Abstract:According to the requirements of flexible and nondestructive grape-picking, a kind of grape-picking device with a grasping and rotary-cut type of underactuated double fingered hand was developed based on the underactuated principle and cooperative working mode of grasping and rotary-cut. To realize the separation of the fruit from the stalk, an electric motor drove a double fingered hand with four knuckles by a linkage mechanism, and the hand approached from the middle of the grape and enveloped it. Its rotary-cut device installed on the hand drove the cutting disc to cut off the stalk by the jigging and telescopic motion. Based on this design idea, the dimensions and rotation angle range of two knuckles of underactuated finger mechanism were firstly determined by analyzing equatorial diameter of grape. Then, the static model of the underactuated finger mechanism was established, and the driven linkage size was determined according to the requirements of optimal force transfer and well-distributed contact force. Combining with the contact force analysis and squeezing-bursting test of the grapes, maximum contact force of grasping 2kg grape without damage was obtained, which was equal to 20N. Thrust force of the electric motor was obtained by solving the static model of the underactuated finger mechanism, and type choice of the electric motor was directed according to the thrust force. The control system of grape-picking device was designed, and the effective control of maximum contact force was realized according to real-time feedback of contact force from four pressure sensors fixed on knuckles. The motion of the rotary-cut device was realized by adopting the control mode of acceleration and deceleration, cutting disc can effectively cut off the stalk with the speed of 1200r/min. By picking 50 bunches of grapes with the equatorial diameter ranged from 95mm to 200mm, the results of grape-picking tests showed that the picking success rate of the device was 100%, extrusion wound rate of grapes was 5.2%, and average cost time of the device was 29.4s without consideration of time consuming of vision localization of grape and its stalk to complete a grasping and rotary-cut motion.
