Development of an automatic control system for a hydraulic pruning robot

•A robotic system was presented for automated geometric pruning of fruit trees.•A control strategy based on rolling observations and continuous prediction algorithm was developed.•The control accuracy of all cylinders and operational bars were within ±1.8°.•The waypoints tracking accuracy and the control lag were investigated. Automatic pruning is capable of trimming trees into complex shapes efficiently and continuously. However, this technology faces more challenges under unstructured agricultural environments. In the present paper, we describe the development of a hydraulic disc-saw mounted pruning robot with a relatively low cost for automated geometric pruning of fruit trees based on a new automatic pruning scenario. A pruning control strategy based on rolling observations and continuous prediction algorithm was designed to precisely control the motion of hydraulic cylinders. A forward kinematic model was described using modified Denavit–Hartenberg (D-H) notation, and the accessible workspace for the end-effector was analyzed in the simulation. The inverse kinematics model based on geometry was established, which provided the foundation for the development of a control system together with the forward kinematics model. Performance tests showed a sufficient control accuracy within ±1.8° of all cylinders. The results of the waypoint tracking tests suggest that an increase in the step-size from 20 mm to 60 mm not only increases the moving speed and improves tracking accuracy, but also increases the fluctuation of the end-effector in the Z-axis direction. The movement of the chassis can introduce new errors and results in a larger average absolute positional error along the X-axis. The average startup and stop lag of the control system was 0.55 s and 0.15 s, respectively. This study provides guidance for pruning robot path planning and lays the foundation for future iterations of autonomous pruning robots.