Design and development of a 5R 2DOF parallel robot arm for handling paper pot seedlings in a vegetable transplanter

Computers and Electronics in Agriculture 166 (2019) 105014

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Design and development of a 5R 2DOF parallel robot arm for handling paper pot seedlings in a vegetable transplanter

T

K. Rahul, Hifjur Raheman , Vikas Paradkar ⁎

Agricultural and Food Engineering Department, Indian Institute of Technology Kharagpur, India

ARTICLE INFO

ABSTRACT

Keywords: Vegetable transplanter Parallel robot arm Paper pot seedlings Automation

This paper presents a complete mechatronic approach in designing and developing a robotic arm for handling paper pot seedlings in a transplanter for the purpose of inclusion of automation and possible reduction of heavy mechanical components in pot seedling transplanters by utilizing microcontrollers and embedded systems. The robot arm developed was of two degrees of freedom (DOF), five revolute joints (5R) with parallel arm structure using 3D printed parts, commercially available embedded microcontrollers and sensors. The gripper which was connected to the parallel robot arm performed pick and place operations at a desired path. For achieving the preferred maximum and minimum reach of the gripper, a suitable method for synthesizing the link length has been discussed. A complete logical technique to program microcontrollers was developed which enabled smooth actuation of both the independent input links simultaneously. Parabolic blends were utilized at the beginning and at the end of the robot input joint trajectory for minimizing dynamic vibrations during continuous operation. The developed robot arm was flexible to change the number of pickup points in a row, pickup and drop coordinates within the workspace envelope. Finally, the robot arm was tested and evaluated for picking and placing 15 numbers (4 replications, 60 runs) of actual paper pots filled with vermicompost, soil and sand. Each pot weighed approximately 80 g with a moisture content of approximately 5%. The robot arm took 2.1 to 2.4 s to pick and drop a pot seedling from a distance of 116.6 mm with a maximum power consumption of 20.47 W. Experimental results showed that no pots were missed in picking and dropping, no visual cracks and damage appeared on the pots. The repeatability of the robot arm was 2.1 mm in X-axis and 3.4 mm in Y-axis.

1. Introduction The need for mechanization in agriculture arises due to the shortage of labor and an increase in demand for food supply every year due to rise in population. Researchers are working to improve the yield of agricultural products by providing proper amount of nutrients and growing healthy crops in the agricultural field. Leskovar et al. (1991) studied the growth of tomato crops transplanted at different days and suggested favorable transplant growth conditions. Alam et al. (2002) investigated and summarized the effect of crop growth transplanted at different days of seedling growth and different seedling raising practice. A technique for growing vegetable seedlings with optimum pot mix (composition of farmyard manure, soil and sand) has been proposed by Hashemimajd et al. (2004), Kumar and Raheman (2012). Manual transplanting is more labor intensive and spacing between consecutive transplanted seedlings is not uniform. This non-uniformity in the agricultural field causes difficulty in implementing mechanization in the subsequent agricultural operations. So, mechanization in the

⁎

transplantation process is adopted by farmers for efficient vegetable seedlings transplantation and to decrease the operating cost and time. The available transplanters are classified as semi-automatic, automatic and robotic vegetable transplanters. Semi-automatic transplanters require one person to control the steering and more than one person are needed to feed the seedlings into the machine. Automatic transplanters have a seedling transfer mechanism which reduces additional labor requirement. The efficiency of these transplanters depends on the picking and metering mechanism employed. Dihingia et al. (2018) reported that most of the walk behind type transplanters uses 6.75 to 10.58 kW tractors as a power source. Compared to the complicated seedling transfer mechanisms used in automatic transplanters, transplanters with robot arm require less power and space (Hu et al., 2014). Additionally, robot arm is flexible to control in terms of speed, rate of transplantation and incorporation of various sensors for effective transplantation. Boa (1984) developed a tractor mounted automatic pot seedling transplanter and achieved a mean spacing between seedlings ranging

Corresponding author. E-mail address: [email protected] (H. Raheman).

https://doi.org/10.1016/j.compag.2019.105014 Received 27 February 2019; Received in revised form 12 September 2019; Accepted 15 September 2019 0168-1699/ © 2019 Elsevier B.V. All rights reserved.

Computers and Electronics in Agriculture 166 (2019) 105014

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from 22 to 28 cm. Hwang and Sistler (1986) used a 5 DOF serial robot arm in a commercial mechanical vegetable transplanter for pepper plant and achieved a transplantation rate of 6 seedlings/min with a transplanting cycle time of 10 s. Gantry type stationary robotic transplanter for bedding plants was developed by Ryu et al. (2001), which utilized machine vision system and a pneumatic pickup unit to assist the manipulation of gripper for the use of stationary plug seedling transplantation in a greenhouse at an average cycle time of 2 s/seedling. Parish (2005) discussed the advantages such as accuracy, precision and effectiveness by the inclusion of automation in the transplantation process. Kumar and Raheman (2011) developed a two row walk behind type hand tractor operated pot seedling transplanter equipped with complicated metering mechanism and feeding conveyor. It was operated at a forward speed of 0.9 km/h at a transplantation rate of 32 pots/ min. Hu et al. (2014) discussed the dimensional synthesis and kinematic simulation of a diamond-shaped two DOF stationary robot in ADAMS software to fetch and plant seedlings grown in plug trays at a desired trajectory for nursery applications. The transplanting cycle time achieved was 1.09 s/seedling. The two most common techniques for growing seedlings for mechanized transplantation method are (a) seedlings grown in plug trays (Han et al., 2018) and (b) seedlings grown in biodegradable paper pots (Prasanna Kumar and Raheman, 2010; Nandede et al., 2014). To handle seedlings grown in plug trays, a pickup mechanism (Tian et al., 2010) approach the seedlings and delicately penetrate into the tray cavity. The pickup finger then separates the seedling along with soil present in the tray by holding the root portion firmly and further the finger is lifted up. In case of handling pot seedlings, a metering mechanism transport the paper pot seedlings individually in upright orientation (Kumar and Raheman, 2011; Nandede and Raheman, (2016) and it is dropped into the soil furrow. Di Benedetto and Klasman (2004) investigated the plug cell volume requirement in plug trays for better seedling growth and the importance of utilizing plug seedlings in the nursery for growing bedding plants. Saito et al. (1998) and Dihingia et al. (2017) studied the requirement of minimum amount of soil block around the seedlings for the use of automatic mechanical transplanters in agricultural field to avoid the transplantation shock and damages caused due to the forces acting on seedlings during dropping of the seedling blocks into the furrow, covering the seedling with soil and compacting the soil around the seedlings. Dihingia et al. (2017) suggested that, in order to avoid the transplantation shock on seedlings and good reestablishment of plant growth by means of mechanized transplantation in the field, paper pot seedlings are better as compared to the seedlings grown on plug trays. In overall, for transplanting seedlings in nursery and greenhouses, tray grown seedlings would be a better choice because of its controlled environmental conditions. To avoid transplantation shock and better seedling growth in an agricultural field, researchers experimented and suggested to go for seedlings with heavy soil blocks or biodegradable paper pots. Most of the pot seedling transplanters developed to work in the field use an IC engine as a power source for propelling the transplanting vehicle. In engine powered automatic mechanical transplanters, the operator needs to engage or disengage a mechanical clutch system to control the transplantation while taking turnings in the field. Frequent engagement and disengagement of clutch and continuous usage of hand operated machines create fatigue and back pain to the operator due to heavy vibration and noise. To reduce the difficulties faced by operators, to exclude heavy complicated clutch mechanisms while taking turns in agricultural field, to eliminate atmospheric pollution created by gasoline engines, to reduce the overall weight of the transplanter and to incorporate complete automation in field operated pot seedling transplantation, IC engine operated propelling vehicle has to be replaced with electric propelling unit (Zhu et al., 2017) and the metering mechanism is needed to be replaced with robotic arm and sensor integration. As well, most of the robotic transplanters developed so far are immobile (Ryu et al., 2001,

Hu et al., 2014) and the associated grippers are designed to transplant plug seedlings in a greenhouse. As the outdoor agricultural field has an uncontrolled environment (Grift et al., 2008; Bechar and Vigneault, 2016) and uneven soil surface which causes vibrations on the propelling vehicle, researchers are working on developing an autonomous robotic system for field applications (Bechar and Vigneault, 2016). Moreover, commercially available industrial robots are not economically viable to use in field operated transplanters. Hence, there is a need to develop a dedicated electronically controlled robotic system which aids in the transplantation of biodegradable pot seedlings at the outdoor environment with good rigidity at reasonable repeatability, which is affordable to medium scale farmers. The advantages of parallel robots over serial robots are described by Pandilov and Dukovski (2014) and Bourbonnais et al. (2015) and are: (a) accuracy of the parallel robot is higher because the joint errors are added up at the output whereas the joint error of serial robot is multiplied. (b) Operating speed, acceleration and rigidity are higher because the endpoints of the parallel robot have a support member. (c) Singularity (loss of DOF) of the parallel robot may occur within the workspace, whereas singularity of serial robot occurs at the workspace boundary. Nabat et al. (2005) developed a 4 DOF parallel manipulator for pick and place operation and obtained a task cycle time of 0.28 s. Li et al. (2014) designed a 4 DOF SCARA pick and place parallel robot of lightweight by dimensional synthesis. It is reported that the virtual prototype was designed for achieving 150 picks/ min. Li et al. (2018) presented a trajectory planning approach for high-speed pick and place parallel robots by utilizing quantic B-spline curves to generate motion. It was observed that the residual vibrations at end effector substantially reduced due to continuous and smooth torque at joints. Thus it is found that, for the application of pick and place of paper pot seedlings at high speed in a hostile environment, a parallel robot would be a good choice than other types of robots. Hence, while designing a parallel robot, proper care and intense analysis are required to avoid singularities within the operating workspace. This paper describes the design, development and testing of a parallel robot arm with mechanical, electronic, logic and program considerations to handle biodegradable paper pot seedlings for the purpose of incorporating automation in vegetable transplanters handling pot seedlings for field application. 2. Materials and methods The 2DOF 5R planner robot consists of two active links of length L1, two passive links of length L2, one fixed link (O1-O2) of length 2 × L0 and five revolute joints (O1, O2, A, C and D) as shown in Fig. 1. Out of these five joints, two joints (O1 and O2) are called active joints, which

Fig. 1. 2D schematic line diagram of a robotic arm for transplanting paper pot seedlings. 2

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K. Rahul, et al.

are connected with the input actuators which make angle a1 and a2 with the horizontal. Likewise, the joints ‘A’ and ‘D’ are called passive joints. The angles ( p1 and p2 ) made by these passive links with the horizontal axis are referred as passive joint angles. The point C (x, y) is the output which is connected with gripper. The reference frame OXY is located at the mid-point of the fixed link. Hence, by varying the active joint angles ( a1 and a2 ), the gripper location, C (x, y) can be positioned at a desired point in the planar coordinate system.

1

l12

= acos

The kinematic analysis describes the behavior of position and orientation of both gripper and input joint angles or vice versa. The calculated joint angles and manipulator position or orientation are fed to the stepper motors to achieve the desired motion. Two types of analysis (a) Forward and (b) Inverse kinematic analysis are used for mathematical representation of position and orientation of 2DOF 5R robot arm. The kinematic analysis for the robot arm is given below.

2

= acos

1

= atan

2

= atan

l12

y = l1sin(

a 1)

a1) + l2 cos(

+ l2 sin(

y |l 0 + x|

Let, ‘m’ be the distance between point A and D parallel to X-axis

m = 2l0 + l1 (cos(

a2 )

cos(

(3)

a1))

Let, ‘n’ be the distance between point A and D parallel to Y-axis

n = l1 |(sin(

a1)

sin(

(4)

a2 ))|

If ‘B’ is the mid-point of the line joining A and D,

ThusthedistancebetweenAandBis

m2 + n2 2

(5)

From triangle ABC, 1

= acos

2

= atan

m2 + n2 2 × l2

(6)

n m

(7)

The orientation angle or passive joint angle i.e. the angle between link AC and horizontal is given as p1

=

1

(8)

2

Hence, substituting the known angles a1 and a2 in the above equations, the orientation angle p1 can be calculated. Thus the position C (x, y) can be calculated by substituting the four angles in Eqs. (1) and (2). 2.1.2. Inverse kinematics The inverse kinematics determines the input joint angles for the given desired gripper position in planar space. Let k be the length of the line connecting O1 and C

k=

(l 0 + x)2 + y2

(l 0

x)2 + y2

a2

=

2

(a) 20°

y |l 0

+

(14)

x|

1

(15)

2

(16)

160°, 20°

a1

a2

160° (to avoid collision between two arms or with the (17)

gearbox) a1

and

a2

(b) (c)

(9)

(m2 + n2) a2

a1

(2L 2) (condition for extreme reachability)

(condition for dynamic stability)

(18) (19)

(d) For better power transmission and good dynamic stability

Let s be the length of the line connecting O2 and C

s=

1

(13)

In actual working conditions, the robot arm is required to operate in 3D space (X, Y and Z coordinates). Automatic pot seedling transplanters generally consist of a conveying unit which continuously supplies pot seedlings to the picking unit. To locate the seedlings present in a row, the robot needs to manipulate in the X-Y plane. For lifting a seedling from the conveyor, a constant pitch (movement in Z-axis) is desired to transport the seedlings from one location to another. As the requirement of pitch movement is small (less than 2 cm lift up or down) to handle pot seedlings, planner (X-Y plane) workspace analysis is sufficient for the pot seedlings to pick and drop the pot seedlings. Also, reduction in robot dimensionality (reduction of DOF) reduces complexity, actuator requirement, overall weight and power requirement of the picking unit. Thus, the dimensionality reduction of robot arm is beneficial from the economic point of view of a transplanter. The workspace of a robot arm is a region in the Cartesian space, which includes all the reachable positions of a robot gripper under given input conditions. The workspace plays an important role in selecting optimal dimensions of robot linkages and based on the known dimensions, the appropriate actuator for input links are selected. Also, it aids in visualizing of robot reachable positions. The forward kinematic equations were employed in determining workspace and the equations given in Section 2.1 are suitable for programming in a software platform to visualize the contours generated through plots. To generate all the possible reachable positions of the gripper, one of the input angles ( a1 or a2) was fixed and other was incremented. MATLAB software was used to generate the appropriate plots for simulating the workspace under the following conditions for the proper functioning of robot arm to pick and place pot seedlings and the generated workspace is shown in Fig. 2.

(2)

p1)

=

(12)

2.2. Workspace and dimensions of the robot arm

(1)

p1)

a1

l22 + s 2 2l1s

Thus for the known lengths L0, L1 and L2 and at the desired known position C(x, y), upon utilizing the above equations of inverse kinematics, the unknown input angles a1 and a2 can be decided.

2.1.1. Forward kinematics The forward kinematics computes the position of gripper for the given input angles. From Fig. 1,

l 0 + l1cos(

(11)

Similarly, using cosine rule at triangle O2DC

2.1. Kinematic analysis of the robot arm

x=

l2 2 + k2 2l1k

during high (10)

speed operation,

transmission angle (angle between active and passive link axis),

Using cosine rule at triangle O1AC

tr

3

40° (Hu etal. , 2014),

(20)

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Mass of pot seedling, M = 80 g (Kumar and Raheman, 2011) The assumed factor of safety = 1.6

Torque = Force × distance = 0.08 kg × 9.81 × (12.5 + 12.5)cm × 1.6 = 31.39 N.cm (22)

L1 = L 2 (Bourbonnais etal. , 2015)

Detent torque of Stepper motor = 2 N·cm 31.39 Gearbox reduction requirement = 2 = 15.69 ≈ 16 Therefore, the gearbox reduction required is 16:1. Since a small change of angle at the input will cause considerable change in manipulator position, the gearbox should have minimum backlash. Also, the space occupied by the gear reducer needs to be as small as possible. Mousavi et al. (2015) reported that planetary gearbox provided good repeatability and offered a high reduction ratio at lesser space. Thus, a two-stage planetary type reducer (1:4 in the first stage and 1:4 in the second stage, thus a total of 1:16 reduction) was selected for the stepper motor output shaft (Fig. 4a). The developed planetary gearbox (Fig. 4b) consisted of a sun gear at the center, a ring gear at outer and three numbers of planet gear meshed with both sun gear and ring gear. A carrier rotated along with the planet gears and ring gear (Figs. 4a and 5). The sun gear is the input gear which was directly coupled with the stepper motor. The planet gear moved freely at an orbit in between ring gear and sun gear. A carrier, which tracked the rotation of planet gears, served as the output of the gear reducer.

(f) For installing stepper motor, gearbox and spacing between

Gearratioforplanetarygearatonestage, i = 1 +

( )

Fig. 2. Simulated workspace of the 2DOF 5R robot.

where,

tr

= acos

l12 + l22 2l1l2

k2

(Fig. 1)

(21)

(e) To avoid voids in the workspace,

them, the length L 0 was considered as 125 mm

Dr Ds

(23)

where

The conceptual schematic diagram for pick and place pot seedlings (G1, G2, G3 and G4) arranged in a row is shown in Fig. 3. The robot arm was required to reach the ideal points Gj, Aj and D as shown in Fig. 3 (where, j = 1, 2, 3 and 4). The distance between G1 and G4 is 200 mm; the distance between G and A is 60 mm. Hence the manipulator needed to cover a rectangular area of 200 × 60 mm. It was found that selecting a length of L1 and L2 as 125 mm satisfied the above conditions and required work area. Fig. 2 shows all the reachable manipulator points (workspace) of the robot arm.

Dr = Diameter of ring gear (60 mm) Ds = Diameter of sun gear (20 mm) Fortwo

stagedrive, gearratioi = i1 × i2 = 4 × 4 = 16

(24)

3. Development of robot arm 3.1. Mechanical considerations

2.3. Selection of actuators

The real-time performance and accuracy of the robot arm depend upon the rigidity of robot links, the accuracy of fabricated components and accuracy of the driving actuator. The robot links were designed using SOLIDWORKS software and it was fabricated with PLA (Polylactic Acid) material using a 3D printer by FDM (Fused Deposition Modeling) technique. A distance plate (Fig. 6) separated motor-1 and 2 at a distance 2 × L0 served as the fixed link. Gearboxes were coupled with stepper motors, which were rigidly mounted on the distance plate. Two shoulder links of equal length L1 were firmly attached with the gearbox output shaft with an intermediate coupler. Similarly, two elbow links of length L2 were coupled with the shoulder links. The revolute joint formed by shoulder link and elbow link rotated freely with the help of bearing support thus reduced the joint friction. The free ends of elbow links were then connected with a revolute joint with the support of an intermediate connecting shaft. A gripper with two jaws, which opened and closed by the control of a servo motor, was rigidly fixed with the intermediate connecting shaft. Hence, the rotation of input stepper motor and servo motor was transferred as the change of position and orientation of gripper in 3D Cartesian space. The requirement of a robot arm is to transfer the pot seedlings in 3D space at various position and orientation within the workspace, but the linkages of 2DOF parallel manipulator move only in 2D space (X and Y coordinates). Therefore the gripper position has to move in the third dimension (Z-direction). A servo motor was used at the gripper to provide rotary up and down motion (pitch) in the Z direction. A rotary pitch movement of 30° to 45° from horizontal is sufficient to lift the pot seedlings from the feeding conveyor. The actual robot arm developed is

2.3.1. Torque requirement of the actuator L1 = 125 mm, L2 = 125 mm

Fig. 3. The conceptual schematic diagram for pick and place task for robot arm. Note: The number of pickup points (‘i’) and coordinate points (c(x, y)) of Gi, Ai and D are user-defined. Depending on the number of seedlings present in a row and seedling orientation at the seedling feeding conveyor, these coordinates of corresponding picking and dropping points may vary from ideal points within the robot workspace. 4

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K. Rahul, et al.

Fig. 4. Gear box to be used in the robotic arm actuator.

Fig. 5. 3D Exploded view of the two-stage planetary gearbox.

shown in Fig. 7. The orientation (yaw) of gripper was controlled with a servo motor connected at the axis of the intermediate connecting shaft of link L2. The pot seedlings were of cylindrical in shape having a diameter of 45 ± 3 mm and the robot was designed to pick up at the circular face. Therefore, the gripping point was curved to increase the contact area while holding the pot seedling.

implementation of controllers, sensors, man-machine interface, program memory design and a communication method for synchronization of individual components and electric power converters. The general block diagram for the developed electronic circuit which consisted of various electronic components to control the mechanical system is shown in Fig. 8. The motion of robot arm input links was controlled by stepper motors and gear reducers. The present position (angle) of such input links was required to track the position of gripper. Potentiometers attached to each of the robot active joint links provided position feedback to the controller. In order to displace the gripper from one location to another location in a defined trajectory, the actuators were required to work independently at varying speed and torque. Hence, two

3.2. Electronic considerations The robot arm consisted of various mechanical components which were powered by actuators. Therefore, precise control of actuators in terms of position, speed and torque is essential. The electronic system for the developed robot arm included proper selection and 5

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Fig. 6. 3D view of the robot arm with gearbox and stepper motors.

location. It computed the actuator position and velocity data for moving the gripper at the desired trajectory and supplied the computed data to the respective slave controllers through a communication bus. Four numbers of push buttons and an LCD display were connected with master microcontroller and were serving as a man–machine interface of the entire system. These push buttons were used to navigate various modes of operation of the robot arm and to change the position coordinates for pickup and placing of seedlings. For accomplishing data transfer between microcontrollers, I2C communication protocol (Beigl et al., 2003) was used to send data from the master controller to the slave controller to perform the required operations. The data transfer rate with I2C was in the range of 100 to 150 kbps. The ATMEGA 328P (Table 1) chip with 16 MHz crystal oscillator was employed and it had 1 Kb of internal EEPROM memory which was utilized to store data such as pickup, drop point coordinates and the total number of pickup positions. Hence, the robot was able to remember the user-defined coordinates irrespective of input power supply availability. The microcontroller had two external interrupts (Int-1 and Int-2 as shown in Fig. 8) which were used as feedback to indicate the master controller whether the slave controllers have completed the given task or not. 3.3. Programming, control and logic considerations After the mechanical and electronic hardware were implemented, the next important step was to enable the robot arm to make decisions to accomplish the task given through the device memory. Since the robot had to follow a specified sequence of continuous tasks, the logic given to the microcontroller need to be computed in less time and memory to make the operation faster. In order to move the gripper from location D to G and back (Fig. 3) without any collision, e.g. to pick and drop the 1st pot seedling, the sequence of robot movement is D-A1-G1Gripper Close-Gripper lift-A1-D-Gripper flat-Gripper open. In order to move the gripper precisely from one location to another at a specified trajectory, the both input joints are required to move independenty at varying speed using a potentiometer feedback. Thus one master (position and speed computation) and multiple slave (to control each actuator with position feedback) microcontrollers are used to control the robot arm. The flow charts shown in Fig. 9a and b represent the logic implemented in the program memory of microcontroller for picking the

Fig. 7. Developed 2 DOF parallel robot arm using a 3D printer.

microcontrollers (1 and 2 as shown in Fig. 8) were used to supply necessary signals to actuate the respective stepper motors. The microcontroller-3 controlled the actuation of three servo motors for wrist yaw, wrist pitch and gripper opening or closing. The microcontrollers which were used for actuator control are defined as slave controllers. A master controller which communicated with the other three slave microcontrollers read the gripper future position data from memory 6

Computers and Electronics in Agriculture 166 (2019) 105014

K. Rahul, et al.

Fig. 8. Block diagram of electronic system for robot arm.

the inter-communication, the computed a1, a2 , corresponding input joint speed (pulse width sequence) and gripper position (yaw, pitch, jaw open/close) data were sent to slave microcontroller-1, salve microcontroller-2 and slave microcontroller-3, respectively. The slave microcontrollers actuated the corresponding stepper motors and servo motors to the desired position commanded by the master microcontroller. Thus, the robot active joints were controlled simultaneously towards the goal position at the computed velocity by means of microcontroller-1 and microcontroller-2. Upon reaching the goal position, an interrupt signal (‘task completed’) was sent by both slave controllers to the master controller (Fig. 8 and Fig. 9b). Then, the master microcontroller followed the same procedure to reach the next location (GiAi-D) to fetch a paper pot and drop the same. After that, the master controller incremented the variable and repeated the process to accomplish the continuous pick and place operations. Finally, when number of picks were satisfied i.e. after completing the pick and place operations at all positions in a row, the variable was reset back to 1. Thus the pick and place process continued until the power supply was turned off. If the pick and drop positions were needed to be modified, by pressing the push buttons, the initialization block could be accessed. Thus the coordinates and the number of pickup positions present in the EEPROM memory could be changed.

Table 1 List of components used in electronic control system of robot arm. S. No.

Component Name

Manufacturer

Model

1 2 3

Microcontroller Buck converter Voltage regulator

Atmel Texas Instruments SparkFun

ATMEGA328P LM2596S LM7805

paper pot seedlings from multiple fixed pickup points and one dropping point. The program logic consisted of two blocks (a) initialization (Fig. 9a and b) operation sequence (Fig. 9b). In the initialization part, the necessary input data were fed into the EEPROM memory by the user. As shown in Fig. 9a, at first, the total numbers of pickup positions were defined. Then, a variable was initialized. It stored the coordinates (x, y) of approach and grip points at the corresponding memory location at EEPROM of the master microcontroller. Then the variable was incremented and the process was repeated until the condition for the total number of the row was true. Then the coordinates of drop point (x, y) were stored in the EEPROM memory by pressing the corresponding push button. Whenever the robot arm was started, the microcontroller read the number of positions from the EEPROM memory and then it initialized a variable. Then the microcontroller read the target coordinates, presentjoint position received from potentiometer and computed the futurejoint angles ( a1 and a2 ), speed (pulse width required for both the input stepper motor) and gripper position (yaw, pitch, jaw open/close) for the desired trajectory (linear trajectory with parabolic blends) by feeding the input data into the inverse kinematic (explained in Section 2) and trajectory algorithm programmed in the microcontroller chip. Through

3.4. Development of gripper One of the critical components in the robot arm is gripper. The minimal damage caused during handling of pot seedlings is a factor which determines the success of seedling growth after transplantation. Since the paper pots were filled with soil, that contains a considerable 7

Computers and Electronics in Agriculture 166 (2019) 105014

K. Rahul, et al.

Fig. 9. Flow diagram of robot arm for programming and control.

amount of moisture, deformation may occur when a force is applied on the surface of the pots due to soil porosity. The deformation should not go beyond a critical point where the pot seedlings are considered as damaged. Yamanobe and Nagata (2010) proposed an efficient grasp planning method for primitive shaped objects using parallel jaw grippers. Also, the grasp planning method was successfully applied to grasp cylindrically shaped can using parallel jaw grippers. Harada et al. (2016) proposed a multi-fingered shape adaptive gripper to grasp everyday objects. Chen and Lan (2017) developed a force regulation mechanism for grasping fragile objects using parallel jaw gripper at a high speed. Experimental results showed that the gripper required 0.04 s to grasp a regular shaped object of 3.05 mm length. Zhou et al. (2017) have developed a soft robotic gripper using a pneumatic power source. It was reported that the object adaptation was enhanced for grasping complicated shaped objects. Hence, for grasping regular cylindrical objects, parallel jaw grippers would be efficient at high-speed operations. Parallel jaw grippers (shown in Fig. 7) are available in the market and most of them are point contact type. Using point tip grippers may cause critical deformation by applying a point force while gripping the paper pots. A specially designed gripper jaw was designed, fabricated and implemented (Fig. 10) to handle cylindrical paper pot seedlings. Upon considering the diameter of ready to plant paper pot seedlings as 45 to 50 mm (Kumar and Raheman, 2011), each gripper jaw were curved to obtain a contact length of 26 mm during gripping and the width of gripper jaw was 15 mm. Thus, the point contact type parallel jaw gripper was modified as surface contact type gripper according to the requirement of pot seedling grasping.

due to the grasping force. 3.5. Trajectory planning for manipulator The stepper motor had 200 steps per revolution (the control resolution of the stepper motor was 1.8° when a high-low electric pulse was given to the motor driver). The resolution was further increased by the planetary gearbox in the ratio of 16:1 i.e., the 1.8° was further divided into 16 more steps which provided an output resolution of 0.1125°. Upon experimenting the stepper motor with different pulse widths, it was found that jerking (missing of steps) started when pulse cycle time was lesser than 1050 μs due to less time for EMF (electromotive force) generation. Hence, the maximum angular velocity of the input joint was limited to 107.14°/s; thus, max was considered as 107.1°/s. Fig. 11 shows an example of two types (pure linear and linear trajectory with parabolic blend) of trajectory obtained with a robot arm input joint angular position with respect to time for the rotation of an experimenting angle (100° to 300°) at a time span of 2 sec. A continuous linear trajectory motion required infinite acceleration and deceleration which caused jerk at the joints. This jerk was reduced by providing higher order splines at the beginning and at the end of a linear segment (Macfarlane and Croft, 2003). To avoid this jerky motion, parabolic blends were given at the beginning and at the end of the trajectory (Constantinescu, 1998; Kunz and Stilman, 2011). The mathematical expression to achieve the second order acceleration and deceleration (parabolic trajectory) is given below: Let i = initial angle, f = final angle, t i = initial time andt f = finishing time a = start of linear trajectory, b= end of the linear trajectory

3.4.1. Testing of the gripper The gripper with a modified jaw was tested for several pick and place cycles of pot seedlings. The pots were filled with soil and vermicompost mix with a moisture content of 5%. For testing the gripper, 15 numbers of pot seedlings were placed on a model feeding conveyor to grip, lift and release as shown in Fig. 7d. A total of four replications were made (15 × 4 = 60 runs) to check the firmness of grasp. During the test, no paper pots were damaged and no visual cracks were found

tf

ti =

|

i|

f max

(25)

Considering 0.15 s parabolic blend (t p ) for acceleration and deceleration, 8

Computers and Electronics in Agriculture 166 (2019) 105014

K. Rahul, et al.

Fig. 10. Functioning of the developed robotic arm gripper.

a b (t )

=

a

+

b

tf

a

2t p

×t

(30)

3.6. Testing of the robot arm for pick and drop pot seedlings The developed robot arm was tested with paper pot seedlings loaded on a feeding conveyor (Fig. 12). The conveyor was of matrix array type which contained 11 rows and 4 columns (G1 to G4). Paper pots were loaded in each cavity of the seedling feeding conveyor and the robot arm was turned ‘ON’ to pick the pots located at position G1….G4 and dropped the same at point D. Once the row of pots was emptied by the robot arm, the conveyor was rotated to bring the next row of paper pots to the robot picking position and the cycle was continued. The robot arm was tested to analyze the positional repeatability of the robot arm. The robot arm was programmed to move linearly between two fixed points within the workspace. For 30 runs, the X and Y deviations from the set point were noted. The deviation parameters are discussed in Section 4.

Fig. 11. Trajectory of the robot joint with a parabolic blend.

I¸ Therefore at point 'a' I¨¸= max deg/sec2 ti-a i

a (t) =

( +if a

f

>

= ¨t p

i i,

± if

1¨2 t 2 i

>

4. Results and discussion

(26)

The robot arm cycle began at point D and ended at point D to fetch a pot seedling located at any one of the points (G1, G2, G3 or G4) and to drop the seedlings at point D (Fig. 3). The analysis is presented below for the complete cycle (to fetch seedling located at position G4) of the robot arm to determine the performance. Also, the simulated displacement (Fig. 13) and velocity (Fig. 14) curves show both the joint motions are smooth and continuous which reduces the dynamic

(27) f

(28) (29) 9

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Fig. 12. Testing of robot arm and feeding conveyor for picking and placing operations of paper pots. Table 2 Robot arm fetching data for seedling G4. Location

XPostion (mm)

YPostion (mm)

D A4 G4 A4 D

0 100 100 100 0

120 120 175 120 120

a1 (deg)

118.17 69.12 52.82 69.12 118.17

a2

(deg)

61.82 14.23 36.12 14.23 61.82

Time (sec)

Cumulative time (s)

0 0.59 0.35 0.35 0.59

0 0.59 0.94 1.29 1.88

vibrations during motion. It can be seen that the linear distance between D and G4 (116.6 mm) was higher than other pickup positions. Table 2 represents the fetching data for seedling G4 under ideal gripping and approach points (D, A4 and G4) which contain X-Y positions with respect to OXY reference frame of the robot arm. The corresponding input angular position of active joints ( a1 and a2 ) were calculated by feeding the gripper positions (X, Y) into inverse kinematic equations programmed in MATLAB. Then, the calculated angular positions were given as input to the robot trajectory planning algorithm which was programmed in MATLAB. Thus the theoretical time requirement for robot for fetching and dropping of paper pot located in position G4 (without considering the time required for gripper open/ close, gripper pitch up and down) was calculated and it is presented in Table 2. It was found that the mean deviation in X and Y directions were 2.4 mm and 3.1 mm.

Fig. 13. Simulated angular displacement curve for two active input joints.

4.1. Angular displacement of input joints The simulated angular displacement of input joints for fetching seedling G4 is shown in Fig. 13. From this figure, it can be seen that the angular displacement of the robot input joints for both the links completing a cycle is smooth and continuous with respect to time. It was found that the total time required to complete the cycle is 1.88 s. 4.2. Velocity profile of input joints The simulated velocity profile of both input active joints (a1 and a2) for completing a fetching cycle of seedling G4 is shown in Fig. 14. It can be seen that the velocity profile increased gradually from rest position to a maximum velocity and it was constant up to a certain point. Then it gradually reduced until it reached the desired location. Thus, it was found that the maximum and minimum velocity of active joint, a1 (represented in gray color) was 107.1°/s and 79.57°/s. The maximum

Fig. 14. Simulated angular velocity profile of two active input joints.

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black line) for fetching a seedling located at position G4 using MATLAB. It can be seen in this figure that, there is a deviation from the actual gripper trajectory and desired trajectory because of the applied smooth parabolic blend. It was observed that due to the deviation in the desired trajectory there was no collision occurred with the robot manipulator and other pot seedlings which were positioned close to each other at points G1…G4. 4.5. Performance and evaluation of the developed robot arm for picking and placing pot seedlings The robot arm was tested by implementing the algorithms into the microcontrollers for seedling pickup and dropping. A total of 15 pot seedlings were loaded on the cavities of the feeding conveyor (Fig. 12) and a total of four replications were made (15 × 4 = 60 runs). The cycle time taken for pickup and dropping of each seedling was found to be between 2.1 and 2.4 s. The deviation of actual and estimated cycle time (1.88 s) was due to the additional time taken for gripper operation (included gripper pitch up, down, jaw open and close), variation in distance between picking and dropping points for each seedling located in feeding conveyor and unaccounted friction between moving parts. In overall, the pickup mechanism didnot create any damage to seedlings and there was no collision observed while picking and dropping the pot seedlings in a row. Further, the robot arm was mounted on a walk behind type propelling vehicle (Fig. 17) to assess its performance of pick and drop under mobile condition in the field. The propelling vehicle was moved on an agricultural land and the robot arm was instructed to pick and drop the paper pots (15 nos.) from the feeding conveyor. It was also found that there was no missing of pot seedlings during the pick and drop operation during the overall (laboratory and mobile) experimental runs and the average time required to pick and drop one pot was 2.3 s.

Fig. 15. Simulated displacement curves of the gripper in X and Y directions.

and minimum velocities of active joint a2 (represented in black color) were 107.1°/s and 103.7°/s, respectively. The differences in velocity of active joints are to synchronize the input joint motion for reaching the different target angular positions at the same time. 4.3. Gripper displacement in Cartesian space The simulated X-Y displacement of the gripper for the change of input joint angles with parabolic blend is shown in Fig. 15. The curve represented in black color provides the gripper displacement (mm) in Xaxis with respect to time (s) and the curve in gray color represents the gripper displacement (mm) in Y-axis with respect to time for fetching a seeding located at position G4 by moving through the points D-A4-G4A4-D.

4.6. Power requirement The developed robot arm consisted of several active electrical and electronic components such as input stepper motors, servo motors, potentiometers, and microcontrollers. Each of these devices works on different input voltage level. The electric circuit contained power converters to supply the required power level for each component. The robot arm was designed to run with a 12 V battery. A buck converter (Table 1), present in the electrical system provided constant voltage to the system irrespective of any input voltage variations provided by the battery. To calculate the power consumption of the robot arm to pick and place the pot seedlings, the battery was replaced with a regulated power supply (RPS) as shown in Fig. 18. A constant 12 V input was supplied, and the current (A) readings were noted while picking and dropping a single pot seedling. It was found that the maximum current consumed for pick and drop actual paper pot seedlings located at positions G1, G2, G3 and G4 was 1.706 A. Thus the maximum power requirement for the robot arm was computed as 20.47 W.

4.4. Gripper actual and desired trajectory Fig. 16 presents the simulated curves of the desired trajectory (represented in dotted line) and actual trajectory obtained (represented in

5. Conclusions

• For the requirement of automated paper pot seedling transplanta-

•

Fig. 16. Simulated curves of the actual and desired trajectory of the manipulator. 11

tion process in the agricultural field, a 2 DOF parallel robot was designed, developed and evaluated. Also, effective equations (kinematics and trajectory planning), suitable for implementation in microcontroller and visualization using software programming platforms (such as MATLAB, Python, etc.,) were explained. Most of the components used in this robot were of 3D printed and commercially available electronics (microcontrollers, sensors and power devices). To avoid jerky movement of arm joints to complete the pick and drop cycle, a linear trajectory with parabolic blend was

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Fig. 17. Image of the robot arm mounted on a walk behind type propelling vehicle.

dropped seedlings into the soil at upright orientation) to automate the entire pot seedling transplantation process. Declaration of Competing Interest None. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compag.2019.105014. References Alam, M.Z., Ahmed, M., Alam, M.S., Haque, M.E., Hossin, M.S., 2002. Performance of seedling ages and seedling raising techniques on yield and yield components of transplant Aman rice. Pak. J. Biol. Sci. 5 (11), 1214–1216. Bechar, A., Vigneault, C., 2016. Agricultural robots for field operations: concepts and components. Biosyst. Eng. 149, 94–111. Beigl, M., Zimmer, T., Krohn, A., Decker, C., Robinson, P., 2003. Smart-its-communication and sensing technology for ubicomp environments, vol. 17. Technical report ISSN 1432-7864. Boa, W., 1984. The design and performance of an automatic transplanter for field vegetables. J. Agric. Eng. Res. 30, 123–130. Bourbonnais, F., Bigras, P., Bonev, I.A., 2015. Minimum-time trajectory planning and control of a pick-and-place five-bar parallel robot. IEEE/ASME Trans. Mechatron. 20 (2), 740–749. Chen, C.C., Lan, C.C., 2017. An Accurate force regulation mechanism for high-speed handling of fragile objects using pneumatic grippers. IEEE Trans. Autom. Sci. Eng. 99, 1–9. Constantinescu, D., 1998. Smooth Time Optimal Trajectory Planning for Industrial Manipulators. Doctoral dissertation. University of British Columbia). Di Benedetto, A.H., Klasman, R., 2004. The effect of plug cell volume on the post-transplant growth for Impatiens walleriana pot plant. Eur. J. Horticult. Sci. 69 (2), 82–86. Dihingia, P.C., Kumar, G.P., Sarma, P.K., Neog, P., 2017. Production of soil block seedlings in plug trays for mechanical transplanting. Int. J. Veg. Sci. 23 (5), 471–485. Dihingia, P.C., Kumar, G.P., Sarma, P.K., Neog, P., 2018. Hand-fed vegetable transplanter for use with a walk-behind-type hand tractor. Int. J. Veg. Sci. 24 (3), 254–273. Grift, T., Zhang, Q., Kondo, N., Ting, K.C., 2008. A review of automation and robotics for the bioindustry. J. Biomech. Eng. 1 (1), 37–54. Han, L., Mao, H., Kumi, F., Hu, J., 2018. Development of a Multi-Task Robotic Transplanting. Workcell for Greenhouse Seedlings. Harada, K., Nagata, K., Rojas, J., Ramirez-Alpizar, I.G., Wan, W., Onda, H., Tsuji, T., 2016. Proposal of a shape adaptive gripper for robotic assembly tasks. Adv. Rob. 30 (17–18), 1186–1198. Hashemimajd, K., Kalbasi, M., Golchin, A., Shariatmadari, H., 2004. Comparison of vermicompost and composts as potting media for growth of tomatoes. J. Plant Nutr. 27 (6), 1107–1123. Hu, J., Yan, X., Ma, J., Qi, C., Francis, K., Mao, H., 2014. Dimensional synthesis and kinematics simulation of a high-speed plug seedling transplanting robot. Comput. Electron. Agric. 107, 64–72.

Fig. 18. Image of RPS for power measurement during operation.

• • •

implemented, which provided good positional repeatability (2.1 and 3.4 mm in X and Y directions) and smoothness in joint motion without collision. The average transplanting time required to pick and drop the paper pots located on the feeding conveyor was 2.25 s. The peak power consumption of robot arm was found to be 20.47 W. This power requirement is much lesser compared to the power requirement of existing vegetable pot seedling transplanters utilizes IC engine and complicated metering mechanism to transport pot seedlings. Further, this work will be extended by installing the developed robot arm along with a feeding conveyor (Fig. 12) onto an suitable electrically operated self propelled vehicle which will consist of a furrow opener, furrow closer and a seedling placement unit (to guide the 12

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