Development of compliant constant-force mechanism for end effector of apple picking robot

Abstract: In order to reduce the damage caused by end effector of picking robot during fruit clamping process, non-destructive fruit picking has gradually become a key technology of fruit harvesters. The application of compliant constant-force mechanism may enable a constant output force within a certain range of operating displacement. The existing compliant constant force mechanism has the defect that the constant force interval is small relative to the size of the mechanism, which makes it difficult to apply in the situations requiring compact structure design such as fruit picking. In this paper, a method to generate compliant constant-force mechanism is introduced. Firstly, based on the shape function, a set of nonlinear ordinary differential equations for buckling deformation of compliant beams under boundary conditions is established. Secondly, the boundary value problem is redescribed as the initial value problem by applying the shooting method combined with the Runge-kutta method, the 3D nonlinear equations are obtained according to three boundary conditions, two unknown variables and one unknown initial value. Thirdly, the nonlinear equations is solved as an optimization problem. To avoid the optimization problem falling into local convergence, the genetic algorithm is employed to globally find the appropriate initial values of the three unknowns in the nonlinear equations, the solution of the differential equations is obtained according to the principle of the shooting method. Finally, the sequential quadratic programming method is implemented to optimize the parameters of the shape function, the optimization objective is to achieve a constant output force within a certain range of displacement input, and the constraints are the design boundary and the maximum permissible stress of the beams. A two-finger apple picking end effector is taken as an example, the initial shape parameters of the compliant mechanism are set and the algorithm is realized in Matlab. After optimization, the constant clamping force of the compliant beam to the fruit is about 7.9 N. In order to verify the correctness of the method, the non-linear finite element simulation is carried out in Abaqus, and the force-displacement experiment is carried out. The simulation results agree well with calculations, compared to the calculations, the average error of force-displacement test is 5.06%, which is acceptable. The main reason for the error may be that the theoretical model only considers the deformation of the neutral axis. The apple picking test is designed to verify the practicality of the method. At constant-force range, the clamping force fluctuates from 7.46-8.42 N, and the average clamping force is about 8.03 N. The clamping force is basically consistent with the calculation results though a certain fluctuation exists. To further verify the flexibility of the clamping mechanism, 20 apples of similar size and intact surface are selected for clamping test. The result shows that the ratio of non-destructive grasping is 95%, no slipping occurred, the compliant mechanism designed can achieve stable clamping of apples. The algorithm can meet the requirements of constant force clamping for different types fruit by properly adjusting the parameters of the compliant beam. The research provides a reference for the effective application of the compliant mechanism in non-destructive fruit-picking.