Effects of Spring-tine Settings and Operational Conditions on the Mechanical Performance of a Weed Harrow Tine

ARTICLE IN PRESS Biosystems Engineering (2005) 91 (1), 21–34 doi:10.1016/j.biosystemseng.2005.02.005 PM—Power and Machinery

Effects of Spring-tine Settings and Operational Conditions on the Mechanical Performance of a Weed Harrow Tine K. Duerinckx; A.M. Mouazen; J. Anthonis; H. Ramon Department of Agro-Engineering and -Economics, Faculty of Agricultural and Applied Biological Sciences, Kasteelpark Arenberg 30, B-3001 Heverlee, Belgium; e-mail of corresponding author: [email protected] (Received 19 May 2004; accepted in revised form 10 February 2005; published online 22 April 2005)

Understanding the mechanical actions of weeding with a spring-tine harrow at different tine settings is necessary to achieve optimal weed control. The mechanical actions of a tine harrow in two different soils at two different locations on the harrow were investigated, pointing out the effects of varied implement settings and operational conditions on the tine weeding performance. A commercially available flexible harrow tine was investigated in sandy and sandy clay loam soils. The tine was pulled in soil bins without plants in order to avoid biological variances. Tine movements and forces acting on the tine were measured with strain gauges and analysed as the mean backward and upward tine torsion, the variation of the tine location around the mean position in sideward and upward direction and the properties of the vibrational frequency in sideward and upward direction during working. In addition, a high-speed camera was used to provide visible description of tine mechanical actions. Results showed that different tine settings and operational conditions and their interaction with soil type and tine location have different responses on certain weeding and tine parameters. High selectivity requires minimising the tine upward and lateral movements, preserving a constant penetration depth and a constant distance from crops. This could be ensured by a low speed, a thin tine and a trailing or vertical tine orientation for a constant depth and trailing or leading tine orientation for constant distance from crops. However, effective weed uprooting would require a high speed, a deep penetration, standard thick tine and a leading tine orientation. As the high selectivity and high effectivity requirement need different tine settings, optimisation of the weed harrow depends on intended effects. r 2005 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd

researchers and manufacturers, inter-row weed remains unsatisfactorily controlled and demands specialised growing schemes (Rasmussen & Ascard, 1995; Melander & Rasmussen, 2001) in addition to the farmers experience. In the worst case, insufficient weed control could lead to the necessity of hand weeding (Landbouwleven, 2003). Field tests performed by ‘Landbouwcentrum voor Voedergewassen’ in Belgium showed that fields with mechanical weed control alone have a lower yield than fields with only chemical or with mixed mechanical-chemical weed control (Haesaert et al., 2003). It is expected that the tine harrow affects the plants both directly by contact between plant and tine and indirectly by soil movement caused by the tine. On the

1. Introduction Harrowing is a common practice for mechanical weeding. The spring-tine harrow used to control intraand inter-row weeds has a high capacity and is relatively cheap. Although it is an environmentally friendly tool, crops can be damaged during weed harrowing, as can happen with all mechanical weed control techniques. Many researchers have been working on assessment of weed control by tine harrowing. Rasmussen (1990) found that high levels of weed control are generally associated with a more than proportional rise in crop damage. In wet conditions, harrowing is insufficiently selective (Koch, 1964; Wilson et al., 1993; Rasmussen, 1990, 1998). In spite of the considerable effort of 1537-5110/$30.00

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r 2005 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd

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Notation a, b c E I k K F M S T Df

proportionality factors damping factor of the tine, Nm s frequency when the movement of the tine has the highest energy, Hz inertia of the tine, Nm s2 spring constant of the tine, Nm mean tine torsion in the soil, N torque exposed to the tine top, Nm mean of the strain gauge signal, V standard deviation of the strain gauge signal, V flexural variation of the tine top position about its mean operational position, mm2 difference between the vibration frequency of the tine in the soil and its eigenfrequency, Hz

other hand, the tine movement is influenced by the presence of plants and variable soil resistance. Kurstjens et al. (2000) indicated that even with a uniform seed depth, the natural variation in emergence created a lot of differences in anchorage strength. Selectivity, identified as the ratio of eliminated weed over crop damage is dependent on several controllable and uncontrollable factors. The latter involves weather, soil properties and economic factors, whereas the former involves timing, implement settings and operational conditions. In order to achieve optimal weed control with the highest selectivity, the controllable factors should be optimised depending on the non-controllable conditions. Many researchers, farmers and manufacturers carried out a large number of experiments aiming to find the optimal utilisation of the weed harrowing systems under various experimental conditions (Rasmussen, 1990, 1992; Peruzzi et al., 1993; Rydberg 1993). Most of these studies explored the effect of different implement settings on the efficiency of weed control and their influence on the yield. Only few papers reported fundamental research on the mechanical impact of the weed harrowing. However, understanding of the mechanisms of the weed harrow would assist the achievement of the optimal weed control performance of the harrow. Kurstjens et al. (2000) and Kurstjens and Perdok (2000) studied the specific impact of the weed harrow on plants. These researches exploited differences in plant properties to achieve selectivity. They examined the effect of working depth, speed and moisture content on achievable selectivity in sandy soils. Since they used a rigid weed harrow tine, the effect of tine bending by soil resistance was eliminated. In fact, bending of the tine usually leads

y y0 y00

angle over which the tine moves, rad angular speed of the tine movement, rad s1 angular acceleration of the tine movement, rad s2

Subscripts g tine working in the soil r tine during the run-up stage, reference data Y movement in lateral direction Z movement in upwards trailing direction

to variable tine depth during operation, which is expected to have considerable consequences on the weeding operation. A rigid tine is less effective in practice, since the selectivity of the weed harrow should result from the tine moving away around crop plants. Examining the effect of a flexible tine performancing on different soil types, Duerinckx and Ramon (2004) found that the tine flexibility induced variance in the tine depth and path. However, a flexible tine is required to move away around crop plants and to enable selective weeding. It is still highly required, to investigate the tine setting and tine operational conditions on harrowing with spring-tine harrows. The variability of results on fields was considered too high to find any significant difference in weeding effect due to changing implement settings. Sources of variability are changing parameters that affect the weeding action. They can be classified in four major groups of plant, soil, implement and external parameters such as the weather and driver competence. This large number of influencing factors and their interaction make it very complicated to adjust the controllable factors towards an optimal weed control performance for a combination of prevailing non-controllable factors. Due to this large amount of factors and possible interactions affecting the harrowing performance, it is necessary to optimise the controllable factors by separating them in different groups. Therefore, it is appropriate to focus first on the action of the weed harrow tine in the soil, so that experiments are to be conducted in soils without crops or weeds. This decreases to a great deal the variability and the number of influencing factors, enabling the research to intensify on the effects of the implement on

ARTICLE IN PRESS EFFECTS OF SPRING-TINE SETTINGS AND OPERATIONAL CONDITIONS

the soil. At this moment, no source of information is available for farmers about the optimal spring-tine setting and optimal operational conditions, including speed and tine depth. This study investigated the effect of tine orientation (angle of penetration), tine type, operational depth and speed on the mechanical action of a single spring tine. The study was conducted in two different soils and for two different locations of the tine on the harrow.

2. Materials and methods 2.1. Experimental equipment and procedure The test rig was installed on a metal cradle that moved on rails (Fig. 1). A single weed harrow tine was mounted on the cradle and the rails ensured the tine following an accurately known path. The manually pushed cradle speed was recorded on-line by means of a speedometer. Of the 20 m length of track, about 9 m was used to accelerate 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 already measuring before the tine was pushed in the soil bins. Before the tine entered the soil, the recorded run-up data were utilised as reference during the experimental analysis. Two bins of 08 m by 12 m were used, filled with soil to a depth of 025 m. The 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. Two soils with different textures were used, namely a sandy loam soil and sandy clay loam (Table 1) with equal moisture content of 01 kg kg1 (dry basis). Before filling the soil bin, soil was first raked with a 21 cm spaced rake to remove clods and big particles. The sandy clay loam contained small clods, which could not be removed by raking. To maintain the resemblance to practice 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 pressure of 250 N on a plate of 300 mm by 333 mm.

Cradle

Tine Wheel with speedometer (a)

Y

Direction of travel

X

Soil bins

Wheel Tine Reflectors

Rails (b)

23

Laser position sensor Fig. 1. Experimental arrangement (a) photograph; (b) plan

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Table 1 Soil texture defined according to the USDA Soil Classification Particle size

Sand, g kg1 (dry basis)

Sandy clay loam, g kg1 (dry basis)

9077 638 285

7501 481 2018

Sand ð450 mmÞ Silt (2–50 mm) Clay ðo2 mmÞ

Direction of travel

Z Tine X Y Cylindrical tube with strain gauges θ

captured the sensor readings at 300 Hz at all channels. Hardware filters were used to remove the high-frequency noise ð4100 HzÞ; preventing aliasing effects. To determine the exact moment when the tine enters the soil, an OMRON laser position sensor of E3JK R4M2 type was mounted on the cradle. The position laser beam was reflected by reflectors mounted at the beginning and at the end of the bins to be received by the OMRON reciever and created an electronic signal that was logged by the Adwin device (Fig. 1). 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 expose the tine to light. Due to the large number of measurement apparatus needed in the experiment, the measurement with the high-speed camera was carried out separately from the remaining measurements with the other sensors. Additionally, the camera was used from two different viewpoints, so that every test had to be performed in three runs. In the first run, all sensors installed were operated except the high-speed camera. In the second run the high-speed camera was used to take images from the front, whereas in the third run the highspeed camera took images from the side.

X-Z movement Fig. 2. Schematic representation of the spring tine with the upward–backward movement indicated

It is presumed that tines mounted on the first row of a harrow would encounter different mechanical soil conditions to those encountered by tines mounted on the second row. To study the effect of the tine location on the response of the tine to changing implement settings and operational conditions, all experiments are repeated for a second tine passing through the same track of the first tine. In the first run, the tine works through a levelled soil, whereas in the second run the tine works with the same settings through the track opened by the tine in the first pass. Tine torsions were recorded around the Y and Z axes with four strain gauges, as shown in Fig. 2. Torsion around the Y axis is related to the movement of the tine in the X and Z direction, whereas, torsion around the Z axis is related to the movement in the Y direction. The strain gauges were installed in pairs, so that each of them recorded the torsion around one of the two axes. They were fixed on a cylindrical tube, on which the tine was mounted. Servo accelerometers were used to measure the vibrations along the Y and Z axes of the boom, on which the tine is mounted. A data acquisition system from the Adwin series of the Ja¨ger Messtechnik

2.2. Influencing factors The influencing factors affecting the weeding action, as considered in this research, were travel speed, working depth, type of tine and tine orientation. Three travel speeds were used: slow, normal and fast harrowing speed (Table 2). Since the cradle was manually moved on rails, the fast experimental speed was limited to 118 km h1. The difference between the high and normal speed was small. During the analysis, the exact speed was calculated with standard deviations shown in Table 2. The standard shallow tine depth of 002 m was varied by setting the tine tip to a deeper position of 003 m. However, these initial depths could not be kept constant during the test, due to the soil resistance that pushed the flexible tine in a backwards and upwards direction. Table 2 Actual speed according to the imposed speed Measured speed, km h1

Speed

Slow Normal Fast

Average

Standard deviation

54 97 118

044 085 060

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The three used tine types were made of flexible spring steel. The tine tip ends straight without any other soil engaging tools. In addition to the commercially available standard tine, two tines with different shape were also used. The standard tine and one of the two others were 7 mm in diameter, whereas the third tine was of 6 mm in diameter. The non-standard tines were attached to the frame by means of an additional aluminium piece, establishing a connection between the tines and the tube with the strain gauges [Fig. 3(a)]. This connection allowed for extra bending around the Y axis. The standard tine eigenfrequency (127 Hz) was lower than the 14 and 15 Hz for the 7 mm non-standard tine and 6 mm tine, respectively. The tine spring factors were 340, 380 and 390 N m1 for the movement in the X direction for the thin tine, the 7 mm diameter non-standard tine and the standard tine, respectively. Those values were 280, 300 and 210 N m1 for the movement in the Y direction. The effect of tine diameter can be assessed by comparing the 7 mm non-standard tine with the thin tine. Three different tine orientations were selected for the standard tine by adopting three various rake angles of the lower part penetrating the soil, as shown in Fig. 3(b).

In the vertical orientation, the tine penetration part was perpendicular to the soil surface, whereas it formed an attacking angle of +2251 for the leading orientation and a 2251 angle for the trailing orientation. Those orientations were set for the tine in its neutral position without external forces. During working in the soil, all orientations tended to more trailing positions, as shown in Fig. 4. Flexural variations of the tine top position did not make the slightly trailing tine to change occasionally to a leading position, as was checked by the high-speed camera (data not included).

2.3. Processing and evaluation of different 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 directions is F ¼ Iy00 þ cy0 þ ky

Spring loop

Spring loop

Tine

Tine

Additional connector

Cylindrical tube with strain gauges

Standarad tine

Non Standarad tines

(a)

Tine Trailing -22.5°

Direction of travel

(1)

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

Direction of travel

Cylindrical tube with strain gauges

25

+22.5° Upright

Leading (b)

Fig. 3. (a) Different types of spring tines with attachment; (b) different orientations of the tine

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Z Direction of travel Y

X

Position during working

Initial position

Leading

Leading FX

FX

F FZ

FZ

F

Vertical F

F

Slightly trailing

FZ FX

Trailing F

F

Trailing

FZ

FZ

FX

FX

Fig. 4. Forces (F) on the tine according to different tine orientations, both in initial position and during working in the soil

N m s; k the spring constant in N m; y the angle over which the tine moves in rad; and y0 and y00 its first and second time derivative. As 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

sideward movement. The X and Z movements could not be analysed separately. From the strain gauge signals (Fig. 5), three different parameters were calculated. The mean tine torsion K in the soil along the X and Z direction was calculated from

y ¼ ðF  Iy00 Þ=k

K ¼ aðM g  M r Þ

(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 gauge signal. The strain gauge 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

(3)

where: Mg is the mean of the strain gauge 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 proportional factor to transfer V to N. The tine torsion K is proportional to the backward shifting of the tine caused by tine–soil forces and is therefore a measure of

ARTICLE IN PRESS EFFECTS OF SPRING-TINE SETTINGS AND OPERATIONAL CONDITIONS

Eg

Tine top position

Mg

Er K

Mr

ðDf ÞY ;Z ¼ E g;Y ;Z  E r;Y ;Z

Time, s

Tine top position

T

Mg

Eg

(b)

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 less predictable and precise track. The flexural variation TY,Z is a measure of the tine displacement, but since y00 is not zero in the case of an oscillating tine, it is not a measure for forces on the tine top. The difference between the vibration frequency and the eigenfrequency for the movements along Y direction and Z direction DY ;Z in Hz was found to be best calculated as

In soil bin

Out of bins (a)

27

Time, s

Fig. 5. (a) Strain gauge signal of the tine location in and out the soil bin; (b) close up of the strain gauge signal in the soil bin: K, mean tine torsion in the soil; Mg, mean of the strain gauge signal while the tine is in the soil; Mr, mean of the strain gauge signal during the run-up stage; T, flexural variation of the tine top position about its mean operation position; Eg, vibration frequency of the tine in the soil; Er, vibration frequency of the tine free in the air

location. However, since the tine oscillates around its mean position, the mean of y00 in Eq. (2) equals zero, and is K 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 bSg;Y ;Z

(4)

(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 eigenfrequency defined when the movement has the highest energy during the run-up stage (reference data) in Hz. The first period of the vibration signal when the tine is entering the bin was a transition stage with larger amplitude than normal, therefore this data was ignored. Owing to the complete factorial experimental design, a multiple analysis of variance (ANOVA) was appropriate. For the calculations, the SAS 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. The prerequisites for the use of general linear model (no correlation between influence factors and the variances of the residues homogeneous) were tested and found to be appropriate. All analyses were conducted with 95% probability. All pair wise comparisons between groups and hence all calculated differences were made by the Tukey-analyses to ensure an overall reliability of 95%. The four influencing factors were studied in combination with the two different soil types and the two different tine locations. At each new experiment, the tine passed through a levelled track. For every combination of the influencing factors, tine location and soil type, two replications were conducted. This low number of replications caused a low power of the experiment for the first level effects of the influencing factors. Therefore, effects of influencing factors and interactions may not be found significant, in spite of their existence. However, in situations were there is no interaction between the studied influencing factor and soil type, statistical calculations can be conducted with four replications due to this hidden replication. From the images taken by the camera, calculations of the tine movements in absolute measures were made.

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Measures for the vibration amplitudes are peak-to-peak values in mm.

3. Results and discussion Tine torsion around the Y axis while working in the soil K originated from forces acting on the immersed part of the tine in the soil. The magnitude of these forces is directly related to soil resistance governed by several factors, namely, soil type, initial bulk density, moisture content, penetration depth and speed. Those forces were needed to overcome soil strength and initiate soil failure, which will increase the tine vibration frequency over the eigenfrequency (Df40). This is because soil failure occurs in successive waves, leading to repeated tine deflection and increasing the vibration frequency. At that moment, the tine will rebound forward and shear the soil, inducing the higher frequency than the eigenfrequency. Soil shear causes a wavy movement of the tine by alternately holding and releasing the tine. However, as the soil is a compactable bulk material, it can also act as a damper. Therefore, the frequency in loose soils will be lower than the undamped eigenfrequency (Dfo0). On the other hand, the external friction and adhesion between the tine and the soil will tend to hold the tine backwards and consequently push it upward, causing torsion around the Y axis. For a standard tine type at upright orientation with 002 m depth of the tip and a normal travel speed (standard settings), the mean upward deflections in sandy and sandy clay loam soil are 7 and 10 mm, respectively, which corresponds backward forces of 5 and 8 N, respectively. The flexural variation of the tine top about its mean operating position along the XZ direction is a measure

of the depth variability TZ. In addition, variation about the mean position in the Y direction TY is important. 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. The 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 (eigenfrequency). For the standard settings, the variation from the mean tine position is 2–4 mm in the Y direction and 3–4 mm in the Z direction. Knowledge about the reasons that introduce variation will help understanding the mechanism of the weeding process. The deviation Df of the tine vibration frequency from the eigenfrequency is a measure of this effect. A tine vibrating at its own eigenfrequency produces zero value of deviation from the vibration frequency, while external influences make the vibration deviate from its eigenfrequency. In addition, 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 might loosen more plants and cover a wider area with soil. Tables 3–6 contain 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 their probability is lower than 005. In the left part of the tables, the interactions of the influencing factors with soil type and tine location are presented. If for a certain response

Table 3 Probabilities of different response parameters to travel speed V and interactions with soil type S and tine location T Response parameter

Probabilities Interactions VST

MTT FV Z FV Y DF Z DF Y

0038 093 095 059 080

VT

033 0997 093 048 040

Effect of travel speed VS

1st tine

2nd tine

1st tine

2nd tine

Sand

scl

Both soils

Both soils

0024 092 067 033 069

082 095 060 0991 073

0011 — — — —

00088 — — — —

— 042 041 023 047

0058 0055 028 027 076

scl, sandy clay loam; MTT, mean tine torsion; FV, the flexural variation of the tine top position about its mean operating position in vertical (Z) and sideway (Y) direction; DF, the difference between the vibration frequency of the tine in the soil and its eigenfrequency position in vertical (Z) and sideway (Y) direction.

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Table 4 Probabilities of different response parameters to imposed depth D and interactions with soil type S and tine location T Response parameter

Probabilities Interactions DST

MTT FV Z FV Y DF Z DF Y

053 089 081 013 063

DT

015 069 059 0071 081

Effect of imposed depth DS

1st tine

1st tine

2nd tine

048 056 044 064 061

083 055 032 002 090

2nd tine

Both soils o00001 092 064 021 036

Sand

scl

Both soils

— — — 0023 —

— — — 030 —

00039 038 030 — 018

scl, sandy clay loam; MTT, mean tine torsion; FV, the flexural variation of the tine top position about its mean operating position in vertical (Z) and sideway (Y) direction; DF, the difference between the vibration frequency of the tine in the soil and its eigenfrequency position in vertical (Z) and sideway (Y) direction.

Table 5 Probabilities of different response parameters to tine type P and interactions with soil type S and tine location T Response parameter

Probabilities Interactions PST

MTT FV Z FV Y DF Z DF Y

021 091 023 013 036

PT

00012 091 096 025 051

Effect of tine type PS

1st tine

2nd tine

1st tine

2nd tine

Sand

scl

Both soils

Both soils

003 084 056 023 014

087 030 019 048 035

00015 — — — —

00015 — — — —

— 013 0083 0087 087

001 002 0086 0070 035

scl, sandy clay loam; MTT, mean tine torsion; FV, the flexural variation of the tine top position about its mean operating position in vertical (Z) and sideway (Y) direction; DF, the difference between the vibration frequency of the tine in the soil and its eigenfrequency position in vertical (Z) and sideway (Y) direction.

parameter, an influencing factor is found to interact with e.g. soil type (probability o005), this means that this influencing factor will have a different effect in the two soil types, and the influencing parameter is analysed separately for the two soil types. The interaction between the influencing parameters and tine location is not presented for the two soil types separately, since analyses found that this effect did not differ significantly for the two soil types.

3.1. Travel speed The tine of the first row encountered larger mean tine torsion in the Y direction K at higher speed, for both soils. In soil–machine systems, soil cutting is generally a dynamic process showing increases in tool forces with

increasing speed. This increase is attributed to the force of inertia referring to soil mass or/and rate effects referring to the effect of strain rate of soil–tool interaction (McKyes, 1984; Shen & Kushwaha, 1998). The differences between the low and normal speed were significant in both soils, while the differences between the normal and high speed were small, corresponding to the smaller difference in actual speed (Table 3). The effect of speed on mean tine torsion was larger in the sandy clay loam soil than in the sandy soil. It has been noticed by many researchers that the cohesion and adhesion of clay increase with an increasing deformation rate, increasing the tool cutting forces (Wismer & Luth, 1972). At higher speed, the inertia force of the soil induced a larger tine force in the sandy soil due to the small cohesion. The large cohesion in the sandy clay loam lead to an increase in tine force introduced by the

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Table 6 Probabilities of different response parameters to tine orientation O and interactions with soil type S and tine location T Response parameter

Probabilities Interactions OST

MTT FV Z FV Y DF Z DF Y

063 085 061 056 081

OT

001 053 060 097 025

Effect of tine orientation OS

1st tine

2nd tine

1st tine

2nd tine

Sand

scl

Both soils

Both soils

022 058 096 0056 018

078 048 050 069 051

015 — — 059 —

00058 — — 0063 —

— 026 020 — 027

078 041 032 084 085

scl, sandy clay loam; MTT, mean tine torsion; FV, the flexural variation of the tine top position about its mean operating position in vertical (Z) and sideway (Y) direction; DF, the difference between the vibration frequency of the tine in the soil and its eigenfrequency position in vertical (Z) and sideway (Y) direction.

strain rate effect. Therefore, in the sandy clay loam the increase in tine torsion around the Y axis was attributed to the effects of both the force of inertia and strain rate. Similarly, the second row tines encountered larger soil resistance at higher speed in both soils. The difference between speeds on tine torsion was less pronounced compared to the first tines, since the second tine passes through the partially opened trench, where forces of inertia and strain rate will have less effect. The effect of speed on tine torsion K changes significantly between the first and second row of tines. In addition, this change also changes with soil type. A significant probability of 0038 was calculated for the interaction among speed, soil type and tine location (Table 3). In both soils and for both tine locations, the variation of the tine top position about the mean position in the Z direction TZ was larger at the fast travel speed. However, the effect was found to be insignificant (Table 3). The increase in variation at higher speed was also found in the measurements with the servo accelerometers mounted on the beam. This indicates that the larger variation was induced by the increase in vibration of the beam, and accordingly it is not a reflection of different tine–soil interaction behaviours at different speed. In addition, the increase in variation of the tine top position due to larger speed is larger for the second row tine than for the first tine. This sensitivity of the second row tines to external vibrations (e.g. beam vibration or tractor vibration) might make the tines in the second row to cause unequal weed efficiency. Similarly, the variation of tine top position in the Y direction TY was found to be originated from the beam vibration. For both tine locations, sideward variation of tine top position increased with increasing speed. Considering the data of the two tine locations together,

an overall increasing in top position variation with increasing speed was found with an almost significant probability of 0057. Increasing side movement may decrease selectivity due to uncontrollable tine paths. The vertical tine vibrational frequency was smaller at the low speed than at the high speed for both tine locations, but the difference in vibrational frequency Df Z was insignificant. Travel speed had no effect on the sideways vibrational frequency difference Df Y :

3.2. Imposed depth The mean tine torsion K around the Y axis increased with depth with a strongly significant probability for both tine locations (Table 4), which can be attributed to the increasing soil resistance with greater depth. Previous classical studies on soil–tool interaction reported an increase in tool forces with increasing cutting depth (Garner et al., 1987; Stafford & Hendrick, 1988; Khalilian et al., 1988; Shinners et al., 1990; Upadhyaya et al., 1984; Mouazen et al., 1999; Mouazen & Ramon, 2002, Rodhe et al., 2004). No interaction between soil type and imposed depth was observed on the variation in K (Table 4). It was shown that the imposed depth had no significant effect on variations of the tine top position TY or TZ in both soils (Table 4). When working deeper, the variation in vertical tine top position became a bit larger while the variation in sideward position decreased slightly for both tine locations in both soils. The frequency of the first tine movement decreased insignificantly in the two directions Df Z and Df Y with increasing tine depth, resulting from a larger soilinduced damping. For both depth settings, the tine vibrated below its eigenfrequency. The second tine

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vibrated vertically Df Z at frequencies higher than the eigenfrequency for both depths in the sandy clay loam soil and for the standard shallow depth of 002 m setting in the sandy soil. The deep setting of 003 m in the sandy soil caused a significant lowering in vibration frequency below the eigenfrequency compared to the standard depth. The sideways frequency of the second row tine occurred at low frequencies under the eigenfrequency at the deep tine setting, whereas the sideways frequency was larger than the eigenfrequency at the standard depth in both soils.

3.3. Tine type For the first row tines, the effect of different tine types on the mean tine torsion K seemed to be similar in both soils, with smaller differences in the sandy soil, leading to a significant interaction between soil type and type of tine (Table 5). The effect of tine type was larger for the tines in the first row than in the second row, resulting in a significant interaction effect between tine location and tine type. The difference in torsion between the three tines was found to be significant. The largest mean tine torsion was found with the standard tine, whereas the smallest was found with the thin tine of 6 mm diameter. For both tine locations and both soil types, the nonstandard tine of 7 mm diameter experienced an intermediate torsion. The smallest torsion with the thin tine of 6 mm diameter can attributed firstly to its smaller spring rate and secondly to the smaller soil resistance due to its smaller volume. The torsion difference between the three tines was larger when working in the sandy loam soil than in the sandy soil, which can be attributed to the presence of small clods in the sandy clay loam. Experiments with the high-speed camera in the sandy clay loam soil showed that the non-standard tines were more sensitive to small clods and obstacles. This was indicated by a smoother movement of the standard tine than the non-standard tine. Equations (2) and (4) show how the tine spring constant is important for the calculation of the tine top position form the strain gauges signal. For the analyses of the variance of the tine top position, the signal was divided by the spring constant of the corresponding tine type, since those constants differ between tine types. Tine design was important for the variation in the tine top position. The thin tine established less variation TZ and TY in comparison with the two thick tines. This was attributed to the combination of the smaller tine kinetic energy and to the smaller soil resistance that resulted from the smaller soil volume moved. The former was also visible during the run-up stage, where the thin tine variation was significantly less than the thick tines (data

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not presented). The latter might result in a more cutting action of thin tines. The location of the tine was not important for the effect of tine type, neither was the soil type. The overall effect of tine type in both soils and for both tine locations was significant with probabilities of 00044 and 00012 for the vibration in XZ direction and Y direction, respectively. The overall effect of tine type on the difference in vertical vibrational frequency Df Z was found to be significant (probability 0023) for both soils and for both tine locations, with more lowering of the tine frequency of 45 Hz obtained with the thin tine compared to the two thick tines. However, the extra difference of vibrational frequency between the two thick tines was insignificant. The thick tine had to move a larger soil volume than a thin one, which introduced a larger shear resistance and accordingly a larger tine vibration frequency. The effect of the tine type was similar for both tine locations. The tine type had no significant effect on sideward vibrational frequency difference Df Y ; which differed with soil type and tine location. Due to the large variances and the low number of replications the specific effect of tine type on sideward vibrational frequency could not be assessed for each situation.

3.4. Tine orientation The leading tine orientation in the first row experienced the significantly largest mean tine torsion K, in comparison with the other two orientations shown in Fig. 3(b). The overall force component acting on the tine penetration part in the soil is pointed downwards at the leading tine orientation, as shown in Fig. 4. The downward force tended to push the tine downwards– backwards around the Y axis, initiating a larger torsion, but with a rather smaller upwards tine deflection. At the vertical and trailing tine orientation, the overall force acts upwards on both tine orientations but with a different magnitude corresponding to the tine penetration angle. Mouazen et al. (1998) found that the soil zone in front of a downward-inclined shank was under compressive loading. In similar mechanism, the harrow tine at the vertical or trailing orientation was subjected to a compressive loading and consequently the tine was pushed upward, resulting in a larger upward tine deflection, but smaller torsion around Y axis. The difference in the overall force acting direction is in-line with what has been found by other researches, working with different tillage tools at similar penetration angles (Chi & Kushwaha, 1990; Gee-Clough et al., 1994; Mouazen et al. 1999). In the second row, there was almost no difference in K for different tine orientations.

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For the comparison of the variations of the tine top position around the Y axis TZ between the different tine orientations, it should be noted that a same rotational angle around the Y axis, which is measured by the strain gauges, creates a larger displacement in the Z direction for the trailing orientation then for the leading orientation [Fig. 3(b)]. The leading tine orientation caused larger variations of the rotational angle than the other orientations for both tine locations in both soils, with too large statistical variances to establish a significant difference (Table 6). The high variations of tine angle with the leading tine orientation were a reflection of the inverse direction between the total force acting on tine pushing the tine downwards–backwards (Fig. 4) and the tine free movement in the upward–backward direction. In addition, the high-speed camera showed the tine with the leading orientation to oscillate up and down, whereas the tine with the trailing orientation moved smoothly in the soil (Fig. 6). The interaction between the tine orientation and soil type on variation of the tine angle was insignificant, as was the interaction with tine location. The vertical tine tended to have a larger variation of the tine top position in the Y direction TY than the inclined tine orientations, due to the larger soil

resistance of the inclined tines. This resistance was larger because for the same imposed depth, a greater part of the tine was immersed in the soil then with the vertical orientations. The leading and trailing orientations have about the same variation in position. The overall effect was not found significant. The interaction effect between the tine orientation and soil type on difference in vertical vibrational frequency Df Z differed slightly from being significant with a probability of 0056 for the first tine (Table 6). In the sandy soil, the effect of tine orientation was insignificant, where small differences in frequency were found. In contrast, the effect was more important in the sandy clay loam soil. In this soil, the trailing tine orientation caused the lowest vibration frequency (5 Hz), which increased with the vertical (117 Hz) and leading orientation (135 Hz). The opposite direction between the overall downwards–backwards forces acting on the leading orientation and the tine upwards–backwards possible movement shown in Fig. 4, caused the leading tine to vibrate at a higher frequency than the other orientations. With the trailing orientation the direction of the overall forces and the possible tine movement were identical, resulting in rather continuous tine trust

Fig. 6. Difference in variation of the tine top position about its mean operating position in the Z direction (T0 Z) in sandy soil (a) trailing orientation highest flexure in the soil; (b) trailing orientation lowest flexure in the soil; (c) trailing orientation neutral position out of the soil, (d) leading orientation highest flexure in the soil; (e) leading orientation lowest flexure in the soil; (f) leading orientation neural position out of the soil; with the mean tine torsion ðK 0 Þ in the soil indicated. The loupes show how the dash-dotted lines are located at the height were a line on the tine is drawn. K 0 and T 0Z are measures, found by image analyses, for the same response parameters as K and TZ in the text; scaling: main photograph, – 10 mm; ellipses: — 10 mm

ARTICLE IN PRESS EFFECTS OF SPRING-TINE SETTINGS AND OPERATIONAL CONDITIONS

in the upwards–backwards direction. However, the tine orientation had no influence on the sideways vibration frequency Df Y : The results reported with the strain gauges coincided well with behaviour of the tine shown by the high-speed camera. In the sandy clay loam soil, the tine at leading orientation was lifted only 2 mm upwards compared to the vertical and the trailing orientation, which were lifted of 10 and 12 mm, respectively. In the sandy soil, the tine at leading orientation was lifted 2 mm, whereas the vertical and trailing orientation are lifted 7 mm. The smaller resistance of the sandy soil caused a smaller lift of the vertical and trailing oriented tines.

3.5. Selectivity versus effectivity The above-discussed experiments confirmed that the soil type and tine orientation had important interactions with the implement setting and operational conditions on the weeding efficiency. By this, soil type and tine location are very important factors to be taken into consideration during optimisation of machine set up and operational conditions to provide optimal selectivity and weed control. An appropriate weeding practice should be sufficiently effective while maintaining large selectivity. The high selectivity can only be satisfied in case the weeding performance is kept unique along the tine path, and hence the horizontal and vertical deviations of the tine should be minimised. The low vertical movement could be achieved by a low driving speed, the use of a thin tine and the tine in the trailing or vertical orientation. Also, the sideward movement of the tine has to be low to permit optimal selectivity, which demanded a low driving speed, a thin tine and trailing or leading orientation. On the other hand, effective weeding might be favoured when the tine exerts sufficient force into the soil that results in large torsion around the Y axis. This was likely to occur at a high driving speed, a deep penetration, a thick and less flexible tine and a leading tine orientation. The effective weeding requirement is in contrast with the selectivity requirement, corresponding to Rasmussen findings (1990). This ensures the complication of optimising implement setting and operational conditions. Proper tine settings and operational conditions should be adopted for certain desired responses (high selectivity or effective weeding). To assess a mathematical optimisation of selectivity versus effectivity, a quantitative study of the effects of tine settings on both parameters would be needed. This study indicates that these effects would vary with soil properties and tine location on the harrow, and it can be

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expected that the effects of different settings also interact themselves.

4. Conclusions For successful weeding, high effectivity should be combined with high selectivity. A high mean backward spring tine torsion, corresponding to large soil-tine forces, might cause high effectivity while small flexural variations of the tine top position about its mean position are expected to be required for sufficient selectivity. Sideway variations of position would lead to an irregular distance from the crop row and upward variations would cause irregular working depth. The effect of the studied tine settings was dependent on soil type and tine location and those factors should be accounted for when optimising weed harrow settings. However, some findings applied for both tested soils and tine locations. High travel speed increased the mean tine torsion, but also raised the variation in tine top position. A larger working depth increased mean torsion and had no effect on the variation but decreased the frequency of tine movement. The largest mean torsion was found with the standard tine of 7 mm diameter, whereas the smallest was found with the thin tine of 6 mm diameter. The thin tine caused least tine top position variations. Because of overall downward forces, the leading tine orientation experienced the largest mean backward torsion, in comparison with the vertical and trailing orientations, and the largest variations about the mean position in the backward direction. Due to the larger soil resistance, the tine with vertical orientation had less variation in its position than the other studied tine orientations. High selectivity could be ensured by a low driving speed, a thin tine and a trailing or vertical tine orientation (constant depth) and leading or trailing tine orientation (constant distance to crops). However, the effective weed uprooting required a high driving speed, a deep penetration, a thick (less flexible) tine and a leading tine orientation. As the high selectivity and high effectivity requirement need different tine settings, optimal settings of the weed harrow depend on intended weeding result.

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