梁喜鳳, 蔡陽陽, 王永維

(1.中國計量學院機電工程學院,杭州310018;2.浙江大學生物系統(tǒng)工程與食品科學學院,杭州310058;3.臺州市匯科農(nóng)業(yè)機械技術開發(fā)有限公司,浙江 臺州318050)

番茄缽苗自動移栽缽體物理機械特性試驗

梁喜鳳1,2*, 蔡陽陽1, 王永維2,3

(1.中國計量學院機電工程學院,杭州310018;2.浙江大學生物系統(tǒng)工程與食品科學學院,杭州310058;3.臺州市匯科農(nóng)業(yè)機械技術開發(fā)有限公司,浙江 臺州318050)

蔬菜缽苗缽體的物理機械特性,是設計全自動蔬菜移栽機取苗機構、栽植器等關鍵部件的重要依據(jù)。以苗齡為23 d的番茄缽苗為試驗對象,利用Instron 5543萬能試驗機對其進行平板壓縮、穿刺和蠕變試驗,并隨機選取30株缽苗進行500 mm高度的散落試驗。研究表明:缽體受壓時其抗壓力與變形關系為非線性變化,缽體屈服點的抗壓力為3.15 N,取苗機構設計時取苗夾取力要超過缽體屈服點的抗壓力;缽體穿刺初期穿刺力與位移呈近似線性關系,隨著變形的增加穿刺力在一定范圍內保持不變,缽體穿刺后期穿刺力顯著增加;缽體蠕變特性符合Burgers模型的蠕變規(guī)律,在加載力為5 N、保持時間為120 s條件下缽體的平均蠕變量為0.398 9 mm;缽體散落試驗顯示缽體平均散落率為22.73%,掉落后缽體保持完整,滿足機械栽植要求。

蔬菜移栽機; 番茄缽體苗; 物理機械特性

Journal of Zhejiang University (Agric. & Life Sci.)， 2015，41(5):616-622

蔬菜可提供人體所必需的多種維生素和礦物質,是人們日常飲食中必不可少的食物之一。我國是世界上最大的蔬菜生產(chǎn)國,2011年全國蔬菜種植面積達到1.97×1011m2[1]。約有60%以上的蔬菜品種采用育苗移栽方式種植,移栽作業(yè)仍以人工為主[2]。然而,傳統(tǒng)的人工移栽方式不僅勞動強度大、生產(chǎn)效率低,而且栽植質量差、種植成本高,難以實現(xiàn)短時間大面積移栽[3]。因此,加大蔬菜移栽機械的研發(fā)力度,實現(xiàn)蔬菜移栽機械化已成為農(nóng)業(yè)生產(chǎn)的迫切需要[4]。

目前,國內蔬菜移栽機主要有鏈夾式[5]、導苗管式[6]、吊籃式[7-9]及撓性圓盤式[10]等,移栽作業(yè)停留在半機械水平,移栽過程中需要人工輔助喂苗,致使移栽效率受到限制。國內學者通過運動分析、樣機試驗等方法對移栽機的結構參數(shù)和運動參數(shù)進行優(yōu)化,試制自動移栽機械,初步實現(xiàn)了蔬菜自動移栽的功能要求[11-14]。然而,蔬菜自動移栽機的設計主要集中在取苗機構和栽植機構的研發(fā)上,設計目標也僅僅是滿足插穴取苗、栽植種苗的功能要求,沒有將結構設計與蔬菜缽苗缽體本身的物理機械特性聯(lián)系起來分析設計[15]。

蔬菜缽苗自動移栽是一項綜合性技術,需要將移栽機的設計與作業(yè)對象相結合。對缽苗缽體的物理機械特性進行研究,進而優(yōu)化設計取苗機構和栽植機構等移栽機核心部件,將更加符合實際需要。本文以番茄穴盤苗為研究對象,測試分析與移栽機械設計直接相關的番茄缽苗缽體物理機械特性。通過缽體壓縮試驗,分析缽體的抗壓力-變形變化規(guī)律,確定缽體在取苗過程中受夾持力特性。經(jīng)過缽體穿刺試驗,分析缽體在取苗夾持時穿刺力作用規(guī)律。通過缽體蠕變試驗并結合Burgers模型描述,分析缽體的蠕變特性。同時對缽體進行500 mm高度的散落試驗,得到缽體散落率,分析缽體移栽過程中的散落特性。

1 材料與方法

1.1 材料與儀器

試驗于2014年1月19日在浙江大學生工食品學院進行。試驗材料選用Alisa番茄穴盤苗,育苗穴盤為隆基育苗穴盤,72孔穴,穴深為45 mm,上口徑尺寸為40 mm×40 mm,下口徑尺寸為20 mm×20 mm。育苗基質采用浙江大學蔬菜所育苗用有機土,草炭和蛭石比例為2∶1。在PRX-350型智能人工氣候箱中培養(yǎng)番茄穴盤苗,育苗時間為2013年12月28日至2014年1月19日,苗齡為23 d。缽苗形成后,缽苗根系在基質里穿插、纏繞、網(wǎng)絡,形成根土復合體即缽體[16]。隨機選取20株番茄苗,利用游標卡尺(精度為0.02 mm)測量番茄缽苗的物理參數(shù)。缽體的含水率采用干濕質量法測量,分別測量缽體的濕質量和干質量,計算缽體的含水率[15]。測得番茄缽苗主要生長指標和缽體幾何參數(shù)見表1。

表1 番茄缽苗主要生長指標和缽體幾何參數(shù)

Table 1 Main growth indexes of tomato seedlings and geometric parameters of the seedling pots

幾何參數(shù)Geometricparameters數(shù)值Values缽苗苗高Seedlingheights/mm120.26～143.64缽苗莖粗Seedlingdiameters/mm2.00～4.06缽體上邊長Lengthsofpotupperedge/mm36.62～39.16缽體下邊長Lengthsofpotloweredge/mm17.46～19.86缽體高度Potheights/mm43.02～44.88缽體含水率Potwatercontent/%80.36～81.75

試驗主要儀器為Instron 5543萬能試驗機(美國Illinois Tool Works公司),直徑為100 mm的P100平板探頭,直徑為5 mm的P5圓柱形探頭;TD型電子天平(精度為0.01 g)。試驗材料和主要儀器見圖1。

A:番茄缽苗缽體;B:Instron 5543萬能試驗機.A: Tomato seedling pot; B: Universal testing machine Instron 5543.圖1 試驗材料和主要儀器Fig.1 Experimental materials and major instruments

1.2 試驗方法

缽體壓縮試驗采用平板壓縮方式,利用Instron 5543萬能試驗機和P100平板探頭,自上而下加載,加載速度為1 mm/s。由于缽體為四棱臺形狀,需要預先構建傾斜載物臺,保持缽體和壓縮平板接觸面水平。試驗時,隨機選取10株番茄缽苗,重復測試20 mm壓縮量下缽體的抗壓特性。

缽體穿刺試驗采用Instron 5543萬能試驗機和P5圓柱形探頭,測試模式為壓縮測試力,加載速度為1 mm/s。缽體在穿刺開始前進行預處理,在每個缽體頂端均勻選取4個穿刺位點進行穿刺試驗[17]。試驗時,隨機選取10株番茄缽苗,重復測試30 mm穿刺位移下缽體的穿刺特性曲線。

缽體蠕變試驗采用平板壓縮方式,利用Instron 5543萬能試驗機和P100平板探頭,加載力設置為5 N,蠕變保持時間為120 s。試驗時,隨機選取10株番茄缽苗,重復測試缽體在特定加載力下的蠕變特性,并得出缽體的平均蠕變量。

在自動移栽過程中,缽體由取苗機構帶至栽植機構上方釋放,缽體下落至栽植機構內基質會造成損失,缽體以一定高度下落后缽體保持完整是機械化植苗的關鍵。為了區(qū)分缽體散落后是否為裸苗,定量分析缽體散落特性,定義散落過程中缽體散落率P,表達式為

式中:m1為散落前缽體的質量,g;m2為散落后缽體的質量,g.

缽體散落試驗測試500 mm跌落高度缽體的基質損失率情況,搭建500 mm高度的散落平臺,隨機選取30株番茄缽苗,剪去缽體外部的缽苗莖葉。試驗時,缽體以自由落體的方式從500 mm高度的散落平臺跌落,記錄散落前后缽體的質量并計算缽體散落率。

2 結果與分析

2.1 缽體壓縮試驗

蔬菜自動移栽時,需要考慮到取苗機構對缽體的擠壓力。為得到缽體受取苗機構作用力時的抗壓力與變形間的關系,選取1組試驗數(shù)據(jù)進行多項式回歸(圖2),回歸決定系數(shù)R2為0.995,回歸方程為

y=0.010 3x3-0.174 9x2+1.447 4x-1.306 5。

圖2 番茄缽苗缽體抗壓力-變形回歸曲線Fig.2 Pressure-deformation regression curve of compression experiment for tomato seedling pots

由圖2可知,缽體抗壓力-變形曲線前期階段為非線性,且抗壓力隨著變形的增加變化較小,其原因是該階段缽體比較松軟,基質間空隙較多,受壓時缽體基質顆粒間出現(xiàn)滑動、坍塌和重新排列,從而使得缽體在受壓變形時其抗壓力變化較小,缽體表現(xiàn)出生物屈服軟化特性。隨著缽體不斷受壓,其內部基質趨于緊密,曲線顯示缽體抗壓力急劇增加,缽體表現(xiàn)出生物壓實硬化特性。在壓縮過程中,缽體抗壓力與變形關系為非線性曲線,沒有明顯的線彈性。

同時對回歸方程進行求導,將變形為5.64 mm的點定義為缽體屈服點M,此時缽體抗壓力為3.15 N。屈服點M的斜率是回歸方程斜率的最小點,其斜率為K=0.46,將此斜率K定義為缽體的壓縮剛度。缽體受壓時在屈服點內會出現(xiàn)生物屈服軟化現(xiàn)象,取苗夾取力在此范圍內容易出現(xiàn)松弛,不利于缽體的夾取,所以缽苗自動移栽時選擇的夾取力要超過屈服點壓力。

2.2 缽體穿刺試驗

由于缽體和育苗穴盤間的間隙極小,移栽時取苗機構無法直接擠壓夾取缽苗,取苗機構一般先通過取苗針對缽體進行穿刺,再夾緊取出缽苗。圖3A是缽體穿刺試驗的穿刺力-位移曲線,表征缽體在穿刺時穿刺力和位移之間的關系曲線。選取圖3A曲線的1組數(shù)據(jù)進行回歸,如圖3B所示,得到缽體穿刺時的穿刺力與穿刺位移的回歸曲線。由圖3A可以看出,在缽體穿刺試驗過程中,其穿刺力隨著穿刺位移的增加而增大,但在不同的位移處表現(xiàn)出來的變化趨勢完全不同。為了更好地分析缽體穿刺規(guī)律,如圖3B所示,分階段對測試數(shù)據(jù)進行回歸分析,回歸方程式為

A:缽體穿刺力-位移曲線;B:缽體穿刺載荷-穿刺位移回歸分析. A: Puncturing force-displacement curve of the pots; B: Puncturing loads-puncturing displacement regression curve of the pots.圖3 番茄缽苗缽體穿刺特性曲線Fig.3 Puncture property curve of puncture experiment for tomato seedling pots

回歸方程的決定系數(shù)R2分別為0.997、0.804、0.997,均大于0.800,說明該方程與試驗擬合良好,試驗誤差較小[18]。通過回歸方程可以知道,當穿刺位移小于13.5 mm時,穿刺載荷隨著穿刺位移的增加而增大,并呈線性關系;當穿刺位移大于13.5 mm且小于19.5 mm時,隨著穿刺位移的增加,穿刺載荷基本保持不變;當穿刺位移大于19.5 mm時,穿刺載荷隨著穿刺位移的增加而顯著增大,穿刺載荷與穿刺位移的關系為非線性曲線。

對于缽苗缽體這種特定的農(nóng)業(yè)物料,內部是缽苗根系和缽體基質的復合體,表現(xiàn)出較復雜的穿刺特性。初始階段的線性變化原因在于隨著穿刺位移的增加圓柱形探頭與缽體的接觸面積增大,且接觸面積的增大與位移呈正比關系,導致摩擦力線性增加;中間階段由于圓柱形探頭進入缽苗根部區(qū)域,受到的阻力相對較小,導致穿刺載荷在缽體中部能夠基本保持不變,此時的穿刺載荷約為5 N;最后階段由于圓柱形探頭與缽體的接觸面積持續(xù)增加,且缽體中部根系的作用,穿刺載荷顯著增大,表現(xiàn)出非線性曲線關系。

2.3 缽體蠕變試驗

在取苗過程中,缽體處于一個定壓力下的蠕變狀態(tài)中,必須明確有關力學特性和確定其基本參數(shù),這樣才能使得取苗機構的設計、計算具有理論性和實用性,達到數(shù)量化的描述。圖4是恒定載荷條件下缽體的蠕變特性曲線,表征缽體的蠕變位移隨時間的變化規(guī)律。

圖4 番茄缽苗缽體蠕變特性曲線Fig.4 Creep property curve of creep experiment for tomato seedling pots

圖5 Burgers模型示意圖Fig.5 Schematic diagram of Burgers model

蠕變模型采用四元件的Burgers模型,如圖5所示,該模型在外力作用下同時呈現(xiàn)出彈性變形和黏性流動變形,可以較好地描述黏彈性物料的性質[15,19-21]。因此,選擇Burgers模型來分析缽體的蠕變特性,其蠕變變形隨時間變化的關系式為

式中:D(t)為任意時間t時的變形量,mm;t為蠕變時間,s;F0為恒定載荷,N;E0和Er為彈性系數(shù),N/mm;e是自然對數(shù)的底數(shù),e=2.718 28;Trel為延遲時間(Trel=η1/Er),s;η0和η1:黏性系數(shù),N·s/mm.

依據(jù)最小二乘法原理,利用Matlab軟件對缽體蠕變試驗數(shù)據(jù)進行擬合,并作回歸分析,求出其蠕變特性參數(shù)及相關系數(shù),缽體擬合方程的決定系數(shù)R2為0.981,由蠕變特性參數(shù)可得缽體蠕變過程Burgers擬合方程為

D(t)=-9.599 4 et+0.000 8t+9.439 2。

圖6為缽苗缽體蠕變試驗和模擬結果,通過比較可以看出蠕變模擬結果與本試驗結果吻合較好,表明Burgers模型能夠很好地描述缽體在壓縮過程中的蠕變特性。此外,在加載力為5 N,保持時間為120 s的條件下缽體的平均蠕變量為0.398 9 mm,可見其蠕變量較小,機械夾取時夾緊松弛受缽體蠕變量變化的影響不大。

圖6 缽苗缽體蠕變試驗結果和模擬結果Fig.6 Experimental and simulation results of seedling pot creep deformation

2.4 缽體散落試驗

蔬菜自動移栽機從穴盤中取苗后,夾持缽體至釋放點,取苗機構釋放缽體使其落至栽植機構內。這是由于受到碰撞和沖擊作用,缽體基質會造成損失,這一過程中缽體基質的損失情況對設計取苗機構和栽植機構具有重要影響。試驗將隨機選取的30株番茄苗缽體標記序號,并稱缽體質量m1,然后依次進行500 mm高度散落后再次稱缽體質量m2,通過計算缽體散落率P,判斷缽體移栽散落過程對缽體基質的影響。試驗結果如表2所示,番茄苗缽體散落后最小散落率為18.36%,最大散落率為28.23%,平均散落率為22.73%。由于缽體是一個根土復合體,在試驗條件下缽體散落后只是表面部分基質損失,掉落后缽體主體形狀完整,依然為缽苗,滿足自動移栽要求。從分析試驗的過程可知,部分番茄苗的根系不夠發(fā)達,缽體基質比較松散,都會造成散落后基質損失率偏大。

表2 番茄苗缽體散落率試驗結果

3 結論

3.1 番茄缽苗缽體受壓時,其抗壓力與變形關系為非線性曲線。當變形為5.64 mm時,缽體屈服點出現(xiàn),對應的缽體抗壓力為3.15 N。取苗時取苗機構的夾取力應超過缽體的屈服點壓力,才有利于夾緊且不容易出現(xiàn)松弛。

3.2 缽體是缽苗根系和基質的復合體,缽體穿刺位移在0～13.5 mm范圍內穿刺力隨穿刺位移呈線性增加,穿刺位移在13.5～19.5 mm范圍穿刺力基本保持不變,當缽體穿刺位移大于19.5 mm時穿刺力急劇增加。

3.3 在加載力為5 N、保持時間為120 s條件下番茄缽苗缽體的平均蠕變量為0.398 9 mm,缽體蠕變特性符合Burgers模型的蠕變規(guī)律,擬合方程的決定系數(shù)R2為0.981。

3.4 苗齡為23 d的番茄缽苗缽體在500 mm散落高度自由跌落,缽體散落率為18.36%～28.23%,平均缽體散落率為22.73%,缽體散落后只是缽體表面基質損失,主體形狀完整,依然為缽苗,有利于移栽作業(yè)的完成。

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Experiment on physical and mechanical properties of tomato seedling pot for automatic vegetable transplanter.

Liang Xifeng1,2*, Cai Yangyang1, Wang Yongwei2,3

(1.CollegeofMechanicalandElectricalEngineering,ChinaJiliangUniversity,Hangzhou310018,China; 2.CollegeofBiosystemsEngineeringandFoodScience,ZhejiangUniversity,Hangzhou310058,China; 3.TaizhouHuikeAgriculturalMachineryTechnologyDevelopmentCompanyLimited,Taizhou318050,Zhejiang,China)

China is the largest country of vegetable production in the world, and more than 60% vegetable varieties are planted by seedling transplanting, which mainly depends on manual work. However, there are some shortcomings in the traditional manual transplanting, such as high labor intensity, low productivity, poor quality and high cost. It is necessary to do some researches on the vegetable transplanters to adapt to the requirements of the mechanization of agricultural production. Currently, the domestic vegetable transplanters are mainly clip-chain transplanter, seedling guide tube transplanter, nacelle-type transplanter and flexible disc transplanter. The transplanters can meet the basic seedling transplanting requirements, but still stays in the semi-mechanization level. The research work of the vegetable transplanter is mainly focused on the design and optimization of the seedling picking and transplanting mechanism. However, automatic vegetable transplanter needs integrated knowledge and technology, the research on which should also combine the structural design with the physical and mechanical properties of vegetable seedling pot. Thus, it will be helpful for further optimal design of the key components of automatic vegetable transplanter, such as the picking seedling mechanism and the transplanting mechanism.

Tomato seedlings with the age of 23 d were used as experiment objects. The physical and mechanical properties of the tomato seedling pot have been tested and analyzed, which were directly related to the transplanting machine design. Tablet compression of seedling pot has been done with a universal testing machine Instron 5543 and P100 flat probe. In the experiment, the regular pressure-deformation was analyzed and the clamping force characteristics of pot during seedling picking were determined. Pot puncture test using a universal testing machine Instron 5543 and P5 cylinder probe has been done to analyze the puncture characteristics of seedling pot during seedling picking. At the same time, to analyze the creep characteristics of seedling pot, creep test has been done using a universal testing machine Instron 5543 and P100 flat probe and combined with the describe of Burgers model. Then a scatter platform with 500 mm height was built and pots were scattered from the platform. The scattering rate of seedling was obtained and the scattering characteristics during transplanting were analyzed. Testing results show that the relationship between the capacity of resistance for compression and deformation was non-linear. The capacity of resistance for compression on yield point was 3.15 N when the deformation of the pot was 5.64 mm, so the picking seedling force should exceed the calculated yield point in the design of picking seedling machinery. In the puncturing process, the relationship between puncturing force and displacement was approximately linear at the displacement of 0 to 13.5 mm, and then it remained constant in the displacement range of 13.5 mm to 19.5 mm , while it increased significantly when the puncturing displacement was greater than 19.5 mm. Seedling pot creep property conformed to the creep law of Burgers model, and the average creep value was 0.398 9 mm in the condition of loading force of 5 N with 120 s retention time. In the scattering process, the tests showed the average scattering rate of seedling was 22.73%, and the seedling pots kept intact after falling, which met the requirements of mechanical planting.

The research results on the physical and mechanical properties of vegetable seedling pot will provide important data for the design on the key components of automatic vegetable transplanters.

vegetable transplanters; tomato seedling pot; physical and mechanical properties

國家高技術研究發(fā)展計劃(863)資助課題(2012AA10A504); 浙江省自然科學基金項目(LQ13E050003)。

2015-03-26;接受日期(Accepted):2015-07-10;網(wǎng)絡出版日期(Published online):2015-09-18

S 223.9

A

*通信作者(Corresponding author):梁喜鳳(http://orcid.org/0000-0001-8373-2482)，E-mail:lxfcjlu@163.com

URL:http://www.cnki.net/kcms/detail/33.1247.s.20150918.1812.024.html