Soft Contact And Flexible Manipulation Mechanism Of Human Fingers In Tomato-picking

Tomato has become one of the important fruits in human diet.However,the contradiction between the increasing tomato planting area and the decreasing labor force is aggravating.Therefore,it is necessary to develop tomato picking robots.However,due to problems such as crushing and falling off,there is currently no commercialized tomato picking robot.It is undeniable that crushing and falling off rarely occur when tomatoes are picked manually.Therefore,in order to improve the picking success rate of tomato picking robots,it is necessary to study the manual picking behavior of tomatoes.(1)In this research,the curve of grasping force in tomato-grasping were firstly analyzed,and a grasping force index 6)was proposed to explore the influencing factors of the gripping force.It was found that the gripping force index 6)was obviously affected by the weight of tomatoes.The greater the mass of the tomato,the greater the gripping force index 6),which means that,the gripping force of the thumb and index finger increased with the weight of tomatoes.When the human hand used the strong grip mode to grasp the vulnerable tomato and the rigid target,the grasping power difference between the thumb and the index finger is significant.The grasping force index 6)of the tomato grasping is small,and the grasping force index 6)of the rigid object is large.In both experiments,it was found that fingers had a significant influence on the grasping force index 6).The grasping force index 6)of the index finger was slightly higher than that of the thumb.(2)Secondly,a three-dimensional compression model of the human index distal finger was constructed to obtain the elastic modulus of the subcutaneous tissue of the finger.Then the Abaqus software was used to explore the mechanical differences in the 14 areas of the hand.The 14 regions were divided into 6 categories by cluster analysis.After that,6 finger areas were selected from the clustered 6 categories for orthogonal experiment,and the multiple linear regression method was used to explore the influence of mechanical parameters and size parameters on soft contact behavior of fingers.There is an obvious nonlinear mechanical behavior of the compression force-displacement percentage curve of the distal region of the index finger.Comparing the experimental data with the finite element simulation data,it was found that when the elastic modulus of the subcutaneous tissue in the finger was 0.17 MPa,the compression force-percentage deformation curve of the index finger distal region model from the simulation approximately lay in the error band of the corresponding data from the experiments.The goodness of fit of the quadratic fitting functions of the compression force-displacement curves of the 14 regions of the hand were all greater than 0.98.Therefore,the coefficient of the quadratic fitting function was defined as the soft contact mechanical coefficient.According to the soft contact mechanical coefficients,the 14 areas of the hand were divided into 6 categories.In addition,it was found that the soft contact mechanical coefficients of most fingers’ regions were in the order of middle region > distal region > proximal region.According to the results of multiple linear regression on six selected regions,it was found that the soft contact mechanical coefficients of I1,L1,M1,R3 and T2 were significantly influenced by the elastic modulus of the skin and subcutaneous tissue,and the Poisson’s ratio and thickness of the subcutaneous tissue.The Poisson’s ratio of the skin and the phalanx,and the Poisson’s ratio of the phalanx did not affect the soft contact mechanical coefficients.Furthermore,by returning the six regions to their representative clusters,it was also found that the effect of the elastic modulus of the subcutaneous tissue on the soft contact mechanical coefficient of each finger region followed the order: middle region ≥ distal region ≥ proximal region.(3)Finally,a five-finger bionic mechanical hand was designed.After that,the influence of finger knuckle length on the grasping dexterity was explored on the basis of the five-finger bionic mechanical hand.A five-finger bionic mechanical hand was designed based on the anatomical size of the human hand.The mechanical hand was able to realize the Power palm-thumb abduction grasping mode which is commonly used in manual tomato picking.The grasping experiment was carried out by changing the length of the finger knuckles.The results proved that the relative size between the finger and the object has an influence on the gripping mode.Use the five-finger bionic dexterous hand to grasp tomatoes with a large diameter range of 68.59～77.05 mm,the tomato fruit can be grasped successfully when the length coefficient is 0.7-1.0.When the length coefficient is 0.6,the grasp of the hand was not stable or partly unsuccessful.When the length coefficient is 1.1or 1.2,the middle finger and thumb had a certain interfere during the grasping.In summary,with the inspiration of the non-damaged picking behavior of human hands to tomato fruit,we studied the soft contact and flexible manipulation mechanism of human fingers in tomato-picking.It plays a role in promoting the development of tomato picking robots.