瞿濟偉,郭康權(quán),2※,高 華,宋樹杰,李翊寧,周 偉
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基于PWM信號的農(nóng)用柔性底盤驅(qū)動與轉(zhuǎn)向協(xié)同控制特性試驗
瞿濟偉1,郭康權(quán)1,2※,高 華1,宋樹杰3,李翊寧1,周 偉1
(1. 西北農(nóng)林科技大學(xué)機械與電子工程學(xué)院,楊凌 712100; 2. 陜西省農(nóng)業(yè)裝備工程技術(shù)研究中心,楊凌 712100; 3. 陜西師范大學(xué)食品工程與營養(yǎng)科學(xué)學(xué)院,西安 710119)
針對四輪獨立驅(qū)動獨立轉(zhuǎn)向的農(nóng)用柔性底盤驅(qū)動轉(zhuǎn)向時需要同時打開和鎖緊電磁摩擦鎖的矛盾,該文提出一種基于脈沖寬度調(diào)制信號(pulse width modulation,PWM)的電磁摩擦鎖控制方法來實現(xiàn)偏置轉(zhuǎn)向軸機構(gòu)的分時步進驅(qū)動和轉(zhuǎn)向,并利用自制偏置轉(zhuǎn)向軸試驗臺,采用雙因素試驗測試了PWM波頻率和占空比對偏置轉(zhuǎn)向軸電磁摩擦鎖脈沖鎖緊力矩的影響,采用三元二次正交旋轉(zhuǎn)組合試驗測試了分時步進驅(qū)動和轉(zhuǎn)向時頻率、占空比和輪轂電機轉(zhuǎn)速對轉(zhuǎn)向特性的影響。雙因素試驗結(jié)果表明:頻率、占空比及其交互作用對脈沖鎖緊力矩均有極顯著影響(<0.01);在頻率4~24 Hz、占空比20%~80%時,鎖緊力矩變化范圍為6.822~40.046 N·m;旋轉(zhuǎn)組合試驗結(jié)果表明:頻率、占空比、兩者交互作用及輪轂電機初始轉(zhuǎn)速對分時步進轉(zhuǎn)向時轉(zhuǎn)向平均角速度均有顯著影響(<0.05),轉(zhuǎn)向平均角速度隨占空比和輪轂電機初始轉(zhuǎn)速增大而減小,隨頻率增大而緩慢增大,在頻率4~24 Hz、占空比20%~80%、初始轉(zhuǎn)速30~120 r/min時,轉(zhuǎn)向平均角速度變化范圍為0~0.514 rad/s。該結(jié)論可為農(nóng)用柔性底盤驅(qū)動與轉(zhuǎn)向協(xié)同控制提供參考。
農(nóng)業(yè)機械;運輸;控制;柔性底盤;PWM技術(shù);偏置轉(zhuǎn)向軸;驅(qū)動與轉(zhuǎn)向;協(xié)同控制
目前國內(nèi)溫室、農(nóng)業(yè)倉儲設(shè)施等狹小封閉農(nóng)業(yè)環(huán)境急需運動靈活且無污染的動力機械[1-4]。農(nóng)用柔性底盤是一種四輪獨立驅(qū)動獨立轉(zhuǎn)向電動底盤,該底盤采用輪轂電機的轉(zhuǎn)向軸與輪胎回轉(zhuǎn)平面偏置的偏置轉(zhuǎn)向軸機構(gòu),將傳統(tǒng)底盤的轉(zhuǎn)向機構(gòu)和驅(qū)動機構(gòu)合二為一,使柔性底盤靈活地實現(xiàn)直行、橫行、斜行及原地回轉(zhuǎn)等運動形式,便于狹小封閉環(huán)境運行作業(yè)[5-7]。為了使輪轂電機的驅(qū)動力矩能夠傳遞到車架,該底盤在偏置轉(zhuǎn)向軸上設(shè)置一電磁摩擦鎖[8],驅(qū)動時鎖緊電磁摩擦鎖,轉(zhuǎn)向時打開電磁摩擦鎖。但是,當需要驅(qū)動和轉(zhuǎn)向同時進行時,便出現(xiàn)矛盾。課題組前期主要進行了柔性底盤固定姿勢運動的動力學(xué)特性[9]及控制策略[10]研究,未涉及驅(qū)動與轉(zhuǎn)向同時進行的運動控制。所以解決此矛盾的方法是柔性底盤運動控制中需要研究的重要問題之一。
在農(nóng)業(yè)領(lǐng)域,輪式移動機械轉(zhuǎn)向系統(tǒng)特性對其工作性能有重要影響[11]。目前關(guān)于輪式移動機械轉(zhuǎn)向系統(tǒng)的研究主要針對轉(zhuǎn)向靈活性、轉(zhuǎn)向精度及轉(zhuǎn)向穩(wěn)定性等展開[12-14]。房素素等[15]開發(fā)了一種線控液壓轉(zhuǎn)向系統(tǒng),采用電磁比例伺服閥控制轉(zhuǎn)向油缸,保證大動力輸出的同時仍具有較好轉(zhuǎn)向靈活性;張聞宇等[16]提出了摩擦輪式拖拉機轉(zhuǎn)向驅(qū)動系統(tǒng),以平行四連桿結(jié)構(gòu)作為驅(qū)動機構(gòu),減少了轉(zhuǎn)向響應(yīng)時間。魯植雄等[17]開發(fā)了雙通道PID轉(zhuǎn)向控制策略,降低拖拉機線控液壓轉(zhuǎn)向跟隨誤差;張京等[18]基于PID算法設(shè)計了轉(zhuǎn)向電機控制策略,提高了農(nóng)用輪式機器人四輪協(xié)同轉(zhuǎn)向控制精度;劉軍等[19]將線控轉(zhuǎn)向和GPS/INS組合導(dǎo)航技術(shù)結(jié)合,并利用轉(zhuǎn)向電機滑模變結(jié)構(gòu)控制提升了系統(tǒng)抗干擾能力。上述底盤很多具有大動力輸出,因結(jié)構(gòu)不同研究重點也各不相同。國外也有諸多研究,如文獻[20]研究了基于加速度傳感器的農(nóng)業(yè)機器人轉(zhuǎn)向模型,實現(xiàn)了慣性導(dǎo)航中轉(zhuǎn)角的獲取。Oksanen等[21]研究了4個非對稱液壓缸引導(dǎo)拖拉機各輪轉(zhuǎn)向的控制策略,實現(xiàn)了無桿轉(zhuǎn)向,使得轉(zhuǎn)向更加輕便;Tabile等[22]設(shè)計了一種農(nóng)業(yè)移動機器人,采用轉(zhuǎn)向電機結(jié)構(gòu)并結(jié)合GIS技術(shù),提升了其在復(fù)雜農(nóng)業(yè)環(huán)境的綜合工作性能。Ettefagh等[23]基于遺傳算法和人工神經(jīng)網(wǎng)絡(luò),對現(xiàn)有的四桿轉(zhuǎn)向機構(gòu)進行綜合優(yōu)化,減小了轉(zhuǎn)向誤差。綜上可知,傳統(tǒng)底盤的轉(zhuǎn)向控制大多涉及機械、液壓助力轉(zhuǎn)向和電機轉(zhuǎn)向[24-27],其轉(zhuǎn)向系統(tǒng)完全獨立于驅(qū)動系統(tǒng),不存在轉(zhuǎn)向與驅(qū)動協(xié)調(diào)的矛盾。
本文研究的偏置轉(zhuǎn)向軸機構(gòu),轉(zhuǎn)向與驅(qū)動動力均源自輪轂電機[28],電磁摩擦鎖雖使轉(zhuǎn)向與驅(qū)動不能同時刻進行,但為轉(zhuǎn)向與驅(qū)動協(xié)同進行提供了條件。故本文提出了一種基于脈沖寬度調(diào)制信號(pulse width modulation,PWM)的電磁摩擦鎖控制方法,使偏置轉(zhuǎn)向軸能夠分時步進驅(qū)動和步進轉(zhuǎn)向,實現(xiàn)轉(zhuǎn)向與驅(qū)動的協(xié)同,有望克服上述矛盾。為此,在自制偏置轉(zhuǎn)向軸試驗臺上搭建測試系統(tǒng),進行了模擬測試,以期獲取偏置轉(zhuǎn)向軸機構(gòu)驅(qū)動與轉(zhuǎn)向協(xié)同控制特性參數(shù),為柔性底盤的運動控制提供依據(jù)。
本文研究的農(nóng)用柔性底盤由4個相對獨立的偏置轉(zhuǎn)向軸機構(gòu)組成[29](圖1)。其中電磁摩擦鎖定片4、偏置轉(zhuǎn)向軸5及車架6三者固連,動片3與偏置臂2固連。定、動片吸合時,輪轂電機1的驅(qū)動力驅(qū)動車架6,定、動片釋放則使偏置臂2轉(zhuǎn)向。
1. 輪轂電機驅(qū)動輪2. 偏置臂 3. 電磁摩擦鎖動片 4. 電磁摩擦鎖定片 5. 偏置轉(zhuǎn)向軸 6. 車架
為使驅(qū)動與轉(zhuǎn)向協(xié)同進行,本文采用電磁摩擦鎖不斷吸合與釋放的方法來實現(xiàn)柔性底盤分時步進驅(qū)動與步進轉(zhuǎn)向。PWM信號高電平時電磁摩擦鎖吸合,輪轂電機驅(qū)動車架;低電平時電磁摩擦鎖釋放,輪轂電機繞偏置軸轉(zhuǎn)向;在連續(xù)PWM信號下,則可實現(xiàn)輪轂電機驅(qū)動輪分時步進式驅(qū)動與轉(zhuǎn)向。如圖2a所示,在PWM頻率(周期為)和輪轂電機轉(zhuǎn)速不變時,占空比大時,使偏置轉(zhuǎn)向軸每步轉(zhuǎn)動角度1小,故轉(zhuǎn)到目標角0所需步數(shù)多,轉(zhuǎn)向時間1長;占空比小時,使每步轉(zhuǎn)動角度2大,故轉(zhuǎn)到同一角度0所需步數(shù)少,轉(zhuǎn)向時間2短。故占空比越小,轉(zhuǎn)向越快。
由以上分析,在輪轂電機轉(zhuǎn)速不變條件下,偏置轉(zhuǎn)向軸驅(qū)動轉(zhuǎn)向需滿足以下條件:
1)電磁摩擦鎖吸合時的鎖緊力矩大于輪轂電機對車架的驅(qū)動力矩M,才能使輪轂電機的驅(qū)動力傳遞到車架。如圖2b所示,輪轂電機驅(qū)動時受到地面對輪胎的反作用力F,即
2)頻率和占空比的組合應(yīng)能夠使電磁摩擦鎖分時釋放,才能使輪轂電機繞偏置軸步進轉(zhuǎn)動,進而實現(xiàn)步進轉(zhuǎn)向。
因此,本研究的關(guān)鍵在于探明頻率、占空比及輪轂電機轉(zhuǎn)速等參數(shù)對電磁摩擦鎖鎖緊力矩和偏置轉(zhuǎn)向軸機構(gòu)轉(zhuǎn)向平均角速度的影響規(guī)律。
注:δ0為目標角度,(°);D1為較大的占空比,%;D2為較小的占空比,%;δ1代表D1時每步轉(zhuǎn)動角度,(°);δ2代表D2時每步轉(zhuǎn)動角度,(°);t1代表占空比為為D1時轉(zhuǎn)動δ0角度所需時間,s;t2代表占空比為D2時轉(zhuǎn)動δ0角度所需時間,s;T為PWM周期,s; Me為鎖緊力矩,N·m;Fx為地面切向反作用力,N; d為偏置距離,m。
通過前期試驗發(fā)現(xiàn),影響鎖緊力矩的因素有:PWM頻率、占空比及電磁摩擦鎖驅(qū)動電壓,其中電磁摩擦鎖驅(qū)動電壓與鎖緊力矩呈線性關(guān)系[10];影響轉(zhuǎn)向平均角速度的因素有頻率、占空比和輪轂電機初始轉(zhuǎn)速。故本試驗選取頻率、占空比以及輪轂電機初始轉(zhuǎn)速為試驗因素。
1)頻率:電磁摩擦鎖存在吸合響應(yīng)時間t和釋放延遲時間t,因此在一定頻率下,電磁摩擦鎖對PWM脈沖控制信號寬度的分辨存在一個最小值[30],頻率需低于某臨界值f,電磁摩擦鎖才能出現(xiàn)開關(guān)特性[31]。通過測試電磁摩擦鎖電流方法獲得t與t分別為19.12 ms與12.13 ms。依據(jù)式(2)[32]
可計算出頻率上限為24 Hz;由于頻率低于4 Hz時,釋放時轉(zhuǎn)向慣性沖擊大,故選取PWM頻率4~24 Hz。
2)占空比:通過大量電磁摩擦鎖鎖緊力矩檢測預(yù)試驗發(fā)現(xiàn),占空比在80%以上,電磁摩擦鎖出現(xiàn)一直鎖緊狀態(tài),無法轉(zhuǎn)向。占空比在20%以下時,電磁摩擦鎖鎖緊時間太短,無法傳遞驅(qū)動力矩;故選取占空比為20%~80%。
3)輪轂電機初始轉(zhuǎn)速:農(nóng)用柔性底盤為低速運動底盤,行駛速度在3.6 m/s以內(nèi)[10],依據(jù)運動學(xué)公式=2π(為輪轂電機轉(zhuǎn)速,為輪胎半徑),3.6 m/s對應(yīng)的輪轂電機轉(zhuǎn)速為120 r/min,考慮初始轉(zhuǎn)速不可能太低,參考Song等[33]的研究,輪轂電機初始轉(zhuǎn)速取30~120 r/min。
2.2.1 電磁摩擦鎖鎖緊力矩檢測
若以輪轂電機為動力檢測鎖緊力矩,只能測出輪轂電機對車架的驅(qū)動力矩,由式(1)知,而此驅(qū)動力矩會小于鎖緊力矩,故要測出鎖緊力矩特性,必須用外在動力為牽引才可測出鎖緊力矩峰值,故本文以液壓升降臺為動力牽引。偏置轉(zhuǎn)向軸機構(gòu)控制系統(tǒng)如圖3所示,中央控制器輸出PWM信號給PWM觸發(fā)開關(guān)模塊,開關(guān)模塊輸出矩形脈沖電壓作用于電磁摩擦鎖,使電磁摩擦鎖處于間歇吸合工作狀態(tài);然后啟動液壓升降臺勻速牽引圖4a所示的鋼絲繩,使其勻速水平拉動自制偏置轉(zhuǎn)向軸試驗臺[9]上“L”形杠桿2的牽引端轉(zhuǎn)動,杠桿中間通過力傳感器1(TJL-1,蚌埠天光傳感器公司,0~500 N,靈敏度1.33 mV/V)垂直拉動偏置臂(圖4a),牽引過程中車輪等的阻力被校正清除。試驗中偏置臂最大轉(zhuǎn)動角度40°,轉(zhuǎn)動速度約0.1 rad/s。用研華工控機(610H,研華科技公司)及數(shù)據(jù)采集卡(USB7648B,中泰研創(chuàng)科技公司)采集數(shù)據(jù),測得牽引力F,得到電磁摩擦鎖鎖緊力矩M(圖4b)。
圖3 偏置轉(zhuǎn)向軸機構(gòu)控制系統(tǒng)簡圖
1.力傳感器 2.“L”形杠桿 3.鋼絲繩 4.多圈電位器
1.Force sensor 2.“L”shaped lever 3.Wirerope 4. Multi turn potentiometer
注:F為鋼絲繩拉力,N;F為力傳感器拉力,N;F為支持力,N;M為鎖緊力矩,N·m;為偏置距離,m。
Note:Fis pulling force of wirerope,N;Fis pulling force of force sensor, N;Fis support force, N;Mrepresents tightening torque, N·m;is off-centered distance, m.
圖4 試驗臺實物圖與偏置臂牽引受力分析圖
Fig.4 Object of test bench and force diagram of off-centered arm
2.2.2 轉(zhuǎn)向平均角速度檢測
采用偏置轉(zhuǎn)向軸試驗臺,以輪轂電機為動力進行單輪轉(zhuǎn)向測試,檢測PWM信號控制下偏置轉(zhuǎn)向軸的轉(zhuǎn)向平均角速度特性。如圖5所示,啟動偏置軸試驗臺的輪轂電機和水平轉(zhuǎn)盤,依據(jù)轉(zhuǎn)速傳感器1、2(D046,龍戈電子,0~1 000 r/min)顯示的轉(zhuǎn)速,通過控制器調(diào)節(jié)水平轉(zhuǎn)盤與輪轂電機同速,因水平轉(zhuǎn)盤與輪轂電機的接觸處到各自回轉(zhuǎn)中心距離相等,故切線速度相同,模擬出勻速直線行駛狀態(tài)。在此狀態(tài)下,由控制器給輪轂電機一方向盤信號,使其相對水平轉(zhuǎn)盤加速,使偏置臂轉(zhuǎn)動,轉(zhuǎn)向目標角度為30°。精密多圈電位器(22HP-10,日本SAKAE公司,0~5 kΩ)檢測偏置臂轉(zhuǎn)動角度,數(shù)據(jù)采集系統(tǒng)的時鐘獲取轉(zhuǎn)向時間,進而計算出轉(zhuǎn)向平均角速度。
1. 轉(zhuǎn)速傳感器1 2. 轉(zhuǎn)盤電機 3. 轉(zhuǎn)速傳感器2 4. 水平轉(zhuǎn)盤 5. 制動盤 6. 多圈電位器 7. 支架
設(shè)計了雙因素五水平試驗測試頻率和占空比對鎖緊力矩的影響,頻率取4、9、14、19、24 Hz,占空比取20%、35%、50%、65%、80%,各5水平、共25組試驗,每組重復(fù)5次取均值。
設(shè)計了三元二次正交旋轉(zhuǎn)組合試驗測試頻率、占空比和輪轂電機轉(zhuǎn)速對轉(zhuǎn)向平均角速度的影響,其因素水平編碼如表1所示。共20組試驗,每組重復(fù)5次取均值。
表1 試驗因素與水平編碼
3.1.1 試驗結(jié)果方差分析
利用SPSS軟件進行進行方差分析的結(jié)果如表2所示。頻率、占空比以及兩者交互作用皆對鎖緊力矩有極顯著影響(<0.01)。同時可以判斷占空比頻率以及頻率和占空比交互作用對鎖緊力矩影響程度排序為:。
利用SPSS軟件進行回歸分析,首先將各因素的二次項以及交互項轉(zhuǎn)換為一次項,即將非線性回歸轉(zhuǎn)換為線性回歸,得出線性回歸方程后再轉(zhuǎn)為非線性方程[34]。本文用逐步回歸方式,得到回歸模型如式(3)所示,決定系數(shù)2=0.955,可見擬合相關(guān)程度較高,可用于預(yù)測電磁摩擦鎖鎖緊力矩在不同PWM波頻率和占空比下的變化情況。
表2 電磁摩擦鎖鎖緊力矩試驗結(jié)果方差分析
3.1.2 頻率和占空比對鎖緊力矩的影響
試驗過程中,不同頻率與不同占空比組合下的電磁摩擦鎖鎖緊力矩及轉(zhuǎn)角動態(tài)變化趨勢相似。舉其中一例如圖6a所示,在頻率4 Hz、占空比50%的PWM控制下,用液壓升降臺牽引偏置臂轉(zhuǎn)動,電磁摩擦鎖鎖緊力矩呈脈沖狀態(tài),偏置轉(zhuǎn)向軸轉(zhuǎn)角呈階梯式增大。脈沖峰值保持在30 N·m上下。
不同頻率與占空比下鎖緊力矩峰值變化如圖6b所示。占空比在20%~80%范圍內(nèi),鎖緊力矩隨著占空比的增大而增大。在占空比為80%時,電磁摩擦鎖鎖緊力矩峰值基本不隨頻率變化,保持在40 N·m上下;當占空比65%~35%時,鎖緊力矩先隨頻率增大而下降;當占空比為20%時,鎖緊力矩隨頻率增大下降到一定值后,接近不變。
總體上看,24 V額定電壓下,在頻率4~24 Hz、占空比20%~80%時,鎖緊力矩變化范圍為6.822~ 40.046 N·m,由圖6可知,通過改變頻率和占空比組合,可實現(xiàn)鎖緊力矩與輪轂電機驅(qū)動力矩的匹配并使偏置臂步進轉(zhuǎn)向。
3.2.1 旋轉(zhuǎn)組合試驗結(jié)果及分析
轉(zhuǎn)向過程中,通過獲取轉(zhuǎn)角從0~30°所用時間,計算出平均轉(zhuǎn)向角速度如表3所示。通過Design expert對表3的試驗結(jié)果進行三元二次回歸分析,建立轉(zhuǎn)向平均角速度與各個試驗因素的回歸模型方程為
回歸模型的方差分析如表4所示,由表4可知回歸模型是極顯著的(P<0.01);且失擬項不顯著(P>0.05),信噪比為25.255,大于4,可見模型較好,能夠用來預(yù)測偏置轉(zhuǎn)向軸機構(gòu)轉(zhuǎn)向平均角速度在不同頻率、占空比及輪轂電機初始轉(zhuǎn)速時的變化情況。
表3 轉(zhuǎn)向平均角速度旋轉(zhuǎn)組合試驗方案及結(jié)果
同時由表4可看到,對于轉(zhuǎn)向平均角速度,占空比、初始轉(zhuǎn)速的影響均極顯著(<0.01),頻率、頻率與占空比交互作用影響顯著(<0.05),其他交互作用的影響不顯著(>0.05),各因素對轉(zhuǎn)向平均角速度影響的主次順序為:占空比、初始轉(zhuǎn)速、頻率與占空比的交互作用、頻率。
表4 轉(zhuǎn)向平均角速度回歸模型方差分析
3.2.2 單因素影響分析
式(4)回歸模型中,將頻率、占空比以及輪轂電機初始轉(zhuǎn)速三因素中任兩者定于編碼中的0水平,便可觀察到單因素對轉(zhuǎn)向平均角速度的影響趨勢。采用此法得到單因素對轉(zhuǎn)向平均角速度影響的關(guān)系式為:
圖7為根據(jù)式(5)繪制出的單因素曲線,頻率在4~24 Hz時,轉(zhuǎn)向平均角速度隨著頻率的增加而呈現(xiàn)緩慢增加趨勢,且總體變化幅度在0.1 rad/s左右。故底盤工作中適當增加頻率可以增大偏置轉(zhuǎn)向軸機構(gòu)轉(zhuǎn)向平均角速度。
圖7 單因素對平均角速度的影響
由圖7b看到,占空比在20%~80%時,隨著占空比的增大,偏置轉(zhuǎn)向軸機構(gòu)轉(zhuǎn)向平均角速度呈現(xiàn)快速減小的趨勢,且占空比為80%時,轉(zhuǎn)向平均角速度為0,說明此時偏置轉(zhuǎn)向軸已無法轉(zhuǎn)動;當占空比為20%時,轉(zhuǎn)向平均角速度為0.465 rad/s,此時鎖緊力矩很小,不能傳遞驅(qū)動力??梢娬伎毡鹊挠行Х秶?0%~80%以內(nèi),工作時占空比可盡量在此范圍的中間值附近選擇。
由圖7c,輪轂電機初始轉(zhuǎn)速在30~120 r/min時,隨著轉(zhuǎn)速增大,轉(zhuǎn)向平均角速度呈減小趨勢,總體變化幅度在0.15 rad/s左右??梢娙嵝缘妆P的前進速度對轉(zhuǎn)向平均角速度有一定的限制作用。
總體上看,頻率在4~24 Hz內(nèi),占空比在20%~ 80%內(nèi),輪轂電機初始轉(zhuǎn)速在30~120 r/min內(nèi)時,偏置轉(zhuǎn)向軸機構(gòu)轉(zhuǎn)向平均角速度的變化范圍為0~0.514 rad/s,且調(diào)節(jié)頻率和占空比,可適應(yīng)偏置臂的不同步進轉(zhuǎn)向速度需求。
3.2.3 頻率與占空比交互作用影響分析
頻率和占空比的交互作用對轉(zhuǎn)向平均角速度的影響顯著,故利用Design expert做出偏置轉(zhuǎn)向軸機構(gòu)平均角速度關(guān)于頻率和占空比的響應(yīng)曲面(圖8),可直觀觀察頻率和占空比耦合作用對于轉(zhuǎn)向平均角速度的影響。由圖8可知,占空比較小越趨近于20%時,隨著頻率增大轉(zhuǎn)向平均角速度緩慢增大;當占空比越趨近于80%時,轉(zhuǎn)向平均角速度隨著頻率的增大而緩慢降低。
圖8 頻率和占空比的響應(yīng)曲面
綜上可知,PWM控制電磁摩擦鎖間歇吸合,實現(xiàn)了輪轂電機分時步進驅(qū)動與轉(zhuǎn)向,且通過改變PWM頻率和占空比,可滿足不同驅(qū)動力矩和轉(zhuǎn)向速度的需求,克服了驅(qū)動與轉(zhuǎn)向不能同時進行的矛盾。
本文利用PWM信號控制電磁摩擦鎖的方法,解決了偏置轉(zhuǎn)向軸機構(gòu)驅(qū)動與轉(zhuǎn)向不能同時進行的矛盾,通過對該方法下鎖緊力矩與轉(zhuǎn)向平均角速度特性的研究,得出以下結(jié)論:
1)PWM信號頻率、占空比及其交互作用對電磁摩擦鎖鎖緊力矩均有顯著影響(<0.05);在頻率4~24 Hz、占空比20%~80%時,鎖緊力矩范圍為6.822~ 40.046 N·m。
2)PWM信號占空比、輪轂電機初始轉(zhuǎn)速對轉(zhuǎn)向平均角速度影響極顯著(<0.01);頻率、頻率和占空比交互作用對轉(zhuǎn)向平均角速度影響顯著(<0.05);占空比影響效應(yīng)最明顯;在頻率4~24 Hz,占空比20%~ 80%、初始轉(zhuǎn)速30~120 r/min時,轉(zhuǎn)向平均角速度變化范圍為0~0.514 rad/s;且轉(zhuǎn)向平均角速度隨占空比增大而快速減小,隨初始轉(zhuǎn)速增大而減小,隨頻率增大而緩慢增大。
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Experiments on collaborative control characteristics of driving and steering for agricultural flexible chassis based on PWMsignal
Qu Jiwei1, Guo Kangquan1,2※, Gao Hua1, Song Shujie3, Li Yining1, Zhou Wei1
(1.712100,; 2.712100,; 3.710119,
Electromagnetic friction lock is an important part of agricultural flexible chassis. When it is closed, flexible chassis will be driven forward, and will steer when opened. In engineering practice, flexible chassis needs to steer while advancing. Therefore, there is conflict between the opening and the closing of electromagnetic friction lock when the 2 motions need to be carried out at the same time. In order to solve this problem, a method based on pulse width modulation (PWM) technology was proposed to control the opening and the closing in this paper. The opening was achieved during low level of PWM wave, while the closing was achieved during high level of PWM wave. In this way, flexible chassis can be driven forward during steering. Further, some experiments were conducted to investigate the influences of PWM frequency and duty cycle on driving and steering performance of flexible chassis. Firstly, based on off-centered steering shaft test bench, two-factor experiments were performed to study the effects of frequency and duty cycle on pulse tightening torque, using a traction device which was composed of a lever and a force sensor. Then tightening torque was calculated from force sensor measurement and arm length value. Secondly, for the purpose of examining the characteristics of time-sharing steering related with the influence of frequency, duty cycle and initial speed of electric wheel, a quadratic orthogonal regression experiment was conducted on off-centered steering shaft test bench. In this test, steering motion was simulated by controlling the speed of horizontal turntable of test bench and electric wheel. Average steering angular velocity was taken as the evaluating indicator of steering characteristics, and it can be attained by measuring the steering time due to the same target angle of steering. Afterwards, results of two-factor experiment showed that frequency, duty cycle and their interaction had highly significant influences on tightening torque (P<0.01). In the process of rotation, the curve of tightening torque showed a pulse change, and the rotation angle of steering arm displayed a step-like rise. When frequency was 4-24 Hz and duty cycle was 20%-80%, the tightening torque of off-centered steering shaft varied from 6.822 to 40.046 N·m. The tightening torque declined as frequency increased when duty cycle was 20%-80% except a few duty cycles. Meanwhile, for the frequency ranging from 4 to 24 Hz, tightening torque rose with the increasing of duty cycle. Then regression analysis was carried out according to the results and a regression model was presented. Results of quadratic orthogonal regression experiment illustrated that the average steering angular velocity was remarkably influenced by the PWM frequency, duty cycle and their interaction as well as the initial rotation speed of in-wheel motor (IWM) (P<0.05). With the increment of duty cycle and the initial speed of IWM, the steering average angular velocity decreased rapidly but slowly increased with the increasing of frequency. When the frequency was 4-24 Hz and the duty cycle was 20%-80%, and the initial speed of IWM was 30-120 r/min, the average steering angular velocity varied from 0 to 0.514 rad/s. Therefore, through changing the PWM frequency, duty cycle and the initial speed of IWM, the process of pulse-driven and time-sharing steering for flexible chassis is able to be well achieved, and these results can provide a basis for cooperative control of flexible chassis.
agricultural machinery; transportation; control; flexible chassis; pulse width modulation; off-centered steering shaft; driving and steering; cooperative control
瞿濟偉,郭康權(quán),高 華,宋樹杰,李翊寧,周 偉. 基于PWM信號的農(nóng)用柔性底盤驅(qū)動與轉(zhuǎn)向協(xié)同控制特性試驗[J]. 農(nóng)業(yè)工程學(xué)報,2018,34(7):75-81. doi:10.11975/j.issn.1002-6819.2018.07.010 http://www.tcsae.org
Qu Jiwei, Guo Kangquan, Gao Hua, Song Shujie, Li Yining, Zhou Wei. Experiments on collaborative characteristics of driving and steering for agricultural flexible chassis based on PWM signal[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2018, 34(7): 75-81. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2018.07.010 http://www.tcsae.org
2017-12-20
2018-03-05
國家自然科學(xué)基金資助項目(51375401)
瞿濟偉,湖北利川人,博士生,主要從事智能農(nóng)業(yè)裝備技術(shù)研究。Email:qujiwei_mail@foxmail.com
郭康權(quán),陜西西安人,教授,博士生導(dǎo)師,主要從事智能農(nóng)業(yè)裝備技術(shù)研究。Email:jdgkq@nwsuaf.edu.cn
10.11975/j.issn.1002-6819.2018.07.010
S229+.1;U463.1
A
1002-6819(2018)-07-0075-07