王姝 張海川 趙軒 宋函錕
摘要:提出了一種融合車輛穩(wěn)定性的路徑跟蹤控制策略,以提高分布式驅(qū)動(dòng)電動(dòng)汽車在高速、低附著等危險(xiǎn)行駛工況下的路徑跟蹤精度和車輛穩(wěn)定性,該控制策略包括路徑跟蹤控制層、穩(wěn)定性控制器決策層、驅(qū)動(dòng)輪轉(zhuǎn)矩分配層。針對(duì)LQR路徑跟蹤控制器在高速大曲率工況下跟蹤精度不足的問題,采用閉環(huán)PID矯正駕駛員模型補(bǔ)償車輛前輪轉(zhuǎn)角,并設(shè)計(jì)穩(wěn)定性控制器用以跟蹤車輛理想?yún)⒖寄P?,基于模型預(yù)測(cè)控制算法決策出附加橫擺力矩,同時(shí)以輪胎負(fù)荷率最小為目標(biāo)優(yōu)化車輪驅(qū)動(dòng)轉(zhuǎn)矩分配。利用自主開發(fā)的分布式驅(qū)動(dòng)電動(dòng)試驗(yàn)車分別在高速高附著和高速低附著雙移線工況進(jìn)行試驗(yàn)。結(jié)果表明:相對(duì)于只運(yùn)用閉環(huán)PID矯正的LQR路徑跟蹤控制器進(jìn)行路徑跟蹤,車輛在干燥的混凝土路面以90 km/h速度行駛時(shí),融合車輛穩(wěn)定性的路徑跟蹤精度的橫向均方根誤差降低了29.7%;車輛在潮濕瀝青路面以70 km/h速度行駛時(shí),均方根誤差降低了10.3%。所提控制策略能夠提高車輛的路徑跟蹤精度,滿足車輛在危險(xiǎn)行駛工況下的橫擺穩(wěn)定性。
關(guān)鍵詞:汽車工程;分布式驅(qū)動(dòng)電動(dòng)汽車;路徑跟蹤;穩(wěn)定性控制
中圖分類號(hào):U461
DOI:10.3969/j.issn.1004-132X.2023.09.003
Research on Path Tracking Control Method of Distributed Drive ElectricVehicles with Integrated Stability
WANG Shu ZHANG Haichuan ZHAO Xuan SONG Hankun
Abstract: A path tracking control strategy with integrated vehicle stability was proposed to improve the path tracking precision and vehicle stability of distributed drive electric vehicles under dangerous driving conditions such as high speed and low adhesion conditions. The hierarchical structure path tracking control strategy with vehicle stability, including the path tracking control layer, the stability controller decision layer and the drive wheel torque distribution layer. To solve the problem of the lower accuracy of LQR path tracking controller under high-speed and large curvature conditions, a closed loop PID correction driver model was used to compensate the front wheel angle of the vehicles. In addition, the stability controller was designed to track the ideal reference model of the vehicles. The controller developed the model predictive control algorithm to generate additional yaw moment. Meanwhile, the controller realized the optimal distribution of the wheel drive torques with the objective of minimizing tire load rate. Based on the independently designed distributed drive electric test vehicle, the double lane change tests were carried out on high-speed high adhesion road surfaces and high-speed low adhesion road surfaces respectively. The results show that, when driving on dry concrete pavement at 90 km/h speed, the lateral root mean square errors of the path tracking precision with integrated dynamics stability reduce by 29.7%, compared to the LQR path tracking controller using only closed-loop PID correction for path tracking. When driving on wet asphalt pavement at 70 km/h speed, the lateral root mean square errors reduce by 10.3%. Therefore, the proposed path tracking control strategy with integrated vehicle stability of distributed drive electric vehicles may improve path tracking accuracy, ensuring yaw stability under extreme conditions effectively.
Key words: automotive engineering; distributed drive electric vehicle; path tracking; stability control
0 引言
路徑跟蹤控制是實(shí)現(xiàn)汽車智能化的關(guān)鍵技術(shù),它通過車輛狀態(tài)信息和預(yù)先規(guī)劃好的期望路徑,實(shí)時(shí)控制前輪轉(zhuǎn)角大小,確保車輛按照預(yù)期路徑行駛[1]。但汽車在高速、大曲率行駛時(shí),輪胎容易達(dá)到飽和狀態(tài),路徑跟蹤控制器決策出的前輪轉(zhuǎn)角很難滿足控制需求。同時(shí),在高速低附著路面下,汽車過彎時(shí)容易發(fā)生側(cè)滑、甩尾等危險(xiǎn)工況,導(dǎo)致車輛的路徑跟蹤精度進(jìn)一步降低。分布式驅(qū)動(dòng)電動(dòng)汽車的4個(gè)車輪驅(qū)動(dòng)/制動(dòng)力矩獨(dú)立可控,為直接橫擺力矩控制提高車輛安全性提供平臺(tái)[2],因此,分布式驅(qū)動(dòng)電動(dòng)汽車在進(jìn)行路徑跟蹤控制時(shí),可以綜合考慮車輛穩(wěn)定性及道路條件。
LEMAN等[3]基于三自由度非線性車輛動(dòng)力學(xué)模型設(shè)計(jì)了模型預(yù)測(cè)路徑跟蹤控制器,但沒有考慮車輛高速行駛穩(wěn)定性問題。張雷等[4]利用軌跡跟蹤與直接橫擺力矩協(xié)調(diào)控制設(shè)計(jì)了上層控制器以生成前輪轉(zhuǎn)角和目標(biāo)橫擺力矩,下層控制器分配4個(gè)車輪驅(qū)動(dòng)轉(zhuǎn)矩,提高了車輛在高速、低附著路面的路徑跟蹤精度和穩(wěn)定性。劉凱等[5]基于包絡(luò)線的輪胎滑移率約束條件設(shè)計(jì)了模型預(yù)測(cè)軌跡跟蹤控制器,滿足高速車輛在各種路面的行駛能力。GUO等[6]針對(duì)分布式驅(qū)動(dòng)自動(dòng)駕駛車輛,設(shè)計(jì)了一種協(xié)調(diào)路徑跟蹤和直接橫擺力矩控制的分層控制系統(tǒng),上層利用模型預(yù)測(cè)算法計(jì)算前輪轉(zhuǎn)角和橫擺力矩,下層利用偽逆算法分配車輪驅(qū)動(dòng)力矩。李軍等[7]通過融合車輛穩(wěn)定性來設(shè)計(jì)路徑跟蹤控制器,利用質(zhì)心側(cè)偏角相平面圖判斷穩(wěn)定性控制器的工作區(qū)域,從而將穩(wěn)定性約束條件從路徑跟蹤控制器中獨(dú)立出來,簡化了路徑跟蹤控制器的復(fù)雜度,提高了系統(tǒng)的工作計(jì)算效率和可靠性。
以上路徑跟蹤控制器的設(shè)計(jì)多是利用模型預(yù)測(cè)控制算法,能解決帶約束控制問題,但存在預(yù)測(cè)時(shí)域較大、耗時(shí)較長的缺點(diǎn)[8]。工業(yè)領(lǐng)域多采用LQR控制算法來跟蹤預(yù)期路徑。XU等[9]利用LQR控制算法設(shè)計(jì)了多點(diǎn)預(yù)瞄控制的車輛路徑跟蹤控制器,提高了路徑跟蹤精度,但沒有考慮車輛高速、大曲率轉(zhuǎn)彎時(shí)車輛行駛工況。陳亮等[10]通過LQR控制器計(jì)算車輪期望側(cè)偏力,利用刷子輪胎模型將其轉(zhuǎn)化為側(cè)偏角,保持了車輛與輪胎模型的非線性特性。此外,針對(duì)車輛的非線性動(dòng)力學(xué)特性,管欣等[11]設(shè)計(jì)了BP神經(jīng)網(wǎng)絡(luò)PID控制器復(fù)合校正模型,克服了輪胎滑移區(qū)跟蹤精度不足的缺點(diǎn)。LI等[12]利用LQR控制算法計(jì)算減小航向角偏差所得到的車輪轉(zhuǎn)角,與位置偏差得到的前輪轉(zhuǎn)角通過一定權(quán)重組合在一起,使車輛適應(yīng)高速、大曲率工況。
上述對(duì)LQR算法的修正并不能克服車輛在高速、低附著路面的側(cè)滑、甩尾等危險(xiǎn)行駛工況,故有必要引入穩(wěn)定性控制策略。HSU等[13]通過最優(yōu)分配車輪驅(qū)動(dòng)/制動(dòng)扭矩,提高車輛路徑跟蹤性能。WANG等[14-15]針對(duì)雙電機(jī)后輪驅(qū)動(dòng)電動(dòng)汽車,提出了基于模型預(yù)測(cè)控制算法的穩(wěn)定性控制策略,試驗(yàn)表明車輛穩(wěn)定性得到提高。但上述方法都只針對(duì)車輛穩(wěn)定性控制,未考慮前輪轉(zhuǎn)角。
針對(duì)上述問題,本文針對(duì)分布式驅(qū)動(dòng)電動(dòng)汽車設(shè)計(jì)了融合車輛穩(wěn)定性的路徑跟蹤模型。基于LQR算法設(shè)計(jì)具有預(yù)瞄特性的路徑跟蹤控制器,同時(shí)采用PID矯正環(huán)節(jié)補(bǔ)償車輛高速大曲率工況下輪胎滑移區(qū)的位置跟蹤偏差;通過設(shè)計(jì)直接橫擺力矩控制器,以整車質(zhì)心側(cè)偏角、橫擺角速度跟蹤理想二自由度參考模型為目標(biāo)計(jì)算附加橫擺力矩,并利用優(yōu)化算法分配車輪驅(qū)動(dòng)力矩來提高車輛在高速低附著路面上的橫擺穩(wěn)定性。
1 融合車輛穩(wěn)定性的路徑跟蹤控制策略
1.1 七自由度車輛動(dòng)力學(xué)模型
高速低附著路面上行駛,相對(duì)于整車側(cè)翻,更容易發(fā)生側(cè)滑、甩尾等危險(xiǎn)行駛工況,因?yàn)榈孛娓街鴹l件的限制,側(cè)滑的臨界條件相對(duì)于側(cè)翻更先達(dá)到,所以主要考慮縱向、橫向、橫擺3個(gè)自由度,同時(shí)考慮到分布式驅(qū)動(dòng)電動(dòng)汽車4個(gè)驅(qū)動(dòng)電機(jī)獨(dú)立可控,本文構(gòu)建七自由度車輛動(dòng)力學(xué)模型,如圖1所示。
式中,m為車輛質(zhì)量;vx、vy分別為車輛質(zhì)心的縱向、橫向速度;ωr為繞z軸的橫擺角速度;Iz為車輛繞z軸的轉(zhuǎn)動(dòng)慣量;a、b分別為車輛質(zhì)心到前后軸的距離;d為整車輪距;δf為汽車前輪轉(zhuǎn)角;Fxij、Fyij分別為4個(gè)車輪的縱向力、側(cè)偏力,ij=fl,fr,rl,rr分別表示左前輪、右前輪、左后輪、右后輪;Jw為車輪轉(zhuǎn)動(dòng)慣量;ωij為4個(gè)車輪的旋轉(zhuǎn)角速度;Tdij為4個(gè)輪胎的驅(qū)動(dòng)力矩;rr為車輪滾動(dòng)半徑。
5 試驗(yàn)驗(yàn)證分析
本文基于分布式驅(qū)動(dòng)電動(dòng)試驗(yàn)車,利用A&D5435半實(shí)物仿真系統(tǒng)及MATLAB/Simulink的代碼自動(dòng)生成技術(shù)搭建了分布式驅(qū)動(dòng)電動(dòng)汽車試驗(yàn)平臺(tái),對(duì)構(gòu)建的基于直接橫擺力矩控制的雙電機(jī)驅(qū)動(dòng)電動(dòng)汽車穩(wěn)定性控制系統(tǒng)進(jìn)行試驗(yàn)驗(yàn)證。A&D5435虛擬控制器替代車輛的整車控制器,控制器輸入信號(hào)包括轉(zhuǎn)向盤轉(zhuǎn)角、轉(zhuǎn)向盤角速度、轉(zhuǎn)向盤扭矩、油門踏板開度、制動(dòng)踏板開度、車速、輪速、電機(jī)扭矩、電機(jī)功率等,輸出信號(hào)包括電機(jī)驅(qū)動(dòng)扭矩、電機(jī)制動(dòng)扭矩、液壓制動(dòng)系統(tǒng)制動(dòng)扭矩。上述輸入信號(hào)可以通過以下傳感器獲得:使用安裝在轉(zhuǎn)向柱上的SensorWay公司的轉(zhuǎn)向盤扭矩、角度傳感器采集轉(zhuǎn)向盤轉(zhuǎn)角、轉(zhuǎn)向盤角速度、轉(zhuǎn)向盤扭矩;使用霍爾式非接觸速度傳感器采集4個(gè)車輪的轉(zhuǎn)速;采用Passat B5雙路加速踏板傳感器采集加速踏板開度;采用制動(dòng)踏板傳感器采集制動(dòng)踏板開度;采用三軸陀螺儀采集車輛的縱向加速度、側(cè)向加速度、橫擺角速度;電機(jī)轉(zhuǎn)速、扭矩、功率信息可以從電機(jī)控制器的CAN信號(hào)中獲取。車輛參數(shù)見表1?;贏&D5435的分布式驅(qū)動(dòng)電動(dòng)汽車試驗(yàn)平臺(tái)如圖6所示。
為了驗(yàn)證所提控制策略的效果,本文在高速高附著和高速低附著雙移線工況進(jìn)行試驗(yàn)。選擇ISO/ TR3888-1規(guī)定的標(biāo)準(zhǔn)雙移線試驗(yàn)軌跡作為參考路徑,如圖7所示。
文中模型1為本文中融合車輛穩(wěn)定性的路徑跟蹤控制器;模型2為閉環(huán)PID矯正的LQR路徑跟蹤控制器,只進(jìn)行橫向跟蹤控制;模型3為LQR橫向路徑跟蹤控制器,只進(jìn)行橫向跟蹤控制。
5.1 干燥的混凝土路面
選擇在附著系數(shù)為0.8的干燥的混凝土路面分別進(jìn)行車速為60,90 km/h的試驗(yàn),如圖8、圖9所示。
由圖8a、圖8b、圖9a、圖9b可以看出,車速為60 km/h時(shí),三種模型均可保證車輛跟蹤參考軌跡,僅在彎道處存在很小偏差,目標(biāo)路徑跟蹤效果良好,模型3最大橫向誤差為0.545 m;當(dāng)車速增加到90 km/h時(shí),模型3最大跟蹤偏差達(dá)到2.341 m,跑偏嚴(yán)重,模型1相對(duì)于模型2均方根誤差減小了29.7%,在減小車身姿態(tài)波動(dòng)的同時(shí),改善了路徑跟蹤精度。圖8c、圖9c表明,兩種車速下,模型1具有最小的前輪轉(zhuǎn)角,最大前輪轉(zhuǎn)角不超過4°,且轉(zhuǎn)角波動(dòng)平緩,有利于提高路徑跟蹤精度和操縱穩(wěn)定性。由圖8d、圖8e、圖9d、圖9g可以看出,60 km/h時(shí),3種模型橫擺角速度與質(zhì)心側(cè)偏角的值接近一致,趨于穩(wěn)定值;當(dāng)車速增加到90 km/h時(shí),模型1相對(duì)于模型2橫擺角速度和質(zhì)心側(cè)偏角幅值降低,說明高速行駛時(shí)模型1整車穩(wěn)定性最好。圖9e所示為90 km/h時(shí)下層控制器分配的四個(gè)車輪驅(qū)動(dòng)力矩,圖9f為相應(yīng)的輪胎負(fù)荷率,根據(jù)式(34),為了追求更高的穩(wěn)定性,前軸車輪利用率權(quán)重系數(shù)小于后軸,因此圖9e中車輪驅(qū)動(dòng)力矩方面前輪明顯高于后輪,且輪胎負(fù)荷率低于0.5,結(jié)果表明,驅(qū)動(dòng)力分配在保證足夠側(cè)向附著能力的同時(shí),提高了整車穩(wěn)定行駛能力。
5.2 潮濕瀝青路面
選擇附著系數(shù)為0.5的潮濕瀝青路面進(jìn)行車速為70 km/h的試驗(yàn),如圖10所示。
圖10a、圖10b表明,模型1控制器均方根誤差相對(duì)于模型2減小了10.3%,跟蹤效果最好,LQR控制的模型3在彎道處可能發(fā)生側(cè)滑,橫向誤差最大,達(dá)到1.415 m。由圖10c可以看出,相對(duì)于模型2、模型3,模型1的前輪轉(zhuǎn)角變化平緩,幅值最小,不超過3°,有利于在低附著路面行駛。圖10d、圖10g表明,模型1跟模型2、模型3相比,橫擺角速度、質(zhì)心側(cè)偏角波動(dòng)小、幅值低,提高了整車冰雪路面的行駛穩(wěn)定性。圖10e、圖10f所示分別為下層控制器分配的四個(gè)車輪驅(qū)動(dòng)力矩和對(duì)應(yīng)的輪胎負(fù)荷率,負(fù)荷率低于0.5,表示可以提供整車轉(zhuǎn)彎行駛所需側(cè)向力。
6 結(jié)語
在高速、大曲率或低附著路面下,采用融合穩(wěn)定性的路徑跟蹤器的車輛路徑跟蹤精度有明顯提高,且具有更好的行駛穩(wěn)定性。本文中LQR控制器的加權(quán)矩陣、預(yù)瞄時(shí)間為人工調(diào)試得到,對(duì)復(fù)雜工況適應(yīng)性差,危險(xiǎn)行駛工況的路徑跟蹤精度也可通過參數(shù)調(diào)節(jié)得到一定程度的改善。
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(編輯 陳 勇)
作者簡介:
王 姝,女,1991年生,高級(jí)工程師。研究方向?yàn)殡妱?dòng)汽車控制。
趙 軒(通信作者),男,1983年生,教授、博士研究生導(dǎo)師。E-mail:zhaoxuan@chd.edu.cn。
收稿日期:2022-02-22
基金項(xiàng)目:國家自然科學(xué)基金(52002034);陜西省科技重大專項(xiàng)(2020zdzx06-01-01);霍英東青年教師基金(171103);陜西省重點(diǎn)產(chǎn)業(yè)創(chuàng)新鏈(群)項(xiàng)目(2020ZDLGY16-01,2020ZDLGY16-02)