Not only is apple picking time-consuming and labor-intensive, but during the harvest season, many growers often face serious challenges in recruiting enough labor due to an aging workforce and a net decrease in the number of mobile workers.
To solve this problem, researchers have explored the use of rigid manipulators such as 6 degrees of freedom robotic arms and linear robots for automated picking. However, these solutions are often costly, which is a huge financial pressure for many growers, and the operational complexity of these solutions in the complex environment of the orchard further increases the difficulty and cost of practical application.
With this in mind, some research teams have turned to the development of apple picking grippers made of flexible materials to provide a safer, cost-effective, and more efficient alternative.
According to the X-robot manuscript that explores the boundaries of cutting-edge science and technology and transmits cutting-edge scientific and technological achievements, researchers from Washington State University recently proposed a lightweight under-driven soft gripper designed for apple picking. With a weight of only 0.306 kg and a successful separation rate of 87.5%, the gripper not only meets the requirements of the soft growing robot in terms of load, but also maintains the simplicity of the design, enables efficient fruit separation, and demonstrates excellent practicality.
这一重要研究成果在《Design and Evaluation of a Lightweight Soft Electrical Apple Harvesting Gripper》这篇论文中详细阐述，并在2024年IEEE第七届软机器人国际会议（RoboSoft）上进行了发表，受到了业内外的广泛关注。
Next, let's explore this research result in depth with the Robotics Lecture Hall!
▍Design and manufacture of light underdrive soft grippers
In the world of fruit picking, ensuring that apples are picked from the stems has always been a technical challenge. Apple-picking requires a higher grip than regular picking and placing tasks, which is a huge challenge for soft grippers made only of silicone. Although the soft properties of silicone can protect the fruit from damage, its limited hardness may cause the apple to slip during the harvesting process, affecting the picking efficiency.
Researchers at Washington State University have successfully overcome this problem with a lightweight, underactuated soft gripper. The main parts of the gripper are made of polylactic acid (PLA) through 3D printing technology, which is not only lightweight but also strong. Its unique cable-driven design, combined with three flexible fingers, greatly enhances grip stability. What's more, the gripper's palm size and finger spacing are precisely designed based on apple sample data from the 2023 harvest season, ensuring optimal harvesting results.
Soft gripper assembly. The soft gripper contains two limit switches to identify when to close and open the finger. The finger is made of silicone with a TPU skeleton embedded inside. Servo motors and pulleys are used to close the fingers. The diagram shows the CAD assembly (a) and the physical prototype (b)
In operation, the gripper is automatically controlled by means of two limit switches located in the palm and on the base. As soon as the apple touches the limit switch in the palm of your hand, the server will quickly activate the cable, so that the finger is tightly closed, and the apple is firmly clamped. When it's time to put down the apple, simply press the limit switch on the base and the gripper will release your fingers and place the apple safely in its intended position.
When making a single finger of a soft gripper, the researchers. First, the negative mold of the finger is printed out by PLA, and then the TPU skeleton is put into the mold for fixation. Next, pour in Dragonskin 30 silicone rubber and wait for it to cure. Once curing is complete, remove your finger and thread it through the cable to securely attach it to the finger end cap. In this process, the researchers paid special attention to the bond strength between the TPU skeleton and the silicone, and by making small incisions in the TPU skeleton, the interlocking effect between the two was effectively enhanced, preventing damage caused by frequent use.
Soft finger assembly. a) The manufacturing process of a single soft finger with a TPU backbone. Silicone is cast into a mold with a TPU skeleton and steel rods to create a cavity for the cable. Remove the steel rod and insert the cable to create a bending motion. b) TPU skeleton pattern. Three types of TPU patterns (Gyroid, Cross3d, and Cross) were tested for gripper design.
The combination of silicone and the 3D-printed TPU skeleton not only protects the apple from damage, but also enhances the rigidity of the soft fingers. To ensure the best results, the researchers evaluated three different TPU skeleton printing modes and ultimately determined the best solution.
On the drive mechanism, the three fingers of the gripper are driven by a common pulley underneath. Due to the limited range of rotation of the servo, the researchers precisely optimized the size of the pulley to ensure complete closure of the fingers within a limited angle. However, during the test, they found that the pulleys made of PLA could not withstand the forces generated by the cable tension. To solve this problem, the research team designed a stronger alternative to 3D-printed pulleys that allows the cable to be wound directly around the bolt and secured by tightening the nut. This improvement not only simplifies the process of replacing cables, but also improves the ease of operation in the field.
Pulley assembly. The pulley design is robust and easy to assemble. The design consists of using nuts and bolts to securely connect the pulley cables. The pulley base is connected to the servo, and the sleeve is wound around the pulley cable. The diagram shows the CAD assembly (a) and the physical prototype (b).
Finally, the size of the gripper's palm is also carefully designed. By randomly measuring the apple circumference of 20 Jazz varieties, the researchers determined that the optimal inner diameter size of the gripper's palm was 9.8 cm. This design not only ensures that the apples can be stably placed in the gripper, but also greatly improves the efficiency and success rate of picking.
▍Performance test of light underdrive soft gripper
In order to fully validate the performance and utility of the lightweight, underactuated soft gripper for real-world picking tasks, the researchers recently conducted a series of well-designed experiments and field tests at the Allan Brothers commercial apple orchard in Prosser, Washington.
A. Strength test
In the strength test, the researchers paid special attention to the force output characteristics of a single finger. The experimental setup shown in the figure below is set up, and the gripper is equipped with a single finger with a load cell attached to the end of the finger to accurately measure the forces generated at different bending angles. This data is essential to assess whether the gripper can hold the apples firmly during the harvesting process. After three replicates of the test, and a detailed record of the bending angle of each finger, the researchers obtained valuable data.
Force test settings. A soft finger is pressed against the load cell to measure its force output. This is repeated four times, once for each TPU backbone hatch pattern and once for the control (no fill).
Experimental data show that fingers with a cross-shaped TPU skeleton generate the greatest force across the entire bending range, while fingers without TPU skeleton generate significantly less force despite bending angles of up to 103°. Based on these test results, the researchers built multiple prototype grippers and conducted field evaluations during the apple harvest season.
Force output and bending angle results. The force output is directly related to the bending angle. Silicone fingers embedded in the TPU produce higher force output.
B. Orchard Assessment
To verify the performance of the gripper in a real-world orchard environment, the research team conducted field tests at Allan Brothers orchards. The orchard has a planar tree structure that allows the fruit to grow in a wall-like arrangement, greatly simplifying the operation of mechanized or robotic platforms. The tests, mainly for the Envy apple variety, were carried out within two days of the start of the harvest date.
In the test, harvested apples are randomly selected, and the gripper is operated by one person to approach the fruit in a direction perpendicular to the apple stem. As soon as the apple triggers the limit switch on the palm of the hand, the servo motor tightens the cable so that the finger wraps the apple tightly. Subsequently, the operator pulls the gripper in an attempt to detach the apple from the branch. Picture (a) below shows a successful harvest with silicone fingers tightly wrapped around the apple, which is successfully plucked.
Orchard assessment results. (a) The top three pictures show the picking sequence of the apples successfully picked by the soft gripper. The bottom three pictures depict a snapshot of the trial in which the soft gripper failed to pick an apple (the apple escaped grasping). During the test, apples that were unsuccessfully picked by the gripper within three attempts were to be hand-picked and collected for further analysis. (b) Box plot showing the diameter of the apples that each gripper is trying to pick. Averages are indicated by black dots. (c) Box plot showing the weight of the apples that each gripper is trying to pick. Averages are indicated by black dots.
After visual inspection, all the apples picked were not damaged by the grippers. A total of 160 apples were collected, and 40 apples were successfully picked for each gripper type without maintenance. Figure (b) and (c) above illustrate the size and weight distribution of the harvested fruits, and the results show that most of the fruits are similar in size and weight, with minimal differences between different gripper types.
It is worth mentioning that although no significant damage to the fruit was observed, the branches supporting the apples suffered some accidental damage during the operation, which was mainly attributed to the operator retracting the gripper too quickly when picking.
Successful crawl rate. The percentage of successful gripping for each gripper type indicates that grippers with a Cross TPU pattern perform best and perform very poorly without a TPU skeleton.
The graph above illustrates the success rate of the various gripper types, with the gripper with a cross-shaped TPU skeleton performing the best with a success rate of 87.5%. This result shows that the soft gripper designed by the researchers not only has excellent apple-friendly gripping ability, but also has a simple mechanism design, is easy to adapt to different sizes of apples, and is easy to maintain.
In addition, the cost of a single gripper is approximately $36, which is more cost-effective than existing pneumatic grippers. In commercial orchards, the gripper has been successfully harvested and can be used for 40 cycles without maintenance.
▍ About the future
Looking ahead, the research team plans to further refine the design of the gripper, including adding a twisting action to mimic the natural way of picking the apple, and exploring ways to increase the friction between the finger and the apple to reduce the possibility of the apple slipping. In addition, they will continue to refine the integration of system components and conduct extensive evaluations of commercial orchards in different weather conditions to optimize the size and performance of the grippers. The ultimate goal is to ensure that the grippers operate reliably throughout the short harvest season, while simplifying the maintenance process and providing strong technical support for automated harvesting.
▍About X-robot
X-robot is an authoritative information release brand column jointly created by Zhongguancun Robot Industry Innovation Center and Robot Lecture Hall, which integrates cutting-edge exploration, industrial research and knowledge popularization, and is committed to actively promoting the generation and development of new quality productivity and helping the prosperity of the robot industry in mainland China and even the world. Based on an international perspective, X-robot vividly demonstrates the cutting-edge technology and cutting-edge achievements of robots through all-round and multi-angle excavation and tracking, providing an important window for academia, industry and the public to gain insight into the future and share technology.
References:
https://ieeexplore.ieee.org/document/10521995