Soil Influences on the Mechanical Actions of a Flexible Spring Tine during Selective Weed Harrowing

ARTICLE IN PRESS Biosystems Engineering (2007) 96 (1), 7–18 doi:10.1016/j.biosystemseng.2006.09.007 PM—Power and Machinery

Soil Influences on the Mechanical Actions of a Flexible Spring Tine during Selective Weed Harrowing A.M. Mouazen; K. Duerinckx; H. Ramon; J. Anthonis Division of Mechatronics, Biostatistics and Sensors (MeBioS), Faculty of Bioscience Engineering, Kasteelpark Arenberg 30, B-3001 Heverlee, Belgium; e-mail of corresponding author: [email protected] (Received 15 September 2005; accepted in revised form 20 September 2006; published online 20 November 2006)

The mechanical action of a flexible weed harrow spring-tine during post-emergence selective treatment was evaluated in different soil textures and properties, and for different tine locations. The tine movements and forces acting on the tine were measured with strain gauges and analysed as mean tine torsion in the soil, the flexural variation of the tine top position and the properties of the vibration frequency during cultivation. In addition, a high-speed camera provided a visible description of the tine penetrating the soil. Experiments were conducted indoors in soil bins to reduce external influences. No plants were involved in the experiments in order to avoid biological variances. Results showed that the soil texture and properties had important effects on the tine mechanical action of the spring-tine harrow. They affected the movements of the tine during soil penetration in the vertical and lateral directions. The tine movement was mainly influenced by means of clods and soil physical conditions, namely, moisture content and soil aggregate size that eventually affect the soil mechanical properties. Also repetition of cultivation through the same track caused different tine behaviour. Tine flexibility induced large variance in tine depth (3–4 mm) and path (2–4 mm). Flexibility made the tine more sensitive to variations in soil properties, which might lead to reductions in selectivity and effectivity of the weed harrow. r 2006 IAgrE. All rights reserved Published by Elsevier Ltd

second group contains the controllable factors, more specifically tine and implement design and settings. Improvement in harrowing is directed towards the modification of the controllable parameters in order to achieve an optimal weed control system. However, the large number of and the interaction between these influencing factors makes it very complicated to adjust the controllable factors towards an optimal weed control performance proper for a combination of prevailing non-controllable factors. Many researchers, farmers and manufacturers have carried out a large number of experiments aiming to find the optimal utilisation of weed harrowing systems under various experimental conditions (Rasmussen, 1990, 1992; Peruzzi et al., 1993; Rydberg, 1993). Most of these studies have explored the effect of different implement settings on the efficiency of weed control and the influences on crop yield. Duerinckx et al. (2005) investigated the effect

1. Introduction Harrowing is a widespread technique for mechanical weed control. The spring-tine harrow is used to control intra- and inter-row weeds. In addition to the high capacity of the spring-tine harrows, they are, comparing to a field sprayer, relatively cheap with respect to investment cost and environmentally friendly as no chemicals are involved which can leave residues in the soil. However, as with all mechanical weeding techniques, the direct interaction with crops can cause damage to the plants during working. Rasmussen (1990) found that high levels of weed control are generally associated with a more than proportional rise in crop damage. The selection of an effective mechanical weed control system depends on many factors, which can be classified in two main groups. The first group, the uncontrollable factors, involves weather and soil properties, while the 1537-5110/$32.00

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r 2006 IAgrE. All rights reserved Published by Elsevier Ltd

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Notation a b c E

proportionality factor proportionality factor damping factor of the tine, N m s calculated frequency of the maximum energy content, Hz F torque exposed to the tine top, N m I inertia of the tine, N m s2 K mean tine torsion around X and Z directions k spring constant of the tine, N m M mean value of strain gauges signal of measured torsion around the Y axis, V m minimal value of response parameter q N adapted effect of soil texture or tine location on tine action R real frequency of the maximum energy content, Hz S standard deviation of the strain gauges signal of measured torsion, V2

of four controllable parameters, namely, driving speed, tine orientation, working depth, and tine diameter on the tine action, while Duerinckx and Ramon (2004) studied the effect of these influencing parameters on soil deformation. These four controllable parameters tested all affected the tine movement and forces, the dimensions of the track formed and soil flow. Kurstjens and Perdok (2000) found that the optimal weed uprooting was achieved when the tine passed 3 mm from the test plants. To achieve such an accurate tine path, the tine top location sideway variation should be minimised. In addition, the tine sideway movement is also a measure for the variable soil resistance to tine penetration. Kurstjens (2000) explained that to increase selectivity (removing weed plants only) of the tool, harrowing force needs to be as constant as possible. Duerinckx and Ramon (2004) found that the vibrational frequency of a spring tine in the soil affects soil loosening and soil transport and that a tine moving at higher frequency loosened more plants and covered a wider area with soil. However, understanding the mechanisms of weed harrowing would benefit the development of tine designs and assist the achievement of optimal weed control. Fundamental research on the mechanics of weed harrowing (Kurstjens et al., 2000; Kurstjens & Perdok, 2000) exploited differences in plant characteristics that resist weed covering with soil and uprooting. However, these studies were restricted to sandy soils and rigid-tine harrows. The latter study eliminated the influences of the soil or crop reaction forces on the tine, which resulted in tine deflection and a variable working depth

SD standard deviation of response parameter q T flexural variation of the tine top position about the mean operation position, mm2 Vi,q value of the response parameter q for the influencing factor i Df difference between the tine vibration frequency and the natural frequency, Hz y angle of tine movement, rad Subscripts i g q r Z Y

influencing factor on tine action tine cultivating soil response parameter tine free in the air movement in the X and Z direction, upward and backward movement in the Y direction, sideward

and tine path. Thus, the mechanical action of a spring tine in different soil textures and properties needs to be explored and is expected to have significant consequences on the weed control operation when spring-tine harrows are used. The optimal settings of the weed harrow depend on the intended weeding result, where an assessment should be made between selectivity and effectivity. This study aims to investigate the effect of the uncontrollable parameter soil texture and physical properties and controllable parameter tine location on the tine mechanical action during post-emergence selective weed harrowing. It is assumed that crop plants are better anchored than weed plants because, due to appropriate tillage, planting or sowing and cultivation, the crop plants have a head start with respect to the weeds. Ideally, the tine should be controlled such that it applies enough force to remove the weeds (effectivity) but not too much force to remove or damage the crop (selectivity).

2. Materials and methods 2.1. Test arrangement and equipment The complete test facility was installed indoors on a metal cradle that was moved on rails (Fig. 1). A single flexible tine was mounted on the cradle and the rails forced the tine to follow exactly the desired path, apart from the recorded movements of the tine itself. The

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indoor set-up with soil in soil bins reduced the number of external influencing factors as weather and preceding field cultivations. Hyvo¨nen et al. (2005) found that management factors had more influence on the presence of weed populations than did soil and physical properties. No weeds or crops were grown in the soil bins. Since the aim of this study was to investigate the effect of soil properties on tine actions and the presence of plants would only cause disturbances in the tine path and forces, increasing variance in the analyses. The cradle was manually pushed, while the speed was recorded on-line by a speedometer. To accurately verify the imposed speed, the actual speed was calculated afterwards from the experimental data. Out of the 20 m long rails, about 9 m was used to speed up the cradle to the desired speed. The next 4 m was used to perform the measurements, while the remaining 7 m was used to stop the cradle. The sensors started measuring before the tine was pushed in the soil containers; these recorded ‘runup’ data were utilised as reference during the experimental analysis. To determine the moment when the cradle with the tine entered the soil, an Omron laser position sensor of E3JK R4M2 type was used. This signal indicated when the tine entered and left the soil bin. Five bins of 08 m by 12 m were filled with soil to a depth of 025 m. The bin walls perpendicular to the driving direction were only 015 m high, which allowed

the tine to enter the soil after the free movement throughout the air during the run-up stage (Fig. 2). Only one type of spring tine was used in the experiments, which is commonly used in practice (manufacturer: Vanhoucke, Belgium). It was made of a flexible round spring steel rod of 7 mm diameter, ending in a straight tip without any other soil engaging tools. The tine has a natural frequency of 127 Hz for the sideward movement and of 14 Hz for the backward–upward movement. Figure 3 presents the tine set-up with the co-ordinate system. Deflections of the tine around the Y and Z axes were recorded with strain gauges. The former is related to the movement of the tine in the X and Z directions, whereas, the latter is related to the movement in the Y direction. Two sets of strain gauges recorded the torsion around the two axes. Since the tine deformations are expected to be large, the strain gauges

Z

Tine during run up stage Tine during measurement stage vertical orientation slightly trailing orientation X

Actual working depth 0.02 m Imposed working depth

Soil surface Soil bin

Direction of travel

Wall perpendicular to driving direction Fig. 2. Section of the experimental set-up with tine flexure

Cradle

Direction of travel Y axis

Torsion spring

Cylindrical tube

Tine

Tine

Wheel with speedometer (a)

Strain gauges

Y



Direction of travel X Cradle

Z Speedometer

Soil bin

Soil bin

X

Tine

Y Reflectors

Laser position sensor

Rails

(b)

Fig. 1. Experimental set-up: (a) photograph and (b) plan

X−Z movement Fig. 3. Schematic representation of the spring tine with the upward–backward movement indicate; y, is angle of tine movement

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were glued on the cylindrical tube on which the tine was mounted instead of on the tine itself. A data acquisition system from the Adwin series of the Ja¨ger Messtechnik captured the sensor readings at 300 Hz sampling frequency at all channels. Hardware filters were used to remove the high-frequency noise in the signals (4100 Hz), preventing aliasing effects. A Redlake digital high-speed camera (MotionXtra HG-110 K) recorded images of the action of the weed harrow tine at 250 Hz. The exposure time was 2998 ns. Two halogen lamps of 500 W were mounted on the cradle to illuminate the tine and surrounding soil.

2.2. Experimental conditions During all experiments, driving speed was 97 km/h with a standard deviation of 085 km/h. The working depth of the tine tip was 002 m and a vertical tine setting, not leading nor trailing, was adopted. Although, the working depth was set to 002 m and the orientation vertically for the tine free in the air without soil resistance, the actual working depth and orientation during cultivation in the soil changed (Fig. 2). This was due to the tine flexibility that allowed the upwards bending caused by the upward reaction forces imposed by the soil. The depth decreased and the orientation changed to slightly trailing. While cultivating, the tine never adopted a leading orientation. The experimental conditions are given in Table 1. On a weed harrow in the field, tines mounted in different rows pass the same track. To investigate the effect of the soil on tines located more to the back of the

harrow, experiments were conducted where the tine passed a second and a third time through the track opened in the first passing.

2.3. Soil preparation Soil texture, moisture content and the presence of clods were investigated. A mixture of soil sample was prepared for texture analysis, which was done using the combination of wet sieve and hydrometer tests. The determination of soil texture was based on the triangle of soil texture classification of the United States Department of Agriculture (USDA). Three soil textures were found, namely, a sandy (S) soil, a sandy clay loam (SCL) soil and a sandy loam (SL) soil (Table 2). Tested soil conditions are given in Table 1. The S soil contained aggregates adhered together by organic macroparticles as small roots. The SCL soil with low organic content formed very hard and big clods up to 010 m in diameter. The largest clods were removed from the two soils by use of a rake with tines spaced 0021 m apart. The SL soil also contained clods, but they were easier to break than the clods of SCL soil. This might be attributed to the bigger expected cohesion of the latter due to the larger clay content. The clods in the SL soil were broken manually until the particles were all less than 003 m in diameter, and the bins were raked and levelled using a leveller. Between the clods no small particles could be detected, while small aggregates were found in the SCL soil. The three soils had equal moisture content of 010 kg/ kg (dry basis), achieved by air drying of the soil. Once

Table 1 Experimental design, soil texture and properties considered during the soil bin test Soil

Moisture content, kg/kg (d.b.)

Presence of clods

Speed, km/h

Depth, m

Tine type

Tine location

010 010 010 027 005

Yes (few) Yes Yes Yes (few) No

97 97 97 97 97

002 002 002 002 002

1 1 1 1 1

3 3 3 3 3

Sandy S Sandy clay loam SCL Sandy loam SL Wet sand WS Sieved sandy clay loam SSCL

Table 2 Soil texture defined according to the USDA Soil Classification Fraction Sand (450 mm) Silt (2–50 mm) Clay (o2 mm) USDA, US Department of Agriculture.

Sand,g/kg

Sandy clay loam,g/kg

Sandy loam, g/kg

9077 638 285

7501 481 2018

6123 2490 1387

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the desired moisture content was reached, this level was maintained by covering the soils with a black foil. The moisture content of the soils was measured and kept constant during the experiments. To maintain the resemblance to practical fields on which soil is kept very loose, only a light pressure of 2500 Pa was applied to compact the soil by applying a static load of 250 N on a plate of 300 mm by 333 mm. The S soil was further tested under more wet conditions of 027 kg/kg moisture content (dry basis). This moisture content was reached by mixing the soil with water, waiting for 3 days and repeating mixing the soil again. In general, the soil cohesion decreases with the moisture content, as reported for sandy loam soil by Mouazen et al. (2002). In this study, no experiments were performed to quantify soil cohesion. As a fifth test soil, SCL was sieved with mesh opening of 28 mm and a moisture content of 005 kg/kg. The difference in particle sizes between the four soil conditions, namely, S, SCL, SL and sieved sandy clay loam (SCLS) soil is shown in Fig. 4.

2.4. Processing of measurement signals The signals measured with the strain gauges were used as a measure of the position of the tine top relative to its attachment. In the equation of motion of the tine, valid for both movements in X and Z direction is: F ¼ I y€ þ cy_ þ ky

(1)

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where: F is the torque exposed to the tine top in N m; I the inertia of the tine in N m s2; c the damping factor in N m s; k the spring constant in N m; y the angle over which the tine moves in rad, and y_ and y€ are its first and second time derivative of y. Constant c is negligible since the tine is made of elastic spring steel. So y, and hence the position of the tine top, can be calculated from   y ¼ F  I y€ =k (2) The numerator of this fraction is measured by the strain gauges for the two considered directions of movement. By this, the position of the tine tip relative to its neutral position is proportional to the strain gauges signal. The strain gauges signal around the Y axis represented the forward–backward (X direction) and upward–downward (Z direction) movements (Fig. 2), whereas the signal around the Z axis represented the sideward movement. The X and Z movements could not be analysed separately. From the strain gauge signals, three different parameters were calculated. The mean tine torsion K along the X and Z direction was calculated from
K ¼ a Mg  Mr (3) where: Mg is the mean of the strain gauges signal of the measured torsion around the Y axis in V while the tine is in the soil; Mr is the mean of this signal during the runup stage in V, when the tine is out of the soil oscillating around its neutral position (reference data); and a is a factor proportional to transfer V to N. The value of K is

Fig. 4. Soils used in the experiment

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proportional to the backward shifting of the tine caused by tine-soil forces and is, therefore, a measure of location. However, since the tine oscillates around its mean position, the mean of y€ in Eqn (2) equals zero, and K is also proportional to the torque on the tine tip, and to the forces working on the tine tip. In this study, K is used as a measure for the tine-soil forces. The flexural variation of the tine top position about the mean operation position along both Y direction and Z direction TY,Z were calculated using the following equation: T Y ;Z ¼ b  S g; Y ;Z

(4)

where: Sg is the standard deviation of the corresponding strain gauges signal, during which the tine is in the soil in V2; and b is a proportional factor to transfer V2 to mm2. A higher standard deviation indicates that the tine acts in a less straight, and therefore, is a less predictable and precise track. The flexural variation TY,Z is an exact measure of the tine location, but since y€ is not zero in the case of an oscillating tine, it is not an exact measure for forces on the tine top. However, TY,Z is a qualitative measure for the variations of the tine-soil force. The difference between the vibration frequency and the natural frequency in Hz for the movements along Y direction and Z direction DfY,Z was calculated from ðDf ÞY ;Z ¼ E g; Y ;Z  E r; Y ;Z

(5)

where: Eg is the frequency in Hz when the vibration of the tine along the corresponding axis in the soil has the highest energy; and Er is the measure of the natural frequency defined when the movement has the highest energy during the run-up stage (reference data) in Hz. The frequencies of the tine movement and the corresponding energy content were calculated from the signal of the strain gauges with a fast Fourier transformation (FFT). The last part of the signal during run-up stage, while the tine is moving freely in the air, was used to calculate the natural frequency of the tine Er and the part of the signal during soil engagement was employed to compute the vibration frequency of the tine moving in the soil Eg. Both signal parts have the same length in order to ensure the same frequency resolution for Er and Eg. Since the time of the tine operating in the soil bin was rather short (less then 2 s), the inaccuracy of the frequency resolution in the FFT was high (larger than 1 Hz) and consequently, the calculated frequency E of maximal energy content could differ from the physical frequency of maximal energy content R. The discretisation of the values due to the high inaccuracy of the calculated frequencies may lead to a non-normal distribution of the calculated values. Statistical analyses of the normal quantiles (not shown) indicated that the discretisation effect was not large and did not threaten

the assumption of normality needed for statistical analysis of variance (ANOVA). However, in a few experiments, the tine made some exceptional shock movements during the run-up stage, which introduced high energy levels in low frequencies. This led to a low calculated value of Er that could not represent the natural frequency. In these cases, the frequency with the highest energy content within acceptable distance from the natural frequency was used, providing good values. In many cases, the frequency of the tine movement in the soil Eg did not differ from the natural frequency, whereas in other cases the tine frequency was higher or lower than the natural frequency. This resulted in positive and negative values of DfZ, with a mean value near zero and a large variance. Because of this, a significant difference between frequency of the tine movement in the soil and the natural frequency (Df 6¼ 0) was rarely found. The absolute values of Df were not used in the analysis of Df, since it is important for physical interpretation to know whether the frequency of the movement is higher or lower than the natural frequency. Owing to the complete factorial experimental design where all soil textures are combined with the three tine locations, a multiple ANOVA was appropriate. For calculations, the statistical software SASs system for windows version 8 was used. Since not all cells (groups of observations with the same settings for all parameters) had the same size, the type III ANOVA table was used. All analyses were conducted with 95% probability. All pairwise comparisons between groups and hence all calculated differences were made by the Tukey analyses to ensure an overall reliability of 95%. From the camera images, the absolute tine movements and soil displacement were calculated. The tine movements obtained by the analyses of the camera images were partially the same as found by the strain gauge analyses. However, the strain gauges provided more precise measurements of the tine movements than did the camera, of which the view was disturbed by soil flowing between the lens and the tine. Therefore, most analyses of the results were performed on data of the strain gauges, while the camera was used to find quantitative information. The images of the camera were used as a verification, e.g. change in frequency or amplitude during run-up stage and tine in the soil.

2.5. Interpretation of the processed signals Tine torsion K along the X and Z directions while working in the soil originated from forces acting backwards or upwards on the immersed part of the tine in the soil. The magnitude of these forces is directly

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3. Results Table 3 provides the probabilities of the effects of the tested influencing parameters on the response factors.

Probabilities are the likeliness of an effect differing from zero by coincidence, on a scale from zero to one. A high probability indicates that the tested influencing parameter has probably no effect on the tested response parameter. Effects are considered as significantly differing from zero as the probability is less than 005. In the second column of the table, the interaction effect of soil texture with tine location (first pass, or second pass or third pass) is presented. If for a certain response parameter, the two parameters interact (probability Po005), this means that soil texture will have a different effect depending on which location the tine is mounted. Figures 5 and 6 show the effects of, respectively, soil texture and tine location on the tine action. The values of the effects are adapted for presentation N and are calculated by N i;q ¼ ðV i;q  mi;q Þ=S Di;q

(6)

where: N is the adapted effect of soil texture or tine location on tine action; Vi,q is the value of the response parameter q for the influencing factor i; subscript i is the influencing parameter (one of the five test soil conditions 3 Adapted effect of soil texture N

related to soil strength that is governed by several factors, namely soil texture, initial bulk density, moisture content, penetration depth and driving speed. The soil shear strength, expressed by the Mohr–Coulomb criterion (McKyes, 1984), is function of the two components, namely, soil cohesion and internal friction angle. The forces needed to overcome soil strength and initiate soil failure in order to cut the soil with the tine cause tine deflection and prestressing of the tine along the X and Z directions. Prestressing increases the natural frequency, which is a known phenomenon in modal analyses. Soil cutting by the tine resulted in a wavy movement of the tine by alternately holding and stressing the tine until the tine forces overcome soil strength and soil failure is induced, which is described by Armstrong-He´louvry et al. (1994) as ‘stick–slip’. As the soil is a compactable bulk material it can also act as a damper. Loose materials can damp mechanical vibrations, and soil can decrease tine vibration amplitude. The flexural variation of the tine top about its mean operating position along the X and Z direction TZ is a measure of the depth variability. Variability in working depth causes variable working effectivity and selectivity (Kurstjens & Perdok, 2000; Kurstjens et al. 2000; Duerinckx & Ramon, 2004). In addition, variation about the mean position in the Y direction TY is important. The flexural variations about the mean position are attributed to the soil reaction (external influences) or/and movements of the tine according to its own mechanical properties (natural frequency). The deviation of the tine vibration frequency from the natural frequency Df is a measure of the effect of the tine on the soil. A tine vibrating at its own natural frequency produces zero value of deviation from the vibration frequency, while external influences make the vibration deviate from its natural frequency.

2 1

−1

Δ fY

Δ fZ

0 K

TY

TZ

−2 −3 −4 Tine action parameters

Fig. 5. Effects of soil texture on tine action for a sandy soil ( , S), sandy clay loam soil ( , SCL), sandy loam soil ( , SL), wet sandy soil ( , WS) and a sieved sandy clay loam soil ( , SSCL) on mean tine torsion (K), flexural variation of the tine top position (T) and difference between the tine movement frequency and the natural frequency (Df) for the upward–backward (Z) and sideward (Y) movement

Table 3 Probabilities of soil texture, tine location and their interaction on the studied response parameters Reference Parameter Mean tine torsion K Vertical flexural variation TZ Vertical frequency difference DfZ Lateral flexural variation TY Lateral frequency difference DfY

Probability Soil texture

Location

Interaction

00003 00001 028 050 043

o00001 053 0013 061 0003

041 074 039 041 089

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3 2 ΔfY

ΔfZ

1 0 −1

TZ

TY

−2 −3 Tine action parameters

Fig. 6. Effects of three tine locations on mean tine torsion (K), flexural variation of the tine top position (T) and difference between the tine movement frequency and the natural frequency (Df) for the upward-backward (Z) and sideward (Y) movement: , first row; , second row; , third row

or one of the three tine locations; first, and second and third) and subscript q is the response parameter (mean tine torsion K, flexural variation of the tine top position T or difference between the tine frequency and the natural frequency Df along the considered axis Y or Z); m is minimal value of response parameter q; and SD is standard deviation of response parameter q. The values m and SD differ depending on the response parameter. For instance, for the adaptation of the mean tine torsion Ni,K, mi,K is the minimal value of the response parameter K over the five tested soil conditions if i is a soil texture or over the three tine locations if i is a tine location, and SDi,K is the standard deviation of those values of K. This calculation ensures that the standardised effects of the same response parameters are equally scaled for both movement directions. The bars in Figs 5 and 6 of mean tine torsions and flexural variations of the tine top positions show how much a certain soil texture or tine location influences the corresponding response parameter, compared to the influencing parameter with smallest value of the response parameter, which is given height zero. The bars of the difference between movement frequency and natural frequency show how much movement frequency changes from the natural frequency. Positive bars indicate an increase in frequency, negative bars a lowering.

3.1. Mean tine torsion in the soil along the X and Z directions The largest mean tine torsion K occurred in the wet sandy (WS) soil (Fig. 5). This is caused by the cohesion due to the wet condition, increasing the forces needed to cut the soil. Soils with clods (SL and SCL) generated

larger mean torsion on the tine than soils without clods (SSCL and S). The hard non-pulverised clods lifted the tine upwards and after moving over the clod, the tine did not move down again immediately, because of its inertia. Since clods could be dragged away by the tine, the tine lifting upwards by clods lasted for long distances, reaching at times 015 m. This created a large mean upward movement of the tine in the soils with clods, and larger mean tine torsion K. Due to the low number of observations in the experiment, the variance was too large to find a statistically significant difference between groups with and without clods. When a flexible tine is pushed aside from its neutral position by a plant, a stone or a clod, it will move back to its neutral position by a harmonic movement according to its damped natural frequency. In fact, the tine returns back to the neutral position after a quarter of its vibration period of 6–10 Hz (0007–0012 m at a driving speed of 10 km/h). Weed plants standing closer than this distance will support each other against the tine, while stones, clods and crop plants also protect weeds. Rasmussen (1992) also found that weed plants support each other during harrowing. In addition, this explains why after the tine encounters a clod, the trench is less deep over a distance up to 7 cm. This implies that during spring tine harrowing, zones are often left untreated after obstacles, decreasing the efficiency of weeding. The untreated surface will be larger in soils with more obstacles as clods. In addition, while a tine drags along a clod, its forces exerted to the soil (mean tine torsion K) are larger and might create a different shape of track. This decreases the homogeneity of the weeding action therefore might lower selectivity. The interaction between soil texture and tine location on the harrow had an insignificant effect on the mean tine torsion K while the separated effects of soil texture and tine location were significant (Table 3). The more a tine was located to the back of the harrow, the smaller the soil resistance (smaller torsion) it encountered (Fig. 6). In the first row, the tine encountered a significantly larger soil resistance than later tines passing through its track. The more a tine was located to the back of the harrow, the smaller was the additional lowering of tine torsion. The first tine made a trench through which the second row tine encountered smaller resistance. This is explained by soil loosening by the first tine and the smaller penetration depth in the track of the second tine. Since the tine penetration depth in soil decreased compared to the first run, the amount of soil being thrown out of the trench during the second pass decreased, leading to a smaller additional effect. The high-speed camera showed that for a location in the first row, the tine was lifted up about 0007 and 0010 m on average when cutting S soil without clods,

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and the SCL with clods, respectively. This means that in the SCL the actual working depth was only half of the imposed 002 m depth. 3.2. Vertical flexural variation of the tine top position round the mean operation position along the X and Z directions

Vertical tine deflection, mm

No effect of the tine location on the harrow was found on the variation of the tine top position along the Z direction TZ. Interaction between the soil texture and tine location had no effect, but the effect of soil texture was found to be significant (Table 3). The S soil and the SSCL soil induced the smallest variation of vertical tine top position TZ, and were almost equal in their effect (Fig. 5). The SL, with a large amount of clods, induced the largest tine top position variations TZ. The WS induced intermediate variations, whereas the SCL induced less positional variation than WS. The SL induced significantly more variation than other soils, WS not included. A study of the movement of the tine itself in the SL soil showed that the variance in the tine position originated from the fact that the tine passed through successive vibration periods. Periods with tine deflection well beyond the mean deflection were alternated with periods with a lower mean deflection (Fig. 7). The difference between those two periods induced a large variation. The periods with a high mean were supposed to reflect the tine dragging a clod, whereas periods with a lower mean reflect the normal tine movement without encountering a clod. The WS soil and S soil differed significantly in their effect on the variation of the tine top position along the Z direction TZ. The movement of the tine around the Y axis is not only a vertical movement, which would reduce the tine to a needle inserting the soil, but also a forward–backward motion constraining the tine to cut the soil. The force needed for soil cutting is dependent on the cohesion of the soil, in this case increased by

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water. When this force is larger, the tine will be lifted more before shearing the soil. The low variations in the mean position TZ in S and SSCL were due to the absence of clods in both soils. In SSCL soil, the tine moved very regularly with low amplitude, while in the non-sieved SCL, the vertical tine top position TZ was more irregular. The vertical deviations from the mean working position of the tine were typically 3–4 mm for S and SCL soils. This value for the vibration amplitude did not cover deflections caused by clods. About 3–4 mm does not seem to be a large variation, but it is significant compared to the real working depth of 13 mm in S soil and only 10 mm in SCL soil according to the camera images. 3.3. Difference between the vibration frequency and the natural frequency for the movements along X and Z directions DfZ An ANOVA analysis could detect no interaction effect of the soil texture and tine location on the harrow on the difference between the vertical vibration frequency and the natural frequency DfZ. However, a study of the values of the difference in frequency, used for the ANOVA analysis, indicated that for a tine passing through the trench, formed by a former tine, the effect of the soil texture on the difference between the vibration frequency and the natural frequency DfZ, was lower than for the first tine (Fig. 6). The large variance in the analysed values of this difference prevented this effect to be proven significantly. Separate analyses on the effect of the tine location show that the frequency at which the first tine moved with maximal energy was lower than that of a tine mounted more to the back of the harrow. For the first tine, a mean lowering in frequency of 54 Hz was observed, compared to the natural frequency of 14 Hz. For a second tine passing on the same track, the vibration frequency increased over the natural frequency

Tine moving normally through the soil

Clod lifting tine

4 0

Clod dug out dragged by tine

−4 0

0.05

0.1

0.15

0.2 Time, s

0.25

0.3

0.35

Fig. 7. Vertical tine movement in sandy loam with effects indicated resulting in high variance:—tine movement, y. mean tine location

ARTICLE IN PRESS A.M. MOUAZEN ET AL.

by 041 Hz. Those frequency values were averaged over the five types of soils. Damping causes lowering in movement frequency, but only a few percentages. Lowering in movement frequencies of several Hz, cannot be attributed to damping. Larger changes in frequency could be attributed to stick–slip (ArmstrongHe´louvry et al., 1994). In WS, no difference between the first pass and the following repetition was observed. This is because the trench formed by the first cultivation action in WS was very narrow and damping did not decrease for the second tine. The overall effect of the number of repetitions was significant. The difference is significant between the first two tine passings (4 Hz), but it was insignificant between the second and the third tine. The extra soil thrown out of the trench during the second pass caused insufficient difference in soil influences. For the first tine location, there was no significant effect of soil texture on the difference between the vibration frequency and the natural frequency DfZ of the tine due to the large variance in the DfZ values. The largest lowering in vibration frequency in the soil compared to the natural frequency was found with the SSCL and S (Fig. 5). In case of the SSCL the frequency of movement for the first tine passing lowered to 55 Hz. The absence of clods in these soils, in addition to the small shear resistance, made the tine move relatively smoothly in the soil without bouncing due to the mechanisms explained by Mouazen et al. (1998). In these conditions, depth variations of the cultivating tine over longer distances caused by local changes in soil properties, causes tine movement at low frequencies to be more influencing the tine top position than the movement along the tines damped natural frequency, which is found in the analyses as a low movement frequency. Both WS and SCL caused lowering as well as increases in frequency of the tine movement, which caused a small mean difference between vibration frequency and natural frequency DfZ. The increases in frequency were still rather small. Both soils had a large shear resistance that created successive failure waves as the tine advanced in the soil, and forced the tine to move at higher frequencies. Due to clods. In SL, the vibration differed little from its natural frequency.

3.4. Sideward flexural variation of the tine top position round the mean operation position along the Y direction Soil texture, tine location and their interaction had insignificant effect on sideward flexural variation TY of the tine top position if data from the three tine locations is analysed together. Figure 5 shows that the effect of soil texture in sideward variation of the tine top position

Ty is smaller than on the vertical variation of position TZ. However, an ANOVA test with only values from the first tine location indicated that in SL the tine top position varied more than in other soils, while it varied the least in the SSCL. Almost no difference in variation was detected between S, WS and SCL. The differences between the five soils were not significant according to a Tukey analysis. Figure 8 shows the difference between the tine movements measured in SL and SSCL. This difference was attributed to clods, because the tine movement in SL with clods is more irregular. The other reason was probably the larger shear strength of the SL soil that increased the amplitude of the sideways vibration TY. The induced compression stress in front of the tine working in SL caused the tine to move not only upwards but also sideways. The sideways variation from the mean working path of the tine was between 2 and 4 mm, which is a relatively large compared to the desired 3 mm distance between the tine and the crop to achieve optimal weed harrowing (Kurstjens & Perdok, 2000). 3.5. Difference between the vibration frequency and the natural frequency for the movements along the Y direction Soil texture had insignificant effects on the difference between the vibration frequency and the natural frequency along the Y direction, DfY, because of the small values of DfY compared to the standard deviations. The analyses indicated that the movement in SSCL occurred at a lower frequency than in SL, caused by the smaller prestress in the sieved soil, leading to decreases in movement frequency (Fig. 8). The interaction between soil texture and tine location was not significant, but tine location had a significant

Sideward tine deflection, mm

16

10 6.8 3.4 0 −3.4 0

0.1

0.2 Time, s

0.3

0.4

Fig. 8. Lateral tine deviation during cutting different soils: y. sandy loam (SL);—sieved sandy clay loam (SSCL)

ARTICLE IN PRESS SOIL INFLUENCES OF A FLEXIBLE SPRING TINE

effect on the difference in sideways vibration frequency. For the first tine, mean decreases of the vibration frequency were detected, while for the second tine passing, an increase in frequency was found. During the first pass in S and SCL soils, the mean frequency of the tine vibration decreased by 25 Hz, compared to the natural frequency, with significant difference. During the second and third passes, there was a rise in the frequency of 08 Hz. It is assumed that the vibration frequency of the tine has an effect on loosening of the soil. A calculation shows that this is incorrect for the spring-tine harrow considered in this study. A typical natural frequency of a harrow tine is between 10 and 15 Hz, and a normal driving speed is about 10 km/h or 28 m/s. During contact with the soil, the natural frequency was lowered to a range of 6–10 Hz. This means that between every period (wave) of the tine vibration, about 025–045 m distance in the travel direction was passed, while not more than 5 mm amplitude in Z and Y directions was observed. This confirms the tine cannot loosen soil and kill weed by vibrating.

4. Discussion An appropriate weeding practise should be sufficiently effective while maintaining large selectivity. The high selectivity can only be satisfied when the weeding performance is kept unique along the tine path, and hence the horizontal and vertical deviations of the tine were minimum. This occurs when the vertical and sideward flexural variations T of the tine top position and the variations in exerted forces are small. On the other hand, effective weeding is favoured when the tine exerts sufficient force into the soil that results in large torsion K in the backward direction and, in addition, to achieve selectivity, an adequate level of force should be exerted. The above-discussed experiments confirmed that the soil texture, soil mechanical properties and tine location on the harrow had important effects on the actions of the flexible spring tine and on the tested selectivity and effectivity parameters during selective treatment. Soil texture, working depth and soil physical conditions influenced the forces needed to penetrate the soil. This is in line with what was reported in other studies for different types of soil tools (Upadhyaya et al., 1984; Rodhe et al., 2004). Since soil conditions vary considerably even within individual fields, selectivity of the weed harrow decreases if tine settings are not adapted to the new situation to meet the new soil conditions. Clods increased the vertical variation in position while loose soils dampened the tine movement. Tine movements are

17

strongly influenced by the soil as soil changes the tine movement frequency. By this, soil texture and conditions are important factors to be taken into consideration during the adjustment of machine set-up and operational conditions to provide optimal weed control with minimal crop damage. To achieve optimal selectivity, it is necessary to readjust tine settings responding to the variable soil conditions (clods, moisture content and texture). The later tine rows are less influenced by changing soil conditions, although the absence of interactions between soil texture and tine location suggests that all rows of tines need a similar adjustment. Probably, the large variances in the measurement data due to the flexibility of the tine prevented the physical interaction from being statistically discovered. Commercial mechanical spring tine harrows are not suited for frequent automated readjusting. Mostly, they are adjusted manually by the farmers before harrowing, which is a time consuming procedure resulting from the need to stop the machine. Although, site-specific spraying is investigated widely (Gerhards & Christensen, 2003; Dammer et al., 1999; Goudy et al., 2001; Vrindts & De Baerdemaeker, 2000; Yang et al., 2003) and proved to be often economically profitable (Barroso et al., 2005) by applying an adapted dose rate of active ingredients, mechanical weeders still cultivate through large inhomogeneous fields with the same settings. If it is to be profitable, machine design of the spring harrow is to be oriented towards automatically adjustable constructions. This means that the design should allow on-the-go adjustment of the harrow responding to the variability of field soil conditions encountered during harrowing.

4. Conclusions Using an indoor soil bin facility, experiments were carried out to evaluate the effect of soil texture and properties and tine location on the harrow on the actions of the flexible spring tine during post-emergence selective weed harrowing. Results proved the following findings and conclusions. (1) Soil texture and soil physical conditions have important effects on tine action during selective harrowing. (2) Vertical and sideward position of the flexible tine varied substantially compared, respectively, to the working depth and desired accuracy of the horizontal tine path. (3) Clods increased the vertical variation in position while loose soils damped the tine movement. (4) The later tine rows are less influenced by changing soil conditions, although the absence of interactions

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A.M. MOUAZEN ET AL.

between soil texture and tine location suggests that all rows of tines need a similar adjustment. Flexibility made the tine more sensitive to variations in soil properties which might reduce the selectivity and effictivity of the weed harrow. Since tine movements are strongly influenced by the soil texture and soil physical conditions, these uncontrollable parameters have to be taken into consideration during the adjustment of machine set-up and operational conditions to provide optimal weed control with minimal crop damage. However, a further study is needed to establish relationships between soil mechanical properties (cohesion and friction angle) and soil-tine interaction properties (adhesion and external friction angle) and tine action at different moisture content, bulk density and textures. Furthermore, the effect of weed plants during the soil bin test should be included, since plant extraction has important effect on tine reaction during harrowing. Different species and growing stages of weeds have to be taken into consideration.

Acknowledgements This project was financed by Institute for the promotion of innovation by science and technology (IWT) in Flanders. Jan Anthonis is financed as postdoctoral researcher by the fund of scientific research Flanders (FWO Vlaanderen). References Armstrong-He´louvry B; Dupont P; Canudas De Wit C (1994). A survey of models, analysis tools and compensation methods for the control of machines with friction. Automica, 30(7), 1083–1138 Barroso J; Fernandez-Quintanilla C; Maxwell B D; Rew L J (2005). Simulating the effects of weed spatial pattern and resolution of mapping and spraying on economics of sitespecific management. Weed Research, 44(1), 413–488 Dammer K H; Schweigert T; Wittmann C H (1999). Probability maps for risk asessment in a patchy weed control. Precision Agriculture, 1, 185–198 Duerinckx K; Ramon H (2004). The response of different soils to changing settings of a flexible spring tine during weed harrowing. EurAgEng Paper No. 508, AgEng04 International Conference, Engineering the Future, Leuven, Belgium, cd rom. Duerincx K; Mouazen A M; Anthonis J; Ramon H (2005). Effects of spring tine settings and operational conditions on the mechanical performance of a weed harrow tine. Biosystems Engineering, 91(1), 21–34, doi:10.1016/ j.biosystemseng.2005.02.2005

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