1、automotive engineering i nternational SAE INTERNATIONAL technology profiles STEERING A Revolution in Steering Technology Daniel J. HoltElectric steering: A revolution in steering technologyOther SAE books of interest: 42 Volt Systems by Daniel J. Holt (Order No. RR-008) Fuel Cell Powered Vehicles by
2、 Daniel J. Holt (Order No. RR-013) For more information or to order this book, contact SAE at 400 Commonwealth Drive, Warrendale, PA 15096-0001; (724) 776-4970; fax (724) 776-0790; e-mail: publicationssae.org; web site: www.sae.org/BOOKSTORE.Electric steering: A revolution in steering technology Dan
3、iel J. Holt Editor-at-Large Automotive Engineering International Society of Automotive Engineers, Inc. Warrendale, Pa. Copyright 2002 Society of Automotive Engineers, Inc. eISBN: 978-0-7680-7079-8Copyright 2002 Society of Automotive Engineers, Inc. 400 Commonwealth Drive Warrendale, PA 15096-0001 U.
4、S.A. Phone: (724) 776-4841 Fax: (724) 776-5760 E-mail: publicationssae.org http:/www.sae.org ISBN 0-7680-0919-7 Library of Congress Control Number: 2001096983 All rights reserved. Printed in the United States of America. Permission to photocopy for internal or personal use, or the internal or person
5、al use of specific clients, is granted by SAE for libraries and other users registered with the Copyright Clearance Center (CCC), provided that the base fee of $.50 per page is paid directly to CCC, 222 Rosewood Dr., Danvers, MA 01923. Special requests should be addressed to the SAE Publications Gro
6、up. 0-7680-0919-7/02-$.50. SAE Order No. T-103Contents Executive summary 3 Preface . 5 Chapter 1 . 9 Steering basics Chapter 2 . 19 Electrohydraulic power steering Chapter 3 . 25 Electric steering Chapter 4 . 37 Four-wheel steering Chapter 5 . 43 The market drivers Chapter 6 . 47 The future Referenc
7、es . 53 1 Executive summary For nearly 50 years after the introduction of the automobile, drivers struggled with the steering wheel during low-speed parking maneu- vers. As automobile performance rapidly improved from the introduction of cars over a hundred years ago, drivers were also seeking impro
8、vements in handling stability at high speeds. The introduction of larger, more powerful engines along with larger and heavier vehicles led engineers to seek a way to help overcome the rack loads that were making it more difficult to turn the steering wheel. In the 1950s, General Motors introduced th
9、e hydraulically assisted power steering system. This system used a recirculating ball steering system coupled with a belt-driven hydraulic pump. The hydraulic pressure created by the rotary vane pump reduced the steering effort by the driver required to overcome the rack load on the steering system.
10、 An important design aspect of the power steering system was that it provided steering stability at high vehicle speeds because the driver had hydraulic pressure assisting in the control of the linkage. A second system, a rack and pinion steering system, was eventually developed and used in many lig
11、hter and sportier vehicles. For many years, this system did not require any power assist but eventually, it too began to use a hydraulic assist. Today, a revolution is taking place in steering technology in the automotive industry. For the past 50 years, steering technology has focused on using hydr
12、aulics to assist the driver in steering the vehicle. Automotive engineers have done a superb job of creating steering systems for all weight and size classes of vehicles ranging from sub-compact through large sport utility 3 vehicles and large trucks. However, during the past decade the focus has be
13、en changing to using electric motors to supply the assist to steering systems. This has been especially true in Europe and Asia, where many of the cars are lighter and smaller than in the United States and fuel economy is a very important consideration. In addition, packaging is an important conside
14、ration in the use of electric steering. A number of electric power assisted systems are under development. The two that are seeing the most widespread use are the electrohydraulic power assisted system and the column-mounted electric power assisted system. A number of other systems are under develop
15、ment for various sized vehicles. Many suppliers are starting to receive orders for electric power assisted steering systems as new models and platforms are developed. The addition of electric power assisted steering can reduce fuel consumption by 5 to 15%, depending on the vehicle. Currently, the ve
16、hicles adopting the use of the column-mounted power assist steering are the lighter (less than 1000 kg 2205 lb) vehicles in the sub- to compact-size range. Electrohydraulic power assisted steering can be used in various vehicles because it requires only the replacement of the belt-driven hydraulic p
17、ump with an electric motor and control electronics. Electric steering is expected to be used on almost 50% of new vehicles in Western Europe by 2005. North America is beginning to use electric steering in some vehicles, but it is expected that it will take another decade before a substantial percent
18、age of vehicles will be equipped with electric steering. The introduction of electric steering is a step in the evolution of the auto- mobile toward steer-by-wire. As the 42 V electric system also is introduced into the mass production market in a few years, it may accelerate the use of electric pow
19、er assisted steering in larger, luxury vehicles. 4Preface Vehicle steering has come a long way from the tiller system used on the first automobiles. The tiller motion was soon replaced with a circular motion using a round handwheel, and thus the modern steering system was born. As vehicles became he
20、avier, tires became wider, and the speed and performance of the vehicles increased, new suspension systems were developed. Steering the vehicle became more difficult, especially during low-speed parking maneuvers, due to the increased forces on the steering system. Early engineers quickly increased
21、the size of the steering wheel, which gave the driver more leverage and eased the steering effort during parking maneuvers. However, the larger steering wheel did little for the performance the consumer demanded at highway speeds. In the mid-1950s with the popularity of the larger, more powerful V-8
22、 engines and the desire to make steering as effortless as possible, a hydraulic power assisted steering system was developed by Delphi Saginaw Steering Systems. This system used hydraulic pressure to assist the driver in turning the wheels, and in doing so, allowed the driver to turn the steering wh
23、eel with very little effort, hence the term “power steering.“ This system allowed more people to enjoy driving, much like the invention of the electric starter whereby people no longer had to hand crank their engines to start them. General Motors decided to add this new system to its Cadillac and Bu
24、ick models in 1953. Since then, about 70% of the vehicles produced are available with power steering. With the introduction of power steering, steering wheels could be downsized because they did not have to be large to give leverage, and their position could be adjusted to obtain a better driver sea
25、ting position. 5 Early units took the standard recirculating ball steering system and added hydraulic boost instead of muscle power to turn the wheels. To accomplish this, engineers had to add the source of hydraulic pressure, a rotary vane hydraulic pump connected by an accessory belt to a rotating
26、 pulley on the engine. Hydraulic pressure created in the power steering pump is used to reduce the driver effort needed to move the steering system. The recirculating ball steering system was used mainly in the heavier vehicles with high steering loads. Lighter vehicles were using the rack and pinio
27、n system because of its flexibility and lower complexity. The rack and pinion steering system could be used on light vehicles without requiring a power assist. However, power assisted rack and pinion steering soon became the norm for many light vehicles. This versatile system could be easily used on
28、 either right-hand or left-hand drive systems by changing the location of the pinion and rack. The rack and pinion steering system is used today in most automobiles, while the recirculating ball system is used in very heavy vehicles, sport utility vehicles (SUVs), and trucks. However, rack and pinio
29、n systems are being developed for the latter market, especially in anticipation of 42 V electrical systems and their ability to supply higher wattage. In the 1980s, high-energy neodymium magnets opened the door to allow motor designers to develop smaller sized, smaller weight, and higher performance
30、 motors. TRW had developed a version of electrically assisted power steering in the mid-1980s. The new electric motors could handle all but the large steering loads or those above 10,000 N rack forces when operating on 14 V electrical systems. Thus, advances in electric motor technology, power elect
31、ronics, and control methods have opened the door for steering systems that get their power assist from an electric motor. The first worldwide production use of electric steering was in the Honda NSX produced in the early 1990s. This fully electric system used a Honda- developed motor that fit concen
32、trically around the rack of the rack and pinion steering system. Honda has used this same type of system in its S2000. 6A number of steering systems using an electric motor have been developed (Figure 1): Electrohydraulic power steering, where the hydraulic pump is powered by an electric motor inste
33、ad of being driven by an accessory belt from the engine Column assist, where the electric motor is mounted on the steering column Pinion assist, where the electric motor is mounted where the pinion gear of the rack and pinion steering system connects to the rack Double pinion, where an additional pi
34、nion is added to the rack and is driven by an electric motor Rack assist, where the electric motor is mounted on the rack Figure 1. Four variations of power steering using an electric motor are shown. From left to right, electrohydraulic power assisted hydraulic steering, rack-mounted electric assis
35、t, column- mounted electric assist, and pinion-mounted electric assist. Not shown is the double pinion electric assist, where the standard pinion gear setup is used and the steering shaft connects to the pinion of a manual system, and a second motor assisted pinion and rack setup is added along the
36、rack. Photo courtesy of TRW, Inc. 7 A fuel economy improvement of 5 to 15% is expected when switching from a hydraulically driven power steering unit to pure electric steering. This savings is realized because the electric motors are on-demand and are used only when the handwheel is being turned by
37、the driver, whereas the con- ventional belt-driven power steering pump is turning as long as the engine is operating. The overall weight of the steering systems can be reduced because electric steering does not require the hydraulic pump, hydraulic fluid, or fluid lines. Electric steering also allow
38、s for precise electronic control of the steering system. Engineers can vary the amount of assist that the electric motor applies to the systems for each car line. The same setup can be used on a number of vehicles, with each model having its own level of assist. In addition, the desired feel and res
39、ponsiveness along with the tactile feedback to the driver can be controlled electronically. Thus, a sport version of the same model vehicle can be adjusted to have different characteristics than the family version by adjusting the control electronics. 8Chapter 1 Steering basics As a vehicle turns, t
40、he front wheels do not track in the same direction. Each wheel actually follows a different circle as generated from the same center. In a right-hand turn, the inside wheel (right front) follows a circle with a smaller radius than the outside wheel (left front). Thus, for the car to turn smoothly, t
41、he inside wheel must be made to turn more than the outside wheel via the steering linkage. Engineers had to develop a steering system that would allow this to happen. As the tiller gave way to the handwheel, a steering system known as the recirculating ball system was developed. The recirculating ba
42、ll steering system was adopted as the system of choice for many years. In fact, today it is still used on sport utility vehicles and trucks, although some models are being developed which use the other type of steering system-rack and pinion. Under the recirculating ball system, the steering wheel i
43、s connected via a steering shaft to a gearbox. This gearbox is connected to the steering linkage through a Pitman arm. The Pitman arm moves left and right corresponding to movement of the handwheel. It is connected to the tie rod for the left front (driver side) wheel and to a track rod, and then th
44、e tie rod for the right front (passenger side) wheel. The tie rods are then connected to steering arms that, in turn, are connected to the wheels. The term “recirculating ball“ reflects the fact that small ball bearings are recirculated within the gearbox (Figure 2). The steering shaft is connected
45、on one end to the handwheel (steering wheel) and to a worm gear on the other. This worm gear passes through a block of metal known as the ball nut rack that contains the small ball bearings. As the wheel is turned, the 9 Figure 2. Cutaway of a recirculating ball steering box with hydraulic assist. N
46、ote the ball bearings that rotate around the worm gear. The sector gear is connected to the Pitman arm. Photo courtesy of Delphi Automotive. steering rod turns and the worm gear rotates in the ball nut rack. The action of the worm gear is such that it does not pass through the ball nut rack but rota
47、tes within the gearbox. The ball bearings traverse up and down the worm gear. The change of rotating motion to linear motion moves the ball nut rack, which is connected to the sector gear. The sector gear is connected to the Pitman arm. As the ball nut rack moves up and down, the Pitman arm moves th
48、e linkage left and right (Figure 3). By choosing the proper ball bearings, the steering movement is smooth as the threads of the worm gear are filled with the ball bearings. The ball bearings tend to reduce friction and wear in the gear and, if their size is correct, there is no slop in the steering
49、 movement. A typical recirculating ball steering gear unit with hydraulic power assist can have a mass of 12.8 kg (28.2 lbs) and have a gear ratio range of 12.5:1 to 17:1. Rack and pinion steering The second type of steering system, found mostly in lighter vehicles, at least when it was first developed, is the rack and pinion steering system. This 10
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