液壓式四輪轉(zhuǎn)向系統(tǒng)設(shè)計(jì)
液壓式四輪轉(zhuǎn)向系統(tǒng)設(shè)計(jì),液壓式四輪轉(zhuǎn)向系統(tǒng)設(shè)計(jì),液壓式,輪轉(zhuǎn),系統(tǒng),設(shè)計(jì)
黑龍江工程學(xué)院本科生畢業(yè)設(shè)計(jì)
附 錄
附錄A
Steering Behavior and Control Technology
0f Four—wheel Steering AutomobiIe
ABSTRACT
This paper analyses the characteristics of car four-wheel steering control to, summarizes the principle of construction four-wheel steering cars. Introduced the four-wheel steering system control strategy. Points out the four-wheel steering system control technology faces difficulties, and prospects the trend of its development.
Keywords: four-wheel steering; Steering characteristics; Working principle; Control; Development;
1. Introduction
With modern road traffic system and the development of modern automobile technology, people on the car's steering control performance and driving stability requirement is increasing day by day. As the most effective improve vehicle handling performance of a kind of active chassis control technology - four-wheel steering technology. In the 1980s began in cars get application, and as the modern automobile industry and continuous development. Car four-wheel steering means in turn, the rear car can be relative to body active steering, that the car can be up to four wheels role. To improve the car turned to maneuverability, handling stability and driving safety.
2. The car's steering four-wheel steering
2.1 4ws cars and 2ws steering process analysis
Ordinary two wheel steering can not only the front car around its own wheel rotation and relative to body deflection around the precision, then wheel rotation and not only deflection. When the driver turned the steering wheel, front wheel steering, changed directions face front tyre, produced a transverse force, through the front wheel effect on body, make the body yawing and produce centrifugal force, the rear wheels cornering, produced change direction, participate in the sports car to. And the rear and front car 4ws as, can rotation can also deflection. When the driver turned the steering wheel, front and rear wheels, that car almost at the same time to change direction, realize steering movement.
In turn, the 2ws car before active steering wheel rotation just makes passive steering. Obviously, 2ws cars in steering process from the steering wheel rotation to rear wheel steering movement in between the existence of certain lag time. This phase lag 2ws automobile steering characteristics make the servo sex becomes poor, and that the car steering radius increased. In addition, 2ws car at a high speed, the steering wheel Angle relative to certain incremental, body yaw-rate and lateral acceleration incremental increases, made the car at a high speed handling and stability becomes poor. And in turn, the car 4ws has done before, rear wheel steering, active in the process of steering, high sensitivity, fast response, effectively overcome the shortcomings.
2.2 4ws automobile steering way
According to the theory analysis and road test showed that a four-wheel steering can improve the maneuverability and steering control stability of high speed, modern 4ws car is developed according to the guiding ideology. Generally speaking, in the process of steering cars in 4ws, according to different driving conditions, front and rear wheel steering Angle should follow certain rules between. At present, the front, rear typical 4ws deflection rules generally are described below:
(a) inverse phase steering
As shown in figure A1 (a) shows, steering wheel Angle at low speeds or larger, front and rear inverse phase shift, realize that the rear wheels with the front deflect deflect instead, and with the steering wheel Angle deflection Angle increases and increases in a certain range. This redirection speed way can improve the manipulation of the car, reduce automobile portability turning radius, making cars maneuverability. Easy car turned turn, obstacle avoidance and driving, garage and parking. As for cars, if rear inverse phase 5 °, then turned can reduce minimum steering radius was about 0.5.
(b) phase with steering
As shown in figure A1 (b) shows, in a high-speed or steering wheel Angle is lesser, front and rear wheels with phase shift, realize that the rear wheels deflect the deflection direction with front wheel. Make the car body yaw-rate greatly reduce automobile body, can reduce the tendency of cornering undergo dynamic, guarantee the overtaking the highway, elevated highway, in and out, in the approach and flyovers insufficient steering state. Now, there are many 4ws cars take improving vehicle handling performance focuses on making cars on high-speed steering stability, without excessive demands in low-speed driving car course-altering maneuvers flexibility. The features is low by front wheel steering car only when only in car speeds, after up to a certain value, rear, only participate in steering phase with four-wheel steering.
（a）Reverse a steering （b）Coincidental a steering
Figure A1 four-wheel steering the front of the car, rear deflection rule
3. Four-wheel steering car components and working characteristics
4ws car is in front wheel steering system, and on the basis of suspension in cars on install a set of rear wheel steering system, both through certain questions of contact, making cars in front wheel steering also participate in the meanwhile, rear wheel steering. After decades of research and development, have molding 4ws car type has a variety of, composition, structure is different, working principle and control methods vary. The typical electric 4ws system mainly by the front wheel steering system, sensors, ECU, rear wheel steering actuators and rear wheel steering transmission mechanism etc. the sensor will front wheel steering motor sports the signal and the signal into ECU were analyzed and calculated, will process after the drive signal to rear wheel steering actuators, rear wheel steering actuators action, through the rear wheel steering transmission mechanism, rear wheel deflection. Meanwhile, ECU when carried out monitoring car operation condition, calculate target steering Angle and rear real-time steering Angle, the difference in value between real-time adjustment to rear wheels corner. Such, can according to the actual sports car, to realize the state car four-wheel steering. General 4ws cars equipped with two turning mode, can enter 4ws state, also can keep the traditional 2ws state, drivers can pass the driving indoor steering mode switch selection. When 4ws car in the course of driving the electronic control system malfunction, rear wheel automatic back into the middle, the automatic into the front wheel steering state, ensure like ordinary front wheel steering car driving safely as automobiles. Meanwhile, dashboard "4ws" indicator, warning drivers, stored on ECU fault condition of warranty, so that by yards.
4. Four-wheel steering control of the car
4ws system not only to realize the need for automotive steering movement, and guarantee the vehicle steering traveling stability. At present, in the research and development of automobile 4ws centered on improving car, the transient manipulation stability as a starting point, explore due to rear brought car in response to changes, and adopts various rear wheel control strategy and produce different effect. Auto four-wheel steering control depends on tires were lateral force, four-wheel steering can make cars in turn, the rear directly involved in for car yawing movement and lateralizing control. Through timely, accurate control of the rear wheel steering Angle, not only can shorten the process of steering the transient response, and can actively control cars the trajectory and attitude. In turn, that the car during its longitudinal direction centerline of the direction, even with the direction of the car horn decreases overlap, attitude Angle of steering bodywork cornering, improves the lateral stability car.
4.1 control goals
That car in turn to basic keep PianJiao zero gravity side car. So can greatly improve the car to the steering wheel input dynamic response characteristics, largely improved yaw-rate and lateral acceleration of transient performance indicators, lower the body posture change. From the PianJiao zero target start. According to certain control program is to realize the rear wheels to function is derived based four-wheel steering. Of course, the modern 4ws cars also have some other control goal request.
4.2 control strategy model based
The general situation of the studies are 4ws system based on a simple dof vehicle model of linear. This is just a kind of utopian mathematical model, when modeling overlooked in some of the dynamic parameters of the car without considering changes, automobile driving produces in the process of many of the random uncertainties and not very accurate. Early 4ws controller design are based on linear dynamic equation with the hypothesis, but due to the above reasons, make the design of control system doesn't meet practical needs, there is no guarantee that the vehicle steering control stability.
4.3 control method
Different cars driving performance requirements of steering and different models of the car wheels deflection 4ws law is not the same. Thus, different 4ws vehicle USES control method is endless and same, various control method has its emphases respectively. At present, in some molding 4ws with car on the control method basically has:
(a). The set than simply steering system 4ws.
(b). Than before, rear wheel steering front-wheel corner function 4ws system.
(c).Than before, rear wheel steering j speed function 4ws system is.
(d). The 4ws with first-order lagging system.
(e).With reversed-phase characteristics 4ws system
(f). With optimal control characteristics of 4ws system.
(g). Has since learning, white the ability to adjust the 4ws system.
Former five kinds of control system belong to classical control theory category, only to satisfy car under certain conditions, but also can't adapt to the need of motor sports of random changes, along with the computer technology and some advanced control theory of development, 4ws system will towards adaptive, intelligent direction.
4.4 4ws system control technology development
At present, for research and development of car 4ws remains constant developing and perfecting stage. Although researchers from structure to control principle of four-wheel steering on have done a lot of research, 4ws technology have made many progress. But, in the use of modern control theory of automotive steering control strategy choice of methods of determination and control, mainly depend on experience, the corresponding theoretical basis is still very lack, 4ws technology not really into wide use in commercial car on stage, no widely used.
In the technology in relatively mature 4ws cars, most use electric hydraulic power 4ws system. With the rapid development of electronic technology, computer technology in cars, the wide application of electronic electric 4ws system will be the trend of the development of automobile 4ws. Although in 4ws system research and development has made great development, however, as the core technical problems 4ws system - 4ws system controller design, what on earth as the best control objectives? Adopt what kind of control method? Still no in this field is the same view. Already, early before of the 4ws and the system based on a simple dof vehicle model, linear controller design are based on 4ws with linear dynamic equation hypothesis, PID control strategy. We know, 4ws system control mainly dependent on tires were transverse force. Early studies is the automobile tires as linear model, general 4ws control is based on the transverse force by tire wheel in proportion to the side PianJiao assumptions, this assumption just in the lateral acceleration smaller range effective. When in the lateral acceleration larger range, tire cornering characteristics of nonlinear area, will go to one side of the PianJiao tyres by transverse force in the response of the relationship of scale, and no longer the longitudinal force by tire vertical load, etc, have relations. In fact, car in turning down the street, tire basically all work in nonlinear area. At this time, garnish with linear control theory for research, appear reluctant to. 4ws vehicle handling dynamics problems is very complicated nonlinear multibody dynamics problems of the control system for 4ws research should be taken into account in the sports car, in-depth study of affecting its state response, establish simulation kinetic parameters of actual sports car mathematical model, adopt more effective control strategy. With the continuous development of control technology, some advanced modern control method has been applied in 4ws system control study, as optimal control and adaptive control, sliding mode control, robust control etc, in recent years, appeared again fuzzy control, based on artificial neural network theory, control method, etc. The study of 4ws control system from linear field gradually transition to nonlinear fields, some multi-degree-of-freedom car dynamics models have proposed 4ws, but mostly in infancy, is not mature. The future of the development of the system is studied 4ws trend for mainly concentrated :
(a). Further research and development of new rear wheel steering actuators and rear wheel steering transmission mechanism, improve the manipulation of steering portability, flexibility and steering Angle of accuracy.
(b). For 4ws system, further development, design high performance, high precision, high sensitivity sensor, in order to correctly detection car sports signal.
(c). Deep study turning the tyre during transient characteristics as the main factors, to join in the mathematic model of the system 4ws.
(d). The control theory and control into method is applied to the 4ws controller study.
附錄B
四輪轉(zhuǎn)向汽車的轉(zhuǎn)向特性及控制技術(shù)
摘要
本文分析比較了四輪轉(zhuǎn)向汽車的轉(zhuǎn)向特點(diǎn)，概述了電控四輪轉(zhuǎn)向汽車的結(jié)構(gòu)原理。介紹了四輪轉(zhuǎn)向系統(tǒng)的控制策略。指出了四輪轉(zhuǎn)向系統(tǒng)控制技術(shù)所面臨的困難，并展望其發(fā)展趨勢(shì)。
關(guān)鍵詞：四輪轉(zhuǎn)向；轉(zhuǎn)向特點(diǎn)；工作原理；控制；發(fā)展；
1.引言
隨著現(xiàn)代道路交通系統(tǒng)和現(xiàn)代汽車技術(shù)的發(fā)展，人們對(duì)汽車的轉(zhuǎn)向操縱性能和行駛穩(wěn)定性的要求日益提高。作為改善汽車操縱性能最有效的一種主動(dòng)底盤(pán)控制技術(shù)——四輪轉(zhuǎn)向技術(shù)。于二十世紀(jì)80年代中期開(kāi)始在汽車上得到應(yīng)用，并伴隨著現(xiàn)代汽車工業(yè)的發(fā)展而不斷發(fā)展。汽車的四輪轉(zhuǎn)向是指汽車在轉(zhuǎn)向時(shí)，后輪可相對(duì)于車身主動(dòng)轉(zhuǎn)向，使汽車的四個(gè)車輪都能起轉(zhuǎn)向作用。以改善汽車的轉(zhuǎn)向機(jī)動(dòng)性、操縱穩(wěn)定性和行駛安全性。
2.四輪轉(zhuǎn)向汽車的轉(zhuǎn)向特性
2.1 4ws汽車與2ws汽車轉(zhuǎn)向過(guò)程分析
普通兩輪轉(zhuǎn)向汽車的前輪既可繞自身的輪軸自轉(zhuǎn)又可繞主銷相對(duì)于車身偏轉(zhuǎn)，而后輪只能自轉(zhuǎn)而不偏轉(zhuǎn)。當(dāng)駕駛員轉(zhuǎn)動(dòng)方向盤(pán)后，前輪轉(zhuǎn)向，改變了行駛方向，地面對(duì)前輪胎產(chǎn)生一個(gè)橫向力，通過(guò)前輪作用于車身，使車身橫擺，產(chǎn)生離心力，使后輪產(chǎn)生側(cè)偏，改變前進(jìn)方向，參與汽車的轉(zhuǎn)向運(yùn)動(dòng)。而4ws汽車的后輪與前輪一樣，既可自轉(zhuǎn)也能偏轉(zhuǎn)。當(dāng)駕駛員轉(zhuǎn)動(dòng)方向盤(pán)后，前、后輪幾乎同時(shí)轉(zhuǎn)向，使汽車改變前進(jìn)方向，實(shí)現(xiàn)轉(zhuǎn)向運(yùn)動(dòng)。
2ws汽車在轉(zhuǎn)向時(shí)，前輪作主動(dòng)轉(zhuǎn)向，后輪只是作被動(dòng)轉(zhuǎn)向。顯然，2ws汽車在轉(zhuǎn)向過(guò)程中，從方向盤(pán)轉(zhuǎn)動(dòng)到后輪參與轉(zhuǎn)向運(yùn)動(dòng)之間存在一定的滯后時(shí)間。2ws汽車的這種相位滯后特性使汽車轉(zhuǎn)向的隨動(dòng)性變差，并使汽車的轉(zhuǎn)向半徑增大。另外，2ws汽車在高速行駛時(shí)，相對(duì)于一定的方向盤(pán)轉(zhuǎn)角增量、車身的橫擺角速度和橫向加速度的增量增大，使汽車在高速行駛時(shí)的操縱性和穩(wěn)定性變差。而4ws汽車在轉(zhuǎn)向時(shí)，前、后輪都作主動(dòng)轉(zhuǎn)向，在轉(zhuǎn)向過(guò)程中，靈敏度高，響應(yīng)快，有效地克服了上述缺點(diǎn)。
2.2 4ws汽車的轉(zhuǎn)向方式
根據(jù)理論分析研究和大量路試表明，四輪轉(zhuǎn)向能夠提高汽車轉(zhuǎn)向的機(jī)動(dòng)靈活性和高速行駛時(shí)的操縱穩(wěn)定性，現(xiàn)代4ws汽車就是根據(jù)這一指導(dǎo)思想研制的。一般來(lái)說(shuō)，4ws汽車在轉(zhuǎn)向過(guò)程中，根據(jù)不同的行駛條件，前、后輪轉(zhuǎn)向角之間應(yīng)遵循一定的規(guī)律。目前，典型4ws汽車前、后輪的偏轉(zhuǎn)規(guī)律一般如下所述。
(a)逆相位轉(zhuǎn)向
如圖B1 (a)所示，在低速行駛或者方向盤(pán)轉(zhuǎn)角較大時(shí)，前、后輪實(shí)現(xiàn)逆相位轉(zhuǎn)向，即后輪的偏轉(zhuǎn)方向與前輪的偏轉(zhuǎn)方向相反，且偏轉(zhuǎn)角度隨方向盤(pán)轉(zhuǎn)角增大而在一定范圍內(nèi)增大。這種轉(zhuǎn)向方式可改善汽車低速時(shí)的操縱輕便性，減小汽車的轉(zhuǎn)彎半徑，提高汽車的機(jī)動(dòng)靈活性。便于汽車掉頭轉(zhuǎn)彎、避障行駛、進(jìn)出車庫(kù)和停車場(chǎng)。對(duì)轎車而言，若后輪逆相位轉(zhuǎn)向5°，則可減少最小轉(zhuǎn)向半徑約0 5m。
(b)同相位轉(zhuǎn)向
如圖B1(b)所示，在中、高速行駛或方向盤(pán)轉(zhuǎn)角較小時(shí)，前、后輪實(shí)現(xiàn)同相位轉(zhuǎn)向，即后輪的偏轉(zhuǎn)方向與前輪的偏轉(zhuǎn)方向相同。使汽車車身的橫擺角速度大大減小，可減小汽車車身發(fā)生動(dòng)態(tài)側(cè)偏的傾向，保證汽車在高速超車、進(jìn)出高速公路、高架引橋及立交橋時(shí)，處于不足轉(zhuǎn)向狀態(tài)。現(xiàn)在，有許多4ws汽車把改善汽車操縱性能的重點(diǎn)放在提高汽車高速行駛的操縱穩(wěn)定性上，而不過(guò)分要求汽車在低速行駛的轉(zhuǎn)向機(jī)動(dòng)靈活性。其工作特點(diǎn)是低速時(shí)汽車只采用前輪轉(zhuǎn)向，只在汽車行駛速度達(dá)到一定數(shù)值后，后輪才參與轉(zhuǎn)向，進(jìn)行同相位四輪轉(zhuǎn)向。
（a）逆向位轉(zhuǎn)向 （b）同向位轉(zhuǎn)向
圖B1 四輪轉(zhuǎn)向汽車的前、后輪偏轉(zhuǎn)規(guī)律
3. 四輪轉(zhuǎn)向汽車的組成及工作特性
4ws汽車是在前輪轉(zhuǎn)向系統(tǒng)的基礎(chǔ)上，在汽車的后懸架上安裝一套后輪轉(zhuǎn)向系統(tǒng)，兩者之問(wèn)通過(guò)一定的方式聯(lián)系，使得汽車在前輪轉(zhuǎn)向的同時(shí)，后輪也參與轉(zhuǎn)向。經(jīng)過(guò)幾十年的研究與開(kāi)發(fā)，已經(jīng)成型的4ws汽車類型有多種，組成、結(jié)構(gòu)不同，控制方式及工作原理也各異。典型的電控4ws系統(tǒng)主要由前輪轉(zhuǎn)向系統(tǒng)、傳感器、ECU、后輪轉(zhuǎn)向執(zhí)行機(jī)構(gòu)和后輪轉(zhuǎn)向傳動(dòng)機(jī)構(gòu)等組成。如圖B2所示，轉(zhuǎn)向時(shí)，傳感器將前輪轉(zhuǎn)向的信號(hào)和汽車運(yùn)動(dòng)的信號(hào)送入ECU進(jìn)行分析計(jì)算，將處理后的驅(qū)動(dòng)信號(hào)傳給后輪轉(zhuǎn)向執(zhí)行機(jī)構(gòu)，后輪轉(zhuǎn)向執(zhí)行機(jī)構(gòu)動(dòng)作，通過(guò)后輪轉(zhuǎn)向傳動(dòng)機(jī)構(gòu), 后輪偏轉(zhuǎn)。同時(shí)，ECU進(jìn)行時(shí)監(jiān)控汽車運(yùn)行狀況，計(jì)算目標(biāo)轉(zhuǎn)向角與后輪實(shí)時(shí)轉(zhuǎn)向角之間的差值，來(lái)實(shí)時(shí)調(diào)整后輪的轉(zhuǎn)角。這樣，可以根據(jù)汽車的實(shí)際運(yùn)動(dòng)狀態(tài)，實(shí)現(xiàn)汽車的四輪轉(zhuǎn)向。一般的4ws汽車設(shè)有兩種轉(zhuǎn)向模式，既可進(jìn)入4ws狀態(tài)，也可保持傳統(tǒng)的2ws狀態(tài)，駕駛員可通過(guò)駕駛室內(nèi)的轉(zhuǎn)向模式開(kāi)關(guān)進(jìn)行選擇。當(dāng)4ws汽車在行駛過(guò)程中電子控制系統(tǒng)出現(xiàn)故障時(shí)，后輪自動(dòng)回到中間位置，汽車自動(dòng)進(jìn)入前輪轉(zhuǎn)向狀態(tài)，保證汽車像普通前輪轉(zhuǎn)向汽車一樣安全地行駛。同時(shí)，儀表板上的“4ws”指示燈亮，警告駕駛員，故障情況被存儲(chǔ)在ECU中，以便于維修時(shí)檢碼。
圖B2 4ws控制工作原理圖
4．四輪轉(zhuǎn)向汽車的控制
4ws系統(tǒng)既要實(shí)現(xiàn)汽車轉(zhuǎn)向時(shí)所需的運(yùn)動(dòng)，又要保證汽車轉(zhuǎn)向時(shí)的行駛穩(wěn)定性。目前，在4ws汽車的研究和開(kāi)發(fā)方面，主要是以改善汽車的瞬態(tài)操縱穩(wěn)定性為出發(fā)點(diǎn)，探索由于后輪參與轉(zhuǎn)向而帶來(lái)的汽車響應(yīng)變化，以及采用各種后輪控制策略而產(chǎn)生的不同效果。汽車四輪轉(zhuǎn)向的控制依賴于輪胎所受的側(cè)向力，四輪轉(zhuǎn)向能使汽車在轉(zhuǎn)向時(shí)，后輪直接參與對(duì)汽車橫擺運(yùn)動(dòng)和側(cè)向運(yùn)動(dòng)的控制。通過(guò)適時(shí)、精確地控制后輪的轉(zhuǎn)向角度，不僅可縮短轉(zhuǎn)向過(guò)程的瞬態(tài)響應(yīng)，而且能主動(dòng)地控制汽車的運(yùn)動(dòng)軌跡和姿態(tài)。在轉(zhuǎn)向過(guò)程中，使汽車的前進(jìn)方向與其縱向中心線的方向一致，即使得汽車的方向角與姿態(tài)角重合，減小轉(zhuǎn)向時(shí)車體的側(cè)偏，提高了汽車的側(cè)向穩(wěn)定性。
4.1控制目標(biāo)
使汽車在轉(zhuǎn)向時(shí)能夠基本保持汽車重心側(cè)偏角為零。這樣能夠大幅度提高汽車對(duì)方向盤(pán)輸人的動(dòng)態(tài)響應(yīng)特性，很大程度上改善了橫擺角速度和側(cè)向加速度的瞬態(tài)性能指標(biāo)，降低車身姿態(tài)的變化。從側(cè)偏角為零的目標(biāo)出發(fā)。按照一定的控制程序?qū)С龊筝嗈D(zhuǎn)向函數(shù)是實(shí)現(xiàn)四輪轉(zhuǎn)向的基礎(chǔ)。當(dāng)然，現(xiàn)代的4ws汽車也有一些其它控制目標(biāo)的要求。
4.2控制策略的模型基礎(chǔ)
一般情況下進(jìn)行的4ws系統(tǒng)的研究都是基于一個(gè)簡(jiǎn)單的二自由度線性車輛模型。這只是一種理想化的數(shù)學(xué)模型，在建模時(shí)忽略了汽車的一些動(dòng)力學(xué)參數(shù)的變化，沒(méi)有考慮汽車行駛過(guò)程中產(chǎn)生的許多隨機(jī)的、不確定因素，因而不是非常精確的。早期的4ws控制器設(shè)計(jì)都是基于跟隨線性動(dòng)力學(xué)方程的假設(shè)，但由于上述原因，使得所設(shè)計(jì)的控制系統(tǒng)不一定滿足實(shí)際的需要，無(wú)法保證汽車轉(zhuǎn)向時(shí)的操縱穩(wěn)定性。
4.3控制方法
不同的汽車對(duì)轉(zhuǎn)向行駛性能的要求不同，不同車型的4ws汽車的車輪偏轉(zhuǎn)規(guī)律也不一樣。因此，不同的4ws汽車所采用的控制方法不盡相同，各種控制方法分別有其側(cè)重點(diǎn)。目前，用在一些成型的4ws汽車上的控制方法主要有：
(a). 定前后輪轉(zhuǎn)向比的4ws系統(tǒng)。
(b)．前、后輪轉(zhuǎn)向比是前輪轉(zhuǎn)角函數(shù)的4ws系統(tǒng)。
(c). 前、后輪轉(zhuǎn)向比是車速函數(shù)的4ws系統(tǒng)。
(d)．具有一階滯后的4ws系統(tǒng)。
(e)．具有反相特性的4ws系統(tǒng)。
(f). 具有最優(yōu)控制特性的4ws系統(tǒng)。
(g). 具有自學(xué)習(xí)、白適應(yīng)能力的4ws系統(tǒng)。
前五種控制系統(tǒng)屬于古典控制理論范疇，只能滿足汽車在某些特定條件下的需要，還不能適應(yīng)汽車運(yùn)動(dòng)的隨機(jī)變化，隨著計(jì)算機(jī)技術(shù)和一些先進(jìn)控制理論的發(fā)展，4ws系統(tǒng)將朝著自適應(yīng)、智能化的方向發(fā)展。
4.4 4ws系統(tǒng)控制技術(shù)的發(fā)展
目前，對(duì)于4ws汽車的研究和開(kāi)發(fā)仍處于不斷發(fā)展和完善階段。盡管科研人員從結(jié)構(gòu)到控制原理上對(duì)四輪轉(zhuǎn)向進(jìn)行了大量的研究, 4ws 技術(shù)已取得不少進(jìn)展。但是，在運(yùn)用現(xiàn)代控制理論進(jìn)行汽車轉(zhuǎn)向控制策略的確定和控制方法的選擇時(shí)，主要是依靠經(jīng)驗(yàn)，相應(yīng)的理論依據(jù)還很缺乏，4ws技術(shù)沒(méi)有真正步入普及應(yīng)用階段，在商用汽車上沒(méi)有得到廣泛應(yīng)用。
在技術(shù)相對(duì)成熟的4ws汽車中, 大多數(shù)采用電控液壓動(dòng)力4ws系統(tǒng)。隨著電子技術(shù)的飛速發(fā)展，計(jì)算機(jī)技術(shù)在汽車中的廣泛應(yīng)用，電控電動(dòng)4ws系統(tǒng)將是4ws汽車的發(fā)展趨勢(shì)。雖然在4ws系統(tǒng)的研究和開(kāi)發(fā)方面已經(jīng)取得了很大的發(fā)展，但是，作為4ws系統(tǒng)的核心技術(shù)問(wèn)題——4ws系統(tǒng)控制器的設(shè)計(jì)，究竟以什么作為最佳的控制目標(biāo)?采用什么樣的控制方法?在該研究領(lǐng)域仍然沒(méi)有較為一致的看法。前已述及，早期進(jìn)行的4ws系統(tǒng)的研究都是基于一個(gè)簡(jiǎn)單的二自由度線性車輛模型，4ws控制器設(shè)計(jì)都是基于跟隨線性動(dòng)力學(xué)方程的假設(shè)，采用PID控制策略。我們知道，4ws系統(tǒng)的控制主要依賴于輪胎所受的橫向力。早期的研究是將汽車輪胎看成線性進(jìn)行建模的, 一般的4ws控制也就基于輪胎所受的橫向力比例于車輪側(cè)偏角的假設(shè)，這種假設(shè)只是在橫向加速度較小的范圍內(nèi)有效。當(dāng)在橫向加速度較大的范圍內(nèi)時(shí)，輪胎的側(cè)偏特性將進(jìn)人非線性區(qū)域，輪胎側(cè)偏角對(duì)輪胎所受橫向力的響應(yīng)不再呈比例關(guān)系，與輪胎所受的縱向力、垂直載荷等都有關(guān)系。實(shí)際上，汽車在轉(zhuǎn)彎行駛時(shí)，輪胎基本上都工作在非線性區(qū)域。此時(shí)，再用線性控制理論來(lái)進(jìn)行研究，就顯得勉為其難。4ws汽車操縱動(dòng)力學(xué)問(wèn)題是非常復(fù)雜的非線性多體動(dòng)力學(xué)問(wèn)題，對(duì)于4ws控制系統(tǒng)的研究應(yīng)綜合考慮汽車的運(yùn)動(dòng)情況，深入研究影響其狀態(tài)響應(yīng)的各種動(dòng)力學(xué)參數(shù)，建立模擬汽車實(shí)際運(yùn)動(dòng)情況的數(shù)學(xué)模型，采取更有效的控制策略。隨著控制技術(shù)的不斷發(fā)展，一些先進(jìn)的現(xiàn)代控制方法已經(jīng)被應(yīng)用于4ws系統(tǒng)的控制研究中，如最優(yōu)控制、自適應(yīng)控制、滑?？刂?、魯棒控制等，近年來(lái)，又出現(xiàn)了模糊控制、基于人工神經(jīng)網(wǎng)絡(luò)理論的控制方法等。對(duì)4ws控制系統(tǒng)的研究逐漸從線性領(lǐng)域向非線性領(lǐng)域過(guò)渡，一些多自由度的4ws汽車動(dòng)力學(xué)模型已有提出，但大多處于研究的初級(jí)階段，尚不成熟。未來(lái)對(duì)4ws系統(tǒng)的研究的發(fā)展趨勢(shì)主要集中為：
(a)．進(jìn)一步研究、開(kāi)發(fā)新型的后輪轉(zhuǎn)向執(zhí)行機(jī)構(gòu)和后輪轉(zhuǎn)向傳動(dòng)機(jī)構(gòu)，提高轉(zhuǎn)向時(shí)的操縱輕便性、靈活性和轉(zhuǎn)向角度的準(zhǔn)確性。
(b)．針對(duì)4ws系統(tǒng)，進(jìn)一步開(kāi)發(fā)、設(shè)計(jì)高性能、高精度、高靈敏度的傳感器，以便于正確地檢測(cè)汽車的運(yùn)動(dòng)信號(hào)。
(c)．深人研究轉(zhuǎn)向過(guò)程中輪胎的瞬態(tài)特性，將其作為主要因素加入到4ws系統(tǒng)的數(shù)學(xué)模型中。
(d).進(jìn)的控制理論與控制方法應(yīng)用于4ws控制器的研究中。
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