Research On Design And Compliant Grasping Of End-Effector Of Apple Harvesting Robot

China is the country with the highest production of apple in the world.At present,apple harvesting is mainly done by hand picking,which has the problems of high labor cost and high labor intensity.As a new agricultural production mode,apple harvesting robot can realize automatic picking,reduce labor cost and effectively improve harvesting efficiency.However,the skin and pulp tissues of apples are relatively fragile.In the process of robot picking,due to the instability and poor compliance of the end-effector grasping action,the fruit is likely to be damaged,thus affecting the harvest quality.Therefore,improving the compliance and stability of the end-effector grasping action is of great significance to reduce the damage and realize the nondestructive picking of apples.This paper will start from the improvement of passive compliance function and active compliance control strategy of the end-effector,the research is carried out around the design of the end-effector,the solution of stable grasping force and the improvement of control strategy.The main contents of this paper are as follows:(1)The end-effector was designed based on the study of biological characteristics of apple.Firstly,the morphological structure,mechanical properties and stem shear mechanical properties of apple were studied,and the picking characteristics of apple and the functional requirements of end-effector design were summarized.Then,based on the above research results,an end-effector suitable for apple picking was designed.The design work of end-effector includes clamping mechanism,shearing mechanism,driving unit and sensor.Finally,establish a mathematical model of the end-effector to provide a theoretical basis for active compliant grasping control.(2)The optimization model of minimum stable grasping force of end-effector was established by improving the constraint conditions of stable grasping.Firstly,establish a gripping constraint model between the end-effector and the apple,and analyze the constraint conditions for stable gripping.Aiming at the unstable state caused by the boundary value of friction cone,an evaluation index of grasping stability was introduced to correct the constraint boundary of friction cone.Then,the optimal model of minimum stable grasping force was constructed for stable grasping planning,and the linear gradient flow algorithm was used to solve the model,so as to obtain the expected grasping force satisfying the grasping stability.Finally,MATLAB is used to simulate the grasping examples,and the feasibility of the grasping force optimization model is verified.(3)A compliance grasping control method of end-effector based on improved adaptive variable damping impedance control tactics was designed.Firstly,by introducing the positionbased impedance control principle,an impedance model considering environmental contact was established,and the influence of impedance parameters on the controller performance was analyzed.Aiming at the problem that the uncertainty of environmental information leads to poor compliance of grasping motion,this paper introduces time-varying damping parameters,improves the traditional position-based impedance control strategy,and proposes an adaptive impedance control method with variable damping coefficient.Based on the deviation of grasping force,an adaptive control rate is designing to adjust the damping parameters,in order to achieve the system adaptability to changes in grasping environment and improve the grasping control performance.Finally,a simulation model was built in MATLAB to simulate the designed adaptive variable damping impedance control method,and respectively in the case of fixed and changing environmental information compared with the traditional impedance control method.The results showed that the adaptive variable damping impedance control method performed better in compliance and anti-interference ability.(4)Complete the design of the end-effector control system,and construct a harvesting experimental platform for experimental verification.The picking performance of the end-effector,the effectiveness of the optimization model of minimum stable grasping force and the efficiency of the adaptive variable impedance control strategy were verified by three groups of grasping experiments.First,by using the designed end effector to grab apples of different sizes,qualities and growing positions,the experimental data such as harvest success rate,damage rate,and time spent are collected to test whether the harvest performance of the end effector meets the design requirements.Then,multiple grasping experiments were conducted before and after setting the stability evaluation index,and the grasping success rate was calculated to verify the effectiveness of the proposed optimization model.Finally,the traditional impedance control method and the variable impedance control method were used for grasping experiments.By comparing the experimental results of the two groups,the control performance of the proposed method in terms of control accuracy,compliance and stability was verified.