The primary goal of this project is the development of a robotic system for apple tree pruning using a robotic end-effector and machine vision system. The project will be focused on the following objectives:
1. Development of an effective robotic pruning end-effector The end effector will be developed by considering the maneuvering capabilities to cut the branches of all orientations and minimum spatial requirements to freely move in a limited workspace. The field test for pruning force measurement will be conducted which is essential for the selection of the end effector components. The design optimization and orientation tracking of the end effector will also be performed. It is expected that the end effector design will be able to produce smooth branch cuts, utilize the minimum workspace, and cut the branches of all orientation.
2. Integration of a robotic pruning system and simulation of spatial requirements for branch accessibility The developed end effector will be integrated with two different robotic arms i.e. three axes linear motion arm and a six degrees of freedom robotic arm. The simulation of the integrated system in a virtual environment will be performed to estimate the space requirements and branch accessibility of both integrated systems. It is expected that the simulation study for branch accessibility will help in developing a collision-free trajectory and path planning for the end effector to reach the target position.
3. Development of a vision system for tree branch detection and optimization of pruning parameters The image acquisition system will be developed to acquire the visual information of the tree canopy, and an image processing algorithm for the identification of branches will be developed. To evaluate the developed machine vision system, a pruning rule of removing branches based on a defined limb-trunk ratio threshold will be used. It is expected that the vision system will be able to detect and identify the base diameter of all branches in a tree and provide the cutting location for the integrated robotic pruning system.
The long term goal of this project is to reduce the dependence of human labor for apple production, thus, to increase the competitiveness and sustainability of the United States tree fruit industry. This proposed study focus on investigation of some key components/technologies for autoamted pruning system for apple trees. The outcomes of this project will provide guideline information for fully automated pruning system.
Tree fruit industry is an important sector for U.S agriculture. Apples represent one of the most valued non-citrus fruit in U.S. A large workforce of seasonal and non-seasonal laborers is required every year for production oeprations. While, the average number of available farm workers is decreasing through the year. Pruning is the second labor intensive task in the apple production just behind harvesting. It would be very important to have an alternative method for the pruning task to reduce human labor dependancy.
Objective #1: Development of an effective robotic pruning end-effector
1. Pruning force/torque measurement
The amount of force required for cutting branches is an important parameter for the pruning end-effector design. The force measurement was performed using a traditional manual pruner and thin force sensor Phidgets-1131 (Phidgets Inc., Calgary, Canada) capable of detecting the forces exerted by the hand (Figure 1). The sensor was attached to the arm of a manual pruner and positioned to coincide with the point of contact of an operator’s finger with the shear handles to investigate the amount of force applied during pruning. The cutting torque was calculated using the measured force and the length of the pruner arm. In the test, 75 cuts were made in the ten branches with different diameters. The maximum force for each cut was recorded, as well as the corresponding branch diameter.
2. Robotic pruning end-effector development
A concept design of a robotic pruning end-effector were carried out in SolidWorks environment (Figure 2). Considering the needed maneuverability of the end-effector, the concept design includes two rotational motors, and a cutter which consists of a cutting blade and an anvil. The pruning end-effector workspace simulation was also conducted to spatial requirement for the end-effector, as well as the capability of aligning with the branches at different orientation.
Objective #2: Integration of a robotic pruning system and simulation of spatial requirements for branch accessibility
1. Simulation on spatial requirement of a pruning robot
The spatial requirements of a robotic arm could be estimated based on the location of the branch to establish the trajectory of the robotic arm. Canopy characteristics such as branch density and branch dimension could possibly affect the path of the robotic arm to reach the object. The study includes the following tasks; 1) Establishing robot kinematic model and obstacle model in MATLAB; 2) Establishing a collision free trajectory for reaching the targeted pruning points.
The robotic manipulator used in the study was a 6 rotational DoF industrial robotic arm (UR-5, Universal Robots, Odense, Denmark). The three-dimensional model of the robotic arm is built in the MATLAB environment. The end-effector cutter tool is considered to be installed on the tip of the robotic arm (Figure 3). A virtual tree with several primary branches also established in the environment. Then a simulation of branch accessibility with the pruning robot is carried out to find a collision-free path for reaching to the targeted branch.
1. Pruning force/torque measurement
The relationship between pruning torque and branch diameter followed a rational 2×2 curve fit with an R2 of 0.9334 (Figure 4). The results are helpful for selecting and optimizing end-effector components such as pneumatic cylinder sizes for cutters, pressure, orientation motor torque, and mounting frames.
2. Robotic pruning end-effector development
These cutter orientation lines depict that the end-effector was capable of aligning the cutter to a wide range of possible orientations in a 3D workspace. The apple tree branches have a wide range of possible orientations, leading to little available space for maneuvering of the end-effector. This simulation shows that the proposed end-effector can be aligned to all possible orientations while utilizing a small workspace for maneuvering within the canopy.
3. Simulation on spatial requirement of a pruning robot
The RRT algorithm was successful in finding a collision free path in the virtual tree structure for most pruning points defined in the simulation. The results suggest that the robotic arm can be used for pruning apple trees; The algorithm was successful to keep the end-effector cutting tool posture aligned perpendicular to pruning branches i.e. yz plane of the tool frame was aligned perpendicular to branch orientation; The RRT smoothing was successful in reducing the time for trajectory generation and also streamlined the joint angles, velocities, and accelerations. The RRT smoothing also reduced the path lengths for all target locations, and; The RRT algorithm performs slow in path finding. Thus, there is a need to introduce optimization algorithm to improve the efficiency of the path finding operation.
Pruning end-effector: The rotation capability of the end-effector along two perpendicular directions (Motor 1 and Motor 2) gave the ability to cut the branches at a wide range of orientation in a given 3D space. For any given target and branch orientation, the designed end-effector was successful in producing smooth and split-free cut branches up to 8 mm and 12 mm diameter for test one and test two respectively. The performance of the end-effector was found to be unsatisfactory to cut branches with a diameter greater than ~12 mm as it required two or three strokes of the pneumatic cylinder. The torque produced by the cutter was insufficient to produce large enough force to cut these primary branches. A more powerful cutter is required to cut the large diameter branches i.e. 12 mm and above. For that purpose, the system may need to be equipped with a large pneumatic cylinder or a modified electric pruner considering the space utilization. The replacement of pneumatic system with electrical pruner will expect to improve the cutter efficiency. It was also observed that while cutting, placement of branches closer to the cutter pivot makes it easier to cut compared to when branches are placed close to the tip of pruner blade. This was an important observation for developing an automatic trajectory and target positioning system.
Spatial requirement simulation: The simulation of the branch accessibility was performed for the developed obstacle model. In the simulation run, the robot was set to home position to find the collision free path. The target pruning points are marked 50 mm from the tree trunk on each branch. The RRT algorithm was able to find a collision free path, i.e. red line path for the selected branches. The orientation of the end-effector was also monitored considering the orientation of the tool frame during the simulation. The results show that the end-effector cutter was successfully aligned perpendicular to branch orientation before making the cut. The outcomes from this simulation study provided guideline information for our future lab test and field test on trajectory generation for cutting branches.
Education & Outreach Activities and Participation Summary
A journal article has been submitted and under review. An conference abstract was submitted recently. Two upcoming presentations will be based on the research results from this project, one is a poster presentation at 2020 Mid-Atlantic Fruit and Vegetable Convention in January, and the other one is Penn State College of Engineering Research Symposium 2020 in April.