17-01-2010, 12:21 AM
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ABSTRACT
Steering and braking are the most critical safety factors in vehicular control. Safe operation of the vehicle demands that the operator be able to maintain absolute control of the vehicleâ„¢s critical operating dynamics:
(1) Control of the direction of motion of the vehicle (steering)
(2) Control of the velocity of the vehicle, i.e. the ability to slow and fully
stop the vehicle (braking)
This presentation provides an overview of electric assisted vehicular
steering including brief description of various conventional systems and the basics of
steering; particular attention in addressed to Electric Power Assisted Steering (EPAS)™™.
Electric power assisted steering has temped automotive engineer since the
1950. Even so, the promise of smaller, lighter, and more efficient systems never quite
matched the low cost and performance of hydraulic power assisted steering (HYPAS)
advance in microelectronics; however have rekindled interest in EPAS. Motor drive stages, electronic control units and torque sensor can now be manufactured relatively
cheaply. These components, coupled with complex control algorithms implemented in
software can rival or better the performance and functionality of conventional hydraulic steering systems. In addition, software can be fine tuned to deliver the desired mix of stability, robustness, and steering ˜™feel™™.
AKNOWLEDGEMENT
I take this opportunity to express my deep sense of gratitude towards my guide Prof. N. V. Borse for his expert guidance during the preparation of this seminars.
I would also like to extend my thanks to the Head of Mechanical Engineering Department, Prof. (Dr.) M. K. Nalawade and all the staff members of Mechanical Engineering department for their valuable guidance, encouragement and co-operation.
I also express my thanks to all of them who directly or indirectly have helped me in preparing this seminars.
Amit Rohidas Bendale
( B.E. Mech)
VIT PUNE Maharashtra
INDEX
1. Abstract 2
2. Steering Basic Concept 4
2.1. Steering Mechanism 4
2.2. Steering Linkage 4
2.2.1. Steering Linkage for Vehicle With Rigid Axel
Front Suspension 6
2.2.2. Steering Linkage for Vehicle With
Independent Front suspension 6
2.3. Steering Gear 6
3. Hydraulic power Assisted Steering 7
4. Electric power Assisted steering 9
4.1. Basic Components and working 9
4.2. Construction and Configuration 11
4.3. Why EPAS? 13
4.4. Control Strategy 15
4.5. Torque Sensor 15
4.6. Electronic Control unit 21
4.7. Motor design 21
4.8. Advantages And Disadvantages 22
4.9. Conclusion 25
References 26
2. STERRING: Basic concepts:
The primary function of steering systems is to achieve angular motion of the front wheels to negotiate a turn. This is done through linkage and steering gear which convert rotary motion of the steering wheel into angular motion of the front road wheels.
Secondary functions of the steering system are:
1. To provide directional stability of the vehicle while going straight ahead.
2. To provide perfect steering condition, i.e. perfect rolling motion of the
wheels at all times.
3. To facilitate straight ahead recovery after completing turn.
4. To reduce the effort required by the driver to steer.
5. To minimize tyre-wear.
2.1. Steering Mechanism:
The perfect steering is achieved when all the four wheels are Rolling perfectly under all condition of running .While taking runs (Illustration-1), the Condition of perfect rolling is satisfied if the axes of front wheels when produced meet The rear wheel axis at one point .This point is the instantaneous centre of the vehicle.
It is seen that the inside wheel is required to through a greater angle than the outer wheel.
To achieve these condition two types of have been devised:
- Davis Steering Mechanism.
- Ackermann steering Mechanism.
For brevity, these have not been discussed in this text.
2.2. Steering Linkage:
Steering Linkage depends on the type of vehicle, whether it is a car which has independent front suspension or a commercial vehicle having generally rigid axle type front suspension.
2.2.1 Steering linkage for vehicle with rigid axel front suspension:
Illustration-2 Shows such a steering linkage. The drop arm (Pitman Arm) is rigidly connected to the cross-shaft of the steering gear at its upper end while its lower end is connected to the page link rod through a ball joint. To the other end of the page link rod to the other end of page link rod is connected to the page link arm through a ball joint. Attached rigidly to the other end is the stub axel on which the road wheel is mounted. Each stub axel has a forged track rod arm rigidly bolted to wheel axis. The other end of the track rod arm is connected to the track rod.
The steering gear provided mechanical advantage so that only a small effort is required at the steering wheel to provide a large force on the steering linkage. When the steering wheel is turned, the swinging action of the drop arm imparts a liner movement to the page link rod. This movement is transmitted through the page link rod arm to the stub axel so as to turn the later about its pivot, which is the king pin. The other wheel is steering through the track rod.
2.2.2. Steering linkage for vehicle with independent front suspension:
Here the two stub axel move up down independent of each other due to which distance between balls joint ends of the two track rod is continuously varying. Hence conventional track rod can not be used.
Illustration-3 depicts this linkage. Here three piece track rod is used, the center portion being called the relay rod, which is connected to the idler arm and drum arm. The relay rod is restricted to move in horizontal plane only.
2.2.: Steering Gear:
The steering gear converts the rotary turning motion of the steering wheel into the To and Fro motion of the page link rod. Moreover it also provides the necessary leverage so that driver is able to steer without fatigue. There are many types and makes of steering gears in use in automobiles. The important ones are mentioned below:
(1)Worm and worm wheel steering gear. (2)Cam and double roller steering gear.
(3)Re-circulating ball type steering gear. (4)Rack and pinion steering gear.
3. HYDRAULIC POWER ASSISTED STEERING:
Large amount of torque is applied by the driver for steering. Hydraulic power steering provides automatic assistance to turning efforts applied to the manual steering system with the help of hydraulic fluid. The system is designed to become operative when effort at wheel exceeds a predetermined value, say 10 N. These systems operate by fluid under pressure as high as 7 Mpa.
The principle of working of all power steering system is same. The slight movement of the steering wheel actuates a valve so that fluid under pressure from reservoir enters the appropriate side of the cylinder, thereby applying pressure on one side of the piston to operate the steering linkage, which steers the wheel in the appropriate direction.
Broadly, the power steering systems are classified into two categories,
The internal type and linkage type.
The main component of internal power steering system consists of a hydraulic pump assembly and a steering gear assembly connected by means of hoses. A rotary valve power steering gear for internal systems uses re-circulating ball type worm and wheel steering gear. (Illustration-4) The steering wheel is connected to the right end of the torsion bar through the steering shaft; the other end is connected to the worm and also to the spool about which the rotary valve is centered. When the force at the wheel exceeds the predetermined value, the spool turns through a small angle, the return valve is closed and the fluid under pressure goes to one side of the rack piston and moves it to effect steering in desired direction. Torsion bar gives the feel of steering to the driver. The rotation of the steering wheel in the opposite direction connects the other side of the steering gear to the pressure line the action of the rotary valve is made by illustration 4a.
4. ELECTRIC POWER ASSISTED STEERING:
As seen earlier, power steering brought about many other changes in automobiles, includes the way people use them. The leverage of huge steering wheel no longer needed, and with smaller wheel the cockpit could be designed with a more relaxed seating position. The hydraulic boost not only reduced the steering effort, it also allowed the quicker steering ratio. As development continued the size, the price and horse power requirements all came down dramatically. In the late ˜80s, advanced hydraulic valve designs made variable rate power steering possible and electronic controls refined it even further. Today variable rate power steering is a standard feature of almost every car sold in the United States.
Even with all these refinements, power steering is still the same basic hydraulic system that was introduced 50 years ago. But for the first time in 1993, Honda introduced full time electric power steering in regular production car, Acura NSX. This is an exotic sports car, competing in the market with Ferrari and Porsche; later this, a small European market economy car, the Fiat Punto will have Delphiâ„¢s electrically boosted E-STEER as standard equipment. Delphi is busily marketing their electric power steering system to the worlds automotive design engineers, and it is expected to show upon several domestic cars in just a few years. There are lots of advantages to using an electric motor to provide steering boost, and many automotive engineers believe we now are seeing the last generation of hydraulic power steering. With a large change just around corner, a look at how electric power steering works imperative.
4.1. Basic components and working:
The system it self comprises main components:
An electric control unit (E.C.U.) , a torque sensor, an electric motor and an intermediate gear with clutch. All these components are integrated into one unit which can be placed on any part of steering column. (Illustration-5)
A database is used to communicate the vehicle speed and engine speed to the EPAS system and the torque sensor detects the force the driver is using to turn the steering wheel. All this information is passed to the ECU.
The ECU uses this information to calculate the additional force required by the EPAS system to achieve the pre programmed steering feel. The steering power is then transmitted by engine to the steering gear by means of an intermediate gear system. The ratio between manual steering torque and electric power controlled in relation to vehicle speed, offering the relevant assistance at varying speeds. At low speed the system offers maximum power, making the steering easy to operate, and at increased speed, when little or no assistance is required reduces amount of power supplied.
Throughout this operation the ECU monitors the system constantly to detect malfunctions and, if necessary, disengages the power assistance with the aid of a built clutch. This ensures that the car still steerable if a fault occurs.
4.2. Construction and configuration:
Currently there are four different types of electric power steering system, all based on rack and pinion steering. Each is differentiated by the placement of the motor, which defines certain application advantages as well as having a big influence on the price.
In the first type (Illustration-6) the motor is built into the steering rack housing, making it the most sophisticated and expensive type of electric power steering system. This type uses permanent magnet direct current (DC) motor, with the armature mounted so that it actually rotates around the rack. At one end of the motor, a gear meshes with another gear to turn a ball screw that is parallel to the rack. A clamp connects a re-circulating ball unit to the rack, so as the unit moves along the length of the ball screw, its motion is transferred directly to the rack. The steering wheel torque sensor is built into the pinion housing. This motor-in-rack layout has the advantage of being extremely compact and can be installed in almost the same space as a hydraulic boosted rack. Disadvantages are higher price, complexity and need to replace the whole rack assembly if the motor, sensor or any mechanical component fails.
Electric power steering (EPS) in the Acura NSX has the armature rotating
around the rack itself. It turns the ball screw which moves a re-circulating ball screw that is clamped to the rack. The torque sensor is built into the pinion housing. (Illustration 1)
Three other configurations are subscribed. On the double pinion type system, the motor is mounted to the rack in separate housing and drives the rack through a second pinion. Since the motor is a separate unit, the rack is less expensive to manufacture than the first type and easier to repair in the field. Also it operates through a separate pinion; itâ„¢s well suited for use on heavier vehicles.
A third basic layout has the motor mounted to the normal pinion housing on the rack, so it acts directly on the existing pinion shaft through an appropriate transmission. Again all components can be serviced or replaced separately, and different
Gear ratio can be used to tune the same basic component for duty in several different
chassis. This system probably will cost less to a op and manufacture, and it take up less space than the double-pinion type.
A fourth version is used on flat pinion. The rack-and-pinion steering assembly is a standard configuration, and an electric motor simply is attached to the steering column. The motorâ„¢s torque is added to the driverâ„¢s steering wheel efforts through a worm gear transmission. The motor can be mounted any where along the column, leaving the design engineers lots of options.
4.3 Why EPAS?
Design options are one of the biggest advantages to the electric power steering. With no pump, mounting bracket, hose or pulley or belt, a lot of under-hood space is liberated for other uses, especially when the servo motor is mounted on the column inside the car. The control unit can be, mounted anywhere on the vehicle, and it can vary the boost level infinitely over a wider range of conditions. Boost level can even become a driver-adjustable feature, and of course boost is available even when engine isnâ„¢t running. The control unit needs a lot of data though, and the power requirement have an impact on the battery and charging system design. Also, the steering wheel torque sensor is a vary sophisticated new device which means its relatively early in its development and therefore, expensive. But when u think about all the hydraulic stuff you donâ„¢t need for steering, the trade-off is quite acceptable from the engineering, service and environmental point of view.
Eliminating power steering fluid from a vehicle provides obvious environmental advantages, along with the fuel saved by not having to drive a hydraulic pump with the engine. On the production line, the time requirement for installation and adjustment is significantly shorter. In the development lab, tuning the system to work in different vehicles takes a few hours of computer type, instead of several days needed to install and different hydraulic valve combinations.
The column-mounted motor is ( so far) the least expensive design, so it will probably be the most common in this generation of electric power steering. But a column-mounted motor is not well suited for use on larger, heavier vehicles. This creates an interesting difference from the way most other new automotive technologies have been introduced. With almost every other new automotive feature, such as air conditioning or automatic transmission, the larger, more expensive cars always had it first. As development continue and cost, size and complexity came down, the new features made their way down to the smaller, less expensive cars. Aside from the Acura NSX, electric power steering technology most likely will start from the bottom a move up to the larger, more expensive cars. Two basic reasons are cited for this.
First, larger, heavier cars require more power to turn the steering wheel, and more expensive rack-mounted-motor design are more suitable for the smaller application.
Second, according to an SAE paper written by Dominke Peter and Ruck Gerhard of ZF Lensysteme (Steering system) in Germany, the present generation of electric steering boost cant deliver the feel and handling qualities that driverâ„¢s expect in larger, heavier cars, which in their country mean expensive cars.
A good answer to both these problems lies in another Ëœnewâ„¢ technology that Delphi and others say is just around the corner: the 42-volt electrical system. Regardless of how an electric steering is configured, current draw would be very high, more than most of todayâ„¢s electrical system can provide for any length of time. All the worldâ„¢s major supplier of automotive electrical components are ready and indeed eager for more voltage because it provides more power. The engineers want to eliminate the belts and pulleys that drive A-C compressors, coolant pump and other accessories because its more economical to drive them with an electric motor. The technology is not difficult; all thatâ„¢s needed is a way to introduce it at a reasonable cost. With things like electric steering, electric (in place of hydraulic) shock absorber and several other drive-by-wire technologies waiting in the wings, a high-voltage electric system is almost inevitable.
When it happens, electric power steering will move Ëœup markerâ„¢ to the heavier cars and maybe even trucks.
4.4. Control strategy:
The steering control unit needs a lot of information to get the boost level and timing just right. Along with vehicle speed and charging system output level, most of the other data is already available from several onboard-computers. There is one new sensor required, a steering wheel torque sensor that tell the control unit how much effort the driver will applying to the steering wheel. A frequent complaint made of HYPAS system is their tendency toover assist the driver at higher speeds. In poorly designed systems, this can result in the vehicle oscillating or yawing around the central line as the driver attains to correct the over steer.
In the new EPAS system, software allows precise control over steering behavior. Algorithms programmed in to the system defined speed sensitivity, yaw damping, and steering self-centering. Added algorithms can give steering a sports feel or offer light load setting. A safety relay incorporated in to the design improves fault tolerance, while the electronic control unit includes diagnostic functions for fault detection n management. If the system fails, its fail stop design cuts or torque assistance and returns the driver to manual steering.
4.5 Torque sensor:
The torque sensor is the heart of the system and is one of the most innovative electronic device. We have seen in recent years. The key to the success of EPAS is an accurate, reliable and low cost torque sensor. However torque measuring devices are complex and expensive. Although there are number of newly developed systems they are usually based on techniques that are require an intermediate compliant member in the steering shaft-usually a torsion bar- and an electrical connection between the steering shaft and the electronics. Non-contact sensors are also used but they are expensive. These being such a critical component of the system the main torque sensor have been briefly discussed here.
Lucas Varity incorporates a dual-channel optical device. Its non-contacting design and mechanical simplicity provides system reliability, while the use of optics offers immunity to EM interference.
To operate, two patterned disks mounted on either end of the torsion bar separating the steering wheel and steering column. Torque applied to the steering wheel creates a relative movement between the two disks. Light intensity reaching the photo detectors varies in proportion to torque. (Illustration-7) Because either detector can be used to measure the light intensity and thus torque, the system is redundant. Offset patterns on the two disks, furthermore, allow the software to calculate the steering wheelâ„¢s relative position and velocity by comparing the two sensor signals.
Researchers at the US departments of energyâ„¢s Ames laboratory say a 1/4-thick ring of the material could be used in an electronic torque sensor to regulate the steering power provided to a cars wheel by an electric motor (Illustration-8).
A sensor using a small ring of the cobalt-ferrite composites would be strategically placed on the steering column. As a driver turn the wheel, the magnetization of the cobalt-ferrite ring would change in proportion to the amount of force applied the driver. The change could be detected by nearby field sensor that would interpret how much force should be applied to turn the wheels and then relay the information to an electrical power-assist motor. Unlike the hydraulic system, the electrical system would consume minimum energy when the steering wheel was not being turned.
What makes the cobalt-ferrite composite ideal for this application is a property known as magnetostriction. Magentostrictive materials undergo slight length changes when magnetized. They take advantage of that property, but in reverse. In their approach, the turn of the steering wheel would apply stress to the cobalt-ferrite ring, producing the change in the magnetic field it emits.
Cobalt-ferrite maintain its Magentostrictive abilities through out the temperature range specified by the auto industry, from -40degree C (-40 F) to 150 C (302 F). Thatâ„¢s necessary because automaker donâ„¢t agree on the best location on the steering column for the torque sensor. Some want it in the passenger compartment while other want it in the engine compartment, where it would be subjected to engine heat well as winter conditions.
Cobalt-ferrite also meets the strength and corrosion resistance requirement for the sensor material. This ceramic metallic composite is similar in concept to materials used in high strength tool bits where excellent mechanical properties are needed and cobalt-ferrite is basically high-class rust, so itâ„¢s hard to corrode any further. The composite is also a cost effective choice.
The third type is the torque sensor on the Acura system. The schematic shows the slider that moves up and down, altering the voltage induced in the secondary coil. This signal can be used to interpret direction and torque on the steering wheel. On the Acura NSX, the torque sensor has a primary coil and next to that, two secondary coils mounted one on top of the other. A slider is positioned between the primary and secondary coil.
Alternating current is passed through the primary coil, which induces current in both secondary coils. When the slider is centered, the secondary coil currents are equal. When the slider moves up or down, it interferes with the current induction in one of the secondary coils, so the two induced becomes unequal. The control unit reads the current difference and uses it to calculate the direction and effort being used to turn the steering wheel.
The other special feature of the steering wheel torque sensor is the ingenious mechanical connection between the electronics and the steering pinion. In a normal rack-pinion steering gear, the pinion shaft is all one piece, with the steering column connected to one end and the pinion gear attached to the other. In this sensor, the pinion shaft is two parts, upper and lower, joined together by a torsion bar that runs through them like an axle in a tube. Imagine holding the pinion gear steering with one hand and twisting the input shaft with the other hand. The torsion bars transfer the torque form one two the other, but allows a calibrated amount of twist. Between the two shafts, half way along a length of torsion bar, a pin protrudes at a right angle from the torsion bar. The pin slots in to the grove in the slider, each are a sleeve or collar around the bar. The grove in a slider is angled, so as the torsion bar twist and the pin moves in the slot, it forces the slider up and down, depending on which way the bar twists. With the correct torsion bar spring rate and the correct grove angle in the slide, vertical movement of the slider can be calibrated to represent a specific amount of torque at steering wheel.
Delphiâ„¢s steering wheel torque sensor is bit different (Illustration-9). It still has an upper and lower pinion shaft, and there is a torsion bar that connects them so they can twist relative to each other. But instead of using a slider to convert twist to a vertical movement, this sensor uses a potentiometer to directly read the amount of twist. Again the rack to tuning the system is proper matching of the torsion bar with steering loads on the front axel. But with this position sensor, a simple DC voltage can be used instead of alternating current, and the return signal is directly proportional to the torque, making the calculation a bit simpler and quicker. In theory, the same potentiometer also could be used to indicate steering wheel direction, but on this system Delphi has chosen a belt-and-suspenders approach, using a separate steering wheel position sensor mounted in the housing.
4.6. Electronic control unit:
The electronic principle is simple, but it relies on to features developed just for this application. First is the very advanced algorithm in the computer program that calculates steering wheel position and torque, essentially giving the machine the ability to interpret the human driverâ„¢s intensions. After that, the control unit simply chooses and appropriate output from a look-up table, similar to the way engine computers control ignition timing.
4.7. Motor Design:
Achieving a smooth, progressive feel at the steering wheel requires a motor with low levels of ripples and cogging torque. Lucas variety, therefore, uses a three-phase inverter to control motor phase current, and hence torque. An array of MOSFETS makeup the circuitry; pulse with modulation(PWM) regulates switching time sequence for the MOSFETS stages.
Since the power-switching stage encompasses the most complex dynamics of the whole EPAS system, optimization requires computer simulation. Lucas uses the saber simulator program from US software house analogy Inc. to analyze alternative PWM strategies, and to model the complex patterns of secondary currents induced when the MOSFETS stages are switched.
The aim is to ensure smooth control of the switching stages and also reduce the ripple currents fed into the battery harness. These currents have to be filter out to protect the electronics in the EPAS control unit. Bye minimizing the ripple current, they are able to use filter currents with lower ripple specifications and, therefore, lower cost.
4.8 Advantages and Disadvantages:
Advantages:
The market for EPAS is growing fast with big supplies such as Delphi, NSX, siemens and TRW involved in its development. It is projected that by 2010 global volumes will exceed 25 million. Manufactures claim that by 2010 the market for EPAS is 15$ billion dollar worldwide.
EPAS system offers a lower component count than HYPAS, is there to five kg lighter than a comparable HYPAS system, and consumers 4-5 % less fuel. Attached to the steering column, the self-contained unit is to be easy to install and particularly cost effective on smaller vehicle. There are often considerable difficulties fitting HYPAS to smaller car. The EPAS system could be installed in four minutes on a production line.
Software allows precise control over steering behavior. Algorithms programmed into the system defined speed sensitivity, yaw damping, and steering self centering. Added algorithms can give steering a sport feel or offer a light load settings. TRW EPS is an environmental friendly fully electric system that eliminates the hydraulic fluid, hardware and potential contamination and leakage problem of conventional hydraulic system.â€saving automakers approximately 190000 liters of hydraulic fluid for every 100000 vehicles produced.
EPAS systems decreases vehicle development time and costs through a unique tunability feature. From a laptop computer in a vehicleâ„¢s interior even while circling a test track engineers can tune in a wide range of steering characteristics. This can save months of work required to achieve the same effect on a conventional hydraulic system. This instant customization also allow the steering to be matched quickly to various suspension packages and easily allows engineers to use steering character to achieve product differentiation and built distinctive brand personality. Once the control unit knows exactly what the vehicle is doing and what the driver wants it to do, controlling the boost is simple matter of controlling current to the motor. The amount of boost can be adjusted instantly and infinitely for any control situation the engineer wants to include in the program.
But in addition to boosting steering response, this system also cab be used to dampen steering response. For instance, in a sudden lane change maneuvers even the best drivers are likely to over control at least a little bit, causing the rear of the vehicle to swing wide. If driver tries to control the swing by turning the other way, this creates what is known as pilot induced oscillation (PIO). In extreme cases or on slick roads, the driver can completely lose control after only two or three oscillations. Delphi has demonstrated how effectively their electric steering system can dampen oscillations, given the appropriate software. Electric power steering can easily become part of existing vehicle stability control system that can detect yaw.
Disadvantages:
Many vehicle manufactures are however not satisfied with current EPAS performance. Right now they donâ„¢t give the comfort and safety car makers want to provide their customers with, feel PSA. There exists a sticking phenomenon meaning that the steering wheel does not go naturally back to its original position after it has been turned.
Ford like PSA is not satisfied with current EPAS performance. They have evaluated 12 of the systems that are currently being developed and none provided a satisfactory dynamics performance. They feel artificial because they suppress feed back from the road. Vehicle response is not what they would like to be. Ford will probably use electro hydraulic technology as a first step towards EPAS because it works with existing hydraulic system.
To-date, technical and product liability concerns have precluded the introduction of such systems in the U.S. market through it is expected that niche application may be expected in the near-to-mid term mix of future vehicles. Such system design have yet to prove themselves sufficiently reliable and safe to prevent dangerous auto steer events. Auto steer has crept into the lexicon as an adjunct to the development of EPAS system. As the name implies auto steer denotes an uncontrolled steering event neither commanded nor stoppable by the vehicleâ„¢s driver due to catastrophic failure in the electron hardware or software. In truth, these systems are control servo systems, similar in function to aircraft control servo systems, and must have multiple redundancy. Although these new EPAS systems are said to have multiple redundancy , their design and broad application within the automotive industry have been, and will continue to be, subject to economic pressure more extreme then found in the aircraft industry. For instance one obvious safety related item has been universally deleted from such system specifications: a clutch for physically disengaging the reduction gear box and drive motor assist assembly from the host steering system in the event of system failure. This means that a driver encountering an EPAS system failure will have to exert additional force to back drive (i.e. manually over ride) the systems reduction gear box and drive motor assist assembly while attempting to maintain control of the vehicle in the absence of normal power steering assist.
Unlike the manual system described above, PAS with the presence of supplementary steering force to that provided solely by the operator introduces additional engineering challenges in terms of maintaining the desired steering linearity described previously. In fact, with respect to steering linearity, a poorly designed power steering assist system may have almost no relationship between the hand wheel torque applied by the operator and the actual required steering force imposed by the wheel or tires. There no longer may exist the uniform, consistent and predictable relationship between the input and outputs to facilitate tactile reference driving. Restated, the tactile sense of the driver to maintain directional control, and the magnitude and modulation of the input force may not bear a direct, proportional relationship to the required output force delivered by the steering system. Tactile reference steering is simply not possible with such vehicles. Rather their drivers must continuously engage in visual reference steering to maintain directional control. The result is that such vehicles are very tiring to drive for any length of time or distance. Further their drivers are constrained to continuously look at the road. If such driver should look away even momentarily (i.e., to check a rearview mirror or a child in the car), he or she has minimal tactile reference as to the actual position of the vehicle during that period of time. This is dangerous because, depending upon the road topography and condition, the vehicle may have moved transversely in significant amount relative to where the driver thought his or her vehicle was positioned. This can and often does lead to serious trouble.
4.9. Conclusion:
These drawbacks are however only temporary barriers for the large scale introduced of these systems in todayâ„¢s cars and it can be unequivocally stated that EPAS is the future in power steering. The advantages far outweigh the disadvantages and as the system components get introduced at more economical prices, EPAS will become most efficient, safe and reliable power steering system.
REFERENCES
1. Automotive Engineering - Dr. Kirpal Singh.
2. Automotive Engineers “ March 2000.
3. Technical Report by Fredric Roos on design of EPAS in 2005
(Royal Institute of Technology, KTH, Stockholm)
4. htpp://motorage.com
5. htpp://scirus.com
