Electric Power Assisted Steering – Mechanical Project

ABSTRACT
Electric Power-Assisted Steering (Epas) Is A New Power Steering Technology That Will Define The Future Of Vehicle Steering. The Assist Of Epas Is The Function Of The Steering Wheel Torque And Vehicle Velocity. The Assist Characteristic Of Epas Is Set By Control Software, Which Is One Of The Key Issues Of Epas. The Straight-Line Type Assist Characteristic Has Been Used In Some Current Epas Products, But Its Influence On The Steering Maneuverability And Road Feel Hasn’t Been Explicitly Studied In Theory.
In This Paper, The Straight-Line Type Assist Characteristic Is Analyzed Theoretically. Then A Whole Vehicle Dynamic Model Used To Study The Straight-Line Type Assist Characteristic Is Built With Adams/Car And Validated With Dcf (Driver Control Files) Mode Of Adams/Car. Based On The Whole Vehicle Dynamic Model, The Straight-Line Type Assist Characteristic’s Influence On The Steering Maneuverability And Road Feel Is Investigated. Based On The Driver’s Request For The Ideal Relationship Between Steering Wheel Torque And Vehicle Velocity, The Vehicle Speed Proportional Coefficient Of The Assist Characteristic Can Be Determined By Making Steering Wheel Torque At Different Vehicle Lateral Acceleration Agree With The Request Of The Driver At A Certain Velocity. The Target Of This Paper Is Analyzing The Influence Of The Straight-Line Type Assist Characteristic On The Steering Maneuverability And Road Feel, And Studying How To Apply Simulation Method To Determine The Straight-Line Type Assist Characteristic Of Epas, Which Will Direct And Benefit The Adjustment Of The Assist Characteristic During The Road Test.

Read more about steering : Steering Gear boxes -Types Of Steering Gear Boxes

INTRODUCTION
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 vehicle’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)

Electric power steering (EPS or EPAS) uses an electric motor to assist the driver of a vehicle. Sensors detect the position and torque of the steering column, and a computer module applies assistive torque via the motor, which connects to either the steering gear or steering column. This allows varying amounts of assistance to be applied depending on driving conditions. Engineers can therefore tailor steering-gear response to variable-rate and variable-damping suspension systems, optimizing ride, handling, and steering for each vehicle. On Fiat group cars the amount of assistance can be regulated using a button named “CITY” that switches between two different assist curves, while most other EPS systems have variable assist. These give more assistance as the vehicle slows down, and less at faster speeds. In the event of component failure that fails to provide assistance, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems.

Electric systems have an advantage in fuel efficiency because there is no belt-driven hydraulic pump constantly running, whether assistance is required or not, and this is a major reason for their introduction. Another major advantage is the elimination of a belt-driven engine accessory, and several high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis. This greatly simplifies manufacturing and maintenance. By incorporating electronic stability control electric power steering systems can instantly vary torque assist levels to aid the driver in corrective maneuvers. The first electric power steering system appeared on the Suzuki Cervo in 1988

NEED OF 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 isn’t running,

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 very sophisticated new device which means it’s relatively early in its development and therefore, expensive. But when u think about all the hydraulic stuff you don’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.

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 Dominique Peter and Ruck Gerhard of ZF Len system (Steering system) in Germany, the present generation of electric steering boost can’t deliver the feel and handling that sriver’s expect in larger, heavier cars, which in their country mean expensive cars.
A good answer to both these problem lies in another “new” 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 today’s electrical systems can provide for any length of time.

BASIC COMPONENT’S AND WORKING
The system itself 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. 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.

FUTURE SCOPE

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-midterm mix of future vehicles. Such system designs have yet to prove themselves sufficiently reliable and safe to prevent dangerous auto steer event. Auto steer has crept into the lexicon as an adjunct to the development of EPS system. As the name implies Auto steer denote an uncontrolled steering event neither commanded nor stoppable by the vehicles 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 redundancies.
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” 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.