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1、Electronic power steering system What it isElectrically powered steering uses an electric motor to drive either the power steering hydraulic pump or the steering linkage directly. The power steering function is therefore independent of engine speed, resulting in significant energy savings.How it wor
2、ks :Conventional power steering systems use an engine accessory belt to drive the pump, providing pressurized fluid that operates a piston in the power steering gear or actuator to assist the driver.In electro-hydraulic steering, one electrically powered steering concept uses a high efficiency pump
3、driven by an electric motor. Pump speed is regulated by an electric controller to vary pump pressure and flow, providing steering efforts tailored for different driving situations. The pump can be run at low speed or shut off to provide energy savings during straight ahead driving (which is most of
4、the time in most world markets).Direct electric steering uses an electric motor attached to the steering rack via a gear mechanism (no pump or fluid). A variety of motor types and gear drives is possible. A microprocessor controls steering dynamics and driver effort. Inputs include vehicle speed and
5、 steering, wheel torque, angular position and turning rate. Working In Detail:A steering sensor is located on the input shaft where it enters the gearbox housing. The steering sensor is actually two sensors in one: a torque sensor that converts steering torque input and its direction into voltage si
6、gnals, and a rotation sensor that converts the rotation speed and direction into voltage signals. An interface circuit that shares the same housing converts the signals from the torque sensor and rotation sensor into signals the control electronics can process.Inputs from the steering sensor are dig
7、ested by a microprocessor control unit that also monitors input from the vehicles speed sensor. The sensor inputs are then compared to determine how much power assist is required according to a preprogrammed force map in the control units memory. The control unit then sends out the appropriate comma
8、nd to the power unit which then supplies the electric motor with current. The motor pushes the rack to the right or left depending on which way the voltage flows (reversing the current reverses the direction the motor spins). Increasing the current to the motor increases the amount of power assist.T
9、he system has three operating modes: a normal control mode in which left or right power assist is provided in response to input from the steering torque and rotation sensors inputs; a return control mode which is used to assist steering return after completing a turn; and a damper control mode that
10、changes with vehicle speed to improve road feel and dampen kickback.If the steering wheel is turned and held in the full-lock position and steering assist reaches a maximum, the control unit reduces current to the electric motor to prevent an overload situation that might damage the motor. The contr
11、ol unit is also designed to protect the motor against voltage surges from a faulty alternator or charging problem.The electronic steering control unit is capable of self-diagnosing faults by monitoring the systems inputs and outputs, and the driving current of the electric motor. If a problem occurs
12、, the control unit turns the system off by actuating a fail-safe relay in the power unit. This eliminates all power assist, causing the system to revert back to manual steering. A dash EPS warning light is also illuminated to alert the driver. To diagnose the problem, a technician jumps the terminal
13、s on the service check connector and reads out the trouble codes. click here to see a bigger Electric power steering systems promise weight reduction, fuel savings and package flexibility, at no cost penalty.Europes high fuel prices and smaller vehicles make a fertile testbed for electric steering,
14、a technology that promises automakers weight savings and fuel economy gains. And in a short time, electric steering will make it to the U.S., too. Its just just a matter of time, says Aly Badawy, director of research and development for Delphi Saginaw Steering Systems in Saginaw, Mich. The issue was
15、 cost and thats behind us now. By 2002 here in the U.S. the cost of electric power steering will absolutely be a wash over hydraulic.Today, electric and hybrid-powered vehicles (EV), including Toyotas Prius and GMs EV-1, are the perfect domain for electric steering. But by 2010, a TRW Inc. internal
16、study estimates that one out of every three cars produced in the world will be equipped with some form of electrically-assisted steering. The Cleveland-based supplier claims its new steering systems could improve fuel economy by up to 2 mpg, while enhancing handling. There are true bottom-line benef
17、its as well for automakers by reducing overall costs and decreasing assembly time, since theres no need for pumps, hoses and fluids.Another claimed advantage is shortened development time. For instance, a Delphi group developed E-TUNE, a ride-and-handling software package that can be run off a lapto
18、p computer. They can take that computer and plug it in, attach it to the controller and change all the handling parameters - effort level, returnability, damping - on the fly, Badawy says. It used to take months. Delphi has one OEM customer that should start low-volume production in 99.Electric stee
19、ring units are normally placed in one of three positions: column-drive, pinion-drive and rack-drive. Which system will become the norm is still unclear. Short term, OEMs will choose the steering system that is easiest to integrate into an existing platform. Obviously, greater potential comes from de
20、signing the system into an all-new platform.We have all three designs under consideration, says Dr. Herman Strecker, group vice president of steering systems division at ZF in Schwaebisch Gmuend, Germany. Its up to the market and OEMs which version finally will be used and manufactured.The large man
21、ufacturers have all grabbed hold of what they consider a core technology, explains James Handysides, TRW vice president, electrically assisted steering in Sterling Heights, Mich. His company offers a portfolio of electric steering systems (hybrid electric, rack-, pinion-, and column-drive). TRW orig
22、inally concentrated on what it still believes is the purest engineering solution for electric steering-the rack-drive system. The system is sometimes refered to as direct drive or ball/nut drive.Still, this winter TRW hedged its bet, forming a joint venture with LucasVarity. The British supplier rec
23、eived $50 million in exchange for its electric column-drive steering technology and as sets. Initial production of the column and pinion drive electric steering systems is expected to begin in Birmingham, England, in 2000.What we lack is the credibility in the steering market, says Brendan Conner, m
24、anaging director, TRW/LucasVarity Electric Steering Ltd. The combination with TRW provides us with a good opportunity for us to bridge that gap. LucasVarity currently has experimental systems on 11 different vehicle types, mostly European. TRW is currently supplying its EAS systems for Ford and Chry
25、sler EVs in North America and for GMs new Opel Astra.In 1995, according to Delphi, traditional hydraulic power steering systems were on 7596 of all vehicles sold globally. That 37-million vehicle pool consumes about 10 million gallons in hydraulic fluid that could be superfluous, if electric steerin
26、g really takes off.The present invention relates to an electrically powered drive mechamsm for providing powered assistance to a vehicle steering mechanism. According to one aspect of the present invention, there is provided an electrically powered driven mechanism for providing powered assistance t
27、o a vehicle steering mechanism having a manually rotatable member for operating the steering mechanism, the drive mechanism including a torque sensor operable to sense torque being manually applied to the rotatable member, an electrically powered drive motor drivingly connected to the rotatable memb
28、er and a controller which is arranged to control the speed and direction of rotation of the drive motor in response to signals received from the torque sensor, the torque sensor including a sensor shaft adapted for connection to the rotatable member to form an extension thereof so that torque is tra
29、nsmitted through said sensor shaft when the rotatable member is manually rotated and a strain gauge mounted on the sensor shaft for producing a signal indicative of the amount of torque being transmitted through said shaft.Preferably the sensor shaft is non-rotatably mounted at one axial end in a fi
30、rst coupling member and is non-rotatably mounted at its opposite axial end in a second coupling member, the first and second coupling members being inter-engaged to permit limited rotation therebetween so that torque under a predetermined limit is transmitted by the sensor shaft only and so that tor
31、que above said predetermined limit is transmitted through the first and second coupling members.The first and second coupling members are preferably arranged to act as a bridge for drivingly connecting first and second portions of the rotating member to one another.Preferably the sensor shaft is of
32、generally rectangular cross-section throughout the majority of its length.Preferably the strain gauge includes one or more SAW resonators secured to the sensor shaft.Preferably the motor is drivingly connected to the rotatable member via a clutch.Preferably the motor includes a gear box and is conce
33、ntrically arranged relative to the rotatable member.Various aspects of the present invention will hereafter be described, with reference to the accompanying drawings, in which :Figure 1 is a diagrammatic view of a vehicle steering mechanism including an electrically powered drive mechanism according
34、 to the present invention,Figure 2 is a flow diagram illustrating interaction between various components of the drive mechanism shown in Figure 1 ,Figure 3 is an axial section through the drive mechanism shown in Figure 1, Figure 4 is a sectional view taken along lines IV-IV in Figure 3,Figure 5 is
35、a more detailed exploded view of the input drives coupling shown in Figure 3, andFigure 6 is a more detailed exploded view of the clutch showing in Figure 3. Referring initially to Figure 1 , there is shown a vehicle steering mechanism 10 drivingly connected to a pair of steerable road wheels The st
36、eering mechanism 10 shown includes a rack and pinion assembly 14 connected to the road wheels 12 via joints 15. The pinion(not shown) of assembly 14 is rotatably driven by a manually rotatable member in the form of a steering column 18 which is manually rotated by a steering wheel 19.The steering co
37、lumn 18 includes an electric powered drive mechanism 30 which includes an electric drive motor (not shown in Figure 1) for driving the pinion in response to torque loadings in the steering column 18 in order to provide power assistance for the operative when rotating the steering wheel 19.As schemat
38、ically illustrated in Figure 2, the electric powered drive mechanism includes a torque sensor20 which measures the torque applied by the steering column 18 when driving the pinion and supplies a signal to a controller 40. The controller 40 is connected to a drive motor 50 and controls the electric c
39、urrent supplied to the motor 50 to control the amount of torque generated by the motor 50 and the direction of its rotation.The motor 50 is drivingly connected to the steering column 18 preferably via a gear box 60, preferably an epicyclic gear box, and a clutch 70. The clutch 70 is preferably perma
40、nently engaged during normal operation and is operative under certain conditions to isolate drive from the motor 50 to enable the pinion to be driven manually through the drive mechanism 30. This is a safety feature to enable the mechanism to function in the event of the motor 50 attempting to drive
41、 the steering column too fast and/or in the wrong direction or in the case where the motor and/or gear box have seized.The torque sensor 20 is preferably an assembly including a short sensor shaft on which is mounted a strain gauge capable of accurately measuring strain in the sensor shaft brought a
42、bout by the application of torque within a predetermined range.Preferably the predetermined range of torque which is measured is 0-lONm; more preferably is about l-5Nm.Preferably the range of measured torque corresponds to about 0-1000 microstrain and the construction of the sensor shaft is chosen s
43、uch that a torque of 5Nm will result in a twist of less than 2 in the shaft, more preferably less than 1 .Preferably the strain gauge is a SAW resonator, a suitable SAW resonator being described in WO91/13832. Preferably a configuration similar to that shown in Figure 3 of WO91/13832 is utilised whe
44、rein twoSAW resonators are arranged at 45 to the shaft axis and at 90 to one another.Preferably the resonators operate with a resonance frequency of between 200-400 MHz and are arranged to produce a signal to the controller 40 of 1 MHz 500 KHz depending upon the direction of rotation of the sensor s
45、haft. Thus, when the sensor shaft is not being twisted due to the absence of torque, it produces a 1 MHz signal.When the sensor shaft is twisted in one direction it produces a signal between 1.0 to 1.5 MHz. When the sensor shaft is twisted in the opposite direction it produces a signal between 1.0 t
46、o 0.5 MHz. Thus the same sensor is able to produce a signal indicative of the degree of torque and also the direction of rotation of the sensor shaft.Preferably the amount of torque generated by the motor in response to a measured torque of between 0-10Nm is 0-40Nm and for a measured torque of betwe
47、en l-5Nm is 0-25Nm.Preferably a feed back circuit is provided whereby the electric current being used by the motor is measured and compared by the controller 40 to ensure that the motor is running in the correct direction and providing the desired amount of power assistance. Preferably the controlle
48、r acts to reduce the measured torque to zero and so controls the motor to increase its torque output to reduce the measured torque.A vehicle speed sensor (not shown) is preferably provided which sends a signal indicative of vehicle speed to the controller. The controller uses this signal to modify t
49、he degree of power assistance provided in response to the measured torque.Thus at low vehicle speeds maximum power assistance will be provided and a high vehicle speeds minimum power assistance will be provided.The controller is preferably a logic sequencer having a field programmable gate array for example a XC 4005 as supplied by Xilinx. Such a controller does not rely upon software and so is able to function more reliably in a car vehicle environment. It is envisaged that a logic se