivel pins. Power assisted steering is designed toreduce the effort the driver has to exert on the steering wheel and to reduce the steering wheel movementfor a given swivel pin angular turn, that is, to make the input (steering wheel movement) to output (swivel

pin movement) more direct.

The power assistance relieves the driver of a large amount of physical exertion when steering, particularlyif the vehicle is large and carries heavy loads, yet there must be sufficient resistance for the driver to overcomewhen moving the steering wheel so that he or she feels that they are exerting an input force corresponding tothe changing road conditions and vehicle load. The driver is therefore able to experience or sense operatingconditions and drives accordingly. Thus a small car may only receive 25% input assistance so that 75% ofthe effort needed to steer the car is provided by the driver, whereas a large truck carrying a full load mayrequire up to 90% assistance, that is, only 10% input effort is supplied by the driver. The integral powerassisted steering gearbox can be used for rigid front axle suspension commonly used on vans and commercial

8/12/2019 65 File 35402

3/12

123

Figure 4-2: Independent Suspension Steering System: Split track rod with relay rod and idler steering linkagelayout

Steering wheel

Steering column

Upper universal joint

Steering link shaft

Lower universal jointl

Pinion shaft

Rack and pinion housing

Rack

Tie rod

Tie rod arm

Wishbone arm

Swivel pin

Stub axle

Figure 4-3: Independent Suspension Steering System: Rack and pinion steering l inkage layout

8/12/2019 65 File 35402

4/12

124

Figure 4-4: Power Assist Steering: Integrated steering box and power assisted steering utilized with rigidaxle front suspension.

vehicles Figure 4-4, and can also be utilized in conjunction with independent front suspension layouts onlarge cars and vans Figure 4-5.

Power assist steering systems requires external hydraulic power which is supplied by the steering pump.The combined reaction control-valve and steering box uses this hydraulic power and acts as an amplifierbetween driver command and actuation power to the steering mechanism. The rack and pinion steeringgear and the reaction control valve are all combined and share a common housing. A reaction valve whichcontrols the charging and discharging is a spool type valve. With no steering effort, the reaction controlvalve will take up a central position determined by the centering springs. Under this conditions fluid fromthe hydraulic pump is free to flow back to the reservoir tank.

Rotating the steering wheel anticlockwise immediately pushes the reaction control valve to the right handside. This causes fluid pressure to act on the right hand end of the rack piston. This fluid pressure assist themanual effort applied by the driver on the steering wheel to rotate the drop-arm in a clockwise direction.The greater the effort applied to the steering wheel by the driver to turn the steering, the greater will be

the steering force acting on the drop-arm. Thus it can be seen that the power assistance provided matchesthe input effort applied by the driver, so that the assistant is progressive and the driver feels that he or sheis in control at all time.

Most power assist steering systems use an engine-driven hydraulic pump. Power steering hydraulic pumpsare usually belt driven from the front crankshaft pulley of the engine. Some power steering pumps are of theslipper or roller design instead of the vane type. When the engine starts, the drive belt rotates the powersteering pump pulley and the rotor assembly inside the power steering pump.

4.4 Electro-Hydraulic Power Assist Steering (EHPAS) System

In a conventional power assist steering systems shown in Figure 4-4 and Figure 4-5 the power steering pump

is driven by the engine crankshaft through a belt drive connection. There is a continuous rotation of oilas long as the engine is running. This continuous oil rotation results in mechanical and hydraulic frictionlosses. A medium sized vehicle will have a loss of approximately 0.3 to 0.4 additional liters per 100 km trav-eled [6]. The phrase power on demand is currently associated with energy efficient electric steering, whilethe phrases hydraulic Feel and hydraulic Power are associated with electro-hydraulic power steering systems.

Considering existing sensors, motor, pumps, valves, and other component technology, and having the goalto provide the best performance at the lowest possible price, it is clear that the EHPAS system which usethe conventional hydraulic power steering component with an electrically driven and electronically controlledpower steering pump satisfies to a great extent the major goals. So referring to Figure 4-6 and Figure 4-7, the

8/12/2019 65 File 35402

5/12

125

Figure 4-5: Power Assist Steering: Integral steering box and power assisted steering utilized with independentfront suspension.

Electrical

Motor

Figure 4-6: Electro-Hydraulic Power Assist: Steering System for Rigid Axle Front Suspension.

main deference between the power assist steering system and the electrical-hydraulic power assist steeringsystem is in the way the power steering pump is driven. Instead of being driven directly by the vehicle enginethrough belt connection, it is driven by an electrical motor for example, which controls the pump upon thesteering command [7]. An electrically driven motor is used to drive the power steering pump. Movement ofthe steering wheel activates the motor and hence supplies the pressurized oil for the steering process. Thispower on demand criteria is the major advantage of electro-hydraulic power assist steering systems.

4.5 Electric Power Assist Steering (EPS) System

Traditional hydraulic power steering systems have provided remarkable performance for many years in termsof handling, cost, and comfort. However, these systems have the drawback of adding drag on the engine

equal to a decrease of 5 horsepower. The hydraulic pump runs regardless of whether steering assistanceis required in fact the power cost of the pump is highest at high speed, when steering assistance is leastneeded. Automotive manufacturers are moving to Electric power Steering ( EPS) to provide the convenienceof steering assist without the cost in engine size and fuel consumption. EPS systems are available today and,by 2006, up to 50have electric instead of hydraulic power steering.

Key Benefits

1. EPS Provides a more flexible and less expensive steering system than a conventional hydraulic system.

2. Supports a single- device solution using an integrated peripheral set.

8/12/2019 65 File 35402

6/12

126

Electrical

Motor

Figure 4-7: Electro-Hydraulic Power Assist: Steering System for Independent Front Suspension.

3. Combines MCU functionality and DSP processing power for high- speed sensor input processing.4. Supports automotive environments (40-C to 125C).

5. Offers a low- cost device and system solution with wider market application availability.6. Out- of- box software frameworks designed to expedite time- to- market and reduce development costs.

Design Challenges

The steering assistance possible for any EPS system is limited by the maximum current available from thevehicle Typical 12V systems can provide a peak maximum current of 80 amps, limiting peak power to only1 Kilowatt, so current applications of EPS are limited to smaller vehicles, such as the compact or smallermedium class of cars However, once, 42V electrical systems are available, EPS can be easily applied to largervehicles, even trucks.

Another challenge is minimizing system cost and maximizing robustness, safety, and flexibility. Choosingthe right components is key to success. EPS is also a cheaper and more flexible solution than a hydraulicpump. Besides providing a five percent improvement in fuel efficiency, EPS is lighter (4 Kg to 6 Kg or 8 lbs

to 13 lbs) and driving for a sports car and smooth, safety- assisted steering for the family sedan simply bychanging the controllers software.

Power steering applications require that the assist device mimic the driver s inputs at the steering wheelThe inputs are typically precise course corrections followed by periods of inactivity. This condition presentsan interesting challenge to any motor design. The motors must also operate for extended periods in anunder- hood environment that can sometimes reach temperatures of to 150 degrees Celsius with little or onmaintenance.

An acceptable motor design with a high efficiency and temperature tolerance, is one that can be preciselymanipulated. Today s multiple- poled brushless DC motors have been designed with tasks like these in mindTheir brushless design moves the electrical windings to the stator, the outer housing, which eliminates theneed for harshest of conditions with a very long service life.

4.5.1 EPS System Configuration

An electric motor is used directly to assist the driver, no hydraulic circuit is required. Omission or simplifica-tion of the hydraulic circuit components (pump, bracket, oil reservoir, pipes) result in an easier maintenanceand reduced manufacturing cost. The electrical steering concept enables to define the vehicles specific steer-ing characteristics and other parameters [6].

Four basic variants of the electrical steering exist as shown in Figure 4-8. The electrical motor locationdefines the type of the EPS system. There are four major EPS systems: column-type, pinion-type, double

8/12/2019 65 File 35402

7/12

127

(a) (b) (c) (d)

Figure 4-8: Electric Power Assist Steering (EPS) Systems: (a) column type, (b) pinion type, (c) doublepinion type, (d) rack type.

pinion-type and rack-type. As shown in Figure 4-8, in the column, pinion and rake types the motor acts di-

rectly on the steering column, the steering pinion or the steering rack respectively. In the double pinion-type,the motor acts in a separate pinion meshed with the steering rake. Particularly the steering column varietyis an economical solution for small vehicles. The pinion principle is suitable for medium class vehicle, thedouble pinion solution constitutes a compromise between the economical pinion solution and the expensiverack solution. The rack solution is primarily suitable for higher-class vehicles. The support torque of theelectric motor is transferred directly to the rod via a planetary gear [6].

A brief description of an EPS system in its column assist type is shown in Figure 4-9, the system is madeup of a steering column, a gear assist mechanism attached to this column, a brushless motor, a controllerand a torque sensor attached to the steering column. The rest of the steering system: steering wheel, inter-mediate shaft, rack and pinion, and the tie rods are also shown. The main purpose of any power steeringsystem is, of course, to provide assist to the driver. This is achieved by the torque sensor, which measuresthe drivers torque and send a signal to the controller proportional to this torque [8].

The controller also received steering position information from the position sensor, the torque and positioninformation is processed in the controller and an assist command is further modulated by the vehicle speedsignal. This command is given to the motor, which provides the torque to the assist mechanism. The gearmechanism amplifies this torque, and ultimately the loop is closed by applying the assist torque to thesteering column [8].

4.6 Steer By Wire (SBW) System

The next generation of steering systems following EHPS and EPS is the steer-by-wire systems (Figure 4-10).The motivation behind developing such systems is a reduction in total system cost, reduced risk of injury bya steering column in the event of an accident, and better overall performance [9]. The main different between

the conventional steering systems and the steer-by-wire systems is at the connection between the steeringwheel and the steering mechanism. We can view the steering system as having two main sub-systems: 1.command input sub-system (steering or hand wheel), 2. steering mechanism (rack and pinion mechanismin case of medium size vehicle, and steering hydraulic power circuit in case of large size vehicles or off-roadvehicles) [10].

The conventional systems include different forms of mechanical and hydraulic connection between thesteering wheel and the steering mechanism. In simplest terms, the steering wheel rotation is proportionallyamplified by the steering mechanism to obtain a proportional articulation angle. Since the steering wheeland the steering mechanism are mechanically or hydraulically coupled, there is a built in inherent force

8/12/2019 65 File 35402

8/12

128

Steering function

Motor Control Power Amplifier

Sensor plausibility

and sensor

evaluation

Battery

Current Feedback

Torque sensorReduction gear

Electric motor

Motor sensor

Steering column

Rack and pinion

mechanism

ECU

Figure 4-9: Electric Power Assist Steering (EPS) System Block Diagram.

feedback to the operator at the steering wheel proportional to the steering load resistance [10].

Like a conventional system, steer-by-wire uses a steering mechanism (electrically powered rack and pin-ion, hydraulic power) to articulte the wheels. Unlike the conventional systems, however, the by-wire systemsteering wheel is connected to the steering mechanism sub-system by an electrical wire, rather than a shaft.A steer-by-wire system has only electrical signals connection between the steering wheel sub-system andthe steering mechanism sub-systems [11]. That alone may be the biggest advantage of the steer-by-wire. Iteliminates the shaft, freeing up packaging space, improving crash worthiness, and increase design flexibilityby allowing engineers to place the steering wheel on the right or left side of the cockpit. Furthermore, itprovides a foundation for such features as automated collision avoidance and lane keeping by enabling thevehicle to steer itself via a computer. Steer-by-wire systems consists of four main elements: a hand wheel,an electro-mechanical force feedback actuator, a micro-processor-based controller; and electrically powered

rack and pinion or hydraulic-power steering mechanism. When the driver turns the hand wheel (so namedbecause it is more like a sensor than a steering wheel), sensors tell the microprocessor how far the wheel hasbeen turned. The microprocessor then sends a signal to an electric motor or a solenoid valve, which will,operates the rack and pinion or the hydraulic-powered steering mechanism. The steering mechanism thenprovides the force needed to turn the wheels [11].

To provide the feedback feel of conventional steering wheel, an electro-mechanical force feedback actuatoris used, which consists of a motor or a brake. As a result, the driver gets the feel of the road. Furthermore,the steer-by-wire enables to tune the steering control system parameters to provide precise vehicle handlingcharacteristics. The bock diagram shown below Figure 10 shows the main structure of a steer-by-wire systemwhich use rack and pinion as its steering mechanism, a torque sensor and a motor are used to emulate thehand wheel resistance feedback [11].

Steer - by- wire, a next - generation steering system in which there is on mechanical link between thevehicles s steering wheel and the front- wheel directional actuator, is receiving much attention recentlyfeatures in comparison with conventional steering include the following .

1. Saving energy over the entire service life with the electric steering for cars, an electric motor that isindependent of the vehicle s driveline generates the serve power assistance . The system can be flexiblyadapted to any steering and vehicle concept and requires 80

2. With a steer by wire steering systems, there is on mechanical connection between the steering wheeland the wheels . The steering power is generated by electric or elector- hydraulic wheel positioningsystems so the column is not thrust toward the driver in the event of a front- end collision .

8/12/2019 65 File 35402

9/12

129

Steering wheel motor

feedback signal

Steering

controller

Steering wheel

controller

Motor

Vehicle speed

Yaw angle speed

Steering angle

Torque signal

Steering wheel

motor current

Steering motor

currentSteering motor

feedback signal

Amp

Amp

Motor

ECM

Figure 4-10: Steer-By-Wire System Structure

8/12/2019 65 File 35402

10/12

130

Figure 4-11: Steer-By-Wire System Main Components

3. Interference with the driver during stability control and automatic steering can be avoided i.e. theactive steering intervenes in cases of extreme situations, in emergencies, and at varying road surfaces ,actively corrects the steering angle and thus stabilizes the vehicle . This kind of intervention is hardlynoticed by the driver.

4. There is more freedom in vehicle design in areas such as steering layout

Components for Steer-By-Wire

Steer- by- wire is based on mechanics, microcontrllers, real- time software , elector motors, power electronicsand digital sensors as follows :

1. sensors for rotation angle and torque of steering wheel and front tires these sensors must be miniaturizedto minimize moving masses and to save energy and space . under the motor hood and must have anbuilt - in standardized digital interface for computer read- out .

2. Actuators for steering - wheel force feed- back and active tire movement, actuators may be electorsmotors

3. Power electronics required to drive the elector motors, power electronics must be transistor - basedsince no AC current is available on board .

4. Controllers are microprocessor - based and contain real- time software and operating system. Con-trollers must receive a feed- back- signal to ensure correct steering and force feed - back .

5. Mechanical components such as gear, overriding gear, steering wheel, pinion, gear rod and tires .

4.7 Electric Assist Steering System

A place where the mechanical-to-electric transformation is taking place, albeit slowly, is in your cars powersteering. Some smaller cars in Europe appeared with electric power steering back in 1998, with volumesincreasing steadily as European automotive manufacturers adapted the new technology. By using an electricmotor to replace the traditional combination of hydraulic pump, fluid, hoses, and fluid reservoirs, you have asystem that, while not significantly less expensive to produce, is smaller and lighter. In addition, the electricsystem is powered by the cars battery, not the engine, which adds to fuel-consumption efficiency and givesthe automobile manufacturer more options in configuring the steering system (Figure 4-12)

8/12/2019 65 File 35402

11/12

131

Figure 4-12: Replacing a traditional hydraulic steering system with an electric motor, as shown here,reducesthe systems size and weight. The electric system also improves fuel efficiency and givesthe car manufacturermore design flexibility. (courtesy Xilinx)

4.8 Electro-Hydraulic Steer-By-Wire System

Automotive and earth moving equipment industry has been developing by-wire systems for steering, brake,and engine throttle control. These systems are expected to appear in the car market and offered to consumersby the year 2007. By-wire systems refer to the physical connection between operator command devices andthe controlled sub-system, such as the connection between the steering wheel and the steering mechanism,the connection between the brake pedal and the brake actuators, the connection between the acceleratorpedal and engine controller.

The main different between the conventional steering systems and the steer-by-wire systems is at theconnection between the steering wheel (or steering lever) and the steering control valve. We can view thesteering system as having two main sub-systems: 1. Command input subsystem (steering wheel and/orsteering lever), 2. Steering power circuit (i.e in the case of electro-hydraulic power steering, this sub-systemincludes the pump, cylinder, and valves). The conventional systems include different forms of mechanical

and hydraulic connections between the steering wheel and the steering valve. In simplest terms, the steer-ing wheel rotation is proportionally amplified by the steering valve to obtain a proportional articulationangle. Since the two systems (steering wheel and steering power system) are mechanically coupled, there isa built in inherent force feedback to the operator at the steering wheel proportional to the steering conditions.

A steer-by-wire system has only electrical signal connection between the steering wheel sub-system and thesteering power sub-systems. This approach has both advantages and disadvantages compared to conventionalsteering systems. Steer-by-wire advantages: - Modular steering sub-system design, simplifies the assembly,lower manufacturing cost - Software customizable to fit different machine needs - The injury risk caused bythe steering column in accident will be diminished. The main disadvantage of SBW system is that there isno direct mechanical feedback to the operator about the steering conditions, which must be emulated by anactive control system.

8/12/2019 65 File 35402

12/12

132

Figure 4-13: Electro-Hydraulic Steer-By-Wire System for Articulated Vehicles