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1、中文3000单词,2000 单词，1万英文字符本文来自于 http:/ conventional hydraulically-assisted power steering used in most cars is soon to be replaced with electric power steering. Already, many manufacturers are using electronically controlled hydraulic systems, while some car manufacturers have recently introduced purel
2、y electric systems to their vehicles. In addition to reducing engine loads, full electric power steering allows steering responsiveness to be automatically varied depending on speed, road conditions - and even the drivers ability!Traditional Hydraulic Power-Assisted SteeringHydraulic Power Assisted
3、Steering has been used in automotive applications for about 50 years. The systems use an engine-driven hydraulic pump, a control valve, steering cylinder and connecting hydraulic hoses. The pump is usually of a vane design with an integrated internal bypass. It is sized so that - even at idle rpm -
4、it delivers enough oil flow to provide a suitable degree of power assistance.The control valve uses a flexible torque-measuring device (such as a torsion bar, spiral spring or leaf spring) to convert the steering torque into a small control movement. This movement is transferred to a valve that regu
5、lates fluid flow to the power assistance mechanism. In rack and pinion steering, a double-ended hydraulic ram mounted parallel to the rack (within the rack assembly) is used, while recirculating ball systems incorporate the mechanism into the steering box. Note that in the Nissan system shown here,
6、the fluid reservoir is incorporated into the pump.A major problem with simple conventional hydraulic steering systems is that the assistance level is not reduced at high speeds, giving a lack of steering feel. American cars of the Fifties and Sixties were particularly well known for their feather-li
7、ght steering effort during parking, a characteristic which resulted in extreme vagueness at high speeds. To overcome this problem, most conventional hydraulic steering systems of the last few decades have incorporated mechanisms that reduce steering assistance as either engine speed increases, or (l
8、ess frequently) as road speed increases. The reason that engine speed was more commonly used as is that such a system can remain purely hydraulic, whereas using road speed as the control variable requires the use of an electronic system.Electronically-Controlled Hydraulic SteeringThe introduction of
9、 electronic speedometers - and subsequently, full engine management - meant that an electronic road speed signal became available, allowing the widespread use of electronically-controlled hydraulic steering systems. These vary steering effort depending on road speed and also, in some cases, other fa
10、ctors as well.A number of different hydraulic approaches to regulating steering assistance are used. These are:Flow ControlA solenoid valve is located on the discharge port of the hydraulic pump. Electronic control is used to control the solenoid valve opening, so regulating the fluid flow. The flow
11、 is reduced at high road speeds, reducing the degree of assistance provided.Cylinder BypassA solenoid valve and associated bypass line is located between the two chambers of the hydraulic cylinder, allowing the reduction of the pressure difference between them. The solenoid valve opening is controll
12、ed electronically, its opening being greater at high road speeds. This reduces the degree of assistance that is provided.Hydraulic Reaction ForceA hydraulic force is enabled that works against the power assistance. As speed increases, the reaction force is increased. Since fluid flow to the power cy
13、linder is not affected, the steering response rate can remain high without reductions occurring in feel.In their electronically-controlled hydraulic steering system, Hyundai use an ECU equipped with an 8-bit microprocessor. Two major inputs - vehicle speed and the speed of steering wheel movement -
14、are used. From these inputs, the ECU determines the driving condition and via a 3-dimensional look-up map, provides the appropriate current flow to a hydraulic solenoid valve.Three different driving conditions are recognised:Parking - maximum current is supplied to the solenoid valve, resulting in m
15、aximum steering assistance.High Speed - minimum current is supplied to the solenoid valve, resulting in minimum steering assistance.Evasive Steering - a large and sudden steering input causes the ECU to supply a current to the solenoid proportional to the speed of the steering input.There are plenty
16、 of speed-sensitive hydraulic steering systems around. This diagram shows the layout of a Mazda MX6 system where the degree of assistance is based on road speed and steering angle. (1) steering wheel; (2) steering shaft; (3) intermediate shaft; (4) steering gear assembly; (5) pressure hose; (6) return hose; (7) oil pump; (8) solenoid valve; (9) ECU; (10) reserve tank; (11) st
