ฉันแปลประโยคยาวๆไม่ค่อยออก...ต้องใช้เวลา..เDevelopment of a mechanism for transplanting rice seedlings
Edathiparambil Vareed Thomas *
Department of Agricultural & Food Engineering, IIT, Kharagpur 721092, India
Received 29 March 2000; accepted 13 September 2001
Abstract
Transplanting of seedlings is a labor intensive operation in the cultivation of rice. It is also a skilled job
and involves working with a stooping posture in a puddled field. There exists a need to mechanize this
operation. For this purpose the design of a mechanism was carried out following the method of analytical
synthesis. A planar four-bar linkage with coupler extension was selected as the basic design. The path
generated by the mechanism was plotted on a computer screen. By varying the dimensions of various links
in the mechanism different paths of output motion of the coupler point were obtained. The potential link
dimensions were identified based on the suitability of the path for picking, conveying and planting of
seedlings as well as the return motion. A four-row self-propelled transplanter using the above mechanism
and an optimized-planting finger was then developed and tested. The machine transplanting system was
found to be technically viable. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
India is predominantly an agricultural country with rice as one of its main food crop. It produces
about 80 million tons rice annually, which is about 22% of the world rice production.
Culturally, transplanting of young seedlings of 20–35 days age in water-inundated field is preferred
over direct seeding. The former leads to better yield due to better crop management
practices that are possible in a transplanted crop. The operation of transplanting requires large
amount of manpower (about 400 man-hour/ha) and the task is very laborious involving working
in a stooping posture and moving in muddy field. Hence, this is considered as an activity that
needs mechanization. Mechanization of transplanting facilitates mechanization of subsequent
activities also in the production of the crop. The machines that are already successful in Japan and
Korea could not be adopted in India because of economic cost constraints and due to the prevailing
cultural practices of this country.
Mechanism and Machine Theory 37 (2002) 395–410
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Operationally machine-transplanting system may be visualized as follows (Fig. 1).
• The seedlings are stacked in a tray.
• A mechanical finger operated by a suitable mechanism pick up seedlings from the tray.
Nomenclature
L1 length of crank AB
L2 length of coupler
L3 length of follower link
L4 length of fixed link
LF length of coupler extension link
xF x-co-ordinate of coupler point F
yF y-co-ordinate of coupler point F
b angle of inclination of fixed link
d included angle between the coupler and the follower link
/1 angle of coupler
/2 angle of the follower link
p the mathematical constant, 3.14. . .
h crank angle
w included angle between coupler and the coupler extension
Fig. 1. Schematic of a mechanical rice transplanter.
396 E. Vareed Thomas / Mechanism and Machine Theory37 (2002) 395–410
• Seedlings are carried to the soil and placed at proper depth in a nearly upright posture.
• The mechanical finger comes back to its original position so that it can repeat the process.
• The machine moves forward to the next location for planting.
In the present investigation a mechanism is designed for the purpose of transplanting rice
seedlings. The mechanism is driven at about 40 rpm by an engine. The same engine drives the
ground wheels of the transplanter. The velocity ratio between the ground wheel and the crank of
the planting mechanism decides the spacing between the plants.
1.1. Design of planting mechanisms used in power operated transplanters
Anon. [1] states that most of the planting devices of power operated transplanters can be
classified as crank and rocker mechanisms of four-bar linkage. A planting finger, which is a part
of the coupler link of the mechanism, separates the seedlings from the seedling tray and places
them in the soil. The curve traced by the planting finger may have an influence on the stability of
the planted seedlings. The kinematic analysis of the planting mechanisms is considered essential
for an understanding of its operation and its further improvements.
1.2. Design of mechanism
Erdman and Sandor [2] state most mechanism tasks require a single input to be transferred to a
single output. Therefore, single-degree-of-freedom mechanisms are the forms used most frequently.
Shigley [6] states that Grubler’s criterion is concerned with the number of links in the
mechanism and with the number and kinds of kinematic pairs. It can be used for determining the
degree of freedom of a mechanism. Erdman and Sandor [2] state that analysis techniques can be
used to replace costly and time consuming building and testing of physical prototypes in a trial
and error design process. Analysis techniques generally form a basic part of most synthesis
methods. Norton [4] states that the four-bar linkage should be among the first solutions to motion
control problems to be investigated. The fewest parts that can do the job will usually give the least
expensive and most reliable solution. Norton [4] states that the Grashof condition can be used as a
very simple relationship, which predicts the behavior of a four-bar linkage, based on the link
lengths. Zimmerman [7] states that a four-bar mechanism is physically impossible if one of the
links has a length greater than the sum of the other three. Hirschhorn [3] states that in a four-bar
linkage distinct types of mechanisms could be obtained by inversion. A crank-rocker mechanism
is obtained by fixing one of the two links paired with the shortest link. Paul [5] suggested that
Newton–Raphson method could used be used to solve the non-linear equations developed for
solving the four-bar linkage position problem. Zimmerman [7] states that one basic mechanism
design problem for which the four-bar chain can provide solutions is that of finding a point of the
coupler of a four-bar mechanism, which describes a path closely approximating the desired one.
2. Methodology
In a mechanical transplanter the finger follow a desired path of motion. A planar four-bar
linkage with all revolute pairs is chosen, as this is very simple, a mechanism made of that may be
E. Vareed Thomas / Mechanism and Machine Theory37 (2002) 395–410 397
easy to maintain and may cost less to manufacture. The input motion is applied to the crank so
that the motion is continuous and rotary. The output motion follow a suitable path in order to
meet the requirements of a transplanter specified below. The mechanism should have one degree
of freedom and a coupler point that is capable of making a loop may be incorporated. The
planting finger will be attached at the coupler point.
2.1. Output motion requirements of the proposed planting mechanism
(i) The seedlings are to be picked up from a tray where these are arranged. The tray is provided
with slots to facilitate an uninterrupted movement of the fingers. During the downward travel,
the fingers pick up the seedlings and carry these to the soil.
(ii) The seedlings are then to be planted at a desired depth.
(iii) The seedlings are to be planted at a nearly upright position. A deviation of about 30 may
be allowed.
(iv) The fingers are to return to the original position after the planting operation. During the
return motion the fingers are required to move without interfering with the seedlings that are
planted. A reversal of the direction of the velocity of fingers at the end of planting is helpful
for achieving this.
(v) During the return motion, a finger has to pass through a path without interfering with the
tray. This is to prevent any possible disturbance of the seedlings arranged in the tray. Thus, the
path of forward travel and the path of return travel shall be necessarily different.
(vi) The finger must not interfere with other members of the machine during its motion.
(vii) The parts of the mechanism must have proper clearance from the field so as to minimize the
accumulation of soil on these parts. The point of picking has to be located at a proper height
from the field level so that the seedling tray can have proper clearance from the field level.
(viii) Planting cycles are continuous as the machine moves with a constant forward speed of
travel.
The dimensions of four links in the four-bar loop, orientation of the fixed link, length of the
coupler extension and orientation angle of the coupler extension are to be decided for the design.
Since these values could not be synthesized directly it was decided to examine various trial values
for the above parameters and study the suitability of the path generated by such mechanism for
the purpose of transplanting. The above parameters were changed systematically using arbitrarily
set ranges. During the present investigation around 0.23 million linkage designs were examined.
The choices of link dimensions were made following a method of analysis and selection. The
link dimensions were varied as stated above. The path of motion of the transplanting finger point
was calculated and displayed on a computer screen for every trial value of link dimensions.
Initially, link dimensions which met the output motion requirements as stated in Section 2.1 were
selected. Details are given in Section 2.4. In order to study the linkage design further and to bring
out paths that may be better, small ranges of link dimensions were set around the above selected
design values. The path of finger point motion was again examined keeping in mind the requirements
stated earlier. Out of several designs, one, which would likely