Review of current mechanical design in agricultural end effector

Agricultural robots have become increasingly crucial in precise agriculture. This paper reviews recent developments in end-effectors applied in the robotic harvesting of fruits and vegetables. The control and harvest function of the end-effector is the focus. Different structures are categorized based on their properties. Advantages and limitations in each category are introduced. For the collection method, the hard-grab method is very popularly for low difficulties and the soft one can protect the fruits adequately and can harvest different fruits of similar size; double-finger type is used widely with the advantages of low cost, while the multi-finger type has the much higher stability. The collection method is divided into the cutting and twisting methods. The former is applied for low difficulties but may spread diseases among plants; therefore, thermal cutting, namely using high temperature to separate stem and crop, and the twisting method is used. Overall, different types of end-effectors show various advantages. With consideration of the target crop physical properties, a proper decision for the harvester can be made.


Introduction
In the past centuries, the application of agricultural machines has considerably increased crop production.Its position is now increasingly important in the aging society, raising the frequency of extreme weather and reduction in the agricultural area due to urbanization.Those problems lead to weaker agrarian capacity; therefore, precise, smart agriculture has become a trend in many countries.Precise agriculture focuses on increasing crop production and efficiency by monitoring the real-time humidity, nutrition in the soil, and other parameters.High technologies, from satellites to a sensor, are used and among them the harvesting robots are critical components to increase production efficiency.For instance, the conifer cone collector [1] designed by Northeast Forestry University can harvest 500kg of larch cones in 6 hours which is 30 to 35 times than that of a human.
The primary research on harvesting robots includes image processing technology to identify crop color, size and shape, end effector, control system, and transportation system.Among them, the end effector acts directly on the target crop and significantly impacts the harvesting robot's performance.Therefore, this paper focuses on the collecting and harvesting methods of the end effector.
In chapter 2, background information about the ideal agricultural environment and the focus on the robot will be introduced; chapter 3 focuses on the controlling method of the end-effector; chapter 4 discusses the harvesting method.

The ideal working environment
The crop growing in the open environment shows a random pattern and high complexity due to the block of branches and leaves.By comparison, the greenhouse crop pattern shows more regularity, and therefore more accessible for the harvester to process.(Picture from: https://www.sohu.com/a/161538536_99929929,https://www.123rf.com/photo_43909369_cultivation-of-watermelon-in-greenhouse.html).

Harvesting Procedure and Methods
The harvesting robot generally follows the following procedure: 1.The robot will locate the mature fruits through a visual recognition system 2. The robotic arm will move the end effector to the corresponding position 3.The end effector controls the selected fruit 4. Separate the fruits from the branch 5. Collect the harvested fruits Obtaining and separation are the two critical actions for the end executor to implement.First, by fetching, suction, hook etc., to obtain the fruit and then use twisting off or cutting to complete separation of the fruit and stem.In this project, steps 3 and 4 are responsible for controlling and harvesting, and therefore will be the focus.

Controlling
The mission of this part is to control the target fruits and keep them stable for the next move, namely, separate the fruit from the stem.The controlling method is generally categorized into the grabbing and suction methods.

Grab Method
The material used on the end-effector finger and the number of fingers is the two primary categorization methods in grabbing.

Material Used
3.1.1.1.Hard-Grab Method.Such a grab method applies the material with high strength to the finger.By such properties, the shape of the fingers can stay almost constant when experiencing high compression (grab) force.Therefore, they are widely used in carrying heavy materials such as watermelons.However, the hard-grab method can cause damage to the fruits.To solve this problem, a rubber ring can be put on the fingers to avoid excess force exerted.An example of the hard grab is the manipulator designed for heavy materials by Chiba University [2], which uses metal for the fingers to  The end effector designed by Jiangsu University [3] is an example of using rubber in hard grab to avoid damage to the fruits.The inner side of the arc surface of the hand is covered with a sponge rubber layer (figure 4) to ensure that the grasping force is evenly distributed during the grasping process and does not damage the apple.The rubber layer can also increase the friction between the apple and the fingers.According to figure 4, the piston rod of the design cylinder relates to the high pair of the chute at the rear end of the two fingers through the pin shaft.Finally, the linear motion of the guide rod is converted into convergence for the two fingers around the rotating shaft.

Soft-Grab Method.
By applying fingers that change the shape, fruits can be picked softly.Such a method is polyfunctional.A single design can be used to pick fruits of similar size (e.g.oranges, lemon) or with an irregular shape, such as dragon fruit and sweet pepper.There are mainly two types of soft grab.The first type uses soft material to change the fingers' shape passively.
An example is the design of an end effector by a group from Nanjing Agriculture University [4].It can harvest most fruits with a diameter of 20~90mm.The end effector contains three mechanical fingers, as shown in figure 5, and are made of an elastic material with high deformation adaptability and can avoid the damage caused by rapid grasping, also wrap the fruit according to the shape of the fruit.The second method holds the fruits by mimicking a human's hand.The Washington State University [5] designed a soft pneumatic end effector to hold the apples.The end effector consists of three highly compliant pneumatic actuators mounted around a soft, flexible palm designed to support an apple against the actuators, as shown in figure 6.
It can be found that the upper two actuators are shorter and can stabilize the apple within the grasp of the end-effector, and the lower actuator is longer, wrapping almost entirely around the apple.
Figure 7 shows the workflow of the functional prototype gripping an apple before harvesting.

Number of Fingers
Another way of sorting the types of grab method is by the number of fingers.This method is feasible in most end-effectors as the current mechanical design contacts the fruit via fingers.This category is further divided into double-finger type and multi-finger type.

Double-Finger
Type.Double-finger types are trendy and used in most kinds of fruit harvesters.
It is widely used in the end effector design for its low difficulties in designing control systems.However, the double-finger system has lower stability than the multiple-finger system.The method controls the fruits in two ways: catching fruit with hard and medium size, such as apples, or catching the long stem of several fruits, such as strawberries and masked melon.The tomato harvester designed by Jiangsu Agriculture University [6] is an instance of the former type.As shown in figure 8, the finger surfaces are designed as arcs and affixed with rubber, which enhances the clamping reliability by increasing the friction and protection of the fruit.The finger clamping mechanism is driven by a DC servo motor which drives a bidirectional screw drive with two sections of left-handed and right-handed threads to close or loosen the fingers.4), ( 8), ( 11) DC servo motor, (5) laser focusing lens, (6) rack, (7) shell, ( 9), ( 10) bevel gear, (12) tooth wheel.
The strawberry harvester designed by a group from China Agriculture University [7] is also a two-finger type.a) The fingers of the mechanical claw gradually close first.b) Then the very long strawberry stem contacts the heating wire placed at the upper part of the fingers and d) is pushed on the plastic wall.
After that, d) the stem is cut at high temperature, and the rest of the stem is still held in the lower part of the fingers.Figure 9 introduces the layout of the end-effector, and figure 10 is the front view and workflow.

Multi-Finger
Type.Multi-finger type has higher catching stability compared to the two-finger ones.They are instrumental in catching fruits with irregular shapes, such as bell pepper.However, the complexity and cost of the machine increase with more fingers, and the fingers can be blocked by the obstacles such as leaves and branches.A good example is the sweet pepper harvester [8], which uses a double-acting long-stroke gripper as the main drive and two single-acting pneumatic drives for cutting, as shown in figure 11.This end-effector first grips the fruit with four fingers and allows the passive adaptation of the fingers to the workpiece geometry.The two lower fingers can rotate around their longitudinal axis for better adaption to different geometries of peppers.

Suction Method
The end effector tries to hold the fruit by a vacuum nozzle and separates the fruits from the branch by twisting the fruits or cutting the stem.The suction method allows compensation to the stability of the fruit controlled compared to the grab method, as the object will be fixed at the nozzle.However, such a method working with small-sized fruits such as strawberries can suck in unmature ones and taking one fruit is time-consuming.The tomato harvester uses the suction method [9].A motor controls the vacuum suction nozzle's extension and contraction to pull tomatoes off the obstacles and then transfer them into the grasping region to be picked.This lessens the likelihood of the end-effector and stalks colliding and causing harm by allowing the fruit to stay in the gripper with better stability.The fruit suction device includes an air compressor, a valve, a cylinder, a vacuum generator, and a suction nozzle.The fingers on the end effector are controlled by wires inside.

Harvesting
At this stage, the end effector will try to separate the fruits from the branches.The primary trend in harvesting methods can be divided into cutting and twisting.

Cutting Method
This method is prevalent for its reliability and low difficulties.The most common way will be using a scissor or slicer.It allows the fruits to drop directly into the collection system or be delivered by the control system introduced in the previous section.However, the sharp end can hurt the fruits, especially those with soft skins like strawberries.Therefore, the higher recognition accuracy of fruit stem and the flexibility of the robotic arm are required.Also, as the scissor is used frequently, diseases caused by bacteria and fungus can be transferred among the fruits.
On the other hand, as mentioned by 'An Autonomous Robot for Harvesting Cucumbers in Greenhouses' [10] in horticultural practice, to avoid virus transformation, a knife used to cut the stem is repeatedly immersed in skimmed milk before each plant contact.As a result, to prevent these issues, the branch is cut using a thermal cutting technique that offers a better cutting approach by applying high voltage when the stem meets electrodes in the design of a sweet pepper harvester made by Kochi University of Technology [11].An electric arc is produced when the stem contacts the electrodes, which causes a quick increase in temperature there, enabling the cutting operation.However, this process is slowed down by the tissue's high-water content.
This strategy offers two clear benefits.First, viruses are killed throughout the cutting process due to the noticeable temperature rise at the cutting surface.Second, cutting closes the incisions on the fruit and plant.As a result, the fruit retains more water and has a longer shelf life.Additionally, it is thought that dressing the wounds will make the plant less susceptible to fungus-related ailments.

Twisting
The twisting method shares the same advantage as the thermal cutting, which will not transfer disease between the plants.Also, the cutting method requires an accurate stem estimate, which must properly fit into the space between the blades.Furthermore, the twisting way has high robustness because it does not require the location of the stem and can harvest fruit as long as it is grasped.
In the design of a tomato harvester [12], an infinity rotational joint and a gripper-based plucking mechanism.The hand initially grasps the fruit with its fingers before rotating.Then, the hand pulls the tomato off when the pedicel, the strongest part of the stem is broken.
Figure 17.The picking process of the end-effector.First, the harvester will approach the target tomato, grasp it, and then rotate until the stem is broken or fragile enough so the harvester can separate the tomato from the stem and collect it.

Conclusion
The agricultural robot has shown growing importance for its high production and efficiency.The most important part of a farming robot, the end effector, is responsible for harvesting and collection.The harvesting method can be categorized into cutting and twisting.Cutting is famous for cheap and low manufacturing difficulties while leading to the spreading of diseases among plants.Therefore, an improved method is thermal cutting, where the twisting process shares the same advantage.
Furthermore, twisting has a low requirement for locating the crop branch.The control part can be classified in two ways.First, if by the grab method, it can be categorized into a hard and soft grab.The former can be used to control solid skin fruit, and the other can be applied to more fruit and can protect the fruit.Second, the end-effector is treated as double-finger and multi-finger types according to the number of fingers.Double finger type is famous for its low price and difficulties, while it does not have the high stability owned by the multi-finger type.In general, different end-effectors have various advantages and disadvantages and are vital factors to consider when designing a proper end effector.

Figure 2 .
Figure 2.An illustration of the layout of the robot.A wire mechanism is used in the collection and control parts, as shown in figure3, which shows the placing procedure.First, a) the fingers are suspended by the wires while remaining closed by gravity.Second, b) the fingers contact the ground after placing the watermelon, so the wire tension becomes zero.Third, c) the DC motor holds the fingers open and reels the wires till d) the effector raises without touching the watermelon.

Figure 3 .
Figure 3.The motion plan for placing using reaction forces.

Figure 6 .
Figure 6.An illustration of the end effector.All the parts are 3D printed.

Figure 7 .
Figure 7.The working procedure of the end effector.a) It first approaches the target fruit and then uses two short fingers to catch the apple.Then c) the long finger will wrap and embrace the apple to give a firmer control.

Figure 10 .
Figure 10.The working procedure for the thermal cutting.

Figure 11 .
Figure 11.The general layout of the peper harvester.

Figure 12 .
Figure 12.Working procedure of the pepper robot.It first uses two upper fingers to hold the pepper, then the lower two fingers, and then cuts the stem with the scissor.

Figure 15 .
Figure 15.System model design of a thermal cutting prototype.

Figure 16 .
Figure 16.Concept of picking and cutting tool.