Design of test platform for robot flexible grasping and grasping force tracking impedance control

Abstract: One of the major challenges of agricultural robots is the grasping and picking fresh fruit and vegetable without any damage under the complex environment. In order to minimize the harm due to robot grasp, this paper mainly introduces a dexterous multisensory gripper design and also develops a new force impedance control algorithm. First, the gripper with dual motor drive was designed, and each of fingers had an independent servo drive system and was integrated with force and tactile sensors. The calibration force sensors were mounted at the root of each finger, and the force signals were obtained by the force sensors. The expected force, position and speed parameters of the fingers can be set separately, so the grip centre and stroke can be controlled for the finger by pre-programming, the movement and forces of grasped objects can be actively controlled to achieve through the two-finger operations. The gripper with both the mechanical mechanism and control flexibility was mounted in the end of the industrial robot. The grasping experiment platform composed of industrial robot and dexterous gripper integrated monocular vision, force sensing and many kinds of sensors. The 2-D vision was able to quickly detect and locate grasped objects at the top of the experiment platform. Second, a force impedance control algorithm was proposed and used for one of fingers, position control was used for another finger, and it can regulate the grasping force by de?ning the target impedance between desired position and contact force. The whole grasping system can be equivalent to impedance & admittance model, and the finger force/position control can be equivalent to the expected target inertia-damping-stiffness model, and the model parameters can be adjustable according to the needs to realize the dynamic relationship between grasping force and position. The contact force errors between expected force value and actual force acquired from the force sensor were as the input of impedance controller and its output can be realized by the reference trajectory correction to the internal position control loop. The proposed algorithm only considered the direction of gripping fruit and vegetable, and the reference trajectory can be determined simply, and then avoids the use of complicated impedance control for multi-degree of freedom manipulators, through which it can improve the real-time control and robustness of the grasping system with model uncertainty or external force disturbance. Robot grasping experiments show that the system runs smoothly and reliably, the force-feedback impedance control is very effective, and the steady state error is maximum range within ± 0.4N in the experiment of grasping the tomatoes and eggs. It can make force track value with small force overshot and fast response simultaneously between the end-effector and fruit and vegetable, so it makes the force controller adaptive to the dynamic grasp process between the end-effector and fruit and vegetable, it can realize the flexible grasping and reduce damages and ensure the quality of the grasped fruit and vegetable. The research provides a key control technology for the compliant grasping of fruit and vegetable.