Obtaining the effective gripper dimensions for a kiwifruit harvesting robot using kinematic calibration procedures

Purpose A harvesting robot is developed as part of kiwifruit industry automation in New Zealand. This kiwifruit harvester is currently not economically viable, as it drops and damages too many kiwifruit in the harvesting task due to the positional inaccuracy of the gripper. This is due to the difficulties in measuring the exact effective dimensions of the gripper from the manipulator. The purpose of this study is to obtain the effective gripper dimensions using kinematic calibration procedures. Design/methodology/approach A setup of a constraint plate with a dial gauge is proposed to acquire the calibration data. The constraint plate is positioned above the robot. The data is obtained by using a dial gauge and a permanent marker. The effective dimensions of the gripper are used as error parameters in the calibration process. Calibration is exercised by minimizing the difference between target positions and measured positions iteratively. Findings The robot with the obtained effective dimensions is tested in the field. It is found that the fruit drops due to positional inaccuracy of the gripper are greatly reduced after calibration. Practical implications The kiwifruit industry in New Zealand is growing rapidly and announced plans in 2017 to double global sales by 2025. This growth will put extra pressure on the labour supply for harvesting. Furthermore, the Covid pandemic and resulting border restrictions have dramatically reduced seasonal imported labour availability. A robotic system is a potential solution to address the labour shortages for harvesting kiwifruit. Originality/value For kiwifruit harvesting, the picking envelope is well above the robot; the experimental data points obtained by placing a constraint plate above the robot are at similar positions to the target positions of kiwifruit. Using this set of data points for calibration yields a good effect of obtaining the effective dimension of the gripper, which reduces the positional inaccuracy as shown in the field test results.

Measurement Configuration Optimization and Kinematic Calibration of a Parallel Robot

This paper presents the kinematic calibration of a 4-DOF high-speed parallel robot. In order to improve the calibration effect by decreasing the influence of the unobservable disturbance variables introduced by error measurement, a measurement configuration optimization method is proposed. Configurations are iteratively selected inside the workspace by a searching algorithm, then the selection results are evaluated through an index associated with the condition number of the identification Jacobian matrix, finally the number of optimized configurations are determined. Since the searching algorithm has been shown to be sensitive to local minima, a meta-heuristic method has been applied to decrease this sensibility. To verify the effectiveness of the algorithm and kinematic calibration, computation validations, pose error estimations and experiments are performed. The results show that the identification accuracy and calibration effect can be significantly improved by using the optimized configurations.

An improved minimal error model for the robotic kinematic calibration based on the POE formula

Summary The conventional product of exponentials $\left(\rm POE\right)$ -based methods dissatisfy the parametric minimality for the kinematic calibration of serial robots due to overlooking the magnitude and pitch constraints. Thus, the minimal kinematic model is presented to solve this problem, which can be developed further. This paper puts forward an improved algorithm for the minimal parameter calibration. An actual kinematic model with the minimal parameters $\left(\rm MP\right)$ is constructed according to the geometric properties of actual joint twists in the auxiliary frames established on the basis of the nominal joint axes. Then, the initial pose error is defined in the tool coordinate frame, which is expressed as the exponential map of the twist, and all twist descriptions are unified, so as to give a unified kinematic model in mathematics. By differentiating the kinematic model, a minimal error model is derived in explicit form. Subsequently, we propose a novel parameter identification method, which identifies the orientation error and position error parameters separately by the iterative least-squares method and updates the MP uniformly. Finally, the simulations and experiments on the different serial robots are conducted to verify the correctness and effectiveness of the proposed algorithm. The simulation results show our calibration algorithm outperforms the existing ones in the accuracy aspect, and the experiment result shows that the absolute pose accuracy of the UR5 industrial robot is upgraded about 9 times under a statistics sense after the calibration.

A modified agile-eye mechanism for robotic manipulation of wristed laparoscopic instruments

<div>Replacement of the exclusively designed instruments of the robotic surgery systems with the commercial hand-held wristed instruments provides advantages such as single-usability and cost reduction. A 4-DOF robotic system, based on a modified non-symmetric 2-DOF agile-eye mechanism, was developed to manipulate the hand-held wristed instruments. The kinematics of the mechanism was analyzed, its dimensions were optimized, and a functional prototype was tested experimentally. The optimized mechanism had a great kinematic isotropy (condition number <1.31) in the target workspace. Experimental studies revealed a high tracking accuracy ($0.27 +- 0.01 deg rms for the worse case) and a reasonably acceptable compliance (0.19 deg/N.m and 0.45 deg/N.m for the first and second kinematic chains respectively). By satisfying the design requirement, the robotic manipulator provides an attractive choice for robotic surgery systems. The performance of the manipulator can be improved further by increasing the stiffness of the second kinematic chain and performing kinematic calibration.</div>