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2007年IEEE的程序 機(jī)器人與仿生學(xué)國(guó)際會(huì)議 2007年12月15日-18，三亞，中國(guó) 割草機(jī)器人多傳感器融合與導(dǎo)航技術(shù)的研究 從明和房波 大連理工大學(xué)機(jī)械工程學(xué)院 大連，116024，中國(guó) congm@dlut.edu.cn 引言——本文提出了一種多傳感器系統(tǒng)從超聲波傳感器和導(dǎo)航相結(jié)合的測(cè)量機(jī)器人割草機(jī)。利用傳感系統(tǒng)使機(jī)器人割草機(jī)來(lái)映射未知的環(huán)境。對(duì)于自動(dòng)割草機(jī)器人能在未知的環(huán)境中進(jìn)行定位和導(dǎo)航執(zhí)行割草任務(wù)是很重要的。由于環(huán)境的復(fù)雜性，簡(jiǎn)單的一種傳感器是不足夠割草機(jī)器人來(lái)完成這些任務(wù)。我們開發(fā)了一個(gè)配有DSPTMS320F2812作為CPU割草機(jī)器人。感測(cè)系統(tǒng)集成由超聲波傳感器，紅外傳感器，碰撞傳感器，編碼器，一個(gè)溫度傳感器和電子羅盤組成。超聲波測(cè)距技術(shù)變換是基于小波變換的精度高來(lái)表示的，以提高超聲波傳感器測(cè)量精度。仿真研究表明，所提出的多傳感器信息融合的方法是非常有效的對(duì)于導(dǎo)航割草機(jī)器人。實(shí)驗(yàn)結(jié)果表明，該傳感系統(tǒng)基于相關(guān)的規(guī)定障礙檢測(cè)和定位顯示出巨大的潛力，為在動(dòng)態(tài)工作條件下的割草機(jī)器人提供一個(gè)強(qiáng)大的高性價(jià)比的解決方案。 關(guān)鍵詞——多傳感器融合，超聲波傳感器，割草機(jī)機(jī)器人，定位，導(dǎo)航。 1.緒論 草坪修剪被許多人認(rèn)為是一個(gè)最枯燥，累人的日常任務(wù)。首先迫切需要執(zhí)行的任務(wù)是能適應(yīng)環(huán)境的機(jī)器人。一些預(yù)測(cè)表明，割草機(jī)器人將是一個(gè)最有前途的個(gè)人機(jī)器人應(yīng)用，并有重大的市場(chǎng)在世界上。因此，智能化的概念割草機(jī)器人（IRM）在1997年度會(huì)議的OPEI（ 戶外電力會(huì)議設(shè)備研究所）上第一次提出[ 1 ]。該機(jī)器人主要面對(duì)一般家庭幫助忙碌的人們和乏力的老人們節(jié)省支付雇傭勞動(dòng)力的報(bào)酬，同時(shí)消除人們來(lái)自噪聲中，花粉和割草刀片的危害。割草機(jī)器人是服務(wù)于家庭護(hù)理的室外移動(dòng)機(jī)器人，是那種真正的智能機(jī)電一體化的環(huán)境清理設(shè)備[ 2 ] [ 3 ]。最重要的是割草機(jī)器人為代表的一些地區(qū)覆蓋的環(huán)保機(jī)器人不僅用于室內(nèi)地面清潔，如[ 4 ]也在危險(xiǎn)的環(huán)境中，例如去地雷，清理輻射點(diǎn)，勘探資源等。與室內(nèi)移動(dòng)機(jī)器人不同，割草機(jī)器人得到很大的挑戰(zhàn)。 在整個(gè)工作區(qū)域內(nèi)，割草機(jī)器人使用傳感器來(lái)感知環(huán)境以及識(shí)別他們的實(shí)時(shí)狀態(tài)下的環(huán)境障礙，地圖構(gòu)建，定位和導(dǎo)航。由于環(huán)境的復(fù)雜性，一種簡(jiǎn)單的傳感器是不足以讓割草機(jī)器人來(lái)完成這些任務(wù)的。因此有必要結(jié)合來(lái)自不同的傳感器上觀察到的傳感器數(shù)據(jù)減少機(jī)器人在任何工作環(huán)境工作的不確定性。為來(lái)自各種傳感器的信息能合并，傳感器魯棒性和實(shí)時(shí)性的融合是必需的[ 5 ]。在傳感器出現(xiàn)誤差或失敗的情況下，多融合傳感器融合也可以減少不確定信息，并提高其可靠性。 低成本的傳感系統(tǒng)，說(shuō)明其低功耗，高性能。超聲波傳感器檢測(cè)范圍是0.3m~ 5m，他們提供良好的范圍信息。然而，環(huán)境引起的鏡面漫反射是超聲波傳感器的不確定因素，讓他們不具吸引力。紅外傳感器的檢測(cè)范圍是0.02m~ 1m，他們可以檢測(cè)在超聲波傳感器的盲區(qū)的障礙。 為了滿足割草機(jī)器人低成本和高精度的測(cè)距技術(shù)的需求，在研究超聲波測(cè)距技術(shù)基于高精度的小波分析變換（WT）的數(shù)據(jù)報(bào)道，提高超聲波傳感器的測(cè)量精度。測(cè)量數(shù)據(jù)從傳感系統(tǒng)集成，實(shí)現(xiàn)規(guī)劃最佳的，可靠地，完全覆蓋整個(gè)工作計(jì)劃的地區(qū)，使割草機(jī)器人避免未知的障礙。 最后，通過(guò)仿真研究和實(shí)驗(yàn)結(jié)果表明該傳感系統(tǒng)的導(dǎo)航效果，障礙物檢測(cè)和割草機(jī)器人定位。 2.信息資源管理系統(tǒng)的硬件 IRM采用DSP TMS320F2812作為其CPU，包括四個(gè)單元：車輛系統(tǒng)，切割系統(tǒng)，傳感系統(tǒng)和控制系統(tǒng)。傳感系統(tǒng)是用來(lái)收集外部動(dòng)態(tài)信息的工作環(huán)境 避障，地圖構(gòu)建，導(dǎo)航與定位。它也可以用來(lái)檢測(cè)車輛系統(tǒng)的運(yùn)動(dòng)參數(shù)和切削機(jī)構(gòu)的工作狀況。該控制器將獲得的信息與數(shù)據(jù)庫(kù)進(jìn)行比較，然后發(fā)出修正后精確的命令讓機(jī)器人完成任務(wù)。信息資源管理的硬件，如圖1所示。 IMR硬件概要圖1 機(jī)器人必須身體強(qiáng)壯，計(jì)算速度快，行動(dòng)準(zhǔn)確和安全。它應(yīng)該有能力 ，而在全部或大部分的割草期間無(wú)需人的干預(yù)。IRM由于模塊化設(shè)計(jì)，各單元的管理是相對(duì)獨(dú)立的。模塊化設(shè)計(jì)使維護(hù)更容易。IRM任何損壞單元都可以直接取代而不影響其它單元的功能。 3.傳感系統(tǒng) A：超聲波傳感器單元 超聲波傳感器可以提供良好的范圍是基于飛行時(shí)間（TOF）信息原理，主要是由于其簡(jiǎn)單性和成本相對(duì)較低，他們已廣泛應(yīng)用于移動(dòng)機(jī)器人的障礙物回避，地圖構(gòu)建等。這種類型的外部傳感器能很好測(cè)量的障礙物的距離。靈敏度函數(shù)的主瓣內(nèi)包含一個(gè)20度角，如圖2所示的【6】。大量的試驗(yàn)結(jié)果表明，傳感器的精度范圍為±2cm。 圖2為超聲波傳感器的典型的強(qiáng)度分布 對(duì)于IRM，我們建立了一個(gè)傳感器陣列由12超聲波傳感器間隔30度的間隔。超聲波信號(hào)可以覆蓋所有的空間，可以要求哪些機(jī)器人檢測(cè)整個(gè)空間的環(huán)境信號(hào)。用基于TOF的測(cè)量的超聲換能器的經(jīng)典技術(shù)，計(jì)算出的距離最近的反射器利用聲音在空氣中的速度從發(fā)射脈沖到回波到達(dá)時(shí)間。距離D為反映對(duì)象的計(jì)算 D =（C×T）/ 2 （1） 其中C是聲音的速度，T是飛行時(shí)間。該TOF法產(chǎn)生一系列的值時(shí)，回波幅度首次超過(guò)臨界值后發(fā)送，忽略第二回波從進(jìn)一步的反射。 超聲波傳感器單元包括一個(gè)觸發(fā)脈沖生成單元，一個(gè)多通道選擇單元和回聲接收單元。傳感器接口電路設(shè)計(jì)發(fā)送和接收超聲波脈沖，捕獲的總是第一個(gè)返回的回聲。一個(gè)對(duì)象相關(guān)的數(shù)據(jù)的范圍要考慮到即使是位于在錐軸離軸。 超聲波頻率通常在40和180千赫之間，而在該系統(tǒng)中超聲波傳感器的頻率使用的是40千赫。光束角度是20度。40千赫PWM脈沖是由通用DSP的定時(shí)器單元產(chǎn)生的。驅(qū)動(dòng)發(fā)射機(jī)有效而不帶來(lái)大的振動(dòng)，40千赫的超聲波一次突發(fā)的時(shí)間是8周期。當(dāng)超聲波脈沖發(fā)射時(shí)，傳感器將經(jīng)歷“振鈴”。振鈴引起的由所發(fā)送的脈沖可以使接收器檢測(cè)到一個(gè)錯(cuò)誤回聲。這個(gè)不能夠捕獲解決DSP中斷問(wèn)題，直到延遲間隔已過(guò)。這意味著在延遲的時(shí)間間隔那測(cè)距儀不能檢測(cè)物體距離該傳感器是少于一半的聲音傳播的距離。這是該超聲波傳感器的盲區(qū)，如圖3所示。 圖3超聲波發(fā)射和接收示意圖 B.：.紅外傳感器裝置和其他傳感器 針對(duì)超聲波傳感器的盲區(qū)，增加了紅外傳感器。紅外傳感器可以檢測(cè)在20cm內(nèi)的障礙，這彌補(bǔ)了超聲波傳感器由于失明問(wèn)題所造成的區(qū)域的問(wèn)題。 這個(gè)單元有16個(gè)紅外傳感器。每個(gè)紅外搜索器范圍有6度，是靈敏度函數(shù)主要的圓錐曲線的視圖。該傳感器具有一個(gè)高精度測(cè)量范圍，有效測(cè)量范圍是一個(gè)目標(biāo)到一米左右。一些測(cè)試表明，該傳感器的測(cè)距精度在±1cm左右。 為了節(jié)省DSP的資源，16個(gè)紅外傳感器采用DSP TMS320F2812的數(shù)據(jù)接口代替IO接口。這種結(jié)構(gòu)也可以同時(shí)讀取傳感器的狀態(tài)，以確保該系統(tǒng)的時(shí)間性能。傳感器接口電路用于發(fā)送和接收紅外脈沖并總是捕獲第一個(gè)回波來(lái)處理其振幅。 割草機(jī)器人在室外環(huán)境中工作時(shí)，其溫度變化迅速。溫度的變化會(huì)影響聲音的速度。因此，溫度傳感器用于保證超聲波傳感器的精度。碰撞傳感器是一組敏感的樣本，采用它是為了防止意外的碰撞造成的損害。由于潮濕的環(huán)境會(huì)危害IRM電路，濕度傳感器被引入用于檢測(cè)濕度環(huán)境。雖然這些傳感器不完全是 一個(gè)自主割草機(jī)器人機(jī)必要的，但他們可以提供有益的功能，讓工作更具有效性和安全性。 4.導(dǎo)航技術(shù) A. 映射 正如圖4所示，基準(zhǔn)方向的X定義和機(jī)器人的坐標(biāo)為，。關(guān)于內(nèi)置電子羅盤對(duì)于機(jī)器人的幫助，角，這是從第一個(gè)傳感器得來(lái)的角度，可容易衡量。實(shí)際上，如果只在角（標(biāo)題的機(jī)器人角）的測(cè)量，從其他傳感器的角度可以發(fā)現(xiàn) 角是我們的世界坐標(biāo)中心。該超聲波傳感器組的最大環(huán)數(shù)為n，半徑為R（在我們的系統(tǒng)中，n = 12和R = 0.25m）。該環(huán)的原點(diǎn)到中心之間的距離是r，并且該向中心的基準(zhǔn)角度是Ω。根據(jù)參考位置機(jī)器人的中心是（，）。這個(gè)距離是從原點(diǎn)到通過(guò)兩個(gè)傳感器數(shù)據(jù)檢測(cè)的二維平面稱之為。 現(xiàn)在讓我們用DMI測(cè)量值來(lái)分別表示從超聲波和紅外傳感器得到的數(shù)據(jù)，用于精確距離。這些值之間會(huì)有一個(gè)誤差 在這項(xiàng)工作中，我們自然假設(shè)是一個(gè)均勻隨機(jī)變量在（W，W）范圍內(nèi)。在這里，W表示最大距離測(cè)量誤差。這里的問(wèn)題是，給定的，，r ，，，，，和，，，，，估計(jì)占用的坐標(biāo)細(xì)胞和（或等價(jià)的）以最有效的方法。涉及檢測(cè)對(duì)象的方程可以寫為 圖4所示機(jī)器人在X-Y段的位置 由于對(duì)象涉及機(jī)器人的方程被寫為 如果我們定義的位置為：=，，，，，然后我們有 將插入到中， 在這里我們有N個(gè)這樣的方程。我們把它們矩陣形式 如果我們引入新的矩陣 ，然后（10）， 可以寫為 在這里，如果我們進(jìn)行最小二乘法估計(jì)，我們得到 因此，我們用最小二乘法估計(jì)找到最好的位置。 B. 仿真研究 基于傳感器導(dǎo)航系統(tǒng)已經(jīng)進(jìn)行了測(cè)試在顯示該傳感器融合方法的有效性的兩種環(huán)境分別如圖5和圖6所示。割草機(jī)是一個(gè)結(jié)構(gòu)化的實(shí)驗(yàn)室初步測(cè)試如圖5所示。開始在（0.3m，0.5m，0°），一個(gè)虛擬的機(jī)器人在虛擬廣場(chǎng)走廊一次。墻在人工環(huán)境中是由真正的地圖表示的。 全車是獨(dú)立的。它有一個(gè)最大的運(yùn)行速度是0.4米/秒。實(shí)驗(yàn)室面積調(diào)查出在10cm精度優(yōu)于1cm為佳。提取映射，提出了一開始的目標(biāo)。機(jī)器人位置和方向是由電子羅盤成立[ 8 ]。 圖5數(shù)據(jù)采集與導(dǎo)航在結(jié)構(gòu)化環(huán)境中的結(jié)果 圖5中的結(jié)果顯示的映射質(zhì)量和該傳感器融合方法的有效性。在測(cè)試中，我們發(fā)現(xiàn)，在估計(jì)的位置的平均誤差（ε）在環(huán)境中的障礙是在[ 0.2 ，0.2]米范圍內(nèi)。 在模擬中，我們看到，在（11）中，實(shí)際上應(yīng)該得到的是不滿足 。在可以為位置更好的估計(jì)的情況下可以表示為 在這種情況下，估計(jì)角不會(huì)改變但估計(jì)距離是縮放到它的最佳估計(jì)。 因此，對(duì)于位置，距離估計(jì)是和以前一樣，而最小二乘估計(jì)的作品只對(duì)角 。仿真結(jié)果表明，這種方法產(chǎn)生更精確的結(jié)果。 圖6仿真結(jié)果的墻下行為 墻后，被選定為初值問(wèn)題域是因?yàn)樗⒁粋€(gè)相當(dāng)簡(jiǎn)單的問(wèn)題評(píng)價(jià)[ 9 ]。它這也奠定了更為復(fù)雜的基礎(chǔ)的問(wèn)題領(lǐng)域，如迷宮的穿越，映射和用于草坪修剪和吸塵全覆蓋路徑規(guī)劃。墻上的仿真結(jié)果—圖6所示的行為后和實(shí)驗(yàn)結(jié)果在圖6表明，該IRM有能力在非結(jié)構(gòu)化的環(huán)境中執(zhí)行它的割草任務(wù)。 在圖5中傳感器的程序?qū)Ш椒抡嫒缦隆? 5 . 超聲波測(cè)距技術(shù)基于小波變換 遺憾的是，由于環(huán)境的復(fù)雜性和噪聲的影響，實(shí)際接收到的多回波具有隨時(shí)間變化的特性，并且是一個(gè)典型的非平穩(wěn)信號(hào)。此外，在超聲波脈沖回聲混合噪聲是非高斯白噪聲，但噪聲，和與目標(biāo)回波相關(guān)。TOF法不能在這樣的條件下直接使用。引用廣義相關(guān)方法估計(jì)時(shí)間延遲的[ 10 ]，我們把提出了廣義自相關(guān)方法基于小波變換的時(shí)延估計(jì)[ 11 ]出現(xiàn)在圖7。 圖7基于小波變換的廣義自相關(guān)延遲估計(jì) 其中（t）是母小波和（t）是女兒小波。該系數(shù)α是規(guī)模（或縮放 因素）和（τ）是時(shí)間位移。小波變換的信號(hào)x（t）是y（t）。實(shí)際上這是一個(gè)過(guò)濾過(guò)程使用大量的帶通濾波器的超聲回波等于的Q值，這相當(dāng)于的白化濾波器對(duì)廣義相關(guān)方法的時(shí)間延遲的估計(jì)，為了消除輸入噪聲的影響做以下處理。可以找到，作為 由于傅里葉變換關(guān)系自相關(guān)函數(shù)之間和他的力譜： 我們獲得的廣義自相關(guān)函數(shù)是： 最后，檢測(cè)到的峰值來(lái)完成TOF的估計(jì)和計(jì)算實(shí)際的超聲波速度。 圖8嘈雜的超聲回波 圖9基于小波去噪的回聲 圖10自相關(guān)函數(shù) 圖11峰值檢測(cè) 嘈雜的超聲回波信號(hào)如圖8所示，和利用小波變換去噪后的超聲回波顯示 圖9。很明顯，該噪聲混入的超聲波回波經(jīng)WT操作后得到有效地消除作。自動(dòng)去除噪聲的超聲波回波的相關(guān)運(yùn)算如圖10所示。圖11顯示了包絡(luò)線，通過(guò)希爾伯特變換。正如我們可以看到，如果每一個(gè)峰的橫坐標(biāo)點(diǎn)確定，TOF估計(jì)可計(jì)算?？紤]的超聲回波衰減和高精度的要求在實(shí)踐中的需求，只有前4回波被用來(lái)估計(jì)TOF。在TOF估計(jì)的值是，，，，，，這是對(duì)稱于X軸。使用這種方法，估計(jì)超聲波速度可以計(jì)算出來(lái)。 到目前為止，障礙檢測(cè)和定位系統(tǒng)成功實(shí)現(xiàn)。運(yùn)用該方法，障礙物檢測(cè)和定位系統(tǒng)已成功實(shí)施。 基于廣義自相關(guān)法小波變換，提出了實(shí)現(xiàn)實(shí)時(shí)超聲波速度測(cè)量，該方法可以消除溫度，濕度和風(fēng)力的影響，超聲波速度測(cè)量可以在機(jī)器人工作的動(dòng)態(tài)條件下完成。在這種傳感系統(tǒng)的基礎(chǔ)上，廣義自相關(guān)方法顯示出巨大潛力提供用于割草機(jī)器人一個(gè)強(qiáng)大的解決方案在動(dòng)態(tài)的工作條件下。 6. 實(shí)驗(yàn)結(jié)果 我們利用超聲波傳感器測(cè)量機(jī)器人和平面之間的距離。測(cè)量結(jié)果和實(shí)際距離如表一所示： 表一 超聲波傳感器的實(shí)驗(yàn)數(shù)據(jù)（單位：厘米） 從表一中，我們可以看到，超聲波傳感器測(cè)量誤差在3%。 然后，基于廣義自相關(guān)法小波變換，提出了實(shí)現(xiàn)實(shí)時(shí)超聲波速度測(cè)量。 通過(guò)上述方法，我們?cè)俅螠y(cè)量機(jī)器人和平面對(duì)象距離的。測(cè)量結(jié)果與實(shí)際距離顯示在表二中。 表二 超聲波傳感器的基于小波變換的數(shù)據(jù)（單位：厘米） 基于小波變換的實(shí)驗(yàn)結(jié)果表明，使用上述的測(cè)量誤差技術(shù)是小于1% 為5m范圍區(qū)域內(nèi)，這種傳感系統(tǒng)的障礙物檢測(cè)和定位擁有巨大的潛力，能作為—個(gè)強(qiáng)大的解決方案用提高于割草機(jī)器人性價(jià)比在動(dòng)態(tài)工作條件下。 7. 結(jié)論 在本文中，我們提出了一個(gè)多傳感器系統(tǒng)結(jié)合超聲波傳感器測(cè)量用于割草機(jī)器人導(dǎo)航。該傳感系統(tǒng)具有低成本，低功耗，高性能，使割草機(jī)器人機(jī)能映射未知環(huán)境。其有效性是通過(guò)仿真研究和實(shí)驗(yàn)結(jié)果得到的。 使用不同種類的傳感器集成在傳感系統(tǒng)可以克服超聲波傳感器的盲區(qū)和多傳感器融合的鏡面反射的缺陷。 一種高精度超聲波測(cè)距技術(shù)的方法基于小波變換已被引入到改善更多的超聲波傳感器的測(cè)量精度準(zhǔn)確的感官信息。該系統(tǒng)應(yīng)用于割草機(jī)器人，證明了實(shí)驗(yàn)的可靠性和實(shí)時(shí)性。 今后的工作將著眼于應(yīng)用所提出的跟蹤技術(shù)的多傳感器融合方案應(yīng)用于在非結(jié)構(gòu)化環(huán)境中的機(jī)器人割草機(jī)控制全覆蓋路徑規(guī)劃[ 12 ]。 參考文獻(xiàn) Ming Cong and Bo Fang School of Mechanical Engineering Dalian University of Technology Dalian 116024 China congm This work is supported by national natural science fund 50675027to Ming Cong Abstract This paper presents a multisensor system for combining measurements from ultrasonic sensors and navigation for robot mowers The proposed sensing system enables robot mowers to mapping unknown environments It is important for an autonomous robot mower to explore its surroundings in performing the task of localization and navigation for mowing Because of the complexity of the environment one simple kind of sensors is not sufficient for robot mower to accomplish these tasks We develop a robot mower equipped with DSP TMS320F2812 as its CPU The sensing system integrates with ultrasonic sensors infrared sensors collision sensors encoders a temperature sensor and an electronic compass A method of high accuracy ultrasonic ranging technology based on wavelet transform is reported to improve the measurement precision of ultrasonic sensors Simulation studies show that the proposed multisensor fusion method is very effective for the navigation of robot mowers Experimental results indicate that this sensing system based on generalized auto correlation method for obstacle detection and localization shows great potential for providing a high performance to price ratio and robust solution for robot mowers in dynamic working condition Index Terms multisensor fusion ultrasonic sensors robot mower mapping navigation I INTRODUCTION Lawn mowing is considered by many to be one of the most boring and tiring routine tasks The environmental robots are needed urgently to perform the task Some predictions indicate that the robot mowers will be one of the most promising personal robot applications and have substantial market in the world Therefore the concept of Intelligent Robot Mower IRM had been proposed for the first time in 1997 s annual conference of the OPEI Outdoor Power Equipment Institute 1 The robots mainly face to the general families to help the busy people and the hypodynamic old folks save the payments for hiring labours also remove people from noise pollen and danger of mowing blade The robot mowers serve for home care as the outdoor mobile robots actually kind of intelligent mechatronics devices for environment clean up 2 3 The important thing is that the robot mowers are representative of some area covering environmental robots used not only for indoor floor cleaning as in 4 but also in hazardous environments such as removing landmines cleaning up radiant points and prospecting for resources etc The robot mowers get great challenges differing from indoor mobile robots The robot mowers use sensors to understand environments as well as their real time states for obstacle avoidance map building location and navigation in the whole work area Because of the complexity of the environment one simple kind of sensors is not sufficient for robot mower to accomplish these tasks It is necessary to combine the observed sensor data coming from different sensors to reduce the uncertainties of the robot in any working environment To merge the information from the various sensors robust and real time sensor fusion is required 5 In cases of sensor error or failure multisensor fusion can also reduce uncertainty in the information and increase its reliability A sensing system of low cost low power consumption high performance is described The detecting range of ultrasonic sensors is 0 3m 5m they provide good range information However uncertainties in ultrasonic sensors caused by the specular reflection from environments make them less attractive The detecting range of infrared sensors is 0 02m 1m they can detect the obstacles within the ultrasonic sensor s blind zone In order to satisfy the needs of robot mowers for the low cost and high accuracy ranging technology the research on the high accuracy ultrasonic ranging technology based on wavelet transform WT is reported to improve the measurement precision of ultrasonic sensors Measurement data gathered from the sensing system are integrated to avoid the robot mower from unknown obstacles and plan an optimum reliable and realizable plan completely coverage of entire working area Finally simulation studies and experimental results show the effectiveness of the sensing system for the navigation obstacle detection and localization of robot mowers II SYSTEM HARDWARE OF IRM The IRM uses DSP TMS320F2812 as its CPU including four units vehicle system cutting system sensing system and control system The sensing system is used to collect the external dynamic information of the working environment for obstacle avoidance map building navigation and localization It is also used to detect vehicle system s movement parameters and cutting mechanism s working status The controller compares the acquired information with the database and then sends out revisory and accurate command to the robot to perform its tasks The hardware of the IRM is shown in Fig 1 Multisensor Fusion and Navigation for Robot Mower 978 1 4244 1758 2 08 25 00 2008 IEEE 417 Proceedings of the 2007 IEEE International Conference on Robotics and Biomimetics December 15 18 2007 Sanya China Fig 1 Hardware overview of IMR The robot must be physically strong computationally fast behaviourally accurate and safety It should have the ability to perform on its own and required no human intervention during the whole or most part of the mowing period The IRM is modularized designed and each unit of the IRM is relatively independent Modularized design makes the maintenance much easier Any broken unit of the IRM can be replaced directly without influencing the functions of other units III SENSING SYSTEM A Ultrasonic Sensor Unit Because ultrasonic sensors can provide good range information based on the time of the flight TOF principle mainly due to their simplicity and relatively low cost they have been widely used in mobile robots for obstacle avoidance map building and so on This type of external sensor is very good in obstacles distance measurement The main lobe of the sensitivity function is contained within an angle of 20 degrees as shown in Fig 2 6 A number of tests showed that the range accuracy of the sensors is in the order of 2cm Fig 2 Typical intensity distribution of an ultrasonic sensor On IRM we set up a sensor array which consists of 12 ultrasonic sensors spaced 30 degrees apart The ultrasonic signals can cover all the space around and satisfy the space requirement about which robot can detect the environmental signals Classical techniques used in ultrasonic transducers are based on TOF measurement which calculates the distance of the nearest reflector using the speed of sound in air and the emitted pulse and echo arrival times The distance d to a reflected object is calculated by 2dct 1 where c is the speed of sound and t is the time of flight The TOF method produces a range value when the echo amplitude first exceeds the threshold level after transmitting ignoring a second echo from a further reflector The ultrasonic sensor unit includes a trigger pulse generation unit a multi channel selection unit and an echo receiving unit A sensor interface circuitry designed to send and receive ultrasonic sound pulses catches always the first returning echo The range data related to an object is considered to be on the conic axes even if it is located off the axes The ultrasonic wave typically has a frequency between 40 and 180 kHz and the frequency of the ultrasonic sensors used in the system is 40 kHz The beam angle is 20 degrees The 40 kHz PWM pulse is generated by the general purpose timer unit of DSP To drive the transmitter effectively and not to bring much vibration an 8 cycle burst of ultrasound at 40 kHz is sent out once a time When the ultrasonic pulse is emitted the sensor will experience ringing Ringing caused by the transmitted pulse can cause the receiver to detect a false echo This problem is solved by not enabling the capture interrupt of DSP until a delay interval has passed This means that the ranger can not detect an object whose distance from the sensor is less than half the distance that sound travels during the delay interval This is the blind zone of the ultrasonic sensor as shown in Fig 3 Trigger pulse Emitted signal Received signal TOF Blind zone Echo Fig 3 The sketch map of ultrasonic transmission and reception B Infrared Sensor Unit and Other Sensors To overcome the ultrasonic sensor s blind zone infrared sensors are added The infrared sensors can detect obstacles within 20cm which patch up the problem caused by the blind zone problem of ultrasonic sensors This unit has 16 infrared sensors Each infrared range finder has a conic view of 6 degrees which is the main lobe of the sensitivity function This sensor has a useful measuring range of a target up to about one meter with high accuracy A number of tests showed that the range accuracy of the sensors is in the order of lcm In order to save the DSP s resource 16 infrared sensors are connected with DSP TMS320F2812 s data interface 418 instead of the IO interface This kind of architecture can also read the sensors status at the same time ensuring the real time capability of the system A sensor interface circuitry designed to send and receive infrared pulses catches always the first retuning echo to process its amplitude Robot mower works in an outdoor environment where the temperature changes rapidly The changing of temperature will affect the speed of sound Therefore a temperature sensor is used to guarantee the precision of the ultrasonic sensor Collision sensor is a group of sensitive swatches which used to prevent the damage caused by unexpected collision Because moist environment do harm to the circuit of the IRM humidity sensors are introduced to detect the humidity of the environment Although these sensors are not absolutely necessary for an autonomous robot mower they can provide helpful functions to make the work availability and safety IV SENSOR BASED NAVIGATION A Mapping As seen in Fig 4 a reference direction x is defined and the robot coordinates are shown as R x R y By the help of an electronic compass built in on the robot 7 the angle i which is the ith sensor s angle from the 1st sensor can be easily measured Actually if only the angle S heading angle of the robot is measured other sensor angles can be found as iSi 2 where i is the angle to the our world coordinate center The number of maximum sensor group on the ultrasonic ring is n and the radius is r in our system n 12 and r 0 25m The distance between the origin and the center of the ring is R and reference angle to the center is The reference position of the robot s center is R x R y The distance from the origin to object which is detected by the ith sensor data on the two dimensional plane is called i R Now let i dm denote measured value which is combined data from the ultrasonic and infrared sensors for the exact distance i R There will be an error i between these values as iii dm d 3 In this work we naturally assume that i is a uniform random variable in the range of W W Here W denotes the maximum distance measurement error Here the problem is given R x R y r 12 n g34 and 12 n dm dm dmg34 to estimate the coordinates of the occupied cells i x and i y or equivalently i R in most efficient way The equations involving the detected object can be written as 222 cos sin iR i i R i i Rxrd yrd 4 22 2 2 cos sin iii RR rd rdx y 22 2 2 cos ii R R R rd rd 5 y x x y R R g455 d d O Fig 4 The robot position on x y section The equations involving the robot due to the object can be written as 222 cos sin iiii i Rxrd yrd 6 222 2 2 cos sin ii i ii ii i Rxy rd rdx y If we define the positions as 11 TT in Ppp p xy g34 then we have 22 2 2 cos sin ii ii ii R R rd rd y P 7 After the inserting the 2 i R in 2 R cos cos sin iiii rd R y P 8 Here again we have n such equations And we write them in matrix form i mAP 9 And if we introduce new matrix as cos sin iiii LP and 0 0 then 10 can be written as 111 1 2 cos cos R nRn nn rdm R L p L rdm R L p g170g186g170 g186g170g186 g171g187g171 g187g171g187 g171g187g171 g187g171g187 g171g187g171 g187g171g187 g171g187g171 g187g171g187 g171g187g171 g187g171g187 g171g187g171 g187g171g187 g172g188g172 g188g172g188 Here if we perform the least squares estimate for i P we obtain 1 TT lsq i PAAAm 11 Thus we find the best squares estimate of the positions B Simulation Studies Sensor based navigation has been tested with simulation to shown the usefulness of this sensor fusion method in the two environments respectively as shown in Fig 5 and Fig 6 The mower has been primarily tested in a structured laboratory as shown in Fig 5 Start at 0 3m 0 5m 0degree a virtual 419 robot was driven around a virtual square corridor one time The walls in the artificial environment are denoted by the real map The entire vehicle is self contained It has a maximum travel speed on 0 4 m s The laboratory area was surveyed out to a 10cm grid with accuracy better than about 1cm To extract the mapping a start and goal points were presented The robot position and orientation were established by the electronic compass 8 Fig 5 Data collection and navigation result in structured environment The result in Fig 5 demonstrates the mapping quality and the usefulness of this sensor fusion method In the tests we find that the average error in estimating the position of the obstacles in the environment was in the range of 0 2 0 2 m In the simulations we see that lsq i P in 11 obtained does not satisfy ilsqi RP which actually should In the case a better estimate for the positions can be given as i ei lsqi lsq i R PP P 12 In this case estimate for the angle i does not change but the estimate for distance i R is scaled to it best estimate Therefore for the position the distance estimate i R remains the same as before while the least squares estimate works only for the angle i Simulations show that this way produces more accurate results Fig 6 The simulation result of wall following behavior Wall following was selected for the initial problem domain because it is a fairly simple problem to set up and evaluate 9 It also lays the groundwork for more complex problem domains such as maze traversal mapping and complete coverage path planning which is used on lawn mowing and vacuuming The simulation result of wall following behavior shown in Fig 6 and the experimental result in Fig 6 demonstrate that the IRM have the capability to perform its mowing task in unstructured environment The program of sensor based navigation simulation in Fig 5 is given below Sub Main Dim PI Fcr Fct X target Y target X Y As Single Dim X grid Y grid i j C As Integer Dim Frx Fry d dist targ rot Fx Fy As Single Dim Fcx Fcy Rx Ry As Single PI 3 1415927 Fcr 1 Fct 1 X target GetMarkX 0 Y target GetMarkY 0 SetCellSize 0 0 1 Set cell size 10 cm x 10 cm SetTimeStep 0 1 Set simulation time step of 0 1 seconds Do Start main loop X GetMobotX 0 Present mobot coordinates in meters Y GetMobotY 0 X grid CoordToGrid 0 X indexes of cells where the Y grid CoordToGrid 0 Y mobot center is MeasureRange 0 1 3 Perform a range scan and update the Certainty Grid max cell value 3 Frx 0 Fry 0 Each ocuppied cell inside the windows of 33 x 33 cells applies a repulsive force to the mobot For i X grid 16 To X grid 16 For j Y grid 16 To Y grid 16 C GetCell 0 i j If C 0 Then d Sqr X grid i 2 Y grid j 2 If d 0 Then Frx Frx Fcr C d 2 X grid i d Fry Fry Fcr C d 2 Y grid j d End If End If Next Next dist targ Sqr X X target 2 Y Y target 2 Fcx Fct X target X dist targ Fcy Fct Y target Y dist targ Rx Frx Fcx Ry Fry Fcy rot RotationalDiff 0 X Rx Y Ry shortest rotational difference between current direction of travel and direction of vector R SetSteering 0 0 5 3 rot mobot turns into the direction of R at constant speed and steering rate proportional to the rotational difference StepForward Loop Until dist targ 0 1 Loop until mobot reaches the target End Sub 420 V ULTRASONIC RANGING TECHNOLOGY BASED ON WT Unfortunately the practical received multi echoes has time varying property and is a typical non stationary signal because the influence of the environmental complexity and the noise Furthermore the noise mixed in the ultrasonic pulse echo is Non Gaussian white noise but colored noise and correlated with the target echo The TOF method can not be used directly in such conditions Referencing the generalized correlation method for estimation of time delay 10 we put forward the generalized auto correlation method for estimation of time of flight based on wavelet transform 11 and present in Fig 7 Fig 7 Delay estimation of generalized auto correlation based on WT Where t is the mother wavelet and a t is the daughter wavelet The coefficient is the scale or scaling factor and is the time displacement The wavelet transform of the signal x t is yt Actually this is a filtering process of the ultrasonic echo using a multitude of bandpass filters of equal Q which is equivalent to the whitening filter of the generalized correlation method for estimation of time delay in order to eliminate the input noise which can influence the following processing yy R can be found as yy xx a a R Eyt yt R t t t As there has the relationship of Fourier transform between auto correlation function yy R and his power spectrume 2 yy yy xx xx GFRG aaG a We obtain the generalized auto correlation function as Last the peak values of yy R are detected to accomplish the estimation of TOF and calculate the real ultrasonic velocity Fig 8 Noisy ultrasonic echo Fig 9 Denoised echo using WT Fig 10 Auto correlation function yy R Fig 11 Peak detection The noisy ultrasonic echo is shown in Fig 8 and the denoised ultrasonic echo by wavelet transform is shown in Fig 9 It is obvious that the noise mixed in the ultrasonic echo is effectively eliminated after WT operation The auto correlation operation yy R of the denoised ultrasonic echo is shown in Fig 10 Fig 11 shows the envelope of yy R through Hilbert transform As we can see if the abscissa of every peak point is determined the estimation of TOF g109 ND can be calculated Considered the attenuation of the ultrasonic echo and the demand of the high precision in practice only the former four echoes are used to estimate the TOF The values of the TOF estimation are g110g110 g109 g108 g109 g109 3 2 2 3DDDDDD which are symmetrical to the x axis Using this method the estimation of the ultrasonic velocity can be calculated So far an obstacle detection and localization system has been implemented successfully By means of above method an obstacle detection and localization system has been implemented successfully The generalized auto correlation method based on wavelet transform is put forward to realize the real time ultrasonic velocity measurement and this method can 11 22 gj j yy yy g yy RG Ged g179g179 421 eliminate the influence of temperature humidity and wind on ultrasonic velocity measurements when the robots are working in dynamic condition And this sensing system based on generalized auto correlation method shows great potential for providing a robust solution for robot mowers in dynamic working condition VI EXPERIMENTAL RESULTS We measure the distance between the robot and plane objects using the ultrasonic sensors The measured results and the actual distances are shown in TABLE I TABLE I THE EXPERIMENTAL DATA OF THE ULTRASONIC SENSORS unit cm Actual distance Measured value1 Measured value2 Average error 30 30 62 30 61 2 50 40 40 70 41 69 1 73 50 50 64 50 67 1 31 60 60 73 60 73 1 22 70 70 81 70 84 1 19 80 81 09 81 04 1 33 90 91 10 91 13 1 24 100 98 82 99 15 1 02 150 148 24 148 37 1 13 200 201 85 201 85 0 93 250 252 71 252 74 1 09 300 302 52 302 58 0 85 350 347 63 347 61 0 68 400 405 75 405 77 1 44 450 456 22 456 26 1 39 500 508 87 508 90 1 77 From TABLE I we can see that the measurement error of the ultrasonic senso