Locomotion-based Litchi Picking Robot Clamping And Cutting Fault Tolerance Mechanism

With the rapid development of the computer and controlling technologies, the fruit and vegetable picking robots have achieved good results in applications, indicating that fruit and vegetable picking develops toward mechanization and intelligence. However, the agricultural environment is very complex which is affected by airflow movement, temperature, crop growth conditions and other factors. It has nonlinear, strong coupling, easy time-varying, multi-disturbing and other features. Therefore, the following problems still exist in the studies of fruit and vegetable picking robots, among which the first one is that disturbance causes big random positioning errors to the visual orientation so that the end configuration is not fault tolerant; the second one is that the clamping and cutting parts cause a lot of damage to fruits and vegetables, with poor universality; the third one is that the uncertainty in vision and environment that affect the behavior control of the robots in time of operating.The studies, with litchi as the study object, proposed a limited universal clamping and cutting parts fault-tolerant mechanism design combining the vision positioning errors and carried out researches on dynamic obstacle avoidance behavior of the picking robots under environmental disturbance. Efficient lossless picking was achieved and a foundation for industrialization of picking robots was laid. The main contents were shown as follows:(1) Analysis on the mapping relationship between the visual positioning errors and terminal structures was run in the studies. Error positioning measurement and analysis methods of dynamic targets was proposed. The errors were classified according to the error variation, and dynamic positioning errors were classified into the system errors and random errors. Statistical methods were adopted for quantitative analysis to determine the positioning capability of the visual system. Results showed that, through positioning experiments under indoor and outdoor environments, the maximum dynamic positioning errors in visual depth and horizontal direction were 60.1 and 17.3 mm. According to visual positioning error variation law, the design theory of a new fault-tolerant method was proposed; measurement errors will be seen as error ellipsoid which is spatially extended and shifted relative to its nominal position, and the shifting distance is determined by the uncertainty; it was determined in the studies that the picking margin of end-effector and the design model of the clamping and cutting parts was given. From the consideration of error relevance and transmission characteristics point of view, the studies adopted mechanism and control methods jointly to eliminate the positioning errors and enabled the clamping and cutting parts to operate within the error range.(2) Analysis on fault-tolerant clamping mechanism and clamping force was run in the studies. A limited universal picking method was also proposed. The studies elaborated the novel humanoid fingers for branch clamping, run analysis on the clamping force closure, developed clip-model under disturbance condition and a new clamping force calculation method. The branch clamping test was operated on press machine. The relationships among clamping force, branch diameter, and maximum snatch force were built, and the quantitative description were also established. Analysis on branch damage was also run. Results showed that the novel humanoid fingers can realize stable clamping on different sizes of branches and have little damage on them. The stable clamping can achieve under disturbance condition. The success rate was 100 % when clamping force was 15 N.(3) Analysis on cutting mechanism which cooperates with fringers was run in the studies. The finite element simulation of explicit dynamics was proposed to obtain the optimal values. For the problem of complicated structure and large size of the traditional fruit and branches separating cutting mechanisms, the new, slight cutting edge cutter was developed. Analyses of the nonlinear movement principle, micro-cut principle and variable cut mathematical models of the cutting blades and simplify the cutting action into elastic mechanics models of blades’ fixed to one end, and the other supported by a spring on the other end were respectively run. The studies proposed simulation in the cutting process and found through orthogonal test that the optimal effects can be achieved when cutting thickness was 1.5 mm, sliding cutting angle was 30° and the speed of rotation was 600 rpm.(4) Analysis on dynamic obstacle avoidance control was run in the studies. For the complexity and randomness of obstacles under natural environment, the global and local dynamic obstacle avoidance algorithm for robots under known targets, obstacles properties and location was developed. On the basis of consideration of positioning errors of obstacles and three-dimensional geometry size of robot arms, the improved pseudo distance method to calculate the C- barriers of space picking robots was adopted; the improved A * search algorithm and the efficiency of obstacle avoidance path planning were improved; the obstacle avoidance strategies for random dynamic obstacles was proposed and the effectiveness and practicability of the avoidance mechanism was verified through simulation experiments.